Craze initiation in glassy polymers: Quantifying the influence of molecular orientation

A study has been made of crazing stress in oriented glassy polystyrene. The aim was to develop a methodology for prediction of crazing stress in glassy polymers with frozen-in molecular orientation. Oriented specimens of two grades of monodisperse polystyrene (PS) and one grade of polydisperse PS were produced by uniaxial melt-drawing and subsequent quenching of compression-moulded bars. Birefringence and crazing stress parallel to the draw direction (in the presence of diethylene glycol) were measured on miniature beam specimens cut from them. The crazing stress increased substantially with orientation, and the magnitude of the increase relaxed approximately on a timescale associated with the longest Rouse time. Specifically, a linear correlation was found, to within experimental scatter, between the increase in crazing stress and the orientation expressed in terms of frozen-in conformational stress, as predicted by the theory of Maestrini and Kramer [13]. The inverse gradient (constant β in the theory) was found to be 0.059 ± 0.002, when inferring the conformational stress from the measured birefringence. Crazing was found to be suppressed in favour of yielding in the most highly oriented specimens, and this could be explained in terms of the differing sensitivities of crazing and yield to molecular orientation.