Effect of Tacticity on Viscoelastic Properties of Polystyrene

The viscoelastic properties of a series of syndiotactic polystyrene (s-PS) melts with high stereoregularity and different molecular weights (<i>M</i>_w = 134–1160 kg/mol) are measured in a wide achievable temperature range (270–310 °C) to determine the entanglement molecular weight (<i>M</i>_<i>e</i>) and flow activation energy (<i>E</i>_<i>a</i>). In addition, four actactic polystyrenes (a-PS, <i>M</i>_w = 215–682 kg/mol) and one isotactic polystyrene (i-PS, <i>M</i>_w = 247 kg/mol) are also studied to elucidate the tacticity effect on the corresponding properties. Using a reference temperature of 280 °C, the master curves of dynamic storage and loss modulus are constructed according to the time–temperature superposition principle. On the basis of the classic integration method, the <i>M</i>_<i>e</i> values are determined to be 14 500 and 17 900 g/mol for the s-PS and a-PS, respectively, which are significantly lower than that for the i-PS, ∼27 200 g/mol, derived from the Wu’s empirical equation. Owing to the difference in <i>M</i>_<i>e</i>, at a fixed <i>M</i>_w, the viscosity of i-PS is about 1 order of magnitude lower than that of s-PS and a-PS. However, when double-logarithmic plotting of the melt viscosity against the <i>M</i>_w/<i>M</i>_<i>e</i> is performed, a self-consistent behavior is seen for all the PS used despite of the differences in the <i>M</i>_w and chain tacticity; the derived exponent is 3.61. According to the Arrhenius plot, the determined <i>E</i>_<i>a</i> for the s-PS is 53 ± 5 kJ/mol, which is apparently lower than that for the other two isomers possessing a similar value of 90–107 kJ/mol.