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The ultraviolet absorption of stretched and unstretched GR-S latex films Mayo, Eleanor Grace 1947

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o f -THE ULTRAVIOLET ABSORPTION 0?  STRETCHED AND UNSTRETCHED  QR-S LATEX EELMS by s. Eleanor Grace Mayo A Thesis submitted i n p a r t i a l f u l f i l l m e n t of the requirements f o r the degree of MASTER OP ARTS i n the department of PHYSICS A p r i l , 1947. ACKNOWLED G-EMEM* . The author wishes to express her thanks to Di*. H. D. Smith f o r his generous help i n the experi-mental work and to Dr. 0. Bluh for h i s assistance i n discussing and formulating the r e s u l t s . She i s also indebted to the National Research Council f o r a Grant and to Dr. L. A. Wood of the Rubber D i v i s i o n of the United States Bureau of Standards f o r supplying the latex. TABLE OF CONTENTS Page I. Introduction . . . 1 I I . Research, on Synthetic Rubbers . . . . . . . . 2 I I I . Properties of GR-S Synthetics . . . . . . . . 4 1. General D e f i n i t i o n . . . . . . . 4 2„ Comparison Between GR-S and Natural Rubbers . . .' . . . . . . . . . . . . . . 5 3. Chemical Preparation . . . . . . . . . . j? (a) GR-S Synthetics i n General j> (b>) Type I I I GR-S Latex . . . . . . . . . 6 TV. Physical Theory of Rubber . 7 V i Theory, of Absorption of Light ' 9 V I . Experimental Procedure . 11 V I I . Discussion of Results 27 1. Observation of the Styrene Band . . . . . 27 2. Absorption and Stress . . . . . . . . . . 2S V I I I . Bibliography . . . . . . . . . . 30 ABSTRACT This work on the u l t r a v i o l e t absorption of stretched and unstretched Type 3 GR-S latex f i l m s , was undertaken on the suggestion of Dr. H. D. Smith and Dr. E. Guth, with the expectation that the u l t r a v i o l e t absorption could give i n -formation on the arrangement of the molecules i n the stretched state as compared with the unstretched latex. Previous i n -vestigators have found that styrene has an absorption band with maximum absorption at 28j>0 angstroms. Since styrene i s one of the constituents of GR-S latex, i t was supposed that th i s absorption band would also appear i n any spectral analysis of the latex. To f i n d the nature of this.band and i t s behavior upon stretching the sample was one of the ob-jectives of t h i s research. The type of spectrograph used f o r t h i s work was a Hilger E496.303 with a wavelength scale, and Spekker photo-meter attachment. Eastman Type II-P spectroscopic plates were used f o r a l l readings. A tungsten s t e e l spark with about 25,000 v o l t s across the electrodes was used as a source fo r a l l plates. The unstretched films were prepared by coating a pane of glass with a ten percent s o l u t i o n of zinc chloride, and when dry, the glass was coated with the latex. A f t e r drying, a very t h i n f i l m of rubber was deposited on the glass which could be peeled o f f as.needed. By t h i s method u l t r a v i o l e t transparent films of i n i t i a l thickness 0.020 cm. were obtained. These films were stretched into the shape of a spherical bubble by means of nitrogen gas. By c a l c u l a t i n g the surface areas of the stretched samples and assuming Poisson's Ratio to be 1/2 f o r rubber, the thicknesses of the" stretched samples could be calculated. A narrow absorption band was found f o r latex of thickness 0.00260 cm. i n the region of 28.50 angstroms which did not appear to s h i f t with further s t r e t c h . A s l i g h t broadening e f f e c t upon stretching might have been present but i t would not amount to more than 3 or 10 angstroms i n either d i r e c t i o n . From t h i s ifr was concluded that the ab-sorption centres of the styrene molecules remained unaffected during stretching. Absorption c o e f f i c i e n t s were also c a l -culated by Lambert's Law on the assumption that the t o t a l l o s s of r a d i a t i o n was due to absorption. I t was found that a decrease i n thickness was followed by an increase i n the ab-sorption c o e f f i c i e n t f o r a constant wavelength. To obtain the true absorption c o e f f i c i e n t s , the c o e f f i c i e n t s computed, by Lambert's Law must, be corrected f o r surface r e f l e c t i o n and body scattering. This may a f f e c t the values of the coef-f i c i e n t s but not the p o s i t i o n of the band. The Increase i n sc a t t e r i n g with a decrease i n thickness i s suggested to be due to the existence of microcrystals when the latex i s under s t r e s s . 