Networks with Semiflexible Chains and Networks Exhibiting Strain-Induced Crystallization

Classical theories of rubber elasticity are based on models of flexible polymer chains that are sufficiently long to exhibit Gaussian behavior as described in chapter 1 and in appendix F. Additionally, the chains are phantomlike in the sense that they do not interact with one other along their contours. The theories described in chapter 2 were based on this picture of the individual chain. In this chapter, we describe the elasticity of networks that depart substantially from those addressed in the classical theories. The departures may result from two sources: (1) the chains may be only semiftexible, as a result of which the segments of neighboring chains compete for space in the deformed network, and choose preferentially oriented configurations, and (2) the chains may form crystallites, upon deformation, as a result of which the homogeneous structure of the classical network model may be transformed into a nonhomogeneous one having microphases of crystalline and amorphous regions. The subject of crystallization under deformation, for networks in general, is relatively old, and has been treated in some detail in previous studies. For this reason, crystallization and some of its effects will be reviewed only briefly at the end of this chapter. The main emphasis will be given to networks with semiflexible chains. Examples of networks with semiflexible chains are those in which the chains have rodlike segments separated by flexible spacers, or those where the chains have bond angles appreciably larger than tetrahedral. Incorporation of these chains into a network structure results in materials that exhibit segmental orientations significantly larger than those shown by classical networks. Specific examples would include networks prepared from aromatic polyamide chains or from chains containing liquid-crystalline sequences along the direction of the backbone. Because of their unique chain structures, these networks are easily orientable, at the molecular level, by macroscopic deformations. The orientational transitions may easily be controlled by application and removal of anisotropic strains, and are therefore of great technological interest for use in optical devices. Other examples of networks with easily orientable chains are those with rigid sequences in the side groups.