Stress Relaxation in Semi-Crystalline PEN Films: Physical Aging Effects

Abstract

The effect of physical aging on the stress relaxation response of a semicrystalline PEN film was studied at temperatures ranging from the secondaryB* transition (-70'C) up to its "amorphous" glass transition temperature (-120'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 powerlaw superposition rule with a characteristic temperaturedependent exponents (= aging rate parameter). They (T) function peaks at -80'C, and it follows a general pattern commonly observed in other polymer systems. Within the temperature range studied, the relaxation function also obeys timetemperature superposition and the corresponding shift factor appears to follow Arrhenius activation. The apparent activation energy is, however, strongly dependent on aging timeit increases nearly threefold over the range of aging times coveredwhich can be rationalized on the basis of the effective time theory.