Tuning the Diffusion-Controlled Crystallization of a Poly(vinylidene fluoride) Ultrathin Film in γ-Form: The Role of Polyester Concentration

Thin-film crystallization of polymers is governed by diffusion processes, often resulting in branching morphologies. According to the theory of branching instability, the characteristic length scale of branch patterns is determined by the diffusion constant (D) and growth velocity (v). In this study, poly(vinylidene fluoride) (PVDF) was blended with several aliphatic polyesters and an aliphatic-aromatic copolyester. The resultant crystalline patterns of γ-PVDF crystals in the blend ultrathin films varied systematically with the polyester content (φp). By analyzing the branch width (λ) and v of γ-PVDF crystals, we established the relationship between φp and D in the depletion zone. Our findings reveal that polyesters stabilize the crystal growth front, with D exhibiting an exponential dependence on φp. Moreover, fitting-parameter K at φp = 1.0, serving as a relative indicator of the diffusion ability across pure polyesters, exhibits a clear inverse correlation with the polyester molecular weight. It is further found that at low polyester concentrations (φp ≤ 50%), the depletion zone ahead of the PVDF crystal growth front contains both mass and concentration gradients, enhancing PVDF chain mobility. However, at higher concentrations (φp > 50%), excessive polyester migration to the PVDF crystal growth front leads to a pure concentration gradient in the depletion zone.