Swelling and elasticity of ionic poly(N-isopropylacrylamide) gels immersed in the melt of poly(ethylene glycol) chains

The equilibrium network concentration ν2 and the elastic modulus G of a series of poly(N-isopropylacrylamide) (PNIPA) gels immersed in the melt of poly(ethylene glycol) (PEG) chains were investigated. 2-Acrylamido-2-methylpropane sulfonic acid sodium salt (AMPS) was used as the ionic comonomer in the gel synthesis. The molecular weight of PEG was varied between 200 and 1000 g/mol. It was found that the decrease in ν2 with increasing charge density of the gels is considerable weaker than expected with the assumption of Gaussian statistics. This deviation is interpreted as the ion pair and multiplet formation from the mobile counter ions of AMPS units of the gel in the presence of PEG chains. It was shown both theoretically and experimentally that the PNIPA gel undergoes a swelling–deswelling transition in PEG, if the chain length of PEG exceeds the critical value ycr=N1/2, where N is the network chain length calculated from the elastic moduli of gels immersed in PEG melt.