Dielectric and dynamic mechanical relaxation behaviour of poly(ethylene 2,6 naphthalene dicarboxylate). I. Amorphous films

The dielectric and dynamic mechanical behaviour of amorphous poly(ethylene 2,6 naphthalene dicarboxylate) (PEN) and of thermally annealed samples is r eported as a function of the morphology. During the treatment at temperatur es close to the melting temperatures the samples become semi-crystalline. d ifferential scanning calorimetry (DSC) shows that the glass transition temp eratures do not change significantly with the thermal treatment. However th e degree of crystallinity as well as the melting temperatures increase with the annealing temperature. Both dielectric (DEA) and dynamic mechanical (D MA) analysis display three relaxation processes. In order of decreasing tem perature the alpha -relaxation due to the glass transition, the beta*-proce ss assigned to the out of plane movements of the naphthalene rings or aggre gates of it and the p-relaxation due to local molecular motions of carbonyl groups. The a-relaxation process shifts to higher temperatures for the sem i-crystalline samples compared to the amorphous one. On the contrary, at a fixed frequency the temperature associated to beta*-relaxation is higher fo r the amorphous sample than for the semi-crystalline ones. The associated a pparent activation energies are rather high and depend on the thermal treat ment and also surprisingly on the method of measurement. It is concluded th at the beta*-relaxation is probably due to cooperative molecular motions of the naphthalene groups which aggregate in the amorphous state and that the se aggregates are prevented from forming when the degree of crystallinity c hanges due to the thermal treatment. Finally, the activation energy for the P-process is nearly independent of the thermal treatment and the value agr ees with that found for poly(ethylene terephthalate) (PET). (C) 2001 Elsevi er Science Ltd. All rights reserved.