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We review simulations of polymeric fluids that report mean-square displacements g(t) of polymer beads, segments, and chains. By means of careful numerical analysis, but contrary to some models of polymer dynamics, we show that hypothesized power-law regimes g(t)∼tα are almost never present. In most but not quite all cases, plots of log(g(t)) against log(t) show smooth curves whose slopes vary continuously with time. We infer that models that predict power-law regimes for g(t) are invalid for melts of linear polymers.

Using molecular dynamics (MD) simulations, this study explores the fluid properties of three polymer melts with the same number of entanglements, Z , achieved by adjusting the entanglement length N e , while investigating the evolution of polymer melt conformation and entanglement under high-rate elongational flow. The identification of a master curve indicates consistent normalized linear viscoelastic behavior. Surprising findings regarding the steady-state viscosity at various elongational rates ( Wi R >4.7) for polymer melts with the same Z have been uncovered, challenging existing tube models. Nevertheless, the study demonstrates the potential for normalizing the steady-state elongational viscosity at high rates ( Wi R >4.7) by scaling with the square of the chain contour length. Additionally, the observed independence of viscosity on the elongational rate at high rates suggests that higher rates lead to a more significant alignment of polymer chains, a decrease in entanglement, and a stretching in contour length of polymer chains. Molecular-level tracking of tagged chains further supports the assumption of no entanglement under rapid elongation, emphasizing the need for further research on disentanglement in polymer melts subjected to high-rate elongational flow. These results carry significant implications for understanding and predicting the behavior of polymer melts under high-rate elongational flow conditions.

Flow-induced disentanglement (FID or CCR-D) and chain tumbling are two molecular mechanisms typically observed in non-equilibrium molecular dynamics simulations of entangled polymer melts under fast shear. As regards quantitative performance, classical tube models exhibit limitations at fast rates presumably due to the negligence of the aforementioned mechanisms. CCR-D or tumbling inclusion is reported in some revised tube models. For example, in Desai–Larson’s (DL) work (J Rheol 58:255–279, 2014 ), which focuses on uniaxial elongation, FID is coupled to the alignment and stretch status of the chains. In Costanzo et al. (Macromolecules 49:3925–3935, 2016 ), tumbling is accounted for via incorporation of a semi-empirical tumbling function in the stretch equation. Nevertheless, CCR-D is neglected. Here, we include tumbling in the DL differential constitutive set and we assess its performance at shear and relaxation following shear. Model predictions are compared against data on various polystyrene melts as obtained by a cone-partitioned-plate fixture.

The influence of axial ultrasonic vibration (the dominant vibration mode) on the filling behavior of polymer melt in microcavities and its effect on microstructure formation remains inadequately understood. Based on the plasticization location and the extent to which the microcavity is covered by the ultrasonic sonotrode action surface, existing ultrasonic plasticization molding technologies were classified into three types—ultrasonic pressing (UP), ultrasonic plasticizing and pressing (UPP), and ultrasonic plasticization injection molding (UPIM). The effects of these configurations on melt fluidity and filling performance were evaluated and compared through slit flow tests. The interaction mechanisms between polymer melts and templates were elucidated based on melt pressure measurements and morphological changes in nickel micropillar arrays and silicon templates after molding. The results indicated that polymer melt exhibits improved flow behavior within microcavities when under the coverage area of the ultrasonic sonotrode action surface and subjected to the axial ultrasonic vibration. Continuous ultrasonic vibration contributed to sustaining melt fluidity during micropore filling. Among the three technologies, the most complex and intense mechanical interactions on the template microstructure were observed in UP, followed by UPP and then UPIM.

Dissipative particle dynamics (DPD) with bond uncrossability shows a great potential in studying entangled polymers, however relatively little is known of applicability range of entangled DPD model to be use as a model for ideal chains and properly describe the full dynamics of entangled melts. Therefore, we perform a comprehensive study on structure, dynamics and linear viscoelasticity of a typical DPD entangled model system, semiflexible linear polymer melt. These polymers obey Flory’s ideality hypothesis in chain dimensions, but their local structure exhibits nonideal behavior due to weak correlated hole effect. Both monomer motion and viscoelasticity relaxation reproduce the full pictures as predicted by reptation theory. The stronger chain length dependent diffusion coefficient and relaxation time as well as dynamic moduli are in close agreement with predictions of modern tube model that accounts for additional relaxation mechanisms besides chain reptation. However, an anomalous sub-diffusive center of mass motion is observed both before and after the intermediate reptation regime and the cross-correlation between chains is not negligible even these polymers obey stress-optical law, indicating limitations of the reptation theory. Hence semiflexible linear entangled DPD model can correctly describe statics and dynamics of entangled polymer melts.

This study utilizes molecular dynamics simulation to investigate the complex dynamics of entangled semi-flexible polymer melts. The investigation reveals a significant stress overshoot phenomenon in the systems, demonstrating the intricate interplay between shear rates, chain orientation, and chain stretching dynamics. Additionally, the identification of metastable states, characterized by a dual-plateau phenomenon in the shear stress-strain curve at specific Rouse-Weissenberg number Wi R , showcases the system’s responsiveness to external perturbations and its transition to stable shear banding states. Moreover, the analysis of flow field deviations uncovers a progression of shear bands with increasing Wi R , displaying distinct behaviors in the system’s dynamics under different shear rates and chain lengths. These findings challenge established theoretical frameworks and advocate for refined modelling approaches in polymer rheology research.

