Elongational Flow Research Articles

A novel PLA/P(3HB-co-4HB)/MWCNT composite featuring enhanced mechanical properties and excellent thermal stability based on elongational rheology

Simple melt blending is the most frequently-used strategy to obtain high-performance PLA composites. However, the thermosensitive PLA displays the defects of poor thermostability, high brittleness and low heat deformation temperature (HDT) in traditional melt-plasticizing equipment based on shear flow field (SFF). In this work, PLA/P(3HB-co-4HB) blends were fabricated via melt blending technology based on the melt plasticization equipment of elongational flow field (EFF) and SFF. Subsequently, Multi-Walled Carbon Nanotube (MWCNT) was introduced into PLA/P(3HB-co-4HB) blends to prepare PLA/P(3HB-co-4HB)/MWCNT high-performance alloy in an eccentric rotor extruder based on EFF. The periodic compression and release of the EFF on the melt promoted P(3HB-co-4HB) in-situ fiberization and formed special microstructure, which greatly enhanced the mechanical properties, HDT, thermal stability and crystallization perfection of PLA. The tensile strength, Young's modulus, elongation at break, impact strength and HDT of the blend containing 20 wt% P(3HB-co-4HB) under EFF were 8.7%, 22.5%, 80.9%, 62.5% and 14.8% higher than those under SFF, respectively. In addition, MWCNT was induced to disperse into P(3HB-co-4HB) phase by EFF, and played a ‘bridge’ role in PLA and P(3HB-co-4HB), thereby obtaining PLA/P(3HB-co-4HB)/MWCNT high-performance alloy with stronger comprehensive performance. The fabricated PLA/P(3HB-co-4HB)/MWCNT alloy provides a reference for preparing biopolymers under EFF and broadens the potential application value of PLA/P(3HB-co-4HB).

Biocomposite PBAT/lignin blown films with enhanced photo-stability

Lignin can be obtained as a byproduct during cellulose-rich pulp fibers production and it is habitually treated as waste or intended for low-value destinations. However, due to UV absorption and mechanical properties, lignin can contribute to the fabrication of biodegradable blown films with superior performances. In this study, it was established the suitability of lignin for manufacturing biocomposite PBAT blown films with higher stiffness and photo-oxidation resistance. The effect of the filler concentration on the melt rheological behavior in non-isothermal elongational flow was investigated. The results allowed us to choose the correct filler concentration for producing films through a film blowing operation. The PBAT/lignin blown film composites displayed an increase of the elastic modulus if compared to neat PBAT films without affecting their elongation at break. Furthermore, the filler delayed the photo-oxidative degradation of PBAT hence potentially allowing open-air applications.

Statistical Dynamics of Flow-Driven Globular Polymers.

The response of collapsed polymers to the effects of linear mixed flow is studied theoretically in this paper using a model of a self-interacting finitely extensible Gaussian chain that evolves stochastically in the presence of random thermal fluctuations and an external fluid velocity gradient. The interactions that produce compact chain configurations are described by a harmonic pair potential of strength κ that acts between nonbonded sites on the chain backbone. Several chain properties are calculated analytically from this model as a function of κ for elongational and shear flows, including the dependence of the chain's steady-state mean-square end-to-end distance on the Weissenberg number of the flow, the time-dependence of the chain's relaxation to equilibrium from a steady-state of given chain extension, and the nature of the force-extension curves that are obtained from the free energy change between unperturbed and flow-stretched states of the chain. For both elongational and shear flows (but to different degrees), it is found that the greater the value of κ (and the more compact the chain), the more difficult it is, in general, for the imposed flow to induce a transition between compact and extended states, in broad agreement with available data from numerical simulations. For the relaxation process, the differences between the two flow types are more marked. The characteristic decay time for relaxation from a state prepared by elongational flow is essentially independent of κ, whereas in the case of a state prepared by shear flow, it is distinctly κ-dependent, the relaxation becoming faster at larger κ.

