Elongational Flow Research Articles

Response of Sulfonated Polystyrene Melts to Nonlinear Elongation Flows

Ionizable polymers form dynamic networks with domains controlled by two distinct energy scales, ionic interactions and van der Waals forces; both evolve under elongational flows during their processing into viable materials. A molecular level insight of their nonlinear response, paramount to controlling their structure, is attained by fully atomistic molecular dynamics simulations of a model ionizable polymer, polystyrene sulfonate. As a function of increasing elongational flow rate, the systems display an initial elastic response, followed by an ionic fraction-dependent strain hardening, stress overshoot, and eventually strain-thinning. As the sulfonation fraction increases, the chain elongation becomes more heterogeneous. Flow-driven ionic assembly dynamics that continuously break and reform control the response of the system.

Instantaneous Formation of Silk Protein Aerosols and Fibers with a Portable Spray Device Under Ambient Conditions.

A variety of artificial silk spinning approaches have been attempted to mimic the natural spinning process found in silkworms and spiders, yet instantaneous silk fiber formation with hierarchical structure under physiological and ambient conditions without post-treatment procedures remains unaddressed. Here, we report a new strategy to fabricate silk protein-based aerosols and silk fibers instantaneously (< 1 s) in situ using a simple, portable, spray device, avoiding complicated and costly advanced manufacturing techniques. The key to success is the instantaneous conformational transition of silk fibroin from random coil to β-sheet right before spraying by mixing silk and polyethylene glycol (PEG) solutions in the spray device, allowing aerosols and silk fibers to be sprayed in situ, with further control achieved via the molecular weight of silk. The spinning process of the spray device is based on the use of green solvents, i.e., all steps of instant conformational transition of silk fibroin are carried out in aqueous conditions or with buffers at ambient conditions, in combination with shear and elongational flow caused by the hydraulic pressure generated in the spray container. The system supports a portable and user-friendly system that could be used for drug delivery carriers, wound coating materials and rapid silk fiber conformal coatings on surfaces.

A strategy of stretching melt to a thin layer: Self-controlled “stretching melt-pancakes” to enhance heat transfer and mixing during polymer extrusion

A novel fabrication strategy with “stretching melt-pancakes” was proposed to enhance heat transfer and mixing capacity during polymer extrusion to resolve a growing challenge of inadequate control of mass transfer and heat management. Finite element analysis of the newly developed screw configurations, melt self-elongation screw, had been carried out and compared with the conventional screws (e.g., single-thread screw; double-thread screw) and the melt self-torsion screw developed by us. Results reveal that it is an effective way to tailor desired flow pattern by controlling the screw configurations. The melt self-elongation screw distinctly introduced radial elongational flow, which is a meaningful improvement on flow patterns established by conventional screws. Elongational deformation induced by the converging wedge-shaped channel of melt self-elongation screw, can enhance the radial mass and heat transfer, obtaining effective mixing of local heat and particles. Sufficient viscous heat generation, effective heat transfer as well as good conveying capacity were achieved by its unique structures of melt self-elongation screw, improving the melting efficiency and extrusion quality. Besides, the radial motion in both melt self-torsion screw and melt self-elongation screw contributed to the synergy interaction between flow field and thermal field, coinciding with the field synergy principle.

Complex polymer topologies in blends: Shear and elongational rheology of linear/pom-pom polystyrene blends

