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J. Phys. Chem. B 2008, 112, 694-697
Surface Structure Relaxation of Poly(methyl methacrylate) Qifeng Li, Rui Hua, Ignatius J. Cheah,† and Keng C. Chou* Department of Chemistry, UniVersity of British Columbia, VancouVer, BC V6T 1Z1, Canada ReceiVed: March 17, 2007; In Final Form: September 24, 2007
Surface structure relaxations caused by temperature changes at the free surface of poly(methyl methacrylate) were studied using IR-visible sum-frequency generation (SFG). A polarization-rotating technique was introduced to enhance the sensitivity of SFG for monitoring the surface structure relaxations during a cooling process. A new surface structure relaxation was observed at 67 °C. This temperature does not match any known structure relaxation temperatures for the bulk and is 40 °C below the bulk glass transition temperature. As expected for a free-surface phenomenon, the surface relaxation temperature was found to be independent of film thickness in the range of 0.1-0.5 µm.
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.1 Previous studies have shown that PMMA displays a complex dynamical behavior. Its glass and sub-glass relaxation processes have been studied by dielectric spectroscopy,2,3 dynamic mechanical analysis,4 neutron scattering,5-8 and NMR spectroscopy.9,10 The main features are the wellknown R- and β-relaxations. The R-relaxation, which is generally regarded as the glass transition, involves cooperative movements of the backbone, whereas the β-relaxation is thermally activated flips of local structural units by external fields. Compared to the bulk relaxation processes, the surface relaxation behaviors are relatively unknown. It is wellestablished that, in simple molecular systems, such as ice, the surface phase transition temperatures are lower than those for the bulk.11 An unresolved issue is whether polymeric materials have similar free-surface effects.12 Many technological applications, such as lithography and nanoimprinting,13,14 rely on the surface properties of PMMA. Therefore, the state of a PMMA surface is an important parameter for both scientific and technological reasons. Another active research area is the study of finite-size effects on the relaxation temperatures in confined systems.15 A reduction in glass transition temperature (Tg) was observed by Keddie et al. using an ellipsometer on polystyrene films with thicknesses less than 100 nm.16 For PMMA, Keddie et al. observed a Tg reduction of ∼6 °C for an ∼30 nm thick film on a gold surface, but the Tg increased with decreasing film thickness on a native oxide of silicon.17 Keddie et al. suggested that a liquid-like layer exists at the air/polymer surface;16,17 however, in these experiments, the polymer-substrate interactions cannot be excluded and, in some cases, can be as significant as the finite-size effect.18-21 Many other studies using various techniques, such as friction-force microscopy,22 X-ray reflectivity,15 fluorescent diffusion,23 positron annihilating,24 optical birefringence,25 and ellipsometry26 have also found a decrease in Tg with decreasing film thickness for thicknesses less than 100 nm. To study the * Corresponding author. E-mail:
[email protected]. † Current address: Department of Chemistry, National University of Singapore.
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.27 Similar Tg depression was also observed in other confined systems,28 such as nanopores29 or artificially roughened films.30 However, these results are not applicable to describing 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 nm. Because the free-surface effect is likely present only at the top few nanometers (if not monolayers) of the surface, answering this question would require an extremely surface-sensitive technique to reduce the bulk signal. Such experiments have become more feasible as surface-sensitive techniques have become available. Jean et al. reported a gradually decreasing Tg with decreasing probing depth on a thick polystyrene film (thickness ca. 1 µm) using positron annihilation spectroscopy.31 However, Xie et al. did not find such a free-surface effect using a similar technique.32 Studies using other surface-sensitive techniques, such as scanning force microscopy,33 near-edge X-ray absorption fine structure spectroscopy (NEXAFS),34 X-ray reflectivity,35 and sum-frequency generation (SFG),36-38 have not been able to confirm a reduced relaxation temperature at the free surface of various polymers with thicknesses greater than 100 nm. In this paper, we revisit this question, using a novel approach: polarization-rotating SFG. SFG is intrinsically surface sensitive because it is a second-order optical process.11 It 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.36 More recently, SFG was employed to study the surface glass transition of poly(vinyl alcohol)37 and polystyrene.38 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 is unclear what effect was introduced to the surface in the rubbing process, it is more desirable to carry out the experiment on a free
10.1021/jp072147j CCC: $40.75 © 2008 American Chemical Society Published on Web 12/29/2007
Surface Structure Relaxation of PMMA
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Figure 1. Experimental setup for polarization-rotating SFG. The IR is p-polarized (defined as 90° polarization angle), the visible beam is linearly polarized with a rotating polarization angle, and the SFG is detected at 45° polarization.
