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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 78 Chapter 6 Conclusions 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 are for individual ones 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 are for individual ones 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 / A3,, for the toluene 20b peaks with different toluene molar 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 F0 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 /F0, when the incident 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 polarization- modulated, 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 30 3.3. SFG intensity vs. visible polarization angle vjc• The solid line is a fitting curve using equation (3.2). SFG reaches a minimum at QQ0,which is used to monitor the surface conformation changes. The data were acquired from a PMMA film with the thickness about 200 nm 32 -vi - 3.4. Measured Q0 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 ± 2 °C 34 3.5. The relationship between the measured Q0 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 CR3 group with respect to the surface normal estimated using the measured Q0. The sample thickness is 200 nm 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 01R are incident on the bottom sample/silica interface from different sides -vii - with incidence angles of O and 81R respectively. The generated SFG signal 0’SFG at angle SFG 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 cm1, 3021 cmt, 3081 cm peaks are assigned to the methyl symmetric 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 cm1, 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 cm1 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) 71 5.7. The comparison of ssp and ppp SF0 spectra of pure toluene on silica. The 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 as metals and semiconductors.’94’However, many of these experimental techniques, such as low-energy electron diffraction (LEED)42, Auger electron spectroscopy (AES)4547, reflection high-energy electron diffraction (RHEED)48’9, can only be 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)5053,Raman scattering5456, and ultraviolet visible absorption spectroscopy5760,do not have sufficient surface —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.6’1R-visible SFG vibrational spectroscopy was first 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 interfaces6369, polymer interfaces7075, gas/solid interfaces7679, liquid/solid interfaces8084,and biological systems8588.A brief theoretical background and 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) (PMMA)89 and buried interface of poly{2-methoxy, 5-ethyl (2’-hexyloxy) para phenylenevinylene] (MEH-PPV)9°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 tm.89 -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 to their chain conformations.9698 As the surface chain conformations are different from 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 interfaces.’00’1 The interfacial sensitivity of SFG comes from the fact that a second-order optical process is forbidden in media with inversion symmetry.6’However, the inversion symmetry is always broken at interfaces. JR-visible SFG is carried out with two pulsed laser beams at frequencies and a2. These 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.’02A 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 KTiOPO4 (KTP) crystal by second harmonics generation (SHG) from the 1064 nm beam. The frequency-tunable visible beam was generated in a BaB2O4 (BBO) optical parametric 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 KTiOAsO4 (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 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) OPG/OPA 355 nm Tunable visible 400—700 nm 532 nm SHG Tunable IR DFG 1200-4000 cm1 Figure 2.1. Layout of OPG/OPA. DM, P.M and BS represent the dichromatic mirror, reflective mirror, and beam splitter, respectively. -6 - and k=k3+1 (2.2) Here the ks are the wave vectors of the electromagnetic fields. Since a, = c / 2 and k = 2zn() / 2, the conditions can be written as --=---+-- (2.3) 2 2 2 and n n n. or n a) —na +n.a). (2.4) 2p 2 pp s S ii 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 n0 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 n0 and e• If n0 > 1e the crystal is called a negative crystal; while if n0 <ne’ the crystal is called a 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 n°(O)=n0 (2.5) I 1+tan2Se(5) 1 (2.6) °\Il+(fl/fl)2tan2 0 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: (2.7a) C ôt VxB=+J (2.7b) c8t c V•E=4irp (2.7c) -9 - V•B=O (2.7d) and the constitutive relations D=80E+P (2.8a) B=p0H+M (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 p0 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) c 8t (2.9b) c at 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 —7lE=_4---P (2.10) [ c atJ cat -10- Under weak electric fields, the polarization of a material can be described by —. —(0) —(1) PP -f P (2.11) P =80%’ECos(O3t) (2.12) —(1) where P is the static polarization, P is the first-order lmear polarization, (1) is the linear susceptibility, s is the permittivity of free space, t 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) —(1) —(2) P=P +P +P +... (2.13) —(2) =60E1cos(o1t)Ek cos(a2t) (2.14) where (2) 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(a1 + a2 )t + cos(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 anda2. These two processes are known as SFG 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 =ai+a2)ocz(21I I (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: (2) = Therefore, (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 a2 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 ksfgkl+k2 (2.17) -12 - where k and k2 are the wave vectors of the incident beams. SFG intensity is given by’°’ = + (02) = 2 (2.18)8s0c cos where fi3 is the exit angle of the SFG output. 2J? is the effective surface nonlinear susceptibility defined as 2’ =[L(co5).ê].(2) :[L(co1.ê] oi (2.19) with ê1 being the unit polarization vector of the optical field at a and L(co1) the tensorial Fresnel factor. L(o,1)describes the relationship between field components in the air and in the medium. For isotropic media, only the diagonal elements of L(a) need to be considered, which are L(a1)= 2n1(a)cos(y (2.20a) n1(wjcos(y)+2(o1)cos(fi) L (a1) = 2n1(a’)cos(fl (2.20b)Qi)cos(fl+n( y L ((0.)= 2n(o1)cos(fl (2.20c) n1(a)cos(y + n2(a1)cos(fi,) n’(o1)) In the above equations, n’(o) is the refractive index of the interfacial layer, n1 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 n1 (os), n2 (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.’09”Zhuang Ct al. did a detailed analysis on this issue using 4”-n-pentyl-4- cyano-p-terphenyl [5CT, CH3(C2)46H3 N].”° 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 (2) Ag (2.21) IR O)q +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 ——-———- aim (222)2(Oq 8q aq where p1, is the dipole moment and a is the polarizability. Hence, in order for 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- The number of non-vanishing (2) 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 aqlmn through a coordinate transformation as Aq,jk = N aqimn ((1 . 