Investigations On Crystallization Behaviors And Applications Of Melt-miscible Poly(L-lactic Acid)(PLLA)/Poly(Oxymethlene)(POM) Blends

Recently, crystalline/crystalline polymer blends have attracted tremendous attention, due to their various crystal structures and complicated crystallization kinetics. In the present work, we have systematically investigated the crystallization behaviors of melt-miscible poly(L-lactic acid) (PLLA)/poly(oxymethylene) (POM) blends. Based on the crystallization characteristics of PLLA/POM blends, we have successfully developed a "banded-spherulitic templating" path to fabricate porous polymeric materials with three-dimensional (3D) interpenetrated internal channels. We have, moreover, introduced multiple-walls carbon nanotube (MWCNT) into PLLA/POM blends and carefully studied the crystallization behaviors of PLLA/POM/MWCNT nanocomposites. Furthermore, we have succeeded in preparing conductive porous polymeric materials (CPMs), via a "block-assembling" strategy based on "nano-hybrid shish-kebab (NHSK)" structures in PLLA/POM/MWCNT nanocomposites. According to key ideas, "from crystallization of polymers (foundation) to fabrication of functional materials (applications)", this work can be simply divided into four parts as follows.Part one, investigations on crystallization behaviors of PLLA/POM blends. PLLA/POM blends are typical melt-miscible polymer systems, exhibiting lower critical solution temperature (LCST) phase behaviors. PLLA and POM are semicrystalline polymers with very close melting temperature (Tm); however, the difference in crystallization kinetics between these two components is quite large. The crystallization rate (vc) of POM is very rapid, but in contrast, that of PLLA is relatively low, and therefore, PLLA/POM blends show various interesting morphologies and superstructures. When POM content (φPOM) is less than 20 wt%, PLLA and POM in binary blends simultaneously crystallize. PLLA crystallize as compact and regular spherulites, while POM crystals form large and ordered banded spherulites. PLLA/POM blends exhibit stepwise crystallization behaviors when φPOM ≥ 20 wt%. POM crystallizes first and volume-fill the whole initial melt space before PLLA could crystallize. Subsequently, PLLA crystallize within the POM scaffolds and "interpenetrated spherulites" of these two components finally generate. It is, moreover, to find that the PLLA crystallization within the POM scaffolds is intensively confined, upon φpom increases to 80 wt%.Part two, investigations on crystallization behaviors of PLLA within the banded-spherulitic POM scaffolds. It is found that PLLA molecular chains are mainly redistributed into the interlamellar region of POM crystals during POM crystallization, due to the rapid vc of POM and the low diffusion ability of PLLA chains. Nano-scaled "co-continuous" structure with highly-continuity composed of solid POM crystals and amorphous PLLA-rich phase, therefore, forms in PLLA/POM blends after POM crystallization. A small amount of POM amorphous chains in the interlamellar regime is found to promote the molecular mobility of PLLA chains effectively and further, significantly accelerate the overall crystallization kinetics of PLLA. Upon increasing of POM content within the interlamellar region, however, the diffusion of PLLA molecular chains from the melt to the growth fronts is increasingly depressed due to the intersified interdiffusion between PLLA and POM chains, thereby decreasing the overall PLLA crystallization kinetics. Most interestingly, the POM components, of which crystals melt primarily, crystallize first in PLLA/POM blends. Therefore, we can directly have a seeing of the crystal morphology of PLLA within the banded-spherulitic frameworks for the first time, by simply melting the POM scaffolds. It is very interesting that the PLLA lamellae cooperatively twist to reversely duplicate the banded-spherulitic structure of POM crystals, implying an elaborate "templating" effect of the POM scaffolds on crystal growth of PLLA.Part three, "banded-spherulitic templated" nanoporous materials with 3D internal channels. The rapid vc of POM and relatively low diffusion ability of PLLA chains, causing the interlamellar inclusion of PLLA chains, lead to generation of a nano-scaled "co-continuous" structure with highly continuity, being consisted of POM crystals and PLLA-rich phase. By selectively removal of PLLA components, we have finally obtained 3D interpenetrated nanoporous POM materials, whose pore structures are expected to be simply modulated by adjusting either crystallization conditions or compositions. Moreover, the obtained porous POM materials can be further exploited as templates with the specific structure to fabricate inorganic/organic hybrid materials or inorganic nanoporous materials.Part four, a "block-assembling" strategy for fabrication of CPMs based on the NHSK structure. MWCNT has been successfully incorporated into PLLA/POM blends to prepare PLLA/POM/MWCNT nanocomposites. It is found that POM crystallizes first onto the surface of MWCNT with formation of NHSKs, resulting in majority of PLLA molecular chains finally locate in the space between adjacent POM "disk-like" lamellae (intra-NHSK inclusion). Upon subsequent crystallization of PLLA within the intra-NSHK regions, the surface of a MWCNT was alternatively patterned with PLLA and POM lamellae, thereby translating NHSK into "ternary-hybrid shish-kebab (THSK)" structure. Similarly, CPMs can be easily achieved by simply and selectively removing of PLLA components in PLLA/POM/MWCNT nanocomposites.