Responsive superabsorbent hydrogels via photopolymerization in lyotropic liquid crystal templates

Abstract

In this work, stimuli-responsive, superabsorbent materials are created by copolymerization of stimuli-responsive poly(n-isopropyl acrylamide) (PNIPAM) in combination with superabsorbent sodium acrylate (SA) via photopolymerization in lyotropic liquid crystal (LLC) templates. Templating PNIPAM in LLC mesophases imparts nanostructure to the polymer that significantly increases transport and swelling when compared to isotropic hydrogels. These materials exhibit twice the equilibrium swelling of analogous non-templated materials and show a dynamic range between the swollen and deswollen state that is 5 times greater. To further augment this stimuli-responsive range, LLC-templated PNIPAM was combined with the superabsorbent monomer sodium acrylate (SA). As SA concentration is increased, significant structure changes are observed during polymerization that leads to less-defined nanostructure and lower stimuli response. Maximum swelling and temperature response are observed at low concentrations of SA (∼2 wt%). These LLC-templated copolymers exhibit stimuli-responsive volume transitions up to 40 times and equilibrium swelling ratios of 60 times their dry mass. This 600% increase in thermal response is due to the combined high swelling capabilities of SA with the enhanced thermal-response behavior induced by the LLC-templated nanostructure. Additionally, the nanostructure induces fast deswelling rates at temperatures above the lowest critical solution temperature of PNIPAM. The high dynamic range and quick response of this composition could allow for the development of superabsorbent, stimuli-responsive hydrogels in a variety of biomaterial, microfluidic, and water remediation applications.