Received March 14, In Final Form: May 10, 2002

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1 Langmuir 2002, 18, Glass Transition Temperature and Conformational Changes of Poly(methyl methacrylate) Thin Films Determined by a Two-Dimensional Map Representation of Temperature-Dependent Reflection-Absorption FTIR Spectra Hyeon Suk Shin,, Young Mee Jung,, Tae Young Oh,, Taihyun Chang, Seung Bin Kim,*,, Do Hyung Lee, and Isao Noda Laboratory for Vibrational Spectroscopy, BK21 Functional Polymer Thin Film Group, Department of Chemistry, Pohang University of Science & Technology, San 31, Hyojadong, Pohang , Republic of Korea, Materials Analysis Team, Research Institute of Industrial Science & Technology, San 32, Hyojadong, Pohang , Republic of Korea, and The Procter and Gamble Company, 8611 Beckett Road, West Chester, Ohio Received March 14, In Final Form: May 10, 2002 Temperature-dependent reflection-absorption Fourier transform infrared spectra of poly(methyl methacrylate) (PMMA) thin films were measured. From these spectra, two-dimensional (2D) maps of the first derivative of the absorbance with respect to temperature over the space of wavenumber and temperature were constructed. These maps were used to determine the glass transition temperature and to investigate the subtle temperature dependence of the population of trans and gauche conformers in three PMMA thin films with different stereoregularities. The glass transition temperatures determined from the 2D maps for isotactic, syndiotactic, and atactic PMMA thin films were approximately 61, 111, and 106 C, respectively. Calculations of the fraction of gauche conformation from the conformational energy showed an abrupt increase in the population of the gauche conformer above the glass transition temperature. The temperature dependence of the fraction of gauche conformation was well reflected in our 2D maps. Furthermore, it was found that molecular reorganization of PMMA occurs even below the glass transition temperature. Introduction The glass transition temperature of polymer thin films has recently received much attention because of the importance of the thermal property in the technological applications of thin films. 1 Conventional methods used to measure the glass transition temperature of bulk polymers such as differential scanning calorimetry and dynamic mechanical analysis are not well suited to the thermal characterization of polymer thin films. Thus, a number of methods have been developed to measure the glass transition temperature of polymer thin films using optical probes, for instance, X-ray reflectivity, 2,3 ellipsometry, 3-6 positron lifetime spectroscopy, 7 Brillouin light scattering, 8,9 optical waveguide spectroscopy, 10 and Fourier transform infrared (FTIR) spectroscopy. 11,12 * Corresponding author. Telephone: Fax: sbkim@postech.ac.kr. Laboratory for Vibrational Spectroscopy, Pohang University of Science & Technology. BK21 Functional Polymer Thin Film Group, Department of Chemistry, Pohang University of Science & Technology. Materials Analysis Team, Research Institute of Industrial Science & Technology. The Procter and Gamble Company. (1) Allara, D. L.; Atre, S. V.; Parikh, A. N. Polymer Surfaces and Interfaces II; John Wiley & Sons: Chichester, (2) Reiter, G. Macromolecules 1994, 27, (3) See, Y.; Cha, J.; Chang, T.; Ree, M. Langmuir 2000, 16, (4) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Europhys. Lett. 1994, 27, 59. (5) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Faraday Discuss. 1994, 98, 219. (6) Grohens, Y.; Brogly, M.; Labbe, C.; David, M.; Schultz, J. Langmuir 1998, 14, (7) Demaggio, G. B.; Frieze W. E.; Gidley, D. W.; Zhu, M.; Hristov, H. A.; Yee, A. F. Phys. Rev. Lett. 1997, 78, (8) Forrest, J. A.; Dalnoki-Veress, K.; Stevens, J. R.; Dutcher, J. R. Phys. Rev. Lett. 1996, 77, For example, the glass transition temperature of polymer thin films has been determined from kinetic ellipsometric scans, 3-6 which measure the change in ellipsometric angle ( ) with temperature (T). In this method, the glass transition temperature of the thin film is identified as a discontinuity in [d /dt], the slope of versus T. Since the ellipsometric angle varies linearly with the thickness (h), its derivative is related to the thermal expansion coefficient [(dh/dt)h -1 ]. Measurement of the glass transition temperature using optical waveguide and X-ray reflectivity is also based