Integrally skinned asymmetric poly(vinylidene fluoride) hollow fibre membranes: A study of gas and vapour transport properties

The gas and organic vapour permeation of integrally skinned asymmetric poly(vinylidene fluoride) (PVDF) hollow fibre membranes with an effective skin thickness of ca. 0.2 ?m was studied. Methanol, dichloromethane and acetone showed fundamentally different behaviour related to their interaction with the polymer phase. Strong plasticization by acetone led to a very pronounced increase in permeance as a function of the vapour activity. In contrast, the permeance of methanol was virtually independent of the vapour activity. Dichloromethane showed intermediate behaviour. The data were supported by vapour and liquid phase sorption measurements and liquid phase dilation measurements on 38.5 ?m thick flat sheet reference samples. X-ray diffraction analysis revealed a significant decrease in crystallinity upon swelling by acetone, whereas none of the liquids caused a notable change in the crystal polymorphs. In addition, differential scanning calorimetry revealed no irreversible changes in crystallinity or crystal polymorphs. A considerable discrepancy between liquid phase sorption and vapour phase sorption at high activity, known as Schröder's paradox, was observed for C-C alcohols. The anomalous transport properties were investigated in detail and were explained in terms of the ENSIC model for polymer-vapour interactions. The results were discussed in light of the potential use of asymmetric PVDF membranes in gas/vapour separation, vapour/vapour separation or pervaporation.