Swelling and mechanical behavior of poly(N-isopropylacrylamide)/Na-montmorillonite layered silicates composite gels

Clay-polymer hydrogel composites have been synthesized based on poly(N-isopropylacrylamide) (PNIPAM) gels containing 0.25-4 wt% of the expandable smectic clay Na-montmorillonite layered silicates (Na-MLS). The morphology of the composite gels has been studied using a polarized optical microscope. The size of Na-MLS aggregates increases with Na-MLS concentration. The swelling ratio of the Na-MLS/PNIPAM composite in water is increased at the low Na-MLS concentration but decreases as the concentration increases. Correspondingly, the shear modulus of the gel is found to exhibit a distinct minimum against clay concentration. For Na-MLS concentrations ranging from 2.0 to 3.2 wt%, the composite gels have larger swelling ratio and stronger mechanical strength than those for a pure PNIPAM. The presence of Na-MLS does not affect the value of the lower critical solution temperature (LCST) of the PNIPAM. However, the gel volume change at the LCST is first increased and then decreased upon the increase of the Na-MLS. No pH induced phase transition is observed for the Na-MLS/PNIPAM composites. The experimental results can be explained by considering that Na-MLS is physically entrapped inside rather than chemically bonded into the gel.