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Nonlinear optical spectroscopic studies of polymer surface properties and competition adsorption of toluene… Hua, Rui 2008

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Nonlinear Optical Spectroscopic Studies of Polymer Surface Properties and Competition Adsorption of Toluene and Heptane on Silica Surfaces By RUT HUA B.Sc., Tsinghua University, China, 2001 M.Sc., Tsinghua University, China, 2004  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY  in  THE FACULTY OF GRADUATE STUDIES (Chemistry)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  JULY 2008  © Rui Hua, 2008  Abstract Surface properties of polymers and competition adsorption of toluene and heptane on silica were studied using IR-visible sum frequency generation (SFG) vibrational spectroscopy. SFG is intrinsically surface sensitive because the second-order optical process is forbidden in media with inversion symmetry, such as bulk polymers and liquids. This nonlinear optical technique provides surface vibrational spectra under ambient conditions without the need of an ultra-high vacuum environment. Polymer surface properties, including surface relaxation temperature of poly(methyl methacrylate) (PMMA) and surface electronic states of poly[2-methoxy, 5-ethyl (2’-hexyloxy) para phenylenevinylene] (MEH-PPV), were investigated. It was found that there are significant differences between the surface and bulk properties for these polymers. For PMMA, a new surface structure relaxation was identified at 67 °C, which does not match any known structure relaxation temperatures for bulk PMMA and is 40 °C below the bulk glass transition temperature. For MEH-PPV, SFG electronic spectra, which were obtained by scanning the frequencies of incident visible and JR beams, indicated that the electronic states at the polymer/solid and air/polymer interfaces are red-shifted with respect to that of the bulk. Finally, SFG was employed to study the competition adsorption of toluene and heptane on silica surfaces. Experimental data showed that heptane adsorbed favorably compared to toluene. Using a Langmuir adsorption isotherm, the changes of Gibbs free energy for the adsorption processes were calculated to be —12.1 ±  1.8 (kJ/mol) for toluene and —16.5  ±  2.3 (kJ/mol) for heptane.  —11  —  TABLE OF CONTENTS Abstract  ii  .  Table of Contents  iii  List of Tables List of Figures Acknowledgements  lix  Chapter 1 Introduction  1  Chapter 2 IR-visible Sum Frequency Generation  4  2.1 Introduction  4  2.2 Experimental Considerations  5  2.2.1 Experimental Setup  5  2.2.2 Basic Principles of OPG/OPA  6  2.3 Interactions of Electric Fields with the Material  9  2.4 Theory of Sum Frequency Generation  12  2.5 IR-visible Doubly Resonant Sum Frequency Generation  16  2.6 2D SF0 Simulation and Discussion  18  Chapter 3 Surface Relaxation Dynamics of Poly(methyl methacrylate)  25  3.1 Introduction  25  3.2 Experimental Setup and Method  28  3.3 Results and Discussion  29  3.4 Summary  41  Chapter 4 Electronic and Conformational Properties of the Conjugated Polymer MEH-PPV at a Buried Film/Solid Interface Investigated by Two-dimensional IR-visible Sum Frequency Generation  4.1. Introduction  42  —111  —  4.2 Experimental Section  .45  4.3. Results and Analysis  46  4.4. Summary  57  Chapter 5 Competition Adsorption of Toluene and Heptane on Silica Surfaces  58  5.1. Introduction  58  5.2. Adsorption Isotherm  59  5.3. Experimental Setup and Sample Preparation  62  5.4. Results and Data Analysis  64  5.4.1 SF0 of Ptire Toluene and Heptane on Silica Surface  64  5.4.2 SF0 of Toluene and Heptane Binary Mixtures on Silica Surface  69  5.5. Conclusion Chapter 6 Conclusions  78 79  Bibliography  -iv  -  List of Tables 3.1 Fitting parameters for ester methyl groups in the ppp and ssp-SFG spectra in Fig. 3.2 31 3.2 Temperatures of the discontinuities in Q for PMMA samples with different film thickness  35  4.1 Fitting parameters for the SFG spectra in Fig. 4.2. The resonance frequency to, the phase çb, and the dephasing constant F are the best fit for all SF0 spectra. The peak amplitudes A and the nonresonant second order nonlinear susceptibility individual ones  are for 47  4.2 Fitting parameters for the SFG spectra in Fig. 4.3. The resonance frequencya, the phase, and the dephasing constantq5 are the best fit for all SFG spectra. The peak amplitudes A and the nonresonant second order nonlinear susceptibility individual ones  are for 49  5.1 Fitting parameters for toluene SF0 spectrum in Fig. 5.2  66  5.2 Fitting parameters for heptane SF0 spectrum in Fig. 5.4  68  5.3 The ssp peak amplitudes A of the toluene 20b peaks obtained by curve-fitting the spectra in Fig.5.5 and the calculated surface coverage toluene at silica surface  70  5.4 The ppp peak amplitudes A of toluene 20b peaks by curve-fitting the spectra in Fig.5.6 75 5.5 The ratios of  /A ,, for the toluene 20b peaks with different toluene molar 3  concentrations in the mixture  76  -v  -  List of Figures 2.1. Layout of OPG/OPA. DM, RM and BS represent the dichromatic mirror, reflection mirror, and beam splitter respectively  6  2.2. Uniaxial crystal in the principle plane  8  2.3. Geometry of SFG setup in the reflection geometry  12  2.4. A schematic representation of doubly resonant IR-Vis and Vis-IR SFG  16  2.5. A schematic representation of the lowest-order doubly resonant SFG. (A) The SFG is resonant with the surface electronic states. (B) The IR and visible frequencies are resonant with the molecular vibrational and electronic states, respectively  18  2.6. The calculated three-dimensional 2D SFG spectra for a single IR-active mode system with S equal to zero and different ratios of Fen (n1) and F 0  20  2.7. The calculated three-dimensional 2D SFG spectra for a single JR-active mode system with different values for Huang-Rhys factor S and I / FCC) = 1  21  2.8. The calculated excitation profiles of 2D SFG with different Huang-Rhys factor S and ratios of dephasing constants for the electronic states F / F , when the incident 0 IR frequency is resonant with the vibrational mode of the system  24  3.1. Experimental setup for polarization-rotation sum frequency generation. The IR is p polarized (defined as 90° polarization angle), the visible beam is polarizationmodulated, and the SFG is detected at 45° polarization angle  28  3.2. SFG vibrational spectra of PMMA in ssp (s-, s- and p-polarized for SFG, visible, and IR, respectively) and ppp configurations. The dots are the experimental data, and the plots are the fitting curves using equation (2.24).The peak at 2955 cm’ is the symmetric stretching mode of the ester methyl group at the surface of a PMMA film with the thickness of 200 nm 3.3. SFG intensity vs. visible polarization angle  30 vjc•  The solid line is a fitting curve  using equation (3.2). SFG reaches a minimum at QQ , which is used to monitor 0 the surface conformation changes. The data were acquired from a PMMA film with the thickness about 200 nm 32  -vi  -  3.4. Measured Q 0 as a function of time and temperature for the PMMA sample with film thickness of 200 nm. The solid line is the fitted curve using equation (3.4). Two discontinuities in molecular orientation were obtained at 107  ±  2 °C and 67  °C  ±  2  34  3.5. The relationship between the measured Q 0 and temperature for the PMMA samples with different film thickness. (A) 500 nm; (B) 200 nm; (C) 100 nm  35  3.6. The tilting angle of the ester CR 3 group with respect to the surface normal estimated using the measured Q . The sample thickness is 200 nm 0  38  4.1. Energy diagrams of (A) JR-resonant SFG and (B) JR-visible doubly-resonant SFG.  I gO>  is the ground state, gi> is the first vibrational excited state, and eO> is the  electronic excited state at vibrational ground state. (C) Experimental SFG setup for probing the buried interface and the structure of MEH-PPV  44  4.2. SFG vibrational spectra of MEH-PPV at the buried interface with various incident visible wavelengths. The dots are experimental data, and the plots are the fitting curves using equation (2.24). The vibrational peaks located at 1593 cm’ are the C-C stretching of benzene rings. The fitting parameters are summarized in Table 4.1 .46 . .  4.3. SFG vibrational spectra of MEH-PPV at the air/MEH-PPV interface with various incident visible wavelengths. The dots are experimental data, and the plots are the fitting curves using equation (2.24). The vibrational peaks located at 1595 cm-i are the C-C stretching of benzene rings. The fitting parameters are summarized in Table 4.2  48  4.4. The absorption spectrum of bulk MEH-PPV film (solid line) and the surface SFG electronic spectra of MEH-PPV at MEH-PPV/solid (.) and air/MEH-PPV interfaces (.).The dashed lines are theoretical fitting curves 53 4.5. Calculated conjugation-length distributions of MEH-PPV at (A) MEH-PPV/solid interface and (B) air/MEH-PPV interface  56  5.1. Experimental setup. The sample is placed between two silica windows and sealed with a Teflon 0-ring. The visible beam with frequency a and JR beam with frequency  0 1 R  are incident on the bottom sample/silica interface from different sides  -vii  -  with incidence angles of O and angle  SFG  1R 8  respectively. The generated SFG signal  at  ’SFG 0  is then measured  63  5.2. The ssp SFG spectrum of pure toluene on silica surface. The dots are the experimental data and the plot is the fitting curve using equation (2.24). The 2864 cm’, 2955 cm , 3021 cm 1 , 3081 cm peaks are assigned to the methyl symmetric t and asymmetric stretching modes, and phenyl 20b and 20a stretching modes respectively. The inset is the SFG spectrum of pure water on silica surface  65  5.3. The phenyl 20a and 20b stretching modes  66  5.4. The ssp SF0 spectrum of pure heptane on silica surface. The dots are the experimental data and the plot is the fitting curve using equation (2.24). The 2864 cm , 1 2955 cm’, 2935 cm’ peaks are assigned to the methyl asymmetric, Fermi resonance, and asymmetric stretching modes respectively  68  5.5. The ssp SFG spectra of toluene/heptane mixtures with changing toluene molar fraction. The dots are the experimental data and the plot is the fitting curve using equation (2.24). The peaks at 3021 cm 1 indicate the amount of the adsorbed toluene on silica surface  69  5.6. The ppp SF0 spectra of toluene/heptane mixtures with changing toluene molar fraction. The dots are the experimental data and the plot is the fitting curve using equation (2.24) 5.7. The comparison of ssp and ppp SF0 spectra of pure toluene on silica. The  71 z\  and V  dots are the experimental data of ssp- and ppp-SFG and the plots are the fitting curves using equation (2.24)  72  5.8. Schematic representation of a fully hydrated silica surface, and of different kinds of configurations of silanol groups  73  5.9. Adsorption isotherm of toluene in the binary mixtures on silica substrate. • is the experimental data, and the solid curve is the fitting result using equation (5.13). .77 . .  -viii  -  Acknowledgements I would like to express my deep and sincere gratitude to my advisor, Prof. Keng Chang Chou. He has been the guide in my PhD study for the past several years. His wide knowledge and creativity in science have been of great value to me. His logical way in thinking and communication, his kindness and patience as an advisor, and his generosity with his time and advice are among the many characteristics that I hope to achieve during my career. Under his guidance, I have grown both as a scientist and as an individual. Prof. Chou’s high standards in research will affect me throughout my life. I have to thank the post-doc fellow, Dr. Qi Feng Li, for his enormous help when I stepped into this new research domain. He taught me a lot in setting up the experiments, such as aligning light beams and adjusting laser, understanding equations, analyzing data, and a lot more. I would like to thank my fellow student, Zheng Yang. I really enjoyed the interesting discussions with him. I can never forget the accomplishment and failure we experienced together. I would also like to thank all my friends and colleagues with whom I have had the pleasure to meet and work at UBC. They have helped me in every aspect, not only academically but also in daily life. Without them, my study and life could not have been such a pleasant experience. Finally, I would like to thank my parents, who experienced both my excitement and frustration during these years and provided me with the greatest emotional support. I wish them good health and an enjoyable retirement.  -ix  -  Chapter 1 Introduction  Surface chemistry is both scientifically and technologically important, because many chemical and physical processes happen at interfaces, such as wetting, -  electrochemical reactions, and biological functions. 1-18 Surface properties of materials are often different from those of the bulk. The differences may include their chemical compositions, molecular structures, and intermolecular interactions. Thanks to the advances in ultra-high vacuum (UHV) technology during the past 50 years, a large number of techniques have been developed to study surfaces at a molecular level. These techniques have allowed researchers to gain a good understanding of solid surfaces, such semiconductors.’ However, many of these experimental techniques, ’ as metals and 94 such as low-energy electron diffraction 42 (LEED) Auger electron spectroscopy , , reflection high-energy electron diffraction , 4547 (AES) 49 can only be ’ 48 (RHEED) operated in UHV environments. In reality, most interfacial phenomena take place under ambient conditions, such as gas/solid, gas/liquid, and liquid/solid interfaces. These surfaces are not accessible by the traditional UHV-based techniques, and our understanding of these interfaces remains very limited. Optical techniques are potentially capable of probing these buried interfaces as these interfaces are, in many cases, accessible by light. However, traditional optical techniques, such as infrared absorption spectroscopy (IRAS) , Raman scattering 5053 , 5456 and ultraviolet visible absorption spectroscopy , do not have sufficient surface 5760 —l  —  sensitivity, and the signals are typically dominated by those from the bulk. On the other hand, second-order optical process, such as sum frequency generation, is intrinsically interface specific because the process is forbidden in media with inversion symmetry, such as bulk liquids and polymers. ’ 1R-visible SFG vibrational spectroscopy was first 6 demonstrated in year 1987 by Shen and his coworkers with a monolayer of coumarin 504 dye on fused silica. 