Para-substituted polystyrenes: Stress relaxation, creep, dynamic mechanical and dielectric analyses

The effects of methyl, ethyl n-propyl and t-butyl para-substituents on relaxations in polystyrene are characterized via multiple thermal techniques. It is shown that the glass transition temperature T-g can be readily obtained from stress relaxation and creep data by locating the intersecting point of the Arrhenius and WLF curves on the logarithm of the shift factor a, vs. temperature plots. The T-g thus obtained from stress relaxation data is correlated well with literature values obtained by isochronal dilatometry. The processes of creep and stress relaxation show two regions of distinct molecular motion patterns with different activation energies determined, Rubbery plateaus for stress relaxation moduli due to chain entanglements are observed for polystyrene, poly (p-methyl styrene) and poly (p-t-butyl styrene). The flexural storage moduli E' for the three polymers show a density dependency, which is correlated by the Rao function. The peak width of loss modulus E '' is affected by the molecular weight distribution of each polymer. Two weak sub-glass transition temperature transitions, beta and gamma, are also observed from dynamic mechanical analysis. The variations in the glass transition temperature and activation energy from dielectric analysis are analyzed with regard to steric hindrance resulting from different alkyl groups.