The mechanics of strain-induced crystallization in poly(ethylene terephthalate)

Abstract

Poly( ethylene terephthalate) (PET) is a thermoplastic polymer found in numerous com­mercial applications. PET exhibits the capability of undergoing strain-induced crystalliza­tion during processing, the result of which is increased stiffness and hardness as well as better dimensional stability. An experimental study of the strain rate, strain state, and tem­perature dependencies of the stress-strain behavior of PET under large strain deformation was conducted over a wide range of strain rates (-0.005/s - 02.0/s) and temperatures (25 ° C - 105 ° C) and in both uniaxial and plane strain compression. The increase in crys­tallinity content with varying strain rate and temperature and for the two different states of strain that developed as a result of these deformation conditions was investigated using Differential Scanning Calorimetry (DSC). The nature and evolution of the crystallo­graphic texture was studied using Wide Angle X-ray Diffraction (WAXD) measurements. The results of the mechanical tests, DSC and WAXD measurements indicated that strain­induced crystallization does not occur in uniaxial compression at temperatures below the glass transition, however, a shift in the cold crystallization exotherm was observed. Alter­natively, in plane strain compression at these temperatures for the fastest strain rates, DSC measurements indicated an increase in crystallinity. At temperatures near the glass transi­tion in uniaxial compression, DSC measurements indicated an increase in crystallinity, whereas WAXD measurements indicated only molecular orientation. In plane strain com­pression, however, both DSC and WAXD measurements indicated an increase in crystal­linity. At temperatures above the glass transition temperature, both DSC and WAXD indicated an increase in crystallinity in both uniaxial and plane strain compression. It was found that the percent crystallinity increased with increasing strain rate and decreasing temperature and that the crystallographic texture that develops is dependent on the state of strain. A constitutive model that predicts the strain rate, strain state and temperature dependence of PET was developed that captures these dependencies very well.