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’ 21112 (2.24) q 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 0IR and a near vibrational and electronic transitions, respectively, SFG can be doubly enhanced.”27By tuning both the 031R 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) — _Ieu) 2IR \ — _eu = ‘0Vis = 0Vis -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) — N/ . ôp fe_S Xk 2 \ Pegflgeh 8q1 co—o+,[’, ____ i n=O n! SFG ‘I °3eg+iT’en gO 0SFG (ii + 1)w COeg +lTen÷i,go (2.25) where N is the surface molecular density, 4 represents the i — component of electronic transition moment, q1 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, 0SFG is the SFG frequency, o, and Weg are the resonant vibrational and electronic frequencies, respectively, and T,jg0 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 as112 (2)(VL—IR) — N! i k 8Pe XUk — 2 \ PegPggh 8q, COIRCOI+ZTI x’— . -_____________ n=O n! 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 0IR +a °‘eg or IR + O4 = ‘0eg + l• These two processes correspond to Fig.2.5 (A) and (B) respectively. Here we assume that the vibrational excitation energy is the same at electronic ground state (g) and excited state (e). 5 F. A F B 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 co c Q 0 -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 X4k are omitted, and pp 1gg is taken as a constant. The vibrational and electronic frequencies oi and COeg 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: CO1=: 1593 cm’, coeg=53O rim =18868 cm, Feo —100 cm’, and ,=20 cm1. 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 Fe0 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)). 1200 -i 1000 -r -, 800 —r 400 - 200 - 1700 1650 1600 --r-- _rN —— I I _I k—— II i_ L. —I_ I3 50 I ___ —— I Il00O 800- _j I I—__ I - :: ::EJ: r 1700 —•-- 650 .—6001600 - — -1550 5501 —1450 Ii Il00O - - - -:i tio6 - - - - -800- I NI —a——800 I — I __— ___ I_ _ 400 _ — — I 200 0 1700 — 6501650 1L 1500 1450 1000 — I :r:---: 400l- 0 0 1700 1650 1450 I I I I 1—_i II — I I 1—_I Il I 50 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 Fe0• Fig. 2.7 shows the 2D SFG spectra with different S values under the condition of F /F0 = 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). —I — — — _I_ I I I —_ 250 — _— II __— __ I —_ 150 — ___ I ——I I100 — ___ I0 ___ 50 1700 650 ;:: — I 1700 650 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 0 ———rh——I———— _-- L_- __N_ H - — — 1 (a) S0 — — I 1000— i___—1 —rb— I __I j__ I I_ I —_I -8004_—r ‘__——j •1___ II _l I 1 I 0 1700 1650 650 1500 __rb—i I _—I_ I — 400 —r I I — = ___,__ I — — i__I 300 —r —I_ — I L_ I — 200 __A— — — I ISO — 4 0 1700 1650’ 1L__ 650 % 1500 — 600 550 550 —3— 600 %1%51550 5501500 500O/(7 1600 fq% 1550’ a 550 -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.3 356 S=0.6 168 z500 550 600 650 Visible wavelength (nm) - (U C LL C,, 2 CU C LI Cl, 122w 60 n ( ren e3) S=0.3 160 S=0.6 — ZJL 500 550 600 650 Visible wavelength (nm) -23 - D0 Li U) 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 / Fe0 , when the incident JR frequency is resonant with the vibrational mode of the system. Visible wavelength (nm) -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.”8Previous studies have shown that PMMA displays a complex dynamical behavior. Its glass and sub-glass relaxation processes have been studied by dielectric spectroscopy”9’2°dynamic mechanical analysis’21, neutron scattering’22425,and NMR spectroscopy’26”7.The main features are the well-known a- 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 nanoimprinting130”31,rely on the surface properties of PMIVIA. Therefore, the state of a 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’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.’35’8 Many other studies using various techniques, such as friction-force microscopy’39,X-ray reflectivity’32, fluorescent diffusion’40, positron optical birefringence’42,and ellipsometry’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’45, such as nanopores’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 technique.’49 Studies using other surface-sensitive techniques, such as scanning force microscopy (SFM)’50, near-edge X-ray absorption fine structure spectroscopy (NEXAFS)151,X-ray reflectivity’52,and sum-frequency generation (SFG)95”534,have 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.95More recently, SFG was employed to study the surface glass transition of poly(vinyl alcohol) (PVA)’53 and polystyrene (PS)’54. In 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 polarization- modulated, 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 (0)2) beams on the surface to generate a third beam with a frequency a = (01 + a2. The polarization of the visible beam was rotated by a half-wave 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 has four independent non-vanishing elements: = x = ,, 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 Gaussian shape.’57 It turned out that the resonances are described well enough by 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 PPP 10 • SSP 8 .ci 1. .‘ 6 U, 0 0 .. -. 1 t- 2800 2900 3000 3100 IR Wavenumber (cm1) 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 Co (cm’) Peak A 0 F (cm’) assignments O-CH3-ss O-CH3-ss ssp-SFG 2955 ppp-SFG 2955 37.4 0.59 1.16 13.2 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 aby 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 - ‘SFG(v18)cc sin S1fl2sFG + COSQSFG = sinQ + cos22 (3.1) The expression can be rewritten as ‘SFG(vL) cc --sin(Q,, — Q0)2 (3.2) with (2) = —arctan(2’;”) (3.3) Xeff,ssp 10 I. U) C ci) .s 0 Li 0 I -180 -120 -60 0 60 120 Visible Polarization Angle c8 (Deg) 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(2v) curve measured at room temperature. Equation (3.2) was used to fit the curve with Q0 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 Q0 with an error of ±0.3 degree. Any change in the surface molecular orientation would change the ratio of and introduce a phase shift in the measured ‘sFG(8)curve. The phase of ‘sFG(,), indicated as Q0 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 Q0 . For example, if the laser intensity decreases, the SFG intensities in Fig 3.3 would decrease, but the Q0 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 experiments.”1”53’60 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 alignment’53”6’or an eternal stretching force162 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 Q0 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 (34) 2 2 where Q1 and Q2 are the low and high limits of Q0 before and after the phase transition, respectively.’53 Based on this fitting method, two discontinuities in molecular orientation were obtained at T0 =107±2 °C and 67±2 °C. Temperature (°C) 140 120 100 80 60 40 20 15T 100 200 300 400 Time (mm.) Figure 3.4. Measured Q0 as a function of time and temperature for the PMMA sample with 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 Q0 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. IA)Ø4.