TOPOLOGICAL PROPERTIES OF POLYMER SPHERULITIC GRAIN PATTERNS FROM SIMULTANEOUS NUCLEATION

Experimental and theoretical characterization of large-scale spherulit ic grain patterns of isotactic polypropylene have been carried out und er simultaneous nucleation conditions. Rigorous image analysis has bee n performed to characterize the topological correlation of grain-bound ary shapes and grain sizes, as well as topological rearrangements duri ng thermal activation experiments. The topological and geometrical asp ects of the spherulitic grains are subjected to a comprehensive analys is, using the characterization methodology commonly employed in studie s of random cellular patterns. A distinguishing feature of polymer gra in patterns is the presence of topological defects. Topological defect s have been identified by using standard computational geometry method such as the multigraphic construction of the grain-boundary network ( GBN) and its relevant dual, the nearest-neighbour network. The topolog ical defects are the mixed configurations of vertices containing three , four, five and six connectives, where the fraction of trivalent vert ices is smaller than 1. It is found firstly that the two-cell correlat ion functions M-k(n) (the average number of k-sided grains adjoining a n n-sided grain), are clearly highly non-linear with n, secondly that the common practice of plotting nm(n) versus n can conceal the non-lin earity of the experimental data, where m(n) is the average sum of the number of sides of the grains immediately adjacent to an n-sided grain and thirdly that the plot of the relation of average area of grains t o the number of sides is non-linear and S-shaped, owing to the polydis perse grain packing. These topological and geometrical characteristics indicate that the polymer GBN does not follow either the mathematical Voronoi diagram or the common random cellular structures displayed in many physical systems. Thermal activation experiments show that the p olymer grain pattern is a topological unstable structure with very slo w dynamics. Finally, these experimental observations are explained in relation to specific polymeric features.