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 reported as a function of the morphology. During the treatment at temperatures close to the melting temperatures the samples become semi-crystalline. differential scanning calorimetry (DSC) shows that the glass transition temperatures do not change significantly with the thermal treatment. However the degree of crystallinity as well as the melting temperatures increase with the annealing temperature. Both dielectric (DEA) and dynamic mechanical (DMA) analysis display three relaxation processes. In order of decreasing temperature the α-relaxation due to the glass transition, the β∗-process assigned to the out of plane movements of the naphthalene rings or aggregates of it and the β-relaxation due to local molecular motions of carbonyl groups. The α-relaxation process shifts to higher temperatures for the semi-crystalline samples compared to the amorphous one. On the contrary, at a fixed frequency the temperature associated to β∗-relaxation is higher for the amorphous sample than for the semi-crystalline ones. The associated apparent activation energies are rather high and depend on the thermal treatment and also surprisingly on the method of measurement. It is concluded that the β∗-relaxation is probably due to cooperative molecular motions of the naphthalene groups which aggregate in the amorphous state and that these aggregates are prevented from forming when the degree of crystallinity changes due to the thermal treatment. Finally, the activation energy for the β-process is nearly independent of the thermal treatment and the value agrees with that found for poly(ethylene terephthalate) (PET).