Flow-Directed Polar Phase Development in Miscible PVDF/PMMA Blends via Synergistic Nucleation and Crystal Growth Control

Blending with thermodynamically miscible polymers presents a promising method for tuning the phase structure of poly(vinylidene fluoride) (PVDF). This study examines the impact of poly(methyl methacrylate) (PMMA) additives on the development of polar phases in PVDF melts under flow conditions, utilizing both ex situ and in situ experiments. Without flow, PMMA has little effect on PVDF melt crystallization, which mainly forms the nonpolar α-phase. However, when subjected to melt stretching, PMMA significantly enhances the formation of polar phases, specifically the γ-phase at low strains and the β-phase at high strains. Consequently, incorporating PMMA notably widens the processing window for achieving high γ-phase content within the temperature-strain space. Two key mechanisms are identified to contribute to this enhancement. First, flow induces chain orientation, facilitating γ-phase nucleation at low strains and β-phase nucleation at high strains. This process occurs independently of PMMA and is attributed to the chain conformational ordering induced by stretching. Second, PMMA confers a significant kinetic advantage for the growth of polar phases over the α-phase following nucleation. It is hypothesized that when both polymer components are oriented, interchain hydrogen bonding reorganizes to favor continuous trans conformations of PVDF, which structurally match the polar β/γ-phases but conflict with the α-phase. This analogous effect, however, is sterically hindered in the absence of flow. These findings reveal a strong synergy between flow and PMMA in directing PVDF polymorphic crystallization, offering practical routes to produce polar-phase PVDF materials through cost-effective and straightforward melt processing.