isotropic polymer surface in its natural state. In the current study, a polarization rotation 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 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 0.10.5 µm. Experimental Setup and Method Atactic PMMA (Mw ) 52 700, Mw/Mn ) 1.08, Scientific Polymer Products, Inc.) films were prepared by spin casting on fused silica windows and were annealed at 100 °C for 12 h before measurements. All films used in this study were thicker than 100 nm to avoid the aforementioned Tg depression caused by the finite-size effect or the polymer-substrate interaction. The film thickness was determined after the SFG studies by measuring the depth of a scratch mark using a scanning force microscope. The sample was sealed in a cell, and the sample temperature was controlled by a feedback program with accuracy better than 0.5 °C. As shown in Figure 1, SFG was carried out by mixing a visible (ω1) beam and an IR (ω2) beam on the surface to generate a third beam with frequency ωs ) ω1 + ω2. The visible and IR laser beams were generated using a Nd:YAG laser at 1064 nm (30 ps at 10 Hz). The laser was used to generate a second harmonic beam at 532 nm and to pump an optical parametric generator/amplifier for generating the IR beam. The 532 nm and IR beams were overlapped both spatially and temporally on the sample, and the SFG was detected by a photomultiplier tube after spectral filtering by a short-pass filter and a monochromator. Both the IR and visible beams were linearly polarized. The polarization of the visible beam was rotated by a half-wave plate mounted on a computer-controlled rotational stage.
Figure 2. SFG vibrational spectra of PMMA in ssp and ppp configurations. The peak at 2955 cm-1 is the symmetric stretching mode of the ester methyl group. The solid lines are fitting curves derived using a Lorentzian line shape. (2) and χeff,ppp , where ssp indicates s-, s-, and p-polarized for SFG, visible, and IR, respectively. The effective nonlinear susceptibilities include the nonlinear susceptibilities and Fresnel factors. (2) The detailed expressions of χ(2) eff,ppp and χeff,ssp can be found in references 41 and 42. Figure 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-1 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.43 As shown in Figure 2, both the ssp and ppp spectra can be fitted by a single Lorentzian line shape I(ωSFG) (2) 2 ∝ |χ(2) NR + A/{ωIR - ωo + iΓ}| , where χNR is the nonresonant contribution, A is the amplitude, Γ is the width, and ωo is the resonant wavenumber at 2955 cm-1. As shown in Figure 1, polarization-rotating SFG was achieved by fixing the IR polarization at p-polarization (defined as a 90° polarization angle), detecting the SFG at a 45° polarization angle, and rotating the polarization of the visible beam Ωvis by rotating a half-wave plate. This approach is mathematically similar to the null-angle method described by Gan et al., in which the polarizations of input beams were fixed and SFG intensities were measured at various polarization angles.44 In our approach, the SFG was always measured at a 45° polarization angle to avoid calibrating the polarization-dependent throughputs of some optics, such as a mirror or a monochromator. The measurements were carried out with the visible wavelength fixed at 532 nm and the IR wavenumber fixed at the resonant frequency of the ester methyl group (2955 cm-1) to monitor discontinuity in the orientation of the surface ester methyl group during a cooling process. The intensity of the measured SFG can be written as a function of the visible polarization angle Ωvis:
2 (2) ISFG(Ωvis) ∝ |χ(2) eff,sspsin Ωvis + χeff,pppcos Ωvis|
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.39 For an azimuthally isotropic surface, the second-order nonlinear susceptibility tensor χ(2) ijk has four independent non-vanishing elements: χxxz ) χyyz, χxzx ) χyzy, χzxx ) χzyy, and χzzz, with z being along the surface normal and x being in the plane of incidence in the laboratory coordinate system.40 With the IR fixed at p-polarization, as shown in Figure (2) 1, the nonzero effective nonlinear susceptibilities are χeff,ssp
(1)
where χ(2) eff is the effective second-order nonlinear susceptibility. The expression can be rewritten as
1 ISFG(Ωvis) ∝ |sin(Ωvis - Ωo)|2 2 with
Ωo ) -arctan
( ) χ(2) eff,ppp χ(2) eff,ssp
(2)
(3)
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Figure 3. A typical curve of SFG intensity vs visible polarization angle Ωvis. The solid line is a fitting curve using eq 2. The phase of the curve, indicated as Ωo, is used to monitor surface structure changes.
Figure 4. Measured Ωo as a function of time and temperature. The solid line is the fitted curve using eq 4.
