Physical ageing and short-term creep in amorphous and semicrystalline polymers

Studies have been made of the effect of physical ageing at 23°C on the short-term creep of polycarbonate (PC), poly(butylene terephthalate) (PBT) and high density polyethylene (HDPE) at creep times between 10 −8 and 10 5 s. The age state of each polymer sample was specified by the elapsed time t e at 23°C between quenching from a temperature at which the structure is close to equilibrium with respect to the α-retardation process and the start of a creep test. With increasing age, a small decrease was evident in the relaxed compliance D R β of the β-retardation process in each polymer and relationships between D R β and t e were derived from quantitative analyses of the data. For the amorphous PC and semicrystalline PBT, this decrease could reflect a slight reduction in the number of groups participating in the localized secondary process. It also appeared for PC and PBT that the average retardation time for the glass-rubber α-process increased with t e, with negligible change in the shape of the retardation time distribution or the magnitude of the α-retardation. For HDPE, the small decrease in D R β with increasing t e for the glass-rubber β-process is similar to that found for polypropylene and might involve conformational changes of amorphous tie-molecules. The results for HDPE deviate from the predictions of Struik's model of ageing in semicrystalline polymers and suggest that, for the crystal-related α-process, changes in retardation magnitude or shape of the retardation time distribution may occur with increasing age.