Traumatic vessel injuries initiating hemostasis generate high shear conditions

AbstractBlood flow is a major regulator of hemostasis and arterial thrombosis. The current view is that low and intermediate flows occur in intact healthy vessels, whereas high shear levels (>2000 s−1) are reached in stenosed arteries, notably during thrombosis. To date, the shear rates occurring at the edge of a lesion in an otherwise healthy vessel are nevertheless unknown. The aim of this work was to measure the shear rates prevailing in wounds in a context relevant to hemostasis. Three models of vessel puncture and transection were developed and characterized for a study that was implemented in mice and humans. Doppler probe measurements supplemented by a computational model revealed that shear rates at the edge of a wound reached high values, with medians of 22 000 s−1, 25 000 s−1, and 7000 s−1 after puncture of the murine carotid artery, aorta, or saphenous vein, respectively. Similar shear levels were observed after transection of the mouse spermatic artery. These results were confirmed in a human venous puncture model, where shear rates in a catheter implanted in the cubital vein reached 2000 to 27 000 s−1. In all models, the high shear conditions were accompanied by elevated levels of elongational flow exceeding 1000 s−1. In the puncture model, the shear rates decreased steeply with increasing injury size. This phenomenon could be explained by the low hydrodynamic resistance of the injuries as compared with that of the downstream vessel network. These findings show that high shear rates (>3000 s−1) are relevant to hemostasis and not exclusive to arterial thrombosis.

Viscoelastic simulations using the closed-form Adaptive Length Scale (ALS-C) model

In this paper we employ the closed-form of the Adaptive Length Scale Model (ALS-C) [Ghosh et al., “A new model for dilute polymer solutions in flows with strong extensional components”, J. Rheol. 46, 1057–1089 (2002)] and we investigate its characteristics and potential to more accurately capture pressure-drop in contraction flows of viscoelastic fluids. The ALS-C model was originally derived based on purely homogeneous elongational flows in order to model coil-stretch hysteresis. However, in its originally proposed form we reveal a number of numerical issues which have not been analysed previously and are reported here considering both standard rheological flows, simple channel flows and complex flows within a 4:1 contraction. We demonstrate a new approach for evaluating the instantaneous change in the adaptive length scale as a result of instantaneous changes in the flow field, overcoming the need to employ other root-finding approaches. Guidelines are provided for the correct use of the employed local Weissenberg number and a modified approach is considered for the evolution equation of the actual extensibility, allowing its efficient use in complex numerical simulations. We illustrate that a suitable combination of the model parameters can produce behaviours that are found experimentally in viscoelastic fluids and we find that pressure-drop enhancements in flows within 4:1 contractions observed experimentally are achievable.

High orientation structure in UHMWPE/BN composites continuously obtained by elongational flow field leading to superior thermal conductivity

AbstractUltra‐high‐molecular‐weight polyethylene/boron nitride (BN) composites have been continuously prepared successfully by eccentric rotor extruder (ERE) based on elongational flow. Optical microscopy (OM), scanning electron microscopy (SEM), x‐ray diffraction (XRD), thermal conductivity test, heat‐dissipation test, and tensile test were performed to investigate the orientation structure, thermal conductivity, mechanical properties, and insulation properties. The orientation factor and thermal conductivity of the composites prepared by ERE were 79.3 and 3.75 W·m−1·K−1, which were much higher than those of composites prepared with a torque mixer (14.7 and 2.73 W·m−1·K−1, respectively). OM and SEM showed that the BN sheets were in contact inside the composites and formed an efficient thermal conductivity network. Besides, the composites prepared by ERE had satisfactory mechanical and insulation properties (>1013 Ω·cm). This study provides experimental and theoretical guidance for industrial preparation of polymer/layered‐filler functional composites.

A Thermodynamically Consistent, Microscopically-Based, Model of the Rheology of Aggregating Particles Suspensions.

