A phenomenological description of large deformations in polymeric glasses (polycarbonate)

Abstract

AbstractA new description of the quasistatic deformation of polymeric glasses is presented. The stress‐strain energy of a glass is defined to comprise two components: a “solid term,” which controls the low‐strain range of deformation, and a “conformational term” proportional to the first strain invariant. By introducing internal variables in terms of classical irreversible thermodynamics, the stress‐strain behavior and the thermoelastic properties of polycarbonate are fairly well described over the whole range of accessible strains. The necking phenomenon is interpreted as a “constrained strain‐induced phase transition” (“one‐way phase transition”). A new version of a Clapeyron‐Clausius equation is derived. Its application yields a new understanding of the temperature dependence of yielding of cold‐drawn polycarbonate. The typical phenomena of a first‐order transition, discussed in the special form of a “one‐way phase transition,” have been fully identified experimentally (by microscopy, stretching‐microcalorimetry, simple extension, and birefringence). Sharp phase boundaries appear just when the new phase is nucleated. The transition then proceeds under a constant force as demanded by thermodynamics. The stress‐strain behavior is shown to be uniquely dependent on temperature at temperatures below the glass‐transition range. General consequences of the phenomenological treatment are discussed.