A. General statements

In this section, the following abbreviations are used: polyvinylacetate (PVA), polystyrene (PS), polymethylmethacrylate (PMMA), polyethylene (PE), polyamide (PA), polyvinylchloride (PVC), polypropylene (PP), polyisobutylene (PIB) and polycarbonate (PC). Polymers form a class of high-molecular compounds with a linear (chain) structure of molecules. The molecular structure may be amorphous (PS, PMMA, etc.) or more or less ordered (crystalline) (PA, PE, etc.). Polymers may be grouped in three states:

• State I: elastorigid (glassy) (PMMA, PS, PVC)
• State II: soft (highly elastic) with large reversible deformation (rubbers and resins)
• State III: plastoviscous (viscofluidity) with irreversible stress deformation (noncured resins).

At different temperatures and depending on the rate of deformation, the same polymer can be in each of three states without changing its chemical structure. The boundaries of the states are specified by glass transition temperature T_g and melting temperature T_M. Widespread thermoplasts are rigid at room temperature and fluid at high temperatures. These properties simplify the production of workpieces by casting, stamping, and extrusion. The high elastic state is implemented in the intermediate range T_g < T < T_M.

The above specific features of polymers substantially affect their thermophysical properties. The data for amorphous polymers can be satisfied with the state equation (Godovsky, 1976):