Crystallization and self-nucleation of PLA, PBS and PCL in their immiscible binary and ternary blends

Three semicrystalline polyesters, poly(lactic acid), poly(caprolactone) and poly(butylene succinate) (PLA, PCL and PBS) were melt blended to prepare binary and ternary systems of selected compositions. In binary blends with a sea-island morphology, each of the polymers is either the major or dispersed phase in the various samples. Analogously, different ternary blends with a “partial-wetting” morphology are prepared, displaying droplets of PLA, PCL or PBS minor phase located at the interface between the other two major components. The crystallization behaviour of the three phases has been investigated under non-isothermal conditions via differential scanning calorimetry. In binary blends, a distinct effect of morphology on nucleation was observed, with the minor phase displaying fractionated crystallization, as a consequence of the droplet concentration being higher than that of nucleating impurities. Partially-wetting droplets of the different polymers in ternary blend show instead non-isothermal crystallization analogous to the bulk material, due to the much larger domain size. The self-nucleation behaviour of the polyesters in the binary and ternary blends was then compared to that of the neat polymers. It was found that the very large number of self-nuclei generated by the self-nucleation protocol (in Domain II or self-nucleation Domain) applied to the samples, completely overrules any effect of blend type or composition, so that the crystallization temperature is exclusively related to the self-nucleation temperature. However, when melting memory is erased and sufficiently high melting temperatures are employed, the role of heterogeneous nucleation is apparent, and the crystallization of the given blend component is highly dependent on the particular morphology.