++ 2b 16 20 140 120 100 80 60 40 20 Temperature (°C) Fig. 3.5 The relationship between the measured Q0 and temperature for the PMMA samples with different film thickness. (A) 500 nm; (B) 200 nm; (C) 100 nm. Table 3.2 Temperatures of the discontinuities in Q0 for PMMA samples with different film thickness. Film Thickness Temperature of the Temperature of the (nm) 1st discontinuity 2nd discontinuity (°C) (°C) 500 107 65 200 107 67 100 108 70 -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 and %, the effect is partially cancelled when only the ratio (or Q0) was measured. Overall, it introduces a small slope in Fig. 3.4, but it is not responsible for the short-range steep changes of Q0. The refraction index of PMMA decreases linearly with temperature, with a slope of approximately -1.4 xl 0 4/°C below the glass transition temperature.’66The slope changes to approximately -3.4 xl04/°C above the glass transition temperature. The discontinuity in the slope can 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 highest- temperature 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.’26The temperature for 13-relaxation of bulk PMMA is near room temperature and decreases with decreasing film thickness.’67’9 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.’34”6°Therefore, a 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 80 60 40 20 100 200 300 400 Time (mm.) Figure 3.6. The tilting angle of the ester CH3 group with respect to the surface normal estimated using the measured Q0. The sample thickness is 200 nm. As shown in Fig. 3.6, the tilting angle, 0, of the ester CH3 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 = x = x, and with along the surface normal and in the plane of incidence in the laboratory coordinate system.’55 The effective nonlinear susceptibility and can be written as = —L5(cot )L (cot )L (a)2)cos /3 cos /3 sinfl2 —L(a))55(a),)L(a2cosfisin,81cosfi2 +L(a)) (1sinficosfi1cosfi2 +L55(a))L(tv2sin,83sin/31sinfi21 (3.5) = L,(a)5)L,,(a)L55(co2) sinfi2 (3.6) where /3j and /32 are the incident angles of visible and IR respectively, /3 is the reflected angle of SFG, and L11 are the tensorial Fresnel factors. The second order nonlinear susceptibility (2) is related to the hyperpolarizability a’2by = N5 ((.aX34X.o))a (3.7) a,b,c where N5 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 C3 symmetry, can be written as’°9 = a[cosO(1 + y) — cos3 0(1 — y)] (3.8) = = -N5a[(cos0 —cos30)(l —y)] (3.9) =N5a[y cos 0 + cos3 0(1 — y)] (3.10) where a = y = abbC / , 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 CR3 group is approximately tilted 600 with respect to the surface normal. The change in 6 for the surface transition is only 10. During the cooling process, the ester CH3 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 angle. Therefore, more generally, a Gaussian distribution f’(8) = Cexp{— (8)] can be adopted, where C is normalization constant and a is the root-mean-square width.’6°However, when using the Gaussian distribution to fit the 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 10. It does not mean that every ester methyl group change its orientation by 10, 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.’70”’Similarly, the molecular weight can affect the PMMA surface relaxation processes. Also, PMMA with different tacticity can be studied, such as isotactic PMMA, as the tacticity of PMMA also affect its glass transition temperature.’72”3Furthurmore, poly(alkyl methacrylates) with longer side chains, i.e. -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 ofpolymer 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 Two- dimensional 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.’77’82 The polymer has found its applications in light- emitting diodes (LED) 177,183 photovoltaics’84 transistors185,and flexible displays183. 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.186”87 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 chains.98”8’0 Mechanisms leading to a finite conjugation length in the polymer due to abrupt flips’91”2 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.113”6Traditionally, JR-visible SFG 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.”2”6As shown in 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) (C) eO) Visible 1R SFG MEHPPV ‘ .1fl)OIR 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 (COIR) on a surface to generate a third beam with a frequency a)SFG = co + (0LR10’ 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 /nn2per 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 CaF2 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. CaF2 was chosen because of its high transmission of IR. To ensure the measured SFG is truly generated at the buried polymer/CaF2interface without a contribution from the air/polymer interface, 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’94,a film thickness of 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 1600 1700 IR Wavenumber IR (cm1) 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. 1500 -46 - Table 4.1 Fitting parameters for the SFG spectra in Fig. 4.2. The resonance frequency (0 , the phase b, and the dephasing constant 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 0 1’ wavelength (cm’) assignments (arb.u.) (cm’) (urn) 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 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 cm1 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 cm1 depending on -47 - the molecular weight.’96 The observed wavenumber at 1593 cm4 is consistent with the previously reported value for MEH-PPV with a molecular weight of 6x104g!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 6 Ie.I.% =660nm co =640nm V’s •• 5 =620nm Vis co=600nm D • •. co=560 nmCl) c 2 .1-’ .E .ri . 580nm co =540 nm0 Vis LII Cl) 0) 520 nm V’s co=500 nm 0 0) =480nm I I 1500 1600 1700 IR Wavenumber OIR (cm1) 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 cm1 are the C- C 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 ç$ , and the dephasing constant 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 Peak A F wavelength (cm1) assignments (arb.u.) (cm’) (nm) 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 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 wavelength-dependent.197Since the Fresnel factors are related to the refractive indexes, -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) X:E(coIR)E(coVIS) (4.1) 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 )L3,,(a)L (COIR ) sinCBIR )X (4.2) with L (2))= 2n1Qv)cos(,6 (43) n1(a)cos(/3 +2(a)cos(y) L(a,)= 2nQi1)cos(fl (4.4) n1 (w) cos(y) + n2 (cot) cosC81 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, n1 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).”2Because the electronic relaxation times are generally much shorter -50 - than the vibrational relaxation times, the contribution of the Vis-IR SFG is generally negligible.’13 Therefore, only the IR-Vis SFG will be considered in the following calculation. Assuming harmonic potential surfaces for the electronic states and the Born— Oppenheimer approximation, the IR-Vis doubly resonant can be described as112 (2) — N / , j ô,t1g’g XUk I’egPge h 8q1 IR1+1”1 1 — 1 n! (DsFG fl0, COeg +ZTen go 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, q1 