Peculiarities of the structure and properties of highly cross-linked polymer networks

Data on the structure and properties of highly cross-linked polymer networks in which the distance between cross-link points is several repeating units or even less than one unit, up to the networks consisting of nothing but cross-link points containing no linear fragments, are generalised. The dependences of a number of physical properties (the glass transition temperature, the modulus of elasticity, etc.) on the degree of cross-linking and the influence of various defects (branchings, dangled chains, etc.) on the properties of highly cross-linked networks are analysed. It is shown that the passage from sparse networks to highly cross-linked networks is accompanied by fundamental changes in their properties and that these properties can be described quantitatively taking into account the chemical structure of the network. The influence of the chemical structure of a cross-link point of the network on the properties is analysed; it is shown that the distances between the cross-link points in such networks can be estimated by nontraditional methods. Particular attention is paid to the cross-linked systems that are the basis for gradient-modulus materials in which the modulus of elasticity, the refractive index, and other properties vary over a wide range. The bibliography includes 93 references.