A study of low temperature mechanical properties and creep behaviour of polypropylene using a new sub-ambient temperature nanoindentation test platform

Abstract

Mechanical properties and creep behaviour of an atactic-polypropylene (aPP) have been studied in the vicinity of its glass transition temperature (−18 °C) via a nanoindentation platform integrated with a sub-ambient temperature capability. All low temperature tests were validated by measurements on a fused silica reference sample from 25 to −30 °C. The fused silica results showed virtually invariant elastic modulus with temperature over this range consistent with the literature measurements by sonic resonance. Hardness and elastic modulus of aPP increased as the test temperature decreased and the amorphous regions went through the glass transition. The creep behaviour was analysed using two approaches: (i) a logarithmic method and (ii) the Boltzmann integral method. The results showed that the creep extent decreased as the temperature was reduced, and for the time constants obtained there were upper-limit values at −10 °C, about 8 °C above the quoted glass transition temperature. The strain rate sensitivity obtained by the logarithmic method also showed a maximum at −10 °C.