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 COeg are the resonant vibrational and electronic frequencies, respectively, F1 and Tengo 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 z in resonance with vibronic transitions with frequencies °eg + na 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.3’116For MEH-PPV, the dephasing times of vibronic transitions are in the femtosecond region.’98 -51 - Therefore, by assuming Fen go >> Teo,go , the non-zero vibronic transitions can be neglected, and equation (4.5) can be simplified as (2) — N / , ____________ 1 \ (2) — — 7 Peg fige X / + XNR (4.6) h 8q1 0IR —CO1+iF, WSFG0eg +lTeogo / 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 (2) A1 (2) • +XJy (DIR co1+iF with A1 describing the electronic resonance. For a system with a single electronic resonance COeg A1 has the following form A1cj (4.8) — 0eg + iTeO gO Equations (4.l)-(4.4) and (4.7) were used to fit the SFG vibrational spectra in Fig. 4.2 and 4.3 with A1, CU1, F,, and X•k as the adjustable parameters. The fitted values of A1 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 - __ 15 .o 10. 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 length.98”867Conjugated polymer 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, Absorption Wavelength (nm) 400 450 590 50 20 • MEH-PPV/Solict Interface • Air/MEH-PPV Interface — Bulk Film 0.02 C.) C CD f,(v 0 C’) .0 SFG Wavelength (nm) -53 - such as medium effects and oligomer interactions205’6,it has produced reasonably good agreement for the optical properties of MEH-PPV207 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 A1 in equation (4.8) can be modified as A1 D(N) (4.9) N WSFG — COe(N) + Te0 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 ofN monomer units can be described by207 EN=E0+2flcos() (4.10) with E0 4.3 eV and /3 = —1.1 eV.202 In this expression, the energy levels with N 2 are located in the UV region. Therefore, only segments with N 3 will be considered in the fitting. Assume a Gaussian conjugation-length distribution function: D(N) = exp[—(N—N02/.2j (4.11) The center conjugation length N0 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 N0 = 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 N0 = 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 N0 = 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 CaF2 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/CaF2 interface features longer wavelength, thus lower energy. The effect from the polar environment from CaF2 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 mixtures.208’10,213-217,224-226 Molecular properties at interfaces of binary systems are usually studied theoretically227 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 coworkers23’have applied 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 critical temperature.23’They found that near the solid surface, n-hexane content is larger 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 air232’3 and silica surfaces.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 volume,235 dynamic viscosity,235’6 density,236 diffusion coefficient,237 surface tension,238 density,238 thermal conductivity,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 solids.241-246 For single solute adsorption on solid surface, the adsorption kinetics can be expressed as —cS)—kdcs (5.1) where Cs 15 the adsorbed solute density on the solid surface, c is the solute concentration in solution, and c1 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 kaC(Ci —C5) = kdCS c = CC1 (5.2) ka c + kd -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 (5.4) kaC+kd KaC+1 where Ka = 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 -AG Ka =exp( RT (5.5) 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 = kaiCi (c,1 — c1 — c2)— kdicSl (5.6a) = kaIC2(CI — C1 — c) — kd2cS (5.6b) Here c1 and c2 are the molar concentrations of the components 1 and 2. And since this is a binary liquid system, we have c1+c2=l (5.7) -60 - Similarly, when the equilibrium is reached, we set equations (5.6a) and (5.6b) equal to zero and solve for c1 and c52. It can be shown that c = 1 + Kai[<a2Cui —Ka1_C2C2) (5.8a)S KaiCi + Ka2C + 1 — c2 (KU2CI + Kai.Ka9C12 ai’<a2CiCii) C52— (• ) KaiCi +Ka2C +1 When the effect of uneven saturation capacities is considered247,the adsorption saturation capacities of components 1 and 2 are related as a=- (5.9) C12 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 dç1 kaiCi (C11 — C51 — ac52)— kdlCSl (5.lOa) =k1C(C —c1—C52) kd2cS (5.lOb) dt a Solving equations (5.lOa) and (5.lOb) gives Ci = Ci(KaiCii +Ka1K21—aKaiKa7c2) (5.lla) KaiCi +Ka2C +1 c2 (Ka2C12 + KaiKa2CiCi — KaiKa2CiCii) = a (5.llb) KaiCi +Ka2C +1 Inserting equation (5.9) into equations (5.11), we obtain C = CiKaiCii (5.12a) K + Ka9C2 + 1 -61 - C 2 = C2Ka12 (5.12b) 1<aiCi + .Ka2C + 1 The surface coverage of component i is defined as 0 = -. Then the adsorption cli isotherms have the form 0 = CiKai (5.13a) KaiCi +Ka2C +1 02 = C2Ka (5.13b) Ka1C +Ka2C +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/nm2248’49 The silica surface will be discussed later in the 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 Silica sample -. - slilca - - WIR 9IR 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 a and JR beam with frequency 0IR are incident on the bottom sample/silica interface from different sides with incidence angles of O and 0IR respectively. The generated SFG signal 0SFG at angle 0SFG is then measured. aVIS eSFG WSFG : -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 modes.250’The 20a and 20b stretching modes are 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 cm1 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,) 3600 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 cm1, 3081 cnf’ peaks are assigned to the methyl symmetric and asymmetric stretches, and phenyl 20b and 20a stretching modes respectively. The inset is the SFG spectrum of pure water on silica surface 2800 3000 3200 3400 Wavenumber (cm1) -65 - Figure 5.3. The phenyl 20a and 20b stretching modes252 Table 5.1 Fitting parameters for toluene SFG spectrum in Fig. 5.2 a Peak A F (cm’) assignments (cm’) 2864 ct-CH3-ss 0.01 1.08 -0.79 7.0 2955 ct-CH3-as 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 cm1 exist on the dried toluene samples. It has been known that water has two stretching modes at 3200 and -3400 cm1, and they are assigned to the bonded OH in ice-like and water-like structures respectively.’57The 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 cm1 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 CH2 groups in all-trans conformation, which represent a centrosymmetric arrangement of 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.253’4 -67 - D -D I U) C G) .1-i C 0 U 0 3600 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 cm4, 2955 cm1 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 A F (cm’) assignments (na4) 2864 c-CH3-ss 0.00 3.01 -1.04 12.4 2935 x-CH3-Fermi 2.87 0.94 13.2 2955 c-CH3-as 3.82 0.11 10.9 2800 3000 3200 3400 Wavenumber (cm1) -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 lfl-J .J-.F W.J -j,j (9 J1PJf Jr r L .31% - - .16% I I 2800 3000 3200 3400 3600 Wavenumber (cm1) 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 system by a coordinate transformation and an average over the angular distribution of the molecules f(2)234: = N f avimn (j l)(J . th)(k (5.14) 1,m,n Based on equation (2.16), the SFG intensity is proportional to AVUk2. From equation (5.14), the amplitude 4 depends on both the number density N8 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. 