Flow birefringence and polydispersity

Flow birefringence data for dilute solutions of various linear and branched polymers of different degrees of polydispersity are discussed. The polymers considered are polystyrene, polypropylene, and polyethylene. By making use of a well‐known stress‐optical relation, reduced elastic normal stresses, which can also be interpreted as coil expansions, are plotted against reduced shear stresses. As elastic normal stresses are rather sensitive to the shape of the molecular weight distribution at high molecular weight, a characteristic, p , related to the skewness of the distribution, is obtained. Experimental values of p are compared with theoretical ones based on special types of distribution functions. Attention is paid to the method of extrapolating experimental results to zero shear stress and concentration.