Viscoelastic behavior of styrene/n‐butyl‐methacrylate/potassium methacrylate polymers

This study concerns the ion containing terpolymers of styrene, n‐butylmethacrylate, and potassium methacrylate (S/n‐BMA/K+MA−) produced by the partial hydrolysis of random S/n‐BMA copolymers. The extent of hydrolysis resulted in terpolymers having 1 to 5 mol % potassium ions. The introduction of ionic sites produces labile crosslinks in these low‐molecular weight amorphous polymers which are reflected in many of the physical properties. The effects of temperature, shear rate, and shear stress on steady state and dynamic rheological properties are examined as functions of molecular parameters (ion content, S/n‐BMA composition, and molecular weight). The dependence of viscosity on composition and molecular weight are complicated by the occurrence of molecular entanglements at lower molecular weights as the proportion of n‐BMA is increased. It is found that the Newtonian viscosity, at constant T‐Tg, varies exponentially with the molar ion content. The rate of log viscosity increase with molar ion content varies as the inverse molecular weight of the S/n‐BMA copolymers in the region of molecular weights studied. At ion contents above about 4 mol % potassium ions, the system is rheologically complex. The viscosity exhibits different shear dependence in steady state and in oscillatory response, and the temperature dependence of the shift factor log aT changes abruptly. These have generally been taken to indicate some degree of ion clustering, although no direct evidence for the existence of clusters by low angle x‐ray studies has so far been observed in these materials.