3600 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. Li Cu > 4-, Cl) ci) 4-, (9 U C,, Wavenumber (cm1) -71 - 0.20 ssP 11 ppp D CU > 0.10 C,) G) .4—’ -9 Cl) 0.05Li. C.l) 4 4 48 48 84 V V8 1V8 v GV VV , V V VVV8 V 0.00 2800 3000 3200 3400 3600 Wavenumber (cm1) 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.5.8.254’5 The terminal H and 0 atoms are charged or partially charged, 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-L0,,H, Hc(’H,H __ __ I __ I __ I __ __ I __ Si Si Si Si Si Si fully hydrated silica surface S I siIoxane t germinal isolated OH groups OH groups OH group 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 CH3 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 and 2 1 = 2 Zeffppp 1112 (5.15a)8&0c cos -73 - =80ccos fl Xs 1112 (5.15b) The effective nonlinear susceptibilities and are associated with the nonnvanishing terms of (2) = —L (cot )L (cot )L (tv2)cos cos /31 sin /32 J — sin fi cosfl2 + L (a )L (a)1)L(a2)sin /3 cos /7 cos fi2X + (a)8)L (a)1 )L88 (a)2)sin /3 sin /1 sinfl2x (5.1 6a) L,,, (a)8 )L,,, (a)1 )L88 (a)2)sin /2%‘ (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 C2, symmetry, and 20b is an asymmetric stretch, the nonvanishing components of microscopic hyperpolarizability are fi J3 fi = and /3l,b /3cbb• Then we have257 = = —;_Nsl3aca((cos ç’) — (cos c’)) (5. 17a) = = = = N/3aca (cos c,) (5.1 7b) = Ns/3,a((cosO)—(cos c’)) (5.17c) where ‘ 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 / A33 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) A0 where O is the surface coverage of toluene in mixture i, A1, and 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) 0.8 1) > g 0.6 0.4 a) C ci) 0 I- 0.0 11•1•1•1 I 0.0 0.2 0.4 0.6 0.8 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 . For a process occurring at constant temperature, it can be shown that AG = 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/CaF2interface, 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 MEH-PPV/CaF2interface is red shifted, which indicates longer conjugation length of polymer chains. The oligomers model was used to analyze the electronic spectrum, and the average conjugation length of MEH-PPV at the MEH-PPV/CaF2 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. D.; Wang, F. C.; Ma, Z.; Wang, C. Y. Rare Metal Materials and Engineering 2007, 36, 727. (3) Antion, C.; Chatain, D. Surface Science 2007, 601, 2232. (4) Khatavkar, V. V.; Anderson, P. D.; Meijer, H. E. H. Journal ofFluid Mechanics 2007, 572, 367. (5) Barzilai, S.; Alzenshtein, M.; Shapiro-Tsoref, E.; Froumin, N.; Frage, N. International Journal ofAdhesion andAdhesives 2007, 27, 358. (6) Aarts, D. G. A. L. Soft Matter 2007, 3, 19. (7) Chen, S. H. Journal ofthe Serbian Chemical Society 2006, 71, 1091. (8) Taylor, C.; Kelly, R. G.; Neurock, M. Journal of the Electrochemical Society 2006, 153, E207. (9) Zhang, M. Q.; Liu, H.; Hu, H.; Xie, S. B.; Jing, P.; Kou, Y.; Shao, Y. H. Chemical Journal of Chinese Universities-Chinese 2006, 27, 1355. (10) Yoshimoto, S.; Tsutsumi, E.; Suto, K.; Honda, Y.; Itaya, K. Chemical Physics 2005, 319, 147. (11) Lu, X. Q.; Hu, L. N.; Wang, X. Q.; Chen, J. Chinese Chemical Letters 2004, 15, 1461. (12) Zhang, Z. Q.; Yuan, Y.; Sun, P.; Su, B.; Guo, J. D.; Shao, Y. H.; Girault, H. H. Journal ofPhysical Chemistry B 2002, 106, 6713. (13) Ostapenko, G. I. Journal ofSolid State Electrochemistry 2002, 6, 139. -82- (14) Bagambisa, F. B.; Kappert, H. F.; Schilli, W. Journal of Cranio-Maxillofacial Surgery 1994, 22, 12. (15) Nebe, J. G. B.; Luethen, F.; Lange, R.; Beck, U. Macromolecular Bioscience 2007, 7, 567. (16) Sabater, S.; Guasch, H.; Ricart, M.; Romani, A.; Vidal, G.; Kiunder, C.; Schmitt Jansen, M. Analytical and Bioanalytical Chemistry 2007, 387, 1425. (17) Branden, M.; Sanden, T.; Brzezinski, P.; Widengren, J. Proceedings of the NationalAcademy ofSciences ofthe United States ofAmerica 2006, 103, 19766. (18) Gnauck, P.; Burkhardt, C.; Wolburg, H.; Nisch, W. Scanning 2006, 28, 65. (19) Somorjai, G. A.; Rupprechter, G. Journal ofChemical Education 1998, 75, 162. (20) Urano, T.; Watanabe, K.; Hongo, S. Applied Surface Science 2001, 169, 88. (21) Mahan, J. E.; Geib, K. M.; Robinson, G. Y.; Long, R. G. Journal of Vacuum Science & Technology a-Vacuum Surfaces and Films 1990, 8, 3692. (22) Saiki, K.; Kono, T.; Ueno, K.; Koma, A. Review of Scientific Instruments 2000, 71, 3478. (23) Forster, H.; Fuess, H.; Geidel, E.; Hunger, B.; Jobic, H.; Kirschhock, C.; Kiepel, 0.; Krause, K. Physical Chemistry Chemical Physics 1999, 1, 593. (24) Cappadonia, M.; Robinson, K. M.; Schmidberger, J.; Stimming, U. Surface Review and Letters 1997, 4, 1173. (25) Nakamura, M.; Matsunaga, K.; Kitahara, K.; Ito, M.; Sakata, 0. Journal of Electroanalytical Chemistry 2003, 554, 175. (26) Rana, S.; Ram, S. Journal ofSolid State Chemistry 2001, 157, 40. -83 - (27) Suzuki, H.; Bemer, S.; Brunner, M.; Yanagi, H.; Schlettwein, D.; Jung, T. A.; Guntherodt, H. J. Thin Solid Films 2001, 393, 325. (28) Gupalo, M. S.; Yarish, I. L.; Zlupko, V. M.; Suchorski, Y. Journal of Vacuum Science & Technology B 1997, 15, 491. (29) Meyer, G.; Bartels, L.; Zophel, S.; Henze, E.; Rieder, K. H. Physical Review Letters 1997, 78, 1512. (30) Hasegawa, Y.; Avouris, P. Japanese Journal ofApplied Physics Part 1-Regular Papers Short Notes & Review Papers 1994, 33, 3675. (31) He, L. Journal of Vacuum Science & Technology a- Vacuum Surfaces and Films 1997, 15, 951. (32) Gobel, H.; Vonblanckenhagen, P. Journal of Vacuum Science & Technology B 1995, 13, 1247. (33) Kroger, J.; Limot, L.; Jensen, H.; Berndt, R.; Crampin, S.; Pehlke, E. Progress in Surface Science 2005, 80, 26. (34) Swift, J. A. Journal ofPhysics E-Scientfic Instruments 1976, 9, 803. (35) Amako, K.; Takeya, K. Journal ofElectron Microscopy 1974, 23, 66. (36) Rickert, K. A.; Ellis, A. B.; Himpsel, F. 3.; Sun, J.; Kuech, T. F. Applied Physics Letters 2002, 80, 204. (37) Oshima, M.; Kawamura, T.; Maeyama, S.; Miyahara, T. Journal of Vacuum Science & Technology a-Vacuum Surfaces and Films 1988, 6, 1451. (38) Dinardo, N. J.; Demuth, J. E.; Clarke, T. C. Chemical Physics Letters 1985, 121, 239. -84 - (39) Frost, M. R.; Harrington, W. L.; Downey, D. F.; Waither, S. R. Journal of Vacuum Science & Technology B 1996, 14, 329. (40) Daolio, S.; Facchin, B.; Pagura, C.; Debattisti, A.; Barbieri, A. Rapid Communications in Mass Spectrometiy 1994, 8, 659. (41) Bubert, H.; Kiockenkamper, R. Spectrochimica Acta Part B-Atomic Spectroscopy 1981, 36, 61. (42) Cho, K.; Kishimoto, J.; Hashizume, T.; Sakurai, T. Japanese Journal ofApplied Physics Part 2-Letters 1994, 33, L125. (43) Achete, C.; Niehus, H.; Losch, W. Journal of Vacuum Science & Technology B 1985, 3, 1327. (44) Palmberg, P. W.; Rhodin, T. N.; Todd, C. J. Applied Physics Letters 1967, 10, 122. (45) Juttner, B.; Wolff, H.; Altrichter, B. Physica