Microstructure and Thermomechanical Properties of Glassy Polylactides with Different Optical Purity of the Lactate Units

Abstract

Microstructure and thermomechanical properties of polylactides (PLA) with different optical purity (OP) of the lactate units were studied in detail. Thermal analysis by DSC revealed that the melting (Tm) and glass transition (Tg) temperatures of PLA can be simulated by the theoretical equations of Flory and Fox, respectively, both decreasing with decreasing OP. The sequential analysis of the PLA samples by 13C NMR spectroscopy suggested that the meso-sequence is more favorably formed than the racemic sequence in the L-rich samples. Their glassy specimens made by injection-moltding showed characteristic behaviors in dynamic mechanical analysis (DMA); the storage modulus (G ′) of the L-rich PLA specimens (L ratio = 99.0, 96.1, and 95.8) dropped and subsequently jumped up above Tg correspondingly to the glass-to-rubber and rubber-to-crystal transitions, respectively. In the G ′ curve of PLA with low L contents (90 and 72.5), only a drop was shown due to the simple glass-to-rubber transition. Wide-angle X-ray diffraction (WAXD) of the former L-rich PLA supported the responsibility of the crystallization for the jump-up of G ′ above Tg. The tensile strength of these glassy PLA specimens decreased with their decreased OP, while the tensile modulus was almost identical irrespectively of OP. On the other hand, the density of the PLA specimens was inversely proportional to OP, being in contrast to the above Tg change. The opposite relationship between Tg and density changes can reasonably be explained by a packing model in which the partial helices are formed along the L-sequenced segments of the PLA chain and randomly packed in solid state to afford a large free volume in between the helices.