Dependence of the Elongational Behavior of Polystyrene Melts on Molecular Weight and Molecular Weight Distribution

The elongational viscosity and the tensile compliance on four polystyrenes of different molecular weights and molecular‐weight distributions were determined by means of an extensional rheometer designed particularly for investigations on a small amount of polymer. The steady‐state elongational viscosity at small strain rates is three times the zero‐shear viscosity according to the Trouton relationship. The Trouton viscosity is proportional to $Mw3.5$ independent of the molecular weight distribution. For the steady‐state tensile compliance in the linear range of elongation no quantitative relationship with molecular data can be given but a strong increase of the compliance due to small amounts of a higher molecular weight component is observed. In the nonlinear range of elongation a small viscosity maximum as a function of tensile stress is observed for three samples whereas one sample shows an increase of the elongational viscosity up to values which are at least five times higher than the elongational viscosity in the linear range. The viscosity increase can be related to a pronounced high‐molecular‐weight tail in the molecular weight distribution. This result gives a hint to an explanation of the different and partly contradictory findings in the literature on the strain‐rate dependence of the elongational viscosity of various polymer melts.