Insight into the formation of a continuous sheath structure for the PS phase in tri-continuous PVDF/PS/HDPE blends

Abstract

This work reports on the morphology development of ternary percolated co-continuous systems in PVDF/PS/HDPE blends in which PVDF and HDPE form two continuous networks, while the PS forms a continuous sheath structure at the PVDF/HDPE interface. By controlling the relative amounts of PVDF, PS and HDPE, continuity data based on gravimetric solvent extraction clearly demonstrate that a PS volume composition as low as 11% results in a very high level of continuity of about 80%. The evolution of PS phase continuity is further studied by changing the component ratios of HDPE and PS with the PVDF phase concentration held at a constant 44% volume fraction. Scanning electron microscopy as well as optical microscopy is used to clearly illustrate and identify the evolution of the PS phase morphology. The results indicate that with a PS phase concentration increase, the evolution of the PS phase morphology in the ternary blends experiences several distinguished stages: when the PS concentration is less than 4 vol%, the PS phase locates at the PVDF and HDPE interface as dispersed droplets; when the PS concentration increases to 7 vol%, the PS phase forms an incomplete interface between PVDF and HDPE; when the PS concentration reaches 10 vol%, most of the PS has clearly and spontaneously structured itself at the PVDF/HDPE interface forming a uniform layer. Additionally, the self-assembly behavior of the PS droplets and the coalescence behavior of the PS layer on the PVDF/HDPE interface are respectively investigated through online observation using optical microscopy under quiescent annealing at 200 °C. The mechanism of the phase morphology evolution under annealing indicates that the movement of the phase interface and interfacial tension play key roles in the phase relaxation and equilibrium.