High-temperature mechanical behavior of semi-crystalline polymers and relationship to a rubber-like “relaxed” state

Abstract

This work addresses the general issue of the mechanical behavior of the confined amorphous phase in rubbery semi-crystalline polymers. The tensile behavior of four semi-crystalline polymers (with various degrees of crystallinity and transition temperatures) was characterized and analyzed at high-temperature (i.e. a few tens degrees below the melting point) and at low strain-rates to try to approach a “relaxed” mechanical behavior. Monotonic loadings followed by different unloading/relaxation/creep stages, and strain-rate jumps experiments were performed on two testing machines equipped with different extensometers. Despite significant differences of micro-structure between the tested materials, a general phenomenon was observed: a non strain-rate sensitive regime was reached both at loading and unloading. However, a “relaxed” equilibrium response cannot account for this regime because of concomitant contradictory features among which (i) the creep and relaxation kinetics which involve the same strain-rates as observed in the non strain-rate sensitive loading regime or (ii) the unusual strain-rate sensitivity observed in strain-rate jump tests. The whole set of experiments has consequences for mechanical modeling. They are discussed as a conclusion of this experimental mechanics work.