Spatially Confined Fabrication of Polar Poly(Vinylidene Fluoride) Nanotubes.

Polar poly(vinylidene fluoride) (PVDF) nanotubes have attracted significant attention due to their excellent piezoelectric and ferroelectric properties, yet a tunable fabrication of homogeneous polar PVDF nanotubes remains a challenge. Here, a simple method is reported to fabricate polar PVDF nanotubes using anodize aluminum oxide (AAO) membranes as templates that are removed by etching in a potassium hydroxide (KOH) solution and then ageing at room temperature. PVDF nanotubes originally crystallized in the AAO membrane are pure α-crystals with very low crystallinity, yet after being released from the templates, the crystallinity of the nanotubes markedly increases with ageing at room temperature, leading to the formation of ß-PVDF crystals in a very short time, with the formation of γ crystals after longer ageing times. A large amount of γ crystals formed when the released PVDF nanotubes are heated to ≈130 °C. The formation of polar PVDF nanotubes released from the AAO templates treated with higher concentrations of alkaline solution results from the reaction of the surface of the PVDF nanotubes with the alkaline solution and structure reorganization under confined conditions. This large-scale preparation of ß- and γ-PVDF opens a new pathway to produce polar PVDF nanomaterials.

The dependence of the ß-to-α phase transition behavior of poly(1,4-butylene adipate) on phase separated morphology in its blends with poly(vinylidene fluoride).

Synchrotron wide-angle X-ray diffraction was used to monitor the melting and ß-to-α transition behavior of ß-poly(1,4-butylene adipate) (ß-PBA) and its blends with poly(vinylidene fluoride) (PVDF). Two kinds of typical phase separated morphologies are prepared: interfabrillar (70/30 PBA/PVDF) and interlamellar (50/50 PBA/PVDF) morphologies. After melt recrystallization at 10 °C, ß-PBA crystals are obtained. The thermal expansion, phase transition and melting behavior of ß crystals highly depend on the phase separated morphology. The ß crystals show the highest melting temperature and ß-to-α phase transition temperature in 70% PBA, which is caused by the thick lamellae of PBA and/or the lower surface energy of chain folding, while the ß crystals show the lowest melting temperature and ß-to-α phase transition temperature in 50% PBA, which is caused by the thinner lamellae of PBA. An unexpectedly high melting temperature of α crystals transited from ß crystals observed in 50% PBA due to the lamellar thickening occuring in the heating process. The different phase separated morphologies lead to different lamellae thicknesses and surface energies of chain folding which consequently alter the stability of the crystals. The phase transition and melting behavior of ß-PBA in PBA/PVDF is also compared with its blends with poly(vinyl phenol) (PVPh). Completely different factors influence the phase transition behavior of ß crystals in these two systems.

Epitaxy-Directed Self-Assembly of Copolymers and Polymer Blends.

Directed self-assembly of materials into patterned structures is of great importance since the performance of them depends remarkably on their multiscale hierarchical structures. Therefore, purposeful structural regulation at different length scales through crystallization engineering provides an opportunity to modify the properties of polymeric materials. Here, an epitaxy-directed self-assembly strategy for regulating the pattern structures including phase structure as well as crystal modification and orientation of each component for both copolymers and polymer blends is reported. Owing to the specific crystallography registration between the depositing crystalline polymers and the underlying crystalline substrate, not only order phase structure with controlled size at nanometer scale but also the crystal structure and chain orientation of each component within the separated phases for both copolymers and polymer blend systems can be precisely regulated.

Branched Crystalline Patterns of Poly(ε-caprolactone) and Poly(4-hydroxystyrene) Blends Thin Films.

The chain organization of poly(ε-caprolactone) (PCL) in its blend with poly(4-hydroxystyrene) (PVPh) in thin films (130 ± 10 nm) has been revealed by grazing incident infrared (GIIR) spectroscopy. It can be found that PCL chains orient preferentially in the surface-normal direction and crystallization occurs simultaneously. The morphology of the PCL/PVPh blends films can be identified by optical microscopy (OM). When crystallized at 35 °C, the blends film shows a seaweed-like structure and becomes more open with increasing PVPh content. In contrast, when crystallized at higher temperatures, i.e., 40 and 45 °C, dendrites with apparent crystallographically favored branches can be observed. This characteristic morphology indicates that the diffusion-limited aggregation (DLA) process controls the crystal growth in the blends films. The detailed lamellar structure can be revealed by the height images of atomic force microscopy (AFM), i.e., the crystalline branches are composed of overlayered flat-on lamellae. The branch width has been found to be dependent on the supercooling and PVPh content. This result differs greatly from pure PCL, in which case the crystal patterns controlled by DLA process developed in ultrathin film or monolayers of several nanometers. In the PCL/PVPh blends case, the strong intermolecular interactions and the dilution effect of PVPh should contribute to these results. That is to say, the mobility of PCL chains can be retarded and diffusion of them to the crystal growth front slows down greatly, even though the film thickness is far more than the lamellar thickness of PCL.