Physical aging in poly(ethylene naphthalene-2,6-dicarboxylate) in relation to sorbed water,enthalpic relaxation,and mechanical properties

Amorphous poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) films (～ 220 μm thick), stored in ambient atmosphere for different periods of time and after annealing at different temperatures below T_g = 123°C, for different times, have been investigated by infrared spectroscopy (FTIR), microhardness, and differential scanning calorimetry (DSC). FTIR spectroscopy and weight measurements reveal the presence of water which is easily removed by annealing of the films. Films again recover their initial weight and absorption bands after 1-2 days storage in ambient atmosphere. Samples annealed at different temperatures T_a for different times t_a show an increasing microhardness for relatively short times of t_a. The microhardness passes through a maximum at an annealing time depending on T_a, and it decreases toward values somewhat larger than the initial ones. The changes observed in the microhardness and in the values of the excess enthalpy with storage time of the samples at room temperature depend on the physical aging as well as on the content of water of PEN films. © 1995 John Wiley & Sons, Inc.