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Colloid Polym Sci (2013) 291:773787
DOI 10.1007/s00396-012-2789-5
ORIGINAL CONTRIBUTION
Cavitation during tensile deformation of isothermally crystallized polypropylene and high-density polyethylene
Andrzej Pawlak
Received: 15 June 2012 /Revised: 26 July 2012 /Accepted: 22 August 2012 /Published online: 5 September 2012 # The Author(s) 2012. This article is published with open access at Springerlink.com
Abstract The cavitation phenomenon was studied in isothermally and non-isothermally crystallized polypropylene and high-density polyethylene. It was found that nano-voids were not present in the crystallized samples, but were formed during their tensile deformation. The process of cavitation was initiated before reaching the yield point. The ellipsoidal voids were initially elongated perpendicularly to the deformation direction, but if the polymer (i.e., high-density polyethylene) was able to deform beyond the yield, then the reorientation of voids into the deformation direction was observed at local strains of 100200 %. This behavior was similar to that observed previously in the samples crystallized without an exact control of solidification conditions. The calculations of Guiniers radius showed that voids in deformed polypropylene samples were characterized by the gyration radii of 2850 nm. Smaller voids were observed in polyethylene. The scale of cavitation during deformation, studied on the example of polyethylene, depended on the preceding crystallization process and was most intensive for the specimens crystallized at the highest temperature of 125 C.
Keywords Polypropylene . Polyethylene . Cavitation .
Tensile properties . Plastic deformation . Isothermal crystallization
Introduction
The cavitation phenomenon was discovered in many semi-crystalline polymers. The generation of voids (cavities) is
usually observed in two different circumstances: during isothermal crystallization [19] and during tensile deformation [1014]. To my knowledge, so far, both cases have been analyzed and described separately. The reason of cavitation during isothermal crystallization is the existence of pockets of melt surrounded by growing spherulites. The volume of molten material in these pockets (called weak spots) is not sufficient for complete crystallization because the density of the crystalline phase is higher than the density of the molten polymer [2]. The negative (i.e., 3D tensile) pressure arises inside the weak spot with time, and as a result, the growth of neighbor spherulites becomes slower and the polymer melt is stretched. Estimations show that the negative pressure in polypropylene (PP) weak spots may be high, even...