Network structure and temperature dependence swelling behavior of PEG-b-Poly (NIPAM-co-AMPSA) hydrogels in water

Temperature sensitive hydrogels were prepared by free radical polymerization of N-isopropylacrylamide (NIPAM) and 2-acrylamido-2-methylpropanesulphonic acid (AMPSA) in presence of polyethylene glycol (PEG) as macroinitiator. The aim of the work reported here was to investigate temperature sensitive swelling and deswelling behaviors of the hydrogels in water in order to investigate the effect of various amounts of AMPSA. The result indicated that the incorporation of a hydrophilic ionizable comonomer (AMPSA) affects the temperature sensitivity, swelling/deswelling, morphology and network structure of the hydrogels. The volume fraction in the swollen state (V_2m), the number average molecular weight between cross-links (\( {\overline M_c} \)), the number of segments between the cross-linked point (N), polymer-solvent interaction parameter (χ), enthalpy (ΔH) and entropy (ΔS) were determined using the Flory-Rehner theory at equilibrium swelling. The negative values of ΔH and ΔS showed that the prepared hydrogels had lower critical solution temperature (LCST). The Flory-Rehner theory of swelling equilibrium was qualitatively satisfied with the experimental data of hydrogels at different temperature.