New Concepts in Thermoplastic Elastomers: The Case of Syndiotactic Polypropylene, an Unconventional Elastomer with High Crystallinity and Large Modulus

Abstract

Syndiotactic polypropylene (sPP), despite high crystallinity, shows unusual elastic properties, associated with a reversible crystal-crystal phase transition. The mechanism behind the crystal-crystal phase transition involved during cyclic elongation and recovery and the time scale of this process is here investigated, to unravel the origin of the elasticity of such an unconventional elastomer. The structural transformation of fibers of sPP during deformation has been studied by wide-angle X-ray diffraction measurements, using the high flux available on the beamline ID11 at the European Syncrotron Radiation Facility (ESRF), Grenoble, France, performing a time-resolved analysis of structural changes occurring while cyclically stretching and relaxing sPP fibers. This analysis indicates that the stress-induced phase transition is a martensitic transformation. As in martensitic phase transitions it occurs readily and directly, supporting the idea that elasticity in sPP is partially of enthalpic nature.