The molecular structure characteristics of long chain branched polypropylene and its effects on non-isothermal crystallization and mechanical properties

摘要

Long chain branched polypropylene (LCBPP) was prepared by melting graft reaction. The molecular structure characteristics such as average molecular weight of long chain branches were estimated by triple-detector gel permeation chromatography. The effects of long chain branches (LCB) on the zero shear viscosity (η_0) were studied, with the ratio of η_0 between LCBPP and its linear precursor isotactic polypropylene (iPP) approximated to (1+LCBI)~(α3) (LCBI was long chain branching index and α_3 is the ratio of the exponent in Mark-Houwink equation of LCBPP and iPP). During non-isothermal crystallization, the nucleation rate of LCBPP was significantly higher than that of iPP but the crystal growth rate was lower. The tensile strength and notch impact strength of LCBPP were slightly increased. The variations of crystallization were due to the introduction of LCB enhanced entanglement between molecular chains. Both the enhanced entanglement and intensive small spherulites increased the mechanical properties of LCBPP.