11)Argon, A. S.; Cohen, R. E.; Gebizlioglu, 0. S.; Schwier, C. E. Adv. Polym. Sci. 1983,52153, 275. Maglio, G.; Palumbo, R. The Role of Interfacial Agenb in Polymer Blends. In Polymer Blends; Kryszewski, M., Galeski, A., Martuscelli, E., Ed.; Plenum: New York, 1982. Haws, J. R.; Wright, R. F. Block Polymers. Ch. 3 in Handbook of Thermovlastic Elastomers: Walker. B. M.. Ed.: Krieger . . -Pub. do.: Melbburne, FL, 1986. Matzner, M.: Noshav, A.: McGrath, J. E. Trans. SOC. Rheol. ' -. . 1977,21,' 273. Hadziioannou, G.; Skoulios, A. Macromolecules 1982,15,267. Cantor, R. Macromolecules 1981,14,1186. Hashimoto, T.; Shibayama, M.; Kawai, H. Macromolecules, 1980,13, 1237.Han, C. C., private communication. See also: Mathushita, Y.; Nakao, Y.; Saguchi, R.; Mori, K.; Choshi, H.; Muroga, Y.; Noda, I.; Nagasawa, M.; Chang, T.; Glinka, C. J.; Han, C. C. The relaxation process is very similar to that postulated for star molecules in ref 12, p 280. As in the star system, the viscosity 7 is not simply the rubber modulus Go times the terminal relaxation time T. Rather, only a small portion of order NIk2Go of the initial stress requires such a time to relax. Thus this mechanism gives q -G,rN/k2. Milner, S. T.; Witten, T. A. Macromolecules 1989,22, 1904. (26) We assume that the solvent casting procedure of ref 8 produces the equilibrium domain thicknew. The equilibrium thickness should in any case be larger than the measured one. (27) Lin, Y.-H. Macromolecules 1987,20, 3080. (28) Taunton, H. J.; Toprakcioglu, C.; Fetters, L. J.; Klein, J. NatureABSTRACT: Dielectric spectroscopy from 10-1 to lo9 Hz was used to investigate bulk amorphous cispolyisoprenes with different molecular weights having narrow molecular weight distributions. A molecular weight dependent normal-mode relaxation process due to reorientation of the end-to-end vector as well as a molecular weight independent segmental-mode process caused by local chain motions were observed. The relaxation time for the normal-mode process exhibits a molecular weight dependence that can be described below a critical molecular weight, M , (=lo4), according to the Rouse theory; but above M , the relaxation time follows the 3.7 power of M,, which is explicable with the reptation theory. Both processes were analyzed in terms of dipole-dipole correlation functions and reveal a Kohlrausch-Williams-Watts (KWW) type of relaxation behavior. The nonexponential decay is explained with the cooperative nature of motions in the bulk amorphous state. The stretched exponential BKWW for the normal-mode process depends on the chain length whereas it remains constant for the segmental process. The results are in agreement with new theoretical and computational approaches.

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Dielectric spectroscopy from lo-' to IO9 Hz was used to investigate bulk amorphous multiarmed stars of cis-polyisoprene and their linear counterparts, both having narrow molecular weight distributions. The star polymers resemble the linear polymers in showing two distinct regions of dielectric dispersion. A molecular weight independent segmental process occurs at nearly the same frequency as in the linear polymer due to dipole moments perpendicular to the chain backbone. The second process is caused by the parallel dipole moments and exhibits a pronounced molecular weight dependence. The relaxation time of the molecular weight dependent normal-mode process for linear polymers can be described according to the Rouse theory below the critical molecular weight, M , (=lo4), but above M , it corresponds to the 3.7 power of M,, which is characteristic for entangled macromolecules. A similar dependence of the relaxation time is observed for the many armed star polymers. It is interpreted by means of the conformational scaling properties of Daoud and Cotton and compared with theoretical and computational approaches.

Poly(methylphenylsi1oxane) with a molecular weight of M , = 28500 Da (Da = daltons) (Tg = -26 O C ) was studied. Experimentally, the quantities measured were the dielectric loss ~" ( w ) and the intensity autocorrelation function by quasielastic light scattering in the melt close to the glass transition temperature. We have fit the ~" ( w ) data at each temperature with the Havriliak-Negami equation and from that computed the dipole-dipole reorientation correlation function. A fit of a stretched exponential (Kohlrausch-Williams-Watts form) to the correlation function is directly compared with the results from light scattering, which are usually given in this functional form. We find quantitative agreement between these two methods when comparing the time correlation functions. The agreement between both methods leads to the conclusion that the collective dynamics measured by light scattering, related to macroscopic mechanical properties of the material, and the molecular dynamics of reorienting dipoles are the same for polymeric melts of high inherent viscosity. IntroductionThe large group of siloxane polymers has served as model materials for highly flexible chain molecules.' Among them, especially poly(methylphenylsi1oxane)

SYNOPSISThin films of poly ( p -phenylene biphenyltetracarboximide) (BPDA-PDA) , prepared by thermal imidization of the precursor poly (amic acid) on substrates, have been investigated by optical waveguide, ultraviolet-visible (UV-VIS ), infrared (IR) , and dielectric spectroscopies. The polyimide films exhibit an extraordinarily large anisotropy in the refractive indices with the in-plane index rill = 1.806 and the out-of-plane index n, = 1.589 at 1064 nm wavelength. No discernible effect of the film thickness on this optical anisotropy is found between films of ca. 2.1 and ca. 7.8 pm thickness. This large birefringence is attributed to the preferential orientation of the biphenyltetracarboximide moieties with their planes parallel to the film surface, coupled with the strong preference of BPDA-PDA chains to align along the film plane. The frequency dispersion of the in-plane refractive index rill is consistent with the results calculated by the Lorentz-Lorenz equation from the UV-visible spectrum exhibiting several absorption bands in the 170-500 nm region. The contribution from the IR absorption in the range 7000-400 cm,-' computed by the Spitzer-Kleinmann dispersion relations from the measured spectra, adds ca. 0.046 to the in-plane refractive index nlI . Tilt-angle-dependent polarized IR results indicate nearly the same increase for the out-of-plane index n,. Application of the Maxwell relation then leads to the out-ofplane dielectric constant t, N 2.7 at 1.2 X 1013 Hz, as compared with the measured value of ca. 3.0 at lo6 Hz. Assuming this small difference to remain the same for the in-plane dielectric constants q, we obtain a very large anisotropy in the dielectric properties of these polyimide films with the estimated in-plane dielectric constant ell N 3.4 at 1.2 X 1013 Hz, and tII N 3.7 at lo6 Hz. 0 1992 John Wiley & Sons, Inc. Keywords: polyimides. stiff, optical and dielectric properties in their films of optical anisotropy and chain orientation stiff polyimide thin films dielectric properties of stiff polyimide thin films ~~ ~