Immiscibility-Miscibility Phase Transformation in Blends of Poly(ethylene succinate) with Poly(L-lactic acid)s of Different Molecular Weights

Using differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and Fourier transformed infrared spectroscopy (FTIR), upper critical solution temperature (UCST) phase behavior with immiscibility-miscibility transformation in blends of poly(ethylene succinate) (PESu) with poly(lactic acid)s (PLAs), such as poly(p,L-lactic acid) (PDLLA), poly(L-lactic acid) (PLLA), poly(o-lactic acid) (PDLA), differing in D/L configurations and molecular weights were investigated. All three binary blends of PDLLA/PESu, PLLA/PESu, and PESu/PDLA exhibit UCST behavior, which means they are immiscible at ambient temperature but can become miscible upon heating to higher temperatures at 240-268 degrees C depending on molecular weights. The PLLAs/PESu blends at UCST could be reverted back to the original phase-separated morphology, as proven by solvent redissolution. The blends upon quenching from above UCST could be frozen into a quasi-miscible state, where the Flory-Huggins interaction parameter (chi(12)) was determined to be a negative value (by melting point depression technique). The interaction between PDLLA and PESu in blend resulted in significant reduction in spherulite growth rate of PESu. Furthermore, blends of PESu with lower molecular weight PLLA or PDLA (M(w) of PLLA and PDLA are 152,000 and 124,000 g/mol, respectively), instead of the higher M(w) of PDLLA (M(w) of PDLLA = 157,000 g/mol), are immiscible with UCST phase behavior, which are affected by molecular weights rather than the ratio of LID monomer in the chemical structure of PLAs. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1135-1147, 2010