on the use of optical measurements to detect the temperature-dependent volume change of polymer thin films. FTIR spectroscopy is an important technique in the measurement of the glass transition temperature of polymers. The utility of FTIR stems from the specificity of the IR probe to different submolecular and segmental constituents of polymeric systems, which gives this method a unique capacity to elucidate the molecular origin of transition phenomena. For example, O Reilly et al. 13,14 reported that the glass transition temperature of bulk polymer could be determined from the conformational energy ( E ) E tg - E tt, where E tg is the energy of the trans-gauche state and E tt is the energy of the transtrans state) calculated using FTIR spectroscopy. The conformational energy obtained from FTIR spectroscopy (9) Forrest, J. A.; Dalnoki-Veress, K.; Stevens, J. R.; Dutcher, J. R. Phys. Rev. B 1997, 56, (10) Prucker, O.; Christian, S.; Bock, H.; Ruhe, J.; Frank, C.; Knoll, W. Macromol. Chem. Phys. 1998, 199, (11) Grohens, Y.; Brogly, M.; Labbe, C.; Schultz, J. Eur. Polym. J. 1997, 33, 691. (12) Grohens, Y.; Brogly, M.; Labbe, C.; Schultz, J. Polymer 1997, 38, (13) O Reilly, J. M.; Mosher, R. A. Macromolecules 1981, 14, 602. (14) O Reilly, J. M.; Teegarden, D. M.; Mosher, R. A. Macromolecules 1981, 14, /la020258y CCC: $ American Chemical Society Published on Web 06/26/2002

2 5954 Langmuir, Vol. 18, No. 15, 2002 Shin et al. was claimed to be consistent with that predicted by the rotational isomeric state calculation. 15 The intensities of some IR bands of polymers showed relatively noticeable changes in the temperature dependence (presumably related to the segmental conformational energy) at the glass transition temperature, so the conformational energies were calculated for the temperatures above and below the glass transition temperature. This conformational energy calculation has also been applied to the study of polymer thin films. Grohens et al. 12 used FTIR spectroscopy to probe conformational changes in poly(methyl methacrylate) (PMMA) chains at gold and aluminum surfaces. Their results indicated an increase in conformational energy at the surface. However, the basic trend of the temperature dependence of bands related to the conformation of PMMA thin films was the same as that reported by O Reilly et al. for bulk PMMA. Thus, they were able to determine the glass transition temperature from the discontinuity in the slope of the plot of the logarithm of absorbance versus (1/T). In the present study, we suggest a new method for determining the glass transition temperature by using FTIR spectroscopy. The method simultaneously probes subtle changes of intensities of individual IR bands by monitoring the first derivative of the absorbance with respect to temperature. We propose the construction of two-dimensional (2D) maps of the first derivative of the absorbance with respect to temperature over the space of temperature versus wavenumber. Such maps should show all changes in the band intensity, thus enabling the identification of subtle changes in the conformations of isotactic, syndiotactic, and atactic PMMA thin films. We test the validity of the glass transition temperatures determined from the 2D maps by comparing these values with the transition temperatures determined by the fraction of the gauche conformation, which is obtained from the conformational energy calculated using difference spectra. 12,13 In addition, it is considered that the decrease of the fraction of the gauche conformation even below the glass transition temperature is related to the reorganization of PMMA. Experimental Section We purchased isotactic PMMA (tacticity > 80%; MW, ) and atactic PMMA (tacticity: syndio 56%, hetero 38%, iso 6%; MW, ) from Aldrich Chemical Co. Ltd. and syndiotactic PMMA (tacticity, 85%; MW, ) from Scientific Polymer Products, Inc. Au-coated silicon wafers from Lance Goddard Associates (USA) were used as the substrates for spin coating. All substrates were cleaned in fresh piranha solution (30% H 2O 2 mixed in a 1:5 ratio with concentrated H 2SO 4) prior to spin coating. To prepare the spin-coated films, about 1 wt % PMMA solution dissolved in toluene was spun onto an Au-coated silicon wafer at 1000 rpm for 30 s. The thicknesses of isotactic, syndiotactic, and atactic PMMA thin films were 371, 228, and 280 Å, respectively. FTIR spectra were recorded at a spectral