Abstract
In blends of high molecular weight polystyrene (PS) (M <sub>w</sub> = 1800 000) with low molecular weight PS (M <sub>w</sub> = 2000) craze microstructure and fibril stability change dramatically with composition. Because the low molecular weight PS is too short to form effective entanglements, crazes become increasingly fragile as the concentration χ of the high molecular weight component decreases, and virtually no stable crazes are observed below χ = 0.3. The craze fibril extension ratio λ <sub>craze</sub> is measured with optical microdensitometry of transmission electron microscope image plates. As χ decreases from 1.0 to 0.5, λ <sub>craze</sub> increases approximately as λ <sub>max</sub> , the maximum extension ratio of a single strand in the entanglement network, but for χ < 0.5, λ <sub>craze</sub> becomes significantly greater than λ <sub>max</sub> . Average craze fibril diameters D and spacings D <sub>0</sub> are measured by low-angle electron diffraction (LAED). The craze fibril diameter D is approximately constant whereas the fibril spacing D <sub>0</sub> increases modestly as χ decreases. Craze fibril stability, as measured by the median strain ε <sub>b</sub> for craze breakdown, decreases almost linearly with χ and approaches zero at χ = 0.3. Dust particle inclusions significantly reduce craze fibril stability but do not change the shape of the eb vs. χ curves. Various other molecular weights, M = 600, 10000, 17 500, 20 400, 37 000, 50 000, and 11 0000, were used as the low molecular weight component at χ = 0.3 to investigate the effect of diluent molecular weight. While the craze fibril stability is still close to zero in the region 2000 ≤ M ≤ 37 000, it rises sharply in the region between 37 000 and 110 000. For the case of M = 600, however, the T <sub>g</sub> of the blends with χ ≤ 0.5 is at or below room temperature; no crazes or local deformation zones form in these blends. From the measured structural parameters, D and D <sub>0</sub> , one can calculate the effective number of entangled strands n <sub>e</sub> in each fibril (after the entanglement loss during fibrillation). While no average molecular weight can correlate the fibril stability of the monodisperse PS's and the blends, a plot of fibril stability vs. n <sub>e</sub> produces superposition of all the data on a single curve. Craze fibril stability was found to increase exponentially with n <sub>e</sub> , in accord with a model which assumes that the n <sub>e</sub> strands must independently break or disentangle for craze breakdown to occur. © 1986 American Chemical Society.