Effect Of Crystal Forms On Dielectric Properties Of Polypropylene

The study on the effect of crystal forms on properties of polypropylene (PP) has attracted much attention, but research efforts basically focus on the mechanical properties. The report about effect of crystal forms on conventional dielectric properties and space charge of PP has not been found in literature.In this thesis, melting process and nonisothermal crystallization dynamics of different crystal form PPs were investigated by Differential Scanning Calorimetry (DSC), and the size and shape of different crystal form PPs were characterized by Polarized Optical Microscope (POM). The conventional dielectric properties of different crystal form PPs were evaluated, especially the space charge distribution and its decay were measured inαandβcrystal form PPs homopolymerized. The effect ofαandβcrystal forms on space charge distribution of homopolymerized PPs was analyzed, and the effect of space charge distribution on short circuit breakdown was studied by DC pre-stress voltage.The results are listed as follow. First there is stillαcrystal form inβcrystal form PPs. Compared withαcrystal form PPs,βcrystal form PPs have lower apparent activation energy, higher crystallization velocity and better nonisothermal crystallization properties. Secondly in homopolymerized PPs, the size of crystal grain inαcrystal form PP is bigger than that inβcrystal form PP, and the gap inαcrystal form PP is bigger and more than that inβcrystal form PP. The size and gap inαandβcrystal form PPs copolymerized are almost same. Thirdly the conventional dielectric properties ofαandβcrystal form PPs both homopolymerized and copolymerized are almost same. Fourthly different crystal form PPs homopolymerized show obviously different space charge distribution, the space charge density inβcrystal form PP homopolymerized is much smaller than that inαcrystal form PP homopolymerized, and its decay with time is more...