resolution of 4 cm -1 and at intervals of 10 C in the range of C (160 C for syndiotactic PMMA) with a Bomem DA8 FTIR spectrometer equipped with a liquid-nitrogen-cooled MCT detector. The Seagull attachment (Harrick Scientific Corp.), which includes a heating block attachment, was used in this study. All external reflection FTIR spectra were obtained with p-polarized radiation at an angle of incidence of 82. To ensure a high signal-to-noise ratio, 1024 scans were coadded. Although the FTIR spectra of the polymer thin films were recorded between 450 and 4000 cm -1, only the region between 1100 and 1300 cm -1 is described in detail in this paper. The sample and source compartments were evacuated to 0.8 Torr. The first derivative of the spectral intensity (15) Sundararajan, P.; Flory, P. J. J. Am. Chem. Soc. 1974, 96, Figure 1. Reflection-absorption FTIR spectra of isotactic (a), syndiotactic (b), and atactic (c) PMMA thin films. Arrows in the inset indicate the direction of the temperature increase. The measured temperature range is , , and C for isotactic, syndiotactic, and atactic PMMA thin films, respectively. with respect to temperature was calculated using the software of MATLAB (The Math Works Inc.). Results and Discussion Reflection-absorption FTIR spectra were measured for three PMMA thin films with different stereoregularities on the gold surface. Figure 1a-c shows the FTIR spectra measured at various temperatures of isotactic (a), syndiotactic (b), and atactic (c) PMMA samples. All of these PMMA spectra show considerable intensity variations in

3 2D Map Representation of PMMA Thin Films Langmuir, Vol. 18, No. 15, Table 1. Average Temperatures of the Temperature Range at Which the Minimum and Maximum of the First Derivative Value of Each Peak in the Range between 1100 and 1300 cm -1 Appear isotactic PMMA syndiotactic PMMA Atactic PMMA peak wavenumber half temp ( C) peak wavenumber half temp ( C) peak wavenumber half temp ( C) average temp 61 average temp 111 average temp 106 the region of cm -1. Although spectra of PMMA have been analyzed in several studies, band assignments in the region of cm -1 are not yet completely clear. However, the absorption bands in that region are known to be assigned to the ν a (C-C-O) mode coupled to the ν(c-o) mode and are very sensitive to conformational changes. Abrupt changes in the intensity of bands in the region of cm -1, which contain information on the polymer conformation of the polymer thin films, are expected below and above the glass transition temperature. Such intensity changes are generally attributed to a large change in polymer chain mobility at the glass transition, resulting in significant bond reorientation. However, the exact temperature at which the intensities of almost all pertinent bands sensitive to the polymer conformation change most rapidly is not obvious from the conventional FTIR spectra, although trends can be seen in the change in intensity with increasing temperature as depicted by arrows in Figure 1. In addition, it is very difficult to estimate the extent of the intensity change of a specific band in a certain temperature range. Our approach for determining the glass transition temperature of a polymer thin film is based on a simple mathematical treatment. We have a data set of spectra A(ν, T), where ν is the wavenumber and T is the temperature. To probe the subtle change of the spectral intensity according to the temperature, we consider the first derivative of A with respect to T at each ν, because it is sensitive to subtle changes in the spectral intensity with changing temperature. By plotting the value of the first derivative, da(ν, T)/dT, as a function of wavenumber and temperature, we obtain a 2D map over the independent axes of wavenumber and temperature. And then, we can easily observe changes in intensities of all bands in the whole spectral range, that is, in the overall mobility of the polymer in the whole temperature range. The position of the maximum and minimum of the first derivative value of each band on this 2D map might be closely involved with the transition temperature because the reorientation of the polymer at the glass transition is expected to result in large changes in IR intensities of almost all bands sensitive to the polymer conformation with respect to temperature. Figure 2a-f shows 2D maps for isotactic (a and b), syndiotactic (c and d), and atactic (e