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Accelerated radiation polymerization of vinyl-divinyl comonomer systems 1973

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ACCELERATED RADIATION POLYMERIZATION OF VINYL-DIVINYL COIvIONOMER' SYSTEMS by MICHAEL M . MICKO BeChem ( E n q * ) s Slovak T e c h n i c a l U n i v e r s i t y , B r a t i s l a v a , 1959 C.Sc* Slovak T e c h n i c a l U n i v e r s i t y , B r a t i s l a v a , 1966 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department F o r e s t r y We accept t h i s t h e s i s as c o n f i r m i n g t o the r e q u i r e d standard» THE UNIVERSITY OF B R I T I S H COLUMBIA A p r i l , 1973 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced deg ree a t the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r a g r ee t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Depar tment o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Depar tment o f The U n i v e r s i t y o f B r i t i s h C o l u m b i a Vancouve r 8, Canada i ABSTRACT Gel - e f f e c t (Ge) acceleration i n r a d i a t i o n polymerization of methyl methacrylate (MMA) - d i v i n y l monomer (DVM) systems and properties of r e s u l t i n g polymer products have been investigated. Four methacrylate esters with variable molecular bridge length between the double bonds, i . e . , ethylene g l y c o l dimethacrylate (EGDMA), diethylene g l y c o l dimethacrylate (DEGDMA), triethylene g l y c o l dimethacrylate (TrEGDMA) and tetraethylene g l y c o l dimethacry- . late (TEGDMA), were used as DVM accelerators and cro s s l i n k i n g agents. The course of polymerization was followed by temperature (T)-time(t) polymerization exotherm curves. A new technique was developed to determine the "Gel-Effect Point" (GEP) and i n d i v i d u a l polymerization parameters associated with gelation i n crosslinked network, i . e . , polymerization rate c o e f f i c i e n t (PRC), curing time ( t i i ^ ) , o v e r a l l curing rate ( l / t ^ ^ ) * and o v e r a l l acceleration constant (K). The o v e r a l l curing rate was found to be proportional to the volume concentration of cro s s l i n k i n g agent only up to 15 to 20% DVM i n the system. Within t h i s concentration i n t e r v a l 1 1 the o v e r a l l a c c e l e r a t i o n constants increased w i t h molecular d i s t a n c e between DVM double bonds i n the f o l l o w i n g order: EGDMA (1.1 x I C T 3 ) , DEGDMA (1.5 x 10-3), TrEGDMA (1.8 x 1 0 ~ 3 ) , and TEGDMA (2.4 x 10~ 3 rnin"^ cone.""-*). Half-time c o n c e n t r a t i o n v a l u e s , i . e . , c o n c e n t r a t i o n of d i v i n y l monomer re q u i r e d t o reduce the cu r i n g time t o one-half of t h a t f o r pure MMA, increas e d i n reverse order. N u m e r i c a l l y , h a l f - t i m e concentra- t i o n values f o r TEGDMA, TrEGDMA, DEGDMA, and EGDMA were 3, 4, 5, and 7% r e s p e c t i v e l y . The c a l c u l a t e d o v e r a l l a c c e l e r a t i o n constant allowed p r e d i c t i o n and c a l c u l a t i o n of curing times for..,. i n d i v i d u a l comonorner mixtures. The agreement between p r e d i c t e d and exp e r i m e n t a l l y measured values was w i t h i n 5% e r r o r . The de r i v e d e m p i r i c a l equation f o r p r e d i c t i o n of cu r i n g time was a l s o a p p l i c a b l e t o the h e a t - c a t a l y s t p o l y m e r i z a t i o n system. A l l d i v i n y l monomers st u d i e d were found t o be e f f i c i e n t c r o s s l i n k i n g agents and improved the thermomechanical and str e n g t h p r o p e r t i e s of the r e s u l t i n g copolymers. The compression s t r e s s , s t r a i n and toughness e x h i b i t e d w e l l defined maxima w i t h i n 5-lC/o of d i v i n y l monomer i n the mixture. W i t h i n t h i s c o n c e n t r a t i o n i n t e r v a l both maximal a c c e l e r a t i o n and superior mechanical p r o p e r t i e s of copolymers were obtained. The numerical value of copolymer connection number (CN ) was found t o be a u s e f u l s t r u c t u r a l parameter r e l a t i n g mechanical and thermomechanical p r o p e r t i e s w i t h copolymer c r o s s l i n k i n g d e n s i t y . i i i TABLE OF CONTENTS c e o < > « » ' « « « o r> f • o • * o » «• TABLE OF CONTENTS.. LIST OF TABLES.... . LIST OF FIGURES... LIST OF SYMBOLS.. . . . . . . . . . . . ACKNOWLEDGEMENTS... «• * * • • «• • o t> « • • # «• 2 © 0 LJ^II2JR./\T0 Piiz R IE V 11_ v •/» 0 « « « » • © * o & » * e © o « • » ^ « » • 2.1 Radical Chain-Growth Po lymer izat ion. . . « a • a 9 * • 0 0 t> o » e o * • 0 e 2.1.1 Free r ad i c a l s ; formation and 2.1.2 Radiat ion polymerizat ion of v i n y l 2.1.2.1 K inet ic c o n s i d e r a t i o n s . . . . . . . . . 2.1.2.2 Overal l rate of polymerizat ion 2.1.2.3 Bulk polymerizat ion of MMA.... 2.2 Polymerizat ion in Gel-Phase M e d i a . . . . . . . . . . . 2.2.1 Autoacce lerat ion; experimental evidence for g e l - e f f e c t (Ge ) . . . . . . . . . . . 2.2.2 Acce le ra t i on wi th in the cross l inked netv^ork; copolymerization cf v i n y l - d i v i n y l comonomer s y s t e m s . . . . . . . . . . . . . 2.2.2.1 Oross l ink ing and ge la t ion; p red i c t i on of ge l - e f f e c t (Ge). 2.2.2.2 Influence of d i v i n y l monomer molecular bridge length „. PAGE i i i i v i v i i i x i x i i i I 6 6 7 9 10 11 13 15 22 23 28 PAGE 2.2.2.3 A c c e l e r a t e d p o l y m e r i z a t i o n i n f o r m i n g wood p o l y m e r c o m p o s i t e s ( i"/PC ) . . . . « « . . « . » . o o o . . o . . o . o . . . 3 l 3«0 MATERIALS AND MEiHODS...»..«.•................... 36 3 o 1 Mon 0 me r S e « o . « . . « o « o A 0 o # « o . o o e o t t . o « o . 0 . o « o o o . . 36 3.1.1 M e t h y l m e t h a c r y l a t e . ••....«<»•••••...«.• • 36 3.1.2 D i v i n y l m o n o m e r s . » . . . . o . . . . . . . . . . . . . . . « 36 3.1.3 C a l c u l a t i o n o f s t r u c t u r a l p a r a m e t e r s . . . 37 3.1.4 P r e p a r a t i o n o f comonomer m i x t u r e s . . . . . . 39 3.2 R a d i a t i o n P o l y m e r i z a t i o n T e c h n i q u e . . . . . . . . . . . 40 3.2.1 D e f i n i t i o n and d e t e r m i n a t i o n o f p o l y m e r i z a t i o n p a r a m e t e r s . . . . . . . . . . . . . . 4 0 3.2.2 P o l y m e r i z a t i o n p r o c e d u r e . 4 2 3.2.3 R e p r o d u c i b i l i t y o f e x p e r i m e n t a l r e s u l t s 43 3.3 P r o p e r t i e s o f P o l y m e r P r o d u c t s . . . . . . . . . . . . ».. 44 3.3.1 T h e r m o m e c h a n i c a l p r o p e r t i e s . . . . . . . . . . . . 44 3.3.2 M e c h a n i c a l s t r e n g t h p r o p e r t i e s . . . , . , . . . , 4 6 s n HT c»ri TOM 5 2 5.1 R a d i a t i o n P o l y m e r i z a t i o n o f t h e M-4A-TEGDMA C onion o (no IT SystGi iu *«6f)««44«<(ti)e*o«r}aar}M«««««« 5.2 I n f l u e n c e o f M o l e c u l a r B r i d g e L e n g t h i n ^ D i v l n y — Monomers* « o . . . . . . . » . . . . . . 4 . o . o o « . . o o . 5.3 P r e d i c t i o n a n d C a l c u l a t i o n o f C u r i n g T i m e s . . . 61 63 5.3.1 A p p l i c a t i o n o f t h e d e r i v e d e q u a t i o n t o p u b l i s h e d results..»• 5.4 A n a l y s i s o f C r o s s l i n k e d P o l y m e r P r o d u c t , r T h e r m o m e c h a n i c a l C u r v e s . . . . . . . . . . . . . . . 1 0 0 5.5 A n a l y s i s o f P o l y m e r P r o d u c t C o m p r e s s i o n S"LxGSo**o\,x*ciu. n u.xv©so & # •» o«**««* * *>«*«0 © * « * » « * 70 V P A G E 6 tt 0 COwO.L< LIS IOl\fS ftft««94a«0£te»«QG>00Q«*eo«9C.«««O0oe>»««» 7 6 7.0 RECOMMENDATIONS F O R F U R T H E R S T U D Y 78 3 o 0 Lj X T H H.A. 1 LIR.E C 1 I E D « « « M « « « 0 « * o i > « t ( i t o « e o t * t i o « * o » » « 7 9 v i LIST OF TABLES TABLE PAGE 2 - 1 . E f f e c t of c o n v e r s i o n l e v e l on the pol y m e r i z a - t i o n c h a r a c t e r i s t i c s of methyl methacrylate ^ 2-2. Some data on p o l y m e r i z a t i o n of d i m e t h a c r y l a t i c e s t e r s ( p h o t o i n i t i a t o r - benzoin 0,2%) ( 8 ) . . . . ^9 3-1» Some p r o p e r t i e s of methyl methacrylate (MMA) and d i v i n y l monomers (DVM) used i n t h i s study- 90 3- 2. Concentrations and r e l a t i v e v i s c o s i t i e s of d i f f e r e n t methyl methacrylate (Mr.'A)-divinyl monomer (DVM) mixtures, .......... 4 - I« Polymerization•exotherm c h a r a c t e r i s t i c s f o r the' MMA-TEGDMA comonomer system (67,69)....... 92 4-2. C a l c u l a t e d p o l y m e r i z a t i o n parameters f o r the MMA--TEGDMA c omonomer system (67,69) ........... 93 4-3. P o l y m e r i z a t i o n exotherm c h a r a c t e r i s t i c s f o r MMA-EGDMA, MMA-DEGDMA and MMA~ TrEGDMA -i QA. c omon oine r sysuems » o . « « . . o . ?. ?. o . . . . . . . . ' • 4-4. C a l c u l a t e d p o l y m e r i z a t i o n parameters f o r MMA- EGDMA, MMA-DEGDMA and MMA-TrEGDMA comonomer 4-5. D i f f e r e n c e s between measured and c a l c u l a t e d c u r i n g times f o r lower c o n c e n t r a t i o n s of d i v i n y l monomer (DVM) i n MMA-DVM systems...... 96 4-6. D i f f e r e n c e s between measured and c a l c u l a t e d c u r i n g times f o r lower c o n c e n t r a t i o n s of TEGDMA i n the styrene (S)-TEGDMA system (70).. ' 97 4-7. D i f f e r e n c e s i n c a l c u l a t e d and p u b l i s h e d c u r i n g times f o r TBS.-di-and t r i - v i n y l comonomer systems ( 5 3 ) . . . . i . . . . . . . . . . . . . « 98 4-8. D i f f e r e n c e s i n c a l c u l a t e d and pu b l i s h e d c u r i n g times f o r h e a t - c a t a l y s t cured MMA-trimethylol propane t r i r n e t h a c r y l a t e (TMPTMA) po l y m e r i z a - u X Oil Sy S*t GHl ( 3 ) a t u p o :» i a 3 » a o « aQ4 + o«oee*i<i6&*e4o V l l PAGE 4-9. R a d i a t i o n p o l y m e r i z a t i o n of TEGDMA at d i f f e r e n t dose 4-10. Thermomechanical p r o p e r t i e s of r a d i a t i o n cured poly(MMA) and c r o s s l i n k e d MMA-DVM polymer prOdliCtS» e « o # » « o » * o a ( > O i » 4 o O ' j e c * « > j i ) i ) 8 i ) e * « » o O ( « 9 « o » 101 4-11. Mechanical p r o p e r t i e s of r a d i a t i o n cured . poly(MMA) and c r o s s l i n k e d MMA-DVM polymer 4-12. C a l c u l a t e d parameters from compression s t r e s s - s t r a i n curves f o r r a d i a t i o n cured poly(MMA) and MMA-DVM polymer products. 103 v i i i LIST'OF FIGURES FIGURE PAGE 3-1. S o l u t i o n of a t y p i c a l p o l y m e r i z a t i o n exotherm curve i n c l u d i n a . d e r i v a t i o n of " G e l - E f f e c t P o i n t " (GEp)» "Cure" (MAX), and showing " A c t i v a t i o n " ( l ) and " A c c e l e r a t i o n " ( I I ) ^ • 3-2, T y p i c a l thermomechanical curves f o r thermo- p l a s t i c ( l ) , p a r t l y c r o s s l i n k e d (2) and f u l l y c r o s s l i n k e d (3) polymer products; and showing f o r (2) s o l u t i o n s f o r g l a s s t r a n s i t i o n temperature (Tg), thermal d i s t o r t i o n tempera- t u r e (TDT') and slope (s) i n the t r a n s i t i o n ^ 3-3. Polymer s t r e s s - s t r a i n curve nomenclature and t y p i c a l c u r ves f o r d i f f e r e n t types of p l a s t i c s ( 9 0 7 9 ) - 1 - 0 0 4-4. .tio n s h i p between t?,^x a n c * volume concen- t r a t i o n of TEGDMA i n the "MMA-TEGDMA comonomer 4-1. Relai  — — — — — - - - ~ — — mixture (6 / , 6 9 ) . . . . . . . . . . . . . . . . . o . . . . . . . . . . . . . 4-2. P o l y m e r i z a t i o n r a t e c o e f f i c i e n t s i n " A c t i v a t i o n " (PRCi) and " A c c e l e r a t i o n " (PRCn) p e r i o d s as f u n c t i o n s of TEGDMA volume concen- t r a t i o n i n the MMA-TEGDMA comonomer mixture (67 69) 1 Q 8 4-3. R e l a t i o n s h i p between o v e r a l l c u r i n g r a t e (l/ts.-w) and TEGDMA volume c o n c e n t r a t i o n i n the MMA-TEGD;- "iA comonomer mixture (67.69)...... R e l a t i o n s h i p between t M A X and d i v i n y l monomer (DVM)'volume c o n c e n t r a t i o n i n MMA-DVM 4-5. R e l a t i o n s h i p between tQEP and divinyl-monomer (DVM) volume c o n c e n t r a t i o n i n MMA-DVM corn0nornGx r n i x t i J X G S o * i » v t f » o * < > 4 o © L > » # « o < » # * e <?©«•<>*»* 4-6. P o l y m e r i z a t i o n r a t e c o e f f i c i e n t s i n " A c t i v a t i o n " (PRCj) and " A c c e l e r a t i o n " ( P R C T J ) p e r i o d s as f u n c t i o n s of d i v i n y l monomer (DVM } volume c o n c e n t r a t i o n i n MMA-DVM comonomer 110 111 112 Ix FIGURE 4 - 7 , R e l a t i o n s h i p between t ^ ^ and t G E p f o r d i f f e r e n t ' MMA-DVM comonomer sys tems and t h e 4 - 8 . R e l a t i o n s h i p between o v e r a l l c u r i n g r a t e UAr/iAx) and DVM c o n c e n t r a t i o n i n MMA-DVM 4 - 9 . O v e r a l l a c c e l e r a t i o n c o n s t a n t (K) and i t s r e c i p r o c a l . (l/l<) as f u n c t i o n s of d i v i n y l monomer c o n n e c t i o n number (CNQYM* T a b l e 3 - 1 ) . . 4 - 1 0 . R e l a t i o n s h i p between o v e r a l l c u r i n g r a t e ( l / t j , / ^ ) and DVM c o n c e n t r a t i o n i n t - b u t y l s t y r e n e (TBS)-DVM m i x t u r e s , E v a l u a t i o n of o r i g i n a l hea t c a t a l y s t d a t a f r o m ( 5 3 ) . . . o o a « o 4 - 1 1 . O v e r a l l c u r i n g r a t e ( l A ^ v O a s a f u n c t i o n of t h e r a d i a t i o n dose r a t e . 4 - 1 2 . Shape of t h e r m o m e c h a n i c a l c u r v e s f o r MMA- EGDMA po l ymer p r o d u c t s . . . . . . . . . . . . . . . . . . . . . . . . . 4 - 1 3 . Shape of t h e r m o m e c h a n i c a l c u r v e s f o r MMA- i E G D i v i A polymex' p r o d u c e s . . . . . . . . . . . . . . . . . . . . . . . . . 4 - 1 4 . G l a s s t r a n s i t i o n t e m p e r a t u r e (T c.) as a f u n c - t i o n of copo l ymer c o n n e c t i o n number ( C M C 0 ) f o r d i f f e r e n t MMA-DVM po l ymer p r o d u c t s 4 - 1 5 . Therma l d e f o r m a t i o n deg ree (TDD) and t h e r m a l d i s t o r t i o n t e m p e r a t u r e (TDT) as f u n c t i o n s of copo l ymer c o n n e c t i o n number ( C M C 0 ) f o r d i f f e r e n t MMA-DVM po lymer p r o d u c t s . . . . . . . . . . . . . 4 - 1 6 . L i n e a r t h e r m o m e c h a n i c a l d e f o r m a t i o n c o e f f i c i e n t (LTDC) as a f u n c t i o n of d i v i n y l monomer (DVM) volume c o n c e n t r a t i o n i n MMA-DVM po lymer p r o d u c t s . . . . . 4 - 1 7 . L i n e a r t h e r m o m e c h a n i c a l d e f o r m a t i o n c o e f f i c i e n t (LTDC) as a f u n c t i o n of copo l ymer c o n n e c t i o n number ( C N c o ) f o r d i f f e r e n t MMA-DVM po l ymer p r o d u c t s . 4 - 1 8 . Shape of c o m p r e s s i o n s t r e s s - s t r a i n c u r v e s f o r MMA-EGDMA po l ymer p r o d u c t s . . . . . . c . . . . . X FIGURE 4-19. Shape of compression s t r e s s - s t r a i n curves f o r MMA-TEGDMA polymer p r o d u c t s . O O 9 O O 9 9 0 O & O 9 4-20, Compression s t r e s s - s t r a i n parameters as f u n c t i o n s of comonomer composition. The MMA- EGDMA comonomer system............. 0 0 0 0 0 * 0 0 0 4-21. Compression s t r e s s - s t r a i n parameters as f u n c t i o n s of comonomer composition. The MMA DEGDMA comonomer system.......... Q G 4 O 0 0 O 0 O 0 O 4-22. Compression s t r e s s - s t r a i n parameters as f u n c t i o n s of comonomer composition. Ihe MMA TrEGDMA comonomer system......,,,. O 0 0 O 0 0 Q 0 0 4-23. Compression s t r e s s - s t r a i n parameters as f u n c t i o n s of comonomer composition. The MMA TEGDMA comonomer system. 4-24. Compression s t r e s s at ru p t u r e as a f u n c t i o n copolymer c o n n e c t i o n number (CM C 0) f o r d i f f e r e n t MMA-DVM polymer products 4-25. R e l a t i o n s h i p between area under the compres- s i o n s t r e s s - s t r a i n curve (F) and p l a s t i c deformation f o r d i f f e r e n t MMA-DVM polymer products, 0 O O * e « « O O 0 O 0 * O 9 « < ) « 0 » O O « « 0 » O O x i LIST OF SYMBOLS a = Number of 2-way connected atoms A MA A l l y l methacrylate AN = A c r y l o n i t r i l e b Number of 3-way connected atoms BMA Butyl" methacrylate c = Number of 4-way connected atoms Conn e c t i o n number CM C N c o — Copolymer c o n n e c t i o n number d = S p e c i f i c g r a v i t y , g/cm3 D — R a d i a t i o n dose, Mrad dp Depth of p e n e t r a t i o n , mm DP - Degree of p o l y m e r i z a t i o n DVB — D i v i n y l benzene DVM D i v i n y l monomer DEGDMA Di e t h y l e n e g l y c o l d i m e t h a c r y l a t e E Modulus of e l a s t i c i t y , kg/cm^ EGDMA Ethyl e n e g l y c o l d i m e t h a c r y l a t e Monomer f u n c t i o n a l i t y 0 f r - Area under the s t r e s s - s t r a i n curve, cmf- Ge — G e l - e f f e c t GEP = " G e l - E f f e c t P o i n t " h = Polymer sample h e i g h t , mm k S e n s i t i v i t y c o n s t a n t , m i l s / i n c h . K O v e r a l l a c c e l e r a t i o n constant, min" . conc k p = P.ropagation r a t e constant k t T e r m i n a t i o n r a t e constant LTDC L i n e a r thermomechanical deformation c o e f f i c i e n t , l/°C M Monomer MAX = Exotherm maximum, "Cure" mf Mole f r a c t i o n M/AA = Methyl methacrylate MW Mo l e c u l a r weight Number of -G-CH2-CH2- u n i t s i n DVM n OAC O v e r a l l a c c e l e r a t i o n constant OCR O v e r a l l c u r i n g r a t e 05 R O v e r a l l g e l a t i o n r a t e P = Extent of o o l y c o n d e n s a t i o n r e a c t i o n , % • P Dead polymer PPC Paper polymer composites PRC = P o l y m e r i z a t i o n r a t e c o e f f i c i e n t , °C/min PPC i n " A c t i v a t i o n " p e r i o d , °C/min PRC! PRC II PRC i n " A c c e l e r a t i o n " p e r i o d , °C/min poly(MMA) Po l y ( m e t h y l methacrylate) p o i y ( S ) P o l y s t y r e n e x i i R* = Primary r a d i c a l Ri = Rate of i n i t i a t i o n RM* = Growing polymer r a d i c a l Rp = Ove r a l l rate of polymerization, / 6/hr s = Slope of thermomechanical curve i n t r a n s i t i o n region S = Styrene t = Time, min T = Temperature, °C TBS = t-Butyl styrene TDD = Thermal deformation degree, % TDT = Thermal d i s t o r t i o n temperature, °C TEGDiMA = Tetraethylene g l y c o l dimethacrylate Tg = Glass t r a n s i t i o n temperature, °C tGEP - Time to onset GEP, min ?GEP = Temperature at GEP, °C tr̂ AX " Time to reach exotherm maximum, min TMAX - Temperature at exotherm maximum, °C TfvlPTA = Trimethylol propane t r i a c r y l a t e TMPTMA = Trimethylol propane trimethacrylate TrEGDMA = Triethylene g l y c o l dimethacrylate VA = V i n y l acetate VAD = V i n y l adipate VC = V i n y l chloride VPC = Veneer polymer composites VS = V i n y l succinate WPC = Wood polymer composites I = "Activation" period II = "Acceleration" period %xel = Relative v i s c o s i t y x i i i ACKNOWLEDGEMENTS I wish t o acknowledge c o n t r i b u t i o n s of a l l my present and p r e v i o u s t e a c h e r s . I t has been my p r i v i l e g e t o have been c l o s e l y a s s o c i a t e d w i t h Dr. L. Paszner, Research A s s o c i a t e , F a c u l t y of F o r e s t r y and Dr. J . W. Wilson, P r o f e s s o r , F a c u l t y of F o r e s t r y . I wish t o express my thanks t o both of them f o r v a l u a b l e a d v i c e , c o n s t r u c t i v e c r i t i c i s m , t h o u g h t f u l c o n s i d e r a t i o n s and h e l p f u l suggestions d u r i n g the i n v e s t i g a t i o n s and p r e p a r a t i o n of t h i s d i s s e r t a t i o n . T h e i r support provided over the past y e a r s i s deeply a p p r e c i a t e d . I wish t o thank the U n i v e r s i t y of B r i t i s h Columbia f o r Graduate F e l l o w s h i p s : the F a c u l t y of F o r e s t r y f o r Research F e l l o w s h i p s and other f i n a n c i a l support; Dr, R. V/. Wellwood and Dr. N. C. Franz f o r support from an NEC op e r a t i n g grant d u r i n g f i n a l stages of t h e s i s p r e p a r a t i o n . A p p r e c i a t i o n i s a l s o expressed t o Miss E. Budyk and t e c h n i c a l s t a f f ; i n p a r t i c u l a r , t o Mr, U. Rumma and G„ Bohnenkamp f o r help with c o n s t r u c t i o n and c a l i b r a t i o n of the thermomechanical a n a l y z e r . Last but not l e a s t , I am g r a t e f u l t o my wife G i t k a and c h i l d r e n M i c h e l l e and Martin-Mike f o r t h e i r g r e a t p a t i e n c e and s a c r i f i c e over the many weekends devoted t o t h i s study and I t s r e p o r t i n g . 