The Effects of Shear Stress on the Crystallization Kinetics of Syndiotactic Polystyrene.

Abstract

Industrial polymer processes involve the application of heat and stress, often resulting in deformations at high shear rates and molecular orientation. This affects both the crystallization rates and the melting temperature of semi-crystalline polymers. Knowledge of the variance of these properties are vital commercially due to their effect on processing techniques, production rates and mechanical properties. An experimental apparatus was designed and constructed that allowed the study of the effects of shear on syndiotactic polystyrene melt samples. This apparatus consisted of a parallel plate rheometer that used changes in shear stress, as well as infrared detection equipment that utilized changes in transmission at specified wavelengths to determine real time changes in crystallinity. Equipment control and data acquisition was achieved through the use of PC based software programs. Syndiotactic polystyrene, the material used exclusively in this study, has only recently been synthesized through the use of new metallocene catalysts. This polymer enjoys intense industrial interest due to several desirable physical properties, including a high melt temperature, high strength, rapid crystallization rates, and excellent solvent resistance. The complex polymorphism that is exhibited by syndiotactic polystyrene is the subject of much on-going research. The rheometer was used to determine a viscosity-temperature relation and induction times at 250$\sp{\circ}$C. A relationship was found to exist between the applied shear rates and final sample crystallinity and crystallography. Real time crystallinity changes, as affected by shear stress and temperature variations, were well described by a crystallization model that incorporated the effects of temperature, shear rate, and stress relaxation. Non-isothermal phase transitions were also examined by analyzing temperature-transmission data.