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 (M_w = 134-1160 kg/mol) are measured in a wide achievable temperature range (270-310 °C) to determine the entanglement molecular weight (M_e) and flow activation energy (E_a). In addition, four actactic polystyrenes (a-PS, M_w = 215-682 kg/mol) and one isotactic polystyrene (i-PS, M_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 M_e 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 M_e, at a fixed M_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 M_w/M_e is performed, a self-consistent behavior is seen for all the PS used despite of the differences in the M_w and chain tacticity; the derived exponent is 3.61. According to the Arrhenius plot, the determined E_a 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.