1 INTRODUCTION Wood i s a valuable material, used t r a d i t i o n a l l y i n i t s natural state. Ease of f a b r i c a t i o n and f i n i s h i n g , pleasing appearance, high strength to weight r a t i o and low cost are c h a r a c t e r i s t i c s contributing to i t s wide use. Numerous attempts have been made to improve inherent wood properties. Finishes have been developed to increase resistance to weathering and mechanical abrasion, treatments have been developed to improve dimensional s t a b i l i t y or to impart greater resistance to decay, f i r e and biodegradation. One treatment of current i n t e r e s t i s the combination of wood with polymers to form wood polymer composites (WPC). A means f o r producing WPC involves impregnation and i n si.tu polymerization of v i n y l monomers i n wood structures. Among techniques available f o r curing WPC, r a d i o l y t i c polymerization has-been much examined (4,5,30,31,49,54,55,56, 82,84,85,90,93,99). After a decade of extensive experimenta- t i o n and development, WPC products have begun to appear i n the market (30). Monomers such as methyl methacrylate (MMA) and styrene (S) are economically a t t r a c t i v e . A major r e s t r i c t i o n imposed by most monomers employed i n WPC manufacture has been long curing schedules, and 2 coxresponding high dosages, required to complete r a d i o l y t i c polymerization. The requirement with S, as example, i s not only beyond reasonable economics, but so large as to degrade the wood substrate. C e r t a i n l y , better r a d i a t i o n e f f i c i e n c y has become a necessity to further process development. Several methods have been used to accelerate r a d i a t i o n polymerization of v i n y l monomers incorporated i n wood matrices. Addition of low molecular weight compounds, p a r t i c u l a r l y carbon t e t r a c h l o r i d e (CC1 4), was found to accelerate the r a d i a t i o n curing of various v i n y l monomers (30,55,90,93). However, secondary e f f e c t s caused by CC1 4, p a r t i c u l a r l y by i t s function as a chain-transfer agent (22), are so disadvantageous that i t i s not considered for technological applications (30,82). It has been known from the theory of monomer gelation (38) that incorporation of small amounts of c r o s s l i n k i n g agents, containing two or more v i n y l groups, allows formation of three-dimensional networks which, during early stages of polymerization reactions would turn the comonomer system into a g e l . R e l a t i v e l y l i t t l e work has been done to u t i l i z e the phenomena of gelation to accelerate r a d i a t i o n polymerization. So f a r only Raff ££. al.. (84) and Kent e_fc_ ad. (55) have paid l i m i t e d attention to acceleration i n v i n y l - d i v i n y l comonomer systems as related to r a d i a t i o n processing of WPC. Recently, during quite 3 advanced stages of t h i s work, Kenaga (53) and Duran and Meyer (33) attempted to u t i l i z e t h i s method i n randomly chosen heat-catalyst systems. A systematic study of g e l a t i o n k i n e t i c s i n v i n y l - d i v i n y l comonomer systems i s completely lacking. Up to t h i s point no quantitative information has been available on the following parameters: 1. differences i n curing rates before and aft e r the Gel-Effect Point (GEP), i i . the temperature p r o f i l e during polymerization, i i i . e f f i c i e n c y of d i f f e r e n t c r o s s l i n k i n g agents, i v . the influence of molecular bridge length between double bonds of d i v i n y l monomer on regulating the propagation and termination reactions i n gelled network, and v. the r e l a t i o n between overal l curing rate (OCR) and composition of comonomer mixtures. The primary objective of the present study was to t e s t the hypothesis that r a d i a t i o n polymerization of v i n y l - d i v i n y l comonomer systems may be accelerated by forming gelled three-dimensional networks. Furt h e r , c h a r a c t e r i s t i c s of these networks should depend on d i v i n y l monomer (DVM) features, such as bridge length between the double bonds. This should allow predictions of the polymerization course 4 and OCR by merely considering the DVM chemical structure. Since the hypothesis involves regulation of i n d i v i d u a l elementary steps of polymerization reactions i t i s expected that such findings would be f u l l y applicable to acceleration of heat-catalyst systems. Further purposes of t h i s work were to: i . develop techniques which allow monitoring of ra d i a t i o n polymerization to complete conversion, determination of GEP and associated parameters, i i . investigate acceleration polymerization across wide ranges of comonomer concentration with various systems, and i i i . elucidate properties of various polymer products available from the systems examined and attempt to r e l a t e these to monomer st r u c t u r a l features. MMA was selected as the basic monomer for the study because of i t s present u t i l i t y i n WPC products and because of wide l i t e r a t u r e available on such composites. Another reason was compatibility of MMA with a commercially available homologous DVM serie s of gl y c o l dimethacrylate esters. The DVM cr o s s l i n k i n g agents were ethylene g l y c o l dimethacrylate (EGDMA), diethylene g l y c o l dimethacrylate (DEGDMA), tr i e t h y l e n e g l y c o l dimethacrylate (TrEGDMA) and tetraethylene g l y c o l dimethacrylate (TEGDMA). To the best of the author 1 s knowledge no l i t e r a t u r e r e l a t e s molecular bridge length with gelation or acceleration i n v i n y l - d i v i n y l comonomer systems. The source of radiation rwas gamma rays from decaying 60Q q. It was chosen on the basis -of i t s a v a i l a b i l i t y , proven effectiveness i n i n i t i a t i o n of MMA polymerization and i t s high penetrating power, normally required for large sections of wood composite products. 6 2.0 LITERATURE REVIEW 2.1 R a d i c a l Chain-Growth P o l y m e r i z a t i o n Polymers of the type important t o t h i s study are macromolecules b u i l t up by the l i n k i n g together of a l a r g e number of unsaturated molecules termed monomers. R a d i c a l chain-growth p o l y m e r i z a t i o n i s i n i t i a t e d by r e a c t i v e s p e c i e s c o n t a i n i n g unpaired e l e c t r o n s , i . e . , f r e e r a d i c a l s . 2.1.1 Free r a d i c a l s ; f o r m a t i o n and r e a c t i o n s Organic molecules c o n t a i n i n g one or more unpaired e l e c t r o n s are termed f r e e r a d i c a l s . They can be e i t h e r monoatomic s p e c i e s or polyatomic aggregates of d i f f e r e n t chemical s t r u c t u r e . Free r a d i c a l s are g e n e r a l l y c o n s i d e r e d t o be h i g h l y u n s t a b l e s p e c i e s because of t h e i r v e r y short l i f e t i m e i n r e a c t i o n systems, p a r t i c u l a r l y i n homogeneous l i q u i d or gaseous r e a c t i o n media. P r o d u c t i o n of f r e e r a d i c a l s may be accomplished by e i t h e r of two g e n e r a l r e a c t i o n s : i . h omolytic decomposition of c o v a l e n t bonds, or i i . g e n e r a t i o n by an e l e c t r o n t r a n s f e r mechanism. Homolytic decomposition of c o v a l e n t bonds i n t o two 7 or more r a d i c a l fragments may be accomplished by absorption of energy i n almost any form; thermal, electromagnetic, e l e c t r i c a l , sonic or mechanical. A l l these energy forms have been used f o r f r e e r a d i c a l production and i n i t i a t i o n of r a d i c a l chain-growth p o l y m e r i z a t i o n (22,36,59,60,78,106). For p r a c t i c a l a p p l i c a t i o n s thermal and electromagnetic energy are the most important means by which f r e e r a d i c a l s are produced. The l a t t e r g e n e r a l l y i n c l u d e s e i t h e r of two major sources, u l t r a v i o l e t l i g h t or high energy r a d i a t i o n . These are termed i o n i z i n g r a d i a t i o n . Free r a d i c a l s take part i n a v a r i e t y of d e s t r u c t i v e chemical r e a c t i o n s , such as o x i d a t i o n and thermal degradation. The most important s y n t h e t i c type r e a c t i o n s i n v o l v i n g f r e e r a d i c a l s are: i . a d d i t i o n t o m u l t i p l e covalent bonds, and i i . r a d i c a l t e r m i n a t i o n r e a c t i o n s . Both r e a c t i o n s are c r u c i a l to r a d i c a l chain-growth p o l y m e r i z a t i o n w i t h v i n y l monomers and w i l l be discussed l a t e r i n r e l a t i o n t o s p e c i f i c problems of the present study. 2»lo2 R a d i a t i o n p o l y m e r i z a t i o n of v i n y l monomers Ra d i a t i o n induced p o l y m e r i z a t i o n of v i n y l monomers i s a d i r e c t a p p l i c a t i o n of r a d i a t i o n chemistry. Such syntheses of high polymers have been used f o r more than three 8 decades and now represent w e l l e s t a b l i s h e d methods f o r producing a v a r i e t y of polymers and copolymers. R a d i a t i o n p o l y m e r i z a t i o n i s not j u s t a l a b o r a t o r y c u r i o s i t y , but i n c l u d e s i n d u s t r i a l a p p l i c a t i o n s . I t s a p p l i c a t i o n t o production of wood polymer composites (WPC), f o r example, i s c u r r e n t l y p r a c t i c e d commercially and under f u r t h e r development (30,55,56,90). I t i s known tha t the i n i t i a t i o n step i n r a d i c a l chain-growth p o l y m e r i z a t i o n r e a c t i o n s r e q u i r e s a p p l i c a t i o n of e x t e r n a l energy. In r a d i a t i o n p o l y m e r i z a t i o n t h i s energy i s s u p p l i e d by i o n i z i n g r a d i a t i o n from a source. Once the c h a i n - growth r e a c t i o n i s s t a r t e d , however, the p o l y m e r i z a t i o n proceeds independent of the energy source according t o conventional k i n e t i c r u l e s derived f o r chain-growth polymeriza- t i o n (22,36,42). Main f e a t u r e s of v i n y l monomer r a d i a t i o n p o l y m e r i z a t i o n are b a s i c a l l y s i m i l a r t o those of conventional heat-catalyzed f r e e r a d i c a l p o l y m e r i z a t i o n r e a c t i o n s . The chemical a c t i o n of r a d i a t i o n i s l i m i t e d t o the primary i n i t i a t i o n step l e a d i n g t o p r o d u c t i o n of f r e e r a d i c a l s and t o a few p e c u l i a r secondary e f f e c t s , such as degradation and c r o s s l i n k i n g . R a d i a t i o n p o l y m e r i z a t i o n mechanisms have been described by Chapiro i n an e x c e l l e n t monograph (22), as w e l l as by others (24,42,74). In a l l these stud i e s . 9 f e a t u r e s of r a d i a t i o n p o l y m e r i z a t i o n k i n e t i c s are discussed. In a d d i t i o n , the present study r e q u i r e s s p e c i a l emphasis on a c c e l e r a t i o n v i a g e l - e f f e c t (Ge) phenomena i n h i g h l y v i s c o u s and c r o s s l i n k e d s t r u c t u r e s . 2 . 1 . 2 . 1 K i n e t i c c o n s i d e r a t i o n s K i n e t i c s of chain-growth p o l y m e r i z a t i o n i s based on i n t e r p r e t a t i o n of experimental r e s u l t s from f r e e r a d i c a l r e a c t i o n s . At l e a s t three types of r e a c t i o n s are described (22) as: i . i n i t i a t i o n , which produces f r e e r a d i c a l s , * i i . propagation, which i n v o l v e s a sequence of i d e n t i c a l r e a c t i o n s repeated many times, and i i i . t e r m i n a t i o n , which stops the chain-growth r e a c t i o n . I t i s a l s o p o s s i b l e t h a t a s i n g l e f r e e r a d i c a l may i n i t i a t e formation of more than one polymer molecule through a r e a c t i o n i n which the a c t i v e s i t e , i . e . , f r e e r a d i c a l , i s t r a n s f e r r e d without l o s s of a c t i v i t y t o another molecule i n the system. This type of r e a c t i o n i s termed a c h a i n - t r a n s f e r r e a c t i o n . Formation of macromoleculas c o n t a i n i n g thousands of c o v a l e n t i y bonded monomer u n i t s i s an o v e r a l l r e s u l t of the above elementary r e a c t i o n s . .10 2.1.2.2 O v e r a l l r a t e of p o l y m e r i z a t i o n The o v e r a l l r a t e of v i n y l monomer p o l y m e r i z a t i o n , i . e . , the r a t e of monomer disappearance i n the r e a c t i o n system, was described (22,42) by the f o l l o w i n g k i n e t i c scheme: Process: Rates: I n i t i a t i o n : X 2 R R^ <........... 0/2™l/ Propagation: R° + M > RM* RMx + M —-> RM; + 1 kp/RMV ' / iV . . . . . . / 2 - 2 / Termination: RM° + RM' -> P x + y k t/RM*/ 2 ........./2-3/ where: X any molecule i n the system, R* = primary r a d i c a l , M = monomer; RM° = growing polymer c h a i n ^ P = dead polymer, = r a t e of i n i t i a t i o n , kp = r a t e constant f o r propagation, and k^ = r a t e constant f o r t e r m i n a t i o n . A f t e r mathematical treatment and steady sta t e assumptions, the o v e r a l l p o l y m e r i z a t i o n r a t e ( Rp) i s given as: -l/2m l / 2 , . , . This c l a s s i c a l equation i n d i c a t e s t h a t r a t e oi p o l y m e r i z a t i o n i s p r o p o r t i o n a l t o the square root of i n i t i a t i o n 11 1/2 r a t e . The quantity k /fc, , a constant value at steady state P <* conditions, changes at higher conversion degree and accelerates the o v e r a l l reaction rate. As a second consequence an increase i n molecular weight (M.W), or degree of polymeriza- t i o n (DPjjis observed. This i s required by the following equation (22): "''*•* DP = k A f / 2 - R i l / 2 « M /2-5/ P t x With respect to a monomer of int e r e s t i n t h i s study, (MMA), the published r e s u l t s on polymerization k i n e t i c s f u l l y confirm /2-4/ and /2-5/ (22,59,106). Bulk polymerization of MMA, e s p e c i a l l y to the high conversion degree, e x h i b i t s deviations from these t h e o r e t i c a l predictions (6,22,64,75,87, 89,103). 2.1.2.3 Bulk polymerization of MMA Much work has been done i n the area of MMA r a d i a t i o n polymerization. The free r a d i c a l mechanism fo r t h i s reaction was established by Chapiro (22). He deduced the mechanism from data which showed that polymerization was i n h i b i t e d by a i r and benzoquinone. During i n i t i a l reaction stages the polymerization process shows a c h a r a c t e r i s t i c Arrhenius temperature dependence, with an a c t i v a t i o n energy of 4.9 kcal/mole (3,22). The constant value of a c t i v a t i o n energy was observed over wide !2 temperature range ( -18 °C to 70 °C) with dose rates 2.3 to 2.5 x 10^ rad/hr,. In a more recent work by Lipscomb and Weber (60), who operated between -49 and -19 °C and at a dose rate of 3.7 x 10 5 rad/hr, the a c t i v a t i o n energy was found to be 3.85 kcal/mole. It should be pointed out, however, that the freezing point of MMA i s -48;2 °C (86), which casts doubt as to v a l i d i t y of the data at -49 °C. The reaction controlled region i s dose rate dependent. Chapiro (22) reported a dependence to the 0.5 power for low dose rates (l e s s than 5.4 x 10^ rad/hr). Above 10^ rad/hr the reaction was said to be dose rate independent. Ballantine et a l . (3) reported dose rate dependence of 0.5 up to 2.0 x 10~* rad/hr. In contrast Lipscomb and Weber (60), working at approximately the same dose rate, reported a power factor of 0.33 at both -49 °C and -19 °C. For MMA, as f o r other v i n y l monomers, the mechanism of free r a d i c a l polymerization changes d r a s t i c a l l y at high conversion degree. A special mechanism, which predominates and controls l a t e r stages of polymerization k i n e t i c s , has been attributed to Ge. Because of i t s p a r t i c u l a r importance t o the present study, Ge phenomena are discussed i n d e t a i l i n a l a t e r section. Attempts to define o v e r a l l reaction orders have been unsuccessful due to e f f e c t s associated with Ge phenomena. 13 Nevertheless, the MMA p o l y m e r i z a t i o n r e a c t i o n i s considered t o be of zero-order up t o about 20% conversion (12). Rabinowitch (83) derived an expression r e l a t i n g r e d u c t i o n of the t e r m i n a t i o n r a t e constant w i t h the r a t e of d i f f u s i o n f o r second order r e a c t i o n s * Vaughan (104) and l a t e r Robertson (87) t r i e d t o c o r r e l a t e the experimental data f o r MMA and styrene using the Rabinowitch equation. The r e s u l t , however, was u n s a t i s f a c t o r y . Attempts t o q u a n t i f y p o l y m e r i z a t i o n r a t e s i n the Ge r e g i o n have been hampered by two major f a c t o r s . F i r s t l y , the onset of Ge does not occur at a s p e c i f i c conversion. Rather, conversion at which Ge begins i s s t r o n g l y dependent upon temperature. Secondly, experimental measurements i n the Ge r e g i o n are i n h e r e n t l y d i f f i c u l t . Isothermal c o n d i t i o n s are almost impossible t o maintain?and a n a l y s i s of the p a r t l y • p l a s t i c i z e d m a t e r i a l r e q u i r e s s p e c i a l experimental techniques f o r recovery and s o l u t i o n of g e l s and i s o l a t i o n of the unconverted monomer. 2.2 P o l y m e r i z a t i o n i n Gel-Phase Media The k i n e t i c scheme of f r e e r a d i c a l p o l y m e r i z a t i o n , discussed i n S e c t i o n 2.1.2.2, a p p l i e s only t o the i n i t i a l r e a c t i o n stage up t o approximately 5 t o 10% conversion. I f r e a c t i o n s proceed t o higher conversion degrees the m o b i l i t y of r e a c t i v e species i s reduced by a massive increase i n l o c a l 14 v i s c o s i t y . The r a t e of d i f f u s i o n i s s i g n i f i c a n t l y reducedc T h i s phenomenon ma n i f e s t s i t s e l f i n Ge, commonly observed i n bulk p o l y m e r i z a t i o n of v i n y l monomers, p a r t i c u l a r l y w i t h MMA (75,103)• The most important consequence of Ge i s a c c e l e r a t i o n of the o v e r a l l p o l y m e r i z a t i o n r a t e , as soon as some c r i t i c a l c o n v e r s i o n l e v e l has been reached,, I f v i s c o s i t y of the r e a c t i o n medium i s f u r t h e r i n c r e a s e d , long range d i f f u s i o n of polymeric r a d i c a l s i s suppressed and only l o c a l m o b i l i t y of polymer segments ensures l i m i t e d r e a c t i o n . F i n a l l y , i f the system s e t s t o g l a s s , a l l polymeric r a d i c a l s remain immobilized and the r e s i d u a l r e a c t i o n , i f any, r e s u l t s only from d i f f u s i o n of small unreacted monomer molecul e s . Under a g i v e n set of r e a c t i o n c o n d i t i o n s v a r i o u s p o l y m e r i z a t i o n r e a c t i o n s may f a l l i n t o any one of th r e e cases as: i . both p r o p a g a t i o n and t e r m i n a t i o n are c o n t r o l l e d by r e a c t i o n r a t e , i i . p r o pagation i s r e a c t i o n - c o n t r o l l e d and t e r m i n a t i o n i s d i f f u s i o n - c o n t r o l l e d * or i i i . both propagation and t e r m i n a t i o n are diffusion-controiled„ The case of d i f f u s i o n - c o n t r o l l e d p r o p a g a t i o n and r e a c t i o n - c o n t r o l l e d t e r m i n a t i o n i s a p h y s i c a l i m p o s s i b i l i t y , 15 since r e a c t i o n between two polymer r a d i c a l segments would be favored over r e a c t i o n between monomer and polymer r a d i c a l s . 