A QUANTITATIVE MODEL DESCRIBING PHYSICAL AGING IN ISOTACTIC POLYPROPYLENES

Physical aging of isotactic polypropylenes (homopolymer and random ethylene-propylene copolymer) at room temperature is investigated by low-strain creep, damping measurements, differential scanning calorimetry, and infra-red spectroscopy. Examination of the results suggests that physical aging produces a decrease of the magnitude of the beta-relaxation (lower glass transition) and an increase of the magnitude and of the relaxation time of the alpha-relaxation (upper glass transition). A model is proposed that accounts for the variations of the fractions of amorphous material which undergo these relaxations. In this model, deformation and increase in the alpha-fraction at the expense of the beta-fraction are expected to be diffusion processes. The agreement with experimental data is good. Moreover, the alpha-form is shown to be an amorphous phase rich in molecular segments having a helical conformation.