Microstructures formed in co-cross-linked networks and their relationships to the optical and mechanical properties of PNIPA/clay nanocomposite gels

PNIPA/clay nanocompoiste gels with co-cross-linked networks were synthesized by in-situ, free-radical polymerization of NIPA (N-isopropylacrylmaide) in the presence of two types of cross-linker, an inorganic cross-linker (clay: hectorite) and an organic cross-linker (N,N'-methylenebis(acrylamide): BIS), with concentrations n and m, respectively, in aqueous media. The optical properties and the tensile and compressive mechanical properties of the resulting hydrogels (NCn-ORm gels) were investigated and are discussed herein in terms of co-cross-linked PNIPA network structures. NCn-ORm gels were all uniform, but their transparencies changed considerably according to n and m and were generally different from the sum of the transparencies of corresponding NCn and ORm gels. NCn-ORm gels generally exhibited pronounced weakness and brittleness in tensile tests, like ORm gels. In contrast, in compressive mechanical tests, large improvements were achieved at high n and low m values (m <= 1: e.g., NC5-OR0.3). Furthermore, abnormal increases in modulus were observed in both mechanical tests. All of these results are explained by the formation of a "microcomplex structure" consisting of exfoliated clay platelets and PNIPA chains with enhanced chemical cross-linking. The mechanism of forming the proposed microcomplex structure is discussed and is based on a preferential distribution of BIS to clay in the reaction solution and the formation of clay-brush particles during synthesis.