Shear-induced crystallization of poly(butylene terephthalate): a real-time small-angle x-ray scattering study
The shear-induced crystallization behavior of poly(butylene terephthalate) (PBT) has been investigated by in-situ small-angle X-ray scattering at temperatures of 198, 203, and 208 Ã‚Â°C as well as by ex-situ atomic force microscopy. Increasing the shear rate and/or the shear strain enhances the crystallization kinetics. The acceleration of the crystallization saturates with increasing shear rate. A pronounced effect of the shear field on the Avrami exponent is observed at 198 Ã‚Â°C but not at 203 and 208 Ã‚Â°C, even though at the latter temperatures the relative reduction of the crystallization time induced by the shear is twice the value at 198 Ã‚Â°C. Compared to some other polymers like isotactic polypropylene, the shear field leads only to a weak orientation of the PBT crystals. The Avrami exponent and the orientation induced by the shear field are interpreted on the basis of a microshish model. Microshish structures are due to the breaking up of shear-induced row nuclei at high temperatures. Finally, the shear field is found to lead to a decrease of the long period.