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Extra info for Crazing in polymers
One of the ways this loss can take place in the current versions of the tube model 79, 80) is by tube relaxation 11a)whereby the process of tube retraction of molecules neighboring the tube in question can lead to a toss of constraint on this tube. This process is more rapid at temperatures just above Tg than tube reptation 79) but is still quite dependent on molecular weight and molecular weight distribution. Experiments in which crazes were grown in a highly oriented PS film qualitatively bear out these predictions 48).
Y o 40 2O i~e? I X/C I crazetip crack tip PP0 5 d I 10 I 15 i_ 20 xlO z5 ~'E (m-3) Fig. 2@a--d. Surface stress profiles for polytertbutylstyrene [PTBS], poly(styrene-26 % acrylonitrile) [PSAN1] and poly(styrene-65 % raethyhmethacrylate) [PSMMA]. The stress at the craze tip St is plotted vs. ve in d, The value of the shear yield stress Y of polycarbonate is indicated Microscopic and Molecular Fundamentals of Crazing 37 craze. ) Part of the reason for this increased tendency toward shear deformation at high entanglement density can be found in the strong increase in the crazing stress with entanglement density.
This extra surface energy hypothesis also has interesting consequences for the temperature (or strain rate -- low strain rates correspond to high temperature) dependence of crazing. As temperature is increased the rates for disentanglement of chains of moderate molecular weight become much larger and entanglement loss by this mechanism, either before or after a chain scission event becomes important. The effective value of F decreases since only v~ = vE - - v~ chains per unit volume must now be broken (v~ is the density of entanglements lost through disentanglement).