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Effect of Tacticity on Viscoelastic Properties of Polystyrene

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journal contribution
posted on 09.08.2011, 00:00 by Chien-Lin Huang, Yu-Chen Chen, Ting-Jui Hsiao, Jing-Cherng Tsai, Chi Wang
The viscoelastic properties of a series of syndiotactic polystyrene (s-PS) melts with high stereoregularity and different molecular weights (Mw = 134–1160 kg/mol) are measured in a wide achievable temperature range (270–310 °C) to determine the entanglement molecular weight (Me) and flow activation energy (Ea). In addition, four actactic polystyrenes (a-PS, Mw = 215–682 kg/mol) and one isotactic polystyrene (i-PS, Mw = 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 Me 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 Me, at a fixed Mw, 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 Mw/Me is performed, a self-consistent behavior is seen for all the PS used despite of the differences in the Mw and chain tacticity; the derived exponent is 3.61. According to the Arrhenius plot, the determined Ea 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.