Characterisation of 3D porous macrostructure of hollow fibre membranes using X-ray tomography-effects of some spinning process conditions

The presence of pores larger than a micron, known as macrovoids, in polymeric membranes is particular lydetrimental for membrane structural integrity and mechanical properties. The theoretical modelling and physical understanding of the mechanisms of initiation and growth of the macrovoids, notably in order to determine processing conditions that would allow to make macrovoids free membranes, have been the subject of a vast literature and a restill controversial. However, experimental data on macrovoids, that could help to discriminate between several ideas are scarce and have been essentially obtained using 2D imaging systems. A very large number of observations are then needed to obtain statistically significant results and the limitation to 2D images does not permitto access to an accurate description of the macroporous structure. In this work, anX-raytomography system was used to image the 3D porous structure of hollow fibre membranes with a spatial resolution of one micron. Image analysis tools have been developed to accurately characterise them acrovoid shapes and spatial distribution on the membrane outer and inner skins as a function of some of the spinning process conditions (concentration of solvent into the bore and airgap length). The main purpose is there fore to quantify effects of some processing conditions on the macroporous structure of a hollow fibre membrane. Also, such an accurate characterisation of the macrovoids spatial distribution and 3D shape, as well as their variations with the process experimental conditions, help to shed light on macrovoids initiation and growth mechanisms.