Strain-induced phase transformation in poly(lactic acid) across the glass transition: Constitutive model and identification

Abstract

Poly(lactic acid) (PLA) is one of the most popular environmental-friendly materials derived from renewable agricultural resources. One of the most important features of this biomaterial is its ability to develop a phase transformation under straining. Two strain-induced phases can potentially form, namely mesomorphic and crystalline, whose respective occurrence is highly temperature and rate-dependent. The aim of the present contribution is to provide a quantitative predictive modeling of this phenomenon. A large-strain constitutive model is proposed to describe the strain-induced phase transformation in PLA along with the stress–strain behavior over a wide range of straining temperatures across the glass transition involving glassy to rubbery response. The material response is decomposed into two physically distinct sources, an elastic–viscoplastic intermolecular resistance to deformation and a viscohyperelastic molecular network resistance to stretching and chain orientation. The effective contribution of the amorphous, mesomorphic and crystalline phases to the intermolecular resistance is treated in a composite framework considering a three-phase representation of the microstructure. Using experimental data of an initially amorphous PLA extracted from the literature, the dual-phase transformation kinetics is designed and the properties of each phase are isolated. The proposed model is found to successfully capture the important features of the experimental observations in terms of strain-induced phase transformation and mechanical response over a large strain range, a wide range of temperatures and two strain rates. The model is then used to discuss some important aspects of the connection between the phenomenon of phase changes and the material response.