Impacts of PVDF polymorphism and surface printing micro-roughness on superhydrophobic membrane to desalinate high saline water

Many studies have focused on increasing the superhydrophobicity of membranes to enhance the sustainable desalination capability of membrane distillation (MD) process. This work focuses on the influences of crystalline β-polymorphs and micro-roughness surface-printing, to transform the surface of hydrophobic poly(vinylidene fluoride) (PVDF) membranes to achieve superhydrophobicity. This study uses two types of PVDF (HV- and LV-PVDF) with different polymer chain lengths and polymer densities. According to FTIR analyses, the membranes synthesized using HV- and LV-PVDF are mainly composed of β-phase polymorphs, with relative fractions of 0.633 and 0.472, respectively. The high content of hydrophilic β-phase polymorphs renders the contact angle (CA) of the non-surface-printed HV-PVDF membrane as low as 87.2º. Through the uniformly distributed micro-scaled structures on the surface-printed PVDF membranes, the surface-printed HV-PVDF membrane successfully surpassed the superhydrophobicity, with a CA of 151.1º and a dynamic sliding angle (SA) of 13º. Regardless of the surface-printing, the HV-PVDF membranes remain a similar surface porosity (42%). The results proved that the layered micro-roughness of surface-printing can resist membrane wetting during MD separation by achieving at least a four-fold increase in average permeation flux from 2.5 kg/m2 h to 10 kg/m2 h and salt rejection of 99.99%. This study qualitatively explains the importance of material chemistry (β-phase polymorphs) and surface roughness (micro-roughness), which affect membrane wetting resistance during MD.