Cavitation during isothermal crystallization of isotactic polypropylene

Abstract

Cavitation during isothermal crystallization of thin films of isotactic polypropylene was investigated systematically by light microscopy. Cavitation results from the negative pressure buildup due to density change during crystallization in the pockets of melts occluded by impinging spherulites. The morphology of such areas was also studied by SEM. The value of the negative pressure at the moment of cavitation was calculated from the drop of the spherulite growth rate. It was shown that the process of cavitation and the value of the negative pressure causing cavitation depend on the crystallization temperature. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2439–2448, 2001

Cavitation during tensile deformation of isothermally crystallized polypropylene and high-density polyethylene

“…In polyethylene, small (a few micrometers in diameter) spherulites grew, so also small weak spots were formed between them and micrometer-sized cavities were not formed during crystallization [5]. The SAXS studies showed that nano-voids were not present in the non-deformed HDPE.…”

“…The SEM photographs showed that weak spots were not very frequent. Nowacki et al [5] observed that the negative pressure buildup was slower in large weak spots formed in polypropylene specimens crystallized at the highest temperatures. The reason was some relaxation of stretched molecules possible during long crystallization and the reduced viscosity of melt.…”

“…Nowacki et al [5] reported that the large weak spots tended to cavitate at lower negative pressure than smaller ones, which resulted from an increased probability of the presence of a cavitation nucleus inside the confined melt. It was also found that the shape of the cavities in polypropylene depended on the temperature of crystallization—at low temperatures, the voids were spherical; at high temperatures (i.e., >135 °C), the voids were elongated (similar to cracks), formed along interspherulitic boundaries [5, 6]. Voiding in weak spots was not observed in thin films of polymers crystallized with a free surface because the deficiency of the crystallizing material was compensated here by the thinning of the film.…”

“…The above discussed experiments were focused on the studies of cavitation in thin films of crystallizing polymers [5–9]. Galeski and Piorkowska [2] examined volume samples of polypropylene which were crystallized isothermally at 125 °C.…”

New understanding for the formation of conductive network in the nanocomposites during the crystallization of matrix

“…Meanwhile the rigid conductive network blocked the motion of molecular and internal stress arose, and resulted in a damage of conductive network. Furthermore, the density of crystal was greater than that of amorphous region, and the growing of spherulites caused local volume deficiencies and negative press which leaded to a damage of conductive network [9][10][11]. During the terminal stage of crystallization, conductive network was compacted by the further growing of spherulite and volume resistivity of all samples decreased before its steady [12,13].…”