2.2.1 A u t o a c c e l e r a t i o n ; experimental evidence f o r g e l - e f f e c t (Ge) Bulk p o l y m e r i z a t i o n of a number of v i n y l monomers, such as MMA (6,13,62,75,87,89,103), methyl a c r y l a t e (MA) (15,64,65), b u t y l methacrylate (BMA) (14), S (8,39,87) and v i n y l acetate (VA) (59), f o l l o w the patterns of a u t o c a t a l y t i c r e a c t i o n s . The p o l y m e r i z a t i o n proceeds smoothly at f i r s t w i t h constant r a t e , as expected from steady state k i n e t i c s /2-4/. A f t e r a c r i t i c a l conversion i s a t t a i n e d the r e a c t i o n r a t e suddenly i n c r e a s e s s e v e r a l times. The r e a c t i o n at t h i s stage shows a d e f i n i t e a u t o a c c e l e r a t i o n c h a r a c t e r . For S p o l y m e r i z a t i o n , f o r example, the o v e r a l l a c c e l e r a t i o n f a c t o r i s 5.2 at 38% and 16.9 at 60% conversion (59). S i m i l a r l y f o r MMA p o l y m e r i z a t i o n , the i n i t i a l p o l y m e r i z a t i o n r a t e of 3.5%/hr in c r e a s e s t o 15.4%/hr at 30% conversion and 24.5%/hr at 50% conversion (43) as shown i n Table 2-1. Simultaneously w i t h increase i n p o l y m e r i z a t i o n r a t e , s e v e r a l secondary e f f e c t s are evident.. These include increased MW and concomitant l o c a l temperature r i s e (22,36). These e f f e c t s are explained by the decreased t e r m i n a t i o n . I t i s g e n e r a l l y accepted t h a t r e d u c t i o n i n the r a t e of t e r m i n a t i o n i s caused by reduced m o b i l i t y of growing polymer chains i n the visc o u s medium. 16 Measurements of i n d i v i d u a l r a t e constants. Table 2-1 (43)., show t h a t the decrease i n t e r m i n a t i o n rate occurs at f a i r l y low conversion, g e n e r a l l y between 10 to 30%, On the other hand, the propagation r a t e i s enhanced only a f t e r 50 t o 60% conversion, i . e . , a f t e r the r e a c t i o n mixture has already g e l l e d . T h i s i s i n accordance w i t h t h e o r e t i c a l c o n c l u s i o n s drawn by Rabinowitch (83). He concluded t h a t r e a c t i o n s having low a c t i v a t i o n energies are more l i k e l y t o become d i f f u s i o n - c o n t r o l l e d . An average a c t i v a t i o n energy f o r t e r m i n a t i o n i s 0 t o 2 kcal/mole, while a c t i v a t i o n energy f o r propagation i s * 5 t o 8 kcal/mole (22,23,36). A s i m i l a r c o n c l u s i o n was reached by Vaughan (104) who c a l c u l a t e d c r i t i c a l v i s c o s i t i e s f o r elementary r e a c t i o n steps. H i s c r i t i c a l v i s c o s i t i e s f o r t e r m i n a t i o n and propagation were 2.0 and 7,0 x 10~* p o i s e , r e s p e c t i v e l y . I t must be emphasized, however, t h a t the v i s c o s i t y e f f e c t i s not due t o changes i n r a d i c a l r e a c t i v i t i e s . Instead, i t a r i s e s from a purely p h y s i c a l e f f e c t caused by the d i f f u s i o n b a r r i e r . Hayden and M e l v i l l e (43) studied v a r i a t i o n of s e v e r a l MMA p o l y m e r i z a t i o n parameters w i t h conversion, i n c l u d i n g average l i f e t i m e of a k i n e t i c c h a i n and rate constants f o r both propagation and t e r m i n a t i o n . They pointed out t h a t the t e r m i n a t i o n r a t e constant decreased by about 100,000-fold from the s t a r t of p o l y m e r i z a t i o n up t o 50% conversion. The e f f e c t of conversion on k i n e t i c v a r i a b l e s has been d i v i d e d i n t o three d i s t i n c t stages as*. 17 i . Stage 1^ up t o 10% c o n v e r s i o n , i i . Stage I I , between 10 t o 70% conversion, and i i i . Stage I I I ? over 70% conversion. In Stage I the r e a c t i o n mixture changes from a mobile l i q u i d t o a vi s c o u s syrup, but the r a t e of polymeriza- t i o n adheres t o steady s t a t e k i n e t i c s represented by /2~4/. In Stage I I the r e a c t i o n mixture changes from a very v i s c o u s f l u i d t o a s o f t s o l i d . Large incre a s e s are obtained i n both p o l y m e r i z a t i o n r a t e and l i f e t i m e of the k i n e t i c chains. Both increase by approximately ten f o l d . In Stage I I I as conversion nears completion, termina- t i o n appears t o become a unimolecular process and even propagation becomes d i f f u s i o n - c o n t r o l l e d as molecules are immobilized i n the g e l - g l a s s y r e a c t i o n medium. Although propagation i n g e l i s hindered, the o v e r a l l e f f e c t i s much smaller since kp values are smaller than k^ by a f a c t o r of 10 4 t o 10^ (Table 2 - l ) . Termination i n v o l v e s the r e a c t i o n of two l a r g e polymer segments /2-3/„ while i n propagation only one l a r g e polymer r a d i c a l and a small monomer molecule are i n v o l v e d /2-2/. High v i s c o s i t y a f f e c t s the former much more than the l a t t e r . Therefore, the quantity , , l / 2 kp/k^. changes during the p o l y m e r i z a t i o n process. This r e s u l t s , i n accordance w i t h /2-4/, i n an increase of the o v e r a l l p o l y m e r i z a t i o n r a t e w i t h i n c r e a s i n g conversion. As a second consequence, and i n accordance w i t h /'2~b/, an increase i n molecular weight i s obtained.. This has been v e r i f i e d e x p e r i m e n t a l l y a l s o f o r numerous v i n y l monomers (13,14,78). Solvents and c h a i n - t r a n s f e r agents, which d i r e c t l y reduce v i s c o s i t y and molecular weight of polymer products, delay or even e l i m i n a t e the onset of 6e, As shown by S c h u l t z and Haborth (89), a u t o a c c e l e r a t i o n i s completely e l i m i n a t e d when the r e a c t i o n system i s d i l u t e d w i t h 60% of an i n e r t s o l v e n t . Molecular weight of polymer formed i n d i f f e r e n t s o l u t i o n s a l s o decreases w i t h i n c r e a s i n g d i l u t i o n . This i s the r e s u l t of decreased k i n e t i c c h a in l i f e t i m e due t o bimolecular c h a i n t e r m i n a t i o n (6,103). I t was f u r t h e r shown th a t a d d i t i o n of an i n e r t polymer, which i n c r e a s e s v i s c o s i t y of the r e a c t i o n media, brought about a premature appearance of Ge (103). Temperature changes of r e a c t i o n media were measured by s e v e r a l i n v e s t i g a t o r s . I t was noted t h a t a c c e l e r a t i o n i s accompanied by considerable overheating of the system. In order t o avoid a d d i t i o n a l complications, such as created by non-isothermal c o n d i t i o n s , S c h u l t z and Harborth (89) polymerized t h i n l a y e r s of MMA onto a mercury surface. R e s u l t s of t h i s experiment c o n c l u s i v e l y demonstrated that Ge a c c e l e r a t i o n i s a t r u l y k i n e t i c phenomenon. Temperature r i s e at the onset of Ge-'is much more pronounced w i t h MMA than w i t h S ( 3 ) , even at very low polymeriza t i o n temperatures, e.g,, -18 °C. D i f u n c t i o n a i methacrylates 19 and a c r y l a t e s , i r r a d i a t e d i n s o l i d s t a t e , showed remarkable temperature r i s e even at -90 °C r e a c t i o n temperature ( 2 2 ) . A d i r e c t consequence of Ge i n p o l y m e r i z a t i o n under constant r a t e of i n i t i a t i o n i s t h a t the c o n c e n t r a t i o n of growing polymer chains increases w i t h t i m e Thus the poly- m e r i z a t i o n system does not f o l l o w s t a t i o n a r y k i n e t i c s due t o v i o l a t i o n of the steady state assumption. O v e r a l l polymeriza- t i o n r a t e i s not p r o p o r t i o n a l t o the square root of i n i t i a t i o n /2-4./c Indeed, a marked departure from conventional k i n e t i c s i s observed and the r e a c t i o n order w i t h respect t o i n i t i a t i o n r a t e r i s e s above the t h e o r e t i c a l 0.5 value. A complete treatment of Ge k i n e t i c s should take i n t o account both the gradual decrease of t e r m i n a t i o n r a t e and the concomitant increase i n f r e e r a d i c a l c o n c e n t r a t i o n . Such a treatment i s s t i l l l a c k i n g ( 2 3 ) . Reduction i n the t e r m i n a t i o n step i n f r e e r a d i c a l p o l y m e r i z a t i o n , and. consequently premature onset of Ge, can be brought about by sev e r a l p h y s i c a l methods. A l l these methods are based on the p r i n c i p l e that t e r m i n a t i o n by the mutual r e a c t i o n of two polymer chains can be prevented by i s o l a t i n g the growing polymer segments from each other. In many cases, i s o l a t i o n of growing polymer fragments has been accomplished by " o c c l u s i o n " of the a c t i v e end groups w i t h i n d i s c r e t e polymer p a r t i c l e s . T h i s phenomenon i s r e s p o n s i b l e f o r the unusual r a t e behaviour observed i n the f o l l o w i n g systems (59): 20 i . polymerization reactions i n i t i a t e d i n gas pha se, i i . polymerization of a monomer i n a medium which i s non-solvent f o r the r e s u l t i n g polymer, termed p r e c i p i t a t i o n polymerization, i i i . emulsion polymerization, i v . polymerization of c r y s t a l l i n e monomers i n the s o l i d systems, and v. polymerization within crosslinked networks, such as v i n y l - d i v i n y l comonomer systems. Systems i i , i i i and v are f a r the most important, p a r t i c u l a r l y for p r a c t i c a l applications involving WPC, as well as paper polymer composites (PPC). If polymerization i s ca r r i e d out i n a medium i n which the r e s u l t i n g polymer i s insoluble, the growing polymer chains separate from the solution to form a second phase by p r e c i p i t a t i o n . Reaction proceeds thereafter i n heterogeneous medium and usually shows a peculiar behaviour similar i n many respects to Ge. Systems which behave i n t h i s manner include: i . pure monomers which do not dissolve t h e i r own polymer, for example a c r y l o n i t r i l e (AN) and v i n y l chloride (VC), i i . monomer solutions immiscible with solvents which are pr e c i p i t a n t s for the polymer, e.g., methanol-S, methanol-MMA. hexane-VA, and 21 i i i . comonomer mixtures which do not d i s s o l v e the r e s u l t i n g copolymer, as the AN-S comonomer system. A c c e l e r a t i o n during e a r l y r e a c t i o n , stages, a pheno- menon i d e n t i c a l w i t h Ge, has been i n t e r p r e t e d as impeded t e r m i n a t i o n i n the presence of a polymer p r e c i p i t a n t (1,16, 22,75). Here r e l a t i v e conversion r a t e i s a f u n c t i o n of monomer/precipitant r a t i o . Maximum a c c e l e r a t i o n i s observed at p r e c i p i t a n t content s l i g h t l y below the p r e c i p i t a t i o n point where phase sep a r a t i o n leads t o a v i s c o u s , g e l - l i k e coagulate h i g h l y swollen by monomer. The o v e r a l l p o l y m e r i z a t i o n r a t e , at t h i s p o i n t , r i s e s sharply and the r e a c t i o n e x h i b i t s non- steady state k i n e t i c s . P h y s i c a l s t a t e of the p r e c i p i t a t e d polymer and the extent of s w e l l i n g seem t o be very important f a c t o r s i n pr e p a r a t i o n of g r a f t copolymers ( 2 l ) . Thus., i t has been reported that a h i g h l y swollen substrate i s necessary t o form l a r g e amounts of g r a f t copolymers, e.g., increased g r a f t i n g e f f i c i e n c y (21,49,57,96). The alcohol-S system has been used i n a p p l i e d studie f o r r a d i a t i o n cured WPC. Siau et. al_. (90) compared g r a f t i n g e f f i c i e n c y of S on red pine (Pinus r e s i n o s g A i t . ) , as w e l l as on yello w - p o p l a r (Llriodencir.pn t u l i p i f e r a L. ). Ramalingam et a l . (85) evaluated e f f e c t s of a three component system, methanol-S-water, on g r a f t copolymerization e f f i c i e n c y . They concluded t h a t , i n a d d i t i o n t o the observed a c c e l e r a t i o n e f f e c t s , t h i s system increased g r a f t i n g e f f i c i e n c y . This i s i n accordance w i t h t h e o r e t i c a l c o n c l u s i o n s drawn from r a d i c a l l i f e t i m e s observed i n v i s c o u s and swollen systems (21,43). 2.2.2 A c c e l e r a t i o n w i t h i n the c r o s s l i n k e d network; copoly- m e r i z a t i o n of v i n y l - d i v i n y l comonomer systems In many cop o l y m e r i z a t i o n r e a c t i o n s d i v i n y l compounds are used as monomeric c o n s t i t u e n t s . The obvious reason f o r i n c l u d i n g such compounds i s to obtain c r o s s l i n k s between l i n e a r macromolecuies. This leads t o three-dimensional net- works. When d i v i n y l monomers (DVM) are incorporated i n a comonomer system, only one of the double bonds reacts//). The other i s l e f t as a pendant v i n y l group. At higher conversion degree the pendant v i n y l group may be i n v o l v e d i n propagation r e a c t i o n s . At such stage a r a t h e r abrupt t r a n s i t i o n i s experienced i n passing from the l i q u i d t o g e l s t a t e . I f s u f f i c i e n t p o l y f u n c t i o n a l c r o s s l i n k i n g agent i s used, a s i g n i f i c a n t departure from l i n e a r i t y i n the o v e r a l l polymeriza t i o n r a t e can be achieved during e a r l y r e a c t i o n stages due t o a premature onset of Ge phenomena. Increase i n p o l y m e r i z a t i o n r a t e i s a d i r e c t consequence of reducing the t e r m i n a t i o n r e a c t i o n step by i s o l a t i n g the growing polymer r a d i c a l seg- ments i n c r o s s l i n k e d three-dimensional networks (59), 23 The formation of such a net w o r k , r e s u l t i n g i n g e l a t i o n of the comonomer mixture, g e n e r a l l y takes place over a very narrow conversion range and v a r i e s i n v e r s e l y w i t h both the amount of c r o s s l i n k i n g agent and average DP (34,36,97,105). F l o r y (34) o u t l i n e d a general method f o r determining and p r e d i c t i n g the extent of r e a c t i o n s i n which such a network i s p o s s i b l e and c a r r i e d out a d e t a i l e d c a l c u l a t i o n , f o r the case of polycondensation r e a c t i o n s . His c a l c u l a t i o n s were i n good agreement w i t h experimental r e s u l t s . He i n d i c a t e d t h a t a s i m i l a r method may be ap p l i e d t o a d d i t i o n p o l y m e r i z a t i o n , p a r t i c u l a r l y f o r v i n y l - d i v i n y l comonomer systems i n which a l l v i n y l groups have the same r e a c t i v i t y . 2.2.2.1 C r o s s l i n k i n g and g e l a t i o n ; p r e d i c t i o n of g e l - e f f e c t (Ge) According t o F l o r y ' s treatment on polycondensation r e a c t i o n s , Ge i n monovinyl and d i v i n y l comonomer mixtures i n which a l l v i n y l groups have the same r e a c t i v i t y may be c a l c u l a t e d from the f u n c t i o n a l i t y equation (34): where: p = extent of r e a c t i o n f = average f u n c t i o n a l i t y DP = degree of p o l y m e r i z a t i o n 24 For a very l a r g e DP, which i s always the case i n a d d i t i o n p o l y m e r i z a t i o n , /2-6/ reduces t o : With b i f u n c t i o n a l monomer molecules, as a consequence of /2 - 7 / , g e l a t i o n does not occur. With t r i - and t e t r a - f u n c t i o n a l molecules g e l a t i o n occurs at 66 and 50% conversion, r e s p e c t i v e l y . However, p r a c t i c a l experimental r e s u l t s do not c o r r e l a t e w i t h p r e d i c t e d v a l u e s . Large d e v i a t i o n s from t h e o r e t i c a l values were observed even f o r p o l y m e r i z a t i o n of r e l a t e d monomers such as V A - v i n y l succinate (VS), MMA-EGDMA, or S - d i v i n y l benzene (DVB) (97,105). Expl a n a t i o n of these d e v i a t i o n s , as pointed out by F l o r y (36) and W a l l i n g (105), l i e s mostly i n two f a c t o r s : i . a p p l i c a t i o n of the condensation p o l y m e r i z a t i o n f u n c t i o n a l i t y equation t o chain-growth poly- m e r i z a t i o n systems, and i i . excessive occurrence of i n t r a - m o l e c u l a r c r o s s - l i n k i n g before onset of Ge, where wastage of i n t e r - m o l e c u l a r c r o s s l i n k s i n t h i s manner i s neglected by theory. • A l f r e y ei. a_l„ ( l ) concluded t h a t c r o s s l i n k i n g . and subsequent g e l a t i o n depends on r e l a t i v e r e a c t i v i t i e s of two double bonds i n DVM. The point at which Ge occurs has been t r e a t e d mathematically f o r s e v e r a l comonomer systems. In a l l 25 i n s t a n c e s i t has been assumed t h a t DVM was presen t i n a low c o n c e n t r a t i o n , s i n c e t h i s i s the case encountered i n most p r a c t i c a l a p p l i c a t i o n s . G e n e r a l l y , t h r e e cases were c o n s i d e r e d ( l ) a c c o r d i n g t o type of c r o s s l i n k i n g agent, as w e l l as v i n y l monomer used: io Case I ? c o p o l y m e r i z a t i o n of symmetrical DVM with a v i n y l monomer of equal r e a c t i v i t y , i i . Case I I , c o p o l y m e r i z a t i o n of a symmetrical DVM w i t h a v i n y l monomer of d i f f e r e n t r e a c t i v i t y , and i i i . Case I I I ^ c o p o l y m e r i z a t i o n of an unsymmetrical DVM w i t h v i n y l monomer of d i f f e r e n t r e a c t i v i t y . In Case I a l l double bonds have the same r e a c t i v i t y and chemical s t r u c t u r e . Examples of such a system are: i . MMA-EGDMAy i i . V A - v i n y l adipate (VAD), and i i i . S-DVB. In Case I I the r e a c t i v i t y and s t r u c t u r e of double bonds i n DVM d i f f e r from the double bond c h a r a c t e r i n the v i n y l monomer. An example of t h i s system i s S-EGDMA or MMA-DV3. A c c e l e r a t i o n i n t h i s system was s t u d i e d p r e v i o u s l y (70). 26 In Case I I I the two double bonds i n dienes have d i f f e r e n t s t r u c t u r e s and r e a c t i v i t i e s . An example of t h i s type i s MMA-allyl methacrylate (AMA). This system i s very complicated and i n e f f i c i e n t . A l l y l type double bends have g e n e r a l l y lower r e a c t i v i t y , thereby c r o s s l i n k i n g and Ge occur only a f t e r c o n s i d e r a b l e conversion has been a t t a i n e d . In some cases r e a c t i o n of one double bond i n the diene r e s u l t s i n a marked r e d u c t i o n i n r e a c t i v i t y of the remaining double bond. The o v e r a l l e f f e c t i s a marked delay i n Ge. An example of t h i s i s cop o l y m e r i z a t i o n of v i n y l monomers w i t h 1,3-dienes. The 1,4 p o l y m e r i z a t i o n of diene leads t o r e s i d u a l 2,3 double bonds, which have lowered r e a c t i v i t y ( l ) . The most studied dienes of t h i s type are isoprene, butadiene and chloroprene (1,44,58,95). Much experimental m a t e r i a l has been accumulated on cop o l y m e r i z a t i o n of symmetrical DVM w i t h v i n y l monomer of i d e n t i c a l double bond s t r u c t u r e (Case I ) . R e s u l t s derived from experimental data on g e l a t i o n were i n good agreement w i t h p r e d i c t e d values ( l ) only w i t h i n a l i m i t e d DVM co n c e n t r a t i o n range. There has been some controversy about these d i s c r e p a n c i e s (9V,105). Storey (97) studied the a c c e l e r a t i o n e f f i c i e n c y of DVB i n the S-DVB comonomer system using h e a t - c a t a l y s t . He found t h a t i n i t i a l p o l y m e r i z a t i o n r a t e v a r i e d l i n e a r l y w i t h 27 DVB c o n c e n t r a t i o n at both temperatures studied (70 °C and 87 °C). The MMA-EGDMA system was studied by Wa l l i n g (105), He a l s o examined the VA-VAD system. In a l l s t u d i e s at higher DVM co n c e n t r a t i o n agreement between expected r e s u l t s and those obtained was poor. The discrepancy may have a r i s e n i n part from i n t r a - m o l e c u l a r chain c y c l i z a t i o n r e a c t i o n s (17,18,19,38, 40,41,61,92) which prevent DVM p a r t i c i p a t i o n i n i n t e r - m o l e c u l a r c r o s s l i n k i n g . W a l l i n g (105), however, d i d not consider t h i s t o be a major f a c t o r . He i n t e r p r e t e d h i s r e s u l t s i n terms as s o c i a t e d w i t h d i f f u s i o n - c o n t r o l l e d c r o s s l i n k i n g r e a c t i o n . According t o h i s hypothesis, r a t e c o e f f i c i e n t of the c r o s s - l i n k i n g process decreases w i t h i n c r e a s i n g DVM c o n c e n t r a t i o n . Simpson et. a l . (91) showed th a t a considerable f r a c t i o n of DVM i s used i n the i n t r a - m o l e c u l a r c y c l i z a t i o n r e a c t i o n . This r e s u l t e d i n delayed g e l a t i o n at comparatively high conversion degrees. Subsequently, Gordon and Roe (40) examined a p p l i c a t i o n of the c l a s s i c a l g e l a t i o n theory t o the MMA-EGDMA comonomer system. They concluded t h a t t h i s theory was a p p l i c a b l e t o t h e i r r e s u l t s only i f f u r t h e r allowances were made f o r i n t e r n a l c y c l i z a t i o n . F u r t h e r , d i f f u s i o n c o n t r o l l e d c r o s s l i n k i n g , as described by Wa l l i n g (105), was unimportant u n t i l a f t e r Ge had been passed. L a t e r , H o l t and Simpson (48) estimated the extent of in t e r - m o l e c u l a r c y c l i z a t i o n on a s e r i e s of d i a l l y l e s t e r s . A l l monomers s t u d i e d gave r i s e t o chain c y c l i z a t i o n under 28 s i m i l a r r e a c t i o n c o n d i t i o n s . The tendency of monomers t o c y c l i z e was found t o be c o n t r o l l e d by the molecular distance between double bonds i n the d i f u n c t i o n a l monomer. This has not been considered as a s i g n i f i c a n t f a c t o r i n g e l a t i o n k i n e t i c s . 