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A first attempt is made to present a detailed mesoscopic‐level description of the conformation and equilibrium dynamic properties of macromolecular chains in ultrahigh molecular weight polyethylene/polypropylene blends driven by extension‐dominated coupled flow, in comparison with shear‐dominated coupled flow. Extension‐dominated coupled flow can be defined as γ˙= 0.005, ε˙≥ 0.1 and shear‐dominated coupled flow as ε˙= 0.1, γ˙≥ 1, as verified by the time‐dependent evolution tendency of cell parameters and velocity profiles along the x, y, z directions. In the case of extension‐dominated coupled flow, the chains are greatly elongated and prefer to align along the flow direction, where the polypropylene chains appear to form a somewhat uniform layer. By comparison, the macromolecular chains assume a highly collapsed conformation and have a greater preference to align themselves perpendicular to the flow direction under shear‐dominated coupled flow, which can be ascribed to the rapid diffusion of polymer chains and a minor role of the weak van de Waals interaction in structural transitions. Analyses of conformational behavior derived from dissipative particle dynamics trajectories can, in general, serve as a useful guideline for experimental investigations of ultrahigh molecular weight polyethylene based materials prepared by eccentric rotor extrusion. © 2020 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

Improving the processability of ultrahigh molecular weight polyethylene (UHMWPE) and understanding the effect of the polymeric chain mobility has long been a challenging task. Herein, we show that UHMWPE without any processing aids can be processed at a lower temperature of 180 °C compared to conventional processing temperatures (~250 °C) under a continuous elongational flow (CEF) by using an eccentric rotor extruder (ERE). By probing the effect of the residence time of UHMWPE samples under a CEF on the morphology, rheological behavior and molecular orientation, we find that the long polymer chains of UHMWPE are apt to orientate under a consecutive volume elongational deformation, thereby leading to a higher residual stress for the extruded sample. Meanwhile, the residence time of samples can regulate the polymeric chain mobility, giving rise to the simultaneous decrease of the melting defects and residual stress as well as Hermans orientation function with increasing residence time from 0 to 60 s. This also engenders the enhanced diffusion of UHMWPE segments, resulting in a defect-free morphology and higher entanglement with lower crystallinity but without causing obvious thermal oxidative degradation of UHMWPE. This interesting result could originate from the fast chain entanglement and particle welding enabled by a desirably short residence time, which could be explained by the empirical, entropy-driven melting explosion mechanism. [ABSTRACT FROM AUTHOR]

Herein, we produced a series of ultrahigh molecular weight polyethylene/polypropylene (UHMWPE/PP) blends by elongational-flow-field dominated eccentric rotor extruder (ERE) and shear-flow-field dominated twin screw extruder (TSE) respectively and presented a detailed comparative study on microstructures and tribological properties of UHMWPE/PP by different processing modes. Compared with the shear flow field in TSE, the elongational flow field in ERE facilitates the dispersion of PP in the UHMWPE matrix and promotes the interdiffusion of UHMWPE and PP molecular chains. For the first time, we discovered the presence of the interlayer phase in blends with different processing modes by using Raman mapping inspection. The elongational flow field introduces strong interaction to enable excellent compatibility of UHMWPE and PP and induces more pronounced interlayer phase with respect to the shear flow field, eventually endowing UHMWPE/PP with improved wear resistance. The optimized UHMWPE/PP (85/15) blend processed by ERE displayed higher tensile strength (25.3 MPa), higher elongation at break (341.77%) and lower wear loss of ERE-85/15 (1.5 mg) compared to the blend created by TSE. By systematically investigating the microstructures and mechanical properties of blends, we found that with increased content of PP, the wear mechanism of blends varies from abrasive wear, fatigue wear, to adhesion wear as the dominant mechanism for two processing modes. [ABSTRACT FROM AUTHOR]

Herein, a novel eccentric rotor extruder (ERE) capable of generating a continuous elongational flow was used to process ultrahigh molecular weight polyethylene (UHMWPE) without any processing aids and then compare with a conventional rotational batch mixer based on a shear flow. The morphological and rheological characterization verify that the technique based on the elongational flow could effectively reduce melting defects and yield more homogeneous morphology within the extruding samples relative to the conventional bath mixing based on a shear flow. The extrusion processing under an elongational flow can largely maintain the viscosity average molecular weight (Mη) of the UHMWPE nascent powder with only a 5.0% decrease at 200 °C, implying considerably low thermal oxidative degradation in sharp contrast to the conventional processing with significantly reduced Mη by 40.3%. Furthermore, the crystallinity for the sample prepared under an elongational flow is lower than that processed under a shear flow. These differences lies in the higher normal stress, rapider heat transfer and shorter duration generated by the ERE. [ABSTRACT FROM AUTHOR]

The property of ultrahigh molecular weight polyethylene (UHMWPE)-based materials are significantly dependent on the morphology and phase behavior evolution subjected to extensional-shear coupled flow fields. In this study, we utilize polypropylene (PP)/UHMWPE blends as model and present a combination of numerical simulation and experimental study on their association behavior under extensional-shear coupled flow field. In terms of simulation, combined molecular dynamics (MD) and dissipative particle dynamics (DPD) method are applied, where two kinds of extensional-shear coupled flow fields are produced, named as coupledⅠand coupled Ⅱ. In response to coupled Ⅰ, shear-stretched alignment of PP chains is observed at shear rate of γ ˙ = 0.01, followed by a quick disordering transition with increasing the shear rates and a final phase-separation at shear rate of γ ˙ = 0.1. When subjected to coupled Ⅱ, the weak segregation start to connect with the adjacent ones, giving a homogeneous phase when further increasing the applied pressure. The presence of pressure-driven flow helps in bridging different chains, causing the percolating polymer network reinforcement. Besides, experimental studies by means of Raman spectroscopy, Raman mapping and rheological measurement are compared with the above simulations. It confirms that it is more efficiently broken for melted drops under an extensional dominated coupled flow than those under a shear dominated coupled flow, in which an effective lubricating phase forms, leading to the induced miscibility in polymer blends and reduction in storage modulus and viscosity. These experimental findings are consistent with simulation results. Image 1 • In response to couple Ⅰ, a final phase separation take place with varying the shear rates. • As imposed by couple Ⅱ, a homogeneous phase can be obtained when further increasing the applied driving pressure. • The obtained results from simulations are in good agreement with these experimental findings. • It forms an effective lubricating phase for the benefit to the viscosity reduction. [ABSTRACT FROM AUTHOR]