and f) PMMA thin (16) Havriliak, S.; Roman, N. Polymer 1966, 7, 387. (17) Willis, H. A.; Zichy, V. J.; Hendra, P. J. Polymer 1969, 10, 737. (18) Spevacek, J.; Schneider, B. Adv. Colloid Interface Sci. 1987, 27, 81. (19) Grohens, Y.; Prud homme, R. E.; Schultz, J. Macromolecules 1998, 31, (20) Grohens, Y.; Carriere, P.; Spevacek, J.; Schultz, J. Polymer 1999, 40, (21) Chen, J.; Zheng, G.; Xu, L.; Zhang, J.; Lu, Y.; Xue, G.; Yang, Y. Polymer 2001, 42, Table 2. Frequencies of Pertinent Bands Sensitive to the Polymer Conformation in 1D Spectra at the Starting Temperature and the 2D Map at the Transition Temperature for the Three Stereoregular PMMA Thin Films Isotactic PMMA Syndiotactic PMMA atactic PMMA 1D 2D a 1D 2D a 1D 2D a a Frequencies obtained from the first derivative peaks of 2D maps at the glass transition temperatures. films. In these maps, negative values of the first derivative, corresponding to decreasing intensity with increasing temperature, are shown as dotted lines, while positive values are shown as solid lines. The spectra at the initial temperature (30 C) in Figure 2a,c,e are supplied as references at the right side of the 2D map. The glass transition temperature would be determined by locating the line parallel to the wavenumber axis on each 2D map at which the first derivatives of almost all pertinent bands sensitive to the polymer conformation change most rapidly with temperature. However, the transition occurs not at a specific temperature but in a narrow temperature range. From Figure 2b,d,f, we can observe a temperature range in which the first derivatives of the intensities of almost all bands sensitive to the polymer conformation in the region of cm -1 change significantly with temperature. For example, the 2D map of isotactic PMMA in Figure 2b shows a peak at 1273 cm -1 at the initial temperature, which is deconvoluted to 1273, 1256, and 1226 cm -1 at 55 C. In this temperature range, the first derivatives of peaks at 1200 and 1140 cm -1 also change greatly. Figure 3 shows the first derivative values as a function of temperature for peaks at 1273 (a), 1256 (b), 1200 (c), and 1140 (d) cm -1 extracted from Figure 2b. Temperatures with arrows in Figure 3 indicate the maximum and minimum of the first derivative values. The glass transition temperature is determined as the average of the average temperatures between the minima and maxima of the first derivative value of each peak. The results for three PMMA samples are summarized in Table 1. The glass transition temperatures were 61, 111, and 106 C for isotactic, syndiotactic, and atactic PMMA thin films, respectively. For syndiotactic and atactic PMMA samples, peaks at 1153 and 1155 cm -1 were considered additionally because the first derivative values of these peaks are also temperature dependent. Furthermore, in Figure 2b,d,f, spectra (dotted lines) near the glass transition temperatures (60, 110, and 100 C for isotactic, syndiotactic, and atactic PMMA thin films, respectively) and spectra (solid lines) at 30 C are supplied on the right side of the 2D map. Intensity changes and frequency shifts of bands in the FTIR spectra are obviously reflected in the 2D maps. The frequencies of some bands are summarized in Table 2. These bands in the cm -1 region are considered to reflect the segmental motion of the polymer because the bands are assigned to the ν a (C-C-O) mode coupled to the ν(c-o) mode which are known to be very sensitive to conformational changes To verify our results, we calculated the conformational energy according to the procedure reported previously. 12,13 Conformational energies were calculated by the van t Hoff equation using the intensities of negative and positive

4 5956 Langmuir, Vol. 18, No. 15, 2002 Shin et al. Figure 2. 2D maps for isotactic (a and b), syndiotactic (c and d), and atactic (e and f) PMMA thin films as a function of the temperature and the wavenumber. Solid and dotted lines are positive and negative values, respectively. The spectral region of cm -1 is expanded in (b), (d), and (f) for more detailed observations. Dotted lines parallel to the wavenumber axis in (b), (d), and (f) indicate the glass transition temperatures. bands of the doublet in the cm -1 region in the difference spectra (the spectrum at temperature T minus the spectrum at the starting temperature, 30 C) based on a data set of FTIR spectra as shown in Figure 1. The population ratio of gauche to trans conformation is related to the conformational energy ( E): 12 n g n t ) e (- E/RT) (1) where n g and n t are the populations of gauche and trans states, respectively, and R is the gas constant. Then, the fraction of sequences containing gauche conformations (f g ) can be calculated from 20 f g ) n g ) e(- E/RT) (2) n g + n t 1 + e (- E/RT) Figure 4 shows f g calculated for the three PMMA systems with different stereoregularities. Taking the temperature of minimum f g to be the glass transition temperature, we obtain glass transition temperatures of approximately 60,

