The extensional and failure properties of polymer melts

摘要

AbstractThe extensional and failure properties of polystyrene melts were studied by pulling sample rods in a special “weight dropping” extensiometer. This apparatus allows pulling to long final lengths and at relatively high rates; except for the highest rates, the experiment is one of constant applied force. Various commercial (broad molecular weight distribution) and special (narrow molecular weight distribution) samples were studied at various temperatures and applied forces. The striking result was that the former (BMWD) samples stretched reasonably uniformly and displayed what has been described as “viscoelastic failure”; the latter (NMWD) samples necked in the final stages and showed what might be called “viscous” failure. In the case of the BMWD material, the stress–time behavior was analyzed theoretically by independently determining the parameters in a nonlinear constitutive equation from GPC and rheogoniometer (shear) data. The theoretical tensile stresses compared quite well with the experimental values. An interesting result came from comparing the complete viscoelastic theory with a viscous (Trouton viscosity) asymptote. These two theoretical curves closely approximated the experimental data until just short of the failure point; at this incipient point, the stresses from the complete theory grew to very large values compared with the viscous stresses. That is, the material could not relax fast enough to allow steady stresses to develop, and the sample failed short