Relationship between molecular structure, crystallization behavior, and mechanical properties of long chain branching polypropylene

The crystallization behaviors and mechanical properties of both linear isotactic polypropylene (iPP) and long chain branching polypropylene (LCBPP) were investigated with differential scanning calorimetry, polarized optical microscopy, and wide-angle X-ray diffraction. The results indicated that the long chain branching (LCB) structure could act as nucleating agent accelerating the nucleation but impeding the crystal growth of LCBPP, and eventually increased the crystallization peak temperature and decreased the spherulite size of LCBPP. Moreover, with the presence of LCB structure, β-form polypropylene was generated in LCBPPs under injection molding conditions, and the content of β-form increased with the increasing of LCB structure content. Benefiting from the combined effect of the LCB structure and β-form crystal, the LCBPPs revealed not only outstanding toughness but also desired stiffness. The impact strength, flexural modulus and tensile strength of HMSPP3 were increased by, respectively, 260.0, 54.7, and 17.2 % than those of linear iPP. These results are helpful to develop new types of high performance polypropylene.