Reversible and irreversible evincing of Rehbinder effect in polymer systems

Changing polyethyleneterephthalate film properties by means of their treatm ent with solvents comprising crystallizing and non-crystallizing components has been studied. It has been found that under the influence of the solven t a microheterogeneous defect structure (and in a narrow range of modifying solvent concentrations, microporous structure) appears. Studying the struc ture is of a special interest because it opens up an opportunity to intensi fy transmembrane fluxes. The cause of micropore appearance is developing of internal stretching tensions locally exceeding material's strength lowered by the adsorption active medium. Permeability and selectivity autooscillat ions have been detected in the process of evaporation of binary mixtures of low molecular weight solvents through an asymmetric polyvinyltrimethylsila ne membrane. The autooscillations were caused by a reversible evincing of R ehbinder effect: periodic emergence and collapse of micropores in the membr ane's active layer. Reversible structural transitions play the role of a fe ed-back relay controlling switching the fluxes in the pores on and off. Con ductometric measurements of swelling of chitosan in water vapour have been carried out. Gradual evolution of kinetic instabilities and macrofluctuatio ns (stochastic autooscillations) of local (belonging to infinite clusters) sorbate concentrations were observed as variations of electrical current. T he phenomenon is connected with a course of relaxation processes resulting in induced porosity genesis. There were no macrofluctuations if the polymer had a relaxed structure. Wavelet analysis was applied for characterization of the swelling dynamics. (C) 1999 Elsevier Science B.V. All rights reserv ed.