1. THE ULTRAVIOLET ABSORPTION OE  STRETCHED AND UNSTRETCHED " GR-S LATEX FILMS I . INTRODUCTION In comparison with the study of chemical, mechani-c a l , thermal and e l e c t r i c a l properties of synthetic rubbers, very l i t t l e research has been c a r r i e d out on the o p t i c a l properties of these substances. Some work has been done on determining the r e f r a c t i v e indices, s c a t t e r i n g e f f e c t s , r double r e f r a c t i o n and absorption i n the i n f r a red, but almost nothing on u l t r a v i o l e t absorption. The present work was undertaken on the suggestion of Dr. H. D. Smith and Dr.< E. Guth. Research on the u l t r a v i o l e t absorption was ex-peeted to give information on the arrangement of the molecules i n the stretched state as compared with the unstretched state of rubber. Previous investigators have found that styrene has an absorption band with maximum absorption at 2850 ang-stroms. Since styrene i s one of the constituents of GR-S latex, which was used i n the i n v e s t i g a t i o n , i t was sup-posed that t h i s absorption band would also appear i n any-spectral analysis of the latex. To f i n d the nature of t h i s band and i t s behavior upon stretching the sample was- one of the objectives of t h i s research. I I . RESEARCH ON SYNTHETIC RUBBERS The search f o r synthetic rubbers started almost at once a f t e r Charles Goodyear i n 1839 discovered the process of vulcanization. A few years previous to t h i s discovery, Faraday i n 1826 demonstrated that rubber was e s s e n t i a l l y a hydrocarbon and found the empirical formula C^Hg. But i t was not u n t i l i860 that anyone was successful i n breaking down rubber into i t s constituents. Williams found that by pyrolysis (a cracking process) a water-white, mobile low b o i l i n g l i q u i d of the same composition as rubber could be obtained. This substance he named isoprene. F i f t e e n years l a t e r , Bouchardat converted isoprene into an e l a s t i c mass and pointed out the monomer polymer r e l a t i o n e x i s t i n g be-tween t h i s material and rubber. From the time of Bouchardat on, the search f o r a synthetic product" gained momentum with the greatest progress made i n the decade preceding the f i r s t World War. This was la r g e l y because of organized research groups. The most tangible accomplishments of t h i s period were: 1. the development by the English groups i n collabora-v t i o n with Auguste Fernbach of the Pasteur I n s t i t u t e of a process f o r the fermentation of starch to give acetone and a series of higher alcohols. 2. the discovery by the German group of organic accelera-tors of vulcanization. This was independent of the discovery of these compounds i n the United States. Both groups evolved workable processes for the production of isoprene and several of i t s homologs and f o r t h e i r polymeri-zation induced by heat or by catalysts such as sodium. The products were poor i n quali t y , i n f e r i o r i n e l a s t i c i t y , and i n resistance to aging and wear. Af t e r the war, synthetic research was v i r t u a l l y given up except f o r work at I . G. Farbenindustrie i n Germany and duPont i n the United States. The main resu l t of t h i s was that many patents were taken out i n Germany; but by 1930 the quality of the synthetic materials was s t i l l unsatisfactory. The approach of the chemists f a i l e d because the exact molecular configuration of rubber, the mechanism of polymerization whereby isoprene i s converted into natural rubber and the way nature i s able to perfect t h i s polymeri-zation has remained e s s e n t i a l l y unknown. A t r u l y