PHYSICAL AGING OF AMORPHOUS AND SEMICRYSTALLINE POLY(ETHYLENE-TEREPHTHALATE)

Thermal and micromechanical properties of amorphous and semicrystallin e poly(ethylene terephthalate) are found to be dependent on ageing eff ects occurring by thermal treatments below T(g). These effects are stu died using two complementary techniques: differential scanning calorim etry and dynamic mechanical spectroscopy at low frequencies (1-10(-4)H z). Experimental results can be described through a physical model ass uming diffusion and annihilation of 'quasi-point defects'. The distrib ution in the mobility of these defects is able to take into account al l the phenomena observed after physical ageing, in particular the effe cts of low-temperature ageing. The ageing effects in the semicrystalli ne material cannot be deduced from those observed for the amorphous on e by a single two-phase rule. Changes in the correlation factor of mol ecular movements and changes in the distribution of defects are necess ary to describe all the observed effects, putting in evidence the role of the crystallites to reduce the segmental mobility.