Mechanical behaviour of isotactic polypropylenes in the lower and the upper glass transitions ranges

 The effects of physical ageing on both glass transitions of isotactic polypropylenes (homopolymer and random copolymer)are investigated by isothermal dynamic mechanical spectroscopy and by low strain creep. The ageing-time dependent expressions of the amorphous fractions which undergo each relaxation are introduced in a rheological model composed by a modified Zener model (associated to the β-relaxation) in series with a γ-element (associated to the α-relaxation). The agreement between the responses of this model and the creep and dynamic results is good over a time (or frequency) range covering up to 15 decades for three kinds of isotactic polypropylenes. Received: 2 January 1996 Accepted: 16 August 1996