Morphological, thermal, and rheological behavior of poly( l -lactide) and poly( meso- lactide) blends

Polylactide (PLA) is one of the most widely studied biodegradable polymers. However, poly(meso-lactide) (PMLA), an amorphous PLA, remains poorly characterized. This study investigates the thermal, rheological behavior, and morphology of poly(l-lactide) (PLLA) and PMLA blends across a wide composition range. Characterizations were performed using differential scanning calorimetry (DSC), small amplitude oscillatory shear (SAOS) rheology, and atomic force microscopy (AFM). Nanoscale heterogeneity is suspected for the 10% and 90% w/w% HMW PLLA/HMW PMLA compositions, while 30 w% HMW PLLA, 50 w% HMW PLLA, and 70 w% HMW PLLA compositions exhibited immiscibility. DSC revealed two glass transition temperatures for 30 w% HMW PLLA, 50 w% HMW PLLA, and 70 w% HMW PLLA blends, corresponding to those of the neat phases, 48.2 ± 0.1 °C for HMW PMLA and 61.8 ± 0.4 °C for HMW PLLA, and a single Tg for the 10% and 90% w/w% HMW PLLA/HMW PMLA composition blends. AFM revealed an emulsion-type morphology at 30 and 70 wt. % HMW PLLA compositions and a co-continuous structure at 50 wt. % HMW PLLA. SAOS results presented a shoulder in the low frequency storage modulus for all blends. The Palierne model was used to fit the dynamic data of the blends and estimated the dispersed phase relaxation time, reinforcing the immiscibility conclusions. Relaxation spectra were obtained for all blends, showing two distinct relaxation modes associated with PLA nature and the interfacial tension of the blend. These results suggest that PMLA and PLLA are largely immiscible.