Extensional flow induced crystallization from solution

Abstract

An experimental apparatus and technique were developed for studying the crystallization of polyethylene from solutions undergoing an oscillatory, constant stretch rate, extensional flow. Investigation of the resulting fiber morphology, obtained at various temperatures and stretch rates, shows that the fibers exhibit a bead and stringlike appearance. Study of the melting, acid etching, and selective dissolution behavior of the fibers leads to the conclusion that fiber growth occurs in a fashion which directly results in the bead and string morphology and that the morphology is not a result of secondary, epitaxial crystal growth as previously believed. A model for the growth process, as suggested by the data, is presented. Evidence is also presented to suggest that at certain temperatures and stretch rates, a triclinic phase transformation occurs during the flow process. The effect is to produce two separate melting entities which appear to be stretch rate sensitive, in line with previously published studies on the effects of stretching on the behavior of bulk polymer.