| Abstract
| - Hydrogels of a semiinterpenetrated polymer network, semi-IPN [based on cross-linkedpolyacrylamide (PAAm) having poly(N-isopropylacrylamide) (PNIPAAm) inside], were synthesized, andtheir properties, such as swelling ratio and compressing elastic moduli, were studied at severaltemperatures. Equilibrium swelling ratios of semi-IPN markedly decreased due to the presence of lesshydrophilic PNIPAAm chains. The semi-IPN presented a greater elastic modulus when compared to thecross-linked PAAm hydrogel. The values of apparent cross-linking density were determined from themechanical compression measurements, made at temperatures that ranged from 25 to 40 °C. The presenceof PNIPAAm chains on the network increases the polymer volume fraction of the swollen gel. At 25 °Cthe effect induces the formation of physical entanglements, increasing the apparent cross-linking densityand then enlarging the compressing elastic modulus. Above 32 °C, the LCST of PNIPAAm in water,semi-IPN hydrogels presented higher values of the compressive elastic modulus. The effect was explainedas being an additional contribution of the PNIPAAm chains, which collapsed around the PAAm networks,to the elastic modulus. By analysis of the swelling ratio, it was concluded that the collapsing of PNIPAAmchains is not accompanied by large shrinking of the gel. It was pointed out that the PAAm networkssupport the collapsed chains. According to the results presented in this work, semi-IPN hydrogels presentbetter mechanical properties than PAAm hydrogel, mainly when the PNIPAAm chains are in a collapsedstate.
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