2.2.2.2 Influence of d i v i n y l monomer molecular bridge l e n g t h I t may be t h e o r i z e d t h a t p o l y m e r i z a t i o n k i n e t i c s of v i n y l - d i v i n y l comonomer systems and p r o p e r t i e s of the polymer products from such systems are determined not only by the nature of r e a c t i v e groups, but a l s o by the chemical s t r u c t u r e and dimension of the DVM molecular b r i d g e , i . e . , the molecular fragment connecting double bonds i n the DVM, D i v i n y l monomers having considerable molecular bridge l e n g t h have higher v i s c o s i t i e s . This may have an i n d i r e c t e f f e c t on d i f f u s i o n processes and p o l y m e r i z a t i o n k i n e t i c s , p a r t i c u l a r l y i n . connec- t i o n w i t h g e l a t i o n and a c c e l e r a t i o n v i a Ge. From experimental data c o l l e c t e d f o r other purposes i n (48,61) and from other s t u d i e s (7,8,9,57,58,76,77) an inference can be made as t o the importance of molecular d i s t a n c e between DVM double bonds. This i s based on: i . c o mpetition between i n t e r - m o l e c u l a r and i n t r a - m o l e c u l a r propagation as r e l a t e d t o the r i n g s i z e 3 i . e . , c y c l l z a t i o n versus c r o s s - l i n k i n g ; and 29 i i . o v e r a l l p o l y m e r i z a t i o n k i n e t i c s , e.g., numerical v a l u e s of i n d i v i d u a l r a t e c o n s t a n t s , as w e l l as o v e r a l l p o l y m e r i z a t i o n r a t e . The importance of i n t r a - m o l e c u l a r c y c l i z a t i o n was emphasized when B u t l e r and co-worker (18,19) found t h a t r a d i c a l p o l y m e r i z a t i o n of d i a l l y l monomers gave s o l u b l e , non- c r o s s l i n k e d polymers w i t h l i t t l e or no r e s i d u a l u n s a t u r a t i o n . E x t e n s i v e work by Simpson and co-workers (48,91,92) has helped t o show t h a t the s i z e of r i n g s t r u c t u r e which can be formed, determines whether i n t e r - m o l e c u l a r p o l y m e r i z a t i o n or i n t r a - m o lecular c y c l i z a t i o n i s the predominant r e a c t i o n f o r a p a r t i c u l a r monomer. As a r u l e , the f o r m a t i o n of 6-membered r i n g s i s a p p r e c i a b l y g r e a t e r than those of l a r g e r s i z e (59,78). C o n t r a r y t o e x p e c t a t i o n s , however, r i n g s t r u c t u r e s c o n t a i n i n g as many as 11 and 17 atoms have been r e p o r t e d (17,33,48,61). The i n f l u e n c e of molecular b r i d g e l e n g t h on p o l y m e r i z a t i o n k i n e t i c s and numerical v a l u e s of i n d i v i d u a l r a t e c o n s t a n t s has not been s t u d i e d s y s t e m a t i c a l l y . Only l i m i t e d data are a v a i l a b l e . Recently, however, S o v i e t s c i e n t i s t s (7,8,9,57,53,76) have p u b l i s h e d comprehensive s t u d i e s on p o l y m e r i z a t i o n k i n e t i c s and p r o p e r t i e s of such polymers. B e r l i n (3) c a l c u l a t e d i n d i v i d u a l r a t e constants of d i m e t h a c r y l a t e e s t e r s of a l k y l e n e g l y c o l s having the general f ormula: CHo Ch" i i C H 2 = C - COO -( C H 2 ) - 00C - C = CH 2 where: x = 4, 6, 10. 30 The i n i t i a l p o l y m e r i z a t i o n rate and propagation r a t e constants sharply increased w i t h length between double bonds. The absolute kp value f o r MMA was 300 l/mole, sec and increased t o 600, 1,2.00, and 1,880 l/mole. sec f o r monomers w i t h x equal t o 4,6, and 10 r e s p e c t i v e l y (Table 2-2). At lower and average conversions, kp f o r these compounds exceeded kp f o r MMA by 5-10 times, although t r u e MMA r a d i c a l r e a c t i v i t y and the double bonds of methacrylate e s t e r s do not depend on the nature of g l y c o l or e s t e r residues (12). From Table 2-2 i t i s evident that the i n i t i a l poly- m e r i z a t i o n r a t e f o r a l l oligomers exceeds p o l y m e r i z a t i o n r a t e • f o r MMA even at 50% conversion where v i s c o s i t y of the polymeriza- t i o n mixture i s much higher. The reason f o r higher p o l y m e r i z a t i o n r a t e i s obvious from the numerical value of k ^ / k ^ ^ . Simple c a l c u l a t i o n of kp/k^.^^, from data at zero conversion f o r e s t e r s w i t h x equal t o 4, 6, and 10 gi v e s 67, 152 and 290 x 10"^, r e s p e c t i v e l y . A l l these values are s i g n i f i c a n t l y higher than the maximum value found f o r MMA as demonstrated i n .Table 2-1 (43). S i m i l a r increase i n i n i t i a l p o l y m e r i z a t i o n r a t e was pointed out f o r p o l y m e r i z a t i o n of dimethacrylates and mixed a l l y l and c a r b o x y a l l y l esters, such as (8) : Rx - 0 ~(CH 2-CH 2-0) I_ 3** R2 where: Rj_ i s : CH ? = C - CO- CH 3 R 2 i s : CH 2 = CH - C H 2 - COO , or C H 2 = CH - C H 2 - 0 - 31 The increased r e a c t i v i t y w i t h increase i n molecular bridge l e n g t h between double bonds was a t t r i b u t e d t o m u l t i p l e ether bonds which increased f l e x i b i l i t y of the three-dimensional framework. Because f l e x i b i l i t y of these molecules favours t i g h t packing, double bonds of a c r y l a t e groups are drawn together and are arranged i n a " k i n e t i c a l l y - f a v o u r a b l e " p a t t e r n and form "bundle a s s o c i a t e d swarms" s i m i l a r t o nematic forma- t i o n i n l i q u i d c r y s t a l s ( 8 ) . The t r a n s f o r m a t i o n from l i q u i d polymerizable e s t e r s t o space-regular c r y s t a l l i n e arrangements i n d i c a t e s broad p o s s i b i l i t i e s of d i r e c t e d s y n t h e s i s and r e g u l a t i o n of both p o l y m e r i z a t i o n k i n e t i c s and p r o p e r t i e s of the r e s u l t a n t polymer products. 2.2.2.3 A c c e l e r a t e d p o l y m e r i z a t i o n i n forming wood polymer composites (WPC) Much work has been published i n connection w i t h a p p l i c a t i o n of v i n y l monomers t o wood and papers. Both r a d i a t i o n and heat c a t a l y s t techniques of p o l y m e r i z a t i o n have been used (4,5,30,31,33,49,53 - 56,82,84,85,90,93,96,101). This simple process converts wood i n t o WPC, a composite m a t e r i a l -with improved p h y s i c a l p r o p e r t i e s such as hardness, abrasion r e s i s - tance and compression strength. F u r t h e r , dimensional s t a b i l i t y and a ssociated wet strength p r o p e r t i e s are improved. So f a r MMA and S have been the most w i d e l y used monomers f o r t h i s purpose. 32 Problems with i n s i t u r a d i a t i o n polymerization of v i n y l monomers r e l a t e to r a d i a t i o n s e n s i t i v i t y of wood constituents, p a r t i c u l a r l y c e l l u l o s e . V i n y l monomers, such as S require massive dosages (10 Mrad) for complete polymeriza- t i o n . The large dose requirements are necessary mainly because of low G-value, 0.6 (22). I t i"s also expected that monomers associated with wood components require more energy f o r polymerization due to influence of wood components and oxygen i n the wood structure. I t i s known (22) that chlorinated compounds, such as C C I 4 , accelerate r a d i a t i o n polymerization of v i n y l monomers. The method has been used for reducing dose requirements i n production of WPC (30,55,82,93). However, secondary e f f e c t s caused by C C I 4 , p a r t i c u l a r l y since i t functions as a very e f f i c i e n t chain-transfer agent (22,42,59), are disadvantageous and l i m i t p r a c t i c a l value of CC1 4 acceleration technique (30,82). Addition of cr o s s l i n k i n g monomers, which contain two or more v i n y l groups, to common v i n y l monomers allows copolymerization and formation of three-dimensional networks. In addition to improved WPC physical properties (30,66) , cro s s l i n k i n g agents quickly turn l i q u i d monomers, deposited i n the wood void spaces, i n t o gels and thus: i . accelerate the polymerization v i a Ge, and i i . eliminate large monomer losses due to evaporation. 33 So f a r , only scant a t t e n t i o n has been g i v e n t o the a c c e l e r a t i n g a b i l i t y of d i f f e r e n t d i v i n y l monomer-s, A sy s t e m a t i c study on g e l a t i o n k i n e t i c s i n v i n y l - d i v i n y l comon- omer systems i s l a c k i n g . Raff et a l . (84) i n v e s t i g a t e d the e f f e c t of 5% DVB on r a d i a t i o n p o l y m e r i z a t i o n of S i n WPC, They found t h a t presence of the c r o s s l i n k i n g agent i n c r e a s e d the c o n v e r s i o n degree f o r a p a r t i c u l a r dosage. U n f o r t u n a t e l y , other comparative data were not g i v e n , Kent et. al_. (55) proposed a d d i t i o n of a DVM (EGDMA) f o r the purpose of i n c r e a s i n g molecular weight, as w e l l as a c c e l e r a t i o n of r a d i a t i o n p o l y m e r i z a t i o n . EGDMA was examined as a d d i t i v e t o MMA, However, the minor a c c e l e r a t i n g e f f e c t at 2/o c o n c e n t r a t i o n was not very c o n v i n c i n g . Kenaga (53) d i s c u s s e d h e a t - c a t a l y s t p o l y m e r i z a t i o n w i t h a s e r i e s of v i n y l - d i v i n y l and v i n y l - t r i v i n y l comonomer systems. E f f e c t of c r o s s l i n k i n g agents on p o l y m e r i z a t i o n of t - b u t y l styrene (TBS) i n basswood ( T i l d a ameripana L t ) at 90°C was f o l l o w e d by the time-temperature exotherm method f o r DVM c o n c e n t r a t i o n s up t o 30%. Based on evidence p r o v i d e d by h i s study t r i m e t h y l o l propane t r i a c r y l a t e (TMPTA) was s e l e c t e d as the most e f f e c t i v e a c c e l e r a t o r . S e v e r a l d i m e t h a c r y l a t e s were shown t o be more e f f e c t i v e than DVB. The l a t t e r c o n t a i n small amounts of d i e t h y l benzene which remained trapped i n wood and caused odor problems. 34 In 1969 Paszner (79) introduced and found TEGDMA t o be a very e f f e c t i v e c r o s s l i n k i n g agent and a c c e l e r a t o r f o r r a d i a t i o n p o l y m e r i z a t i o n of MMA, and S. Dose requirements f o r complete p o l y m e r i z a t i o n , as fo l l o w e d by the time-temperature exotherm method, were found t o be lower than t h a t f o r an optimum mixture of S-AN. Subsequently, the a c c e l e r a t i n g a b i l i t y of TEGDMA was t e s t e d s y s t e m a t i c a l l y In numerous other comonomer systems (.11,25,26,52,67-71,79-81,98,99). Recently, Duran and Meyer (33) adopted a systematic approach f o r e v a l u a t i n g the a c c e l e r a t i n g a b i l i t y of trimethylo/ propane t r i m e t h a c r y l a t e (TMPTMA) w i t h MMA i n basswood composites. Using maxima of time-temperature exotherm curves ( 3 l ) as i n d i c a t o r s of r e a c t i o n r a t e s showed tha t time t o exotherm peak decreased r a p i d l y as percentage of c r o s s l i n k i n g agent increased across low co n c e n t r a t i o n l e v e l s (1-20%). They a t t r i b u t e d the increase i n r e a c t i o n r a t e , i . e . , the decrease i n time t o exotherm peak, t o formation of a three-dimensional g e l s t r u c t u r e . In our l a b o r a t o r i e s , as reported e a r l i e r (68-70,79-81, 98) time-temperature exotherm curves were used f o r q u a n t i t a t i v e c h a r a c t e r i z a t i o n of a c c e l e r a t i n g a b i l i t y of c r o s s l i n k i n g agents i n r a d i a t i o n p o l y m e r i z a t i o n of v i n y l - d i v i n y l comonomer systems. The most a t t r a c t i v e comonomer systems have been a p p l i e d t o f a b r i c a t i o n of WPC, PPC and veneer polymer composites (VPC), as w e l l as proposed f o r tough, h i g h l y r e s i s t a n t surface c o a t i n g s (25,26,52,79-81,98,99). 35 In summary, much work has been done t o c l a r i f y Ge a c c e l e r a t i o n phenomena i n v i n y l monomer homopolyrnerization. V i n y l - d i v i n y l comonomer systems, however, have not been s t u d i e d s y s t e m a t i c a l l y . I t i s c l e a r t h a t t h e o r e t i c a l t r e a t - ment of Ge du r i n g p o l y m e r i z a t i o n of v i n y l - d i v i n y l comonomer systems i s more complex than t h a t accepted f o r condensation p o l y m e r i z a t i o n . A c c e l e r a t i o n y_ia c r o s s l i n k i n g agents i s based not only on s t a t i s t i c a l treatment of c r o s s l i n k p r o b a b i l i t i e s , but i n v o l v e s many other f a c t o r s , such as i n t r a - m o l e c u l a r c y c l i z a t i o n , d i f f u s i o n , r e a c t i o n c o n d i t i o n s and chemical s t r u c t u r e of d i v i n y l monomers. As example, the molecular b r i d g e l e n g t h between double bonds may have s i g n i f i c a n t e f f e c t s . Much work remains t o be done before Ge i s understood i n v i n y l - d i v i n y l systems. 36 3.0 MATERIALS AND METHODS Methods and some monomers used i n t h i s study were described i n d e t a i l e a r l i e r (67,69,79). Only general monomer c h a r a c t e r i s t i c s , b r i e f d e f i n i t i o n s of p o l y m e r i z a t i o n para- meters, as w e l l as thermomechanical and physical-mechanical t e s t s f o r polymer products are repeated below. 3.1 Monomers • Two kinds of monomers were used i n t h i s study. The v i n y l monomer was methyl methacrylate (MMA), whi l e a commercially a v a i l a b l e s e r i e s of g l y c o l d i m ethacrylates were used as d i v i n y l monomers. 3.1.1 Methyl methacrylate Methyl methacrylate was the b a s i c monomer used i n a l l experiments. I t contained a commercial I n h i b i t o r (50 p.p.m. hydroquinone) as supplied by Eastman Co. No monomer p u r i f i c a - t i o n was attempted f o r these experiments. 3.1.2 D i v i n y l monomers Four d i f f e r e n t d i v i n y l monomers (DVM) of the common g l y c o l dimethacrylate s t r u c t u r e , w i t h v a r i o u s molecular chain 37 lengths between t e r m i n a l v i n y l double bonds, were s e l e c t e d . These comonomers served as c r o s s l i n k i n g agents and a c c e l e r a t o r s f o r MMA p o l y m e r i z a t i o n . A l l DVM are c h a r a c t e r i z e d by the general formula: The index "n" v a r i e d from 1 t o 4. The four DVM used i n t h i s study were: P r o p e r t i e s of the MMA and DVM used are given i n Table 3-1. A l l DVM were purchased from Borden Chemical Co., P h i l a d e l p h i a , Pa. Higher homologs of the s e r i e s were not a v a i l a b l e at the time. 3.1.3 C a l c u l a t i o n of s t r u c t u r a l parameters To t r a n s f e r from monomer i n t o polymer, each carbon atom i n the polymer chain or i n the three-dimensional network must be connected t o at l e a s t two other atoms by covalent bonds. L i n e a r one-dimensional (thermoplastic) macromolecules, such as poly(MMA) f o r example, only c o n t a i n 2-way connected carbon atoms and thus have a connection number (CN) equal t o i i i . i v . i i . i . Ethylene g l y c o l dimethacrylate (EGDMA), Diethylene g l y c o l dimethacrylate (DEGDMA), T r i e t h y l e n e g l y c o l dimethacrylate (TrEGDMA),and Tetraethylene g l y c o l dimethacrylate (TEGDMA). 38 2,000. Thermosettings c r o s s l i n k e d polymers or copolymers c o n t a i n both 2-vvay and 3-way connected carbon atoms. T h e i r CN i s between 2.000 and 3.000. Re c e n t l y , H o l l i d a y and coworker (45,46) proposed and demonstrated the u s e f u l n e s s of CN f o r simple t o p o l o g i c a l a n a l y s i s of random polymer networks. CM d e s c r i b e s an average s t r u c t u r a l f e a t u r e of polymers and can be extended f u r t h e r t o p r e d i c t p h y s i c a l p r o p e r t i e s of polymer products. An average CN can be c a l c u l a t e d from the chemical s t r u c t u r e of monomer r e p e a t i n g u n i t s as: 2a + 3b + 4c a + b + c where: a, b and c are, r e s p e c t i v e l y , 2-way, 3-way and 4-way connected atoms i n the r e p e a t i n g monomer u n i t s . C a l c u l a t e d v a l u e s f o r members of the present s e r i e s are g i v e n i n Table 3 X o The average CN can be extended f u r t h e r t o copolymers as copolymer c o n n e c t i o n number (CN )«. C N c o p r o v i d e s an average number of network bonds, i . e . , c r o s s l i n k i n g d e n s i t y . The numerical v a l u e of C N C 0 depends on molar composition of the comonomer mixture. I t i s c a l c u l a t e d as: CN c o ( C N ^ ^ A X n i f ^ ^ ) + (C^DVN * m^DVM^ ••••••• «/3-2/ where: CN, V 1 M A and C N D V M are c o n n e c t i o n numbers f o r MMA and DVM. 3 9 m^MMA a n c* m^DVM a r e r n°l e f r a c t i o n s of MMA and DVM i n the comonomer system, r e s p e c t i v e l y . In c a l c u l a t i n g an average CN only those connections are considered which form part of polymer network. Thus, side s u b s t i t u e n t s such as CH^- or CH^OOC- i n poly(MMA), f o r example, were excluded. 3.1,4 P r e p a r a t i o n of comonomer mixtures Mixtures of MMA and r e s p e c t i v e DVM were prepared on a volume/volume b a s i s i n l o t s of 50 ml t o minimize experimental e r r o r due to mixing. These were stored at -10 °C p r i o r t o po l y m e r i z a t i o n . Volume c o n c e n t r a t i o n s , as w e l l as mole r a t i o s are given i n Table 3-2. R e l a t i v e v i s c o s i t i e s (£ x ei) of the pure monomer and comonomer mixtures were measured w i t h a Ubbelohde l / a v i s c o s i - meter (K = 0.05394) at 25 °C and c a l c u l a t e d as: t tMMA mixture . where: ^mixture a n c i M̂MA a r e e f f l u x times f o r the comonomer mixture and MMA, r e s p e c t i v e l y , R e l a t i v e v i s c o s i t i e s f o r pure monomers are given i n Table 3-1 and f o r the comonomer mixtures i n Table 3-2. 40 3,2 R a d i a t i o n P o l y m e r i z a t i o n Technique Although r a d i a t i o n induced p o l y m e r i z a t i o n i s now a w e l l e s t a b l i s h e d technique, only scant a t t e n t i o n has been g i v e n t o p o l y m e r i z a t i o n k i n e t i c s as r e l a t e d to Ge and p r o d u c t i o n of WPC. One reason has been l a c k of proper methods f o r f o l l o w i n g the t r a n s f o r m a t i o n of monomers t o polymers, e s p e c i a l l y w i t h i n the r a d i a t i o n f i e l d . D i f f i c u l t i e s i n studying p o l y m e r i z a t i o n k i n e t i c s t o h i g h degree of c o n v e r s i o n are w e l l documented. Davies et al.. (31) s t u d i e d the k i n e t i c s of MMA r a d i a t i o n p o l y m e r i z a t i o n i n y e l l o w b i r c h ( B e t u l a a l l e g h a n i e n s i s B r i t t . ) and red pine (P,i,nu,$, r e s i n p s a A i t . ) by a method p r e v i o u s l y d e s c r i b e d f o r h e a t - c a t a l y s t b u l k p o l y m e r i z a t i o n of v i n y l monomers ( 5 , 8 8 ) . In h i s study the exothermic heat r e l e a s e d d u r i n g p o l y m e r i z a t i o n was recorded a u t o m a t i c a l l y . From the shape of time-temperature exotherms, k i n e t i c a l l y important stages of p o l y m e r i z a t i o n were d e r i v e d and these were r e l a t e d t o i n h i b i t i o n , Ge phenomena and complete c o n v e r s i o n at peak temperature. The same technique was adopted f o r t h i s study. D e t a i l e d d e s c r i p t i o n s and d e t e r m i n a t i o n of p o l y m e r i z a - t i o n parameters have been g i v e n p r e v i o u s l y (67,69,70). 