Compatibilizing hydrophilic and hydrophobic polymers via spray coating for desalination

Abstract

The incompatibility between hydrophobic polymers such as polytetrafluoroethylene (PTFE) and hydrophilic polymers such as polyvinyl alcohol (PVA) is typically overcome by reducing the large surface energy difference between these materials via complex protocols or using bespoke chemicals. In this study, we deployed the simple technique of spray coating to solve this incompatibility, depositing polyvinyl alcohol (PVA) onto PTFE porous support layers. This was achieved as sub-micron sized PVA solution droplets infiltrated and filled up PTFE inter-fiber pores, forming a tightly bound PVA layer on PTFE fibrous supports. The defect-free thin hydrophilic PVA layer and the porous hydrophobic PTFE substrate of the composites were exploited for desalination in both direct contact membrane distillation (DCMD) and pervaporation (PV) modes. When deployed to separate 3.5 wt% NaCl from water at 75.0 ± 0.9 °C with a permeate side pressure of 100 Pa in PV mode, these thin film composites demonstrated ultra-high water fluxes of 143.4 ± 8.9 kg m⁻² h⁻¹, outperforming state-of-the-art PV membranes. Moreover, when the membrane was tested in DCMD mode with a cooling stream at 20.6 ± 0.3 °C, a water flux of 64.2 ± 2.9 kg m⁻² h⁻¹ was obtained and was on par with the best DCMD membranes. With excellent organic and ionic fouling resistances, these thin film composites can be potentially deployed to treat polluted brine mixtures, even under harsh operating conditions.