62 Since then, rn-visible SFG vibrational spectroscopy has attracted much attention because surface chemistry at buried interfaces can be studied in unprecedented detail. The technique has been applied to study a broad range of systems, such as air/liquid interfaces , polymer interfaces 6369 , gas/solid interfaces 7075 , 7679 liquid/solid interfaces , and biological systems 8084 . A brief theoretical background and 8588 the experimental setup of rn-visible SFG spectroscopy are described in chapter 2. Applications of rn-visible SFG to study free surfaces of poly(methyl methacrylate) 89 and buried interface of poly{2-methoxy, 5-ethyl (2’-hexyloxy) para (PMMA) phenylenevinylene] 9 (MEH-PPV) are described in chapters 3 and 4. Polymer science is ° an active research area because of its broad applications. However, the surface properties of polymers remain relatively unknown. For example, whether the surface glass transition temperature of a polymer is the same as that of the bulk remains controversial. 9195 In chapter 3, a polarization-modulation SFG technique was introduced to increase the sensitivity of SFG for studying the surface structure relaxation of PMMA. A new structure relaxation on the free surface of PMMA was observed at 67 °C which is 40 °C below the bulk glass transition temperature and is independent of film thickness in the range of 0.1  —  0.5  89 tm.  -2  -  Chapter 4 describes applications of two-dimensional (2D) IR-visible SFG to study surface electronic states of MEH-PPV. Conjugated polymers, such as MEH-PPV, have been studied intensively because of their applications in organic devices. Their electronic band gaps are determined by the delocalization of the it-electrons and are highly sensitive 9698 As the surface chain conformations are different from to their chain conformations. those in the bulk, it is expected that the surface band gaps would also be different. However, no previous study has been reported because of the lack of a technique to probe the surface electronic states of a buried interface. 2D SFG, with the capability of tuning both the incident IR and visible frequencies, has made such study possible. SFG electronic spectra showed that the electronic transition energies at MEH-PPV/solid and MEH-PPV/air interfaces are different from that for the bulk. Theoretical analysis based on an oligomer model indicates that the average conjugation-length is roughly 5.8 monomer units at the MEH-PPV/solid interface and 5.1 monomer units at the MEH PPV/air interface. 90 Chapter 5 reports studies of competitive adsorption of toluene and heptane on silica surfaces. The adsorption processes are important for the proposed nonaqueous extraction of oilsands to reduce the water usage in the current extraction process. 99 With SFG’s submonolayer sensitivity, the coverage of toluene on silica surfaces was measured as a function of the bulk molar fraction, and the adsorption isotherm was obtained. Based on a simple Langmuir adsorption isotherm, the change of Gibbs’s free energy can be calculated. It was found that heptane adsorbs on silica surface preferentially in comparison to toluene.  -3  -  Chapter 2 JR-visible Sum Frequency Generation  2.1 Introduction JR-visible sum frequency generation (SFG) is a surface-specific optical technique that provides vibrational spectra of molecules at ’ 10 The interfacial 00 interfaces.’ sensitivity of SFG comes from the fact that a second-order optical process is forbidden in media with inversion symmetry. ’ However, the inversion symmetry is always broken at 6 interfaces. JR-visible SFG is carried out with two pulsed laser beams at frequencies and a . These 2  two  beams are overlapped spatially and temporally at an interface, and  SFG with a frequency of a  co  + to  is detected. The intensity of SFG is enhanced  when the frequency of the JR beam is resonant with the vibrational modes of molecules at the interface. If both the incident visible and JR frequencies are scanned, SFG signal will be doubly enhanced when the visible and JR frequencies are resonant with the surface electronic and vibrational transitions, respectively.’ 02 A brief discussion of SFG theory is presented in this chapter while more detailed descriptions can be found in the literature. 101,103-107  -4  -  2.2 Experimental Considerations 2.2.1 Experimental Setup As shown in Fig. 2.1, the SFG experiments were carried out with a visible 532 nm beam (or a tunable visible beam from 400 to 700 nm) and an JR beam tunable from 1200 to 4000 cm’, which were generated using a Nd:YAG (yttrium aluminum garnet) laser system (1064 nm, 10 Hz, and 30 ps). The 532 nm beam was produced in a KTiOPO 4 (KTP) crystal by second harmonics generation (SHG) from the 1064 nm beam. The frequency-tunable visible beam was generated in a BaB 4 (BBO) optical parametric O 2 generator/amplifier (OPG/OPA) pumped by the 355 nm beam, which was generated by the third harmonics of the laser fundamental frequency. A portion of the 532 nm beam was used to pump a KTP OPG/OPA to generate a frequency-tunable beam around 1300 nm, which was used to generate the tunable JR beam in KTiOAsO 4 (KTA) crystals by difference frequency generation (DFG) with the 1064 nm beam. A brief introduction for the principle of OPG/OPA is presented in the following section. Both the visible and ER beams had a pulse duration of 30 Ps and a repetition rate of 10 Hz. The JR beam frequency was calibrated by the absorption lines of polystyrene film (Thermo Electron Corp.). The ER and visible input beams were overlapped both spatially and temporally at the sample. The SF0 output passed through a series of bandpass filters to eliminate the noise and background, and then was detected by a HAMAMATSU R3896 photomultiplier tube (PMT). The signal intensity was recorded by a gated integrator (SR 280, Stanford Research Systems Inc.) and digitized by a computer.  -5  -  RM OPG/OPA Tunable visible 400—700 nm  532 nm 355 nm SHG  DFG  Tunable IR 1200-4000 cm 1  Figure 2.1. Layout of OPG/OPA. DM, P.M and BS represent the dichromatic mirror, reflective mirror, and beam splitter, respectively.  2.2.2 Basic Principles of OPG/OPA In a second-order nonlinear optical process, two conditions, both energy and momentum conservations need to be satisfied. In the process of OPG, an input pump photon with frequency co is split into two photons with lower frequencies in the nonlinear crystal. The commonly used definition is to denote the one having larger energy the signal beam (co,), and the one having lower energy idler beam (o). In this process, the frequencies of the output photons follow energy conservation and phase matching conditions, which can be written as (2.1)  -6  -  and +k 3 k=k 1  (2.2)  Here the ks are the wave vectors of the electromagnetic fields. Since a, k  =  =  c / 2 and  2zn() / 2, the conditions can be written as --=---+--  2  2  2  n  n  n. 2  (2.3)  and  p 2  a) —na or n pp +n.a). s S ii  (2.4)  Because of dispersion, it is generally not possible to satisfy these two conditions in a medium with a single value of refractive index. However, these two conditions can be valid in a birefringent nonlinear optical crystal, such as KTP and BBO. In uniaxial crystals, such as BBO crystal, there exists a special direction. Light with different polarization has the same velocity only when propagating in this direction. This direction is called the optical axis, and usually labeled the Z axis. The plane containing the Z axis and the wave vector k of the light wave is termed the principle plane, as shown in Fig. 2.2. A light beam with polarization normal to the principle plane is called an ordinary ray, or 0-ray, while a beam with polarization parallel to the principle plane is called an extraordinary ray, or e-ray. The refractive index of an 0-ray  5  independent of the propagation direction, whereas that for the e-ray depends on the propagation direction. Thus, the refractive index in a nonlinear optical crystal generally depends on both the light polarization and the propagation direction. The difference between the refractive indices of the 0-ray n 0 and e-ray n, is known as birefringence, An.  -7  -  As shown in Fig. 2.2, tn is zero along the Z-optical axis, and maximum in the X,Y plane. The refractive indices in the X,Y-plane for 0-ray and e-ray are labeled as n 0 and e• If n 0  >  n <ne’ the crystal is called a 1e the crystal is called a negative crystal; while if 0  positive crystal.  Z-optical axis  Figure 2.2. Uniaxial crystal in the principle plane  For a light beam propagating at an angle S with respect to the Z-axis, the refractive indices are 0 n°(O)=n e(5)  I S 2 1+tan 1 2 tan °\Il+(fl/fl) 20  (2.5)  (2.6)  It can be seen from equations (2.l)-(2.6), when the 0 changes, the phase matching condition is altered, so the frequencies of the signal and idler need to change in  -8  -  order to satisfy the new conditions. In this way, by tuning the angle of the nonlinear crystals, tunable frequencies can be obtained. The OPG process can only generates weak signal and idler beams. To achieve high intensity of these beams, the beams were amplified in an OPA, in which the pump beam passes the nonlinear crystal together with either the produced signal or idler beams. In the OPG/OPA for visible light, 355 nm beam is the pump to pump BBO crystals. A small portion of 355 nm beam is spilt into a signal beam in the visible range (4OO-’7OO nm) and an idler beam in the range between 720—3 160 nm. The pump beam and the signal beam are separated by a dichromatic mirror, and then reflected by back collinearly to BBO crystals to amplify the intensity of the signal beam. In the OPG/OPA for JR light, similarly, 532 nm beam is the pump to pump KTP crystals. The generated signal between 755-.955 nm is reflected to interact with 532 nm pump again to amplify the intensity of the idler beam in the range of 1200- 1800 nm.  2.3 Interactions of Electric Fields with the Material All electromagnetic phenomena are governed by the Maxwell’s equations for the electric and magnetic fields E and B:  C  ôt  (2.7a)  VxB=+J c8t c  (2.7b)  V•E=4irp  (2.7c)  -9  -  V•B=O  (2.7d)  and the constitutive relations E+P 0 D=8  (2.8a)  H+p B=p M 0  (2.8b)  J=oE  (2.8c)  where J and p are the current and charge densities respectively, D and H are the intensities of the electric and magnetic fields respectively, s and p 0 are the electric permittivity and magnetic permittivity of free space respectively, F and M are the polarization and magnetization of the medium respectively, and c is the speed of light. For the interactions between light and dielectric materials, there are no magnetization, current, and charge, that is M =0, J =0, and p  =  0. Then equations  (2.7) and (2.8) can be simplified as (2.9a)  8t  c  c at  (2.9b)  cãt  (2.9c) VB=O  (2.9d)  Applying a curl operation to equation (2.9a), and substituting (2.9b) into the former resultant equation, we get the wave equation Evx(vx)+Poo  [  c  lE=_4---P 7 — atJ  cat  (2.10)  -10-  Under weak electric fields, the polarization of a material can be described by —.  P  where P  —(1)  —(0)  PP =  -f  P  (2.11) (2.12)  %’ECos(O3t) 0 8 —(1)  is the static polarization, P is the first-order lmear polarization,  linear susceptibility, s is the permittivity of free space,  t  (1)  is the  is time, and E cos(at)  describes the electric field. As equation (2.12) indicates, the frequency of light does not change as it passes through a medium. Under strong electric fields, such as those produced by lasers, the second order polarization can be significant. In this case, the polarization can be described by the following equations. —  —(0)  P=P —(2) =  where  (2)  —(1)  +P  —(2)  +P  (2.13)  +...  1 cos(o E 0 6 t) 2 t)Ek cos(a 1  (2.14)  is the second-order nonlinear susceptibility. The subscripts i, j, and k refer to  the axes of the lab coordinate system. Equation (2.14) can be rewritten as —(2)  E Ek [cos(a 1  +  2 )t + cos( a 1  —  &2  )t]  (2.15)  Equation (2.15) indicates that it is possible to generate a polarization with a frequency equal to the sum and difference of a and a . These two processes are known as SFG 2 and difference frequency generation (DFG), respectively. In this thesis, we focus on SFG, although DFG also occurs. Therefore, the output SFG intensity can be written as 1(o 2 )ocz(21 =ai+a I iI  (2.16)  It should be noted that in the theory described above, the electric dipole approximation is assumed, and the effects of optical magnetic fields and of multipoles (e.g., quadrupoles)  —11  —  are neglected. 107 Under this assumption, there is no SFG signal from a medium with inversion symmetry. In a medium with inversion symmetry,  (2)  is invariant under  inversion symmetry. However, the electric field and polarization must change sign as they are vectors. Using equation (2.14), the inversion operation gives: Therefore, (2)  =  (2) =  0, and SFG is not allowed. At an interface, inversion symmetry is  broken and SFG is always allowed.  24 Theory of Sum Frequency Generation Fig. 2.3 shows a typical SFG setup in the reflection geometry. The visible and IR beams, with frequencies aj and a 2 respectively, are overlapped spatially and temporally at an interface, and SFG at the frequency of  +  is generated.  +  2  Figure 2.3. Geometry of SFG setup in the reflection geometry  The sum-frequency field radiates in the direction given by the condition 2 ksfgkl+k  (2.17)  -12  -  where k and k 2 are the wave vectors of the incident beams. SFG intensity is given by’°’ =  where  3 fi  + (02)  =  (2.18)  c cos 2 0 8s  is the exit angle of the SFG output.  2J?  is the effective surface nonlinear  susceptibility defined as :[L(co ] 1 ) ] [L(oi .ê ).ê .(2) 2 5 =[L(co 2’ ]  (2.19)  with ê 1 being the unit polarization vector of the optical field at a and L(co ) the tensorial 1 Fresnel factor. L(o, ) describes the relationship between field components in the air and 1 in the medium. For isotropic media, only the diagonal elements of L(a) need to be considered, which are ) 1 L(a  L (a ) 1  =  =  L ((0.)=  ( 2n ) 1 ) cos(y a (wjcos(y n ) 1 +1 ( 2 n ) cos(fi) o  (2.20a)  ( 2n ) 1 ) cos(fl a’ Q n ) ( 2 ) ) 1 cos(fl cos(y +n o i  (2.20b)  ( 2 2n ) 1 ) cos(fl o 11 n )cos(y + n (a ) 2 (a )cos(fi,) n’(o 1 )) 1  (2.20c)  In the above equations, n’(o) is the refractive index of the interfacial layer, n 1 is the refractive index of medium i, /3 is the incident angle, and Ti is the refracted angle. Since the interfacial layer is only one or a few monolayer thick, its refractive index can be different from that of its own bulk material and difficult to measure. 108 It is therefore an usual practice that n’ (as) is chosen to be either n 1 (os), n 2 (a,), or the bulk refractive index of the material at the interface.’ 