Surface morphology changes during initial usage of hydrophobic, microporous polypropylene membranes

Surface morphology changes of hydrophobic Celgard 2500 and Accurel 1E-PP membranes during initial contact with water were examined using scanning electron microscopy (SEM) and image analysis. The surfaces of both membranes showed increases in several of the following morphology parameters after 72 h of contact: porosity, pore area, pore length, pore breadth, pore equivalent diameter and pore spread factor. This was attributed to non-wetting intrusion of the water meniscus into some pores with a resulting enlargement of pore entrances. The results also suggested that the larger pores became larger and the smaller pores became smaller. A model based on more effective intrusion of liquid water into the larger pores and a resulting lateral displacement of the fibrils that formed the boundaries with neighbouring, smaller pores was invoked to explain the latter observation. Celgard 2500 membrane was also subjected to contact with 30% w/w CaCl2 solution but no significant morphology change was observed in this medium after 72 h. The absence of a morphology change was attributed to a lower degree of intrusion of the CaCl2 solution relative to that of water due to the higher surface tension of the former.