| The essential work of fracture (EWF), a tool for characterizing fracture toughness of ductile materials, is becoming popular due to its simplicity and usefulness.Two series of amorphous polymers, polyurethanes and copolyesters, are employed. The polyurethanes synthesized by myself are disclosed to have a homogenous morphology while their network characteristics vary with the hard-to-soft segment ratio and the treatment of gamma-ray irradiation with different dosages. The molecular structure of the copolyesters was modified with the incorporation of stiff moieties into the main chain of poly(ethylene terephthalate). The results revealed that at low deformation rates the specific essential work of fracture, we, is mostly determined by the properties of the chain segments between entanglements/crosslinks. The longer the segments, the larger is we. At high deformation rates, due to the influences of deformation restriction and deformation mechanism, we changes. At T < Tg, the glass transition temperature of a polymer, we is nearly insensitive to test temperature. The specific plastic work of fracture, wp, exhibits the same temperature dependence of the yield stress of polymers, sigmay. The wp/sigma y ratio was found to be essentially a constant, independent of temperature but dependent on the molecular structure of polymers.wp is proposed to be the energy for fully extending the networks in the plastic zone, which can be predicted by wp = sigmay(1.2 Cinfinity - 1), where Cinfinity is the characteristic ratio defined as the ratio between the mean-square end-to-end distance of an unperturbed coiled real chain and that of a freely jointed chain while we is proposed to be the energy for elastically stretching and breaking the skeletal covalent bonds of the highly orientated chains near the fracture plane, which can be predicted by we = (140 x 10-18J) x N e x O, where Ne is the number of the skeletal bonds in a chain segment between two adjacent entanglement junctions and O is the chain density crossing the fracture plane. With these models, the experimental observations of the dependences of the EWF parameters on intrinsic and extrinsic factors can be explained successfully. The theoretically estimated values of the EWF parameters are in good agreement with those from experiments for a variety of glassy polymers published by our team and other researchers. (Abstract shortened by UMI.)... |
