Modeling brittle fracture in elongational flow of polymer melts with different chemical side groups

Fracture data in elongational flow of four polymer melts with different chemical side groups are analyzed by the enhanced relaxation of stretch model, which is extended here to include the effects of finite chain extensibility and the entropic fracture hypothesis. Two melts, poly(4-methyl styrene) and poly(4-vinyl biphenyl), contain aromatic side groups, which are more rigid than the alkyl side groups of the other two melts, poly (n-butyl methacrylate) and poly(methyl methacrylate). The elongational viscosity data were obtained by a filament stretching rheometer, and at high strain rates the stretched filaments fail by rupture before reaching the steady-state elongational viscosity. The analysis confirms that brittle fracture in elongational flow is related to the number of Kuhn segments per entanglement strand, rather than the number of entanglements per chain. For the two melts with aromatic side groups and a high number of Kuhn segments per entanglement strand, maximal chain extensibility has to be considered as a dynamic property depending on the equilibration time of the entanglement segment. The fracture data are in agreement with the assumption that fracture is caused by scission of primary C–C bonds of polymer chains when the strain energy reaches the bond-dissociation energy of the covalent bond. For poly(4-vinyl biphenyl) the π–π stacking of the phenyl groups stabilizes the polymer chain, and a much higher fracture energy is needed for bond-dissociation.