Heterogeneous polyelectrolyte gel membranes: Effect of morphology on stimulus-responsive permeation control

Poly(methacrylic acid)-poly(dimethyl siloxane) (PMAA-PDMS) composites and interpenetrating polymer networks (IPNs) were prepared in order to examine the effect of a heterogeneous hydrogel-elastomer morphology on the variable transport properties of stimuli-responsive membranes. The composites consisted of PMAA particles (≅40 μm diameter) dispersed within a PDMS network and IPNs were composed of PMAA channels (≅0.1 μm diameter) which spanned the thickness of the PDMS network.; The mechanism of permeation control for the composite was based on the percolation or connectivity of the dispersed PMAA particles. The fast swelling-deswelling rates of the surface-resident PMAA particles resulted in a dynamic permeation response to pH change in the order of minutes and an ON/OFF (pH 7/pH 3) permeability ratio of 160 for vitamin B12. This compared favourably to the ON/OFF permeability ratio of 7 estimated for homogeneous PMAA membranes.; The mechanism of permeation control for IPNs was based on size exclusion of the permeant from the PMAA channel. Although the ON/OFF permeability ratio (1400) improved over both PMAA and composite membranes, the response time for the dynamic 24 h permeation cycle was very slow. The slow swelling kinetics of the membrane-spanning PMAA channel from pH 3 to 7 resulted in a lag time of 15 h before permeation resumed at pH 7. A combination of the composite and IPN morphologies is expected to improve upon the deficiencies of the two membrane types. For example, a hydrogel-PDMS IPN can be prepared where the gel channel in the surface region is stimuli-responsive and that within the bulk remains hydrated and non-responsive.; A novel monomer immersion method was developed to prepare stimuli-responsive, bicontinuous, PMAA PDMS IPN membranes. The method involved immersing the pre-IPN in methacrylic acid (MAA) during IPN formation in order to obtain a uniform MAA concentration profile within the pre-IPN.; A unique approach was used to examine PMAA-PDMS IPN morphology with the laser scanning confocal microscope and fluorescent probes of varying diameters. The results revealed complex, superimposed structures of PMAA domains of varying sizes and spatial distributions. These observations had not been reported previously and present a new understanding of morphology development in IPNs.