09 However, the determination of molecular  -13  -  orientation in SFG analysis is subjected to the values of the local field factor n’(a) at the interface.’ 0 Zhuang ” 09 cyano-p-terphenyl  Ct  [5CT,  al. did a detailed analysis on this issue using 4”-n-pentyl-46 ( 3 CH ( ) 2 ) 4 H 3CN].”° CH C  It  was  concluded  that  n= 1.18 .t 0.04 is appropriate for terminal methyl groups, and this value has been generally adopted in analyzing SFG spectra for methyl groups. This value is later confirmed by Wang et al. experimentally. ’ 11 The vibrational spectrum of an interface is acquired by scanning the JR frequency. SFG intensity is proportional to the square of the effective second-order nonlinear susceptibility, as shown in equation (2.18). The effective susceptibility described in equation (2.19) contains components of resonant and nonresonant parts. When the visible beam frequency is fixed, the resonant susceptibility, which originates from vibrational modes on the surface, is described by Ag  (2) IR  O)q  (2.21)  +ZFq  where Ag is the strength of the qth vibrational mode,  aIR  is the frequency of the infrared  laser beam, aq is the frequency of the qth vibrational mode, Fq is the damping constant of the qth vibrational mode. The amplitude Ag in equation (2.21) can be written as follow (1)  ——-———-  A 2(Oq  8q  aim  aq  (222)  where p , is the dipole moment and a is the polarizability. Hence, in order for 1  to  be non-zero, the vibrational mode must satisfy both the JR and Raman selection rules. 62 (2)  is a third rank tensor. It consists of 27 elements, whose values are the  property of the medium under investigation and are invariant under symmetry operations.  -14-  (2)  The number of non-vanishing  elements is often reduced because of the symmetry of  the medium. For example, there are only four nonequivalent and non-vanishing (2)  values on an isotropic surface. With the lab coordinates chosen such that z is along  the interface normal and x in the incidence plane, these four terms are:  z  and  =  )J.  =  They can be deduced by measuring SFG with four  different input and output polarization combinations. These values are associated with the molecular orientation at the interfaces. Therefore, the information about molecular orientation can be obtained. 62 The amplimdeAq, as shown in equation (2.21) is related to the molecular hyperpolarizability Aq,jk  =  aqimn ((1  N  .  aqlmn  through a coordinate transformation as  l)(J th)(k. ii))  (2.23)  .  l,m,n  N donates the molecule surface density. The subscripts i,  j,  and k refer to the lab  coordinates, and the subscripts 1, m, and n, refer to the axes for the molecular coordinate system.  (1. l)()  )  is the coordination transformation from molecular fixed  coordinates to laboratory fixed coordinates, and  ()  indicates average over molecular  orientations. When taking into account the resonant and nonresonant parts of the second order nonlinear susceptibility, and their relative phases, the Lorentzian shaped SFG signal can be expressed by Aq +  e’ q  21112  (2.24)  IRq”q  -15  -  where  are the phases for the vibrational modes. Equation (2.24) is used to fit the SFG  vibrational spectra.  2.5 IR-visible Doubly Resonant Sum Frequency Generation With  IR 0  and a near vibrational and electronic transitions, respectively, SFG  can be doubly enhanced.” 27 By tuning both the  1R 03  and co, SFG is a highly selective  spectroscopic probe of an interface. In general, there are two types of SFG processes. 112 The first type starts with an electronic transition followed by a vibrational transition (Vis IR), and the second type begins with a vibrational transition followed by an electronic transition (IR-Vis), as shown in Fig. 2.4. g and e represent the electronic ground and excited states, respectively, and v and u denote the ground and excited vibrational states, respectively.  _eu)  — —  2IR  _Ieu) \  _eu  = ‘ Vis 0  = 0 Vis  -v)  H) IR-Vis Case  -) Vis-IR Case  Figure 2.4. A schematic representation of doubly resonant IR-Vis and Vis-IR SFG.  -16  -  The theory of doubly resonant SFG has been previously reported by Shen and coworkers, assuming the Born—Oppenheimer approximation and harmonic potential surfaces for the electronic states.” 2 For IR-Vis case, the second order nonlinear susceptibility can be expressed as (2)(IR-fl)  ôp N/ 2 \ Pegflge 1 8q h  —  fe_S  .  Xk  n=O  n!  SFG  ‘I  co—o+,[’,  ° e 3 g +iT’ en gO  SFG 0  i (ii + 1)w 1  COeg +lTen÷i,go  (2.25) where N is the surface molecular density,  4 represents the i  —  component of electronic  transition moment, q 1 is the normal coordinate, S is the Huang—Rhys factor, n labels the vibrational state, g and e label the ground and excited electronic states, respectively, SFG 0  is the SFG frequency, o, and  frequencies, respectively,  Weg  are the resonant vibrational and electronic  and T,jg 0 are the damping constants, the angular brackets  indicate an average over molecular orientations. For Vis-IR case, the second order nonlinear susceptibility can be described as 112 (2)(VL—IR)  XUk  —  —  N! i k 8 Pe 2 \ PegPgg 8q, h  x’— n=O  n!  COIRCOI+ZTI  .  w  0l0egPen,go  -_____________  SFG —(n +  l)COi••COeg+l(Ten+igo  +  (2.26) where  el  denotes the vibrational dephasing rate constant for the JR active mode 1 of the  electronically excited state e.  -17-  The contribution of the Vis-IR SFG is generally negligible, because the electronic relaxation times are generally much shorter than the vibrational relaxation times.” 3 Therefore, only the IR-Vis SFG will be considered. Equation (2.25) can be used to fit the 2D SFG data. The IR-Vis doubly resonant feature of 2D SFG can be easily seen from equation (2.25). For example, for the lowest order process (n=O), the SFG intensity will be doubly enhanced when  IR 0  +a  °‘eg  or IR + O4  to Fig.2.5 (A) and (B) respectively.  = ‘ eg 0  + l•  These two processes correspond  Here we assume that the vibrational excitation  energy is the same at electronic ground state (g) and excited state (e). 5 F.  A  B  F  co  c  Q  0  Figure 2.5. A schematic representation of the lowest-order doubly resonant SFG. (A) The SFG is resonant with the surface electronic states. (B) The IR and visible frequencies are resonant with the molecular vibrational and electronic states, respectively.  26 2D SFG Simulation and Discussion There are several factors in equation (2.25) that affect the shape of the 2D SFG  -18  -  spectrum, namely the Huang—Rhys factor 5, the resonant vibrational and electronic frequencies o., and  and the dephasing constants for vibrational and electronic states.  For the theoretical simulation, the non-resonant contributions gg 1  pp  COeg  X4k  are omitted, and  is taken as a constant. The vibrational and electronic frequencies oi and  determines the position of the SFG peaks, whereas the Huang—Rhys factor S and the  dephasing constants are the important factors that determine the shapes of the 2D SFG peaks. To understand some of the basic properties of the 2D SFG spectra, a model system consisting of one single IR mode is discussed here. The parameters are chosen as follows:  =: 1 CO  1593 cm’, coeg= O rim =18868 cm, 53  Feo  —100 cm’, and ,=20 cm . 1  Here all the parameters are transformed into wavenumber units. The Huang—Rhys factor S and dephasing constant Fen for en (n?:l) states are the variables for the simulation. For simplicity, all the  en  for different n are assumed to have the same values.  Fig. 2.6 shows the 2D SFG spectra as a function of the visible wavelength and IR wavenumber of the incident laser beams. In Fig. 2.6, different SFG spectra are shown with different ratios of F, 1 and Fe 0 when S is set to be zero. The ratios are chosen to be 1, 3, 6, and 10. The bigger the ratio is, the short time an electron can stay without dephasing at the en (n?1) states compared to that at eO state. The two peaks centered at 530 rim and 580 rim are resultant from the processes shown in Fig. 2.5 (A) and (B) respectively. In the following discussion, these two peaks are labeled as peak (A) and (B) respectively. When the lifetime of electron at en states is the same as that at eO state, the  -19  -  2D SFG spectrum features two peaks with almost the same intensity, as shown in Fig. 2.6 (a). If the lifetime of electron at en states is third of that at eO state, the peak A is much weaker than the peak B, as shown in Fig. 2.6 (b). With the electron lifetime at en states getting even shorter, the peak A is becoming weaker (Fig. 2.6 (c)) and nearly disappears when the ratio of the dephasing constants for en and eO states is 10 (Fig. 2.6 (d)).  -,  --r--  1200  -i  1000  -r  800  —r  ——  ——  -  200  -  ___  k  _I I  L.  400  I l00O  _rN I I  I  ——  800-  I i_  I  _j II—__  I  I  :: ::EJ: r  —I_  -  3 I  1700  1700 1650  50  1600  1600  650  —•--  —  -  -  1550 1  .—600 550  1450—  l00O  -  Ii  -  800-  -  -:i tio6  -  800  -  I  __— _  _  ___  I_  —  —  —  I  :r:---:  - -  NI  —a—— I  I  400  1000  I  -  I I  400l0  200 0 1700  1—_i  I  1—_I I  II  I  —  —  I  I  I  Il  0 1700 1650 —  650  1650  50  1L 1500 1450  1450  Figure 2.6. The calculated three-dimensional 2D SFG spectra for a single JR-active mode system with  S equal to zero and different ratios of ‘en (nl) and Fe • 0  Fig. 2.7 shows the 2D SFG spectra with different S values under the condition of F /F 0  =  1. Several values for Huang-Rhys factor S are used: 0, 0.3, 0.6, and 1.0. When  S is smaller than 0.3, the processes described in Fig. 2.6 are dominant. With -20  -  increasing 5, multiple peaks at shorter visible wavelength appear because of the higher order vibronic transitions. For example, when S =0.6, the third peak around 490 nm is obvious. The multiple peaks come from higher order vibronic transitions. It should be noted that in the case of S =1, the peak at 530 nm disappear. This is due to the fact that for S =1, the Franck-Condon overlap integral contributions of the transition of gO—*el and g0—e0 cancel each other. This is can be easily seen from equation (2.25).  __rb—i  ———rh——I———— _--  —  —  1000—  L_-  1 (a) S0 i___—1  I  8004_—r I  —  —  __N_ I  H  I —rb— I_  j__  I  ‘__——j  _l  I  1  -  __  I —_I •1___ I I  -  _—I_  I  400  —r  =  —r  I  __A—  200  0  —  I  —  i__I  — —  —I_ L_ I  —  — —  —  I  4  ISO 0 1700  I  ___,__  300  I  I  —  0 1700 1650  1650’ 1L__  650 — 550  1500  600  %  650 1500  550 —3—  —I  —  —  _I_ I  —  250  I — I  150 0  I I  _—  —  ___  — ___  ___  I  ;:: —_  —_ ——  I  100  I  __  __—  I  I  I  50 —  1700  1700 650 600 %1%51550  1500  7 O/(  550  650  1600  fq% a  1550’  550  500  Figure 2.7. The calculated three-dimensional 2D SFG spectra for a single JR-active mode system with different values for Huang-Rhys factor  S and Fen I  =  1.  To see more clearly the dependence of SFG peaks on the Huang-Rhys factor S and the ratio  efl ‘eO’  Fig. 2.8 shows the calculated excitation profiles of 2D SFG when  -21  -  the incident JR beam is resonant with the vibrational frequency of the system. When the dephasing rate constants for eO and en states are the same, as shown in Fig. 2.8 (a), high order peaks appear when S is nonzero. When S increases, the intensities of the peak A at 530 nm and peak B at 580 nm decrease correspondingly, When S equal to 1, there is no peak at 530 nm. When the ratio of the dephasing rate constants for en and eO states increases to 3, as shown in Fig. 2.8 (b), the excitation profiles of 2D SFG are much different. No matter what the value of S is, the peak B at 580 nm is the only dominant one without any other peaks with comparable intensity. Although the multiple peaks can still be seen when S is larger, they are all much weaker. Furthermore, when the ratio is set to be 10, which means the life time of electrons at en state is 10 times shorter than that at eO state, the excitation profiles are shown in Fig. 2.8 (c). In this case, only the peak due to the process (B) in Fig. 2.5 (B) is visible. In Raschke’s study of Rhodamine 6G with 2D SFG spectroscopy, they only saw one peak for each JR-active mode. 113 The reason for not seeing peaks from higher order vibronic transitions can be attributed to the fact that the vibronic transitions of Rh6G have significantly shorter dephasing times. In chapter 4, the excitation profiles of 2D SFG of MEH-PPV at MEH-PPV/solid and air/MEH-PPV interfaces will be discussed.  -22  -  1000  s=o  60S=O.3  2  CU  356 S=0.6  C  LI Cl,  z  168  500  550  600  650  Visible wavelength (nm)  122w  ( ren  e3)  n  60  S =0.3  -  S=0.6  (U  —  C LL  C,,  160  ZJL 500  550  600  650  Visible wavelength (nm)  -23  -  D  0 Li U)  Visible wavelength (nm)  Fig. .2.8. The calculated excitation profiles of 2D SFG with different Huang-Rhys factor  S and ratios of dephasing constants for the electronic states en / Fe 0 when the incident ,  JR frequency is resonant with the vibrational mode of the system.  -24  -  Chapter 3 Surface Relaxation Dynamics of Poly(methyl methacrylate)  3.1 Introduction Poly(methyl methacrylate) (PMMA), a glass-forming polymer, has been widely used in scientific and technological applications because of its special mechanical, thermal, and optical characteristics.” 8 Previous studies have shown that PMMA displays a complex dynamical behavior. Its glass and sub-glass relaxation processes have been studied by dielectric spectroscopy” ° 2 ’ 9  dynamic mechanical analysis’ , neutron 21  , and NMR . 22425 scattering’ 27 The main features are the well-known a” 26 spectroscopy’ and f3-relaxations. The cL-relaxation, which is generally regarded as the glass transition, involves cooperative movements of the backbone whereas the n-relaxation is thermally activated flips of local structural units by external fields. Compared to the bulk relaxation processes, the surface relaxation is relatively poorly understood. It is well established that in simple molecular systems, such as ice, the surface phase transition temperatures are lower than those for the bulk.’ 28 An unresolved issue is whether polymeric materials have similar free-surface effects.’ 29 Many technological applications, such as lithography and 31 rely on the surface properties of PMIVIA. Therefore, the state of a ” 130 nanoimprinting , PMMA surface is an important parameter for both scientific and technological reasons.  -25  -  Another active research area is the study of finite-size effects on the relaxation temperatures in confined systems.’ 32 A reduction in glass transition temperature (Tg)  WdS  observed by Keddie et al. using an ellipsometer on polystyrene films with thicknesses less than 100  133  For PMMA, Keddie et al. observed a Tg reduction of  6 °C for a  -  30 nm thick film on a gold surface, but the Tg increased with decreasing film thickness on a native oxide of silicon.’ 