Figure 3 shows a typical ISFG(Ωvis) curve measured at room temperature. Equation 2 was used to fit the curve, with Ωo and an additional proportional constant as the fitting parameters. The signal-to-noise ratio, presented in Figure 3, allows us to determine the values of Ωo with an error of (0.3 degree. Any change in the surface molecular orientation would change the (2) ratio of χ(2) eff,ppp/χeff,sspand introduce the phase shift described by eq 3. The change in Ωo was used to monitor the surface structure changes, instead of using the absolute SFG intensity. Therefore, long-term laser intensity fluctuations can be excluded from our measurements. This approach does not involve changing the polarizations of multiple laser beams commonly used to determine the orientation of the surface functional groups.37,39,41 Therefore, it is highly efficient and allows us to carry out a real-time recording of structure relaxation on an isotropic polymer surface, without the need to introduce a rubbinginduced alignment37,45 or an external stretching force46 to enhance the magnitude of surface structure changes. Figure 4 shows the measured Ωo as a function of temperature and time. Initially, the sample was kept above 140 °C for 90 min before SFG measurements were taken to stabilize the temperature of the cell. The temperature was then decreased at a rate of -0.3 °C/min. For each structure relaxation, data points were phenomenologically fitted by a hyperbolic tangent function:37
Ωo(T) )
(
)
Ω1 + Ω2 Ω1 - Ω2 T - To tanh 2 2 ∆T
(4)
where Ω1 and Ω2 are the low and high limits of Ωo before and after the relaxation, respectively. On the basis of this fitting method, two relaxation temperatures were obtained, at To )
107 ( 2 °C and 67 ( 2 °C, with ∆T ∼ 7 ( 2 °C and 3 ( 1 °C, respectively. The first structure relaxation at 107 °C is the bulk-induced structure relaxation, as it agrees well with the bulk Tg of atactic PMMA.47 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.48 Currently, there is no theory that can 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 glass transition demonstrates that the polarization-rotating 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 × 10-5/°C.49 As the temperature changes, it slightly changes the Fresnel factors, which are included in the effective nonlinear susceptibility in eq 3.42 Because the Fresnel factors appear in both χ(2) eff,ppp and (2) χeff,ssp , the effect is partially cancelled when only the ratio (2) χ(2) eff,ppp/χeff,ssp (or Ωo) was measured. Overall, it introduces a small slope in Figure 4, but it is not responsible for the shortrange steep changes of Ωo. The refraction index of PMMA decreases linearly with temperature, with a slope of approximately -1.4 × 10-4/°C below the glass transition temperature.50 The slope changes to approximately -3.4 × 10-4/ °C above the glass transition temperature. The discontinuity in the slope can produce a very small kink in the measured Ωo at the glass transition temperature, but not a step-like change as seen in Figure 4. Therefore, the observed Ωo change at 107 °C is mostly due to the bulk structure relaxation, instead of the changes in refraction indexes. 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 bulk relaxation transitions of polymers are generally labeled as R, β, γ, and so forth, in alphabetical order with decreasing temperature. For PMMA, the highest-temperature relaxation, the R-relaxation temperature, is generally regarded as the glass transition temperature and is associated with the long-range cooperative motion of the backbone. The β-relaxation is associated with the local movements of side-chains.9 The temperature for the β-relaxation of bulk PMMA is near room temperature and decreases with decreasing film thickness.51-53 The