Formation of porous poly(vinylidene fluoride) membranes with symmetric or asymmetric morphology by immersion precipitation in the water/TEP/PVDF system

The phase equilibrium boundaries of the membrane forming system, water/triethyl phosphate (TEP)/PVDF, at 25 °C were determined experimentally using cloud-point and equilibrium absorption methods. Based on the phase diagram, appropriate dope and bath compositions were selected to prepare microporous membranes by means of the isothermal immersion-precipitation technique. As a metastable casting dope with respect to crystallization was adopted, the formed membranes exhibited a uniform cross-section composed of interlocked crystal elements coexisting with the network of continuous pores, as was revealed by high resolution FESEM imaging. Morphologies of the membranes' top surfaces were found to depend heavily on the bath strength, which was controlled by the TEP content. By changing the bath gradually from pure water to 70% TEP, the top surface evolved from a dense skin (asymmetric membrane) to a totally porous morphology (symmetric membrane). Wide angle X-ray diffraction analysis indicated that PVDF crystallized into α-type structure for all of the synthesized membranes. The crystallinity as determined from diffraction peak deconvolution was ≈65%, which value was confirmed by Differential Scanning Calorimetry (DSC). The obtained thermograms also showed a similar melting peak temperature (Tm ≈ 169 °C) for all membranes. Furthermore, water fluxes and tensile strengths of the membranes were measured. The results were found to correlate with the morphologies of the membranes.