Two series of nanoclay reinforced, thermoresponsive hydrogels were prepared, one based on poly(Nisopropylacrylamide) (PNIPA) and the other on semi-interpenetrating networks containing PNIPA and poly(N-vinyl pyrrolidone) (PVP), designated as SIPNs. The gels were crosslinked with 1, 3, and 5 wt % inorganic clay (hectorite) and SIPN gels additionally contained 1 wt % of PVP. The hydrogels were tested in the ''as-prepared state,'' i.e., at 10 wt % PNIPA concentration in water and at equilibrium (maximum) swelling. Increasing the concentration of nanoclays increases crosslink density, modulus, tensile strength, elongation (except in equilibrium swollen gels), hysteresis and with decreases in the degree of swelling, broadening of the phase transition region, and a decrease in elastic recovery at high deformations. The presence of linear PVP in the networks increases porosity and the pore size, increases swelling, deswelling rates, and hysteresis, but decreases slightly lower critical solution temperature (LCST), tensile strength, elongation, and elastic recovery. The strongest hydrogels were ones with 10 wt % PNIPA and 5 wt % of nanoclays, displaying tensile strengths of 85 kPa and elongation of 955%. All properties of hydrogels at the equilibrium swollen state are lower than in the as-prepared state, due to the lower concentration of chains per unit volume, but the trends are preserved.
Three series of novel semi-interpenetrating polymer networks, based on crosslinked poly(N-isopropylacrylamide), PNIPA, and different amounts of the linear poly(N-vinylpyrrolidone), PVP, were synthesized to improve the mechanical properties and thermal response of PNIPA gels. The effect of the incorporation of the linear PVP into the temperature responsive networks on the temperature-induced transition, swelling/deswelling behavior, and mechanical properties was studied. Polymer networks with four different crosslinking densities were prepared with varying molar ratios (25/1 to 100/1) of the monomer (N-isopropylacrylamide) to the crosslinker (N,N 0 -methylenebisacrylamide). The hydrogels were characterized by determination of the equilibrium degree of swelling, the dynamic shear modulus and the effective crosslinking density, as well as tensile strength and elongation at break. Furthermore, the deswelling kinetics of the hydrogels was studied by measuring their water retention capacity. The inclusion of the linear hydrophilic PVP in the PNIPA networks increased the equilibrium degree of swelling. The tensile strength of the semi-interpenetrating networks (SIPNs) reinforced with linear PVP was higher than that of the PNIPA networks. The elongation at break of these SIPNs varied between 22% and 55%, which are 22 -41% larger than those for pure PNIPA networks.
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