rubberlike polymer was not r e a l i z e d u n t i l chemists struck out to de-velop new polymers that might possess the,essential proper-t i e s of the natural product but would be derived from monomers d i s t i n c t l y d i f f e r e n t i n t h e i r chemical composition 4. from isoprene. No synthetic polymer of isoprene has yet been developed that approaches natural rubber i n quality or i s comparible with the synthetics now being produced. The f i r s t r e a l l y successful product was developed i n 1931 and was c a l l e d pplychloroprene ( l a t e r c a l l e d duprene or neoprene). P h y s i c a l l y the structure of th i s synthetic approaches that of natural rubber but chemically i t contains 40 percent chlorine. In 1932 Thiokal, an alkylene poly-sulphide e l a s t i c appeared followed i n 1933 by Koroseal, a p l a s t i c i z e d p olyvinyl chloride, and i n 1933 hy the German buna synthetic rubbers. The buna synthetics or GR-S as they are c a l l e d i n the United States are by f a r the best synthe-t i c s developed and as a re s u l t were produced i n the greatest quantities during the second World War. I I I . PROPERTIES OF GR-S SYNTHETICS 1. General D e f i n i t i o n Synthetic rubbers are those organic substances that possess the property of f o r c i b l y r e t r a c t i n g to approxi-mately t h e i r o r i g i n a l size and shape a f t e r being greatly d i s t o r t e d . The above d e f i n i t i o n would also cover the term "elastomer". 5. 2. Comparison Between GR-S and Natural Rubbers GR-S which used to be known as Buna-S i s also made under the proprietary names of Butaprene-S, Chemigum IT, Hycar TT and Buton-S. I t has the general properties of natural rubber but i n some respects i t i s superior to i t . I t s properties can be compared i n the following table: Superior In I n f e r i o r In 1. Abrasion 1. Dynamic f l e x cracking 2. Ageing 2. Heat b u i l d up 3. Reversion on overeure 3. B r l t t l e n e s s 4. Tendency to scorch i n 4. Rate of vulcanization processing 3 i Chemical Preparation . (a) GR-S Synthetics i n General. The basic material f o r a l l GRrS synthetics i s butadiene, a substance that i s a gas above -3° Centigrade. Butadiene i t s e l f can be prepared by four main processes: 1. pyro l y s i s (or cracking) of higher b o i l i n g components of petroleum 2. dehydrogenation of some of the lower b o i l i n g com-ponents of petroleum i 3. conversion of alcohol 2C 2H^0H—s>CH 2 = CH-CH = CH 2 + 2H2G + Hg alcohol butadiene 4. from acetylene by the su b s t i t u t i o n of hydrogen 6. + CaO > 0aC2 + CO coke lime calcium carbide — + H 20 3>HC a CH + CaO acetylene — + HgO —a>CHj- - CHO acetaldehyde 1 CH^ - CHO -—^CH^ - CHOH - CH 2 - CHO a l d o l 1 — + 2H CH5 - CH( OH) - CH 2 - CH2OH butanediol or butylene g l y c o l 1 3> CH 2 = CH - CH = CH 2 + 2H20 butadiene I t i s very d i f f i c u l t to obtain butadiene of high purity from petroleum products e s p e c i a l l y when they are products of cracking operations or from a g r i c u l t u r a l pro-ducts. The process f o r obtaining butadiene from acetylene Is by f a r the best method of obtaining a y i e l d of high purity since i t i s free of side reactions. GR-S i s then made from butadiene by an emulsion polymerization process with 20% - 50% styrene added. This involves the formation of new carbon-to-carbon linkages, (b) Type I I I GR-S Latex Type -III GR-S latex as was used i n the following research consists of: r 7. Material Parts by Weight Butadiene 30 Styrene 30 Emulsifier 3 Potassium per sulphate (KgSgOg) 0.6 Mereaptan 0.4-3 Water 140 The emulsifier i t s e l f contains: Dressinate 731 1 Dressinate 212 9 The c h a r a c t e r i s t i c feature of t h i s type of latex i s that buta-diene and styrene e x i s t i n equal quantities. I t i s a white l i q u i d of about the same consistency as creamy milk but has a very strong odor of styrene. IV. PHYSICAL THEORY OP RUBBER Elastomers i n addition to t h e i r well known long range r e v e r s i b i l i t y exhibit other general c h a r a c t e r i s t i c s , (a) Heat i s produced upon stretching