Tunable optical and swelling/deswelling properties associated with control of the coil-to-globule transition of poly(N-isopropylacrylamide) in polymer - Clay nanocomposite gels

Polymer-clay nanocomposite gels (NC gels) consisting of poly(N-isopropylacrylamide) (PNIPA) and inorganic clay (hectorite) were investigated in terms of their optical and swelling/deswelling properties. Depending on the clay concentration (C_clay), NC gels exhibit unique changes in optical transmittance, optical anisotropy, and swelling/deswelling behaviors, all of which were distinct from those of chemically cross-linked hydrogels (OR gels). The optical transparency and its temperature-induced change differed greatly between NC and OR gels. The decrease in transmittance associated with the coil-to-globule transition of PNIPA occurred at higher temperatures than the normal transition temperature in NC gels as the clay concentration increased. On the assumption of the uniform and random dispersion of exfoliated clay platelets, the critical C_clay (C* _clay) for spontaneous clay aggregation (layer stacking) in NC gels was calculated to be 10 × 10^-2 mol/L H₂O. C*_clay was consistent with the experimental results, including the mechanical properties of strength and modulus, the transparency changes induced by temperature, and the appearance of optical anisotropy. Optical anisotropy was observed in NC gels with clay concentrations above C*_clay, and by increasing the water content, it could be reversibly changed to isotropy when C_clay was close to C*_clay. The swelling of NC gels at 20 °C and their deswelling at 50 °C were both depressed greatly with increasing C _clay. Furthermore, NC gels with C_clay higher than that of NC12 gel exhibited swelling even at 50 °C, and it is expected that NC gels whose volume remains unchanged regardless of temperature can be produced by further increasing C_clay. All of these unique changes in the properties of NC gels can be explained by the partial phase separation due to the coil-to-globule transition of PNIPA and the dispersion morphology of clay platelets.