34 Keddie et al. suggested that a liquid-like layer exists at the air/polymer surface’ ; 3 ” 33 4 however, in these experiments the polymer-substrate interactions cannot be excluded and, in some cases, can be as significant as the finite-size effects.’ 3 ’ 35 8 Many other studies using various techniques, such as friction-force , X-ray reflectivity’ 39 microscopy’ , fluorescent diffusion’ 32 , positron 40 , and ellipsometry’ 42 optical birefringence’ 43 have also found a decrease in Tg with decreasing film thickness for thicknesses less than 100 nm. To study the pure finite-size effect without the polymer-substrate interactions, Forrest et al. measured the Tg of freely standing polystyrene films using Brillouin light scattering, and confirmed a decreasing Tg with decreasing film thickness for films less than 70 nm thick.’ Similar Tg depression was also observed in other confmed systems’ , such as nanopores’ 45 46 or artificially roughened films’. However, these results are not applicable to describe the free surface of a thicker film, as the nature of the finite-size effect is fundamentally different from that of the free-surface effect. It remains an open question whether the surface relaxation temperatures are lower than the bulk relaxation temperatures at the free surface of a polymer film thicker than 100  fliTi.  Because the free-surface effects are likely present only within a few nanometers,  if not monolayers, of the surface, answering this question would require an extremely  -26  -  surface-sensitive technique to reduce the bulk signal. Such experiments have become more feasible as surface-sensitive techniques were developed. Jean et al. reported a gradually decreasing Tg with decreasing probing depth on a thick polystyrene film (thickness ca. 1 .im) using positron annihilation spectroscopy (PAS).’ 48 However, Xie et al. did not find such a free-surface effect in an experiment based on a similar 49 Studies using other surface-sensitive techniques, such as scanning force technique.’ microscopy (SFM)’ , near-edge X-ray 50  absorption fine  structure  spectroscopy  , X-ray reflectivity’ 151 (NEXAFS) , and sum-frequency generation , 52 95 (SFG) 5 ” 3 have 4 not been able to confirm a reduced relaxation temperature at the free surface of various polymers with thicknesses greater than 100 nm. We revisit this question, using a novel approach: polarization-modulation SFG (PMSFG). SFG vibrational spectroscopy was first applied by Gracias et al. to study the surface glass transition of polypropylene, but no difference between the surface and bulk glass transition temperatures was observed. 95 More recently, SFG was employed to study the surface glass transition of poly(vinyl alcohol) (PVA)’ 53 and polystyrene (PS)’ . In 54 these studies, an alignment of surface chains was introduced by rubbing the polymer films to increase the sensitivity of SFG to surface structure relaxations. In both studies, it was concluded that the rubbed polymer surface has the same Tg as the bulk. As it was unclear what effect was introduced to the surface in the rubbing process, it is more desirable to carry out the experiment on a free isotropic polymer surface in its natural state. In the current study, a polarization modulation technique was employed to improve the sensitivity and efficiency of SFG in detecting the surface structure relaxation at a free isotropic PMMA surface. We observed a structure relaxation on the free surface of  -27-  PMMA at 67 °C, which does not match any known relaxation temperature for the bulk,  and is 40 °C below the bulk Tg. As expected for a surface property, this surface relaxation temperature was found to be independent of film thickness in the range of 0.1  —  0.5 tm.  3.2 Experimental Setup and Method Visible (polarization modulated) SFG  prized  —PMMA  Figure 3.1. Experimental setup for polarization-rotation sum frequency generation. The IR is p-polarized (defmed as 90° polarization angle), the visible beam is polarizationmodulated, and the SFG is detected at 45° polarization angle.  Atactic PMMA (Mw  =  52,700, Mw/Mn  =  1.08) was purchased from Scientific  Polymer Products Inc. PMMA was dissolved in tetrahydrofuran with a concentration of 2% w/v. Polymer films were prepared by spin casting on fused silica windows and were annealed at 100 °C for 12 hours before measurements. Several PMMA films with thicknesses of 100, 200 and 500 nm were used in this study. The thicknesses were determined after the SFG studies by measuring the depths of the scratch marks using a scanning force microscope. The thicknesses are all above 100 nm to avoid the aforementioned Tg depression caused by the finite-size effect or the polymer-substrate -28  -  interactions. The sample was sealed in a temperature-controlled cell, which was filled with Ar. The sample temperature was controlled by a homemade feedback program with accuracy better than 0.5 °C. As shown in Fig. 3.1, SFG was carried out by mixing a visible (cot) and an IR frequency a  = (01  +  (0)2)  beams on the surface to generate a third beam with a  . The polarization of the visible beam was rotated by a half-wave 2 a  plate mounted on a computer-controlled rotational stage.  33 Results and Discussion Previous studies by Wang et al. have demonstrated that the SFG signal from a PMMA film on silica is dominated by the air/PMMA interface, with the SFG from the PMMA/silica interface being negligible.” For an azimuthally isotropic surface, the second-order nonlinear susceptibility tensor elements:  =  x=  ,,  has four independent non-vanishing and  x’  with z being along the surface  normal and x being in the plane of incidence in the laboratory coordinate system.’ 55 With the JR fixed at p-polarization, as shown in Fig. 3.1, the nonzero effective nonlinear susceptibilities are  and  x,.  Fig. 3.2 shows the SFG vibrational spectra of PMMA in ssp and ppp configurations. The symmetric stretching mode of the ester methyl group at 2955 cm’ dominates the SFG spectra. The slightly different line shape of the ppp spectrum can be explained by the interference between the resonant and nonresonant SFGs.’ 56 As shown in Fig. 3.2, both the ssp and ppp spectra can be fitted by a single Lorentzian line shape using equation (2.24). The fitting parameters are summarized in Table 3.1. It should be noted that Lorentzian line is an appropriate description for SFG peaks, although there  -29  -  exist inhomogeneous broadening for the SFG peaks. This broadening effect has a 57 It turned out that the resonances are described well enough by Gaussian shape.’ Lorentzian lines, and it has been a usual practice that this inhomogeneous broadening effect is omitted for simplicity, rather than make more sophisticated fits using superposition of Lorentzians and Gaussians superfluous.’ 58  A  •  10  .ci 1. .‘  U,  PPP SSP  8 6  0  0 2800  ..  1 2900  -.  3000  t  -  3100  IR Wavenumber (cm ) 1  Figure 3.2. SFG vibrational spectra of PMMA in ssp (s-, s- and p-polarized for SFG, visible, and IR, respectively) and ppp configurations. The dots are the experimental data, and the plots are the fitting curves  using equation (2.24). The peak at 2955 cm’ is the  symmetric stretching mode of the ester methyl groups at the surface of a PMMA film with the thickness of 200 nm.  -30-  Table 3.1 Fitting parameters for ester methyl groups in the ppp and ssp-SFG spectra in Fig. 3.2 Peak  Co (cm’)  A  0  F (cm’)  assignments ssp-SFG  2955  -ss 3 O-CH  37.4  0.59  1.16  13.2  ppp-SFG  2955  -ss 3 O-CH  11.2  0.56  1.80  20.7  As shown in Fig. 3.1, PMSFG was achieved by fixing the IR polarization at p polarization (defined as  900  polarization angle), detecting the SFG at 45° polarization  angle, and rotating the polarization of the visible beam a by rotating a half-wave plate. This approach is mathematically similar to the null-angle method described by Gan et al., in which the polarizations of input beams were fixed and SFG intensities were measured at various polarization angles.’ 59 In our approach, the SFG was always measured at a polarization angle of 45° to avoid calibrating the polarization-dependent throughputs of some detection optics, such as a monochromator. The measurements were carried out with the visible wavelength fixed at 532 mn and the IR wavenumber fixed at the resonant frequency of the ester methyl group (2955 cm’) to monitor the discontinuity in the orientation of the surface ester methyl group during temperature changes. As seen in equation (2.24), SFG intensity is proportional to the square of the effective second-order nonlinear susceptibility  In this experiment, the SFG signal  and visible input beam have both s- and p- polarized components. And since SFG was measured at 45° polarization angle, the intensity of the measured SFG can be written as a function of the visible polarization angle Q,,..  -31  -  ) 8 ‘SFG(v1  cc  sin  =  sinQ +  sFG 2 S1fl  +  COSQSFG  (3.1)  2 cos2  The expression can be rewritten as ‘SFG(vL)  cc  --sin(Q,,  —  2 ) 0 Q  (3.2)  with (2)  =2 —arctan( ’ ;”) Xeff,ssp  (3.3)  10  I.  U) C  ci)  .s 0  Li 0  -180  -120  -60  I  0  Visible Polarization Angle  60  120  8 (Deg) c  Figure 3.3. SFG intensity vs. visible polarization angle Q. The solid line is a fitting curve using equation (3.2). SFG reaches a minimum at  =  2, which is used to  monitor the surface conformation changes. The data were acquired from a PMMA film with the thickness about 200 nut  Fig. 3.3 shows a typical ISFG( v) curve measured at room temperature. Equation 2 (3.2) was used to fit the curve with Q 0 and an additional proportional constant as the  -32-  fitting parameters. The signal-to-noise ratio, presented in Fig. 3.3, allows us to determine the values of Q 0 with an error of ±0.3 degree. Any change in the surface molecular orientation would change the ratio of measured  ) 8 ‘sFG(  curve. The phase of  and introduce a phase shift in the ‘sFG(,),  indicated as Q 0 in equation (3.2),  was used to monitor the surface conformation changes, instead of using the absolute SFG intensity. In this approach, long-term laser intensity fluctuations only effect the SFG intensities but not the Q 0  .  For example, if the laser intensity decreases, the SFG  intensities in Fig 3.3 would decrease, but the Q 0 will stay the same. PMSFG does not involve changing the polarizations of multiple laser beams commonly used to determine the orientation of surface functional groups in conventional SFG ’ 5 ” 1 experiments.” 1 60 3 Since the SFG spectra with different polarization combinations are taken at different time, the laser intensity can not be guaranteed to be identical. Therefore, the deduction of the information on the orientation angles of the functional groups has relatively large uncertainties. Small changes in the orientation of the surface functional groups may not be observed. To observe the small change, some researchers introduced rubbing-induced ’ or an eternal stretching force 6 ” 53 alignment’ 162 to generate larger SFG signal changes. PMSFG allowed us to carry out a real-time recording of structure relaxation on an isotropic polymer surface without the need of introducing a rubbing-induced alignment or an external stretching force to enhance the structure changes. Fig. 3.4 shows the measured Q 0 as a function of temperature and time for a PMMA film with a thickness of 200 nm. Initially, the samples were kept above 140 °C for 90 mm  before SFG measurements to stabilize the temperature of the cell. The  -33  -  temperature was then decreased at  —  0.3 °C/min. For each structure relaxation, data points  were phenomenologically fitted by a hyperbolic tangent function’ 63 Q(T)=Q12_Q1Q2tanh1T  2  (34)  2  where Q 1 and Q 2 are the low and high limits of Q 0 before and after the phase transition, 53 Based on this fitting method, two discontinuities in molecular orientation respectively.’ were obtained at T 0 =107±2 °C and 67±2 °C.  140  Temperature (°C) 100 80 60  120  40  20  15T  100  200  300  400  Time (mm.) Figure 3.4. Measured  0 as a function of time and temperature for the PMMA sample with Q  film thickness of 200 nm. The solid lines are the fitted curves using equation (3.4). Two discontinuities in molecular orientation were obtained for these curves at 107 ± 2 °C and 67 ± 2°C.  Fig. 3.5 shows the relationship between the measured Q 0 and temperature for the PMMA samples with different film thickness, i.e. 100, 200, and 500 nm. As shown in Fig. 3.4, although the data qualities vary for some samples, the two discontinuities in the molecular orientation of the ester methyl groups at the surface of these PMMA films are  -34-  obvious. The temperatures for the two discontinuities for these samples are summarized in Table 3.2. Within the fitting uncertainties, they are considered to agree with each other. These two temperatures are averaged at 107.3±2.7 and 67.3±2.7 °C. The following discussions will be focused on the results from the 200 inn sample.  I  A)Ø4.++  2b  20  16 140  120  100  80  60  40  20  Temperature (°C)  Fig. 3.5 The relationship between the measured  0 and temperature for the PMMA samples Q  with different film thickness. (A) 500 nm; (B) 200 nm; (C) 100 nm. Table 3.2 Temperatures of the discontinuities in  0 for PMMA samples with different film Q  thickness.  Film Thickness  Temperature of the  (nm)  1st discontinuity  2nd discontinuity  500  (°C) 107  (°C) 65  200  107  67  100  108  70  Temperature  of the  -35  -  The first structure relaxation at 107 °C is bulk-induced, as it agrees well with the bulk Tg of atactic PMMA.’ 63 The coherence length of SFG in the current study is about 30 nm. Although SFG is surface-sensitive, it is known that SFG is not totally free from bulk contributions. 164 Currently, there is no theory that can be used to determine the percentage of the bulk contribution. However, the bulk contribution does not affect our ability to detect surface structure changes. On the other hand, the observation of the bulk ass transition demonstrates that the polarization-modulation technique is sensitive to the structural changes within the probing depth of SFG. It is worth pointing out that the refraction index of silica increases proportionally to temperature with a slope of 1 xl 0 165  As the temperature changes, it slightly changes the Fresnel factors, which are  included in the effective nonlinear susceptibility in equation (3.3). 109 Because the Fresnel factors appear in both ratio  and  %, the effect is partially cancelled when only the  (or Q ) was measured. Overall, it introduces a small slope in Fig. 0  3.4, but it is not responsible for the short-range steep changes of Q . The refraction index 0 of PMMA decreases linearly with temperature, with a slope of approximately -1.4 xl 0 /°C below the glass transition temperature.’ 