Formation of PLA stereocomplex crystals during melt-blending of asymmetric PLLA/PDLA/PMMA blends of varying miscibility

Abstract

Polylactide (PLA) enantiomers of poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) were melt-blended with poly(methyl methacrylate) (PMMA) and compression-molded at a temperature between the melting points of PLA homocrystals (Tm,HC) and stereocomplex (SC) crystals (Tm,SC), causing the selective formation of SC crystallites. The degree of crystallinity of the SC crystals (χc,SC) did not change with the PMMA weight fraction but did vary with changes in the weight ratio of PLLA to PDLA. From differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) measurements, the miscibility of PLA and PMMA was confirmed, but the formation of SC crystals during melt-blending induced phase separation into PLA-rich and PMMA-rich phases. The formation of homocrystals was hindered by increases in the weight fraction of PMMA and χc,SC. The thermal and viscoelastic properties of the PLLA/PDLA/PMMA blends were also affected by the PMMA weight fraction and χc,SC. According to the DMA results, the storage modulus of the ternary blends with higher χc,SC values showed a gentler decrease at the glass transition temperature; the ternary blends also exhibited a higher storage modulus than the PLLA/PMMA blends at high temperatures near Tm,HC. PDLA weight fraction dependence of stereocomplex crystallinity χc,SC for PLLA/PDLA/PMMA blends with PDLA weight fractions of 5, 10, and 20 wt%.