Gelation is the cross-linking process that leads to the network structures required for rubberlike elasticity. In some cases, gelation can be reversible. There have been numerous studies involving theory and simulations exploring gelation and the mechanical properties of the resulting networks. Cross linking with free radicals is still quite common. Radiation has often been used to carry out the cross linking, as have new techniques known as “click” chemistry. Hydrosilylation is also popular. Networks have even been designed with movable cross links. Finally, reactive groups can be placed at the chain ends or within the chains themselves. Related studies have involved polydimethylsiloxane (PDMS)-based organogelators, web-to-pillar transitions of gels, and silica aerogels. There has also been interest in polysiloxanes in interpenetrating hydrogels with high oxygen permeabilities and viscoelastic magnetic gels. Organic-inorganic hybrids with relatively low melting temperatures also exist, some of which can be made to be self-healing. Gels are also formed in swelling experiments, which are useful for equilibrium experiments to characterize network structures. One of the recent topics in this area involves stimuli-responsive gels, under the descriptive title of “self-walking gels” “wormlike motion of gels,” and “peristaltic motion of gels.” The earliest studies of networks formed in solution were undertaken to investigate some subtle aspects of the elastic free energy expression— whether or not an additional term in the logarithm of the volume was required. Other studies focused on the properties of networks in general. As can be gathered from chapter 4, it is difficult to obtain information on the topology of a network. Some studies have therefore taken an indirect approach. Networks were prepared in a way as to simplify their topologies, and their properties were measured and interpreted in terms of reduced degrees of network-chain entanglement. The two techniques employed involved separating the chains prior to cross linking by either dissolution or stretching. After cross linking, the solvent is removed or the stretching force is relaxed, and the network is studied (unswollen) with regard to its stress-strain properties, typically in elongation.
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