Nanoscopic replication of cross-linked hydrogel in high-porosity nanoporous silica

Nanoscopic network structure of N-isopropylacrylamide (NIPAM) gel cross-linked with N,N′-methelenbisacrylamide (BIS) was replicated in sol-gel derived silica matrix. As a result of that, the silica became a nanoporous solid with a high-porosity, which reaches 60-80%. The porosity was totally due to mesopores whose size was within 10 nm. Electron microscope observation revealed that the nanoscale network structure were replicated in the silica matrix as considered to be constituted from the main polymer chains and their interconnecting cross-linkers. The mesh size observed in the TEM micrograph matches the span length estimated from the molar ratio (∼80) of the main chain moiety (NIPAM) to that of cross-linker (BIS). When the weight ratio of NIPAM to silica matrix is smaller than unity, the pore size distribution measured by nitrogen adsorption/desorption has a sharp peak at 2 nm indicating that the worm-like nanopore is formed as a replica of an individual NIPAM chain. The peak position in the pore size distribution shifts to a larger value as the weight ratio of NIPAM was increased, suggesting the nanoscale segregation of NIPAM chains in the silica matrix. Therefore, the resultant size of the nanopores are considered to be determined at the moment of the structural freezing caused by the completion of the cross-linking in NIPAM gel. The completion of cross-linking (gellation) eliminates the mobility both of silica and NIPAM/BIS moiety. The TEM micrograph of a sample prepared at a large weight ratio also shows that larger nanopores tend to be generated when the template NIPAM moiety is more abundant.