4 66 The slope changes to approximately -3.4 xl 0 /°C above the glass transition temperature. The discontinuity in the slope can 4 produce a very small kink in the measured  at the glass transition temperature, but not  a step-like change as seen in Fig. 3.4. Therefore, the observed Q change at 107 °C is mostly due to the bulk structure relaxation, instead of the changes in refraction indexes. To the best of our knowledge, the surface relaxation at 67 °C has never been previously reported for PMMA. Bulk PMMA does not have any known structure relaxation near 67 °C. The relaxation transitions in polymers are generally labeled as a,  13,  -36-  y, etc., in alphabetical order with decreasing temperature. For PMMA, the highesttemperature relaxation, the a-relaxation temperature, is generally regarded as the glass transition temperature and is associated with the long-range cooperative motion of the backbone. The 13-relaxation is associated with the local movements of side-chains.’ 26 The temperature for 13-relaxation of bulk PMMA is near room temperature and decreases with  decreasing film 67 thickness.’ 6 ’ 9 The observed surface structure relaxation at 67 °C is lower than the bulk a-relaxation temperature and higher than the bulk 13-relaxation temperature. The same measurements were repeated for various film thicknesses of PMMA between 0.1 and 0.5 pm. Within measurement errors, this surface structure relaxation temperature is independent of the film thickness, as one would expect for a true free-surface property. What are the possible origins of the observed relaxation at 67 °C on a free PMMA surface? It is generally believed that the surface molecules have more freedom to adjust their position and are expected to have a lower relaxation temperature.’ ° Therefore, a 6 ” 34 decreased surface a-relaxation temperature is a plausible explanation for the observed structure relaxation at 67 °C. However, current SFG studies cannot rule out the possibility that an increased surface 13-relaxation temperature is responsible, even though such an explanation may not be consistent with the general expectation. This uncertainty is due to the fact that the ester methyl group monitored in the current study is located at the side-chain. Therefore, in principle, the measured SFG could be sensitive to both the a- and 13-relaxations. Similar studies could not been carried out on other vibrational modes of PMMA because of their much lower signal-to-noise ratios.  -37-  When looking at the relaxation times of these relaxation processes, the measurements indicated that the surface polymer chains have a significantly higher mobility compared to the bulk chains. As shown in Fig. 3.4, the full structure relaxation time (4z17) associated with the bulk glass transition temperature at 107 °C was 96 mm while the surface relaxation at 67 °C took only 36 mm. Although not under the same condition, the measured bulk/surface relaxation time ratio of 2.7 (96 mm  /36 mm) is  almost identical to the measurements carried out by Wu et al. using NEXAFS, which showed the surface relaxation is approximately 2.6 times faster relative to the bulk for a stretched polystyrene film.’ 62  Temperature (°C) 140  120  100  100  80  40  60  200  300  20  400  Time (mm.)  Figure 3.6. The tilting angle of the ester CH 3 group with respect to the surface normal estimated using the measured Q . The sample thickness is 200 nm. 0  As shown in Fig. 3.6, the tilting angle, 0, of the ester CH 3 group with respect to surface normal can be estimated using the ratio of  and  described in  equation (3.3). Assuming an azimuthally isotropic surface, the macroscopic susceptibility  -38  -  tensor  has only four independent non-vanishing elements, which are  x = x,  =  with  and  along the surface normal and  x  =  x  in the plane  of incidence in the laboratory coordinate system.’ 55 The effective nonlinear susceptibility  and  can be written as =  ) cos /3 cos /3 sin fl 2 5 (cot )L (cot )L (a) —L 2 ) 5 —L(a) ) 2 (a),)L(a) 55 1 cosfi cosfi L sin,8 2 5 +L(a) 1 ) 2 sinfi L(a) cosfi 1 cosfi 2  =  1 sinfi 21 3 ) 5 +L ) 1 (a) 55 ) 2 sin,8 L(tv L sin/3  (3.5)  L,(a) ) 5 L,, (a )L 2 ) sin fi (co 55 2  (3.6)  where /3j and /32 are the incident angles of visible and IR respectively, /3 is the reflected angle of SFG, and L 11 are the tensorial Fresnel factors. The second order nonlinear susceptibility =  (2)  is related to the hyperpolarizability a’ 2 by  5 ((.aX34X.o))a N  (3.7)  a,b,c  where N 5 is the surface density of molecules, and 1JI and â describe the laboratory  and molecular coordinates, respectively. Assuming a delta function distribution of 0, for the symmetric stretch of a methyl group with C 3 symmetry, a[cosO(1 + y)  =  =  =  where a  =  =  3 0(1 cos  a[(cos0 —cos 5 -N 3 0)(l —y)]  a[y cos 0 + cos 5 N 3 0(1 y  —  = abbC  /  ,  —  y)]  —  y)]  9 can be written as’° (3.8) (3.9) (3.10)  and 0 is the angle of the molecular symmetry axis ê with  respect to 2 in the laboratory coordinate system.  -39-  The tilting angles 6 shown in Fig. 3.6 were derived using equations (3.3), (3.5), (3.6), (3.8), (3.9), and (3.10). The estimate shows that the surface ester CR 3 group is approximately tilted  600  surface transition is only  with respect to the surface normal. The change in 6 for the 10.  During the cooling process, the ester CH 3 group reoriented  slightly toward the surface normal. It should be noted that during the above analysis, we assumed a ö-distribution of the orientation angles of ester methyl groups. This is a assumption commonly used when deriving molecule orientation. Usually, surface functional groups do not have the same orientation f’(8)  =  angle.  Cexp{—  Therefore,  (8)]  more  generally,  a  Gaussian  distribution  can be adopted, where C is normalization constant and a is  the root-mean-square width.’ ° However, when using the Gaussian distribution to fit the 6 SFG spectra, one can not get the values of Oo and a simultaneously. Assuming a delta distribution, the change in 6 for the surface transition is ester methyl group change its orientation by  10,  10.  It does not mean that every  but it indicates that the orientation  change is very small during the surface relaxation. Whether surface relaxation temperatures are generally lower than the bulk relaxation temperatures for other polymers remains a subject for future study. The surface relaxation processes can be further studied on PMMA samples with different molecular weights. It has been shown that the glass transition temperature of PMMA is affected by the molecular weight.’ ’ Similarly, the molecular weight can affect the PMMA surface 7 ” 70 relaxation processes. Also, PMMA with different tacticity can be studied, such as isotactic PMMA, as the tacticity of PMMA also affect its glass transition 73 Furthurmore, poly(alkyl methacrylates) with longer side chains, i.e. ” 72 temperature.’ -40  -  poly-(n-butyl methacrylate) (PBMA), poly(isopropyl methacrylate) (PPMA), and poly(ethyl methacrylate) (PEMA) can also be studied. It has been found that the cooperativity of the side chains and the backbone are different for Poly(alkyl methacrylates) with different side chains. 174-176 It is expected that the length of side chains would also change the effect of polymer back bone on the functional groups on the side chains.  34 Summary In summary, we have identified a new surface relaxation at 67 °C on the free surface of PMMA films. This temperature does not match any known bulk structure relaxation temperature and is independent of the film thickness in the range of 0.1  —  0.5  p.m. It is assigned to a depressed surface a-relaxation due to the free-surface effect. Our time-resolved measurements indicated that the surface polymer chains have a higher mobility in comparison to the bulk chains with a structure relaxation about 2.7 times faster than that of the bulk.  -41  -  Chapter 4 Electronic and Conformational Properties of the Conjugated Polymer MEH-PPV at a Buried Film/Solid Interface Investigated by Twodimensional JR-visible Sum Frequency Generation  4.1. Introduction The optical and electronic properties of poly[2-methoxy, 5-ethyl (2’-hexyloxy) para-phenylenevinylene] (MEH—PPV) have been studied intensively because of its broad applications in organic devices.’ 82 The polymer has found its applications in light’ 77 emitting diodes (LED)  177,183  , transistors 84 photovoltaics’ , and flexible displays 185 . 183  MEH-PPV is characterized by a it-conjugated backbone, in which the it-electrons are delocalized over several monomer units along the carbon chains, forming it-bands. 87 ” 186 Because the delocalized orbitals are half-filled, the energy gap between the filled and empty bands results in semiconducting properties. The extent of delocalization of the  it-  electrons, the so-called conjugation length, determines the energy gap, which plays a major role in the optical and electrical properties of the materials and the performance of the organic devices they are used in. The longer the conjugation length, the lower the gap energy is. The bulk electronic and optical properties of MEH-PPV have been studied  -42  -  extensively by UV/visible absorption and photoluminescence spectroscopy, and have been shown to be highly dependent on the conformation of the polymer ’ 80 ” 98 chains. 9 8 Mechanisms leading to a finite conjugation length in the polymer due to abrupt flips’ 92 ” 91 and conformational disorder’ 93 have been proposed. Despite enormous efforts, these organic semiconducting materials and optimization of the organic devices using them are still not well understood. Both experimental and theoretical investigations are required to meet this challenge. Compared to the bulk properties, the optical and electronic properties of conjugated polymers at a buried polymer/solid (film/solid) interface remain unexplored. In an organic device, the charge carriers, both electrons and holes, have to be injected through polymer/solid interfaces. Therefore, the band gaps of conjugated polymers at buried interfaces are important factors that affect the charge injection and overall efficiency of the organic devices. Because of the surface confinement effect at a polymer/solid interface, the surface chain conformation and the surface band gap are expected to be different from those in the bulk. However, it has been a great challenge to measure the buried interfacial electronic states because of a lack of a suitable probing technique. Traditional techniques based on ultra-high vacuum are not applicable to a buried interface, and absorption and emission spectroscopy do not have the necessary surface sensitivity. Recent developments in two-dimensional (2D) JR-visible sum frequency generation (SFG) spectroscopy have made it possible to study the optical properties of the conjugated polymer at a buried interface. 6 Traditionally, JR-visible SFG ” 113 vibrational spectroscopy has been carried out by tuning the incident JR frequency to  -43  -  obtain a surface vibrational spectrum, which reveals the surface chemical species. In this case, the visible frequency is fixed. Recently, it has been shown that one can obtain a surface electronic spectrum by tuning the incident visible frequency.” 6 As shown in ” 2 Fig. 4.1B, SFG intensity is doubly enhanced when the JR is resonant with the vibrational state and the SFG is resonant with the surface electronic state. With the capability of  tuning both the incident JR and visible frequencies, 2D SFG spectroscopy becomes a highly selective surface probe for studying the surface electronic states coupled to a specific vibrational mode. In this study, 2D SFG was used to measure the buried surface electronic states associated with the C-C stretching mode of benzene rings at the backbone of MEH-PPV, as indicated in Fig. 4.1C. Based on the measured SFG electronic spectrum, the conjugation-length distribution of MEH-PPV at the buried interface was estimated using an oligomer model.  (A)  (B) eO)  (C) Visible 1R  SFG  MEHPPV ‘  .fl)OIR 1 .  Figure 4.1. Energy diagrams of (A) JR-resonant SFG and (B) JR-visible doubly-resonant SFG.  I gO>  is the ground state, I gi> is the first vibrational excited state, and  I eO>  is the electronic excited state at vibrational ground state. (C) Experimental SFG setup for probing the buried interface and the structure of MEH-PPV.  -44  -  4.2 Experimental Section As shown in Fig. 4.1C, 2D SFG involves mixing a frequency-tunable visible beam  (j)  and a frequency-tunable infrared beam  beam with a frequency  a)SFG  =  co +  ’ 10 (0LR  (COIR)  on a surface to generate a third  The visible and IR beams were overlapped  spatially and temporally on the sample at incident angles of 45° and 55°, respectively. Because of the large absorption coefficient of MEH-PPV in the visible region, the fluence of the visible beam was kept below 10 /nn 2 per pulse to avoid photo-damage. All SFG spectra were normalized against a z-cut quartz crystal. MEH-PPV (molecular weight  55,000) was purchased from Sigma-Aldrich, Inc.  The polymer films were spin-coated at —2000 rpm on CaF 2 windows from a 2% w/v tetrahydrofuran solution. Films were annealed at 100 C for several hours to evaporate the solvent in the film before spectroscopic measurements were taken. CaF 2 was chosen because of its high transmission of IR. To ensure the measured SFG is truly generated at 2 interface without a contribution from the air/polymer interface, the buried polymer/CaF the spin-casting process was repeated several times to obtain a thicker film. Because of the large absorption coefficient of MEH-PPV in the visible region, the incident visible beam and the SFG generated from the air/polymer interface were mostly absorbed by the film. Based on the absorption coefficient of MEH-PPV’ , a film thickness of 94  1 pm  would guarantee that the incident visible beam and the SFG generated from the air/polymer interface in the reflected direction are at least 95% blocked for wavelengths shorter than 580 nm. However, the visible absorption spectra of MEH-PPV were measured using thin films.  -45  -  4.3. Results and Analysis  6  U) C -a  C  (3  LI (I)  2  0  1500  1600  1700  IR Wavenumber IR (cm ) 1  Figure 4.2. SFG vibrational spectra of MEH-PPV at the buried interface with various incident visible wavelengths. The dots are experimental data, and the plots are the fitting curves using equation (2.24). The vibrational peak centered at 1593 cm’ is the C-C stretching of benzene rings. The fitting parameters are summarized in Table 4.1.  -46  -  Table 4.1 Fitting parameters for the SFG spectra in Fig. 4.2. The resonance frequency (0 the phase ,  the dephasing constant  b, and  F are the best fit for all SFG spectra. The peak amplitudes A and the nonresonant  second order nonlinear susceptibility  ‘5 are for individual ones.  Visible  a  Peak  A  wavelength  (cm’)  assignments  (arb.u.)  480  1593  c-c  2.04± 1. 13  0.26  -0.91  16.3  500  1593  c-c  3.66±0. 76  0.33  -0.91  16.3  520  1593  c-c  5.95±1.01  0.28  -0.91  16.3  540  1593  c-c  8.89±1.23  0.42  -0.91  16.3  560  1593  c-c  13.20±0. 60  0.46  -0.91  16.3  580  1593  c-c  16.03±1.07  0.36  -0.91  16.3  600  1593  c-c  13.48±1. 07  0.31  -0.91  16.3  620  1593  c-c  11.27±1.00  0.30  -0.91  16.3  640  1593  c-c  7.85±1.24  0.29  -0.91  16.3  660  1593  c-c  4.28± 1. 17  0.34  -0.91  16.3  0  1’ (cm’)  (urn)  Fig. 4.2 shows the SFG vibrational spectra from the MEH-PPV/solid interface with various visible wavelengths in the ssp configuration (s-, s- and p-polarized for SFG, visible, and IR, respectively). The vibrational band centered near 1593 cm 1 is assigned to the  c-c  stretching of benzene rings located at the backbone of MEH-PPV. 195 The  wavenumber of this mode has been reported between 1583 and 1593 cm 1 depending on  -47  -  the molecular weight.’ 96 The observed wavenumber at 1593 cm 4 is consistent with the previously reported value for MEH-PPV with a molecular weight of 6x10 4 g!mol.’ 96 Similarly, the ssp-SFG vibrational spectra from the air! MEH-PPV interface with various visible wavelengths are shown in Fig. 4.3.  7  •  I  I  •  Ie.I.%  =660nm  6  co V’s =640nm ••  5  Vis  =620nm  co=600nm  •  D  .ri  Cl)  c  •.  580nm  .  co=560 nm  2  .1-’  .E 0  co Vis =540 nm  LII  Cl)  0) V’s 520 nm co=500 nm 0) =480nm  0 I  1500  I  1600  IR Wavenumber  1700  OIR  ) 1 (cm  Figure 4.3. SFG vibrational spectra of MEH-PPV at the airIMEH-PPV interface with various incident visible wavelengths. The dots are experimental data, and the plots are the fitting curves using equation (2.24). The vibrational peaks located at 1595 cm 1 are the CC stretching of benzene rings. The fitting parameters are summarized in Table 4.2.  -48  -  Table 4.2 Fitting parameters for the SFG spectra in Fig. 4.3. The resonance frequency 0), the phase the dephasing constant  ç$ and ,  F are the best fit for all SFG spectra. The peak amplitudes A and the nonresonant  second order nonlinear susceptibility  Visible  %5 are for individual ones. Peak  A  F  ) 1 (cm  assignments  (arb.u.)  (cm’)  480  1593  c-c  5.27±0. 73  0.06  1.97  18.0  500  1593  c-c  7.49±0. 77  0.07  1.97  18.0  520  1593  c-c  9.53±0.96  0.03  1.97  18.0  540  1593  c-c  12.00± 1. 12  0.09  1.97  18.0  560  1593  c-c  15.35±1.05  0.08  1.97  18.0  580  1593  c-c  16.63±1.01  0.06  1.97  18.0  600  1593  c-c  13.73±1. 03  0.11  1.97  18.0  620  1593  c-c  10.81±1. 06  0.07  1.97  18.0  640  1593  c-c  7.07±1.04  0.03  1.97  18.0  660  1593  c-c  5.88±0. 86  0.08  1.97  18.0  wavelength (nm)  As shown in Fig. 4.2 and 4.3, when the incident visible beam is near 580 nm, the SFG intensity was enhanced. Referring to Fig. 4.1B, the SFG wavelength is at 530 run with a visible wavelength of 580 inn and an JR wavenumber of 1593 cm’. Because of the electronic resonance, the refractive index of MEH-PPV in the investigated region is 197 Since the Fresnel factors are related to the refractive indexes, wavelength-dependent.  -49-  as shown in equations (2.20), the Fresnel factors need to be considered to obtain the dispersion relation of the second-order nonlinear susceptibility. In the electric-dipole approximation, the SFG arising from the second-order polarization can be written as’° 9 I(OSFG  = WIR +a)  (4.1)  X:E(coIR)E(coVIS)  where %? is the effective second-order nonlinear susceptibility tensor, and  E(aJR)  and  E(a) are the input fields. In the ssp configuration, the effective second-order nonlinear susceptibility  can be written as =  (°‘SFG  ,, (a )L 3 )L  (COIR  ) sinCBIR )X  (4.2)  with (2))=  Q 2n ) 1 ) cos(,6 v ( n ) ) 1 cos(/3 a+n (a)cos(y) 2  (43)  L(a,)=  Q 2 2n ) 1 ) cos(fl i 1 (w) cos(y) + n n 2 (cot) cosC8 ) 1  (4.4)  L  where  is the yyz component of the second-order nonlinear susceptibility in the  laboratory coordinate (defined in Fig. 4.1 C),  (a) is the Fresnel factor, n 1 is the  refractive index of medium i, /9 is the incident angle, and y. is the refracted angle. As described in section 2.5, there are two types of processes in IR-visible SFG. The first type starts with an electronic transition followed by a vibrational transition (Vis IR), and the second type begins with a vibrational transition followed by an electronic transition (IR-Vis).” 2 Because the electronic relaxation times are generally much shorter  -50  -  than the vibrational relaxation times, the contribution of the Vis-IR SFG is generally 13 Therefore, only the IR-Vis SFG will be considered in the following negligible.’ calculation. Assuming harmonic potential surfaces for the electronic states and the Born— Oppenheimer approximation, the IR-Vis doubly resonant  (2)  XUk  —  /  N j , I’egPge h  ô,t1g’g  1 8q  IR1+1”1  1 n!  can be described as 112  —  (DsFG fl0, COeg +ZT en  go  1 WSFG —(n + l)a, COeg + iTen+i gO j  /  NR,zjk  (4.5) where N is the surface molecular density, p represents the i  —  component of electronic  transition moment, q 1 is the normal coordinate, S is the Huang—Rhys factor, n labels the vibrational state, g and e label the ground and excited electronic states, respectively, COSFG  is the SFG frequency, a and  frequencies, respectively, F 1 and  COeg  engo T  are the resonant vibrational and electronic  are the damping constants, the angular brackets  indicate an average over molecular orientations, and  describes the non-resonant  contributions. Equation (4.5) includes all the vibronic transitions series (Huang-Rhys series). There are successive resonant terms describing transitions with frequencies  °eg +  z  in resonance with vibronic  n a for n=O, 1, 2, etc. However, experimentally the  non-zero vibronic transitions have not been previously observed because the vibronic transitions have much shorter dephasing times than the zero-vibration transition. 116 For ’ 3 MEH-PPV, the dephasing times of vibronic transitions are in the femtosecond region.’ 98  -51  -  Therefore, by assuming F en go >> T eo,go  ,  the non-zero vibronic transitions can be  neglected, and equation (4.5) can be simplified as (2)  —  —  —  /  N , 7 Peg fige 8q h 1  X 1 +iF, IR —CO 0  1 WSFG 0 eg +lTeogo  \/ /  (2)  +  XNR  (4.6)  It is worth pointing out that a significantly larger Ten go also suppresses the aforementioned Vis-IR SFG, which starts with an electronic transition followed by a vibrational transition. Based on equation (4.6), the vibrational spectra shown in Fig.4.2 can be fitted using A 1 (2) • +XJy +iF co 1  (2) (DIR  with A 1 describing the electronic resonance. For a system with a single electronic resonance COeg A 1 has the following form cj 1 A —  eg 0  +  iT eO gO  (4.8)  Equations (4.l)-(4.4) and (4.7) were used to fit the SFG vibrational spectra in Fig. 4.2 and 4.3 with A , 1  , 1 CU  F,, and  X•k  as the adjustable parameters. The fitted values of A 1 for  various visible wavelengths are plotted in Fig. 4.4. The 2D SFG electronic spectrum at air/MEH-PPV interface is red shifted with respected to an absorption spectrum, which peaks at 500 nm. As discussed earlier, the red shift shows that the electronic band gap at surface is lower than that for the bulk film, which indicates longer conjugation lengths at the surface.  -52  -  Absorption Wavelength (nm) 590 450 50  400 20  • • —  MEH-PPV/Solict Interface Air/MEH-PPV Interface Bulk Film  15  0.02 C.) C CD  .o f,(v  10.  0 C’) .0  SFG Wavelength (nm)  Figure 4.4. The absorption spectrum of bulk M.EH-PPV film (solid line) and the surface SFG electronic spectra of MEH-PPV at MEH-PPV/solid (a) and airfMEH-PPV interfaces (.).The dashed lines are theoretical fitting curves.  For MEH-PPV, multi-electronic resonances should be considered. The band gap of a conjugated polymer is related to the conjugation ” 98 length. 8 6 Conjugated polymer 7 chains consist of a series of connected segments, each of which has a different extent of t-electron delocalization. The extent of the conjugation is limited by the twists in the polymer backbone. The longer the segment is, the smaller the band gap. The theoretical methodology for describing the properties of conjugated polymers remains an active research area. 199-203 Most theoretical work on predicting the optical properties of a conjugated polymer use an oligomer approach. 204 In this method, the properties of oligomers of various chain lengths are calculated and then treated as separated subunits. Although the oligomer model does not have a full description of the material properties, -53  -  such as medium effects and oligomer , 206 it has produced reasonably good ’ 205 interactions agreement for the optical properties of MEH-PPV 207 and will be adopted in the following analysis. To estimate the corresponding conjugation-length distribution at the interfaces based on the SFG electronic spectra in Fig. 4.4, it is assumed that the surface polymer chains consist of oligomers with various conjugation lengths. For a conjugation-length distribution function D(N), the SFG electronic spectrum is a sum of contributions from oligomers of different lengths, and the amplitude A 1 in equation (4.8) can be modified as D(N)  1 A N WSFG  —  (4.9)  COe(N) + Te 0  where coe(N) = EN/h is the electronic transition frequency associated with an oligomer of N monomer units. For MEH-PPV, Chang et al. have shown that the energy levels for oligomers of N monomer units can be described by 207 +2flcos() 0 EN =E with E 0  4.3 eV and /3  =  (4.10)  —1.1 eV. 202 In this expression, the energy levels with N  located in the UV region. Therefore, only segments with N  2 are  3 will be considered in the  fitting. Assume a Gaussian conjugation-length distribution function: D(N) = exp[—(N—N 2 /.2j ) 0  (4.11)  The center conjugation length N 0 and the distribution width  for MEH-PPV at the  interface can be derived by using equations (4.9)(4.1 1) to fit the SFG electronic spectrum in Fig. 4.4. The corresponding fitting curve is shown in Fig. 4.4. For the MEH PPV/solid interface, the best fit was obtained with N 0  =  5.8  ±  0.2, o  =  0.9  ±  0.1, and  -54  -  eO  =640±40 cni’. The conjugation-length distribution curve is shown in Fig. 4.5A. For  comparison, 2D SFG measurements were also carried out at the air/MEH-PPV interface. As shown in Fig. 4.4, the SFG electronic spectrum at the air/MEH-PPV interface is slightly broader than that at the MEH-PPV/solid interface. The best fit was obtained for the air/MEH-PPV interface with N 0  =  5.1  ±  0.2 and o-  1.3  ±  0.1.  A theoretical estimate for the conjugation-length distribution for a bulk film is not available for direct comparison with the current fitting results. However, these fitting results are within a reasonable range when compared to N 0  =  5 and o  =  1.8 obtained by  Chang et al. for MEH-PPV in chloroform solution. A comparison of the polymer/solid and air/polymer interfaces indicates that a rigid surface confinement at the solid surface produces a longer average conjugation length with a narrower distribution width. A non  rigid surface confinement at the air/polymer interface shows a shorter average conjugation length with a larger distribution width. The difference is larger than the fitting error, since the error bars for the SFG electronic spectra do not overlap, as shown in Fig. 4.4.  -55  -  0.6  0.4  D02 1  C  0.0 I-  Cl)  0  0.4 ci)  0.2  0.0 2  3  4  5  6  7  8  9  10 11  12 13  Conjugation Length (No. of Monomer)  Figure 4.5. Calculated conjugation-length distributions of MEH-PPV at (A) MEH PPV/solid interface and (B) air/MEH-PPV interface.  There might be some other possibilities for the differences in the PMMA electronic transition energies in the bulk and at interfaces. For example, the electronic transition energies of t electrons are also associated with the chemical environments of the polymer backbone. If the excited-state dipole moment is smaller than that of the ground state, then increasing the polarity of the environment will stabilize the ground state more than the excited state, resulting a blueshift of the transition energy. 98 Conversely, if the excited-state dipole moment is larger than that of the ground state, then increasing the solvent polarity will stabilize the excited state more than the ground state, producing redshifted transition energy. 98  For MEH-PPV, it has been experimentally  -56  -  measured that the ground state of most of the chromophores in annealed MEH-PPV films possess a larger dipole moment than the excited state. 98 This indicates that if MEH-PPV is exposed to a polar environment, such as CaF 2 window we used in this experiments, the electronic transition energy should be increased. However, as shown in Fig. 4.3, the interfacial electronic transition energy at MEH-PPV/CaF 2 interface features longer wavelength, thus lower energy. The effect from the polar environment from CaF 2 can not give a satisfactory explanation. Our discussion on the conjugation length provides a reasonable possibility. At the interfaces, especially the MEH-PPV/air interface, the polymer chains tend to have increased conjugation lengths because of the asymmetric forces from the two bulk media. Since the polar environment effect does exist at the interface, our analysis about the longer polymer chain conjugation lengths at interfaces might be underestimated.  4.4. Summary The optical, electronic and conformational properties of MEH—PPV located at interfaces were studied by 2D fR-visible SFG. Surface SFG electronic spectra obtained at the MEH-PPV/solid and air/MEH-PPV interface show that the surface band gaps and conjugation-length distributions of MEH-PPV are sensitive to surface effects. Based on the measured SFG electronic spectra and an oligomer model, the oligomers at the buried interface were estimated to have an average conjugation length of 5.8 monomer units and a distribution width of 0.9 monomer unit. Similar surface effects were also observed at the air/polymer interface with a shorter average conjugation length of 5.1 monomer units and a broader distribution width of 1.3 monomer units.  -57  -  Chapter 5 Competition Adsorption of Toluene and Heptane on Silica Surfaces 5.1. Introduction The extraction of bitumen from oilsands with minimum usage of water is of great interest. One approach is to use organic solvents, which must be selected and applied to maximize the recovery of both bitumen and solvent from the oilsands. Therefore, the interfacial properties at solvent/mineral interfaces need to be better understood to make the extraction process efficient and energy-saving. Liquid binary systems have been studied theoretically and experimentally. 208223 The research results have shown that their physicochemical properties significantly deviate from ideal 208 mixtures. 10,213-217,224-226 2 ’ Molecular properties at interfaces of binary systems are usually studied theoretically 227 230  The main reason for the limited number of experiments is due to the lack of effective  methods to probe such interfaces. Recently, Bowers and coworkers ’ have applied 23 neutron reflectometry to study the composition of the film on octadecylcoated silicon surface formed from n-hexane-d14/perfluoro-n-hexane mixture above and below the temperature. They found that near the solid surface, n-hexane content is larger ’ critical 23 and decays toward the bulk. But because of the limited z resolution, the measurements might not be performed exactly at the interface. Sum frequency generation (SFG) is interface sensitive thus very suitable in these cases. Recently, SFG has been used in studying the water-alcohol binary liquid mixtures at air 233 and silica surfaces. ’ 232 234  -58  -  Currently, SFG is the only technique to yield interface-specific vibrational spectra for a buried interface. In this chapter, we investigate the competitive adsorption of heptane and toluene on fused silica using SFG vibrational spectroscopy. The bulk properties of heptane/toluene binary mixtures have been previously studied, including the molar 236 density, ’ 235 235 dynamic viscosity, volume, 236 diffusion coefficient, 237 surface tension, 238 238 thermal conductivity, density, 239 isothermal compressibility, 240 and sound speed. ° 24 Currently, very little is known about interfacial properties at solvent/mineral interfaces. The objective of this study is to establish a methodology to obtain the adsorption isotherm for binary solvents at a liquid/solid interface.  52. Adsorption Isotherm The Langmuir isotherm was developed by Irving Langmuir in 1916 to describe the pressure dependence of the surface coverage of molecules on a solid surface. The isotherm has also been applied to liquid adsorption on 24 solids. 1-246 For single solute adsorption on solid surface, the adsorption kinetics can be expressed as (5.1)  —cS)—kdcs  where  Cs  15  the adsorbed solute density on the solid surface, c  is the solute  concentration in solution, and c 1 is the solute adsorption saturation density on the solid surface. When the adsorption process reaches equilibrium, the derivative (5.1) is equal to zero. So we have ) = kdCS 5 kaC(Ci —C  c  =  1 CC  ka c + kd  (5.2)  -59  -  The fraction of sites that are occupied on silica is (5.3) Cl  Inserting equation (5.3) into equation (5.2), we have kC  =  =  kaC+kd  where Ka =  KaC  (5.4)  KaC+1  is the adsorption equilibrium constant. Equation (5.4) is the Langmuir  adsorption isotherm for liquid on solid. And Ka is associated with the adsorption free energy of the solute on solid AG by Ka =exp(  -AG  (5.5)  RT  where R is the gas constant and T is temperature. For a binary liquid mixture, there would be competing adsorption. Let us assume the components can only adsorb to the available surface sites, which are free of adsorbates. Under this assumption, the process of the replacement can be considered as a two step process: the desorption of component 1 and the absorption of component 2. Therefore, the adsorption kinetics is then expressed as dc  1 = kaiCi (c,  =  (CI kaIC 2  —  1 c  —  1 C  —  ) 2 c  —  —  c)  kdicSl  —  cS kd 2  (5.6a)  (5.6b)  Here c 2 are the molar concentrations of the components 1 and 2. And since this is 1 and c a binary liquid system, we have +c 1 c = 2 l  (5.7)  -60  -  Similarly, when the equilibrium is reached, we set equations (5.6a) and (5.6b) equal to zero and solve for c 1 and . 52 It can be shown that c =  cS  1  + Kai[<a Cui C 2  —  K_C 1 Ka 1 C 2 ) 2  KaiCi + Ka C +1 2  —  22 c CI + Kai.Ka (KU 12 1 C 9  ai’<a2CiCii)  C52—  KaiCi +Ka C 2  +1  (5.8a)  (• )  When the effect of uneven saturation capacities is considered , the adsorption 247 saturation capacities of components 1 and 2 are related as a=-  (5.9)  12 C  The factor a means that to adsorb on the surface, one component 1 molecule needs a times larger surface area than that of one component 2 molecule. The adsorption kinetics is then 1 dç 11 kaiCi (C  =  dt  C(C 1 k  —  51 C  —  ) 52 ac  —  kdlCSl  —c 2 1 cS )—kd 52 —C a  (5.lOa)  (5.lOb)  Solving equations (5.lOa) and (5.lOb) gives ci 2 c 7 —aKaiKa ) KaiCi +Ka C +1 2  = Ci(KaiCii 2 K 1 +Ka 1 C 1 a Ci  2 (Ka c 12 + 2 C 2 CiCi KaiKa =  —  (5.lla)  CiCii) 2 KaiKa a  KaiCi +Ka C +1 2  (5.llb)  Inserting equation (5.9) into equations (5.11), we obtain C  =  CiKaiCii  K  + Ka 2 +1 C 9  (5.12a)  -61  -  = C 2  K 1 C 2 2 a <aiCi 1  + .Ka C + 2  (5.12b)  1  The surface coverage of component i is defined as 0  =  -.  Then the adsorption  cli  isotherms have the form 0  02  =  CiKai  (5.13a)  KaiCi +Ka C +1 2 =  Ka C 2 C +Ka 1 Ka C 2  (5.13b)  +1  Equations (5.5), (5.7), and (5.13) will be used to fit the experimental results and calculate the adsorption free energies of toluene and heptane on silica surface.  5.3. Experimental Setup and Sample Preparation The fused silica windows (CASIX  )  were prepared by dipping in detergent  (Extran AP12 alkaline) and a mixture of sulfuric acid (Fisher Scientific) and nitric acid (Fisher Scientific) for ten and three hours respectively, then rinsing in distilled water with a resistance of 18 M2cm. Then the silica windows were blow dried by nitrogen gas. This procedure is known to produce fully hydroxylated silica surface with a surface density of silanol of ‘.-5/nm 249 The silica surface will be discussed later in the ’ 2248 discussion part. To minimize the presence of water molecules on the silica windows, they were baked in a home-made heating cell at 180°C with the presence of nitrogen gas for 1 hour before use. Toluene (HPLC grade, Fisher Scientific) and heptane (HPLC grade, Fisher Scientific) were used as received. The water content is 0.005% in toluene and  -62  -  0.002% in heptane. The solvents were mixed with different ratios before they were transferred to the spectroscopic cell with silica windows. The experin:ntal setup is shown in Fig. 5.1. The visible beam wavelength is fixed at 532 nm with the energy of 350 J. The JR beam is tunable from 2800 cm’ to 3900 cm ‘with the energy of  -.‘  200 tJ. The incident angles of visible and JR light are 45° and 55°  respectively.  WV’S  WSFG aVIS  eSFG  Silica sample -. -  slilca  -  -  :  IR 9  WIR  Figure 5.1. Experimental setup. The sample is placed between two silica windows and sealed with a Teflon 0-ring. The visible beam with frequency  IR 0  a and JR beam with frequency  are incident on the bottom sample/silica interface from different sides with incidence  angles of  O and  IR 0  respectively. The generated SFG signal  SFG 0  at angle  SFG 0  is then  measured.  -63  -  5.4. Results and Data Analysis 5.4.1 SFG of Pure Toluene and Heptane on Silica Surface The SFG spectrum of pure toluene on silica surface is shown in Fig. 5.2. The fitting parameters are summarized in Table 5.1. The spectrum was obtained using ssp polarization combination (s-, s-, and p-polarizations for SFG, visible input, and JR input, respectively). The dots are the experimental data and the solid curve is the fitting result using equation (2.24). The resonant JR frequencies a, dephasing rate constants F, peak amplitudes  A, and the relative phases 0 of the five peaks displayed in Fig.5.2, together  with the nonresonant second order nonlinear susceptibility  z  are the fitting parameters.  The peaks at 2864 cm’ and 2955 cm’ are assigned to the asymmetric and symmetric stretch of the methyl group, and the peaks at 3021 cm’ and 3081 cm’ are assigned to the 20b and 20a phenyl group stretch ’ 25 The 20a and 20b stretching modes are 250 modes. sketched in Fig. 5.3. The broad peak around 3200 cm’ comes from the trace water. By comparison with SFG spectrum of pure water on silica surface, as shown in the inset of Fig. 5.2, the water content at silica surface can be estimated. The water peak amplitudes of pure water are 12.9 and 11.7 at 3200 and 3400 cm’ respectively. The peak amplitude of water peak at 3200 cm 1 in toluene is 4.2. The water peak amplitude for pure water is about 6 times stronger than that for toluene. Since SFG amplitude is proportional to the molecule numbers being probed, as shown in equation (2.23), it is estimated that the water coverage at silica surface is roughly 15%. Here, we assume water molecules have similar structure on the silica surface when they are in pure water and organic solvents. It is difficult to completely remove water molecules on silica surface under ambient  -64  -  conditions, therefore, the adsorption of heptane and toluene only take place on silica surface where no water molecule is present. The water peak stayed roughly constant during the experiment period, therefore it is reasonable to assume that the number of available sites for heptane and toluene are roughly constant. No peaks from silanol groups have been observed in the current experiments, which may be too weak to be detective.  D  .r I— Cu  U)  0)  (9  LL C,)  2800  3000  3200  3400  3600  Wavenumber (cm ) 1 Figure 5.2. ssp SFG spectrum of pure toluene on silica surface. The dots are the experimental data and the plot is the fitting curve using equation (2.24). The 2864 cm’, 2955 cm’, 3021 cm , 3081 cnf’ peaks are assigned to the methyl symmetric and 1 asymmetric stretches, and phenyl 20b and 20a stretching modes respectively. The inset is the SFG spectrum of pure water on silica surface  -65  -  Figure 5.3. The phenyl 20a and 20b stretching modes 252  Table 5.1 Fitting parameters for toluene SFG spectrum in Fig. 5.2  a  Peak  (cm’)  assignments  2864  -ss 3 ct-CH  2955  A  F (cm’)  0.01  1.08  -0.79  7.0  -as 3 ct-CH  3.63  -0.36  16.9  3021  Phenylring2ob  3.57  0.77  15.7  3081  Phenyl ring 20a  1.43  0.21  22.4  3170  water  4.16  -0.39  138.4  We tried to use sodium metal to react with and thus eliminate water in the toluene and heptane samples, but this drying process did not make any difference on our SFG spectra. The water peaks around 3200 cm 1 exist on the dried toluene samples. It has been known that water has two stretching modes at 3200 and -3400 cm , and they are 1 assigned to the bonded OH in ice-like and water-like structures respectively.’ 57 The fact that only the peak from ice-like structures is present indicates that the trace water molecules are well ordered at the silica surface. The relatively strong interaction from  -66  -  silanol groups aligns the water molecules at the silica surface. This might be the reason why it is difficult to eliminate water molecules from the silica surface. Fig. 5.4 shows the typical ssp SFG spectrum of heptane on silica surface. The fitting parameters for the spectrum are summarized in Table 5.2. The peaks at 2864 cm’ and 2955 cm’ are assigned to the symmetric and asymmetric stretching modes of the methyl groups of heptane, and the 2935 cm 1 peak is assigned to the Fermi resonance of the overtone of the methyl bending mode with the methyl symmetric stretching mode. It should be noted that in heptane SFG spectrum, the methylene peaks are much weaker than those of the methyl groups and even undetectable. Heptane is a molecule with the 2 groups in all-trans conformation, which represent a centrosymmetric arrangement of CH the methylene groups. When it is adsorbed on the surface, the methylene groups at the interface are in the same environments and experience identical forces, so their  (2)  cancel out. Therefore, there is no obvious SFG signal for the methylene groups. Other SFG experiments on n-normal alkane and alcohol have reported the same phenomena. 2 ’ 253 54  -67  -  D -D  I  U) C  G)  .1-i  C  0  U 0  2800  3000  3200  3400  3600  Wavenumber (cm ) 1 Figure 5.4. The ssp SFG spectrum of pure heptane on silica surface. The dots are the experimental data and the plot is the fitting curve using equation (2.24). The 2864 cm’, 2935 cm , 2955 cm 4 1 peaks are assigned to the methyl symmetric, Fermi resonance, and asymmetric stretching modes respectively.  Table 5.2 Fitting parameters for heptane SFG spectrum in Fig. 5.4  a  Peak  (cm’)  assignments  2864  -ss 3 c-CH  2935 2955  A  F ) 4 (na  0.00  3.01  -1.04  12.4  -Fermi 3 x-CH  2.87  0.94  13.2  -as 3 c-CH  3.82  0.11  10.9  -68  -  5.4.2 SFG of Toluene and Heptane Binary Mixtures on Silica Surface We have studied toluene and heptane binary mixtures with toluene volume fractions of 12.5%, 25%, 37.5%, 50%, 62.5%, 75%, and 87.5%. With the toluene density 0.8669g/ml and molar mass 92.14g/mol, and heptane density 0.6849g/ml and molar mass 100.2lgIml, the toluene molar fractions in the mixtures were 16%, 31%, 45%, 58%, 70%, 81%, and 91% respectively. The SFG spectra at the interfaces of the binary solutions and silica were collected under the ssp and ppp polarization combinations. The ssp SFG spectra with different toluene molar fraction are shown in Fig. 5.5. The SFG spectra were fitted using equation (2.24). The amplitudes of the 20b peak of these samples are summarized in Table 5.3.  fJLL ..  i _jcI  (9  lfl-J  .J-.F  J1PJf  W.J  Jr  -j,j  r L  .31% -  -  I  2800  .16%  I  3000  3200  3400  3600  Wavenumber (cm ) 1 Figure 5.5. The ssp SFG spectra of toluene/heptane mixtures with changing toluene molar fraction. The dots are the experimental data and the plot is the fitting curve using equation (2.24). The peaks at 3021 cm_i indicate the amount of the adsorbed toluene on silica surface.  -69  -  Table 5.3. The ssp peak amplitudes A of the toluene 20b peaks obtained by curve-fitting the spectra in Fig.5.5 and the calculated surface coverage of toluene at silica surface  Toluene molar fraction x  Peak amplitude A (arb.u.)  Surface coverage 6  1.00  3.57±0.34  100  0.91  2.05±0.37  57.4±11.7  0.81  1.61±0.23  45.1±7.7  0.70  1.10±0.35  30.1±10.2  0.58  0.57±0.14  16.0±4.2  0.45  0.36±0.20  10.1±5.7  0.31  0.08±0.18  2.2±5.0  0.16  0.01±0.16  0.3±4.5  (%)  Since both toluene and heptane have methyl peaks, complicating the analysis, we only consider the toluene phenyl peaks to obtain the surface coverage of toluene. The 20b and 20a peaks are both from phenyl group. We chose to work with the 20b peaks, which are stronger in amplitude in all the spectra, to minimize the fitting error. The amplitude  4  of the SFG peaks in the laboratory-fixed coordinate system (x,y,z)  is related to the molecular hyperpolarizability, aylmn, in the molecule-fixed coordinate by a coordinate transformation and an average over the angular distribution  system  : 234 of the molecules f(2) =  N  f  avimn (j  l)(J  .  