MOLECULAR THEORY OF THE FRACTURE-TOUGHNESS OF LOW-MOLECULAR WEIGHT POLYMERS

Recent experiments by Robertson show that the fracture toughness GIC of glassy polystyrene PS does not decrease to the ideal brittle value 2γ (where γ is the surface energy for PS) at molecular weights Mw below Mc the entanglement molecular weight. Instead GIC is more than an order of magnitude above 2γ at Mc and decreases slowly below Mc. It is postulated that a small craze exists at the crack tip in such low molecular weight glassy polymers. However, since entanglements do not occur single molecules must span this craze; if they do not the craze becomes unstable and the crack advances. Under these conditions a critical craze surface displacement exists and GC can be computed to be GIC=Sc(λ-1) 〈R2〉1/2, where λ and Sc are the craze fibril extension ratio and craze surface drawing stress observed in high molecular weight crazes (both quantities should be only weak functions of Mw) and 〈R2〉1/2 is the root mean square end-to-end distance of the PS molecule in the glass from neutron scattering measurements. The fracture toughness is predicted to decrease as Mw1/2; this prediction and the absolute magnitude of GIC are in excellent agreement with experiment. © 1979 Chapman and Hall Ltd.