Effect of temperature-dependent microstructure evolution on pore wetting in PTFE membranes under membrane distillation conditions

In certain applications, such as membrane distillation (MD), one key parameters affecting the process performance is liquid entry pressure (LEP) of the porous membranes. In this work, three 0.2-μm commercial expanded polytetrafluroethylene (PTFE) membranes were evaluated for wetting behavior under MD conditions. The effect of temperature on microstructure and stability of the membranes was investigated in relation to its impact on LEP. In the MD feed temperature range of interest, 25–70 °C, experimental LEP results for all three membranes showed deviations from theoretically estimated values obtained by the well-established Laplace (Cantor) equation, reflecting a changing geometric factor with temperature. While thermogravimetric analysis (TGA) measurements showed that the membranes had no degradation at temperatures less than 350 °C, differential scanning calorimetry (DSC) measurements revealed possible relaxation of internal stresses in the interconnected continuous fibrils of the expanded PTFE membranes. Further evidence of microstructure evolution due to fibril distortions in all three membranes was revealed by comparison of high resolution, secondary electron images taken using scanning electron microscopy at progressively increasing temperatures from 25 °C to 50 °C. Additional evidence was revealed through pore size analysis. Therefore, a temperature dependent geometric correction factor must be accounted for in the Laplace (Cantor) formulation for LEP estimation to predict the wetting of PTFE membranes under non-isothermal conditions. Present results indicate that, in addition to membrane pore size and hydrophobicity, microstructure evolution with temperature significantly impacts the wettability of the expanded PTFE membranes.