Modelling the Mechanical and Strain Recovery Behaviour of Partially Crystalline PLA.

John Sweeney,Phil Coates,Paul Spencer,Karthik Nair

doi:10.3390/polym11081342

Abstract

This is a study of the modelling and prediction of strain recovery in a polylactide. Strain recovery near the glass transition temperature is the underlying mechanism for the shape memory in an amorphous polymer. The investigation is aimed at modelling such shape memory behaviour. A PLA-based copolymer is subjected to stress–strain, stress relaxation and strain recovery experiments at large strain at 60 °C just below its glass transition temperature. The material is 13% crystalline. Using published data on the mechanical properties of the crystals, finite element modelling was used to determine the effect of the crystal phase on the overall mechanical behaviour of the material, which was found to be significant. The finite element models were also used to relate the stress–strain results to the yield stress of the amorphous phase. This yield stress was found to possess strain rate dependence consistent with an Eyring process. Stress relaxation experiments were also interpreted in terms of the Eyring process, and a two-process Eyring-based model was defined that was capable of modelling strain recovery behaviour. This was essentially a model of the amorphous phase. It was shown to be capable of useful predictions of strain recovery.

Highlights

Polylactic acid (PLA) and related polymers and blends are a focus of increasing attention, arising both from their potential for degradability within the natural carbon cycle for disposable products [1,2], and from their bioresorbable properties for prosthetic implants [3,4,5,6]
This is the motivation of this paper, for which a fundamental study was made of the mechanical behaviour of PLA, with particular emphasis on aspects relevant to shape memory and strain recovery
Elastic finite element modelling partially crystalline material shows that crystal phase

Summary

Introduction

Polylactic acid (PLA) and related polymers and blends are a focus of increasing attention, arising both from their potential for degradability within the natural carbon cycle for disposable products [1,2], and from their bioresorbable properties for prosthetic implants [3,4,5,6]. In the latter category, there are applications for ‘smart’ implants that make use of the polymer’s shape memory properties [7]. There have been two notable recent studies of molecular orientation effects arising from biaxial deformations [11,12]

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Results

Conclusion