Effect of a polymeric additive on the pore-size distribution and shrinking process of a hydrogel network

A systematic comparison of the effect of architectural modifications to the network structure on the internal microstructure of N-isopropylacrylamide (NIPA) based hydrogels showed that the addition of a second component to the network significantly increased the proportion of macropores in the network. The second components considered were short poly(N-isopropylacrylamide)(PNIPAM) chains grafted to the network backbone, high-molecular-weight polyacrylamide (PAM) chains, or microsphere particles of PNIPAM. Structures are proposed for each of the modified gel networks taking into account the new structural information. Through combination of the pore size and network structure data reported here, and with the shrinking data obtained previously, shrinking mechanisms are proposed for each of the modified network structures. In all cases, the enhanced shrinking rates were directly caused by the presence of the second component, which acted as nuclei for shrinking (graft-PNIPAM and PNIPAM microspheres) or as water-release channels (PAM gel), and indirectly caused by the second components via their affect on the network microstructure.