Effect of Shearing on the Crystallization Behavior of Poly (Butylene Terephthalate) and PBT Nanocomposites

Abstract

Poly (butylene terephthalate) (PBT) is an engineering thermoplastic polyester with excellent mechanical properties and a fast crystallization rate widely processed via extrusion and injection molding. Such processes require very complex deformation histories, which can influence the ultimate properties of the processed material and parts. For such systems, flow-induced structural changes in the material as a function of processing are of increasing interest in the field of polymer processing. Linear viscoelastic material functions, including the storage and loss moduli and magnitude of complex viscosity, are very sensitive to the structural changes occurring in the polymer melt. This initial study focuses on the shear-induced crystallization of PBT and PBT nanocomposites with multi-walled carbon nanotubes (MWNTs). (Shear-induced crystallization is a subset of the more general flow-induced crystallization behavior which is the long-term goal of this research.) The effects of shear history on the isothermal crystallization behavior of these materials were investigated. Time sweep experiments at constant frequency, temperature and strain amplitude were carried out employing small-amplitude oscillatory shear within a parallel-plate geometry. Samples obtained upon quiescent crystallization suggested that the rate of crystallization and crystallization temperatures were modestly affected by the presence and concentration of the nanotubes, consistent with the findings of the earlier reports. However, the characterized shear-induced crystallization behavior of the nanocomposites presented here indicate more significant changes in the crystallization temperature and the rate of crystallization occur as a result of the incorporation of the carbon nanotubes. The shear-induced crystallization behavior was affected by the deformation rate, temperature, and the concentration of the carbon nanotubes. These findings indicate that shear-induced crystallization of polymer nanocomposites (and in general flowinduced crystallization effects due to arbitrary flow fields in the melt state during processing) should be an integral part of attempts to generate a comprehensive understanding of the development of the microstructural distributions and the coupled ultimate properties of polymer nanocomposites.