Effect of Drawing Temperature on Structure and Properties ofα'-Phase of Poly(lactic acid)

Abstract

Poly(lactic acid) (PLA) films consisting of α'-forms were prepared and uniaxially drawn. The effects of the drawing temperatures above the glass transition temperature on molecular weight and molecular weight distribution, chain conformation, degree of crystallinity and crystalline phase transformation were investigated by a combination of vibrational spectroscopy (infrared and Raman), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). The general appearance 776 化 学 学 报 Vol. 70, 2012 of stress-strain behaviour of drawn PLA, and in particular its yield stress and Young’s modulus, is strongly affected by the drawing temperatures. The values decrease with increasing the stretching temperatures. The GPC results showed that the molecular weight decreases and its distribution becomes wider after drawing at various drawing temperatures. It was established that the α'-forms phase of PLA films does not transform into either an α or β crystals upon uniaxial drawing with various temperatures at a fixed draw ratio of 4. However, compared with undrawn α'-forms PLA films, the crystallinity and the degree of orientation of PLA products are greatly influenced by the drawing temperatures. The crystallinity is significantly increased upon deformation. The crystallinity and orientation of PLA products increase with increasing the draw temperature when the stretching temperature is <100 °C, however, the degree of crystallinity and deformation will decrease when the temperature is ≥100 °C. While the overall changes in physical properties can be attributed to changes in the degree of crystallinity as a function of drawing temperatures, polarized Raman spectra and XRD clearly illustrated changes and the differences in the amorphous and crystalline orientation as a function of processing conditions.