3.2.1 D e f i n i t i o n and d e t e r m i n a t i o n of p o l y m e r i z a t i o n parameters The temperature ( T ) , °C versus time ( t ) , . m i n r e c o r d s obtained, such as shown i n F i g , 3-1, were analysed t o provide the f o l l o w i n g p o l y m e r i z a t i o n c h a r a c t e r i s t i c s : 41 i . I n i t i a l c o n d i t i o n of the system (T35, t 0 ) ; i i . " G e l - E f f e c t P o i n t " (GEP) by geometric s o l u t i o n ( F i g . 3-1) as the exotherm curve i n t e r s e c t i o n by the p e r p e n d i c u l a r through the common p o i n t obtained by e x t r a p o l a t i n g e a r l y and l a t e curve stages, and wit h components ( T G E p , t G E p ) ; i i i . "Cure" at the exotherm maximum (MAX) wit h components (T^^x, ^^/J^) I i v . " A c t i v a t i o n " ( i ) p e r i o d as time i n t e r v a l t o reach GEP and c a l c u l a t e d as (t G£p - t ), and v. " A c c e l e r a t i o n " ( I I ) p e r i o d as time i n t e r v a l between GEP and the exotherm maximum, and calculated (t^^ ~ "^GEP^ * As shown a l s o i n F i g . 3-1, dose (D) may be s u b s t i t u t e d f o r time ( t ) , thereby d e s c r i b i n g the i r r a d i a t i o n requirements at the s i n g l e dose r a t e examined i n the present study. To compare r e l a t i v e r a t e s of p o l y m e r i z a t i o n i n " A c t i v a t i o n " (I) and " A c c e l e r a t i o n " ( I I ) p e r i o d s , the f o l l o w i n g p o l y m e r i z a t i o n c o e f f i c i e n t s (PRC) were c a l c u l a t e d : i . P o l y m e r i z a t i o n r a t e c o e f f i c i e n t f o r the " A c t i v a t i o n " (PRCj) p e r i o d as: TGEP - T: 36 /3-4/ PRC T « - 0 0 0 0 0 0 0 « i £ O o « * o 0 0 « « . 0 0 O 0 « o « a ~ t o 42 i i . P o l y m e r i z a t i o n r a t e c o e f f i c i e n t f o r the " A c c e l e r a t i o n " (PRGn) p e r i o d as: T T MAX - GEP . , P R C I I ~ tMAX - ^ E P The r a t i o of these c o e f f i c i e n t s , PRCjj/PRCj, n u m e r i c a l l y expresses the i n c r e a s e i n p o l y m e r i z a t i o n r a t e due t o the Ge phenomena. R e c i p r o c a l of time needed t o a t t a i n exotherm maximum ( l / t j Y ^ ) i s used as a measure f o r c h a r a c t e r i z i n g the o v e r a l l c u r i n g r a t e (OCR): 1 OCR. — — — 0 4 0 0 O 0 0 0 0 0 O 3 9 0 O 9 ^* 6 / u MAX By analogy, r e c i p r o c a l of the time t o r e a c h GEP ( l / t ^ p p ) , r e f e r s t o the o v e r a l l g e l a t i o n r a t e (OGR) and c h a r a c t e r i z e s the e f f i c i e n c y of the c r o s s l i n k i n g agent t o onset Ge phenomena. 3.2.2 P o l y m e r i z a t i o n procedure For d e t e r m i n a t i o n of p o l y m e r i z a t i o n parameters 5 ml of monomer or comonomer mixture (volume/volume) were pl a c e d i n a s m a l l g l a s s v i a l , which was then f l u s h e d w i t h n i t r o g e n and 43 s e a l e d . R a d i a t i o n p o l y m e r i z a t i o n was done i n a Gammacell 220 w i t h dose r a t e at 0,82 Mrad/hr, as determined by F r i c k e dosimetry (27). In two experiments, r a d i a t i o n dose r a t e a t t e n u a t o r s were used t o eval u a t e i n f l u e n c e of dose r a t e on OCR. Comonomer mixtures were f i r s t c o n d i t i o n e d t o the ambient Gammacell chamber temperature (36 ° C ) , then set i n a styrofoam seat which provided thermal i n s u l a t i o n and r e p r o d u c i b l e p o s i t i o n i n the chamber. In each case, exotherm temperatures were f o l l o w e d by i n s e r t copper-constantan thermocouples, w i t h s i g n a l s sent t o a time based s t r i p c h a r t r e c o r d e r c a l i b r a t e d a g a i n s t an ice-water r e f e r e n c e j u n c t i o n . A s i m i l a r technique has been d e s c r i b e d i n d e t a i l (67,71). Use of the method i n v o l v e s assumptions t h a t p o l y m e r i z a t i o n r a t e r e l a t e s t o heat e v o l u t i o n and t h a t comparisons may be made between the systems s t u d i e d , 3.2.3 R e p r o d u c i b i l i t y of experimental r e s u l t s R e p r o d u c i b i l i t y of p o l y m e r i z a t i o n parameters, as d e r i v e d from r a d i a t i o n p o l y m e r i z a t i o n exotherms by t h r e e independent operators, with the MMA-TEGDIvA comonomer system ( l l ) , was t e s t e d on s i x t e e n i d e n t i c a l experiments, done as d e s c r i b e d above. The f o l l o w i n g standard d e v i a t i o n v a l u e s were c a l c u l a t e d (50): i . "tQEP = 17.5 -: 1.5 rnin, i i . t j i ^ = 23.2 - 1.7 min, 44 i i i . T G E P - 63.0 t 2,5 °C, and i v . Tmx = 143.0 t 7.8 ° C . 3.3 P r o p e r t i e s of Polymer Products P r o p e r t i e s of polymer products were e v a l u a t e d on the b a s i s of t h e i r thermomechanical and mechanical s t r e n g t h b e h a v i o u r s . The i n d i v i d u a l c h a r a c t e r i s t i c s , determined from unconventional thermomechanical curves, as w e l l as standard compression s t r e s s - s t r a i n curves are d e s c r i b e d below. 3.3,1 Thermomechanical p r o p e r t i e s Thermomechanical p r o p e r t i e s of the polymer products y^ere obtained by a simple d e v i c e c o n s t r u c t e d f o r r e c o r d i n g deformation of loaded and heated specimens (67,69,80). Polymer samples (3 mm high) were r e s t e d on a tube support and were loaded v i a a rod 3.2 x 10"*2 cm 2 i n c r o s s - s e c t i o n . The rod e x t e n s i o n was coupled w i t h a probe l e a d i n g i n t o the core of a l i n e a r v a r i a b l e d i f f e r e n t i a l t r a n s d u c e r . High-temperature s i l i c o n e o i l was used as h e a t i n g medium, The o i l temperature was r e g u l a t e d t o i n c r e a s e the sample temperature at constant r a t e (12 t o 15 °C/min), f o l l o w e d by an i n s e r t copper-constantan thermocouple. S i g n a l s generated by changes i n temperature and p o s i t i o n of the t r a n s d u c e r probe were fed t o a r e c o r d e r i n the X-Y mode. 45 G l a s s t r a n s i t i o n temperature (Tg) and thermal d i s t o r t i o n temperature (TDT) were determined as demonstrated i n F i g , 3-2, In a d d i t i o n , the magnitude of r e l a t i v e l y c o nstant deformation a c r o s s the broad rubbery s t a t e temperature range, c h a r a c t e r i s t i c of c r o s s l i n k e d polymers, was used t o c a l c u l a t e thermomechanical deformation degree (TDD), As c a l c u l a t e d here, TDD v a r i e s from 0 t o 1 f o r t h e r m o p l a s t i c t o f u l l y c r o s s l i n k e d polymer p r o d u c t s . Numerical v a l u e s of TDD were c a l c u l a t e d as: h - dp TDD = ... »•..!• j i i . , ., • . I.,. . . i « 4 O * O • .1 • • « » 9 O • 9 O 9 fl O O « »/3*'7/ where: h = sample h e i g h t , mm, and dp = depth of probe p e n e t r a t i o n , mm. The l i n e a r thermomechanical deformation c o e f f i c i e n t (LTDC) was c a l c u l a t e d a c c o r d i n g t o Holsworth (47) as: k x s h where: k = the s e n s i t i v i t y ( m i l s / i n c h of r e c o r d e r c h a r t p a p e r ) , s = slope of thermal deformation curve i n the t r a n s i t i o n r e g i o n ( F i g , 3-2), and h = sample h e i g h t . 46 3.3.2 Mechanical s t r e n g t h p r o p e r t i e s S t r e s s - s t r a i n c h a r a c t e r i s t i c s i n compression were determined on a 5 S000 kg c a p a c i t y INSTRON Model TM-L U n i v e r s a l T e s t i n g Instrument. I t was equipped with a CCM l o a d c e l l and standard p o t e n t i o m e t r i c - t y p e graph r e c o r d e r ( 5 l ) . The f o l l o w i n g t e s t i n g c o n d i t i o n s were used f o r a l l samples: i . cross-head speed at 0.1 cm/min, i i . r e c o r d e r c h a r t speed at 1*0 cm/min, and i i i . f u l l s c a l e l o a d at 5,000 kg. Standard compression t e s t specimens were manufactured as c y l i n d e r s from cured polymers. P o l y m e r i z a t i o n s were done i n a separate experiment w i t h 10 ml of monomer or comonomer mixture. The sample l e n g t h t o diameter r a t i o ivas 2:1, a c c o r d i n g t o ASTM Standard D 695-68T f o r d e t e r m i n a t i o n of compressive p r o p e r t i e s of r i g i d p l a s t i c s ( 2 ) . Test specimen dimensions w i t h uniform c i r c u l a r c r o s s - s e c t i o n a l area, as machined on a metal l a t h e , were as f o l l o w s : i , diameter ~- 1.0 cm, i i . h e i g h t =2,0 cm, i i i , r a d i u s of g y r a t i o n = 0.25, and i v . s l e n d e r n e s s r a t i o = 8.0 V a r i o u s types of s t r e s s - s t r a i n c u r v e s , obtained w i t h d i f f e r e n t polymers, and standard nomenclature are shown i n F i g . 3-3 (20,72). 47 4,0 RESULTS R a d i a t i o n p o l y m e r i z a t i o n of the MMA-TEGDMA comonomer system was i n v e s t i g a t e d over the c o n c e n t r a t i o n range 100:0 t o 0:100, R e s u l t s f o r i n d i v i d u a l p o l y m e r i z a t i o n parameters of these mixtures, as d e r i v e d from p o l y m e r i z a t i o n exotherm curves, are summarized i n Tables 4-1 and 4-2, G r a p h i c a l i n t e r p r e t a t i o n s f o r some of these r e s u l t s are g i v e n i n F i g , 4-1 and 4-2, The r e l a t i o n between OCR ( l / t ^ v , ) and TEG DMA. volume c o n c e n t r a t i o n i n MMA-TEGDMA comonomer mixtures i s p l o t t e d i n F i g . 4-3. R e s u l t s w i t h the MMA-TEGDMA comonomer system demonstrated t h a t most a c c e l e r a t i o n was obtained w i t h i n a narrow c o n c e n t r a t i o n range, e s p e c i a l l y between 2.5 t o 10% DVM c o n c e n t r a t i o n . At higher TEG DMA c o n c e n t r a t i o n s OCR. wa.s not p r o p o r t i o n a l t o volume c o n c e n t r a t i o n of the c r o s s l i n k i n g agent. For t h i s reason emphasis i n f u r t h e r experiments was pl a c e d only on studying DVM c o n c e n t r a t i o n s up t o 30% i n comonomer mixtures. A c c o r d i n g l y , p o l y m e r i z a t i o n c h a r a c t e r i s t i c s f o r MMA with the other three c r o s s l i n k i n g agents are summarized i n T a bles 4-3 and 4-4. G r a p h i c a l i n t e r p r e t a t i o n of tMAX a n c * t(3£p, as f u n c t i o n s of DVM c o n c e n t r a t i o n i n these systems, i s gi v e n F i g . 4-4 and 4-5, P o l y m e r i z a t i o n r a t e c o e f f i c i e n t s i n " A c t i v a t i o n " (PRCj) and " A c c e l e r a t i o n " (PRCjj) p e r i o d s as a f u n c t i o n of DVM c o n c e n t r a t i o n are p l o t t e d i n F i g , 4-6. 48 I l l u s t r a t i o n s i n F i g . 4-7 show the r e l a t i o n s h i p between t j V ^ and time r e q u i r e d t o onset Ge (t^pp) f o r the four, systems s t u d i e d , i n c l u d i n g r e s u l t s w i t h the S-TEGDMA system as taken from e a r l i e r work (70). The OCR ( l / t M A V ) i n c r e a s e d p r o p o r t i o n a l l y w i t h DVM MAX c o n c e n t r a t i o n . L i n e a r r e l a t i o n s h i p s between OCR and DVM c o n c e n t r a t i o n f o r the f o u r systems are give n i n F i g . 4-8. For comparison, OCR val u e s as c a l c u l a t e d from r e s u l t s of Duran and Meyer (33) are i n c l u d e d . The (l/t^ A x / ^ ' \ o n c ^ p r o p o r t i o n a l i t y c o n s t a n t s c a l c u l a t e d from slopes i n F i g . 4-8 r e p r e s e n t the o v e r a l l a c c e l e r a t i o n c o n s t a n t (K) f o r i n d i v i d u a l DVM. These are i n v e r s e l y r e l a t e d t o DVM co n n e c t i o n number (CNj^y^) as seen i n F i g . 4-9. The l i n e a r r e l a t i o n s h i p between OCR and DVM c o n c e n t r a t i o n a l l o w s c a l c u l a t i o n and p r e d i c t i o n of c u r i n g times ( t ^ x ) f ° r MMA-DVM mixtures. D i f f e r e n c e s between measured and p r e d i c t e d c u r i n g times f o r the f o u r systems at low DVM c o n c e n t r a t i o n s are summarized i n Table 4-5. S i m i l a r c a l c u l a t i o n s f o r the 5-TEGDMA comonomer system are given i n Table 4-6 as taken from e a r l i e r work (70). The r a t h e r good agreement between p r e d i c t e d and e x p e r i m e n t a l l y measured c u r i n g times f o r d i f f e r e n t comonomer systems of the present and p r e v i o u s (70) s t u d i e s suggested 49 f u r t h e r a p p l i c a t i o n of t h i s new technique t o p u b l i s h e d heat- c a t a l y s t r e s u l t s . Such data from q u i t e d i s s i m i l a r experiments (53) are examined i n Table 4-7. F u r t h e r , OCR as a f u n c t i o n of c r o s s l i n k i n g agent c o n c e n t r a t i o n f o r these data i s demonstrated i n F i g . 4-10. D i f f e r e n c e s between h e a t - c a t a l y s t c u r i n g times p u b l i s h e d by Duran and Meyer (33) and c a l c u l a t e d by the new method are giv e n i n Table 4-8. The o v e r a l l a c c e l e r a t i o n c o n s t a n t and p r e d i c t e d c u r i n g times were c a l c u l a t e d only from two randomly chosen experiments (the "two-point method"). One problem encountered w i t h the h e a t - c a t a l y s t c u r i n g process i s t h a t r e a c t i o n temperatures are v i r t u a l l y u n c o n t r o l l a b l e and peak temperature i n c r e a s e s v e r y r a p i d l y up t o 180-190 °C (53). Table 4-9 demonstrates t h a t , using a c o n t r o l l e d r a d i a t i o n c u r i n g method, p o l y m e r i z a t i o n r a t e can be a l t e r e d over wide range and the a s s o c i a t e d peak exotherm temperature can be c o n s i d e r a b l y reduced. The OCR v a r i e d l i n e a r l y w i t h the square ro o t of r a d i a t i o n dose r a t e , as demonstrated i n F i g . 4-11. Shapes of thermomechanical curves* obtained by a simple d e v i c e used are reproduced f o r MMA-EGDMA and MMA-TEGDMA systems i n F i g . 4-12 and 4-13, r e s p e c t i v e l y . Thermomechanical parameters such as T g, TDD and TDT, as w e l l as c r o s s l i n k i n g d e n s i t y as re p r e s e n t e d by copolymer c o n n e c t i o n numbers ( C N C 0 ) , 50 axe g i v e n i n Table 4-10 f o r the f o u r systems s t u d i e d . T , TDT and TDD r e l a t e m o b i l i t y of heated polymer segments. V a r i a t i o n of these parameters w i t h c r o s s l i n k i n g d e n s i t y , c a l c u l a t e d as C N C 0 , i s re p r e s e n t e d i n F i g . 4-14 and 4-15. LTDC c h a r a c t e r i z e s deformation r a t e i n the t r a n s i t i o n r e g i o n . The e x p o n e n t i a l r e l a t i o n s h i p between LTDC- and DVM c o n c e n t r a t i o n f o r the two extreme systems i s i l l u s t r a t e d i n F i g . 4-16. The r e l a t i o n s h i p between LTDC and CN f o r the f o u r systems i s giv e n i n F i g . 4-17. Compression s t r e s s - s t r a i n curves f o r products from the MMA-EGDMA comonomer system are simulated i n F i g . 4-18, For comparison s i m i l a r curves f o r the MMA-TEGDMA system are shown i n F i g , 4-19. Numerical v a l u e s f o r some p h y s i c a l - m e c h a n i c a l parameters of the polymer pr o d u c t s , as d e r i v e d from compression s t r e s s - s t r a i n t e s t s f o r the f o u r systems examined, are summarized i n Table 4-11. R e l a t i o n s h i p s between compression s t r e s s at r u p t u r e , compression s t r a i n at ru p t u r e and area under the s t r e s s - s t r a i n curve as an estimate of toughness, and DVM c o n c e n t r a i o n , are shown f o r the v a r i o u s systems i n F i g . 4-20 t o 4-23, Compression s t r e s s a t r u p t u r e , as a f u n c t i o n of C N c o f o r the f o u r systems examined, i s given i n F i g , 4-24. The slope of the i n i t i a l s t r a i g h t l i n e p o r t i o n of s t r e s s - s t r a i n curves r e p r e s e n t s the e l a s t i c modulus ( E ) , Numerical v a l u e s of E, p l a s t i c deformation and area under s t r e s s - s t r a i n curve (F) f o r the d i f f e r e n t polymer products are summarized i n Table 4-12. C o r r e l a t i o n between p l a s t i c d e formation and toughness as estimated by area under the s t r e s s - s t r a i n curve i s demonstrated i n F i g . 4-25. 52 5.0 DISCUSSION Some e a r l y r e s u l t s on a c c e l e r a t e d r a d i a t i o n p o l y m e r i z a t i o n of MMA-TEGDMA comonomer mixtures (79) showed improved c u r i n g times and s t r e n g t h p r o p e r t i e s of the r e s u l t i n g polymer p r o d u c t s . These p r e l i m i n a r y experiments encouraged f u r t h e r systematic s t u d i e s on r a d i a t i o n p o l y m e r i z a t i o n of v i n y l - d i v i n y l comonomer systems. Subsequently, the MMA-TEGDMA and S-TEGDMA comonomer systems were s t u d i e d i n d e t a i l over f u l l c o n c e n t r a t i o n ranges of the mixtures (67,69,70), These r e s u l t s , b r i e f l y summarized as f o l l o w s , served as the i n i t i a l observa- t i o n s on which the present study was developed. 5.1 R a d i a t i o n P o l y m e r i z a t i o n of the MMA-TEGDMA Comonomer System R a d i a t i o n p o l y m e r i z a t i o n parameters f o r the MMA-TEGDMA system were d e r i v e d at d i f f e r e n t comonomer compositions ( v o l / v o l mixtures) from p o l y m e r i z a t i o n exotherm c u r v e s . These v a l u e s obtained from bu l k p o l y m e r i z a t i o n s are summarized i n Tables 4-1 and 4-2. The a c c e l e r a t i n g a b i l i t y of TEGDMA i s i l l u s t r a t e d over a wide c o n c e n t r a t i o n range by a h y p e r b o l i c decrease i n t ^ ^ as shown i n F i g . 4-1, Pronounced a c c e l e r a t i o n accompanying TEGDMA a d d i t i o n as measured by t ^ ^ 1 S p r i m a r i l y r e s t r i c t e d t o a narrow c o n c e n t r a t i o n i n t e r v a l , A d d i t i o n s of TEGDMA up t o 10% are of 53 p a r t i c u l a r i n t e r e s t ; amounts i n excess of 10% are l e s s e f f e c t i v e . For example, a d d i t i o n of 10% TEGDMA t o MMA decreased t ^ ^ ^ r o m H 7 m i n » f ° x pure MMA, t o 31 min. F u r t h e r TEGDMA a d d i t i o n s i n f l u e n c e d t j V 1 A X only moderately. The 50:50 comonomer composition, however, represented the most e f f i c i e n t mixture over the whole c o n c e n t r a t i o n range w i t h t j ^ x a"t only 15 min. Comonomer mixtures w i t h h i g h e r c o n c e n t r a t i o n of c r o s s l i n k i n g agent were l e s s r a d i a t i o n s e n s i t i v e . I t i s assumed t h a t at h i g h c o n c e n t r a t i o n s the c r o s s l i n k i n g agent f u n c t i o n s as a d i f f u s i o n b a r r i e r l i m i t i n g t r a n s l o c a t i o n of monomer t o a c t i v e r a d i c a l s i t e s . The a c c e l e r a t i n g a b i l i t y of TEGDMA as r e l a t e d t o Ge may be expressed q u a n t i t a t i v e l y by PRC as determined i n " A c t i v a t i o n " (PRCj) and " A c c e l e r a t i o n " (PRCTJ) p e r i o d s , i'.e., b e f o r e and a f t e r the onset of GEP, r e s p e c t i v e l y . Both c o e f f i c i e n t s vary w i t h composition of the comonomer mixture as shown i n F i g . 4-2. Up t o 20% TEGDMA a d d i t i o n both c o e f f i c i e n t s i n c r e a s e d v e r y r a p i d l y due t o reduced t e r m i n a t i o n i n v i s c o u s and p a r t l y c r o s s l i n k e d media (43). F u r t h e r i n c r e a s e i n TEGDMA co n c e n t r a - t i o n a f f e c t e d not only the r a t e of t e r m i n a t i o n , but a l s o i n f l u e n c e d the propagation r a t e . Both PRCj and PRCJ-J- became s t a b i l i z e d w i t h i n the in t e r m e d i a t e range of TEGDMA c o n c e n t r a t i o n (20-50%). In h i g h l y c r o s s l i n k e d systems, i . e . , over 50% TEGDMA, c h a i n growth pr o p a g a t i o n becomes f u l l y d i f f u s i o n - c o n t r o l l e d ; the r e a c t i o n system was immobilized and consequently l e s s 54 r a d i a t i o n s e n s i t i v e . The dose r e q u i r e d t o o b t a i n a s o l i d copolymer w i t h the 50:50 mixture was 0.2 Mrad (Table 4-2). Such a comonomer composition i s 7.7 times more r a d i a t i o n s e n s i t i v e than pure MMA. The r e c i p r o c a l of t ^ ^ ^^MAX^ c h a r a c t e r i z e s OCR /3-6/. From the r e l a t i o n s h i p between OCR and TEGDMA c o n c e n t r a t i o n ( F i g . 