Mechanical Properties and Elastic Behavior of High-Molecular-Weight Poorly Syndiotactic Polypropylene

Abstract

An analysis of the mechanical properties and elastic behavior of low-stereoregular and nearly amorphous syndiotactic polypropylene (sam-PP), prepared with heterocycle-fused indenyl silyl amido dimethyltitanium complexes, is presented. High-molecular-weight poorly syndiotactic sam-PP samples show good elastic behavior at room temperature in a large range of deformation. While highly stereoregular and crystalline syndiotactic polypropylene shows good elastic properties only for previously stretched oriented fibers, sam-PP samples present good elasticity even for unoriented compression-molded films during the first stretching. Because of the very low crystallinity, these samples experience a negligible irreversible plastic deformation and show a typical behavior of thermoplastic elastomers. The small crystalline domains in the amorphous matrix act as physical knots of the elastomeric lattice, preventing the viscous flow of the amorphous chains. The elastic behavior is associated with a reversible polymorphic transition between the trans-planar mesomorphic form and helical form I, which occurs in the crystalline domains during successive stretching and relaxing of fiber specimens, giving an enthalpic contribution which assists the elastic recovery of the samples. The entropic effect of the conformational transition of the amorphous chains, connecting the crystalline domains as tie chains, is, however, mainly responsible for the elasticity. The comparison with the mechanical properties of high-molecular-weight amorphous polypropylene samples has shown that the latter present lower strength and experience rapid viscous flow of the chains at high deformations and/or by application of stresses for long time. The presence of crystallinity in the sam-PP samples increases the strength, producing interesting thermoplastic elastomeric materials.