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 100–200 %. This behavior was similar to that observed previously in the samples crystallized without an exact control of solidification conditions. The calculations of Guinier’s radius showed that voids in deformed polypropylene samples were characterized by the gyration radii of 28–50 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.
Highlights
The cavitation phenomenon was discovered in many semicrystalline polymers
We showed that there is a competition between two processes possible at yield in the semicrystalline polymer: plastic deformation of crystals by a chain-slip mechanism and the breaking of the amorphous phase [10]
The crystalline structures of polypropylene films, crystallized according to the procedures described in Table 2, were determined using the differential scanning calorimetry (DSC) and SAXS methods and observed by the polarized light microscopy
Summary
The cavitation phenomenon was discovered in many semicrystalline polymers. The generation of voids (cavities) isWhen the negative pressure reaches the level of cohesive strength of melt, a rapid break of melt occurs and 1-μm or more micrometer voids are formed. The cavitation phenomenon was discovered in many semicrystalline polymers. When the negative pressure reaches the level of cohesive strength of melt, a rapid break of melt occurs and 1-μm or more micrometer voids are formed. The presence of a cavitation nucleus (e.g., impurities) supports the initiation of the process at lower pressure. The fracture of melt is observed at the same time in adjacent weak spots, which suggests that the propagation of an acoustic wave emitted in the cavitation act, resulting from a sudden pressure change, induces the cavitation in other weak spots. The acoustic emission from the crystallizing polymer was recorded by Galeski et al [4]. Cavitation relaxes stresses inside the weak spot and the growth rate of spherulite increases to the initial level [1]
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