th)(k  (5.14)  1,m,n  Based on equation (2.16), the SFG intensity is proportional to (5.14), the amplitude  4  . 2 AVUk  From equation  depends on both the number density N 8 and the orientation of  adsorbed molecules. If the adsorbed molecules preserve the same orientation in different concentrated mixtures,  4  is proportional to the adsorbed molecules on the surface.  -70  -  Therefore, further analysis of the molecular orientation is necessary to obtain the coverage of toluene on silica.  Li Cu >  4-,  Cl)  ci)  4-,  (9 U  C,,  3600  Wavenumber (cm ) 1 Figure 5.6. The ppp SFG spectra of toluene/heptane mixtures with changing toluene molar fraction. The dots are the experimental data and the plot is the fitting curve using equation (2.24).  The ratio of  and  of the methyl symmetric stretch can be used to  estimate the orientation of methyl groups, as discussed in chapter 3. From the curve fitting and analysis of the spectra in Fig. 5.7, it is found that the methyl group of adsorbed toluene is tilted by 34 degree with respect to the silica surface normal.  -71  -  0.20 ssP  11  D  ppp  CU > C,)  0.10  G)  .4—’  -9 Cl) 0.05 Li. C.l)  4  4  48  48  V GV  0.00 2800  84  8 V  V8 1  v V  VV ,  3000  8 V VVV  V  3200  3400  3600  Wavenumber (cm ) 1 Figure 5.7. The comparison of ssp and ppp SFG spectra of pure toluene on silica. The  t  and V dots are the experimental data of ssp- and ppp-SFG and the piots are the fitting curves using equation (2.24).  This tilting angle is reasonable. A toluene molecule has a dipole moment of 0.36 D, pointing from the phenyl ring to the methyl group. The silica surface is characterized by a silanol layer of Si-OH after the preparation process, which exhibit several configurations shown in Fig.5.8. 255 The terminal H and 0 atoms are charged or partially charged, ’ 254 which means the silica surface has a dipole moment as well. More area of the silica surface is terminated by hydrogen atoms than oxygen atoms. Since the toluene molecule is much bigger than the OH groups on the silica surface, one toluene molecule has to reside above several OH groups. The overall dipole from these OH groups might be almost parallel to the surface normal. This dipole will align the toluene molecules near  -72  -  the silica surface to be almost parallel to the surface normal. The deduced toluene tilting angle of about 34° from the surface normal might be the result of the competition between different toluene molecules and other heptane molecules when they try to adsorb on the silica surface.  c(  I-L , 0 ,H,  I  Si  Si  H  I  I  Si  Si  c(’  H, H  Si  I  Si  fully hydrated silica surface  S  I  siIoxane  t isolated OH groups OH group  germinal OH groups  Figure 5.8. Schematic representation of a fully hydrated silica surface, and of different kinds of configurations of silanol groups.  256  Since both toluene and heptane have CH 3 vibrational modes, the aforementioned method to determine molecular orientation does not work for binary solutions. In binary solutions, the orientation of toluene can be verified using the ratio of the phenyl 20b stretch intensity under different polarization combinations, such as ppp and ssp. As discussed in chapter 2, the ppp and ssp SFG signal intensity is proportional to the effective second order susceptibilities  1  2 =  c cos 0 8&  2  Zeffppp 1112  and  (5.15a)  -73  -  =  8 c 0 cos  fl  Xs  (5.15b)  1112  The effective nonlinear susceptibilities nonnvanishing terms of  and  (2)  = —L (cot )L (cot )L (tv ) cos 2  cos /31 sin /32  J  sin fi cosfl 2  —  +  are associated with the  L (a )L (a) 1 )L (a ) sin /3 cos /7 cos fi2X 2 8 )L (a) (a) 1 )L 88 (a) ) sin /3 2  +  sin /1 sin fl x 2  1 )L 88 (a) L,,, (a) 8 )L,,, (a) ) sin /2 %‘ 2  (5.1 6a) (5.1 6b)  where Lfl (a),) are the diagonal elements of the tensorial Fresnel factor defined in equations (2.20). Since the phenyl group in toluene has C , symmetry, and 20b is an 2 asymmetric stretch, the nonvanishing components of microscopic hyperpolarizability are  fi J3 fi  and  =  l,b 3 /  =  = —;_Nsl3aca((cos ç’)  =  =  =  =  = Ns/3,a((cosO)—(cos where  ‘  cbb• 3 /  —  Then we have 257  (cos  c’))  N/3aca (cos  c’))  c,)  (5. 17a)  (5.1 7b) (5.17c)  is the tilting angle of the toluene.  From equations (5.15), (5.16), and (5.17), it is seen that I, /1 is the function of the tilting angle ç of toluene. Therefore, we can measure the intensity ratio of the phenyl 20b peaks under the ppp and ssp polarization combinations to examine the  -74  -  orientation of toluene. This approach has been previously used by Zhang and coworkers for alcohol and water binary mixtures. 234 In the current study, the ssp SFG spectra of each mixture were measured whereas only the ppp SFG spectra of those mixtures with higher toluene concentrations were measured, as shown in Fig. 5.6. This is because the ppp SFG spectra are much weaker in intensity because of the smaller  The ssp and ppp SFG  of pure toluene on silica are shown in Fig. 5.7 to illustrate the intensity difference. The amplitudes of the toluene 20b peak for x=0.70, 0.81, 0.91, and 1.00 are summarized in Table 5.4. The ratios of  /  under different bulk molar fractions are summarized in  Table 5.5. The ratios remain approximately constant. This indicates that the orientations of the toluene molecules are independent of its molar concentration in the mixtures. This means that the change in the amplitudes of the toluene phenyl peaks is mainly due to the surface coverage variation. We used this to deduce the surface coverage of toluene molecules from  Table 5.4. The ppp peak amplitudes A of toluene 20b peaks by curve-fitting the spectra in Fig.5.6  Toluene molar fraction x  Peak amplitude A (arb.u.)  1.00  1.50±0.23  0.91  0.85±0.12  0.81  0.69±0.18  0.70  0.49±0.27  -75  -  Table 5.5. The ratios of  /A 33 for the toluene 20b peaks with different toluene molar concentrations  in the mixture  Toluene molar fraction x  Peak amplitude ratio  1.00  0.42±0.08  0.91  0.41±0.10  0.81  0.43±0.13  0.70  0.45±0.28  /  Assuming that the toluene coverage 9 on silica is 1 for pure toluene, 0 for toluene in other mixtures can be determined by the ratios of the 20b peaks as (5.18)  0 A  where O is the surface coverage of toluene in mixture i, A , and 1  4  are the amplitudes of  the 20b peak for the mixture i and pure toluene. The results are shown in Fig. 5.9. The adsorption isotherm can now be fitted using Equations (5.5), (5.7), and (5.13) with the adsorption free energies AG of toluene and heptane on the silica surface as the fitting parameters. The best fit was obtained with the adsorption free energies of toluene and heptane on the silica surface equal to -12.1±0.5 kJ/mol and -16.5±0.5 kJ/mol, respectively. Considering the error propagation from the uncertainty of the SFG amplitudes in Table 5.3, toluene and heptane adsorption energies are -12.1±1.8 kJ/mol and -16.5±2.3 kJ/mol, respectively. Based on these adsorption free energies changes, heptane shows preferential adsorption to toluene on the silica substrate.  -76  -  1.0  a) 1) >  g  0.8 0.6  0.4 a)  C ci) 0  I0.0 11•1•1•1  0.0  0.2  0.4  0.6  0.8  I  1.0  Toluene bulk molar fraction  Figure 5.9. Adsorption isotherm of toluene in the binary mixtures on silica substrate. • is the experimental data, and the solid curve is the fitting result using equation (5.13).  The preferential adsorption of heptane deduced from the adsorption free energies seems to contradict with the interactions of heptane and toluene molecules with the silica surface. Toluene has a dipole moment of 0.36 D, whereas heptane has no dipole moment. The main interactions between the toluene and silica are ion-dipole and dipole-dipole interactions. Whereas for heptane, the main interactions are van der Waals forces, which in general are weaker than ion-dipole and dipole-dipole interactions. This suggests that the interaction of toluene and silica surface is stronger than that of heptane. However, the above discussion has not considered the interactions between the adsorbed molecules and the molecules in the bulk solutions. Molecules with non-zero dipole moments have stronger intermolecular interactions and may prefer to stay in the bulk to minimize their energies. On the other hand, the AG derived here is also related to the change of entropy  -77  -  AS AG  .  =  For a process occurring at constant temperature, it can be shown that All  —  TzSS. It is relatively inaccurate to consider AG purely from the discussion of  AH. Further experimental data with temperature dependence will allow us to derive  All and AS for the adsorption processes.  55. Conclusion Using sum frequency generation, we have studied competitive adsorption of toluene-heptane binary mixtures on a silica surface. The amplitudes of the unique SFG peaks of toluene allow us to determine its surface coverage as a function of the bulk molar concentrations. Using the Langmuir adsorption mechanism for binary mixture, the toluene adsorption isotherm was constructed, and the adsorption free energies AG of toluene and heptane on the silica surface are calculated to be —12.1±1.8 kJ/mol and —16.5±2.3 kJ/mol.  -78  -  Chapter 6 Conclusions In this thesis, we studied several interfacial properties of polymer films, such as the surface relaxation process of Poly(methyl methacrylate) (PMMA) films and the interfacial  electronic  states  of  poly[2-methoxy,  5-ethyl  (2’-hexyloxy)  para  phenylenevinylene] (MEH-PPV) films, and the competition adsorption mechanism of toluene and heptane in binary systems on silica surface. We used the interfacial specific and sensitive nonlinear optical method, sum frequency generation (SFG) spectroscopy, to investigate these systems. First, we investigated the surface relaxation process of PMMA films. A polarization modulation technique was introduced. This technique utilizes continuously variable polarization of the incident visible beam, rendering it the ability to detect small changes in the orientation of the interfacial molecules. This method enhances the SFG sensitivity and greatly shortens the data acquisition time, which allows it to be used to carry out a real-time recording in dynamic interfacial studies. We have studied PMMA films with thicknesses of 100-500 nm to avoid the substrate-polymer interaction. The ester methyl groups at surfaces were found to change their orientations at around 67 °C, which is about 40 °C lower than the bulk PMMA glass transition temperature, indicating a new surface relaxation mechanism for these PMMA films. This was the first measurement of the surface relaxation time for a free surface under no external field. The surface relaxation of PMMA has been found to be fast relative to the bulk PMMA. Our results are informative for many applications relying on the surface properties of PMMA,  -79  -  such as lithography and nanoimprinting. In the future, further studies are needed to investigate whether the surface phenomena generally exist on other poiymer surfaces. We also studied the interfacial electronic states of MEH-PPV films. Doubly resonant two-dimensional (2D) SFG utilizing frequency-tunable JR and visible beams was adopted to study such systems. Different to the conventional vibrational SFG spectroscopy, 2D SFG can reveal not only the resonance of the vibrational modes but also the associated electronic transition energies. These merits together with its interfacial specificity make it very suitable in the studies of the interfacial electronic states. The electronic transitions associated with the C=C vibrational mode in the backbone of MEH PPV were studied. At the MEH-PPV/CaF 2 interface, MEH-PPV is electronically resonant when the incident visible wavelength is at 580 urn, which corresponds to the electronic transition energy 2.3 eV. By comparing the absorption spectrum of bulk film and the interfacial SFG electronic spectrum, it is found that the electronic transition energy at the 2 interface is red shifted, which indicates longer conjugation length of MEH-PPV/CaF polymer chains. The oligomers model was used to analyze the electronic spectrum, and the average conjugation length of MEH-PPV at the MEH-PPV/CaF 2 interface was estimated to be 5.8 monomer units and a distribution width of 0.9 monomer unit. Similar surface effects were also observed at the MEH-PPV/air interface with a shorter average conjugation length of 5.1 monomer units and a broader width of 1.3 monomer units. Our results showed that the surface band gaps and conjugation-length distributions of MEH PPV films are sensitive to their surface effects. We have demonstrated that 2D SFG spectroscopy is applicable in the studies of the interfacial electronic states of polymer films. Since conjugated polymers are widely used in organic devices, and their interfacial  -80  -  electronic properties are important in these applications, more systems related to the polymer interfacial electronic properties can be studied in the future. Finally, the competition adsorption processes on silica surfaces of heptane and toluene in binary mixture systems were studied. The SFG spectra revealed the existence of heptane and toluene on the surfaces. With changing concentrations of the binary mixtures, the surface coverage of heptane and toluene were found to change as well. Using a simple Langmuir model to analyze the adsorption processes, we have found that heptane preferably adsorbs on the silica surface. In binary solutions, we obtained that the adsorption free energies change AG is —12.1±1.8 kJ/mol for toluene and —16.5±2.3 kJ/mol for heptane. These results are valuable to the oil extraction processes, and pointed a direction for further investigation of the competition adsorption processes of other chemical species. In conclusion, we have successfully investigated the surface relaxation process of PMMA films, the interfacial electronic properties of MEH-PPV films, and the competition adsorption processes of toluene and heptane in the binary mixture systems. Our studies showed that these properties at interfaces and surfaces of both polymer films and liquid samples are different from those in the bulk systems. These results are valuable to further studies of polymer interfacial properties, and applications in industrial processes, such as oil extractions.  -81  -  Bibliography (1)  Chem, T. S.; Tsai, H. L. Materials Chemistiy and Physics 2007, 104, 472.  (2)  Wang, Y. W.; Yu, X. 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