A theory of necking in semi-crystalline polymers

TL;DR

This paper develops a theoretical model explaining necking in semi-crystalline polymers, linking it to crystalline unfolding, and analyzes elasticity, viscoelasticity, and heat effects during neck propagation.

Contribution

It introduces a novel continuum model connecting crystalline unfolding to necking in semi-crystalline polymers, enhancing fundamental understanding.

Findings

Necking involves crystalline unfolding in semi-crystalline polymers.

Reversible elasticity explains slow neck propagation.

Viscoelastic effects and heat generation are significant at higher speeds.

Abstract

Necking or cold drawing is a smoothed jump in cross-sectional area of long and thin bars (filaments orfilms) propagating with a constant speed. The necks in polymers, first observed about seventy years ago, arenow commonly used in modern processing of polymer films and fibers. Yet till recently there was a lack infundamental understanding of necking mechanism(s). For semi-crystalline polymers with co-existingamorphous and crystalline phases, recent experiments revealed that such a mechanism is related tounfolding crystalline blocks. Using this idea, this paper develops a theoretical model and includes it in ageneral continuum framework. Additionally, the paper explains the forced (reversible) elasticity observedin slowly propagating polymeric necks, and also briefly analyses the viscoelastic effects and dissipative heatgeneration when polymer necks propagate fast enough.