The molecular weight distribution in polypropylene

Light scattering, osmotic pressure, and dilute solution viscosity measurements were utilized to determine the weight‐average, number‐average, and viscosity‐average molecular weights of 10 unfractionated commercial and pilot unit polypropylene samples. Weight‐average molecular weights evaluated from angular scattering data by the method of Zimm were found to agree with those determined by means of the “dissymmetry” method when proper solution clarification procedures were employed. As would be predicted, the z ‐average dimension, {article}{empty}{document}RZ ²{document} , for these polydisperse samples was found to be larger than those reported by Chiang for fractionated samples of comparable weight‐average molecular weight. A general trend toward increased breadth of molecular weight distribution with increasing average molecular weight was detected from the variation of M̄ w / M̄ n values with average molecular weight, and is supported by the [η]‐ M̄ w relationship found for these unfractionated samples as compared with the relationship established by Chiang for fractionated samples. The log‐normal distribution function of Wesslau was applied to data on M̄ w , M̄ n , and M̄ v , demonstrating that commercial Ziegler‐type polypropylene displays this type of molecular weight distribution within the molecular weight range studied.