Controlling Of The Dispersed Phase And Structure-Property Relationship For Impact Polypropylene Copolymer

As a representative of the high-performance polypropylene (PP) material, impact polypropylene copolymer (IPC) is widely applied due to excellent impact toughness and good comprehensive properties. Many researchers paid attention to developing new IPC with more excellent performance and lower cost. However, many scientific issues, such as relationship of "component-structure-property", toughening mechanisms and so on, remain to be elucidated.Although much work has been reported on characterizations of composition, micro-structure, morphology and thermal behavior of IPC, few investigations on "component-structure-property" relationship were conducted due to the difficulty in obtaining samples with regular changing in composition or micro-structure. By contrast, blending is a simple, economical and effective method to solve this problem.According to a two-step preparation technique consisting of temperature-gradient extraction fractionation (TGEF) and subsequent solution-mixing, a series of polypropylene/ethylene-propylene random copolymer/ethylene-propylene block copolymer (HPP/EPR/EbP) blends were prepared to explore the influences of composition on phase morphology, crystallization, microstructure and mechanical properties, preparation for simulating behavior of IPC.For HPP/EPR/EbP blends with a constant content of EbP at 10 wt%, the dispersed phases with core-shell structure whose microstructures were similar to IPC could be rebuilt in certain composition; however an excessive EPR would led to a bicontinuous phase structure. The crystallization and melting behavior of HPP in blends exhibited special composition-dependences, for example, the melting point increased markedly with the increase of EPR content until it turned down at a critical content (about 30 wt%). This was mainly ascribed to the different primary nucleation abilities which were related to compatibility and phase structure. The correlation between structural parameter of crystals and crystallization behavior revealed that primary nucleation behavior determined by phase morphology played a major role in overall crystallization rate while secondary nucleation exponent was inclined to affect growth rate.When the content of HPP (70 wt%) and total dispersed phases (30 wt%) in all HPP/EPR/EbP blends were fixed, the weight ratio of EPR/EbP in dispersed phase was regulated. As a result, the number, the size and layer thickness of multilayered core-shell dispersed particles were regularly controlled. With the decrease of EbP content, the outer interfacial thickness of core-shell particles decreased and the blends presented an elevated melting point, which might be ascribed to enhanced nucleation ability. It was found that co-crystallization occurred at the outer layers between HPP and EbP. Meanwhile, the special simultaneous depression of glass-transition temperatures (Tg) of HPP and EPR was observed in all HPP/EPR/EbP blends compared with neat components, which was attributed to the enlarging free volume by imperfect co-crystals and infiltration of long ethylene chain segments of EbP component at interface.To investigate the toughening mechansm of the blends containing core-shell dispersed particles and to fabricate a PP blend with simultaneous enhancement of strength and toughness, high-density polyethylene (HDPE) was introduced into IPC. Morphological observations showed that a series of multilayered core-shell dispersed particles appeared when the content of HDPE was less than 40%, on the contrary the continuous network structure turned up beyond that. With increasing the content of HDPE, the size of the core increased and the number of dispersed particles with incompletely encapsulated PE cores rose. More valid ’bridge’ made up of segmented ethylene-propylene copolymer (sEbP) appeared and connected the PE core with PP matrix. Meanwhile, co-crystallization occurred in the core domain, between long ethylene chain segments of the joined HDPE and sEbP in multi-component IPC. The increased HDPE in blends reduced defective co-crystals, and in turn led to an increased average lamellar thickness and decreased amorphous thickness of PE. Partial inserted ethylene-propylene random sequences were constrained by narrowed PE amorphous layer. The connection between PP matrix and dispersed phase was strengthened by co-crystals,’bridge’and restriction effects. Thus both excellent strength and toughness for IPC was achieved by consuming the energy significantly as the stress could be transferred effectively and induce shearing deformation of PE core.Besides phase morphology, the states of crystalline and amorphous phase in HPP matrix could also influence the final properties. By introduction of β nucleating agent (β-NA) and annealing treatment, the crystallization and chain mobility of amorphous phase in IPC were profoundly investigated to reveal toughening mechanism. The β-NA induced the formation of β-PP crystals and consequently chain mobility of amorphous phase was strengthened markedly in either RAF or MAF comparing with neat IPC. After annealing between Tg and Tm, lamellae thickening and and a fi-a transition of crystals occurred and in this case, the chain mobility in RAF was constrained further by lamellar thickening. The impact test results showed that IPC was toughened effectively with adding P-NA but deteriorated after annealing, as ft was bad for the fracture energy to transfer and dissipation with thickening lamellae and reduced motive chains.