Effect of initial monomer concentration on the equilibrium swelling and elasticity of hydrogels

The linear swelling ratio α and the effective network chain length N of a series of poly(N,N-dimethylacrylamide) (PDMAAm) hydrogels were investigated as a function of the gel preparation concentration image. PDMAAm hydrogels were prepared at a fixed cross-linker ratio but at various initial monomer concentrations. It was found that α is not a monotonic function of image. As image is increased, α first decreases up to about image and remains constant in a narrow range of image, but then it increases continuously. The image-dependence of α is due to the variation of the network chain length N depending on the gel preparation concentration. In the range of image below 0.1, N follows the scaling relationship image, while at higher concentrations, N varies only slightly with image. The increase of α with N obeys the relation image, as predicted by the Flory–Rehner theory.