In this work, we outline the development of a thermodynamically consistent microscopic model for a suspension of aggregating particles under arbitrary, inertia-less deformation. As a proof-of-concept, we show how the combination of a simplified population-balance-based description of the aggregating particle microstructure along with the use of the single-generator bracket description of nonequilibrium thermodynamics, which leads naturally to the formulation of the model equations. Notable elements of the model are a lognormal distribution for the aggregate size population, a population balance-based model of the aggregation and breakup processes and a conformation tensor-based viscoelastic description of the elastic network of the particle aggregates. The resulting example model is evaluated in steady and transient shear forces and elongational flows and shown to offer predictions that are consistent with observed rheological behavior of typical systems of aggregating particles. Additionally, an expression for the total entropy production is also provided that allows one to judge the thermodynamic consistency and to evaluate the importance of the various dissipative phenomena involved in given flow processes.

Flow-accelerated polycondensation reaction to prepare rigid rodlike poly(p-phenylene-cis-benzobisoxazole)

Under the conditions specified in this work, the polycondensation reaction to prepare rigid rodlike poly(p-phenylene-cis-benzobisoxazole) (PBO) is diffusion-controlled. This work aims to study the flow-driven polycondensation reaction of PBO. The effect of mixing efficiency through changing the stirring speed on the polycondensation reaction of PBO was first investigated in a paddle impeller reactor (PIR). In the initial stage of polymerization, the increase of stirring speed is obviously conducive to accelerating the polycondensation reaction. However, the promoting effect of stirring speed on the polycondensation reaction is greatly reduced in the middle and late stages of polymerization. The effect of the flow pattern with different proportions of shear flow, elongational flow and rotational flow on the polycondensation reaction of PBO was further investigated by comparing three reactors including Couette-Taylor reactor (CTR) with a complete shear flow, PIR with strong shear and elongational flows, and double helical ribbon reactor (DHR) with strong shear and rotational flows. Result shows that the improvement of proportions of rotational flow and elongational flow is beneficial to accelerate the polycondensation reaction of PBO in the middle and late stages of polymerization, especially for the rotational flow.

Probing structure development in Poly(vinylidene Fluoride) during “operando” 3-D printing by small and wide angle X-ray scattering

We have investigated the crystallization of the thermoplastic polymer Poly(vinylidene Fluoride) (PVDF) during “operando” 3D printing Fused Filament Fabrication (FFF). The performance of the 3D printing set-up and the corresponding methodology for performing simultaneous SAXS/WAXS with synchrotron radiation have been discussed. Simultaneous SAXS and WAXS experiments were performed across the printed line with a resolution of 50 μm. The experiments indicate that crystallization is faster at the polymer-air interface than in other points within the printed line. The final crystallinity varies with position being lower at both the interfaces, i.e. polymer-air (13%) and welding zone (11%), while being higher (18%) in the middle part of the printed line. Orientation of the crystalline lamellae is higher at both interfaces, suggesting higher shear rate than in the bulk of the printed line where elongational flow is dominant. The final crystallinity levels in the different locations of the printed line are relatively low, as compared with reported values in melt pressed samples (≈30%), due to the extremely fast crystallization kinetics involved in the solidification of PVDF 3D printed lines. Therefore, it is expected that 3D printed PVDF pieces will exhibit significant structural modifications due to the potential crystallization that eventually will proceed during storage because the glass transition of PVDF is well below room temperature.

