Molecular dynamics simulation of the diffusion behavior of water in poly(vinylidene fluoride)/silica hybrid membranes

Abstract

A molecular dynamics (MD) simulation is employed to investigate the effects of the concentration and size of silica particles on the diffusion behavior of water in poly(vinylidene fluoride) (PVDF)/SiO₂ hybrid membranes. The membranes with different concentrations of SiO₂ particles (class I) and with different sizes of SiO₂ particles (class II) are designed and studied. The X-ray diffraction patterns and mean-square displacement (MSD) values of PVDF, the fractional free volume (FFV) characteristics and diffusion coefficients of water are discussed. The results revealed that the incorporation of silica particles into PVDF increased the polymer chain mobility and FFV in PVDF/SiO₂ hybrid membranes. The diffusion coefficient of water presented an increase as the concentration of SiO₂ increased (0–14.6 wt%). With the same concentration of SiO₂, when the sizes of the SiO₂ particles are different, the diffusion coefficient of water presented an increase first and then decreased as the size of the silica increased (radius: 0.794–1.26 nm). When the radius of the silica is nearly 1 nm, the diffusion coefficient of water in the membranes is at its largest (4.50 × 10⁻¹² m² s⁻¹).