The microstructure evolution of isotactic polypropylene （iPP） during annealing is reported. A few amount of poly（ethylene oxide） （PEO） which exhibits much lower melt temperature compared with iPP was introduced into iPP in this work. The crystalline structure of iPP was detected using differential scanning calorimetry （DSC） and wide-angle X-ray diffraction （WAXD）, and the relaxation of iPP was characterized using dynamic mechanical analysis （DMA）. The variation of PEO morphology was investigated by scanning electron microscopy （SEM）. The results show that the crystallization, including the primary crystallization and second crystallization during annealing, as well as the relaxation of iPP matrix is promoted with the presence of PEO.
The effect of externally applied mechanical pre-conditioning, i.e. pre-impact treatment, on the fracture resistance was investigated for polypropylene （PP）. Impact strength was obtained via notched and/or unnotched samples. It has been shown that the pre-impact treatment is favorable to the improvement of the fracture resistance. The impact strength increases linearly with the applied pre-impact energy. Both optical and SEM results show that there are at least two possible mechanisms for the improvement of the fracture resistance. One is the crack blunting effect which is introduced to notched sample by pre-impact treatment, reducing the sensitivity of PP to the applied notch. The other is the formation of large amount of microvoids induced by pre-impact treatment, which changes the stress distribution and induces intensive plastic deformation of PP at the second impact measurement, leading to the improvement of the fracture resistance.
As a part of a serial work about the annealing inducing improvement of fracture toughness of polypropylene （PP） articles, in this work, a highly efficient mobilizer was introduced into PP and the injection-molded samples were annealed at different temperatures. The mobility of chain segments of PP was investigated by measuring the glass transition temperature. Differential scanning calorimetry （DSC） and wide angle X-ray diffraction （WAXD） were used to characterize the variation of crystalline structure of PP during the annealing process. The fracture behaviors including notched Izod impact fracture and universal tensile fracture were investigated to detect the mechanical properties in response to the variations of both chain segments mobility and crystalline structures. It was found that the mobilizer greatly improved the chain segments mobility. Further results showed that the mobilizer also induced apparent changes of the glass transition temperature and the degree of crystallinity of PP during the annealing process. Consequently, the annealed PP samples containing a few amount of mobilizer exhibited largely increased fracture toughness.