Cartesian grid method with a consistent direct discretization approach for numerical simulation of non-Newtonian fluid flow

This paper proposes an extended version of the discretization scheme for a consistent direct discretization approach in a Cartesian grid method for flow simulations of incompressible non-Newtonian fluids. Based on the concept of the original method, which was developed for Newtonian fluid flow, the Navier–Stokes and the pressure Poisson equations are discretized directly in the vicinity of a solid boundary while maintaining the consistency between the velocity and pressure fields. The main difficulty in the discretization arises from the evaluation of the velocity gradients on the fluid–solid interface, which is necessary for handling the viscous term for spatially distributed shear-dependent viscosity. In the extended method, a new procedure for the calculation of the velocity gradient using the relation that holds on a rigidly moving solid surface is proposed. The validity of the method is ascertained in fundamental test problems of non-Newtonian fluids: laminar Poiseuille/Couette flows in a concentric annulus and flows around a circular cylinder. It is found that a second-order spatial accuracy with the desired numerical stability is achieved in both the shear-thinning and shear-thickening viscous properties, and the numerical error is considerably decreased compared with some conventional methods. As a realistic application of the extended method, a polymer mixing simulation inside a co-rotating twin-screw extruder is demonstrated by applying the method to the moving boundary problem. In a screw arrangement with a sequential combination of a kneading disk (KD), a cylindrical ring (CR), and a backward mixing screw (BMS), large- and small-scale structures of bifurcating and converging flows are respectively formed in the KD and BMS zones, whereas elongational flow structures and velocity fluctuations are still maintained in the CR zone due to flow interaction with the upstream and downstream screw elements, which presumably contributes to moderate mixing at low shear stress. • A Cartesian grid method for non-Newtonian fluid flow is developed. • The present method extends a consistent direct discretization approach. • A procedure for evaluating velocity gradients on an immersed boundary is proposed. • Second order spatial accuracies for velocity and its derivatives are ascertained. • A polymer mixing simulation in a co-rotating twin-screw extruder is demonstrated.

Analysis of elongational flow of star polymers

Star polymers with three arms are the simplest example of branched polymers. Elongational rheology data of three well-characterized monodisperse polystyrene melts, a symmetric star, an asymmetric star, and a linear polymer with the same span molecular weight of 180 kg/mol reported by Huang et al. (Macromolecules 49:6694−6699, 2016) are analyzed by the enhanced relaxation of stretch (ERS) model (Wagner and Narimissa, J Rheol 65:1413–1421, 2021). All three melts show the same elongational stress growth coefficient and the same steady-state elongational viscosity in fast extensional flows when the stretch-related Weissenberg number {Wi}_R=dot{varepsilon}{tau}_R>1. Excellent agreement between experimental data of elongational stress growth coefficient and model predictions is obtained, based exclusively on the linear-viscoelastic characterization of the polymer systems. Stress relaxation following steady elongational flow depends on the presence of the branch point and the length of the arm, and a new process regarding relaxation of the orientation of the stars is identified.

Nonlinear Elongation Flows in Associating Polymer Melts: From Homogeneous to Heterogeneous Flow

Numerical simulations explore how material properties of polymers are altered as clusters of strongly interacting groups along the chain break apart and reform under flow, an insight that can help control complex fluids.

Solvation-Free Fabrication of PEO/LiTFSI/SiO2 Composite Electrolyte Membranes with High Ionic Conductivity Based on a Novel Elongational Flow Field

Solvation-Free Fabrication of PEO/LiTFSI/SiO₂ Composite Electrolyte Membranes with High Ionic Conductivity Based on a Novel Elongational Flow Field

Elucidating the role of network topology dynamics on the coil-stretch transition hysteresis in extensional flow of entangled polymer melts

