Compressive elastic modulus of polyacrylamide hydrogels and semi-IPNs with poly(N-isopropylacrylamide)

Hydrogels of a semiinterpenetrated polymer network, semi-TPN [based on cross-linked polyacrylamide (PAAm) having poly(N-isopropylacrylamide) (PNIPAAm) inside], were synthesized, and their properties, such as swelling ratio and compressing elastic moduli, were studied at several temperatures. Equilibrium swelling ratios of semi-TPN markedly decreased due to the presence of less hydrophilic PNIPAAm chains. The semi-IPN presented a greater elastic modulus when compared to the cross-linked PAAm hydrogel. The values of apparent cross-linking density were determined from the mechanical compression measurements, made at temperatures that ranged from 25 to 40 degreesC. The presence of PNIPAAm chains on the network increases the polymer volume fraction of the swollen gel. At 25 degreesC the effect induces the formation of physical entanglements, increasing the apparent cross-linking density and then enlarging the compressing elastic modulus. Above 32 degreesC, the LCST of PNIPAAm in water, semi-IPN hydrogels presented higher values of the compressive elastic modulus. The effect was explained as being an additional contribution of the PNIPAAm chains, which collapsed around the PAAm networks, to the elastic modulus. By analysis of the swelling rat io, it was concluded that the collapsing of PNIPAAm chains is not accompanied by large shrinking of the gel. It was pointed out that the PAAm networks support the collapsed chains. According to the results presented in this work, semi-IPN hydrogels present better mechanical properties than PAAm hydrogel, mainly when the PNIPAAm chains are in a collapsed state.