Influence of Stretching on the Performance of Polypropylene-Based Microporous Membranes

We analyze the pore structure changes during the fabrication of polypropylene-based microporous membranes via the stretching method. The membranes were prepared via melt extrusion followed by annealing and stretching at room temperature and at an elevated temperature (cold and hot stretching steps, respectively). Understanding the pore formation mechanisms is important for effective control of the membrane performance. Hence, the pore structure along the membrane surface and across the thickness, which determined the size, number, and interconnectivity of the pores, was analyzed to quantify the effect of stretching on the membrane performance. The cold stretching step was found to be the important one for promoting interconnection between the pores. Furthermore, it was shown that applying a low strain rate improved the permeability of the membranes. Finally, no maximum was observed in the permeability by increasing the stretch ratio during the hot stretching step.