PLA, TPS and PCL binary and ternary blends: structural characterization and time-dependent morphological changes

Abstract

One-step melt mixing procedure was used to generate binary and ternary blends containing different compositions of poly-l-lactide (PLA), thermoplastic starch (TPS) and poly(ɛ-caprolactone) (PCL) polymers. The structural characterization of the blends was carried out using X-ray diffraction, Raman and infrared spectroscopy, solid-state nuclear magnetic resonance (NMR) and atomic force microscopy (AFM). In addition, time-dependent evolution in structure and morphology of the blends was studied. Binary blend of PLA50PCL50 exhibited reduction in crystallinity of PLA due to some extent of intermixing of PCL and PLA phases. Diffuse Raman bands, X-ray diffraction signals as well as broadening of crystalline NMR resonances of PLA confirmed the phenomenon. In PLA50TPS50 blend, TPS did not mix with PLA and had no effect on the crystallinity of PLA. In the ternary blends, the addition of PCL was observed to improve the dispersion of TPS in PLA. The ternary blends with different compositions did not exhibit any significant differences among each other, and no strong interactions between the components were observed. Raman mapping was also demonstrated to accurately map the dispersion of components in the blend. Morphology of the blends was observed to exhibit physical aging phenomenon as a function of time. PLA50PCL50 blend had significant changes in topography, reduction of polymer domains and interfacial separation in the aged samples due to the relaxation of the polymer domains. Due to the glassy nature of PLA and TPS phases, the contraction of these phases co-existed with subsequent expansion of PCL phase due to its rubbery state at room temperature. In the ternary blends, the physical aging resulted in similar loss of ultrastructure in PCL and phase separation, although the ultrastructure of PLA and TPS was not significantly affected.