Formation and Evolution of the Lath-Shaped γ Poly(vinylidene fluoride) Single Crystals in Its Blends with Poly(1,4-butylene adipate)

Morphology of poly(vinylidene fluoride) (PVDF) single crystals grown in its blends with poly(1,4-butylene adipate) (PBA) has been studied in detail. Another type of γ-PVDF single crystal with morphology differing from that of the previously reported ellipse-shaped ones has been discovered. The newly found γ-PVDF single crystal exhibits a lath-like shape with pronounced overgrowth as well. The lath-shaped γ-PVDF single crystal shows a smaller curvature, weaker (111) electron diffraction spots, but more stable middle sector and ordered arrangement of the overgrowth than the ellipse-shaped one. The evident unstable side sector growth fronts lead to serrated edges and ordered arrangement of overgrowth. Moreover, the presence of large quantities of molten PBA can enhance the mobility of PVDF molecules for diffusing the growth fronts. Thus, a single crystal with smooth edges, such as an ellipse-shaped one, can be obtained. It is further found that though PVDF is miscible with PBA in the molten state because of the polydispersity of components, phase separation can take place in some microdomains of the PVDF/PBA blend, especially for supercooled PVDF. In some domains, the PBA component prefers to segregate over the PVDF, and the diffusion ability of underlying PVDF decreases greatly. As a result, the growth front of the crystal is serrated, and the overgrowth is easy to scroll but arranged more orderly, especially along ±b-axis directions. This indicates that the molten component of low melting temperature changes the diffusion rate of the crystalline process and thus is capable of tuning the morphology of polymer crystals in ultrathin films of binary crystalline/crystalline polymer blends.