Status Solidi a-Applied Research 1975, 27, 403. (46) Sicking, G. Surface Technology 1980, 10, 321. (47) Fukuhara, M.; Harada, T. Denki Kagaku 1985, 53, 693. (48) Peneva, S. K.; Djuneva, K. D. Thin Solid Films 1984, 113, 297. (49) Frank, N.; Springholz, G.; Bauer, G. Journal ofCrystal Growth 1995, 150, 1190. (50) Therien, M.; Dallaire, R. Canadian Metallurgical Quarterly 1982, 21, 377. (51) Peremans, A.; Maseri, F.; Darville, J.; Gilles, J. M. Surface Science 1990, 227, 73. (52) Dunn, D. S.; Mcclure, D. 3. Journal of Vacuum Science & Technology a-Vacuum Surfaces and Films 1987, 5, 1327. -85 - (53) Nishimura, K.; Ohnishi, R.; Kunimatsu, K.; Enyo, M. Journal of Electroanalytical Chemistry 1989, 258, 219. (54) Kruszewski, S. Vacuum 1997, 48, 363. (55) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R.; Feld, M. S. Physical Review Letters 1997, 78, 1667. (56) Quagliano, L. G.; Orani, D. Surface Science 1996, 368, 108. (57) Canamares, M. V.; Garcia-Ramos, J. V.; Sanchez-Cortes, S. Applied Spectroscopy 2006, 60,1386. (58) Jang, S. M.; Kim, S. I.; Shin, S. M.; Joo, S. W. Surface and Interface Analysis 2004, 36, 43. (59) JO0, S. W. Chemistry Letters 2004, 33, 60. (60) Geddes, N. 3.; Urquhart, R. S.; Furlong, D. N.; Lawrence, C. R.; Tanaka, K.; Okahata, Y. Journal ofPhysical Chemistry 1993, 97, 13767. (61) Shen, Y. R. The Principles ofNonlinear Optics; Wiley-Interscience: New York, 1984. (62) Zhu, X. D.; Suhr, H.; Shen, Y. R. Physical Review B 1987, 35, 3047. (63) Ishiyama, T.; Morita, A. Journal ofPhysical Chemistry A 2007, 111, 9277. (64) Knock, M. M.; Bell, G. R.; Hill, E. K.; Turner, H. J.; Bain, C. D. Journal of Physical Chemistry B 2003, 107, 10801. (65) Scatena, L. F.; Brown, M. G.; Richmond, G. L. Science 2001, 292, 908. (66) Morita, A.; Hynes, J. T. Journal ofPhysical Chemistry B 2002, 106, 673. (67) Aliaga, C.; Baldelli, S. Journal ofPhysical Chemistry B 2007, 111, 9733. -86 - (68) Van Loon, L. L.; Minor, R. N.; Allen, H. C. Journal of Physical Chemistry A 2007, 111, 7338. (69) limori, T.; Iwahashi, T.; Kanai, K.; Seki, K.; Sung, J. H.; Kim, D.; Hamaguchi, H. 0.; Ouchi, Y. Journal ofPhysical Chemistry B 2007, 111, 4860. (70) Miyamae, T.; Akiyama, H.; Yoshida, M.; Tamaoki, N. Macromolecules 2007, 40, 4601. (71) Chen, Z.; Shen, Y. R.; Somorjai, G. A. Annual Review of Physical Chemistry 2002, 53, 437. (72) Chen, C. Y.; Loch, C. L.; Wang, J.; Chen, Z. Journal of Physical Chemistry B 2003, 107, 10440. (73) Ji, N.; Ostroverkhov, V.; Lagugne-Labarthet, F.; Shen, Y. R. Journal of the American Chemical Society 2003, 125, 14218. (74) Asong, N.; Dukes, F.; Wang, C. Y.; Shultz, M. J. Chemical Physics 2007, 339, 86. (75) Chen, Z. Polymer International 2007, 56, 577. (76) Yang, M.; Somorjai, G. A. Journal of the American Chemical Society 2004, 126, 7698. (77) Rupprechter, G.; Unterhalt, H.; Morkel, M.; Galletto, P.; Deliwig, T.; Freund, H. J. Vacuum 2003, 71, 83. (78) Yudanov, I. V.; Sahnoun, R.; Neyman, K. M.; Rosch, N.; Hoffmann, J.; Schauermann, S.; Johanek, V.; Unterhalt, H.; Rupprechter, G.; Libuda, J.; Freund, H. J. Journal ofPhysical Chemistry B 2003, 107, 255. (79) Cimatu, K.; Baldelli, S. Journal of the American Chemical Society 2006, 128, 16016. -87 - (80) Bordenyuk, A. N.; Weeraman, C.; Yatawara, A.; Jayathilake, H. D.; Stiopkin, I.; Liu, Y.; Benderskii, A. V. Journal ofPhysical Chemistiy C 2007, 111, 8925. (81) Ostroverkhov, V.; Waychunas, G. A.; Shen, Y. R. Chemical Physics Letters 2004, 386, 144. (82) Lu, 0. Q.; White, J. 0.; Wieckowski, A. Surface Science 2004, 564, 131. (83) Vidal, F.; Busson, B.; Tadjeddine, A.; Peremans, A. Journal of Chemical Physics 2003, 119, 12492. (84) Kolasinski, K. W.; DeWitt, K. M.; Harrison, I. Physica Status Solidi a- Applications and Materials Science 2007, 204, 1356. (85) Chen, X. Y.; Boughton, A. P.; Tesmer, J. J. 0.; Chen, Z. Journal of the American Chemical Society 2007, 129, 12658. (86) Chen, X.; Sagle, L. B.; Cremer, P. S. Journal of the American Chemical Society 2007, 129, 15104. (87) Chen, X. Y.; Wang, J.; Paszti, Z.; Wang, F. L.; Schrauben, J. N.; Tarabara, V. V.; Schmaier, A. H.; Chen, Z. Analytical and Bioanalytical Chemistiy 2007, 388, 65. (88) Sartenaer, Y.; Tourillon, 0.; Dreesen, L.; Lis, D.; Mani, A. A.; Thiry, P. A.; Peremans, A. Biosensors & Bioelectronics 2007, 22, 2179. (89) Li, Q. F.; Hua, R.; Cheah, I. J.; Chou, K. C. Journal ofPhysical Chemistiy B 2008, 112, 694. (90) Li, Q.; Hua, R.; Chou, K. C. Journal ofPhysical Chemistiy B 2008, 112, 2315. (91) Jean, Y. C.; Zhang, R. W.; Cao, H.; Yuan, J. P.; Huang, C. M.; Nielsen, B.; AsokaKumar, P. Physical Review B 1997, 56, R8459. -88 - (92) Xie, L.; Demaggio, G. B.; Frieze, W. E.; Devries, J.; Gidley, D. W.; Hristov, H. A.; Yee, A. F. Physical Review Letters 1995, 74, 4947. (93) Ge, S.; Pu, Y.; Zhang, W.; Rafailovich, M.; Sokolov, J.; Buenviaje, C.; Buckmaster, R.; Ovemey, R. M. Physical Review Letters 2000, 85, 2340. (94) Weber, R.; Zinimermann, K. M.; Tolan, M.; Stettner, J.; Press, W.; Seeck, 0. H.; Erichsen, 3.; Zaporojtchenko, V.; Strunskus, T.; Faupel, F. Physical Review E 2001, 6406. (95) Gracias, D. H.; Zhang, D.; Lianos, L.; thach, W.; Shen, Y. R.; Somorjai, G. A. Chemical Physics 1999, 245, 277. (96) Westenhoff, S.; Beenken, W. J. D.; Yartsev, A.; Greenham, N. C. Journal of Chemical Physics 2006, 125. (97) Dykstra, T. E.; Kovalevskij, V.; Yang, X. J.; Scholes, G. D. Chemical Physics 2005, 318, 21. (98) Schwartz, B. J. Annual Review ofPhysical Chemisti’y 2003, 54, 141. (99) Bryant, E. M.; Bowman, R. S.; Buckley, 3. S. Journal ofPetroleum Science and Engineering 2006, 52, 244. (100) Bain, C. D. Journal of the Chemical Society-Faraday Transactions 1995, 91, 1281. (101) Shen, Y. R. Nature 1989, 337, 519. (102) Huang, J. Y.; Shen, Y. R. Physical Review A 1994, 49, 3973. (103) Du, Q.; Superfine, R.; Freysz, E.; Shen, Y. R. Physical Review Letters 1993, 70, 2313. (104) Shen, Y. R. Surface Science 1994, 300, 551. -89 - (105) Conboy, J. C.; Messmer, M. C.; Richmond, G. L. Journal ofPhysical Chemistiy 1996, 100, 7617. (106) Johal, M. S.; Ward, R. N.; Davies, P. B. Journal ofPhysical Chemistry 1996, 100, 274. (107) Lambert, A. G.; Davies, P. B.; Neivandt, D. J. Applied Spectroscopy Reviews 2005, 40, 103. (108) Casson, B. D.; Bain, C. D. Langmuir 1997, 13, 5465. (109) Zhuang, X.; Miranda, P. B.; Kim, D.; Shen, Y. R. Physical Review B 1999, 59, 12632. (110) Wei, X.; Hong, S. C.; Zhuang, X. W.; Goto, T.; Shen, Y. R. Physical Review E 2000, 62, 5160 (111) Wang, J.; Chen, C. Y.; Buck, S. M.; Chen, Z. Journal of Physical Chemistry B 2001, 105, 12118 (112) Hayashi, M.; Lin, S. H.; Raschke, M. B.; Shen, Y. R. Journal of Physical Chemistry A 2002, 106, 2271. (113) Raschke, M. B.; Hayashi, M.; Lin, S. H.; Shen, Y. R. Chemical Physics Letters 2002, 359, 367. (114) Chou, K. C.; Markovic, N. M.; Kim, J.; Ross, P. N.; Somorjai, G. A. Journal of Physical Chemistry B 2003, 107, 1840. (115) Belkin, M. A.; Shen, Y. R. Physical Review Letters 2003, 91. (116) Chou, K. C.; Westerberg, S.; Shen, Y. R.; Ross, P. N.; Somorjai, G. A. Physical Review B 2004, 69. -90 - (117) Caudano, Y.; Silien, C.; Humbert, C.; Dreesen, L.; Mani, A. A.; Peremans, A.; Thiry, P. A. Journal ofElectron Spectroscopy and Related Phenomena 2003, 