Rheo-Optical Studies on the Deformation Mechanism of Semicrystalline Polymers XVI. Alpha and Beta Mechanical Dispersions of Spherulitic Polypropylene and Polybutene-1 and the Dynamic Orientation Distribution Function of Crystallites

Abstract

The uniaxial deformation mechanism of spherulitic polypropylene and polybutene-1 was investigated in regard to changes in the birefringence and orientation distribution function of crystal grains with the extension ratio of the bulk specimen. This mechanism can be represented by a spherulite deformation model combining the affine orientation of the crystal lamellae with the reorientation of the crystal grains within the orienting lamellae. The latter is caused by three kinds of intra-lamellar shearing mechanisms; i.e., lamellar detwisting mostly in the equatorial zone of the uniaxially deformed spherulites, and lamellar tilting and micronecking mostly in the polar zone of the spherulites. In contrast to spherulitic high-density polyethylene in which the lamellar detwisting and tilting mechanisms are dominant, the lamellar tilting and micronecking occur dominantly in spherulitic polypropylene and polybutene-1. The investigation was extended to dynamic measurements and good correspondence of the mechanical and optical dispersions designated as α and β dispersions was found, in contrast to the case of spherulitic high-density polyethylene for which an additional α2 mechanical dispersion having no correspondence to the optical dispersion was observed. The β dispersion became clearer the a dispersion did less so as the specimen changed from polyethylene to polypropylene and polybutene-1. The β dispersion was attributed to the dynamic orientation dispersion of the crystal lamellae constituting the spherulitic crystalline texture for any poly-alpha-olefin specimen, i.e., inter-lamellar crystal-grain-boundary phenomena possibly associated with orientational and/or distorsional dispersions of noncrystalline material between the lamellae. The α dispersion was ascribed to the dynamic reorientation dispersion of the crystal grains within the orienting lamellae, which occurs with a preferential orientation of the c-axis caused by dynamic lamellar detwisting and tilting in the case of polyethylene and only by dynamic lamellar tilting for polypropylene and polybutene-1, i.e., intra-lamellar crystal-grain-boundary phenomena associated with the inter-crystal friction at the boundaries.