4-3), i t i s evident t h a t f o r lower TEGDMA c o n c e n t r a t i o n s (up t o 10%) OCR i n c r e a s e d l i n e a r l y . At higher TEGDMA c o n c e n t r a t i o n s OCR d e v i a t e d from p r o p o r t i o n a l i t y as a c c e l e r a t i n g e f f i c i e n c y decreased, p o s s i b l y due t o a d i f f u s i o n b a r r i e r set i n the more h i g h l y c r o s s l i n k e d networks. For c o n c e n t r a t i o n s f a l l i n g w i t h i n the p r o p o r t i o n a l i t y range i t was p o s s i b l e t o p r e d i c t the c u r i n g times of other mixtures a c c o r d i n g t o the f o l l o w i n g e m p i r i c a l equation: l/t^ A x (mixture) = l/tMAX(MMA) + K /DVM/ . . . . . . . . , / 5 - l / where: /DVM/ = v o l - c o n c e n t r a t i o n of TEGDMA, and K = o v e r a l l a c c e l e r a t i o n constant (OAC), slope as i n F i g . 4-3. Using t h i s equation i t i s p o s s i b l e to p r e d i c t r e q u i r e d c u r i n g times w i t h i n the most important c o n c e n t r a t i o n r e g i o n . The p r o p o r t i o n a l i t y constant (K) c h a r a c t e r i z e s o v e r a l l a c c e l e r a t i n g a b i l i t y of a p a r t i c u l a r DVM or c r o s s l i n k i n g agent. I t s value f o r the MMA-TEGDMA system (0 t o 10% TEGDMA) as 55 calculated from the l i n e a r portion of F i g . 4-3 i s 1.1 x 10~ J . -1 -1 rain .cone . Deviation of OCR from the propor t i o n a l i t y l i n e within 10 to 20% TEGDMA concentration i n t e r v a l ( F i g . 4-3) i s i n good agreement with r e s u l t s , derived from ESR studies by Szocz and Lazar (100). They studied decay and segmental mobility of polymeric chains, as well as monomer molecules within crosslinked polyglycol methacrylate network. For comonomer mixtures having higher d e n s i t i e s of cro s s l i n k i n g , i . e . , at higher TEGDMA concentrations, acceleration according to Ge has to be considered as the ov e r a l l e f f e c t of two concurrent f a c t o r s : i . DVM acceleration r e s u l t i n g from decreased termination i n the partly crosslinked network, and i i . DVM retardation r e s u l t i n g from d i f f u s i o n b a r r i e r s and consequent d i f f u s i o n - c o n t r o l l e d propagation i n highly crosslinked networks. Gradual increases of TEGDMA up to 10% creates reaction conditions i n which i predominates over i i . A d d itional increase i n DVM concentration increasingly enlarges 11 causing deviation of OCR from the propor t i o n a l i t y l i n e ( F i g . 4-3). 56 5.2 Influence of Molecular Bridge Length i n D i v i n y l Monomers Crosslinking density of gelled MMA-DVM network depends on: i . frequency of c r o s s l i n k s on the polymer backbone i . e . , poly(MMA), and i i . molecular bridge length between DVM double bonds. It seemed reasonable to assume that v a r i a t i o n i n DVM molecular bridge length would influence the DVM accelerating a b i l i t y and a f f e c t the delay or premature onset of GEP. This hypothesis was tested with a series of dimethacrylate esters selected to act as cros s l i n k i n g agents. Accelerating a b i l i t i e s of EGDMA, DEGDMA and TrEGDMA are given i n Tables 4-3 and 4-4, while graphical interpretations of these r e s u l t s are i l l u s t r a t e d i n F i g . 4-4 and 4-5. Changes i n the accelerating a b i l i t y of the d i f f e r e n t MMA-DVM systems studied, as shown by t^AX ̂ n F I 9 » 4 ~ 4 » indicate that accelerating a b i l i t y changed according to DVM molecular bridge length. With increasing bridge length .mono- (EGDMA), d i (DEGDMA), t r i - (TrEGDMA) to tetraethylene g l y c o l dimethacrylate (TEGDMA) \ t j ^ decreased over the whole concentration range studied. Acceleration by DVM i n MMA-DVM mixtures i s thus shown to be d i r e c t l y related to DVM molecular bridge length. 57 For examples at 2*5% DVM c o n c e n t r a t i o n t i V l A X decreased from 83 min'for MMA-EGDMA t o 60 rain f o r the MMA-TEGDMA comonomer system. Numerical v a l u e s f o r MMA-DEGDMA and MMA-TrEGDMA systems, at the same c o n c e n t r a t i o n , were 77 and 72 min, r e s p e c t i v e l y . S i m i l a r r e s u l t s were obtained by e v a l u a t i o n of h a l f - t i m e c o n c e n t r a t i o n v a l u e s , i . e . , DVM c o n c e n t r a t i o n s r e q u i r e d t o reduce the c u r i n g time t o one-half of t h a t f o r pure MMA. E x t r a p o l a t e d v a l u e s f o r h a l f - t i m e c o n c e n t r a t i o n s i n c r e a s e d w i t h d e c r e a s i n g DVM molecular b r i d g e l e n g t h . Such v a l u e s f o r the f o u r system s t u d i e s were ( F i g . 4-4): i . ' M M A - E G D M A 1%} i i . MMA-DEGDMA 5%, i i i . MMA-TrEDGMA A%, and i v . MMA-TEGDMA 3%. Comparison of h a l f - t i m e c o n c e n t r a t i o n s f o r the two extreme systems of t h i s study (MMA-EGDMA and MMA-TEGDMA) shows t h a t when E G D M A r e p l a c e d TEGDMA, twice as much DVM was r e q u i r e d t o reduce t ^ A Y t o one-half t h a t f o r pure MMA. Higher EGDMA c o n c e n t r a t i o n s i n the MMA-EGDMA system i n c r e a s e d c r o s s l i n k i n g . d e n s i t y of the g e l l e d network, c r e a t i n g d i f f u s i o n b a r r i e r s i n the system. The pr o p a g a t i o n r e a c t i o n i s assumed t o become d i f f u s i o n - c o n t r o l l e d at an e a r l i e r r e a c t i o n stage f o r MMA-EGDMA than i n the MMA-TEGDMA system. Consequently, t^AX i n c r e a s e s p r o g r e s s i v e l y . D i f f e r e n c e s In c r o s s l i n k i n g d e n s i t i e s i n these two systems can be demonstrated by the average segment molecular 58 weight between two cro s s l i n k s on the poly(MMA) backbone. For example, mixtures of 93:7 MMA-EGDMA and 97:3 MMA-TEGDMA represent comonomers of i d e n t i c a l curing times, equalling one- hal f of that needed to cure MMA. However, the cros s l i n k frequency i s considerably higher i n 93:7 MMA-EGDMA system i n proportion to differences i n DVM- concentration and respective CNj3YM» The estimated average segment molecular weights between two c r o s s l i n k s , assuming random d i s t r i b u t i o n of DVM i n the system, i s nearly 10,000 for the 97:3 MMA-TEGDMA system, but i n the 93:7 MMA-EGDMA system only. 1,600-3,400 (based on MMA/DVM mole r a t i o s given i n Table 3-2). In addition the length of the molecular bridge between double bonds, which i s four times longer i n TEGDMA, assures higher segmental mobility and f l e x i b i l i t y i n the MMA-TEGDMA mixtures due to the numerous f l e x i b l e C - 0 - C linkages ( l O ) . Additional substituents or branching of the molecular bridge increases the e f f e c t i v e d i f f u s i o n b a r r i e r and, consequently, t^x» This i s also evident from lower accelerating a b i l i t y of the t r i f u n c t i o n a l c r o s s l i n k - ing agent used by Duran and Meyer (33), as demonstrated i n F i g . 4-4. Formation of ring structures a r i s i n g from i n t e r n a l c y c l i z a t i o n may also influence curing times. Considering EGDMA as an example, formation of the following ring structure i s t h e o r e t i c a l l y possible: 59 CH 2 (CH q)C CO CO 0 - CH. l2 - CH, 2 - 0 V (CH 0)C CH 2 - CO CO 0 - CH - CH 0 - 0 2 2 An a d d i t i o n a l e t h y l e n e g l y c o l u n i t i n the DVM molecular b r i d g e r e s u l t s i n enlargement of the r i n g s i z e by thr e e members. Thus, the 8-membered r i n g of EGDMA e n l a r g e s t o 11-, 14-, and 17- membered r i n g s f o r DEGDMA, TrEGDMA and TEGDMA, r e s p e c t i v e l y . f u n c t i o n as a d d i t i o n a l d i f f u s i o n b a r r i e r s . In a d d i t i o n , the i n t e r n a l l i n k a g e c o n s t i t u t e s a l o s s of i n t e r m o l e c u l a r bonding which would otherwise p a r t i c i p a t e i n c r o s s l i n k i n g and g e l fo r m a t i o n . The o v e r a l l r e s u l t of both e f f e c t s i s delayed GEP and i n c r e a s e d c u r i n g time. Formation of 6-membered r i n g s i s most f a v o u r a b l e (59,78) and p o s s i b i l i t y of r i n g f o r m a t i o n decreases w i t h i n c r e a s i n g r i n g s i z e . T h i s would e x p l a i n the de l a y i n onset of GEP f o r DVM s t u d i e d i n the sequence from TEGDMA t o EGDMA, i . e . , 23 t o 41 min at 10% DVM c o n c e n t r a t i o n ( F i g . 4-5 and Tab l e s 4-1 and 4-3). Once GEP has been passed, f o r m a t i o n of such i n t e r n a l l i n k s becomes p r o g r e s s i v e l y l e s s The r i n g s t r u c t u r e s , surrounding a c t i v e r a d i c a l s i t e s 60 important as the d i s t i n c t i o n between i n t e r n a l and e x t e r n a l l i n k s f a d e s . Consequently, P R C J J i s independent of the s t r u c t u r e of DVM ( F i g , 4-6) and t-,,^ i s d i r e c t l y p r o p o r t i o n a l t o tQgp r e g a r d l e s s of DVM molecular b r i d g e l e n g t h ( F i g , 4 - 7 ) , S i m i l a r c o n c l u s i o n s were drawn from r e s u l t s on a c c e l e r a t e d r a d i a t i o n p o l y m e r i z a t i o n of the S-TEGDMA comonomer system ( 7 0 ) , The f o r m a t i o n of r i n g s t r u c t u r e s i n DVM p o l y m e r i z a t i o n s has been proposed as a reason f o r poor c o r r e l a t i o n between p r e d i c t e d and a c t u a l g e l a t i o n ( 3 8 , 4 0 , 4 8 , 9 1 ) . Subsequently, proposed a l t e r n a t i n g i n t e r - i n t r a molecular propagation f o r these p o l y m e r i z a t i o n systems has been v e r i f i e d f o r a number of DVM ( 1 7 , 1 8 , 4 0 ) . However, there i s not enough d i r e c t evidence at t h i s time s u p p o r t i n g i n t e r n a l r i n g s t r u c t u r e s i n s t u d i e d c r o s s l i n k e d polymer products. F u r t h e r s t r u c t u r a l analyses of copolymers may p r o v i d e the necessary evidence to back up these c o n t e n t i o n s . The r e l a t i o n s h i p between PRCj and P R C J J at v a r i o u s DVM c o n c e n t r a t i o n s i s i n d i c a t e d i n F i g . 4-6, Both, PRCj and P R C J J i n c r e a s e d w i t h i n c r e a s i n g amount of DVM i n the comonomer mixture over the c o n c e n t r a t i o n range s t u d i e d (up t o 3 0 % DVM). A b s o l u t e PRCJJ_ v a l u e s at the same c o n c e n t r a t i o n l e v e l d i f f e r e d c o n s i d e r a b l y from PRCj v a l u e s , as expected from two separate a c c e l e r a t i n g e f f e c t s i n the system s t u d i e d : i . a c c e l e r a t i o n by i n c r e a s e d v i s c o s i t y as r e p r e s e n t e d by PRCj, and 61 i i . a c c e l e r a t i o n by Ge as repr e s e n t e d by PRCjr. Numerical v a l u e s of P R C J J are at l e a s t one order of magnitude hig h e r than those of PRCj (Tables 4-2 and 4 - 4 ) . T h i s i s i n a good agreement w i t h r a t e c o n s t a n t s observed b e f o r e and a f t e r onset of GEP r e p o r t e d by Barthalemy ( 4 ) . P R C J J was found t o be independent of DVM s t r u c t u r e , while PRCj i n c r e a s e d w i t h i n i t i a l v i s c o s i t y of the comonomer mixtures, g i v e n i n Table 3-2. 5.3 P r e d i c t i o n and C a l c u l a t i o n of C u r i n g Times In p r e v i o u s s t u d i e s (67,69) an e m p i r i c a l method was suggested f o r p r e d i c t i n g c u r i n g times ( t ^ ^ ) f ° r "the MMA-TEGDMA comonomer system. The e m p i r i c a l equation / 5 - l / was found t o be v a l i d f o r TEGDMA c o n c e n t r a t i o n up t o '\Q% i n the MMA-TEGDMA comonomer mixture. A p p l i c a t i o n of / 5 - l / t o the present data d e r i v e d f o r the MMA-DVM s e r i e s , a l l o w s i n t e g r a t i o n and comparison of p r e v i o u s data w i t h r e s u l t s of the present study. Mathematical treatment of data i n Tab l e s 4-1 and 4-3 g i v e s the f o l l o w i n g OAC, as c a l c u l a t e d from l i n e a r r e l a t i o n s h i p s between OAC and DVM c o n c e n t r a t i o n i n F i g . 4-8: . -1 -1 mm • cone i a ^MMA-EGDMA * 1 " L X 1 0 _ r- '-3 1 1 • ^MMA-DEGDMA ~ l o 5 X 1 0 i i i . ^MMA-TrEGDMA = 1 , 8 x 1 0 3 l v * KMMA-TEGDMA = 2,4 x 10" 3 62 Numerical v a l u e s of OAC f o r the d i f f e r e n t MMA-DVM systems demonstrate t h a t a c c e l e r a t i n g a b i l i t y i n c r e a s e s w i t h DVM molecular b r i d g e l e n g t h . As i l l u s t r a t e d i n F i g . 4-9 OAC v a l u e s are i n v e r s e l y r e l a t e d t o DVM co n n e c t i o n number ( C N Q Y ^ K A f t e r c a l c u l a t i o n of CN , using / 3 - l / , one can estimate OAC from the f o l l o w i n g equation: 1//KMMA~DVM = 5 , 2 X C NDVM - 10.55 ....... ./5-2/ where: CNnw,,, i s c o n n e c t i o n number of r e s p e c t i v e DVM. The l i n e a r r e l a t i o n s h i p g i v e n by /5-2/ i s i l l u s t r a t e d i n F i g , 4-9. R e l a t i v e a c c e l e r a t i n g a b i l i t y of the DVM s t u d i e d , expressed as OAC r a t i o s w i t h r e s p e c t t o the l e a s t e f f e c t i v e i n the s e r i e s (EGDMA), i n c r e a s e d as: EGDMA : DEGDMA : TrEGDMA : TEGDMA = 1.0 : 1.36 : 1.64 : 2.18 C o r r e l a t i o n of OAC wi t h the DVM molecular b r i d g e l e n g t h i s i n good agreement w i t h r e c e n t data on p o l y m e r i z a t i o n of d i m e t h a c r y l a t e e s t e r s having v a r i a b l e b r i d g e d i s t a n c e s ( 8 ) . Increase i n a b s o l u t e v a l u e s of propagation c o n s t a n t s (k ) wi t h P molecular b r i d g e l e n g t h i s demonstrated i n Table 2-2. 63 From c a l c u l a t e d OAC of i n d i v i d u a l comonomer systems i t i s p o s s i b l e t o p r e d i c t occurrence of maxima on p o l y m e r i z a t i o n exotherm c u r v e s . D i f f e r e n c e s i n c u r i n g times ( t j y ^ ) c a l c u l a t e d from / 5 - l / g and those measured e x p e r i m e n t a l l y , are g i v e n i n Table 4 - 5 f o r the f o u r systems s t u d i e d . These r e s u l t s demonstrate the u s e f u l n e s s of / 5 - l / , f i r s t d e r i v e d f o r the MMA-TEGDMA system ( 6 7 , 6 9 ) , l a t e r a p p l i e d t o the S-TEGDMA system (70) as recorded i n Table 4 - 6 . The equation can be a p p l i e d t o a v a r i e t y of v i n y l - d i v i n y l comonomer systems. The average e r r o r f o r p r e d i c t i n g c u r i n g times of the f o u r systems s t u d i e d was t 5 .7% (Table 4 - r 5 ) . 5 , 3 . 1 A p p l i c a t i o n of the d e r i v e d e q u a t i o n t o p u b l i s h e d r e s u l t s Kenaga (53) and r e c e n t l y Duran and Meyer (33) used the p o l y m e r i z a t i o n exotherm technique t o evaluate the a c c e l e r a t i n g a b i l i t y of v a r i o u s d i - and t r i f u n c t i o n a l c r o s s l i n k i n g agents polymerized by h e a t - c a t a l y s t systems. Both MMA (33) and styrene-type monomers (53) were polymerized i n wood. Due t o s i m i l a r i t i e s between the c o n c l u s i o n s of these authors and the i m p l i c a t i o n s of r e s u l t s from the present study, i t was of i n t e r e s t t o t e s t a p p l i c a b i l i t y of Eq. / 5 - l / t o p r e d i c t i o n of c u r i n g times from the data p u b l i s h e d . Numerical v a l u e s of OAC, c a l c u l a t e d from Kenaga 1s data (53) ; were p l o t t e d a g a i n s t c o n c e n t r a t i o n of c r o s s l i n k i n g agents. The expected l i n e a r r e l a t i o n s h i p s f o r these comonomer systems are g i v e n i n F i g . 4 - 1 0 . The f o l l o w i n g OAC v a l u e s were 64 c a l c u l a t e d f o r these systems: min"""*" cone, i . KTBS-TMPTMA 2.6 x 10~ 4 i i . KTBS-Tr EGDMA = 3,6 x 1 0-4 i i i . KTBS-EGDMA 4.0 x 10~ 4 i v . KTBS-TEGDMA 4,8 x l O " 4 -1 Only small d i f f e r e n c e s were found between OAC of i n d i v i d u a l systems used by Kenaga (53), The reason f o r t h i s may be the l i m i t e d number of experiments c a r r i e d out over the r a t h e r wide c o n c e n t r a t i o n range ( o n l y f o u r experiments w i t h i n the 0-30% c o n c e n t r a t i o n i n t e r v a l ) . However, OAC f o r a l l t h r e e d i - f u n c t i o n a l c r o s s l i n k i n g agents were higher than those c a l c u l a t e d f o r the t r i - f u n c t i o n a l TMPTMA, T h i s suggests t h a t s u b s t i t u t i o n on the molecular b r i d g e f u n c t i o n s as a d i f f u s i o n b a r r i e r f o r the propagation r e a c t i o n . C a l c u l a t e d OAC from data of Kenaga (53) were used t o p r e d i c t c u r i n g times f o r i n d i v i d u a l comonomer mixtures a c c o r d i n g t o / 5 - l / . D i f f e r e n c e s i n c u r i n g times ( t ^ ^ ) between c a l c u l a t e d and p u b l i s h e d v a l u e s are summarized i n Table 4-7. Good agreement was found between a c t u a l and p r e d i c t e d v a l u e s , r e g a r d l e s s of d i f f e r e n c e s i n p o l y m e r i z a t i o n c o n d i t i o n s and i n i t i a t i n g system used. The average e r r o r of t ^ ^ - was l e s s than t 5%. 65 Only two experiments have t o be run i n order t o p r e d i c t OAC. Expected c u r i n g times f o r d i f f e r e n t comonomer mixtures w i t h i n c o n c e n t r a t i o n range covered by the two experiments can be c a l c u l a t e d w i t h the a i d of / 5~ l A T h i s "two-point method" was t e s t e d by using experimental data p u b l i s h e d by Duran and Meyer (33), The f o l l o w i n g tv/o e x p e r i - ments were randomly chosen: 93:2; 91:9; Based on these two experiments, the c a l c u l a t e d OAC value (K„,„ T,. n T,,, = 1.03 x 10"""3 min" cone." ) was used f o r MMA-TMPTMA p r e d i c t i n g c u r i n g times of v a r i o u s MMA-TMPTMA compositions. D i f f e r e n c e s between c a l c u l a t e d and pub l i s h e d v a l u e s (33.) are summarized i n Table 4-8. Agreement again, was q u i t e good. By i n t e g r a t i n g experimental r e s u l t s from t h i s work w i t h data a l r e a d y p u b l i s h e d , i t i s demonstrated t h a t a c c e l e r a t i o n a b i l i t y and c u r i n g times f o r d i f f e r e n t comonomer systems, r e g a r d l e s s of the p o l y m e r i z a t i o n c o n d i t i o n s , can be estimated w i t h i n the most d e s i r a b l e p r a c t i c a l c o n c e n t r a t i o n r e g i o n by / 5 - l / w i t h a high l e v e l of c o n f i d e n c e . From d i r e c t comparison of the r e s p e c t i v e OAC va l u e s i t i s apparent t h a t a l l DVM s t u d i e d were more e f f e c t i v e a c c e l e r a t o r s than the t r i f u n c t i o n a l TMPTMA. T h i s i.s g r a p h i c a l l y i l l u s t r a t e d i n F i g . 4-4 and 4-8, i . comonomer composition, MMA:TMPTMA = c u r i n g time, t,,.., = 108.3 min; and 3 MAX i i . comonomer composition, MMA:TMPTMA = c u r i n g time, t j v y ^ = 60.8 min. 6 6 A f u r t h e r disadvantage of t r i f u n c t i o n a l c r o s s l i n k i n g agents i s t h a t they evolve more heat dur i n g the p o l y m e r i z a t i o n p r o c e s s . Peak temperature ( T ^ A X ) i n c r e a s e d v e r y r a p i d l y t o 180-190 °C (33) which may be i n j u r i o u s t o WPC. T h i s c o u l d be avoided by l o w e r i n g OCR i f r a d i a t i o n i s used as the energy source f o r p o l y m e r i z a t i o n . Data i n Table 4-9 and F i g . 4-11 demonstrate t h a t OCR v a r i e s l i n e a r l y w i t h the square r o o t of r a d i a t i o n dose r a t e . At lower dose r a t e s the temperature r i s e s more g r a d u a l l y and T ^ ^ i s c o n s i d e r a b l y lower. Another disadvantage of t r i f u n c t i o n a l c r o s s l i n k i n g agents i s t h e i r h i g h tendency t o develop d i f f u s i o n b a r r i e r s . P o l y m e r i z a t i o n may cease due to l a c k of monomer d i f f u s i o n i n the h i g h l y c r o s s l i n k e d network. T h i s problem i s r e a d i l y demonstrated w i t h the MvlA-TMPTMA system. For example, "tjvsAX at 15 and 20% TMPTiMA i n the system i s e s s e n t i a l l y the same at 55 and 49.4 min, r e s p e c t i v e l y . C o n v e r s i o n degree at these c o n c e n t r a t i o n s , however, decreased from 85.6% t o 82.3% (33). In a d d i t i o n , t r i f u n c t i o n a l c r o s s l i n k i n g agents form b r i t t l e polymer products w i t h lower a b r a s i o n r e s i s t a n c e ( 6 3 ) . T h i s was r e p o r t e d f o r WPC by Kenaga (53). 