Study On Liquid-Liquid Phase-Separation Behavior And The Effect On The Crystallization Of Blend Composed Of Polypropylene/Ethylene-Propylene Rubber

Blending with elastomer is one of the most effective ways to improve the impact property of isotactic polypropylene(iPP). iPP and ethylene-propylene rubber (EPR) are also the basic composition for the polypropylene catalloy. As a typical crystallize/amorphous material, the relationship between the structure and the property of iPP/EPR has a great theoretical significance, and attracts the attention of many researchers. In this thesis, the liquid-liquid phase-separation kinetics, phase morphology, the effect on the crystallization and the formation of β-iPP for iPP/EPR blend were investigated.The liquid-liquid phase-separation (LLPS) and phase morphology of iPP/EPR were investigated by using small angle laser light scattering (SALLS) and phase contrast microscope (PCM). The results reveal that LLPS of (50/50 wt/wt) iPP/EPR blend happens at temperatures above the melting point of iPP and its kinetic follows Spinodal Decomposition (SD) mechanism. The apparent diffusion coefficient (D_app) and the spinodal temperature (T_s) for the iPP/EPR(50/50 wt/wt) sample were calculated through Cahn-Hilliard theory. Moreover, it is found that the D_app increases with the increase of temperature, indicating the LCST behavior of iPP/EPR. The development of phase morphologies could be observed by quenching the iPP/EPR samples subjected to different annealing time at 200 ℃ in liquid N₂. On the other hand, the structure of iPP/EPR melt appears nearly homogenous at the beginning of LLPS. It is suggested that the co-continuous phases form and the phase connectivity grows until macroscopic spherical texture generates.The isothermal crystallization behavior and the crystalline morphology were investigated by using differential scanning calorimetry (DSC) and polarized optical microscope (POM). It is found that both the increases of phase separation time (t_ps) and phase separation temperature (T_ps) result in the decrease of the crystallization rate. It is ascribed to the decrease of the amount of EPR as the nucleation agent and the increase of the viscosity of iPP melt when LLPS happens. Furthermore, the results ofthe kinetic analysis indicate that the Avrami model is valid for describing the isothermal crystallization of iPP/EPR. It is believed that the degree of phase-separation has little effect on the crystallization kinetic and the crystallization gives priority to instantaneous nucleation and three-dimension-growth. It is revealed that the spherulite of iPP/EPR (50/50 wt %) has the co-connectivity structure, and with the increase of the degree of LLPS, the structure sizes of the crystallizable and the amorphous become larger. Maltese Cross of the spherulite could be seen obviously at higher degree of LLPS. It is ascribed to the difference of limitation degree of iPP during the crystallization. The growth rate of spherulite decreases with the increase of the degree of LLPS.The effects of preparation method, composition and thermal condition on formation of β-iPP in iPP/EPR blends were studied by using DSC, wide angle X-ray diffraction (WAXD) and PCM. It is found that the α-iPP and β-iPP can simultaneously form in the melt-blended samples while only α-iPP exists in the solution-blended samples. The results show that the formation of β-iPP in the melt-blended samples is related to the crystallization temperature and the critical isothemal temperature is from 114 ℃ to 134 ℃. Furthermore, it is found that the amount of β-iPP in melt-blended iPP/EPR samples is dependent on the composition and the maximum amount of β-iPP forms when the composition of iPP/EPR blends is 85:15 in weight. The results through examining the effect of annealing for iPP/EPR samples at melt state indicate that this annealing may eliminate the susceptibility to β-crystallization of iPP. The phase contrast microscopy images demonstrate that an obvious phase-separation happens in both melt-blended and solution-blended iPP/EPR samples, implying that compared with the disperse degree of EPR in iPP, the preparation method plays a dominant role in formation of β-iPP. It is suggested that the origin of formation of β-iPP results from the thermomechanical history of the EPR component in iPP/EPR.