The shear and elongational rheology of linear and pom-pom shaped polystyrene (PS) blends was investigated experimentally and modeled using constitutive models such as the Doi–Edwards and the molecular stress function (MSF) model. The pom-pom molecule is the simplest topology to combine shear thinning with strain hardening in elongational flow. A PS pom-pom with a self-entangled backbone (Mw,bb = 280 kg mol−1) and 22 entangled sidearms (Mw,a = 22 kg mol−1) at each star was blended with two linear PS with weight average molecular weights of Mw = 43 and 90 kg mol−1 and low polydispersities (Ð &amp;lt; 1.05). A semilogarithmic relationship between the weight content of the pom-pom, ϕpom-pom, and the zero-shear viscosity was found. Whereas the pure pom-pom has in uniaxial elongational flow at T = 160 °C strain hardening factors (SHFs) of SHF ≈100, similar values can be found in blends with up to ϕpom-pom = 50 wt. % in linear PS43k and PS90k. By blending only 2 wt. % pom-pom with linear PS43k, SHF = 10 can still be observed. Furthermore, above ϕpom-pom = 5–10 wt. %, the uniaxial extensional behavior can be well-described with the MSF model with a single parameter set for each linear PS matrix. The results show that the relationship between shear and elongational melt behavior, i.e., zero-shear viscosity and SHF, can be uncoupled and customized tuned by blending linear and pom-pom shaped polymers and very straightforwardly predicted theoretically. This underlines also the possible application of well-designed branched polymers as additives in recycling.

Stretch-orientation-induced reduction of friction in well-entangled bidisperse blends: a dual slip-link simulation study

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.

Micro-structure and tensile properties of microfluidic spinning konjac glucomannan and sodium alginate composite bio-fibers regulated by shear and elongational flow: experiment and multi-scale simulation

Microfluidic spinning has been widely used to produce bio-fibers with excellent tensile performances by regulating the conformation of biological macromolecules. However, the effect of channel shapes on fiber tensile performances is unclear. In this study, bio-fibers were prepared using konjac glucomannan and sodium alginate by five channels. The micro-morphology and tensile performance of fibers were characterized and measured. Then, the dynamical behaviours of macromolecule clusters in flow fields were simulated by multi-scale numerical methods. The results show that the elongational flow with increasing extension rates produced fibers with a tensile strength of 32.34 MPa and a tensile strain of 18.72 %, which were 1.37 and 1.55 times that for a shear flow, respectively. The difference in tensile performances was attributed to the micro-morphology regulated by flow fields. The continuously increasing extension rate of flow was more effective than the shear rate or the maximum extension rate for the stretching of macromolecule clusters. We conclude that the channel shapes significantly influence flow fields, dynamical behaviours of molecule clusters, the morphology of fibers, and tensile performances. This study provides a novel numerical method and understanding of microfluidic spinning, which will promote the optimization and applications of bio-fibers.

Investigation the effect of processing condition and dispersed phase content on rheology–morphology relationship in polypropylene/polyethylene terephthalate microfibrillar composite

AbstractMorphological evolutions during the production of microfibrillar composites can be investigated by considering two main steps: (1) the melt blending in the extruder and (2) the drawing process. Deformation, coalescence and break‐up of dispersed phase droplets under shear and elongational flow fields applied in these two main steps lead to the formation of fibrillar morphology. Therefore, it can be stated that the fibrillar morphology is controlled by dispersed phase content, rheological and interfacial properties of the components as well as dispersed phase domains relaxation. In this study, the effect of dispersed phase content on the morphology of the polypropylene/polyethylene terephthalate microfibrillar composite was investigated. Also, three following processing parameters were considered to evaluate the kinetics of in‐situ formation of the microfibrils: Extruder die temperature, water bath temperature and the drawing path length. Linear and nonlinear rheological tests were used as an efficient tool to investigate the rheology–morphology relationship. Among the processing parameters studied, the drawing path length and die temperature had the most and least effect on microfibrillar morphology, respectively. By increasing the drawing path length from 20 to 30 cm, the break‐up of the fibrils and non‐uniformity in the cross section of the fibrils caused a weaker strain recovery after stress removal and also a rapid reduction of stress after cessation of steady shear flow field. Increasing the die temperature has a similar but weaker effect on the final morphology. The rheological results in the linear viscoelastic range all indicated the formation of a physical network of microfibrils within the matrix.

