GLASS-RUBBER CONSTITUTIVE MODEL FOR AMORPHOUS POLYMERS NEAR THE GLASS-TRANSITION

The constitutive behaviour of amorphous polymers near the glass transi tion contains many features general to this class of materials. A new, physically based, three-dimensional constitutive model has been devel oped for simulating this wide range of features in models of polymer p roducts and processes. In particular, the model displays glassy respon se at low temperatures and short time-scales, and rubber-like response at high temperatures and long time-scales, and is therefore an exampl e of a glass-rubber constitutive model. Its basis is the assumed addit ivity of free energies of bond distortion and conformation perturbatio n. For the elastic bond distortion stress-strain law and flow model, a nd the conformational entropy function, the model employs linear elast icity, Eyring viscous flow and the Edwards-Vilgis entropy function, re spectively. Glass structure and temperature dependence are introduced through the Vogel-Tammann-Fulcher and Arrhenius equations for viscosit y, respectively. With parameters obtained for poly(ethylene terephthal ate) in a companion paper, the model was solved numerically to simulat e a variety of uniaxial strain sequences, and found to replicate well the characteristic patterns of behaviour of amorphous polymers in the temperature region of interest, over a wide range of experimental situ ations from small to large strains. The only major deficiency, resulti ng from the simplifying assumption of a single activation barrier heig ht, is its being too localized in the time domain. At high temperature s and long times, applicability of the model is limited by the onset o f conformational relaxation.