Blending Effects on Polymorphic Crystallization of Poly(l-lactide)

The effects of miscible blending on the crystallization kinetics and crystalline structure of poly(l-lactide) (PLLA) were investigated by differential scanning calorimetry (DSC), polarized optical microscopy (POM), Fourier transform infrared (FTIR) spectroscopy, and wide-angle X-ray diffraction (WAXD). It was confirmed that the blends of PLLA and poly(d,l-lactide) (PDLLA) are miscible at all compositions. Due to the dilution effect, the crystallization of PLLA was hindered with the presence of PDLLA. The PLLA/PDLLA blends show the irregular or ring-banded spherulites. The polymorphic behavior of PLLA is influenced by the miscible blending with PDLLA. FTIR and WAXD data indicates that the formation of the α-form crystals of PLLA is favored in the blends. With incorporation of 50 wt % PDLLA, the critical temperature for the formation of α′- and α-form crystals decreases from ∼110 (of neat PLLA) to ∼80 °C. The factors affecting the formation of the PLLA α′- and α-form crystals were discussed from the thermodynamic and kinetic considerations. It was proposed that the formation of the metastable α′-form crystals of PLLA is kinetically preferential, while that of the thermally stable α-form crystals is thermodynamically favored. The blending effects on the crystalline structure of PLLA are ascribed to two factors, that is, the lowered equilibrium melting point and decreased crystallization rate.