the elastomer and a cooling e f f e c t i s noted upon relaxation. This i s known as the Joule Heating E f f e c t . (h>) Stress f o r a given degree of s t r e t c h i s a l i n e a r function of temperature. (c) Elastomers when cold harden and at high temperatures they tend to he thermoplastic. They also exhibit both temporary and permanent p l a s t i c i t y . They show a c r y s t a l l i n e structure at high stretches which may also occur at low temperatures i n the unstretched state, x A l l types of synthetic rubber consist of atomic chains of very great length (giant molecules) that are b u i l t up by the r e p e t i t i o n of some unit configuration. These long chains are nearly always formed by the polymerization of the molecules of c e r t a i n l i q u i d s . The double bond attached to a carbon atom at the end of the molecule opens' to form the necessary valence bond f o r attachment to the next unit. These long chain molecules have f r e e l y r o t a t i n g l i n k s and weak secondary forces extending around them, with the r e s u l t that a loose three dimensional network forms. From a study of the e l a s t i c behavior of rubber, we can make a number of inferences about the siz e and shape of the molecules. The f a c t that rubber can be stretched almost re v e r s i b l y up to 1000 percent with l i t t l e change i n density shows that rubber must contain strong filaments that are • normally crumpled or twisted to less than one-tenth of t h e i r extended length. These filaments can s l i d e past one another with very l i t t l e f r i c t i o n over most of t h e i r length but at a few points they are interlocked i n a -three dimensional net-work to l i m i t the degree of deformation or flow. There must also be some mechanism that tends to make the filaments and 9. network return s u b s t a n t i a l l y to t h e i r o r i g i n a l crumpled state a f t e r the external stress i s removed. Recently methods of s t a t i s t i c a l mechanics have been applied to t h i s problem of e l a s t i c i t y by E. Guth. V. THEORY OF ABSORPTION OE LIGHT When a beam of l i g h t f a l l s on matter, the l i g h t i s either transmitted, r e f l e c t e d or absorbed by the substance. A substance i s said to exhibit "general absorption" i f i t reduces the i n t e n s i t y of a l l wavelengths of l i g h t by nearly the same amount. This means f o r v i s i b l e l i g h t that the transmitted l i g h t shows no marked color change. There i s merely a reduction of the t o t a l , i n t e n s i t y of the white l i g h t . No substance i s known that absorbs a l l wavelengths equally; but some, such as films of platinum, approach t h i s condition over a f a i r l y wide range of wavelengths. A substance that absorbs c e r t a i n wavelengths of l i g h t i n preference to others i s said to show "se l e c t i v e absorption". The color of p r a c t i c a l l y a l l colored substances i s due 'to the existence of s e l e c t i v e absorption i n some part or parts of the v i s i b l e spectrum. When l i g h t i s absorbed, i t s energy may be trans-ferred i n three ways: 1. the energy may be re-emitted as fluorescent l i g h t 10. 2. the molecules that have absorbed the l i g h t being now i n a higher energy or excited state may enter into chemical reaction or may dis s o c i a t e . U l t r a v i o l e t r a d i a t i o n i s known to be very e f f e c t i v e i n promoting chemical changes 3. the absorbed energy may be changed into heat energy. Lambert's Law states that each layer of equal thickness ab-sorbs an equal fraction, of t h e ^ l i g h t that traverses i t . I f layers of the thickness of a single molecule are considered, then each molecule absorbs an equal f r a c t i o n of the l i g h t that passes by i t . This can be stated i n the form of an equation. I = I G e-K* where . I 0 = i n t e n s i t y of the l i g h t entering the layer, x = thickness of the layer. I = i n t e n s i t y of the l i g h t a f t e r passing throught the layer. E = absorption c o e f f i c i e n t since i t i s a measure of the rate of loss of