Miscibility and phase structure of binary blends of poly(L-lactide) and poly(vinyl alcohol)

Blend films of poly(L-lactide) (PLLA) and poly(vinyl alcohol) (PVA) were obtained by evaporation of hexafluoroisopropanol solutions of both components. The component interaction, crystallization behavior, and miscibility of these blends were studied by solid-state NMR and other conventional methods, such as Fourier transform infrared (FTIR) spectra, differential scanning calorimetry (DSC), and wide-angle X-ray diffraction (WAXD). The existence of two series of isolated and constant glass-transition temperatures (T-g's) independent of the blend composition indicates that PLLA and PVA are immiscible in the amorphous region. However, the DSC data still demonstrates that some degree of compatibility related to blend composition exists in both PLLA/atactic-PVA (a-PVA) and PLLA/syndiotactic-PVA (s-PVA) blend systems. Furthermore, the formation of interpolymer hydrogen bonding in the amorphous region, which is regarded as the driving force leading to some degree of component compatibility in these immiscible systems, is confirmed by FTIR and further analyzed by C-13 solid-state NMR analyses, especially for the blends with low PLLA contents. Although the crystallization kinetics of one component (especially PVA) were affected by another component, WAXD measurement shows that these blends still possess two isolated crystalline PLLA and PVA phases other than the so-called cocrystalline phase. 13C solid-state NMR analysis excludes the interpolymer hydrogen bonding in the crystalline region. The mechanical properties (tensile strength and elongation at break) of blend films are consistent with the immiscible but somewhat compatible nature of these blends. (C) 2001 John Wiley & Sons, Inc.