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• Additives affect recrystallization from the melt of commercial linear low density polyethylenes (LLDPE). • Additives are consumed after melt-reprocessing and the effect of melt-memory or self-nucleation on recrystallization can be followed by DSC. • The effect of melt-memory on recrystallization of LDPEs and metallocene LLDPEs differs from the behavior of broadly distributed Ziegler-Natta LLDPEs. • Studying the effect of melt-memory on recrystallization of LLDPEs by DSC is a simple avenue to identify if LLDPEs undergo liquid-liquid phase separation (LLPS) on cooling from the melt. Compared with a constant recrystallization rate of commercial linear low density polyethylenes (LLDPE) as a function of melt temperature, copolymers reprocessed in an open twin-screw extruder, display the expected strong melt-memory features upon melting followed by recrystallization. Precipitation from dilute solution had the same effect on melting-recrystallization of commercial copolymers. In the twin-screw melt re-processing, a small content of long-chain branches and crosslinks are incorporated to the copolymer's structure, but especially a further consumption of additives uncovers self-nucleation features that speed up recrystallization and are related to the major copolymer microstructure. The observed difference in re-crystallization behavior between metallocene type and Ziegler-Natta (ZN) type copolymers, both with equivalent average comonomer content and molecular mass, is due to the inter-chain comonomer distribution. While for metallocene-made and low density polyethylenes, below a critical melt temperature the recrystallization rate increases continuously with decreasing melt temperature, in broadly distributed ZN copolymers there is an inversion of the temperature gradient of the rate at the onset of liquid-liquid phase separation (LLPS). The mass fraction of highly branched molecules required to observe the onset of LLPS via melting followed by recrystallization was evaluated from the TREF profiles, and the effect of self-nucleated melts at different levels on the overall spherulitic morphology was observed via polarized optical microscopy. [Display omitted] [ABSTRACT FROM AUTHOR]

In recent years, rubber toughened polypropylene (PP) blends have been extensively investigated with special regard to the importance of PP matrix properties (e.g., crystal morphology and structure) in tailoring the toughening efficiency. Unfortunately, although high melt flowability is indispensable to the processing of polymers into complex thin-wall products, it remains a huge challenge to achieve a marvelous balance between stiffness and toughness in the blends with relatively low matrix molecular weight. Herein, taking PP/EPR (ethylene-propylene copolymer) blends as an example, the combined effects of matrix molecular weight and crystallinity on the mechanical properties of PP/EPR blends have been examined in both experiment and theory. The experimental results show that decreasing molecular weight leads to significant deterioration in notched impact toughness, tensile strength and Young's modulus. Impressively, the toughness loss caused by the decrease of molecular weight can be well-compensated by a slight reduction in the matrix crystallinity, without evidently sacrificing the strength and modulus, demonstrating an improved stiffness-toughness balance. Theoretical analysis indicates that the impact toughness of PP/EPR blends is heavily dependent on the chain entanglement density (v e) of PP matrix, and a linear relationship between critical interparticle distance (ID c) and the v e 1/3 has been verified from both experiment and theory. Moreover, it is interesting to find that decreasing matrix crystallinity can increase the v e of low-molecular-weight PP matrix, which enables the effective toughening at lower concentration of EPR and thus gives rise to less loss in the strength and modulus. We believe this work not only gives a new insight into the role of chain entanglement in the toughening but also provides a promising guidance for the design of high-performance PP with high melt flowability. [Display omitted] • Combined effects of M n and crystallinity on the toughening efficiency of PP/EPR blends were studded. • The relationship between the ID c and v e of PP matrix i.e. ID c ∼ v e 1/3 , was given for the first time. • Matrix with higher M n or lower crystallinity has higher v e , exhibiting larger ID c. • Toughness loss caused by decreasing M n can be compensated by reducing crystallinity. • Low-viscosity PP/EPR blends with good toughness-stiffness balance were obtained. [ABSTRACT FROM AUTHOR]