observed surface structure relaxation at 67 °C is lower than the bulk R-relaxation temperature and higher than the bulk β-relaxation temperature. The SFG measurements were repeated for various film thicknesses of PMMA, between 0.1 and 0.5 µm. Within measurement errors, this surface structure relaxation temperature is independent of film thickness, as one would expect for a surface property. What are the possible origins of the observed relaxation at 67 °C on a free PMMA surface? It is generally believed that surface molecules have more freedom to adjust their position, and are expected to have decreased relaxation temperatures.17 Therefore, a decreased surface R-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 β-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 R- and β-relaxations. Similar studies could not been carried out on other
Surface Structure Relaxation of PMMA vibrational modes of PMMA because of their much lower signal-to-noise ratios. Summary The sensitivity of IR-visible SFG to surface structure changes was improved by measuring the SFG intensity as a function of the incident visible polarization angle. This approach significantly reduced the long-term laser intensity fluctuations and allowed us to detect surface structure changes on an isotropic PMMA surface. A new surface structure relaxation was observed at 67 °C, which does not match any known bulk structure relaxation temperature and is independent of film thickness in the range of 0.1-0.5 µm. This temperature is lower than the bulk R-relaxation temperature and higher than the bulk β-relaxation temperature. While a decreased surface R-relaxation temperature is a plausible interpretation for the observed structure relaxation at 67 °C, the SFG study cannot exclude the possibility of an increased surface β-relaxation temperature being responsible for the observed relaxation. Acknowledgment. This study was financially supported by the Natural Sciences and Engineering Research Council of Canada and the University of British Columbia (UBC). I.J.C. was an exchange student from the National University of Singapore through the Professional Placement Programme supported by the Lee Foundation. The authors thank Mu Chiao in the Department of Mechanical Engineering at UBC for measuring the PMMA film thickness and Daniel B. Murray at UBC Okanagan for his valuable comments. References and Notes (1) Billmeyer, F. Textbook of Polymer Science; Wiley & Sons: Singapore, 1984. (2) Bergman, R.; Alvarez, F.; Alegria, A.; Colmenero, J. J. Chem. Phys. 1998, 109, 7546. (3) Bergman, R.; Alvarez, F.; Alegria, A.; Colmenero, J. J. Non-Cryst. Solids 1998, 235, 580. (4) Alves, N. M.; Ribelles, J. L. G.; Tejedor, J. A. G.; Mano, J. F. Macromolecules 2004, 37, 3735. (5) Genix, A. C.; Arbe, A.; Alvarez, F.; Colmenero, J.; Farago, B.; Wischnewski, A.; Richter, D. Macromolecules 2006, 39, 6260. (6) Genix, A. C.; Arbe, A.; Alvarez, F.; Colmenero, J.; Schweika, W.; Richter, D. Macromolecules 2006, 39, 3947. (7) Moreno, A. J.; Alegria, A.; Colmenero, J.; Frick, B. Phys. ReV. B 1999, 59, 5983. (8) Moreno, A. J.; Alegria, A.; Colmenero, J.; Frick, B. Macromolecules 2001, 34, 4886. (9) Schmidt-Rohr, K.; Kulik, A. S.; Beckham, H. W.; Ohlemacher, A.; Pawelzik, U.; Boeffel, C.; Spiess, H. W. Macromolecules 1994, 27, 4733. (10) Kuebler, S. C.; Schaefer, D. J.; Boeffel, C.; Pawelzik, U.; Spiess, H. W. Macromolecules 1997, 30, 6597. (11) Wei, X.; Miranda, P. B.; Shen, Y. R. Phys. ReV. Lett. 2001, 86, 1554. (12) Mayers, G. F.; Dekoven, B. M.; Seitz, J. T. Langmuir 1992, 8, 2330. (13) Gottschalch, F.; Hoffmann, T.; Torres, C. M. S.; Schulz, H.; Scheer, H. C. Solid State Electron. 1999, 43, 1079. (14) Chou, S. Y.; Krauss, P. R. Microelectron. Eng. 1997, 35, 237. (15) Orts, W. J.; Vanzanten, J. H.; Wu, W. L.; Satija, S. K. Phys. ReV. Lett. 1993, 71, 867. (16) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Europhys. Lett. 1994, 27, 59.
