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 (φ POM ), 3 wt % < φ 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 φ 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 % ≤ φ POM < 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 φ POM ≥ 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.

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Poly(oxymethylene)/poly(L-lactic acid) (POM/PLLA) blends are typical melt-miscible binary systems. During isothermal crystallization at various temperatures, in the presence of amorphous PLLA chains, POM crystallizes into banded spherulites with different band spaces, which forms a continuous crystalline phase and serves as a sturdy frame in the final porous materials. On the other hand, the amorphous PLLA chains are simultaneously expelled out from POM crystal lamellae to generate the other continuous phase during the crystallization of POM. Consequently, the interpenetration of the POM lamellae and the amorphous PLLA phase construct a cocontinuous phase structure. All the PLLA constituents are fully included in the interlamellar or interfibrillar of POM crystals. Thus, nanoporous POM materials with hierarchical patterned surface and 3D interpenetrated internal channels have been successfully obtained by extracting the amorphous PLLA phase. It is further found that the POM crystal morphologies in the blends are much dependent on the crystallization conditions. Therefore, the hierarchical patterned structure and the size of internal channels (pore size) can be modulated by adjusting the crystallization conditions.

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