Molecular weight and molecular weight distribution from dynamic measurements of polymer melts

A method for determining the molecular weight distribution (MWD) of a polymer melt has been developed using the dynamic elastic modulus ( G '), plateau modulus ( G ), and zero shear complex viscosity (η ). The cumulative MWD was found to be proportional to a plot of ( G '/ G ) 0.5 vs. measurement frequency (ω). Frequency (ω) was found to be inversely proportional to ( MW ) 3.4 , as expected. Results were scaled to absolute values using the empirical relationship η ∝ ( M̄ w ) 3.4 , where M̄ w is the weight‐average MW. M̄ w , M̄ n (number‐average MW ) and M̄ w /M̄ n calculated from melt measurements were found to agree with size exclusion chromatography usually well within 10 percent for broad and bimodal distribution samples. M̄ w /M̄ n tended to be approximately 20 percent higher for narrow distribution samples (M̄ w /M̄ n < 1.2) because we did not account for a finite distribution of relaxation times from a collection of monodisperse polymer chains. We also did not account for the plasticizing effect of short chains mixed with long ones which caused peak positions to be closer together for Theological vs , size exclusive chromatography (SEC) determinations of MW for bimodal distribution blends.