Secondary transitions of aryl-aliphatic polyamides II. High-resolution solid-state C-13 NMR investigation

High-resolution solid-state C-13 NMR experiments were performed on the two series of aryl-aliphatic copolyamides whose dielectric relaxation investiga tion is reported in the first paper of this series. The local motions were studied by following the temperature dependence of C-13 chemical shift anis otropies and C-13-H-1 dipolar couplings. In the copolyamides that contain a lactam-12 sequence, the lactam-12 sequences were shown to be involved in s ignificant motions, even at low temperatures. In contrast, in the copolymer s whose aliphatic part is a methylpentane sequence, the methylpentane seque nces do not exhibit detectable motions at temperatures below 100 degrees C. In all the systems investigated, the aromatic rings were shown to undergo oscillations whose amplitudes and axis depend of the relative meta- or para - positions of the substituents. In addition to these oscillations, para-su bstituted rings are also involved in pi-flips. The motions of the lactam-12 sequences were associated with the gamma-relaxation processes whereas both oscillations and pi-flips occur in the temperature and frequency ranges of the beta-transition observed by dielectric relaxation. (C) 2000 Elsevier S cience Ltd. All rights reserved.