Dynamics and Microstructure Analysis of N-Isopropylacrylamide/Silica Hybrid Gels

The mechanical properties, gelation process, and microstructure of N-isopropylacrylamide (NIPAm)/silica hybrid gels were investigated by using mechanical measurements, dynamic light scattering (DLS), and contrast-variation small-angle neutron scattering (CV-SANS). The three different hybrid gels, containing an equal total volume fraction of silica particles of different sizes (NS_S: small = 20-30 angstroms radius; NS_M: medium = 40-55 angstroms; and NS_L: large = 100-150 angstroms), were employed for comparison. An obvious effect on the compressive properties due to the size of the silica particles was observed, with harder materials produced from particles of smaller size. The gelation process was strongly dependent on the competition between silica aggregation and NIPAm polymerization, as measured by using the DLS methods. In the case of NS_S, the rate of aggregation was larger than NIPAm polymerization, resulting in the formation of an aggregate-dominant microstructure. The aggregation rate for NS_M and NS_L, however, was comparable to the NIPAm polymerization rate, leading to simultaneous aggregation and polymerization. CV-SANS determined that the NIPAm adsorption layer on the silica surface was clearly affected on a microscopic scale by the silica particle sizes.