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One class of multiphase elastomers are those capable of undergoing strain-induced crystallization, as was discussed separately in chapter 12. In this case, the second phase is made up of the crystallites thus generated, which provide considerable reinforcement. Such reinforcement is only temporary, however, in that it may disappear upon removal of the strain, addition of a plasticizer, or increase in temperature. For this reason, many elastomers (particularly those which cannot undergo strain-induced crystallization) are generally compounded with a permanent reinforcing filler. The two most important examples are the addition of carbon black to natural rubber and to some synthetic elastomers, and the addition of silica to siloxane rubbers. In fact, the reinforcement of natural rubber and related materials is one of the most important processes in elastomer technology. It leads to increases in modulus at a given strain, and improvements of various technologically important properties, such as tear and abrasion resistance, resilience, extensibility, and tensile strength. There are also disadvantages, however, including increases in hysteresis (and thus of heat buildup) and compression set (permanent deformation). Another problem in this area is the absence of a reliable molecular theory for filler reinforcement, in general, and even simple molecular pictures of the origin of the reinforcement are lacking. The subject is not even discussed in what has long been the standard reference book on rubberlike elasticity! On the other hand, there is an incredible amount of relevant experimental data available, with most of these data relating to reinforcement of natural rubber by carbon black. Recently, however, other polymers such as poly(dimethylsiloxane), and other fillers, such as precipitated silica, metallic particles, and even glassy polymers, have become of interest. These studies have shown that materials which act as fillers can vary substantially with respect to the chemical nature of their surfaces, and probably most solid, finely divided materials may advantageously be incorporated into an elastomer. In fact, this is one of the ways the crystallites discussed in chapter 12 improve the mechanical properties of an elastomer. Experimental evidence indicates that the extent of the reinforcement depends strongly on particle size.

This chapter is a brief overview of the topics treated in the book. It is aimed, in particular, at providing some qualitative information on rubber elasticity theories and their relationships to experimental studies, and at putting this material into context. The following chapter describes in detail the classical theories of rubber elasticity, that is, the phantom and affine network theories. The network chains in the phantom model are assumed not to experience the effects of the surrounding chains and entanglements, and thus to move as “phantoms.” Although this seems to be a very severe approximation, many experimental results are not in startling disagreement with theories based on this highly idealized assumption. These theories associate the total Helmholtz free energy of a deformed network with the sum of the free energies of the individual chains—an important assumption adopted throughout the book. They treat the single chain in its maximum simplicity, as a Gaussian chain, which is a type of “structureless” chain (where the only chemical constitution specified is the number of bonds in the network chain). In this respect, the classical theories focus on ideal networks and, in fact, are also referred to as “kinetic” theories because of their resemblance to ideal gas theories. Chain flexibility and mobility are the essential features of these models, according to which the network chains can experience all possible conformations or spatial arrangements subject to the network’s connectivity. One of the predictions of the classical theories is that the elastic modulus of the network is independent of strain. This results from the assumption that only the entropy at the chain level contributes to the Helmholtz free energy. Experimental evidence, on the other hand, indicates that the modulus decreases significantly with increasing tension or compression, implicating interchain interactions, such as entanglements of some type or other. This has led to the more modern theories of rubber elasticity, such as the constrained-junction or the slip-link theories, which go beyond the single-chain length scale and introduce additional entropy to the Helmholtz free energy at the subchain level.