Change of modulus and yielding properties of syndiotactic polypropylene with the glass transition

Abstract

Semi-crystalline polymers like syndiotactic polypropylene pass during tensile deformation different stages with cross-overs at four critical points. Temperature variation above ambient changes the stresses at the critical points, but leaves the strains constant. Cooling down to the glass transition leads to the following changes: Fracture occurs before the onset of strain hardening. Yield point strain shifts to lower values. Young's modulus increases by a factor on the order of the inverse of the crystallinity, as can be expected when the amorphous regions also contribute to the transmission of forces.