Tacticity effects on viscoelastic properties of polystyrenes*

Polystyrenes (PS) with a similar molecular weight and distribution but different tacticities, i. e. syndiotactic (s-PS), atactic (a-PS), and isotactic (i-PS), have been used in this study for a fair comparison to reveal their differences in the viscoelastic properties. Our attention was focused on the influence of the PS tacticity on the rheological properties, especially the plateau shear modulus $G_N^0 $ and entanglement molecular weight $M_{e}$. Based on the time-temperature superposition principle, the master curves of dynamic storage modulus $G'(\omega )$, dynamic loss modulus $G?(\omega )$, and \textit{tan}$\delta $ at a reference temperature of 280$^{o}$C were constructed. The feasibility of conventional approaches for determining $G_N^0 $, i. e. minimum \textit{tan}$\delta $criterion, the integration method, and an empirical equation derived by Wu, has been discussed and compared; the corresponding $M_{e}$ is then derived by the classical relation: $M_e =\rho RT/G_N^o $. Our results show that the measured $M_{e}$ and activation energy for flow are similar for s-PS and a-PS, but the highest for i-PS. In addition, the zero shear viscosity of i-PS is ca. one order larger than that of s-PS and a-PS.