Two component polymer membranes

Abstract

We prepared membranes from synthesized grafted polymers consisting of hydrophobic macromolecules and hydrophilic grafts. We studied especially polyacrylic acid (PAA) grafted on polypropylene (PP) and on ethylene-propylenediene rubbers (EPDM). From a technological point of view, the PP grafted membranes can be classified in three groups: membranes obtained from solution, from bioriented films and from not oriented films. EPDM grafted membranes were prepared from solution. The structure of membranes from solution is characterized by spheroidical domains (diameter ≅ 5.10 -2μm) situated in a rigid or elastomeric matrix. Good salt rejections can be obtained in the case of grafted PP with small fluxes and high thickness. Ultrafiltration of molecules larger than NaCl can be successful when using low thickness rigid membranes, with high fluxes. Elastomeric membranes, which show a low water permeability, do not allow any flux under pressure. Bioriented PP films after grafting show a layer-like structure with detached layers, partially fractured. This structure allows very small fluxes. The original band-like structure of not oriented PP films is heavily modified by grafting: the resulting structure is characterized by a uniform distribution of PAA, which allows the best salt rejection of all our membrane types, but with rather small fluxes. We explain the behaviour of the membranes from solution, using a mechanical model based on a single swelling sphere immersed in a rigid or elastomeric matrix. Internal stresses due to the PAA swelling are calculated and related to water adsorption. Calculated internal stresses justify fracture in the rigid matrix, thus allowing high fluxes, but not in the elastomeric matrix; this explains its very low permeability. Consequence of the existence of microfractures in the rigid matrix is the high dilatation rate of these membranes when immersed in water: the elastomeric membranes show a much lower dilatation rate. A mathematical model is proposed which relates the elongation to the diffusion coefficient of water and of salt both in the rigid and elastomeric membranes. Diffusion water coefficient is of the order of 10 -8 cm 2 s -1 and 10 -10 cm 2 s -1 respectively in the rigid and elastomeric membranes. Salt diffusion coefficient is of the order of 10 -12 cm 2 s -1 in both cases.