Stress relaxation in semi-crystalline PEN films: Physical aging effects

The effect of physical aging on the stress relaxation response of a semi-crystalline PEN film was studied at temperatures ranging from the secondary beta* transition (similar to 70 degrees C) up to its 'amorphous' glass transition temperature (similar to 120 degrees C). Within this temperature range, the response is thermorheologically simple, i.e., the effect of physical aging is to shift the stress relaxation function along the log timescale to longer times following a simple power-law superposition rule with a characteristic temperature-dependent exponent mu (= aging rate parameter). The mu(T) function peaks at ca. 80 degrees C, and it follows a general pattern commonly observed in other polymer systems. Within the temperature range studied, the relaxation function also obeys time-temperature superposition and the corresponding shift factor appears to follow Arrhenius activation. The apparent activation energy is, however, strongly dependent on aging time - it increases nearly threefold over the range of aging times covered - which can be rationalized on the basis of the effective time theory.