Dissipative particle dynamics (DPD) simulations are performed on coarse-grained replicas of linear, monodisperse entangled polyethylene melts C1000H2002 and C3000H6002 undergoing both steady-state and transient planar elongational flow (PEF). The fidelity of the DPD simulations is verified by direct comparison of flow topological and rheological properties of a 334-particle chain liquid against the united-atom C1000H2002 liquid, simulated using nonequilibrium molecular dynamics (NEMD). These DPD simulations demonstrate that a flow-induced coil-stretch transition (CST) and its associated hysteresis caused by configurational microphase separation, as observed in previous NEMD simulations of PEF, can be replicated using a more computationally efficient coarse-grained system. Results indicate that the breadth of the CST hysteresis loop is enlarged for the longer molecule liquid relative to the shorter one. Furthermore, relaxation simulations reveal that reducing the applied flow Deborah number (De) from a high value corresponding to a homogeneous phase of highly stretched molecules to a De within the biphasic region results in a two-stage relaxation process. There is a fast initial stratification of the kinetically trapped highly stretched chains into regions of highly extended and less extended chains, which displays similar behavior to a system undergoing a spinodal decomposition caused by spatial configurational free energy fluctuations. After a short induction period of apparently random duration, the less extended chain regions experience a stochastic nucleation event that induces configurational relaxation to domains composed of coiled molecules over a much longer time scale, leaving the more highly extended chains in surrounding sheetlike domains. The time scales of these two relaxation processes are of the same order of magnitude as the Rouse and disengagement times of the equilibrium liquids.

Modelling elongational viscosity overshoot and brittle fracture of low-density polyethylene melts

The Hierarchical Multi-mode Molecular Stress Function (HMMSF) model predicts the elongational and multiaxial extensional viscosities of polydisperse linear polymer melts based exclusively on their linear viscoelastic characterization and a single nonlinear material parameter, the so-called dilution modulus {G}_{D}. For long-chain branched (LCB) polymer melts such as low-density polyethylene (LDPE), the HMMSF model describes quantitatively the elongational stress growth coefficient up to the maximum of the elongational viscosity but fails to predict the existence of the maximum and the following steady-state viscosity. By taking into account branch point withdrawal in elongational flow of LCB melts, we extend the HMMSF model and show that the maximum of the elongational viscosity can be characterized by a single additional parameter, the characteristic stretch {overline{lambda }}_{m}, while the steady-state tensile stress and the elongational viscosity depend only on the dilution modulus {G}_{D} as in the case of linear polydisperse melts. Comparison of predictions of the Extended Hierarchical Multi-mode Molecular Stress Function (EHMMSF) model to experimental data of 5 LDPE melts with widely different molecular weights, polydispersities and densities, and a model polystyrene pom-pom polymer shows good agreement within experimental accuracy in constant elongational-rate flow as well as stress relaxation after steady and reversed elongational flow. For the LCB melts considered, we report differences in the specific Hencky strain at the maximum of the tensile stress as quantified by the characteristic stretch {overline{lambda }}_{m}, and we discuss correlations between polydispersity, dilution modulus {G}_{D}, and strain hardening potential of the LDPE melts. We also extend the fracture criterion for brittle fracture of monodisperse polymer melts to the case of polydisperse polymers and find reasonable agreement with experimental evidence.

Melt Intercalation in Montmorillonite/Polystyrene Nanocomposites

AbstractAtactic polystyrene (PS) was used to study the effect of flow field (shear and/or elongational) on the intercalation of polymer/clay nanocomposites (PNC). Three grades of (PS), with different molecular weights, were compounded with an ammonium-modified montmorillonite (Cloisite 10A) in a twin-screw extruder (TSE). The compounds were subsequently fed to a single screw extruder, fitted with one of three specially designed torpedo-attachments. The attachments were designed to provide combinations of different levels of shear and elongational deformations. The resins, TSE compounds, and final PNC’s were characterized for the degree of intercalation, degradation, rheological behavior, and mechanical properties. The data showed that the thermal decomposition of the quaternary ammonium intercalant caused severe damage to both PNC components: a collapse of the organoclay interlayer spacing, and the thermo-oxidative degradation of PS. In spite of these detrimental effects, the attachment employing combined elongational and shear flow resulted in generally larger gallery spacing and more improvement of the mechanical properties than the other two attachments.

