POLYDISPERSITY EFFECTS ON DILUTE-SOLUTION DYNAMIC PROPERTIES OF LINEAR HOMOPOLYMERS

The bead-spring model of Zimm is generalized to predict the effects of molecular weight polydispersity in samples of linear homopolymers. The predictions agree quantitatively with the frequency dependence of the modified birefringence coefficient, SΔ, measured for several polydisperse samples (1.06 < Mw/Mn < 2.5) of atactic polystyrene dissolved in Aroclor 1248. At low levels of polydispersity the effects are only seen in the low-frequency transition zone; at higher levels the effects are seen at all frequencies. Estimations of the number-, weight-, and z-average molecular weights from the frequency dependence of the intrinsic birefringence are possible after calibration with monodisperse standards. Consideration of sample polydispersity (and approximating Se with S’∞) leads to a 15% reduction in past estimates of the molecular weight of a Gaussian submolecule for polystyrene dissolved in Aroclor 1248 at 25.00 °C. The similarity between the frequency dependence of SΔ for a sample of linear chains with a bimodal molecular weight distribution and that for a monodisperse sample of regular combs underscores the need to assess sample polydispersity when using dynamic properties to interpret long-chain structure. © 1990, American Chemical Society. All rights reserved.