Characterization of molecular weight distributions in high polymers

Abstract

AbstractIn this review paper, these types of molecular weight distribution found in typical polymers are described. It is pointed out that procedures for fractionating polymers usually yield incomplete information on molecular weight distribution unless the fractions are characterized for molecular weight. In addition, measurement of different average molecular weights can itself give limited information on molecular weight distribution breadth. Experimental methods for molecular weight determination are described with emphasis on range of applicability, precision, accuracy, convenience, and cost. For measuring number‐average molecular weight, the thermodynamic methods of cryoscopy, ebulliometry, and osmometry have proven absolute accuracy. The thermoelectric differential vapor pressure technique, while a transport rather than a thermodynamio method, is simple and, in most cases, accurate. Weigh average molecular weight can be measured by light scattering, equilibrium ultracentrifugation, and approach‐to‐equilibrium ultracentrifugation, all thermodynamic methods. Transport properties such as the sedimentation constant, the diffusion constant, the intrinsic viscosity, or the Newtonian melt viscosity, can be empirically correlated with molecular weight. Although in principle these properties can be combined in pairs to yield molecular weights without resort to outside calibration, as in the sedimentation transport method, the results of such treatments must be applied with considerable caution to random coil polymers.