Injection Molding of Reinforced Thermosets

Abstract The orientation of glass fibers (initial length 20 mm) and the mold filling of reinforced unsaturated polyester compounds have been studied. Observations of the fiber orientation have been made in three molds where shear and elongational flows occur in variable ways. A part of the fibers are split and broken into filaments. Reinforcement can be more or less buckled. A thin skin layer without fibers is observed. The great thickness of a core region with an orientation perpendicular to the flow direction shows the importance of negative elongational flows. Efficiency of positive elongational flows to suppress buckling and to give a well flow aligned orientation is also observed. On the contrary the efficiency of shear flows is limited compared to elongational flows. The incidence of injection conditions on the pressure inside a rectangular plaque has been studied both experimentally and theoretically. Experimental difficulties due to the material heterogeneity leads to measure viscosity data of the uncured material on a special capillary rheometer and rheological kinetic data on a simplified compound. Numerical results are in good agreement with experimental measurements. For instance the effect of crosslinking which leads to a pressure rise at low flow rate or high mold temperature is well predicted.

Design and Analysis of a Mandrel Process to Control the Properties of Poly(p-Phenylene Terephthalamide) Films

Abstract Kinematic and rheological considerations have been used to design and analyze the processing steps for obtaining biaxial chain orientation in poly(p-phenylene terephthalamide) (PPTA) films. A semi-quantitative approach to the mandrel design allows one to predict the type and degree of molecular orientation obtained. For the case of PPTA dissolved in H2SO4, the basic steps are extrusion of the liquid crystalline solution through an annular die followed by expansion and elongational flow over a mandrel. The mandrels used include conical, hyperbolic and ogival shapes. Molecular orientation as well as mechanical properties were found to be dependent on mandrel shape and presence. Wide angle x-ray diffraction (WAXS) techniques using White-Spruiell orientation factors and pole figures were used to characterize the films. Mechanical properties as well as the above techniques indicate a high degree of biaxial orientation can be obtained with a bias in certain film directions depending on the mandrel used. Tensile strength and Young's modulus values were found to be on the order of 30 000 psi and one million psi respectively. They also can be equally distributed in the film plane depending on mandrel shape. White-Spruiell orientation factors were as high as 0.49 and 0.5 for f 1 B a n d f 2 B $f_{1}^{B}\text{ }and\text{ }f_{2}^{B}$ respectively. Scanning electron microscopy was used to observe the film superstructure as a function or processing conditions. All films exhibited a skin-core structure. Uniaxially drawn films are fibillar while the biaxial films appear to be more homogeneous.

Study on the exfoliation mechanism of graphene nanoplatelets in the polypropylene/graphene nanoplatelets composites under the elongational flow generated by convergent-divergent channels

Study on the exfoliation mechanism of graphene nanoplatelets in the polypropylene/graphene nanoplatelets composites under the elongational flow generated by convergent-divergent channels

Toughening and reinforcing waste polypropylene with POE/nano‐SiO2 under elongational flow

AbstractDue to the white pollution, the recycling of waste plastics has attracted more and more attention. In this research, waste polypropylene/Ethylene‐octene elastomer/nano‐SiO2 composites (wPP/POE/SiO2) were melt‐blended using an Eccentric Rotor Extruder (ERE) dominated by elongational flow field and a Torque Rheometer (TR) dominated by shear flow field, respectively. The micromorphology, mechanical, crystallization, thermal and rheological properties of wPP/POE/SiO2 blends were studied. Compared to the blends fabricated by TR, the wPP/POE/SiO2 blends with different compositions fabricated by ERE showed higher tensile strength, flexural strength, and modulus. Particularly, without sacrificing the tensile strength, the notched impact strength of the wPP/POE/SiO2 blends fabricated by ERE increased to 13.53 kJ/m2, a factor of 3 times larger than that of neat wPP. The enhancement of impact strength of the wPP/POE/SiO2 blends fabricated by ERE could be ascribed to the good dispersion of POE under elongational flow and the increase of PP crystallinity. The finding from this work provides an efficient way to recycle wPP.