Molecular-level evaluation and manipulation of thermal conductivity, moisture diffusivity and hydrophobicity of a GO-PVP/PVDF composite membrane

Hydrophobic porous membranes are widely used in various processes like distillation-based seawater desalination. For such applications, membranes with a higher moisture permeability, lower thermal conductivity and higher hydrophobicity are highly desired. Previous material modifications mainly relied on macro-scale graphene oxide (GO) filler modifications. Here, a molecular-level manipulation technique based on molecular dynamics simulations (MDS) is proposed for the performance improvement of a GO-PVP/PVDF composite membrane. The wetting behavior on the surface, the thermal conductivity and the moisture diffusivity through the composite membrane are optimized via atomic modifications inside the material while considering the molecular structures, pore structures, contacted face and/or interfacial resistance. Combining MDS with a macro-scale resistance-in-series/parallel model for the bulk performance, the overall heat conductivity and moisture diffusivity are evaluated. For the first time the molecular heat and mass transport mechanisms through the graphene stacks are disclosed. It is found that the common six-membered ring of graphene oxide is difficult for moisture to cross directly. To solve this problem, a novel GO membrane where water molecules can penetrate directly through the pores of graphene oxide, is fabricated. Moisture diffusivity is increased by 38%.