J. Phys. Chem. B, Vol. 112, No. 3, 2008 697 (17) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Faraday Discuss. 1994, 98, 219. (18) Zheng, X.; Sauer, B. B.; Vanalsten, J. G.; Schwarz, S. A.; Rafailovich, M. H.; Sokolov, J.; Rubinstein, M. Phys. ReV. Lett. 1995, 74, 407. (19) Grohens, Y.; Brogly, M.; Labbe, C.; David, M. O.; Schultz, J. Langmuir 1998, 14, 2929. (20) Overney, R. M.; Buenviaje, C.; R., L.; Dinelli, F. J. Therm. Anal. Calorim. 2000, 59, 205. (21) Sharp, J. S.; Forrest, J. A. Eur. Phys. J. E 2003, 12, S97. (22) Hammerschmidt, J. A.; Gladfelter, W. L.; Haugstad, G. Macromolecules 1999, 32, 3360. (23) Frank, B.; Gast, A. P.; Russell, T. P.; Brown, H. R.; Hawker, C. Macromolecules 1996, 29, 6531. (24) DeMaggio, G. B.; Frieze, W. E.; Gidley, D. W.; Zhu, M.; Hristov, H. A.; Yee, A. F. Phys. ReV. Lett. 1997, 78, 1524. (25) Schwab, A. D.; Agra, D. M. G.; Kim, J. H.; Kumar, S.; Dhinojwala, A. Macromolecules 2000, 33, 4903. (26) Kawana, S.; Jones, R. A. L. Phys. ReV. E 2001, 63, art. no. 021501. (27) Forrest, J. A.; Dalnoki-Veress, K.; Stevens, J. R.; Dutcher, J. R. Phys. ReV. Lett. 1996, 77, 2002. (28) Alcoutlabi, M.; McKenna, G. B. J. Phys.: Condens. Matter 2005, 17, R461. (29) Simon, S. L.; Park, J. Y.; McKenna, G. B. Eur. Phys. J. E 2002, 8, 209. (30) Kerle, T.; Lin, Z. Q.; Kim, H. C.; Russell, T. P. Macromolecules 2001, 34, 3484. (31) Jean, Y. C.; Zhang, R. W.; Cao, H.; Yuan, J. P.; Huang, C. M.; Nielsen, B.; AsokaKumar, P. Phys. ReV. B 1997, 56, R8459. (32) Xie, L.; Demaggio, G. B.; Frieze, W. E.; Devries, J.; Gidley, D. W.; Hristov, H. A.; Yee, A. F. Phys. ReV. Lett. 1995, 74, 4947. (33) Ge, S.; Pu, Y.; Zhang, W.; Rafailovich, M.; Sokolov, J.; Buenviaje, C.; Buckmaster, R.; Overney, R. M. Phys. ReV. Lett. 2000, 85, 2340. (34) Liu, Y.; Russell, T. P.; Samant, M. G.; Sto¨hr, J.; Brown, H. R.; Cossy-Favre, A.; Diaz, J. Macromolecules 1997, 30, 7768. (35) Weber, R.; Zimmermann, K. M.; Tolan, M.; Stettner, J.; Press, W.; Seeck, O. H.; Erichsen, J.; Zaporojtchenko, V.; Strunskus, T.; Faupel, F. Phys. ReV. E 2001, 64, 61508. (36) Gracias, D. H.; Zhang, D.; Lianos, L.; Ibach, W.; Shen, Y. R.; Somorjai, G. A. Chem. Phys. 1999, 245, 277. (37) Zhang, C.; Hong, S.-C.; Ji, N.; Wang, Y.-P.; Wei, K.-H.; Shen, Y. R. Macromolecules 2003, 36, 3303. (38) Schwab, A. D.; Dhinojwala, A. Phys. ReV. E 2003, 67, art. no. 021802. (39) Wang, J.; Chen, C. Y.; Buck, S. M.; Chen, Z. J. Phys. Chem. B 2001, 105, 12118. (40) Guyot-Sionnest, P.; Hunt, J. H.; Shen, Y. R. Phys. ReV. Lett. 1987, 59, 1597. (41) Wei, X.; Zhuang, X.; Hong, S.-C.; Goto, T.; Shen, Y. R. Phys. ReV. Lett. 1999, 82, 4256. (42) Zhuang, X.; Miranda, P. B.; Kim, D.; Shen, Y. R. Phys. ReV. B 1999, 59, 12632. (43) Miranda, P. B.; Shen, Y. R. J. Phys. Chem. B 1999, 103, 3292. (44) Gan, W.; Wu, B.; Chen, H.; Guo, Y.; Wang, H. Chem. Phys. Lett. 2005, 406, 467. (45) Tsang, O. C.; Tsui, O. K. C.; Yang, Z. Phys. ReV. E 2001, 63, art. no. 061603. (46) Wu, W. L.; Sambasivan, S.; Wang, C. Y.; Wallace, W. E.; Genzer, J.; Fischer, D. A. Eur. Phys. J. E 2003, 12, 127. (47) Polymer Handbook, 4th ed.; Brandrup, J., Immergut, E. H., Grulke, E. A., Eds.; John Wiley & Sons: New York, 2003. (48) Held, H.; Lvovsky, A. I.; Wei, X.; Shen, Y. R. Phys. ReV. B 2002, 66, 205110. (49) Wray, J. H.; Neu, J. T. J. Opt. Soc. Am. 1969, 59, 774. (50) Cariou, J. M.; Dugas, J.; Martin, L.; Michel, P. Appl. Opt. 1986, 25, 334. (51) Perez, J.; Cavaille, J. Y.; David, L. J. Mol. Struct. 1999, 479, 183. (52) Fukao, K.; Uno, S.; Miyamoto, Y.; Hoshino, A.; Miyaji, H. Phys. ReV. E 2001, 64, 051807. (53) Quinson, R.; Perez, J.; Germain, Y.; Murraciole, J. M. Polymer 1995, 36, 743.
