The spherulitic crystallization of isotactic polypropylene from solution: On the evolution of monoclinic spherulites from dendritic chain-folded crystal precursors

Polypropylene can be crystallized in the form of monoclinic (Natta) spherulites from moderately concentrated solutions of the polymer in different solvents. A study is presented involving both optical and electron microscopy which has led to the characterization of the unusual structure and morphology of the dendritic precursor crystals from which such spherulites evolve, as well as the manner in which these precursors degenerate progressively into spherulites. The overall shape of the above mentioned polypropylene dendrites approximates that of a rectangular parallelepiped (reference axes x, y, z, where x/y ≈ 1.1 and y> 2z). These dendrites consist of a dense but not compact network of monolayer chain-folded lamellar branches which traverse the dendrite diagonally with respect to its rectangular x, y cross section, the fold surfaces of the individual lamellar branches (i.e., those faces between which the constituent molecules in each branch fold back and forth) being normal to the x, y cross section. Electron diffraction data indicate that the orientation of the b-crystallographic axis is unique throughout the cross-hatched array of lamellar branches and is parallel to z, the latter axis corresponding to the direction of slowest growth of the dendrite as a whole as well as its constituent branches. It has also been deduced on the basis of the above features coupled with electron diffraction data and consideration of two different but plausible model twinned dendrites that the fold surfaces of the lamellar branches are (001) and that the angle between the c-crystallographic axis in intercrossing branches is circa 80°. The possible origin of this unusual mode of twinning, which involves an 80° change in the orientation of the chain axes in offspring branches from that in parent branches, is briefly presented. The process of evolution of monoclinic polypropylene spherulites from the unusually twinned dendritic crystal precursors is contrasted with the evolution of spherulites in other polymers; furthermore, the relevance of the observations presented in this study to an understanding of the origin of the previously reported atypical fine structures exhibited by monoclinic spherulites of polypropylene crystallized from the melt, is discussed.