l i g h t from the d i r e c t beam. This equation may be rewritten as . I_ _ „-Kx o T - « e l o g 1 0 I_ = -0.4343 Kx l o g 1 0 -|- = 0.4343 Kx = E where T = transmission = l o g ^ _ E = extincti o n or o p t i c a l density = l o g Jo_ . I . . l o g 1 0 E = l o g 1 0 O.4343 K + l o g 1 0 x. Since the absorption c o e f f i c i e n t K varies with the wavelength and thickness x, the shape of the absorption or transmission curve depends upon the term l o g 1 G 0.4343 k and the height of the curve upon the term l o g ^ 0 x • In a s o l u t i o n the absorption depends upon the.con-centration and thickness of the layer traversed. This may-be expressed by an equation which i s known as Beer's Law. I = I 0 e - a c x where a = absorption c o e f f i c i e n t of unit concentration, x = thickness, c = concentration. I 0 = entering i n t e n s i t y of l i g h t beam. I = i n t e n s i t y of l i g h t a f t e r passing through the solution. No exceptions have ever been found to Lambert's Law, but Beer's Law holds only f o r c e r t a i n ranges of concentration; i . e . the absorbing power of a molecule i s influenced by the proximity of i t s neighbors. 71. EXPERIMENTAL PROCEDURE The type of spectrograph used f o r t h i s work was a Hilger E496.303 with a wavelength scale and Spekker photo-meter attachment. Eastman Type II- F spectroscopic plates 12. were used f o r a l l readings. ..A tungsten s t e e l spark with about 35,000 v o l t s across the electrodes was used as a source f o r a l l plates. This source was obtained by using a transformer with a 110 v o l t primary and 20,000 v o l t secondary winding i n p a r a l l e l with four Leyden j a r condensers i n a series p a r a l l e l hookup. I t was found that t h i s gave a very intense source. 1 « F i g . 1. Preliminary experiments were conducted on t h i n sheets of f a i r l y transparent Vulcanized X-224 GR-S of t h i c k -ness 0.0635 cm. to determine the amount of s t r e t c h that could be expected from such a sheet and to determine the best method of stretching. I t i s known fo r v i s i b l e l i g h t that with a one way st r e t c h the rubber sample becomes cloudy or "milky" and transmits only a small portion of the incident l i g h t beam. However, i f stresses are applied both l o n g i -t u d i n a l l y and l a t e r a l l y simultaneously, the rubber remains transparent. A surface e f f e c t ressembling oxidation was also noted on the surface of the rubber closest to the spark source but on the opposite surface no such e f f e c t could be detected. So i t was expected that i f the experiments were Rubber Sample Quartz Window Section A-A F I G conducted i n an oxygen free atmosphere such an e f f e c t could be eliminated. It was decided to use a nitrogen atmosphere which could also serve as the stretching force. A suitable clamp f o r the rubber was b u i l t which allowed.equilateral stress and a nitrogen atmosphere. The holder ( f i g . 2) consisted of a piece of brass stock d r i l l e d to allow a beam of l i g h t one square centimeter i n area to pass through. A quartz window was glued onto one end and a brass plate f i t t e d onto i t to make a strong w a l l . The rubber sample was placed on the other end and held r i g i d l y i n p o s i -t i o n by means of another brass plate into which grooves were cut. Similar grooves were cut into the main part of the holder and into these the rubber was supposed to "flow" when the plate was tightened up. Into the hollow central part a lead f o r the nitrogen was d r i l l e d and also one f o r a con-nection to a mercury manometer. By simply increasing the nitrogen pressure the rubber sample was made to str e t c h into the form of a sphere or balloon and i n t h i s way an,equal force i n a l l directions was exerted on the sample. The next problem encountered was how to measure the thickness of the sample when i t was stretched. I t was decided to use an o p t i c a l device to measure t h i s by simply obtaining r e f l e c t i o n s of a l i g h t beam of f both surfaces of the rubber sample. 15. V F i g . 