MAGIC-ANGLE C-13 NMR-STUDY OF SOLID POLY(ETHYLENE NAPHTHALENE-2,6-DICARBOXYLATE)

Amorphous (1) and semicrystalline (2) samples of poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) have been investigated by cross-polarization/magic angle spinning (CP/ MAS) C-13 NMR at 26 degrees C (1 and 2), 100 degrees C (1) and 120 degrees C (2) in order to study the phase structure and the local motion of polymer chain segments at temperatures below and close to T-g (120 degrees C). The lineshape of the ethylene unit C-13 signal in sample 2 is consistent with the presence of two components which were assigned to trans and gauche conformations The first component arises mainly from the crystalline regions and the second one from the amorphous part. Cross-polarization curves were traced by changing the contact time between carbon and proton reservoirs. T-CH (cross relaxation time) and proton T-1 rho (spin-lattice relaxation time in the rotating frame) values were obtained as best fit parameters by fitting calculated curves to the experimental data. All C-13 NMR data are consistent with the presence of highly rigid ethylene units in both semicrystalline and amorphous samples within the temperature range (T) investigated. This result is in disagreement with the H-1 NMR wide line spectra which showed a noticeable narrowing of the linewidth with increasing temperature in the same range, hence indicating a great mobility of the chain segments. To account for this discrepancy a qualitative model based on the existence of two distinct dynamic regions, one where motion is highly restricted and the other one where large reorientations of ethylene group torsional angles take place, is suggested. The NMR results led to the conclusion that three structural phases are present in PEN: crystalline, very rigid amorphous, and very mobile amorphous. (C) 1995 John Wiley & Sons, Inc.