Hydrophobic modification of polyvinylidene fluoride membranes in supercritical carbon dioxide fluids

Polyvinylidene fluoride (PVDF) membranes exhibit exceptional chemical stability and find applications in various fields, including building membranes, flexible sensors, blood purification, and seawater desalination. Nevertheless, the hydrophobic performance of PVDF membranes in these applications is subpar and requires enhancement. Supercritical carbon dioxide (scCO2) fluid demonstrates a beneficial expansion effect on PVDF membranes and exhibits good solubility with fluorine reagents. This study employs scCO2 as a medium to introduce a fluorine reagent onto the surface of PVDF membranes, forming a layer of low surface energy substance that significantly improves the membrane's hydrophobicity. The results indicate the feasibility of hydrophobic modification of PVDF membranes using scCO2 as a medium, leading to improved hydrophobic properties. When using a fluorine reagent with a relatively large molecular weight, PVDF membranes in scCO2 fluid at 110 °C and 20–25 MPa for 4 h results in a substantial improvement in the hydrophobicity of the PVDF membrane, with the contact angle reaching 117.2° (compared to the primitive membrane's 80°). Scanning electron microscopy (SEM) analysis confirms the presence of a layer of low surface energy material on the modified PVDF membrane surface. Thermogravimetric (TG) and differential thermogravimetric (DTG) analyses demonstrate that the hydrophobic modification using scCO2 enhances the crystallinity and thermal stability of the PVDF membrane, raising the thermal cracking temperature from 425.3 °C to 450.5 °C. Fourier-transform infrared spectroscopy (FT-IR) reveals clear absorption peaks of the fluorine reagent at 1740 cm−1 and 1211 cm−1. Nuclear magnetic resonance (NMR) confirms the presence of characteristic peaks of hydrogen on the fluorine reagent, confirming the embedding of the hydrophobic reagent onto the PVDF membrane surface. This modification method holds significant potential for guiding the development of hydrophobic surfaces, contributing to carbon neutrality, and meeting environmental sustainability goals.