The aim of the present paper is to analyse the differences between tube-based models which are widely used for predicting the linear viscoelasticity of monodisperse linear polymers, in comparison to a large set of experimental data. The following models are examined: Milner–McLeish, Likhtman–McLeish, the Hierarchical model proposed by the group of Larson, the BoB model of Das and Read, and the TMA model proposed by the group of van Ruymbeke. This comparison allows us to highlight and discuss important questions related to the relaxation of entangled polymers, such as the importance of the contour-length fluctuations (CLF) process and how it affects the reptation mechanism, or the contribution of the constraint release (CR) process on the motion of the chains. In particular, it allows us to point out important approximations, inherent in some models, which result in an overestimation of the effect of CLF on the reptation time. On the contrary, by validating the TMA model against experimental data, we show that this effect is underestimated in TMA. Therefore, in order to obtain accurate predictions, a novel modification to the TMA model is proposed. Our current work is a continuation of earlier research (Shchetnikava et al., 2014), where a similar analysis is performed on well-defined star polymers.

Samples of two commercial low-density polyethylene melts were investigated with respect to their fracture behavior in controlled uniaxial extensional flow at constant strain rate in a filament stretching rheometer. In order to assess the possible influence of grain boundaries on fracture, the samples were prepared by three different types of pre-treatment: by compression molding of (1) virgin pellets used as received, (2) pellets homogenized in a twin-screw extruder, and (3) pellets that were milled into powder by cryogenic grinding under liquid nitrogen. The elongational stress growth data were analyzed by the Extended Hierarchical Multi-mode Molecular Stress Function (EHMMSF) model developed by Wagner et al. (Rheol. Acta 61, 281-298 (2022)) for long-chain branched (LCB) polymer melts. The EHMMSF model quantifies the elongational stress growth including the maximum in the elongational viscosity of LDPE melts based solely on the linear-viscoelastic relaxation spectrum and two nonlinear material parameters, the dilution modulus G D and a characteristic stretch parameter λ ¯ m . Within experimental accuracy, model predictions are in excellent agreement with the elongational stress growth data of the two LDPE melts, independent of the preparation method used. At sufficiently high strain rates, the fracture of the polymer filaments was observed and is in general accordance with the entropic fracture criterion implemented in the EHMMSF model. High-speed videography reveals that fracture is preceded by parabolic crack opening, which is characteristic for elastic fracture and which has been observed earlier in filament stretching of monodisperse polystyrene solutions. Here, for the first time, we demonstrate the appearance of a parabolic crack opening in the fracture process of polydisperse long-chain branched polyethylene melts.

We investigated the rheological properties of bidisperse entangled-polymer blends under high-deformation-rate flows by slip-link simulations with a friction reduction mechanism. The friction reduction mechanism induced by the stretch and orientation (SORF) is important to predict the viscoelasticity under uniaxial elongational flows. To test the applicability of this mechanism for bidisperse systems, we incorporated an expression of friction reduction (Yaoita et al. Macromolecules 45:2773–2782 2012 ) into the Doi-Takimoto slip-link model (DT model) (Doi and Takimoto Philos Trans R Soc Lond A 361:641–652 2003 ). For six experimental bidisperse systems, i.e., four polystyrene blends and two polyisoprene blends, the extended DT model where the order parameter of the friction reduction mechanism is evaluated through the component averages succeeds in reproducing the data under uniaxial elongation and shear. This success is due to the suppression of the stretch of the longer chains using the statistical average over each component. Through this study, the SORF expression improves the rheological prediction for bidisperse entangled polymer melts under uniaxial elongational flows with strain rates comparable to or larger than the inverse of the Rouse relaxation time of the longer chains. Additionally, the predictions with the SORF using the component average for the stretches reproduce the steady viscosities because under elongational flows, the states of the components with different molecular weights clearly differ from each other depending on their Rouse relaxation time. The finding means that for chain dynamics, the friction coefficient is determined by the state of the surrounding polymer chains and the state of the chain.

We revisit and improve the technique of piezo-operated sliding-plate rheometry in order to provide a versatile platform for measuring the linear viscoelastic properties of various soft matter systems at frequencies from 10 to 1.000 Hz. The sensitive loss angle measuring loop is validated explicitly against reference data from entangled amorphous polymer melts obtained with conventional rotational rheometers by means of time-temperature superposition (tTS). Frequency range limiting factors such as sample and tool inertia are discussed while errors are traced and theoretical correction is shown to be feasible when strong nonlinear behavior of the measuring cell is present. This gives confidence in measuring more complex systems where tTS does not apply. We also demonstrate the ability to probe the short-time dynamics of hard-sphere colloidal glasses. Important high-frequency features such as the behavior of the elastic modulus, G ′, the moduli crossover frequency f c related to β-relaxation, and the associated limiting in-phase (with strain-rate), dynamic viscosity η ∞ ′, are captured. This validates the suitability of this high-frequency rheometric technique to provide insights into interactions at nanometric particle separations.