5.4 A n a l y s i s of C r o s s l i n k e d Polymer Product Thermomechanical Curves Thermomechanical p r o p e r t i e s of c r o s s l i n k e d polymer products depend p r i m a r i l y upon the t i g h t n e s s of the network 67 s t r u c t u r e , i . e . , c r o s s l i n k i n g d e n s i t y caused by a gi v e n c o n c e n t r a t i o n of c r o s s l i n k i n g agent. A l l DVM s t u d i e d here f u n c t i o n e d as e f f i c i e n t c r o s s l i n k i n g agents and transformed l i n e a r and t h e r m o p l a s t i c poly(MMA) t o f u l l y or p a r t l y c r o s s - l i n k e d p r o d u c t s . L i n e a r , f u l l y and p a r t l y c r o s s l i n k e d polymer pr o d u c t s e x h i b i t d i f f e r e n t thermomechanical curves as demonstrated i n F i g . 3-2. C r o s s l i n k i n g e f f i c i e n c y of i n d i v i d u a l DVM s t u d i e d was not i d e n t i c a l but v a r i e d w i t h molecular b r i d g e l e n g t h . I t i n c r e a s e d a c c o r d i n g t o the sequence: TEGDMA <^ TrEGDMA <f DEGDMA < ^ EGDMA T h i s i s demonstrated by shape of thermomechanical curves f o r copolymer products formed from the two DVM having extreme CN v a l u e s , i . e . , EGDMA wit h CN 2.200 ( F i g . 4-12) and TEGDMA.with CN 2.105 ( F i g . 4-13). Comparison of F i g . 4-12 and 4-13 shows t h a t small a d d i t i o n s of e i t h e r DVM t o MMA r a p i d l y changed the l i n e a r poly(MMA) t o a c r o s s l i n k e d thermoset p l a s t i c w i t h i n c r e a s e d network s t r e n g t h at higher temperatures, T r a n s f o r m a t i o n e f f i c i e n c y of EGDMA was, however, s i g n i f i c a n t l y h i g h e r . D i f f e r e n c e s i n c r o s s l i n k i n g e f f i c i e n c y f o r these two DVM can be demonstrated by comparison of r e s p e c t i v e thermo- mechanical parameters as obtained at the same DVM c o n c e n t r a t i o n 68 l e v e l , as shown i n Table 4-10, At 10% EGDMA i n the MMA-EGDMA system, f o r example, T , TDD and TDT were 127 °C, 77% and 202 °C. The same parameters f o r 10% TEGDMA i n the MMA-TEGDMA system were c o n s i d e r a b l y lower; T^ 113 °C,.TDD 52% and TDT 165 °C„ To o b t a i n the same parameter v a l u e s w i t h TEGDMA, i t s c o n c e n t r a t i o n has t o be i n c r e a s e d to approximately 30% (Table 4-10 and F i g . 4-12 and 4-13). A n a l y s i s of thermomechanical data f o r the f o u r systems s t u d i e d , Table 4-10, l e a d s t o the o v e r a l l o b s e r v a t i o n t h a t i n c r e a s e d DVM c o n c e n t r a t i o n i n f l u e n c e d thermomechanical parameters i n two ways: i . i n c r e a s e d T Q, TDT and TDD, and i i . • decreased LTDC. Tg i s an important parameter which i s r e l a t e d t o m o b i l i t y of macromolecular segments (72,73). As expected, replacement of van der Waal's f o r c e s between polymer c h a i n s by carbon-carbon bonds reduced m o b i l i t y cf polymer segments • and i n c r e a s e d T . As i l l u s t r a t e d i n F i g . 4-14, i t i s p o s s i b l e t o assess the c o n t r i b u t i o n of DVM molecular b r i d g e l e n g t h t o o v e r a l l m o b i l i t y of the c r o s s l i n k e d network i n the f o u r MMA-DVM systems s t u d i e d . At the same c r o s s l i n k i n g d e n s i t y (as expressed by copolymer CN) the s h o r t e r DVM b r i d g e l e n g t h gave lower segmental m o b i l i t y and higher T ( F i q . 4-14). S t a b i l i z a t i o n of Tg at h i g h e r c r o s s l i n k i n g d e n s i t i e s ( F i g . 4-14) can be i n t e r p r e t e d by o v e r l a p p i n g of two independent e f f e c t s as r e l a t e d t o T_ as: 69 i . c r o s s l i n k i n g e f f e c t , and i i . c o p o l y m e r i z a t i o n e f f e c t . The c r o s s l i n k i n g e f f e c t always i n c r e a s e s T^,whereas the c o p o l y m e r i z a t i o n e f f e c t decreases T . Both e f f e c t s are a d d i t i v e ( 6 1 ) . The c r o s s l i n k i n g e f f e c t p r e v a i l s at lower C N C 0 , At h i g h e r DVM c o n c e n t r a t i o n the c o n t r i b u t i o n of the c r o s s l i n k i n g e f f e c t i s balanced by the c o p o l y m e r i z a t i o n e f f e c t . The o v e r a l l r e s u l t i s T s t a b i l i z a t i o n ( r i g . 4 - 1 4 ) . R e s i s t a n c e of i n d i v i d u a l polymer products t o high temperature deformation can be expressed by TDD, while s t r e n g t h of the c r o s s l i n k e d network i s p r o p o r t i o n a l t o TDT v a l u e s . As demonstrated f o r the fo u r stystems s t u d i e d ( F i g , 4 - 1 5 ) , both parameters (TDD and TDT) are dependent..on copolymer c o n n e c t i o n number ( C N C 0 K r e g a r d l e s s of the s i z e of the molecular b r i d g e of the c r o s s l i n k i n g agent used. Both parameters Increa s e d r a p i d l y w i t h i n the narrow DVM c o n c e n t r a - t i o n i n t e r v a l . F u r t h e r a d d i t i o n of DVM t o induce higher c r o s s l i n k i n g d e n s i t i e s changed TDD and TDT only moderately. T h i s i s an important o b s e r v a t i o n and s u g g e s t i v e l y the e f f e c t c o i n c i d e s w i t h both optimal a c c e l e r a t i o n and thermomechanical p r o p e r t i e s of polymer products at low DVM c o n c e n t r a t i o n s (compare Tables 4 - 3 and 4 - 1 0 ) . The l i n e a r thermomechanical deformation c o e f f i c i e n t (LTDC) c h a r a c t e r i z e s behaviour of heated polymer products w i t h i n the t r a n s i t i o n r e g i o n . LTDC r e p r e s e n t s the r a t e of polymer 70 deformation a s s o c i a t e d w i t h the t r a n s i t i o n from p l a s t i c t o rubbery s t a t e . Deformation of c r o s s l i n k e d polymer products i n the rubbery r e g i o n i s constant a c r o s s a broad temperature i n t e r v a l u n t i l the polymer l o s e s coherence at temperatures e q u a l l i n g TDT ( F i g . 3-2, 4-12 and 4-13). I t i s t o the advantage of c r o s s l i n k e d polymer products of the present study t h a t LTDC decreased e x p o n e n t i a l l y w i t h DVM c o n c e n t r a t i o n i n the system. T h i s i s demonstrated f o r the two system extremes (MMA-EGDMA and MMA-TEGDMA) using the s e m i - l o g a r i t h m i c p l o t i n F i g . 4-16, F u r t h e r , the r e l a t i o n s h i p between LTDC and polymer c r o s s l i n k i n g d e n s i t y as r e p r e s e n t e d by copolymer c o n n e c t i o n number (CN C 0) can be used t o great advantage i n p r e d i c t i n g thermomechanical behaviour of complex c r o s s l i n k e d polymer products d e r i v e d from comonomer mixtures. 5.5 A n a l y s i s of Polymer Product Compression S t r e s s - S t r a i n Curves Compression s t r e s s - s t r a i n curves were found t o f o l l o w p a t t e r n s r e p r e s e n t e d by curves i l l u s t r a t e d i n F i g . 3-3. These show a wide v a r i a t i o n i n p r o p e r t i e s ranging from e l a s t i c , s e m i - r i g i d , hard, tough t o b r i t t l e m a t e r i a l s (20,72). In g e n e r a l , s o f t and weak polymers ( F i g . 3-3a) e x h i b i t low modulus of e l a s t i c i t y ( E ) , low y i e l d p o i n t and moderate deformation at r u p t u r e . Hard and b r i t t l e m a t e r i a l s show hi g h E, no w e l l - d e f i n e d y i e l d p o i n t and low deformation at ru p t u r e ( F i g . 3-3b). S o f t and tough p l a s t i c s ( F i g . 3-3c) show 71 low E and y i e l d p o i n t but high deformation and r u p t u r e s t r e s s . Hard and strong p l a s t i c s ( F i g . 3-3d), on the other hand, e x h i b i t h i g h E and y i e l d p o i n t but only moderate deformation* F i n a l l y , hard and tough m a t e r i a l s ( F i g . 3-3e) show E, y i e l d p o i n t , deformation and rup t u r e s t r e s s a l l t o be h i g h . T y p i c a l compression s t r e s s - s t r a i n curves f o r the two extreme DVM systems s t u d i e d over the f u l l c o n c e n t r a t i o n range (MMA-EGDMA and MMA-TEGDMA), are reproduced i n F i g . 4-18 and 4-19. L i n e a r poly(MMA) e x h i b i t e d compression s t r e s s - s t r a i n p r o p e r t i e s s i m i l a r t o those of poly ( S ) (20,70); i . e . , i t was hard and moderately tough ( F i g . 3-3d). H i g h l y c r o s s l i n k e d homopolyrners of pure EGDMA ( F i g . 4-18) and TEGDMA ( F i g . 4-19) e x h i b i t e d behaviour c h a r a c t e r i s t i c of hard and b r i t t l e polymers ( F i g . 3-3b). I t was of p a r t i c u l a r i n t e r e s t t o study shapes of compression s t r e s s - s t r a i n curves as obtained f o r the wide range of MMA-DVM comonomer mixtures. Experiments were concentrated w i t h i n the most a t t r a c t i v e c o n c e n t r a t i o n i n t e r v a l , i . e . , up t o 10% DVM i n the r e s p e c t i v e c.omonomer systems. As shown i n Tables 4-1 and 4-3 and F i g . 4-4, t h i s r e g i o n was of p a r t i c u l a r i n t e r e s t because of s i g n i f i c a n t l y a c c e l e r a t e d c u r i n g . OCR w i t h i n t h i s c o n c e n t r a t i o n i n t e r v a l was p r o p o r t i o n a l t o DVM volume c o n c e n t r a t i o n ( F i g . 4-8). As demonstrated i n F i g . 4-18 and 4-19, a d d i t i o n of EGDMA or TEGDMA t o MMA caused change i n shape of polymer compression s t r e s s - s t r a i n curves a c c o r d i n g t o amount of DVM added. 72 They changed from hard and b r i t t l e - t y p e s ( F i g . 3-3b) t o hard and tough-types ( F i g . 3-3e). These changes i n s t r e s s - s t r a i n response i n d i c a t e improvement i n mechanical p r o p e r t i e s as f o l l o w s : i . r u p t u r e s t r e s s , p l a s t i c deformation and area under the s t r e s s - s t r a i n curve were much i n c r e a s e d with 2.5 t o 10% DVM a d d i t i o n t o MMA. f, i i . development of w e l l - d e f i n e d y i e l d p o i n t s at hi g h e r MMA c o n c e n t r a t i o n , and i i i . no s i g n i f i c a n t change i n slope of the i n i t i a l Hookean p o r t i o n o c c u r r e d . Numerical data of i n d i v i d u a l mechanical parameters, d e r i v e d from compression s t r e s s - s t r a i n curves f o r the f o u r MMA-DVM systems s t u d i e d are summarized i n Tables 4-11 and 4-12. G r a p h i c a l i n t e r p r e t a t i o n of compression s t r e s s and s t r a i n at r u p t u r e and area under the s t r e s s - s t r a i n curve, as a f u n c t i o n of comonomer composition f o r the f o u r systems s t u d i e d , are g i v e n i n F i g . 4-20 through 4-23. By comparison of the r e s u l t s compiled i n Tables 4-11 and 4-12 the f o l l o w i n g o b s e r v a t i o n s can be made: i . compression s t r e s s - s t r a i n p r o p e r t i e s of poly(MMA) wi t h r e s p e c t t o r u p t u r e , y i e l d and toughness parameters were higher than those of any DVM homopolymers s t u d i e d ; i i . p r o p e r t i e s of products from comonomer mixtures are not a d d i t i v e s i . e . , the components do not c o n t r i b u t e s t r e n g t h i n p r o p o r t i o n t o t h e i r presence; i i i a compression s t r e s s and s t r a i n , as w e l l as area under s t r e s s - s t r a i n c u r v e s , f o r a l l systems s t u d i e d , e x h i b i t e d w e l l - d e f i n e d maxima ( F i g . 4-20 through 4-23); i v . s u p e r i o r s t r e n g t h p r o p e r t i e s f o r the system* s t u d i e d were r e s t r i c t e d t o the narrow . c o n c e n t r a t i o n r e g i o n , w i t h i n 5 t o 10% DVM i n the system; and v. the c o n c e n t r a t i o n i n t e r v a l showing s u p e r i o r s t r e n g t h p r o p e r t i e s broadened w i t h i n c r e a s i n g DVM molecular b r i d g e l e n g t h , i . e . , i n the sequence from EGDMA t o TEGDMA. I n d i v i d u a l MMA-DVM systems d i f f e r e d only i n s p e c i f i c d e t a i l s . C o n s i d e r , as an example, l o c a t i o n of the maxima on p r o p e r t i e s / c o m p o s i t i o n curves ( F i g . 4-20 through 4-23). They appeared at approximately 5% EGDMA c o n c e n t r a t i o n i n the 'MMA- EGDMA system ( F i g . 4-20). However, i n the MMA-TEGDMA system the maxima were l o c a t e d near 10% TEGDMA c o n c e n t r a t i o n . I t can. be estimated from the r e s p e c t i v e MMA/DVM mole r a t i o s , i . e . , 34.3 f o r the MMA-EGDMA and 24,4 f o r the MMA-TEGDMA.system (Table 3-2), t h a t copolymers w i t h s u p e r i o r s t r e n g t h p r o p e r t i e s possess approximately one c r o s s l i n k f o r every 25 t o 35 monomer u n i t s on the poly(MMA) backbone. C o n s i d e r i n g the MW of MMA t o 74 be equal t o 100, a number average molecular weight of 2,500 t o 3,500 i s obtained f o r poly(MMA) segments between two s u c c e s s i v e j u n c t i o n s . These v a l u e s are i n v e r y good agreement wi t h averages d e r i v e d f o r c r o s s l i n k e d rubbers and thermoset copolymers (73), as c a l c u l a t e d from s w e l l i n g e q u i l i b r i a measurements. From the l i m i t e d d i f f e r e n c e s i n segmental MvV between c r o s s l i n k s , estimated f o r the two extreme comonomer systems s t u d i e d , i t was suggested t h a t compression s t r e n g t h parameters can be r e l a t e d t o comonomer c o n n e c t i o n number (CN ). CM C 0 of i n d i v i d u a l copolymers, as c a l c u l a t e d from /3-2/,are i n c l u d e d i n Table 4-11. Compression s t r e s s at r u p t u r e , as demonstrated i n F i g . 4-24 f o r a l l f o u r systems s t u d i e d , i s s t r o n g l y r e l a t e d t o C N C 0 , i r r e s p e c t i v e of the b r i d g e l e n g t h of c r o s s l i n k i n g agent used. The c r o s s l i n k i n g d e n s i t y of polymer networks as expressed by CN i s a u s e f u l s t r u c t u r a l parameter. Although CN t e l l s n othing about the c o n f i g u r a t i o n a l a spects of the c r o s s l i n k e d networks, i t does q u a n t i t a t i v e l y c h a r a c t e r i z e r e s i s t a n c e t o both mechanical and thermomechanical deformations. P h y s i c a l and thermomechanical p r o p e r t i e s of polymer products are assumed t o be f u n c t i o n s of c r o s s l i n k i n g d e n s i t y as d e s c r i b e d by CM co O b s e r v a t i o n of maxima at low DVM'concentrations ( F i g . 4-20 through 4-23) demonstrates an important d i s c o v e r y ; t h a t s t r e n g t h p r o p e r t i e s of s o l i d c r o s s l i n k e d polymer products v a r y w i t h c o n c e n t r a t i o n of c r o s s l i n k i n g agent i n the same manner as s t r e n g t h p r o p e r t i e s of c r o s s l i n k e d rubbers and elstomers (28,29,32,35,3?,63,94,102). For p r a c t i c a l a p p l i c a t i o n s , a narrow DVM c o n c e n t r a t i o n i n t e r v a l (up to. 10%) i s the most a t t r a c t i v e . W i t h i n t h i s c o n c e n t r a t i o n range both maximum a c c e l e r a t i o n and s u p e r i o r mechanical and thermo- mechanical p r o p e r t i e s were o b t a i n e d . I n t r o d u c t i o n of DVM w i t h MMA w i t h i n t h i s range a l s o r a p i d l y i n c r e a s e d the area under the compression s t r e s s - s t r a i n c urve. The r e s u l t i n g copolymers are capable of c o n s i d e r a b l e energy storage before f a i l u r e . I t was a l s o observed t h a t f a i l u r e of these copolymers occurred w i t h tremendous sound e f f e c t s due t o r e l e a s e of the energy s t o r e d . In s p i t e of the s i g n i f i c a n c e and importance of polymer toughness, s t a n d a r d i z e d procedures f o r i t s e v a l u a t i o n are s t i l l l a c k i n g (72). From the o b s e r v a t i o n t h a t small s t r e s s e s i n the p l a s t i c range produced c o n s i d e r a b l e deformation i n tough polymer products ( F i g . 4-18 and 4-19), i t was assumed t h a t p l a s t i c deformation i s i n d i c a t i v e of toughness. T h i s i s demonstrated by l i n e a r r e l a t i o n s h i p between the area, under the compression s t r e s s - s t r a i n curve and p l a s t i c d eformation for' the f o u r systems s t u d i e d i n F i g . 4-25. Toughness of these copolymers seems t o be independent of DVM molecular b r i d g e l e n g t h . A s i m i l a r c o n c l u s i o n was r e p o r t e d f o r the S-TEGDMA comonomer system (70). 76 6.0 CONCLUSIONS lo A c c e l e r a t i o n i n r a d i a t i o n p o l y m e r i z a t i o n of v i n y l - d i v i n y l comonomer systems via. g e l - e f f e c t (Ge) i n c r o s s - l i n k e d networks was r e l a t e d t o the chemical s t r u c t u r e of the c r o s s l i n k i n g agents, i n p a r t i c u l a r t o t h e i r molecular b r i d g e l e n g t h . The o v e r a l l c u r i n g r a t e (OCR) was found t o be p r o p o r t i o n a l t o volume c o n c e n t r a t i o n of d i v i n y l monomer (DVM). The o v e r a l l a c c e l e r a t i o n constant (K) i s i n v e r s e l y r e l a t e d t o DVM con n e c t i o n number (CNgy^) as: 1/K = 5,2 x C N D V M - 10,55 2. The r e l a t i v e a c c e l e r a t i o n a b i l i t y of DVM st u d i e d i n c r e a s e s w i t h DVM molecular b r i d g e l e n g t h i n the order: EGDMA : DEGDMA : TrEGDMA : TEGDMA = 1.00: 1 036 : 1,64 : 2.18. The h a l f - t i m e c o n c e n t r a t i o n v a l u e s f o r EGDMA, DEGDMA, TrEGDMA and TEGDMA were approximately 7, 5, 4, and 3% ( v o l / v o l ) , r e s p e c t i v e l y . 3. S u b s t a n t i a l decrease i n c u r i n g time ( t ^ ^ ) was obtained with up t o 10% DVM i n the r e s p e c t i v e comonomer mixtures. W i t h i n t h i s c o n c e n t r a t i o n i n t e r v a l s u p e r i o r mechanical and thermomechanical p r o p e r t i e s of copolymers were a l s o observed. 4. Tfie e m p i r i c a l e q u a t i o n f o r c a l c u l a t i n g the o v e r a l l 7 7 a c c e l e r a t i o n constant and p r e d i c t i o n of c u r i n g times was d e r i v e d and a p p l i e d t o the f o u r systems s t u d i e d . I t s e x t e n s i o n t o p u b l i s h e d r e s u l t s on a c c e l e r a t i o n i n heat- c a t a l y s t systems was found t o be f u l l y a p p l i c a b l e . Agreement between p r e d i c t e d and p u b l i s h e d r e s u l t s was withini5/o e r r o r f o r a l l copolymer systems observed. b. Copolymer c o n n e c t i o n number (CN ) was i n t r o d u c e d and found t o be a u s e f u l s t r u c t u r a l parameter f o r c r o s s l i n k e d copolymers. I t c h a r a c t e r i z e s and c o r r e l a t e s q u a n t i t a t i v e l y the mechanical and thermomechanical p r o p e r t i e s with copolymer c r o s s l i n k i n g d e n s i t i e s . 6. The MMA-TEGDMA comonomer system was found to be the most s u i t a b l e and e c o n o m i c a l l y a t t r a c t i v e . I t r e p r e s e n t s a w e l l balanced compromise of improved p o l y m e r i z a t i o n parameters and copolymer p r o p e r t i e s , s u i t a b l e f o r wood-polymer composite p r o d u c t s . 78 7.0 RECOMMENDATIONS FOR FURTHER STUDY Based on t h e o r e t i c a l c o n s i d e r a t i o n s and experimental r e s u l t s of the present study the f o l l o w i n g are recommended as de s e r v i n g of f u r t h e r r e s e a r c h : i . extend a c c e l e r a t e d p o l y m e r i z a t i o n t o the MMA- methyl a c r y l a t e system t o examine i n f l u e n c e of -CH3 groups on Ge a c c e l e r a t i o n ; i i . extend the present MMA-DVM systems t o higher homologous members (hexa-, deca-, e t c . , po l y e t h y l e n e g l y c o l d i m e t h a c r y l a t e ) t o examine e f f e c t s of Ge onset and delayed d i f f u s i o n - c o n t r o l l e d p r o p a g a t i o n ; i i i . examine s t r u c t u r e of polymer products t o c o l l e c t evidence f o r p o s s i b l e i n t e r n a l r i n g f o r mation; i v . move from b u l k c o p o l y m e r i z a t i o n t o a c c e l e r a t e d p o l y m e r i z a t i o n w i t h i n wood s t r u c t u r e s ; v. study the e f f e c t of a t h i r d component i n the MMA-DVM system, f o r example t o i n c r e a s e OCR i n the presence of oxygen and/or t o impart f i r e r e s i s t a n c e p r o p e r t i e s (68); v i . f u r t h e r extend u t i l i z a t i o n of exothermic heat as p r e s e n t l y proposed f o r one-step p r o d u c t i o n of veneer polymer o v e r l a i d plywoods (52,81). 