Transient cooperative processes in dewetting polymer melts

We compare the high velocity dewetting behavior, at elevated temperatures, of atactic polystyrene (aPS) and isotactic polystyrene (iPS) films, with the zero shear bulk viscosity (η(bulk)) of aPS being approximately ten times larger than iPS. As expected, for aPS the apparent viscosity of the films (η(f)) derived from highshear dewetting is less than η(bulk), displaying a shear thinning behavior. Surprisingly, for iPS films, η(f) is always larger than η(bulk), even at about 50 °C above the melting point, with η(f)=η(bulk) following an Arrhenius behavior. The corresponding activation energy of  ~ 160 +/- 10 kJ/mol for iPS films suggests a cooperative motion of segments which are aligned and agglomerated by fast dewetting.