Effect of molecular weight and its distribution on stretching of polyacrylonitrile gel

Abstract The present studies were designed to secure information about the effect of molecular weight and its distribution on the hot‐stretchability (defined as maximum hot draw ratio possible without breakage under given condition) of the coagulated gel obtained by extruding a concentrated solution of polyacrylonitrile dissolved in 70% nitric acid into dilute nitric acid solution. After washing, the coagulated threads thus obtained were stretched at 100–140°C. In the case of fractionated polyacrylonitrile, the results obtained under the above conditions showed a maximum hot‐stretchability at M = 80,000. Below this point, hot‐stretchability was lowered with decrease of molecular weight. For polymers having a molecular weight larger than this value, it was also depressed as molecular weight increased and approached a certain value. For whole polymers, hot‐stretchability distribution curves obtained from molecular weight distribution curves by substituting molecular weight with corresponding hot‐stretchability were studied. It was found that the maximum stretchability of a whole polymer was the weight average value of those of its constituent polymer species. The effect of molecular weight of the polymer on the mechanical properties of fibers and their fine structure was investigated by means of tensile and x‐ray measurement.