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 ν20. 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 ν20. As ν20 is increased, α first decreases up to about ν20=0.10 and remains constant in a narrow range of ν20, but then it increases continuously. The ν20-dependence of α is due to the variation of the network chain length N depending on the gel preparation concentration. In the range of ν20 below 0.1, N follows the scaling relationship N≈ν20-2, while at higher concentrations, N varies only slightly with ν20. The increase of α with N obeys the relation α≈ν20N1/5, as predicted by the Flory-Rehner theory.