Multi-responsive nanocomposite hydrogels with high strength and toughness
Journal of Materials Chemistry B, 2016, DOI: 10.1039/C6TB00135A
Multi-responsive hydrogels with high strength have great significance for potential applications in smart soft devices. However, it remains a challenge to incorporate multiple responsive moieties with energy dissipation mechanisms. Herein, multi-responsive nanocomposite hydrogels with high compressive strength and toughness were synthesized via in situ copolymerization of N-isopropylacrylamide (NIPAM) and acryloyloxyethyltrimethyl ammonium chloride (DAC) in aqueous dispersion of exfoliated Laponite RDS with a minute amount of N, N’-methylenebisacrylamide (MBAA) as crosslinker. The combined use of clay and MBAA is demonstrated favorable for the high strength and toughness, and helped avoiding precipitation of clay nanosheets that otherwise occurred upon addition of cationic DAC. The effect of NIPAM/DAC molar ratio, MBAA and clay content on the properties of the hydrogels have been systematically investigated. Compression tests showed a compressive strength up to 6.2 Mpa, with fracture strain higher than 90%. The presence of ionic DAC moieties in the hydrogels rendered a very high swelling ratio up to 40 (g/g). These hydrogels were responsive to temperature changes due to the presence of NIPAM units, with transition temperature (LCST) dependent on the molar ratio of NIPAM and DAC monomers. The internal electrostatic repulsion of NIPAM/DAC copolymer network was changed upon exposure to solutions with different pH and/or ion strength. Cyclic swelling-shrinking was demonstrated by shuttlting the gels between pure water and 0.1 mol/L NaCl solution.