3. The rubber sample was set i n such a p o s i t i o n that the l i g h t beam was incident upon the surface at an angle of 45° and the r e f l e c t i o n s from both the upper and lower surfaces were then r e f l e c t e d up into a microscope. By measuring the distance between these beams and knowing the magnification of the microscope, the thickness of the sample could be determined. It was found that t h i s was a very e f f i c i e n t method f o r ob-ta i n i n g the thickness of an unstretched sample but when the sample was stretched i t s thickness was so reduced that the r e f l e c t i o n s from the two surfaces appeared i n the microscope as a single l i n e . Thus t h i s method was not applicable. The method of measurement that was f i n a l l y used was to photograph the bubble against a white background and thus obtain a silhouette of i t s outline on a photographic plate. By having the camera set up at such a distance that the silhouette obtained was i n a r a t i o of one to one with the bubble, the radius of the spherical surface could be measured d i r e c t l y from the plate. With the radius known the surface area of the f i l m could be calculated. By assuming 16. Poisson's Ratio f o r rubber to be 1/2, that i s , that the volume of the unstretched rubber i s the same as the volume of the stretched rubber, the thickness of the stretched f i l m could be determined. °It was o r i g i n a l l y intended to make determina-tions of the s i z e of the bubble as a function of the pressure exerted upon i t by the nitrogen gas. Then by simply reading the pressure from the mercury manometer when the spectro-graphic plates were being taken, the bubble size could be read d i r e c t l y from the predetermined graphs. But i t was found that such a function was not always reproduceable since many factors entered i n such as the rate at which the nitrogen gas was a l -lowed to flow into the holder. Thus the bubble si z e had to be determined at the same time as the spectral analysis was done. Absorption curves were obtained f o r vulcanized £-224 GR-S samples of i n i t i a l thickness 0.06j55 cm. but i t was found that with maximum st r e t c h f o r these samples, readings could not be obtained below about 3200 angstroms. These > samples then, were not of much value since knowledge of the absorption was desired p a r t i c u l a r l y i n the region of 2800 - 2900 angstroms. As these were the thinnest trans-parent samples that could be obtained from the United States Bureau of Standards i t was decided to make very t h i n f i l m s from Type I I I GR-S latex. The unstretched films were prepared by coating a pane of glass with a ten percent sol u t i o n of zinc chloride. When t h i s was dry the glass was coated with the latex. Upon 17. drying a very t h i n f i l m of rubber was deposited on the glass which could be; peeled o f f as needed. I t was found that with practice films of a very uniform thickness could be made. By t h i s method u l t r a v i o l e t transparent films of i n i t i a l thickness 0.020 cm. were obtained which gave the p o s s i b i l i t y of obtaining experimental r e s u l t s i n the u l t r a v i o l e t region. TABLE I Readings and Calculations Unstretched Sample Thickness = 0.020 cm Surface Area = 1.432 cm2 Volume * .0286 cm? Wavelength A (Angstroms) Extinction C o e f f i c i e n t E Percent Transmission Log Percent Transmission Tot a l Absorption C o e f f i c i e n t 6700 .7 20.0 1.301 .80 x 10 2 cm"1 5800 .8 15.9 1.201 .92 5100 .9 12.6 1.100 1.04 4750 1.0 10.0 1.000 1.16 4360 1.1 7.95 .900 1.27 4220 1.2 6.32 .801 1.38 4060 1.3 5.02 .701 1.50 . 3980 1.4 3.99 .601 1.61 3880 1.5 3.16 .500 1.73 3800 1*6 2.52 .401 1.84 3740 1.7 2.00 .301 1.96 3670 1.8 1.59 .201 2.08 3650 1.85 1.43 .154 2.13 3630 1.9 1.26 .100 2.19 3600 1.95 1.12 .049 2.25 3585 2.0 1.0 0.0 s 2.31 TABU! I I Radius of bubble «* 1.00 cm 'b Surface Area = 2* 1 4 *L dx 1/2 A ^ (1.0 dx - 2tf I r dx J -.75 = 2*(1.0)(1.0 + .75)= 11.00 cm2 Poisson's Ratio gives f i n a l volume = i n i t i a l volume. . ' . 