Compressive Elastic Modulus of Polyacrylamide Hydrogels and Semi-IPNs with Poly(N-isopropylacrylamide)

Hydrogels of a semiinterpenetrated polymer network, semi-IPN [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-IPN 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 °C. The presence of PNIPAAm chains on the network increases the polymer volume fraction of the swollen gel. At 25 °C the effect induces the formation of physical entanglements, increasing the apparent cross-linking density and then enlarging the compressing elastic modulus. Above 32 °C, the LCST of PNIPAAm in water, semi-IPN hydrogels presented higher values...