Morphologies and Crystallization Behaviors in Melt-Miscible Crystalline/Crystalline Blends with Close Melting Temperatures but Different Crystallization Kinetics

Poly(L-lactic acid) (PLLA) and poly(oxymethylene) (POM), with very close melting temperatures (T-m), can crystallize simultaneously or separately in their blends depending on composition and crystallization temperature (T-c), resulting in various types of morphology. It is mainly attributable to the greatly different crystallization kinetics of PLLA and POM. At a content of POM (phi(POM)), 3 wt % < phi(POM) < 20 wt %, PLLA crystallization kinetics are comparable to POM, and therefore two type spherulites exhibit side-byside simultaneous growth with the penetration of PLLA spherulites into POM crystals. Although crystal growth rate (v(c)) of POM is still a bit faster than that of PLLA, for phi(POM) = 3 wt %, the nucleation of POM is restrained and POM spherulites can only develop on the propagating PLLA growth fronts with the generation of novel core shell blended spherulites. For 20 wt % <= phi(PCM) < 80 wt %, interspherulitic growth of PLLA inside the pre-existing matrix of POM spherulites causes the formation of interpenetrated blended spherulites, owing to the large discrepancy in kinetics. At phi(PCM) >= 80 wt %, PLLA molecular chains are redistributed into the interlamellar level regimes within the POM spherulites and can only crystallize into tiny crystals (owing to strong confinement). PLLA/POM blends provide a perfect example and new insights for understanding the crystallization of miscible crystalline/crystalline polymer blends (with very similar T-m's), in which kinetic factors could play a significant role in crystallization behaviors and morphology.