0 2 8 6 cm3 = 11.00 x t t =• .00260 cm = thickness of sample. Amount of stretch = 11-00 - 1.4?2 = 688f, 1.452 Wavelength A E x t i n c t i o n Percent Log Percent Total (Angstroms) Coefficient E Transmission Transmission Absorption Co e f f i c i e n t 6800 " .2 63.2 1.801 1.77 x-102 cm-1 5600 • .4 39.9 1.601 3.55 5070 .6 25-2 1.401 5.32 2990 • 1 20.0 1.301 6.20 2915 .8 15.9 1.201 7.09 2885 .9 12.6 1.100 7.98 8.86 2865 1.0 10.0 1.000 2850 1.0 10.0 1.000 8.86 2855 1.0 10.0 1.000 8.86 2820 1.2 6.32 .801 10.63 2800 1.3 5.02 .701 11.52 2775 1.4 3.99 .601 12.41 2760 1.3 3.16 .500 13.30 2750 1.6 2.52 .401 14.20 2745 1.7 2.00 .301 15.07 2730 1.8 1.59 .201 15.96 2720 1.9 1.26 .100 16.83 2710 2.0 1.0 0.0 17.72 TABLE I I I Radius of Bubble • 1.25 cm Surface Area '- 17.55 cm 2 Thickness = .OOI65 cm Amount of Stretch »• 1110$ Wavelength A .< (Angstroms) Ext i n c t i o n .Coefficient E : Percent Transmission Log Percent Transmission Total Absorption C o e f f i c i e n t 5700 .2 63.2 1.801 2.80 x 10 2 cm-1 3500 .4 39.9 1.601 5.59 5050 .6 25.2 1.401 . 8.37 2990 .7 20.0 1.301 9.76 2970 . .75 17.8 1.250 11.0 2950 .8 15.9 1.201 11.15 2925 .82 15.2 1.182 11.44 2915 .84 14.5 1.161 11.72 2910 .86 13.8 1.140 12.0 2900 .90. 12.6 1.100 12.56 2890 .92 12.0 1.079 12.83 2880 .96 11.0 1.041 13.39 2880 .98 10.5 1.021 13.67 2880 • 1.00 10.0 1.000 13.95 2875 1.05 8.92 .950 14.7 2870 1.10 7..95 .900 15.3V 2865 1.15 7.08 .850 16.1 2850 1.15 7.08 .850 16.1 2849 1.10 7.95 .900 15.3 2847 1.05 8.92 .930 14.7 2845 1.05 8.92 .950 14.7 2840 1.10 7.95 . 900 15.3 2830 1.15 7.08 .850 16.1 2830 . 1.20 .6.32 .801 16.7 2810 1.30 5.02 .701 18.2 TABLE IV Surface Area =24.86 em^ Thickness = .00115 cm Amount of Stretch = 1635% Wavelength A Ext i n c t i o n Percent Log Percent Total (Angstroms) Coefficient E Transmission Transmission Absorption C o e f f i c i e n t 3600 .2 * 63i2 1.801 4.0 x 10 2 cm - 1 -3070 .4 39.9 1. 601 8.0 2890 .7 20.0 1.301 14.0 2880 .74 18.4 1.265 14.8 2875 .8 15.9 1.201 16.0 2870 .82 15.2 1.182 16.4 2870 .84 14.5 1.161 16.8 2865 .86 13.8 1.140 17.2 2860 .9 12.6 1.110 18.0 2850 .9 12.6 1.110 18.0 2849 .86 13.8 1.140 17.2 2848 .84 14.5 1.161 16.8 2847 .82 15.2 1.182 16.4 2845 .8 15.9 1.201 16.0 2840 .8 15.9 1.201 16.0 2840 .82 15.2 1.182 16.4 2857 .84 14.5 1.161 16.8 2855 .86 13.8 1.140 17.2 2830 ,9 12.6 1.110 18.0 2815 1.0 10.0 1.000 20.0 2800 1.1 7.95 .900 22.0 2790 1.2 6.32 .801 24.0 22. 20 o u_ U_ LU O O,.o O h -O p e s X L U (1-o l o \ r l V G \ \ — i — \ V \ OJ )20 cm. -• ^ > 0 .0 0.0016! ) 2 6 0 cm. i cm. 0.00115 cr i . •,. 2500 3000 3500 4 0 0 0 4 5 0 0 5 0 0 0 W A V E L E N G T H A . U . 5 5 0 0 23. 24. 80 2500 3000 3500 4000 4500 5000 ' , WAVE LENGTH A.U. 25 27. A VII. DISCUSSION OF RESULTS 1, Observation of the Styrene Band I t may be noticed that there i s a large difference between the thickness of the unstretched sample and the thicknesses of ihe stretched samples. The d i f f i c u l t y of ob-taining thicknesses between 0.020 cm. and 0.00260 cm. i s due to the f a c t that latex, exhibits a tendency to flow; i . e . when under pressure the latex f i l m tended to increase i n sur-face area i n a way out of our control. A c e r t a i n minimum pressure had to be applied i n order to s t r e t c h the films at a l l ; however, i t was possible to produce films of thick-nesses 0.0026 cm., O.OOI65 cm. and 0.00115 cm. Although the range of thicknesses f o r stretched samples was r e l a t i v e l y narrow, the accuracy of the method used seemed to be high enough to disclose any possible difference i n absorption ef-f e c t s . A narrow absorption band was found f o r latex of 0.00260 cm. thickness i n the region of 2850 angstroms;, i . e . at a wavelength where previous investigators have found a band i n styrene. As i t was impossible to observe t h i s band i n unstretched latex (because of the i m p o s s i b i l i t y of pre-paring s u f f i c i e n t l y t h i n unstretched films of latex) we can only compare the absorption e f f e c t s r e l a t i v e to varying > 28. stretch. The absorption band does not appear to s h i f t with further s t r e t c h but we must not exclude the p o s s i b i l i t y of a s l i g h t broadening e f f e c t being present. I f there i s a broadening of the band, i t w i l l not be greater than j> or 10 angstroms i n either d i r e c t i o n over a change of thickness of 100 percent. From t h i s i t can be concluded that the absorp-t i o n centre of the styrene molecule remains unaffected when the molecule i s stretched. The .general shape of the trans-mission curve seems to be of the same form as those found f o r natural rubber of s i m i l a r thicknesses; there i s a gradual increase i n absorption with a decrease i n wavelength^". 