Strong effect of precursor preparation on the morphology of semicrystalline phase inversion poly(vinylidene fluoride) membranes

This work documents the role of precursor preparation as an overlooked important factor in the control of membrane structures obtained by the phase inversion (PI) of solutions of crystallizable polymers. The effect of precursor-solution preparation temperature on the structure of a phase inversion poly(vinylidene fluoride) (PVDF) membrane is examined. The phase inversion example explored is the coagulation of a PVDF solution in dimethyl formamide (DMF) by the action of 1-octanol, which is a soft non-solvent. Polymer dissolution temperature is varied from 50 to 110 °C and the immediate consequence is a substantial variation in the density of nuclei available for the initiation of crystallization. Membrane structure is defined through a competition between solid–liquid (s–l) and liquid–liquid (l–l) equilibria; such competition can be strongly affected by changes in the density of crystallization nuclei. As a result, it is found possible to generate a wide spectrum of membrane morphologies without resorting to a variation of any of the standard parameters, such as dope and coagulant compositions and temperatures, usually employed for the control of phase inversion membrane morphology. It is also found that aging time can be used as a further structure-control parameter and a ‘maturation time–dissolution temperature’ equivalence is demonstrated.