Synthesis and rheological characterization of linear and star-shaped polydimethylsiloxanes

Linear polydimethylsiloxanes (PDMS) and 3- and 4-arm star-shaped PDMS of na rrow molecular weight distribution (MWD) have been synthesized by anionic p olymerization. The MWD was obtained by Size Exclusion Chromatography (SEC), Some of the anionically synthesized PDMS showed a relative broad MWD, espe cially when they were terminated after 3 or more days of reaction. In order to analyze the influence of the extent of reaction on the MWD, a polymeriz ation with sample extractions at different conversions was performed. The M WD of the polymers synthesized at different extents of reaction were studie d from a statistical point of view. Linear viscoelastic parameters of the s ynthesized polymers, such as storage (G ') and loss (G ") moduli, were obta ined at different temperatures as functions of frequency (omega). Zero shea r rate viscosity (eta (0)) was calculated from the data obtained in the ter minal relaxation zone. The molecular weight dependence of eta (0) shows a g ood agreement with classical models. Preliminary results obtained from the star polymers suggest an exponential increase in eta (0) with the number of entanglements per arm.