| In processes, such as melt spinning, the crystallization behavior of syndiotactic polypropylene (sPP) is found to be substantially different from that of most other linear polymers. The anisotropic stress field in such processes leads invariably to extension as well as alignment (orientation) of the chains in the melt, both of which contribute usually to dramatic enhancement in the rate of crystallization. However, since the primary structure of sPP chain in its preferred crystal form is comprised of a “coiled helical”, -(T2G2)2-, sequence, stress-induced chain extension can lead to conformational sequences that are not favorable for crystallization in this form. As a consequence, process conditions that generate higher stress levels can cause a diminution in the rate of crystallization of this polymer. Such conformation-related aspects of oriented crystallization of sPP have been addressed through an analysis of the structure and properties of melt-spun fibers, produced over a range of spinning speeds. The results serve to identify a refinement needed in current models of oriented crystallization and also a mechanism to promote the nucleation of crystallization of sPP.; Blends of sPP with isotactic polypropylene, a polymer that exhibits facile oriented crystallization, have been explored as a possible mechanism to promote its crystallization, through a combination of extensive morphological characterization, and measurement of mechanical and thermo-rheological properties. The results show that, while iPP does have a measurable nucleating effect on crystallization of sPP, its efficacy is limited by the fact that it forms a single phase with sPP in the melt, at least up to a concentration ∼10% w/w. The consequent kinetic limitations that arise from the required demixing renders it ineffective, especially in processes, such as melt spinning, that combine orientation-inducing anisotropic stress field with a high rate of cooling.; A global crystalline network of iPP is formed when a blend of sPP and iPP is cooled from the melt, even at a low concentration of 2.5% w/w. In oriented fibers as well as in quiescent bulk, this network is sustained when the solid polymer is heated well above the melting of sPP, till the onset of melting of iPP. |