79 8.0 LITERATURE CITED 1. 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W a l l i n g , - C . 1945, G e l f o r m a t i o n i n a d d i t i o n p o l y m e r i z a - t i o n . J . Am. Chem. Soc. 67: 4 4 1 - 4 4 7 . 106. . 1957. Free R a d i c a l s i n S o l u t i o n . John W i l l e y , New York. 631 pp. 88 Table 2-1. E f f e c t of c o n v e r s i o n l e v e l on p o l y m e r i z a t i o n c h a r a c t e r i s t i c s of methyl methacrylate (MMA) at 2 2 . 5 0 C ( 4 3 ) . C o n v e r s i o n , FL lP' /o • / O / %/hz k p k t x 1 0 ~ 5 ( k p / k t I ^ 2 ) x l 0 2 0 3 . 5 384 442 5 . 7 8 10 2 . 7 234 273 4 . 5 8 20 6 . 0 267 7 2 . 6 8 . 8 1 30 1 5 . 4 303 1 4 . 2 25*60 40 2 3 . 4 368 8 . 9 3 3 3 . 9 0 50 2 4 . 5 258 4 . 0 3 40o60 60 2 0 . 0 74 0 .498 3 3 . 2 0 70 1 3 . 1 16 0 . 0 5 6 4 2 1 . 3 0 80 2 . 8 1 0 .0076 3 . 9 5 Table 2-2. Some data on p o l y m e r i z a t i o n of d i m e t h a c r y l a t i c e s t e r s ( p h o t o i n i t i a t o r - b e n z o i n 0*2%) (.8). N The compound Symbol M. Viscosity The degree of The tempera- W _ 105 M (sec- 1) K, c.p. transformation (%) ture of poly- merization 1. mole" ' s e c - 1 1. mole"' sec I Methylmcthacrylale M M A 100 50 50 25 0-2 _ 2 Dimctlmcryiaic of butyk'iicKlycoi 226 4 0 0 25 ' 3-8 600 8.10s 3 Dinielhaciyiate of biaylciicsjlyci.il M U 226 — 30 25 4 0 112 2-3.104 4 Dinictliitcrylalc of butylcneglycol MB 226 — 5 25 . 33-0 17 83.10* 5 Diriicthacrylafehe.xa- mcihylciu'ijlycol MO 244 4-9 0 25 13-0 1200 6-2.105 r> Diiv.eiliacrylatelicxa- nicihylcncglycol M G 244 — ' 5 25 7-0 245 3-7.104 7 Dimelliacrylalehexa- mclliylcncylycol MG 244 — 30 25 4-0 26 1 -3.10J 8 Diir.ctlucrylatedeea- mciliylcncglycol MD 300 7-7 0 25 160 1880 • 4-2.10s 9 Dimethacrylalcdoca- inclhylciicglycol M D 300 — 5 25 24-0 1400 1 -5.10s 10. Dimcthacrylatcdcca- mcthylcncglycol M D 300 — 30 25 29-0 S40 2-5.104 11 D i m c 111 a c r y 1 a 1 c (bis- dicthylencglycol) plithalolc MGPh 566 51-0 0 30-6 20-0 2470 4-8.105 12 Dimethacrylate (bis- diclliylcncylycol) plithalolc) MGI 'h 566 — 5 30-6 24-0 2300 2-7.105 13 DiiiK-lbaciylatc (/;;> diclhylcncglycol) plithalote MGPh 566 — 30 30-6 54-0 1750 3-2.10* CO vO Notes: (I) l o r M M A at a 0 per cent Kn 300 I. mole" ' sec" 1 (T 25°). (2) The kinetics of polymerization of MB, MG, MD, M G F were studied by methods of precision dilatomctry and thermometry. 90 Table 3-1. Some p r o p e r t i e s of methyl methacrylate (MM) and d i v i n y l monomers (DVM) used In t h i s study* Name, (abr„) n Mw d, CM g r e l formula g/cm 25°C M e t h y l m e t h a c r y l a t e , (MMAJ C sH 0 — 100 0.940 2.000 1,0 5 9 2 Ethylene g l y c o l d i m e t h a c r y l a t e , (EGDMA) 1 198 1,045 2.200 4,2 C 1 0 H 1 4 ° 4 D i e t h y l e n e g l y c o l d i m e t h a c r y l a t e , (DEGDMA) 2 242 1.062 2,150 6.9 C 1 2 H 1 8 ° 5 Trmethylene g l y c o l dimethacrylate,(TrEGDMA) 3 286 1,090 2.125 10.5 C 1 4 H 2 2 ° 6 T e t r a e t h y l e n e g l y c o l d i m e t h a c r y l a t e , (TEGDMA) 4 330 1.140 2.105 23.4 C 1 6 H 2 6 ° 7 91 Table 3-2. C o n c e n t r a t i o n s and r e l a t i v e v i s c o s i t i e s of d i f f e r e n t methyl methacrylate (MMA)-divinyl monomer (DVM) mixtures. DVM DVM f r a c t i o n i n Mole r a t i o s comonomer mixture volume mole DVM/MMAxlO MMA/DVM 25° C 0.025 0.013 0.14 70.5 1.04 0.050 0.028 0.29 34.3 1.08, 0,100 0.059 0.63 15.9 1,14 EGDMA 0.150 0.087 1.00 10.0 1.24 0.200 0.123 1.40 7,1 1.33 0,300 0.194 2.40 4.2 .1.41 0.025 0.012 0.13 83.5 1,05 0,050 0.024 0,25 40,5 1.11 DEGDMA 0.100 0.049 0.52 19.2 1,18 0.150 0.077 0.83 12,0 1,27 ,0.200 0.104 1,16 8.6 1,34 0.300 0.167 2.00 5.0 1.55 0.025 0.011 0.12 85.4 1.08 0.050 0.020 0.21 47.0 1.20 Tr EGDMA 0.100 0.043 0.45 22.2 1.28 0.150 0.065 0,69 14.4 1,42 0.200 0.092 1.01 9.9 1 o 53 0.300 0.144 1,69 5.9 1.83 0.025 0.009 0.09 102.0 1.14 0.050 0.019 0.19 57.8 1,29 TEGDMA 0.100 0.039 0.41 24.4 1.48 0.800 0.138 1.88 5.3 ' 2.54 92 Table 4-1. P o l y m e r i z a t i o n exotherm c h a r a c t e r i s t i c s f o r the MMA-TEGDMA comonomer system (67,69). Comonomer G e l - E f f e c t P o i n t composition, (GEP) MAX v o l . % MMA TEGDMA ^GEP, TGEP, H'AX, TMAX, min °C min °C 100 0 106 62 117 120 97.5 2.5 52 69 60 159 95 5 30 60 45 160 90 10 23 57 31 167 70 30 14 54 20 165 50 50 10 53 15 157 30 70 13 57 18 149 0 100 16 61 22 130 93 Table 4 - 2 . C a l c u l a t e d p o l y m e r i z a t i o n parameters f o r the MMA-TEGDMA comonomer system ( 6 7 , 6 9 ) . Comonomer Exotherm peak P o l y m e r i z a t i o n P RC T T Rate X V Composition, parameters C o e f f i c i e n t (PRC)., v o l % °C/min MMA TEGDMA Dose, Mrad £ T , °c P R C I PRC X I 100 0 1.50 84 0.24 5.2 21.6 95,5 2 .5 0.81 123 0.64 5.0 ' 7 .9 95 5 0.61 124 0.80 6.7 8.4 90 10 0.42 131 0.91 13.8 15.2 70 30 0.27 129 1.28 18.3 14.3 50 50 0.20 121 lo64 20.0 12.7 30 70 0.24 111 1.60 16.7 9.8 0 100 0.30 94 1.56 11.5 7.3 94 Table 4-3. P o l y m e r i z a t i o n exotherm c h a r a c t e r i s t i c s f o r MMA-EGDMA, -MMA-DEGDMA and MMA-TrEGDMA comonomer systems. Comonomer G e l - E f f e c t P o i n t composition, (GEP) MAX DVM v o l % MMA t „ T t T DVM GEP, GEP, MAX, MAX, min OC min °C 97.5 2.5 69 68 83 149 95 5 55 62 66 156 90 10 41 57 50 171 EGDMA 85 15 34 57 41 174 80 20 30 56 37 176 70 30 25 59 31 . 176 0 100 21 52 37 114 97.5 2.5 64 66 77 156 95 5 50 64 61 164 90 10 28 59 39 176 DEGDMA 85 15 25 58 33 178 80 20 23 56 29 176 70 3D 21 60 27 179 0 100 18 56 30 • 128 97.5 2.5 59 68 72 154 95 5 44 64 54 160 90 10 27 57 36 174 TrEGDMA 85 15 22 56 29 174 30 20 20 56 27 134 70 30 18 56 24 175 0 100 18 57 28 127 95 Table 4 - 4 . C a l c u l a t e d p o l y m e r i z a t i o n parameters f o r MMA-EGDMA, MMA-DEGDMA and MMA-TrEGDMA comonomer systems. DVM Comonomer Exotherm peak P o l y m e r i z a t i o n PRC composition, Rate / v o l % parameters C o e f f Icient(PRC), p R C 0C / r r n m MMA DVM Dose, AT, PRCj PRC Mrad °C I I 9 7 . 5 2 . 5 0 . 9 1 113 0 , 4 6 5 . 8 1 o 9 95 5 0 . 7 3 120 0 . 4 7 8 , 5 1 8 , 1 90 10 0 . 5 5 .135 0 . 5 1 1 2 . 6 2 4 . 8 EGDMA 85 15 0 . 4 5 138 0 . 6 2 1 6 . 7 2 6 , 9 80 20 0 , 3 9 140 0o66 1 7 . 1 2 5 . 8 70 30 0 . 3 4 140 0 . 9 2 1 9 . 5 2JL Q 2 0 100 0 . 3 9 78 0 . 7 6 3 . 9 5 . 1 9 7 . 5 2 . 5 0 . 8 5 120 0 , 5 3 ' 6 . 9 1 3 . 0 95 5 0 , 6 7 128 0 , 5 6 9 . 1 1 6 . 2 90 10 0 , 4 3 140 0 . 8 0 1 1 . 7 1 4 . 7 DEGDMA 85 15 0 . 3 6 142 0 , 8 8 1 7 , 2 X 9 <> !D 80 • 20 0 , 3 2 140 0 , 9 0 1 5 , 0 1 6 . 6 70 30 0 . 3 0 143 1 . 1 0 1 9 . 8 1 8 . 0 0 100 0 . 3 3 92 1 .11 6 , 0 5 . 4 9 7 . 5 2 . 5 0 . 7 9 118 0 . 5 4 6 . 6 1 2 . 2 95 5 0 . 5 9 124 0 . 6 4 9 . 6 J. 3«5 TrEGDMA 90 10 0 , 3 9 133 0 . 7 7 1 3 . 0 1 6 . 9 85 15 0 . 3 2 138 0 , 9 1 1 6 . 9 18*5 80 20 0 . 3 0 146 1 , 0 0 1 8 . 0 1 8 . 0 7 0 . . 30 0 , 2 6 139 1 . 2 0 1 9 , 0 1 5 . 8 0 100 0.3.1 91 1 .16 7 , 0 6 . 0 96 Table 4-5. D i f f e r e n c e s between measured and c a l c u l a t e d c u r i n g times f o r lower c o n c e n t r a t i o n s of d i v i n y l monomer (DVM) i n MMA-DVM systems* DVM Comonomer com p o s i t i o n , v o l 0/ t MAX, m m D i f f e r e n c e MMA DVM' c a l c u l a t e d measured min 0/ EGDMA ,97.5 95 90 85 2.5 5 15 88 70 49 38 83 66 50 41 +5 +4 -I -3 +6.0 + 6.1 -2.0 -7.3 97.5 2,5 81 77 +4 +4.9 DEGDMA 95 5 62 61 + 1 + 1,6 90 10 42 39 +3 +7.1 85 15 31 33 -2 -6,5 97.5 2.5 76 72 +4 + 5.5 TrEGDMA 95 5 56 54 +2 +3.7 90 10 37 36 + 1 + 2,8 85 15 27 29 -2 -6.9 97.5 2.5 68 60 +8 • 13 o 5 TEGDMA 95 5 48 45 + 3 +6,7 90 10 30 31 -1 -3.3 'An average e r r o r f o r p r e d i c t i n g c u r i n g time f o r the f o u r systems s t u d i e d . 97 Table 4-6, D i f f e r e n c e s between measured and c a l c u l a t e d c u r i n g times f o r lower c o n c e n t r a t i o n s of TEGDMA i n the styrene (S)-TEGDMA system (70), Comonomer composition, v o l % min D i f f e r e n c e S TEGDMA c a l c u l a t e d measured min % 95 5 887 822 ,+ 55 +6.5 90 10 445 462 -17 -3.8 80 20 222 228 -6 -2.6 80 20 222 222 0 0.0 70 30 148 135 +13 +9.6 60 40 110 97 + 13 +13.2 50 50 69 87 -18 -20.6 98 Table 4-7, D i f f e r e n c e s i n c a l c u l a t e d and p u b l i s h e d c u r i n g times f o r TBS - d i - and t r i - v i n y l comonomer systems (53). C r o s s l i n k i n g • monomer • tMAX,• min .Difference type 0/ '° measured c a l c u -l a t e d min /° 5 52 53 +1 +1.9 EGDMA 10 46 48 + 2 +4.3 20 40 40 0 0 30 34 35 + 1 +3,1 5 36 36 0 0 TrEGDMA 10 34 34 0 0 20 30 30 0 0 30 27 27 0 0 5 40 41 +1 . +2.5 TEGDMA 10 35 37 + 2 + 5.7 • 20 34 32 -2 -5,9 30 27 28 +1 + 3.7 5 50 49 -1 -2.1 TMPTMA 10 48 46 -2 -4.2 20 40 42 + 2 +5,0 30 37 37 0 0 99 Table 4-8. D i f f e r e n c e s i n c a l c u l a t e d and p u b l i s h e d . c u r i n g times f o r h e a t - c a t a l y s t cured MMA - t r i r n e t h y l o l ' p r o p a n e - t r i m e t h a c r y l a t e (TMPTMA) p o l y m e r i z a t i o n system (33 ) . TMPTMA, t MAX, min D i f f e r e n c e s /o c a l c u l a t e d •mea sured • min ,°/ yo 1 123,0 123.4 -0.4 -0 o3 2* 108.3 108.3 0.0 0.0 5 81 o 5 81.3 + 0.2 + 0.3 7 69.8 72.5 -2.7 -3,7 9* 60.8 60.8 0.0 0.0 12 51.3 55«5 -4.2 -7,6 * Values used f o r c a l c u l a t i o n of o v e r a l l a c c e l e r a t i o n constant by "two-point method," 100 Table 4-9. R a d i a t i o n p o l y m e r i z a t i o n of TEGDMA at d i f f e r e n t dose r a t e s . Dose r a t e MAX l / 2 t T r ad/sec ( r ad/sec) MAX, "MAX, l / t xlO MAX m m °C 220 14.8 202 14.1 156 12.5 22 130 4.55 31 107 3.22 89 88 1.12 101 Table 4-10. Thermomechanical p r o p e r t i e s of r a d i a t i o n cured p o l y (MMA) and c r o s s l i n k e d MMA-DVM polymer p r o d u c t s . Comonomer Composition, LTDCxiO 4, T TDD, TDT, Monomer v o l % CM CO 0 C MMA DVM 1/°C °c 0/ /o MMA 100 0 2.000 551 108 0 125 97.5 2.5 2.003 388 115 49 175 95 5 2.006 302 123 60 198 EGDMA 90 70 10 30 2.012 2,039 125 15 127 130 77 92 202 214 50 50 2.072 4 130 94 250* 97.5 2.5 2.002 437 114 41 158 95 5 2.004 350 115 46 164 DEGDMA 90 70 10 30 2.007 2.025 263 82 119 122 61 82 175 200 50 50 2.048 21 125 87 198 97.5 2.5 2.001 476 111 25 155 95 5 2.003 375 113 50 165 TrEGDMA 90 10 2.005 275 116 63 180 70 30 2,018 102 119 83 • 195 50 50 2.036 57 122 92 207 95 5 2.002 425 110 37 . 160 90 10 2.004 326 113 52 165 TEGDMA 85 15 2.007 275 115. 61 185 70 30 2.014 126 117 81 203 50 50 2.029 63 117 88 197 Over 250 °C, maximum f o r equipment 102 Table 4-11. Mechanical p r o p e r t i e s of r a d i a t i o n cured p o l y (MMA) and c r o s s l i n k e d MMA-DVM polymer product s. Comonomer YIELD RUPTURE co m p o s i t i o n , v o l . % CN Monomer 0 0 MMA DVM STRESS, STRAIN, STRESS, STRAIN, •kg/cm2 % kg/cm 2 % MMA 100 0 2.000 1,080 6,0 2,100 50.5 97,5 2.5 2,003 1,080 7,0 4,450 62.2 95 5 2.006 1,060 6.7 4,770 67.5 90 • 10 2.012 1 9080 7.0 3,950 55.0 EGDMA 80 20 2.024 1,020 6.0 2,550 38.5 50 50 2,072 955 5.0 1,530 20.0 40 60 2.092 — — 1,330 17.5 20 80 2.138 — — 1,210 12,5 0 100 2.200 895 5.5 97,5 2.5 2.002 1,150 7.5 3,915 60,5 95 5 2.004 1,050 7,0 4,680 65.5 DEGDMA 90 10 2.007 1,050 7.0 3,950 54.0 70 30 2.025 1,050 6.5 2,770 42.5 50 50 . 2.048 1,020 6.0 1,910 27.0 20 80 2,099 895 5.5 1,400 15.0 0 100 2,150 — — - 825 5.5 97,5 2.5 2.001 1,020. 7,0 3,630 63.0 95 5 2,003 1,050 7.5 4,650 66.0 TrEGDMA 90 10 2.005 1,020 7.6 4,580 62,5 70 30 2.018 1,000 7.0 3,120 48.0 60 40 2.026 960 6,5 2,510 44.5 50 50 2.036 950 6.0 2,230 36.0 30 70 2.061 950 5.5 1,520 20,0 0 100 2.125 — — - 1,250 11.5 95.5 2.5 2.001 925 6.0 4,710 67.0 95- 5 2., 002 955 6.5 5,100 68.5 TEGDMA 90 10 2,004 1,080 7.0 5,030 67.0 70 30 2,014 1,050 7.0 3,890 59.5 50 50 2.029 955 7.0 2,450 47,0 40 60 2,037 860 6,0 2,250 41.0 20 80 2.062 765 5.0 1,520 2.7.0 • .0 100 2,105 510 2.5 .1,660 26.5 103 Table 4-12. C a l c u l a t e d parameters from compression s t r e s s - s t r a i n curves f o r r a d i a t i o n cured p o l y (MMA) and MMA-DVM polymer products. Comonomer composition, E, P l a s t i c F v o l , % Deforma- Monorner , _3 t i o n , kg/cm^xlO MMA • DVM % cm' MMA 100 0 19.0 44.5 32.3 97.5 2.5 20.0 55.0 49.5 95 5 19.0 61.0 55.7 90 10 19.0 48.0 45.8 EGDMA so 20 19,0 32.5 27.8 '50 50 18.5 15.0 11.1 40 60 18.0 6.8 20 SO 18.0 —— 6.1 0 100 16.5 —— 1.0 97.5 2.5 19.0 53.0 50.9 95 5 19.5 58.5 56.6 90 10 19.0 44.0 48.1 DEGDMA 70 30 18.0 36.0 30.4 50 50 18.0 21.0 17.1 20 80 17,0 9.5 7.3 0 100 17.0 —— 1.5 97.5 2,5 18.0 56.0 47.5 95 5 19.5 58.5 56,5 90 10 20.0 55,0 54.1 TrEGDMA 70 30 17.0 41.0 34.8 60 40 16.5 38,0 26,6 50 50 17.0 30.0 23.3 30 70 17.0 14,5 10.9 0 100 17.0 7.7 97.5 2,5 18.5 61.0 55.6 95 5 18.5 62.0 59.5 90 10 20,0 60,0 57.5 TEGDMA 70 30 19.5 52.5 49.4 50 50 17,0 40.0 34.8 40 60 16.0 35,0 25.3 20 80 15.0 22.0 1.4,5 0 100 15.0 24.0 13,9 104 F i g u r e 3-1. S o l u t i o n of a t y p i c a l p o l y m e r i z a t i o n exotherm curve i n c l u d i n g d e r i v a t i o n of " G e l - E f f e c t P o i n t " (GEP), "Cure" (MAX), and showing " A c t i v a t i o n " (I) and " A c c e l e r a t i o n " ( I I ) p e r i o d s (67,69,71). t t t 0 GEP MAX Time (t)» min D D D 0 GEP MAX Dose (D), Mrad 105 F i g u r e 3 - 2 . T y p i c a l thermomechanical curves f o r t h e r m o p l a s t i c ( I ) , p a r t l y c r o s s l i n k e d ( 2 ) and f u l l y c r o s s l i n k e d ( 3 ) polymer products; and showing f o r ( 2 } s o l u t i o n s f o r g l a s s t r a n s i t i o n temperature ( T g ) , thermal d i s - t o r t i o n temperature (TDT) and slope (s) i n the t r a n s i t i o n r e g i o n ( 6 7 9 6 9 ) . c o Temperature ( T ) , °C 106 Figure 3-3. Polymer s t r e s s - s t r a i n curve nomenclature and t y p i c a l curves for d i f f e r e n t types of p l a s t i c s (20,72). S_train at rupture_ stress at rupture P l a s t i c Deformation s t r a i n y soft and weak hard and b r i t t l e soft and tough r 1 hard and / strong / hard and tough 107 F i g u r e 4-1. R e l a t i o n s h i p between "̂ MAX and volume con c e n t r a - t i o n of TEGDMA i n the MMA-TEGDMA comonomer mixture ( 6 7 , 6 9 ) . 108 F i g u r e 4-2. P o l y m e r i z a t i o n r a t e c o e f f i c i e n t s i n " A c t i v a t i o n " (PRCj) and " A c c e l e r a t i o n " ( P R C J J ) p e r i o d s as f u n c t i o n s of TEGDMA volume c o n c e n t r a t i o n i n the MMA-TEGDMA comonomer mixture (67,69). 0 20 40 60 80 100 TEGDMA, v o l . % 109 F i g u r e 4-3. R e l a t i o n s h i p between o v e r a l l c u r i n g r a t e ( l / t j ^ ) and TEGDMA volume c o n c e n t r a t i o n i n the MMA- TEGDMA comonomer mixture (67,69), 60 c • H e o < 50 40 30 20 10 0 0 10 20 30 40 50 TEGDMA, v o l . % 110 F i g u r e 4-4. R e l a t i o n s h i p between tMAX and d i v i n y l monomer (DVM) volume c o n c e n t r a t i o n i n MMA-DVM comonomer mixtures. 0 10 20 30 DVM, v o l . % I l l 112 Figure 4-6. Polymerization rate c o e f f i c i e n t s i n "Activation" (PRCj) and "Acceleration" (PRG__) periods as functions of d i v i n y l monomer (DVM) volume concentration i n MMA-DVM comonomer mixtures. O MMA - EGDMA X MMA - TrEGDMA A MMA - DEGDMA . Q MMA - TEGDMA DVM, v o l . % 113 F i g u r e 4-7. R e l a t i o n s h i p between WMX and ''GEP f o r d i f f e r e n t MMA-DVM comonomer systems and the S- TEGDMA system. 70. 60 50 40 30 20 10 0 0 10 20 30 40 50 ^GEP, min 114 F i g u r e 4-8. R e l a t i o n s h i p between o v e r a l l c u r i n g r a t e - ( l / t w A ^ ) and DVM c o n c e n t r a t i o n i n MMA-DVM mix t u r e s . 1 O MMA - EGDMA H MMA - TEGDMA A MMA - DEGDMA . © MMA - TMPTMA (33) * MMA - TrEGDMA A MMA 0 5 10 15 DVM, v o l . % 115 Figure 4 - 9 . Overall acceleration constant (K) and i t s r e c i p r o c a l (l/K) as functions of d i v i n y l monomer connection number (CNr,.,,,, Table 3 - l ) . UVM O MMA-EGDMA X MMA-TrEGDMA A MMA-DEGDMA IS MMA-TEGDMA 2.100 2.150 2.200 C NDVM 116 Figure 4-10. Relationship between o v e r a l l curing rate ( l / t ^ y ) and DVM concentration i n t-butyl styrene (TBS/-DVM mixtures. Evaluation of o r i g i n a l heat-catalyst data from (53). B A X 0 TBS-EGDMA TBS-TrEGDMA TBS-TEGDMA TBS-TMPTMA 40 c •H E CO O 30 20 10 0 20 30 Crosslinking agent, % 117 F i g u r e 4-11, O v e r a l l c u r i n g r a t e ( l / t ^ ) of TEGDMA as a f u n c t i o n of r a d i a t i o n d o s e ' r a t e . Figure 4-12. Shape of thermomechanical curves f o r MMA-EGDMA polymer products. Numbers indicate per cent of EGDMA i n comonomer mixture and copolymer connection number ( C N ) . 100 c o •H +> 3 m o M-i cu Q 75 5 0 25 50 100 RO• 0070 ^ 0 : | 2.POO, 150 200 Temperature, °C co Figure 4-13. Shape of thermomechanical curves f o r MMA-TEGDMA polymer products. Numbers indicate per cent of TEGDMA i n comonomer mixture and copolymer connection number ( C N C 0 ) 0 50 100 150 200 Temperature °C F i g u r e 4-14. G l a s s t r a n s i t i o n temperature (T ) as a f u n c t i o n of copolymer co n n e c t i o n number ( C N C 0 ) f o r d i f f e r e n t Mf'M-DVM polymer pr o d u c t s . F i g u r e 4-15. Thermal deformation degree (TDD) and thermal d i s t o r t i o n temperature (TDT) as f u n c t i o n s of copolymer c o n n e c t i o n number ( C N c o ) f o r d i f f e r e n t MMA-DVM polymer products* 122 F i g u r e 4-16, L i n e a r thermomechanical deformation c o e f f i c i e n t (LTDC). as a f u n c t i o n of d i v i n y l monomer (DVM) c o n c e n t r a t i o n i n MMA-DVM polymer p r o d u c t s . 0 10 20 30 40 50 DVM, v o l , % F i g u r e 4-17. L i n e a r thermomechanical d e f o r m a t i o n c o e f f i c i e n t tLJ/DC) as a f u n c t i o n of copolymer c o n n e c t i o n number (CN ) for. d i f f e r e n t MMA-DVM polymer p r o d u c t s . 2.000 2.010 2.020 2.030 2.040 CN co 124 F i g u r e 4-18. Shape of compression s t r e s s - s t r a i n curves f o r MMA-EGDMA polymer p r o d u c t s . Rate of s t r a i n 0.1 cm/min. 125 F i g u r e 4-19. Shape of compression s t r e s s - s t r a i n curves f o r MMA-TEGDMA polymer p r o d u c t s . Rate of s t r a i n 0.1 cm/min. 126 Figure 4-20. Compression s t r e s s - s t r a i n parameters as functions of comonomer composition. The MMA-EGDMA comonomer system. (Rate of s t r a i n 0.1 cm/min.) 0 Compression stress at rupture, kg/cm 2xl0~2 •Jf Compression s t r a i n at rupture, % A Area under s t r e s s - s t r a i n curve, F, cm2. EGDMA MMA concentration, v o l . % 127 F igure 4 -21 . Compression s t r e s s - s t r a i n parameters as funct ions of comonomer composi t ion. The MMA-DEGDMA comonomer system. (Rate of s t r a i n 0.1 cm/min.) 0 Compression s t ress at rupture , kg/cm^xl0~2 •X* Compression s t r a i n at rupture , % A Area under the s t r e s s - s t r a i n curve F, cm^. T x DEGDMA 0 20 40 60 80 100 MMA 100 80 60 40 20 0 concent ra t ion , v o l . % 128 F igure 4-22. Compression s t r e s s - s t r a i n parameters as funct ions of comonomer composi t ion. The MMA-TrEGDMA comon- omer system. (Rate of s t r a i n 0.1 cm/min). © Compression stress at rupture , kg/cm2xl0~ •ft Compression s t r a i n at rupture , % A Area under the s t r e s s - s t r a i n curve F, cm^ TrEGDMA 0 20 40 60 80 100 MMA 100 80 60 40 20 0 concent ra t ion , v o l . % 129 Figure 4-23. Compression s t r e s s - s t r a i n parameters as functions of comonomer composition. The MMA-TEGDMA comonomer system. (Rate of s t r a i n 0.1 cm/min.) © Compression stress at rupture, kg/cm2xl0""2 •ft Compression s t r a i n at rupture, % A Area under the s t r e s s - s t r a i n curve F f cm 2 TEGDMA 0 20 40 60 80 100 MMA 100 80 60 40 20 0 concentration, v o l . % oer 131 F i g u r e 4-25, R e l a t i o n s h i p between area under the compression s t r e s s - s t r a i n curve (F ) and p l a s t i c deformation f o r d i f f e r e n t MMA-DVM polymer p r o d u c t s , 0 15 30 45 60 P l a s t i c deformation, %

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