Immiscibility-miscibility phase transitions in blends of poly(L-lactide) with poly(methyl methacrylate)

Background: The nature of phase transitions and apparently irreversible phase homogenization upon heating in blends of biodegradable poly(L-lactide) (PLLA) with poly(methyl methacrylate) (PMMA) were proven using differential scanning calorimetry, polarized optical microscopy, scanning electron microscopy and ¹H NMR spectroscopy. The complex phase behaviour in this blend system is puzzling and is a matter of debate; this study attempts to clarify the true nature of the phase behaviour. Results: A PMMA/PLLA blend is immiscible at ambient temperature but can become miscible upon heating to higher temperatures with an upper critical solution temperature (UCST) at 230°C. The blends, upon rapid quenching from the UCST, can be frozen into a quasi-miscible state. In this state, the interaction strength was determined to be X₁₂ = -0.15 to -0.19, indicating relatively weak interactions between the PLLA ester and PMMA acrylic carbonyl groups. Conclusion: The absence of chemical exchange reactions above the UCST and phase reversibility back to the original phase separation morphology, assisted by solvent re-dissolution, in the heat-homogenized PLLA/PMMA blend was shown. Verification of UCST behaviour, phase diagrams and solvent-assisted phase reversibility were experimentally demonstrated in PMMA/PLLA blends.