The double-reptation mixing rule is widely used to calculate the rheological behavior of linear polymers from their molecular weight distributions. Double-reptation models with a fixed set of parameters, however, do not accurately predict the low-frequency, elastic response of polystyrene for the entire range of weight-average molecular weights of Dow's current product line. Accurate rheological models facilitates the design of polymer products with specific melt processing performance.

A modification to existing double-reptation models is demonstrated, in which the relaxation exponent is a function of the degree of entanglement as opposed to a parameter with a fixed value of ~ 3.4. This modification enables accurate calculation of the low-frequency storage modulus, as illustrated for a series of commercial polystyrene samples with weight-average molecular weights ranging from 130 to 300 kg/mole. The modified rheological model is shown to be very useful in the formulation of polymer blends with specific low frequency, elastic behavior.