2. Absorption and Stress The absorption c o e f f i c i e n t f o r a range of wave-lengths was calculated by Lambert's Law as i f the t o t a l l o s s of r a d i a t i o n was due.to true absorption. These absorption c o e f f i c i e n t s are entered i n Tables I to IV. When l i g h t passes through a medium i t i s r e f l e c t e d and scattered as well as being absorbed. The true absorption! c o e f f i c i e n t s can be calculated only i f the amount of l i g h t l o s t by r e f l e c t i o n and s c a t t e r i n g i s known. The r a t i o of the i n t e n s i t i e s of r e f l e c t e d and incident beams at each surface i s equal to i s^_z_ i l^ where n i s the r e f r a c t i v e index of the (n + l ) d latex. As latex has a r e f r a c t i v e index of approximately 1.6 i n the u l t r a v i o l e t region, the loss of i n t e n s i t y due to r e -1 L . A. Wood, "The Optical Properties of Rubber", J . App. Phys. 12, 119-126, 1941. -29. f l e c t i o n on both surfaces w i l l be about IG to 12 percent. In the graph of absorption c o e f f i c i e n t versus wave-length i t w i l l be noticed that a decrease In thickness i s followed by an increase i n the absorption c o e f f i c i e n t f o r constant wavelength. As the d i f f e r e n t thicknesses are pro-duced by d i f f e r e n t stretch, we must assume that the change i n the absorption c o e f f i c i e n t i s due to the changed stress since the absorption c o e f f i c i e n t f o r a uniform material should be constant f o r a given wavelength regardless of the thickness of the material. This apparent increase i n absorption could be due to a greater scattering of l i g h t i n the latex with i n -creased stress. I t i s well known that rubber tends to be c r y s t a l l i n e at high degrees of str e t c h which would r e s u l t i n greater scattering. Williams and Taschek 1 and Williams and Dale 2 found s i m i l a r changes i n the absorption c o e f f i c i e n t i n the i n f r a red region which they explained i n the same way. «f, App. Phys., 8, 497-505 (1937). App. Phys., 15, 585, (1944)/ 30, VTII. BIBLIOGRAPHY 1. Jenkins and White, "Fundamentals of Physical Optics" 2. Wood, "Physical Optics" 3. Brode, "Chemical Spectroscopy" 4. Desha, "Organic Chemistry" 5. Barron, "Modern Synthetic Rubbers" 6. Memmler, "Science of Rubber".• 7. M. Kroger and H. Staude, "The Light Absorption of 1 Stretched and Unstretched Rubber and of Isoprene", Gummi-Ztg. 43, 22 (1928) 8. L. A. Wood, "The Optical Properties of Rubber", J.App. Phys. 12, 119-126 (1941) 9. L. A. Wood, "Synthetic Rubbers: a Review of Their Com-positions, Properties and Uses", Nat.Bur.Stand. Ci r c u l a r , C427 10. J. Crabtree and A. R. Kemp, "Weathering of Soft Vulcani-zed Rubber", Ind.and Eng.Chem. 38, 278-296 (1944) 11. H. I. Cramer, "I n d u s t r i a l Progress i n Synthetic Rubber-l i k e Polymers", Ind.and Eng.Chem. 34, 243 (1944) 12. H. A. Schwarzenbach, "Light Scattering i n Stretched Rubber", Rubber Chem. Tech. 13, 285 (1940) 13. W. Wittstadt, ^External Influence and the Internal State of Rubber'', Rubber Chem. Tech. 12, 488 (1939) 14. E. Guth, "The Problem of the E l a s t i c i t y of Rubber and of Rubberlike Materials", Am. Assn. f o r Advancement of Science, No. 21, pp. 103-127. 15. D. Williams and R. Taschek, "The Eff e c t s of E l a s t i c Stretch on the Infra Red Spectrum of Rubber", J.App. Phys. 8, 497-505 (1937) 16. D. Williams and B. Dale, "Further Studies of the Infra Red-Absorption of Rubber", J.App. Phys. 15, 585 (1944) 

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