5 2D Map Representation of PMMA Thin Films Langmuir, Vol. 18, No. 15, Figure 3. The first derivative values as a function of temperature for bands at 1275 (a), 1256 (b), 1200 (c), and 1140 (d) cm -1 extracted from Figure 2b. Temperatures indicated with arrows are maxima and minima of the first derivative values. Figure 4. The fraction of sequences containing gauche conformations (f g) for three stereoregular PMMA thin films. 105, and 100 C for isotactic, syndiotactic, and atactic PMMA thin films, respectively. These values are consistent with the results obtained from the 2D maps, which are 61, 111, and 106 C for isotactic, syndiotactic, and atactic PMMA thin films, respectively. The value of f g decreases gradually with increasing temperature up to the glass transition temperature. This strongly suggests that all of the PMMA samples undergo substantial molecular reorganization even below the glass transition temperature. We conjecture from Figure 4 that the reorganization occurs in the temperature range of C for isotactic PMMA and C for syndiotactic PMMA. In the case of isotactic PMMA, we cannot determine exactly the starting temperature of the reorganization because its spectra were not measured at sufficiently low temperatures. Although the starting temperature of the reorganization for atactic PMMA can be estimated to be 40 C as shown in Figure 4, determination of the starting temperature of the reorganization process is hindered due to the uncertainty of f g calculation by the low absorbances at low temperature in the difference spectra. If we assume that the extensive reorientation of the polymer backbone does not freely occur below the glass transition temperature, we are left with the conclusion that the side groups are responsible for the reorganization. Above the glass transition temperature, the fraction of gauche conformations gradually increases because of the mobility or reorientation of PMMA chains. Finally, the fraction of gauche conformation reaches a constant value. We expect that the gauche fractions for syndiotactic and atactic PMMA thin films also reach constant values at higher temperatures, although this could not be tested

6 5958 Langmuir, Vol. 18, No. 15, 2002 Shin et al. because of the temperature limit of the heating block attachment. The present results indicate that the flexibility of PMMA chains above the glass transition temperature is related to the increase in the fraction of gauche conformations. From this, we can infer that the increasing fraction of the gauche conformation increases the free volume. The free volume above the glass transition temperature is also related to the thermal expansivity. 22 Grohens et al. 6 observed from ellipsometry that the thermal expansivity increases in the sequence of syndiotactic, atactic, and isotactic PMMA. Indeed, the slopes of the curves in Figure 4 show the same sequence above the glass transition temperature. The change in slopes on going from syndiotactic to isotactic PMMA may be due to the different contributions of the backbone and side groups to the conformational change in each system. Isotactic PMMA easily reaches the equilibrium between the backbone and side groups; 19 hence, its slope is large and it quickly reaches a constant value. In other words, the conformational energy of the backbone can be easily compensated for by the side chain rotations. In addition, the large slope, which accounts for the fast conformational change and the increase in flexibility by the motion of chain segments, implies the existence of the cooperativity in isotactic PMMA. In the case of syndiotactic PMMA, on the other hand, the localized strong interactions of the side groups reduce the compensatory effects of side chain rotation. 