Flow behavior of narrow‐distribution polydimethylsiloxane

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Abstract
The flow behavior of α,ω‐dihydroxypolydimethylsiloxanes, having a weight‐average number‐average molecular weight ratio of 1.1–1.2, was studied with a Cannon‐Manning viscometer and an Instron rheometer. Comparison of the flow behavior of samples with narrow and broad molecular weight distributions indicated that the onset of non‐Newtonian behavior occurred at a much higher shear rate for narrow‐distribution polydimethylsiloxanes than for polydisperse polydimethylsiloxanes. A plot of reduced viscosity versus \documentclass{article}\pagestyle{empty}$ \dot \gamma \eta _0 {M \mathord{\left/ {\vphantom {M T}} \right. } T} $ gave two experimental master curves, one for polymer of narrow distribution and the other for polydisperse polymer. The experimental master curve obtained from the narrow‐distribution polymer was found to fit the theoretical master curve derived from Graessley's entanglement theory. The viscosity–molecular weight relationship for the higher molecular weight polydimethylsiloxanes was found to be the same for both hydroxydimethylsilyl‐ and trimethylsilyl‐endblocked polymers. However, at low molecular weight, the viscosity–molecular weight curve deviated from linearity because of the association of polydimethylsiloxanols, which apparently is not significant at higher molecular weights. The critical molecular weight of entanglement, Mc, was found to be about 30,000.