Localized volume deficiencies as an effect of spherulite growth. II. The three‐dimensional case

As in the two‐dimensional case, the density change due to crystallization leads to a buildup of internal strain in some regions of a polymer melt occluded by growing spherulites. The occluded parts of the sample are called “weak spots.” Computer simulation of spherulite growth in bulk samples shows that the largest weak spots have the size of an average spherulite. The total volumes of weak spots are 0.47, 0.094, and 0.119% of the sample for athermal, thermal, and mixed primary nucleation, respectively. The weak spots in the bulk material exhibit distance correlation. Within weak spots, internal strain is released (Raman spectroscopy), and holes develop. Polypropylene bulk spherulite samples contain holes distinctly visible under a microscope with infrared optics and illumination. The number of visible holes is in agreement with the computer prediction. The increase of impact strength with decreasing spherulite size is explained in terms of changes in the number and size of weak spots.