18,23,24 The comparison of the 2D maps in Figure 2 with Table 2 with the fraction of the gauche conformation in Figure 4 reveals that the peaks at 1275 and around 1256 cm -1 are characteristic for trans and gauche conformations of the PMMA thin films, respectively. 13 This comparison also shows that the reorganization region in Figure 4 coincides with the abrupt changes of the first derivatives at 1275 cm -1 in the 2D maps. As mentioned above, it is difficult to determine the starting temperature of the reorganization process in atactic PMMA from Figure 4. From the abrupt change of the first derivative in Figure 2f, however, the approximate temperature range of the reorganization in atactic PMMA is C. Dynamic mechanical analysis has shown an additional broad dispersion peak at 50 C below the glass transition temperature in PMMA, of which the glass transition temperature was about 105 C. 22 This peak is called the secondary or β-transition. Thus, the reorganization process is assumed to occur after the β-transition. In addition to the doublet at 1275/1260 cm -1 in atactic PMMA and that at 1275/1259 cm -1 in syndiotactic PMMA, two doublets at 1155/1142 and 1751/1742 cm -1 in atactic PMMA and corresponding ones at 1153/1140 and 1751/ 1740 cm -1 in syndiotactic PMMA also show a clear temperature dependence. The first derivative of one peak (22) Aklonis, J. J.; MacKnight, W. J. Introduction to Polymer Viscoelasticity, 2nd ed.; John Wiley & Sons: Chichester, (23) Vacatello, M.; Flory, P. J. Macromolecules 1986, 19, 405. (24) Sundararajan, P. R. Macromolecules 1986, 19, 414. at 1155 or 1742 cm -1 for atactic PMMA and the corresponding one at 1153 or 1740 cm -1 for syndiotactic PMMA are negative, whereas the first derivative of the other peak at 1142 or 1751 cm -1 for atactic PMMA and the corresponding one at 1140 or 1751 cm -1 for syndiotactic PMMA are positive. O Reilly et al. suggested in their model II that below the glass transition temperature the doublets around 1153/1140 and 1751/1740 cm -1 are associated with changes in side groups. 13,19 As shown in Figure 2, the first derivatives of both peaks of the doublet at 1160/1140 cm -1 in isotactic PMMA are positive continuously from the initial temperature and those at 1752/1740 cm -1 are positive and negative, respectively. The behavior of the doublets in isotactic PMMA is different from that in atactic and syndiotactic PMMA. Hence, the different temperature dependence of the doublets in isotactic PMMA may be due to conformational changes involving a large contribution from the side groups. One major advantage of representing the temperaturedependent FTIR spectra of PMMA in a 2D map with coordinates of temperature and wavenumber is the ability to compare temperature-dependent spectral changes even to subtle differences in the molecular responses as well as overall intensity change of each band sensitive to the PMMA conformation. The 2D maps in Figure 2 clearly show that there is a small but noticeable gap in the onset temperatures reflecting the population shift of trans and gauche conformers, indicating the complexity of the conformer population distribution of PMMA thin films, which cannot be adequately described by a simple model based strictly on the trans-gauche transition of the main chain backbone. Such subtle but information-rich spectral responses can be easily missed if one simply plots da/dt versus T only for the selected bands. The 2D map representation provides a quick visual survey of molecular level responses in three PMMA thin films. Conclusion Two-dimensional maps of the first derivative of the IR absorbance with respect to temperature with coordinates of wavenumber and temperature were successfully utilized to determine the glass transition temperature based on the whole macromolecular status and to probe the temperature-induced conformational changes in three PMMA thin films with different stereoregularities. The results in this study suggest the potential of the 2D map format for relating temperature-dependent spectral changes to subtle differences in the molecular responses. The present 2D map method will also provide a deeper insight into spectral changes induced by changes in other variables such as pressure and concentration. Acknowledgment. This work was supported in part by the Research Institute of Industrial Science & Technology and the Korean Ministry of Education (BK 21 Project). T.C. also acknowledges the support from KOSEF (Center for Integrated Molecular Systems). LA020258Y