129, 139. (118) Billmeyer, F. Textbook ofPolymer Science; Wiley & Sons: Singapore, 1984. (119) Bergman, R.; Alvarez, F.; Alegria, A.; Colmenero, J. Journal of Chemistry Physics 1998, 109, 7546. (120) Bergman, R.; Alvarez, F.; Alegria, A.; Colmenero, 3. Journal ofNon-Crystalline Solids 1998, 235, 580. (121) Alves, N. M.; Ribelles, J. L. G.; Tejedor, J. A. G.; Mano, J. F. Macromolecules 2004, 37, 3735. (122) Genix, A. C.; Arbe, A.; Alvarez, F.; Colmenero, 3.; Farago, B.; Wischnewski, A.; Richter, D. Macromolecules 2006, 39, 6260. (123) Genix, A. C.; Arbe, A.; Alvarez, F.; Colmenero, J.; Schweika, W.; Richter, D. Macromolecules 2006, 39, 3947. (124) Moreno, A. J.; Alegria, A.; Colmenero, 3.; Frick, B. Physical Review B 1999, 59, 5983. (125) Moreno, A. J.; Alegria, A.; Colmenero, J.; Frick, B. Macromolecules 2001, 34, 4886. (126) Schmidt-Rohr, K.; Kulik, A. S.; Beckham, H. W.; Ohiemacher, A.; Paweizik, U.; Boeffel, C.; Spiess, H. W. Macromolecules 1994, 27, 4733. (127) Kuebler, S. C.; Schaefer, D. J.; Boeffel, C.; Paweizik, U.; Spiess, H. W. Macromolecules 1997, 30, 6597. (128) Wei, X.; Miranda, P. B.; Shen, Y. R. Physical Review Letters 2001, 86, 1554. -91 - (129) Mayers, G. F.; Dekoven, B. M.; Seitz, 3. T. Langmuir 1992, 8, 2330. (130) Gottschalch, F.; Hoffmann, T.; Torres, C. M. S.; Schulz, H.; Scheer, H. C. Solid State Electronics 1999, 43, 1079. (131) Chou, S. Y.; Krauss, P. R. Microelectronic Engineering 1997, 35, 237. (132) Orts, W. 3.; Vanzanten, J. H.; Wu, W. L.; Satija, S. K. Physical Review Letters 1993, 71, 867. (133) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Europhysics Letters 1994, 27, 59. (134) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Faraday Discussions 1994, 98, 219. (135) Zheng, X.; Sauer, B. B.; Vanalsten, 3. G.; Schwarz, S. A.; Rafailovich, M. H.; Sokolov, J.; Rubinstein, M. Physical Review Letters 1995, 74, 407. (136) Grohens, Y.; Brogly, M.; Labbe, C.; David, M. 0.; Schultz, J. Langmuir 1998, 14, 2929. (137) Overney, R. M.; Buenviaje, C.; R., L.; Dinelli, F. Journal of Thermal Analysis and Calorimetiy 2000, 59, 205. (138) Sharp, J. S.; Forrest, J. A. European Physical JournalE 2003, 12, S97. (139) Hammerschmidt, J. A.; Gladfelter, W. L.; Haugstad, G. Macromolecules 1999, 32, 3360. (140) Frank, B.; Gast, A. P.; Russell, T. P.; Brown, H. R.; Hawker, C. Macromolecules 1996, 29, 6531. (141) DeMaggio, G. B.; Frieze, W. E.; Gidley, D. W.; Zhu, M.; Hristov, H. A.; Yee, A. F. Physical Review Letters 1997, 78, 1524. (142) Schwab, A. D.; Agra, D. M. G.; Kim, J. H.; Kumar, S.; Dhinojwala, A. Macromolecules 2000, 33, 4903. -92 - (143) Kawana, S.; Jones, R. A. L. Physical Review E 2001, 63, art. no. 021501 (144) Forrest, J. A.; Dalnoki-Veress, K.; Stevens, J. R.; Dutcher, J. R. Physical Review Letters 1996, 77, 2002. (145) Alcoutlabi, M.; McKenna, G. B. Journal ofPhysics-Condensed Matter 2005, 17, R461. (146) Simon, S. L.; Park, J. Y.; McKenna, G. B. European Physical Journal E 2002, 8, 209. (147) Kerle, T.; Lin, Z. Q.; Kim, H. C.; Russell, T. P. Macromolecules 2001, 34, 3484. (148) Jean, Y. C.; Zhang, R. W.; Cao, H.; Yuan, J. P.; Huang, C. M.; Nielsen, B.; AsokaKumar, P. Physical Review B 1997, 56, R8459. (149) Xie, L.; Demaggio, G. B.; Frieze, W. E.; Devries, J.; Gidley, D. W.; Hristov, H. A.; Yee, A. F. Physical Review Letters 1995, 74, 4947. (150) Ge, S.; Pu, Y.; Zhang, W.; Rafailovich, M.; Sokolov, J.; Buenviaje, C.; Buckmaster, R.; Overney, R. M. Physical Review Letters 2000, 85, 2340. (151) Liu, Y.; Russell, T. P.; Samant, M. G.; StoThr, J.; Brown, H. R.; Cossy-Favre, A.; Diaz, J. Macromolecules 1997, 30, 7768. (152) Weber, R.; Zimmermann, K. M.; Tolan, M.; Stettner, J.; Press, W.; Seeck, 0. H.; Erichsen, J.; Zaporojtchenko, V.; Strunskus, T.; Faupel, F. Physical Review E 2001, 64, 61508. (153) Zhang, C.; Hong, S. C.; Ji, N.; Wang, Y. P.; Wei, K. H.; Shen, Y. R. Macromolecules 2003, 36, 3303. (154) Schwab, A. D.; Dhinojwala, A. PhysicalReviewE2003, 67, art. no.021802 -93 - (155) Guyotsionnest, P.; Hunt, 3. H.; Shen, Y. R. Physical Review Letters 1987, 59, 1597. (156) Miranda, P. B.; Shen, Y. R. Journal ofPhysical Chemistiy B 1999, 103, 3292. (157) Shen, Y. R.; Ostroverkhov, V. Chemical Reviews 2006, 106, 1140 (158) Himmeihaus, H.; Eister, F.; Buck, M.; Grunze, M. Journal ofPhysical Chemistry B 2000, 104, 576. (159) Gan, W.; Wu, B.; Chen, H.; Guo, Y.; Wang, H. Chemical Physics Letters 2005, 406, 467. (160) Wei, X.; Zhuang, X.; Hong, S. C.; Goto, T.; Shen, Y. R. Physical Review Letters 1999, 82, 4256. (161) Tsang, 0. C.; Tsui, 0. K. C.; Yang, Z. Physical Review E 2001, 63, art. no. 061603 (162) Wu, W. L.; Sambasivan, S.; Wang, C. Y.; Wallace, W. E.; Genzer, J.; Fischer, D. A. European Physical Journal E 2003, 12, 127. (163) Brandrup, J.; Immergut, E. H.; Grulke, E. A. Polymer Handbook, 4th Ed. ed.; John Wiley & Sons, 2003. (164) Held, H.; Lvovsky, A. I.; Wei, X.; Shen, Y. R. Physical Review B 2002, 66, 025110 (165) Wray, J. H.; Neu, 3. T. Journal ofthe Optical Society ofAmerica 1969, 59, 774. (166) Cariou, J. M.; Dugas, J.; Martin, L.; Michel, P. Applied Optics 1986, 25, 334. (167) Perez, J.; Cavaille, J. Y.; David, L. Journal of Molecular Structure 1999, 479, 183. -94 - (168) Fukao, K.; Uno, S.; Miyamoto, Y.; Hoshino, A.; Miyaji, H. Physical Review E 2001, 64, 051807. (169) Quinson, R.; Perez, J.; Gennain, Y.; Murraciole, J. M. Polymer 1995, 36, 743. (170) Casalini, R.; Roland, C. M.; Capaccioli, S. Journal of Chemical Physics 2007, 126, 184903. (171) Roth; C. B.; Pound, A.; Kamp, S.W.; Murray, C.A.; Dutcher, J.R. European Physical Journal E 2006, 20, 441. (172) Soldera, A.; Grohens, Y. Polymer 2004, 45, 1307. (173) Soldera, A.; Grohens, Y. Polymer-Plastic Technology and Engineering 2002, 41, 561 (174) Floudas, G.; Stepanek, P. Macromolecules 1998, 31, 6951. (175) Kuebler, S. C.; Schaefer, D. J.; Boeffel, C.; Paweizik, U.; Spiess, H. W. Macromolecules 1997, 30, 6597. (176) Tanaka, S.; Machida, S.; Yamashita, T.; Hone, K. Macromolecular Chemical Physics 1996, 197, 4095. (177) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Bums, P. L.; Holmes, A. B. Nature 1990, 347, 539. (178) Skotheim, T. A.; Elsenbaumer, R. L.; Reynolds, J. R. Handbook of Conducting Polymers, 2nd ed.; Marcel Dekker: New York, 1998. (178) Swager, T. M. Accounts of Chemical Research 1998, 31, 201. (179) Yu, G.; Gao, J.; Hummelen, 3. C.; Wudi, F.; Heeger, A. J. Science 1995, 270, 1789. -95 - (180) Nguyen, T. Q.; Martini, I. B.; Liii, J.; Schwartz, B. J. Journal Physical Chemistry B 2000, 104, 237. (181) Coakley, K. M.; McGehee, M. D. Chemistry ofMaterials 2004, 16, 4533. (182) Gustafsson, G.; Cao, Y.; Treacy, G. M.; Kiavetter, F.; Colaneri, N.; Heeger, A. J. Nature 1992, 357, 477. (183) Brabec, C. 3.; Sariciftci, N. S.; Hummelen, J. C. Advanced Functional Materials 2001, 11, 15. (184) Huitema, H. E. A.; Gelinck, G. H.; van der Putten, J. B. P. H.; Kuijk, K. E.; Hart, C. M.; Cantatore, E.; Herwig, P. T.; van Breemen, A. 3. 