Morphology and performance of polyvinylidene fluoride (PVDF) membranes prepared by the CCD method: Thermodynamic considerations

The combined crystallisation and diffusion method (CCD) has recently demonstrated its superiority in producing high-performance PVDF membranes over the traditional membrane preparation methods. The CCD method involves a unidirectional cooling step in its membrane preparation. While the role of kinetics behind the CCD method has been thoroughly studied by varying the cooling rate, the cooling temperature itself has never been changed from −30 °C. This study therefore explores the thermodynamic implications of changing the cooling temperature during the CCD process on the morphology and performance of the resultant membranes. Two different molecular weights of PVDF were studied and different batches of membranes were prepared by progressively changing the cooling temperature from −30 °C to 0 °C. The results showed that for both the PVDF types, with an increase in cooling temperature, the separation surface pore sizes and membrane permeances increase but more so for the one prepared using the higher molecular weight PVDF. This showcases the ability to tailor the properties of the CCD membranes by merely changing the cooling temperature. The membranes were then also characterised for their morphology, mechanical strengths and crystallinity to help further understand the role of cooling temperature in affecting these membrane properties.