Morphology of polyethylene based ion exchange membranes

Abstract

Commercial low density polyethylene films were cold drawn up to ∼ 450% elongation and changes in their morphology due to drawing were followed by DSC, infrared dichroism and birefringence. Thus obtained films were immersed in CCl 4, and the solvent swollen samples were subjected to chlorosulphonation. The initial morphology of the polyethylene films was not changed significantly as result of the chlorosulphonation and of the subsequent hydrolysis of the chlorosulphonic groups. Some conformational changes in the polymeric backbone were, however, noted at a very high degree of chlorosulphonation. The analysis of the IR spectra of such samples indicated an increase of the gauche-gauche sequences in the polymeric chains. Considerable changes in the orientation of the polyethylene precursors were obtained only at elogations exceeding 250%. A pronounced orientation of the amorphous regions as well as of the crystalline lamellae was observed in such samples. The overall crystallinity of the cold drawn samples seemed to be, however, uneffected by the treatment. The rates of the chlorosulphonation and chlorination reactions (chlorination takes place simultaneously with chlorosulphonation) depend on the orientation of the polyethylene films. E.g. the chlorosulphonation rate decreased by at least 30% and the chlorination rate by at least 50% as result of 425% drawing of the precursor. The properties of the products of chlorosulphonation strongly depend on the initial orientation of the polyethylene precurors. The most striking effect was observed for the rates of hydrolysis of the chlorosulphonyl groups. Thus, only 50% of the chlorosulphonyl groups was hydrolyzed in the highly oriented sample while 99% was hydrolyzed in an unoriented one after 20 days and 1 day, respectively, of treatment at R.T. with in NaOH. (Both samples contained 0.8mM/g of SO 2Cl and l.5mM/g of Cl). This drastic decrease in the rates of hydrolysis may be explained, in terms of restrictions in the diffusion of the OH- ions towards the So 2cl groups,' shielded by the oriented polymeric network. Consideration of the orien- tation of the SO 2Cl groups in respect to polymeric chains explains why the diffusional restriction are so severe. Namely, it may be concluded from the dichroic ratios of the S-CI and S=0 stretching modes, that the orientations of the S-CI bonds/are nearly parallel, to the polymeric chains. Hence, they are so effectively shielded in the oriented material. Analysis of the morphology of the hydrolyzed materials revealed several interesting features. The orientation of the sulphonic groups and of the chlorosulphonic precursor are closely related.. Subsequently, the ionic bonds in the resulting ion exchange membranes are oriented perpendicularly to the direction of the plymeric chains. (The dichroic ratio of the symmetric SO 3 stretching mode is negative and increases with the drawing ratio of the pllyethylene precursor). Interesting results were provided by the fluorescence polarization measurements of these ion exchange membranes (large asymmetric counterions were used as fluorescence probes). Such measurements, conducted with water swollen membranes, indicated that free counterions formed, as result of the dissociation of the polar bonds, were oriented in parallel to the initial orinetation of the plymeric chains. Obviously, the water swollen ion exchange membrane “remembers” the orientation of the polyolefinic precursor. Indeed the dimensional changes due to the swelling are assymetric and depend on the drawing ratios of the precursors. Similarly assymetric behavior is revealed by the measurements of the electrical conductivity of the membrane. E.g. 450% drawing of the precursor caused a two fold increase in the conductance of the final ion exchange membrane in drawing direction and only 20% increase in the direction perpendicular to the initial drawing (ion exchange capacity of these membranes was 2.4meq/g).