The effect of uni-axial stretching on microporous phase separation membrane structure and performance

The goal of this research was to measure and predict changes in the material properties and performance characteristics of phase separation (PS) microfiltration membranes (MF) due to uni-axial stretching. Material properties such as pore aspect ratio, pore area, porosity, membrane thickness, surface properties, and polymer phase transition temperatures were measured as a function of membrane strain. Membrane performance was characterized in terms of pure water flux through the membrane and particle rejection using either silicon dioxide particle or polystyrene latex microspheres. In order to predict membrane pore deformation two different mathematical models were constructed using ABAQUS finite element software. Poly(vinylidene fluoride) (PVDF) PS membranes supplied by Pall Corporation and Millipore Corporation were tested. The PVDF membranes had a broad distribution of pore sizes and were characterized by the size of particle they reject with 99% efficiency. The membranes were stretched at temperatures between 80ºC and 160ºC and at strain rates between 0.1 cm/s and 3 cm/s. Membrane surface pore characteristics were measured using SEM and digital image analysis. This analysis found that membrane stretching significantly alters both pore shape and size. Pore deformation occurred in three stages: (1) initial membrane stretching aligns pores in the direction of strain; (2) continued stretching results in uniform deformation of pores in the direction of stretching; (3) at a critical level of strain small polymer fibrils in the membrane will fracture resulting in the formation of macro voids in the membrane. This deformation will continue with additional strain until catastrophic failure of the membrane structure occurs. Performance testing of membranes showed improvement of pure water flux due to membrane stretching regardless of membrane type. Particle challenge experiments did not yield conclusive positive or negative results. The mathematical model was able to predict the alignment of membrane pores in the initial stretching phase and the deformation of the membrane structure. Membrane material properties such as non-linearity and visco-elasticity were taken into account in the construction of the finite element model. Model limitations did not allow for the prediction of membrane failure points and did not taken into consideration the full complexity of the membrane geometry.Chemical Engineerin