Kinematic forms of isothermal deformation of resins or other "cross-linked" elastomers are discussed: homogeneous uniaxial tension, simple homogeneous shear deformation, and biaxial orthogonal homogeneous deformation. It is shown that not only the conformational entropy of the state of the micromolecular structure changes, but also variation in the internal energy of this structure is possible during equilibrium isothermal deformation of "cross-linked" elastomers.Elasticity, which determines the deformation behavior of the majority of polymeric materials not only during production processing on various types of equipment, but also during practical use of articles fashioned from these materials, is one of their basic properties. Voluminous researches have been devoted to study of the elasticity property [1,2].Based on the classical theory of elasticity, the majority of state equations developed to describe the deformation behavior of elastic media supplement the theory with correction factors of a denomenological character; this is associated with the large number of not always substantiated assumptions adopted in the classical theory. These assumptions introduce the concept of an ideal elastic medium, which is a defect-free homogeneous three-dimensional grid formed by the "cross-linked" macromolecular structure of the polymer (rubber), and which also exhibits properties of incompressibility (constancy of volume during deformation), and an affinity for the deformation of its chains, links of which experience free internal rotation.These assumptions enable us to consider that the internal energy of an ideal elastic medium does not vary during its isothermal deformation. Where familiar laws of thermodynamics and statistical methods are employed, classical elasticity theory will, as a result of these assumptions, lead to the conclusion that the equilibrium isothermal deformation of an ideal elastic medium is characterized by the work associated only with a change in the entropy of state of its structure:where W is the elastic potential, A and Q are the specific work and specific heat, T is the absolute temperature, ∆S is the change in the specific entropy of state of the medium, G 0 is the elastic shear modulus, and I 1 is the first invariant of the elastic-strain tensor.The following, however, should be noted: firstly, polymeric materials are very slightly compressible media, and, secondly, in an actual polymer molecule, there is no free rotation of links of its chain about the valent bonds, since, as we