3. M.; de Leeuw, D. M. Nature 2001, 414, 599. (185) Hoizer, W.; Penzkofer, A.; Gong, S. H.; Bradley, D. D. C.; Long, X.; Bleyer, A. Chemical Physics 1997, 224, 315. (186) Bredas, 3. L.; Cornil, J.; Beljonne, D.; dos Santos, D.; Shuai, Z. G. Accounts of Chemical Research 1999, 32, 267. (187) Schwartz, B. J. Annual Review ofPhysical Chemistry 2003, 54, 141. (188) Pope, M.; Swenberg, C. E. Electronic Processes in Organic Crystals and Polymers; Oxford University Press: New York, 1999. (189) Shi, Y.; Liii, J.; Yang, Y. Journal ofApplied Physics 2000, 87, 4254. (190) Collison, C. J.; Rothberg, L. J.; Treemaneekarn, V.; Li, Y. Macromolecules 2001, 34, 2346. (191) Kohier, B. E.; Samuel, I. D. W. Journal ofChemical Physics 1995, 103, 6248. (192) Yaliraki, S. N.; Silbey, R. J. Journal ofChemical Physics 1996, 104, 1245. (193) Rossi, G.; Chance, R. R.; Silbey, R. Journal ofChemical Physics 1989, 90, 7594. -96 - (194) Koynov, K.; Bahtiar, A.; Ahn, T.; Bubeck, C.; Horhold, H. H. Applied Physics Letters 2004, 84, 3792. (195) Wu, X. F.; Shi, G. Q.; Qu, L. T.; Zhang, J. X.; Chen, F. E. Journal of Polymer Science, Part A: Polymer Chemistiy 2003, 41, 449. (196) Wu, X. F.; Shi, G. Q.; Chen, F. E.; Han, S. H.; Peng, J. B. Journal of Polymer Science, Part A: Polymer Chemistiy 2004, 42, 3049. (197) Tammer, M.; Monkman, A. P. Advanced Materials 2002, 14, 210. (198) Yang, X. 3.; Dykstra, T. E.; Scholes, G. D. Physical Review B 2005, 71, art. no. 045203. (199) Friend, R. H.; Bradley, D. D. C.; Townsend, P. D. Journal ofPhysics D: Applied Physics 1987, 20, 1367. (200) Hagler, T. W.; Pakbaz, K.; Heeger, A. J. Physical Review B 1994, 49, 10968. (201) Oberski, 3. M.; Greiner, A.; Bassler, H. Chemical Physics Letters 1991, 184, 391. (202) Yu, J. W.; Fann, W. S.; Kao, F. J.; Yang, D. Y.; Lin, S. H. Synthetic Metals 1994, 66, 143. (203) Gierschner, J.; Comil, J.; Egelhaaf, H. J. Advanced Materials 2007, 19, 173. (204) Mullen, K.; Wegner, G. Electronic Materials: The Oligomer Approach Mullen; Wiley-VCH: Weinheim, Germany, 1998. (205) Salzner, U. Current Organic Chemistry 2004, 8, 569. (206) Scholes, G. D.; Larsen, D. S.; Fleming, G. R.; Rumbles, G.; Bum, P. L. Physical Review B 2000, 61, 13670. -97 - (207) Chang, R.; Hsu, J. H.; Fann, W. S.; Liang, K. K.; Chiang, C. H.; Hayashi, M.; Yu, J.; Lin, S. H.; Chang, E. C.; Chuang, K. R.; Chen, S. A. Chemical Physics Letters 2000, 317, 142. (208) Scott, R. P. W. Analyst 2000, 125, 1543. (209) Dangelo, M.; Onori, G.; Santucci, A. Journal of Chemical Physics 1994, 100, 3107. (210) Wakisaka, A.; Abdoul-Carime, H.; Yamamoto, Y.; Kiyozumi, Y. Journal of the Chemical Society-Faraday Transactions 1998, 94, 369. (211) Dixit, S.; Poon, W. C. K.; Cram, 3. Journal ofPhysics-Condensed Matter 2000, 12, L323. (212) Wilson, K. R.; Schaller, R. D.; Co, D. T.; Saykally, R. 3.; Rude, B. S.; Catalano, T.; Bozek, 3. D. Journal ofChemical Physics 2002, 117, 7738. (213) Guo, J. H.; Luo, Y.; Augustsson, A.; Kashtanov, S.; Rubensson, 3. E.; Shuh, D.; Zhuang, V.; Ross, P.; Agren, H.; Nordgren, J. Journal of Electron Spectroscopy and Related Phenomena 2004, 137, 425. (214) Guo, J. H.; Luo, Y.; Augustsson, A.; Kashtanov, S.; Rubensson, 3. E.; Shuh, D. K.; Agren, H.; Nordgren, J. Physical Review Letters 2003, 91, -. (215) Dixit, S.; Cram, J.; Poon, W. C. K.; Finney, 3. L.; Soper, A. K. Nature 2002, 416, 829. (216) Dougan, L.; Bates, S. P.; Hargreaves, R.; Fox, 3. P.; Cram, J.; Finney, J. L.; Reat, V.; Soper, A. K. Journal of Chemical Physics 2004, 121, 6456. (217) Finney, J. L.; Bowron, D. T.; Daniel, R. M.; Timmins, P.; Roberts, M. A. Biophysical Chemistry 2003, 105, 391. -98 - (218) Ferrario, M.; Haughney, M.; Mcdonald, I. R.; Klein, M. L. Journal of Chemical Physics 1990, 93, 5156. (219) Allison, S. K.; Fox, J. P.; Hargreaves, R.; Bates, S. P. Physical Review B 2005, 71, 024201. (220) Tarek, M.; Tobias, D. J.; Klein, M. L. Journal of the Chemical Society-Faraday Transactions 1996, 92, 559. (221) Chang, T. M.; Dang, L. X. Journal ofPhysical Chemistry B 2005, 109, 5759. (222) Matsumoto, M.; Takaoka, Y.; Kataoka, Y. Journal of Chemical Physics 1993, 98, 1464. (223) Stewart, E.; Shields, R. L.; Taylor, R. S. Journal ofPhysical Chemistry B 2003, 107, 2333. (224) Wakisaka, A.; Ohici, T. Faraday Discussions 2005, 129, 231. (225) Yilmaz, H.; Athorne, C. Journal ofPhysics a-Mathematical and General 2002, 35, 2619. (226) Barraclough, C. G.; Metigue, P. T.; Ng, Y. L. Journal of Electroanalytical Chemistry 1992, 329, 9. (227) Castellanos, A. J.; Toro-Mendoza, 3.; Urbina-Villalba, G.; Garcia-Sucre, M. Fluid Phase Equilibria 2007, 262, 87. (228) Howes, A. J.; Radke, C. J. Langmuir 2007, 23, 11580. (229) Paul, S.; Chandra, A. Journal ofPhysical Chemistry B 2007, 111, 12500. (230) Hiester, T.; Dietrich, S.; Mecke, K. Journal of Chemical Physics 2006, 125, 184701. -99 - (231) Bowers, J.; Zarbakhsh, A.; McLure, I. A.; Webster, J. R. P.; Steitz, R.; Christenson, H. K. Journal ofPhysical Chemistry C 2007, 111, 5568. (232) Ma, G.; Allen, H. C. Journal ofPhysical Chemistry B 2003, 107, 6343. (233) Chen, H.; Gan, W.; Lu, R.; Guo, Y.; Wang, H. F. Journal ofPhysical Chemistry B 2005, 109, 8064. (234) Zhang, L. N.; Liu, W. T.; Shen, Y. R.; Cahill, D. G. Journal of Physical Chemistry C 2007, 111, 2069. (235) Iloukhani, H.; Rezaei-Sameti, M.; Basiri-Parsa, J. Journal of Chemical Thermodynamics 2006, 38, 975. (236) Al Gherwi, W. A.; Nhaesi, A. H.; Asfour, A. F. A. Journal ofSolution Chemistry 2006, 35, 455. (237) Pandey, J. D.; Mishra, R. K. Physics and Chemistry ofLiquids 2005, 43, 49. (238) Kahi, H.; Wadewitz, T.; Winkelmann, 3. Journal of Chemical and Engineering Data 2003, 48, 580. (239) Lei, Q. F.; Lin, R. S.; Ni, D. Y.; Hou, Y. C. Journal of Chemical and Engineering Data 1997, 42, 971. (240) Pandey, J.D.; Jam, P.; Vyas, V. Pramana-Journal ofPhysics 1994, 43, 361. (241) Olafadehan, 0. A.; Susu, A. A. Adsorption Science & Technology 2005, 23, 195. (242) Derylomarczewska, A.; Jaroniec, M.; Oscik, J.; Marczewski, A. W. Journal of Colloid and Interface Science 1987, 117, 339. (243) Belton, G. R. Metallurgical Transactions B-Process Metallurgy 1976, 7, 35. (244) Belton, G. R. Jom-Journal ofMetals 1975, 27, A21. -100- (245) Dabrowski, A.; Jaroniec, M. Journal of Colloid and Interface Science 1980, 73, 475. (246) Dabrowski, A. Zeitschrfl Fur Physikalische Chemie-Leipzig 1986, 267, 494. (247) Gu, T. Y.; Tsai, G. J.; Tsao, G. T. Aiche Journal 1991, 37, 1333. (248) Woifrum, K.; Lobau, J.; Laubereau, A. Appplied Physics A: Materials Science and Processing 1994, 59, 605. (249) Du, J. C.; Cormack, A. N. Journal of the American Ceramic Society 2005, 88, 2532. (250) Gautam, K. S.; Schwab, A. D.; Dhinojwala, A.; Zhang, D.; Dougal, S. M.; Yeganeh, M. S. Physical Review Letters 2000, 85, 3854. (251) Lu, R.; Gan, W.; Wu, B. H.; Zhang, Z.; Guo, Y; Wang, H. F. Journal ofPhysical ChemistryB2005, 109, 14118. (252) Hommel, E. L.; Allen, H. C. Analyst 2003, 128, 750-755. (253) Shen, Y. R.; Ostroverkhov V. Chemical Reviews 2006, 106, 1140. (254) Lu, R.; Gan, W.; Wu, B. H.; Chen, H.; Wang, H. F. Journal ofPhysical Chemistry B 2004, 108, 7297. (255) Voumard, P.; Zhan, Q.; Zenobi, R. Langmuir 1995, 11, 842. (256) Ostroverkhov, V.; Waychunas, G. A.; Shen, Y. R. Physical Review Letters 2005, 94, 046102. (257) Du, Q.; Freysz, E.; Shen, Y. R. Physics Review Letters 1994, 264, 826. (258) Wang, H. F.; Gan, W.; Lu, R.; Rao, Y.; Wu, B. H. International Reviews in Physical Chemistry 2005, 24, 191-256. -101 -

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