Improving stretchability of associative polymers through tuning density of the secondary interactionsa)

The macroscopic stretchability of ionomer melts strongly relies on the structural evolution during the elongational flow. It has been noted that the introduction of the secondary associations weaker than the ionic association can improve the stretchability. To understand the mechanism, this study examines the stretchability of unentangled ionomers containing a fixed number of ionic groups per chain, fion≅4, but a varied number of hydrogen bonds per chain, fH = 5.5–27. The stretchability that is reflected in the maximum Hencky strain achieved before rupture shows nonmonotonous change with fH: the stretchability improves with increasing fH from 5.5 to 14 while it decreases upon further increasing fH to 27. The former improvement is attributed to the slowing down of chain retraction after the strain-induced dissociation of ionic groups. The slowing down would suppress the formation of defects or small cracks that potentially grow into the fracture. This mechanism, i.e., strain-induced dissociation followed by the chain retraction, holds only in a window where the elongational rate is faster than the ionic dissociation rate but slower than the chain retraction rate. This window narrows down with increasing fH, which probably leads to the decrease of stretchability at high fH = 27.

Flow behavior of impounded fly ash slurries using flow test

Coal ash is one of the largest industrial waste products generated in the United States. Since 1967, several failures in ash impoundments have been reported, releasing millions of gallons of impounded fly ash slurry and resulting in loss of life and mass environmental destruction. Several studies have been carried out to understand the flow behavior of fly ash slurries. However, several questions remain unanswered regarding the flow behavior of fly ash slurries after containment loss. In this study, a new flow test, a miniature version of a cylindrical slump test, was developed to investigate flow behavior of six different fly ash slurries. The flow behavior of these slurries was evaluated based on the yield stress derived from the flow test results. The results showed that some fly ash slurries flowed at higher solids content (>60 %), while others flowed at lower solids content (<50 %) and stopped flowing at higher solid content. Furthermore, some slurries did not flow at all. It was found that the chemical compositions of fly ash, the state of hydration (hydrated or non-hydrated), original solids content when received, and initial solids content of fly ash slurries at the start of the flow test played a major role in the flow behavior of fly ash slurries. The interactions between these factors were complex, and it was difficult to identify the effect of a single factor. Most of the fly ash slurries showed shear flow, and some slurries at high solids content showed elongational flow. The fly ash slurries with shear flow had larger flow areas after containment loss. The addition of an anionic polyacrylamide significantly increased the yield stress by 2.8 – 3.5 times and decreased the flow of fly ash slurries.

Non-isothermal direct bundle simulation of SMC compression molding with a non-Newtonian compressible matrix

Compression molding of Sheet Molding Compounds (SMC) is a manufacturing process in which a stack of discontinuous fiber-reinforced thermoset sheets is formed in a hot mold. The reorientation of fibers during this molding process can be either described by macroscale models based on Jeffery's equation or by direct mesoscale simulations of individual fiber bundles. In complex geometries and for long fibers, direct bundle simulations outperform the accuracy of state-of-the-art macroscale approaches in terms of fiber orientation and fiber volume fraction. However, it remains to be shown that they are able to predict the necessary compression forces considering non-isothermal, non-Newtonian and compaction behavior. In this contribution, both approaches are applied to the elongational flow in a press rheometer and compared to experiments with 23% glass fiber volume fraction. The results show that both models predict contributions to the total compression force and orientation reasonably well for short flow paths. For long flow paths and thick stacks, complex deformation mechanisms arise and potential origins for deviation between simulations models and experimental observations are discussed. Furthermore, Jeffery's basic model is able to predict orientations similar to the high-fidelity mesoscale model. For planar SMC flow, this basic model appears to be even better suited than the more advanced orientation models with diffusion terms developed for injection molding.

Constructing synergistically strengthening-toughening 3D network bundle structures by stereocomplex crystals for manufacturing high-performance thermoplastic polyurethane nanofibers reinforced poly(lactic acid) composites

As one of the most promising biodegradable materials, poly(lactic acid) (PLA) is seriously restricted by its notorious inherent brittleness and weak heat distortion resistance. In this work, a novel and simple methodology is proposed using an eccentric rotor extruder (ERE) with a predominantly elongational flow field to manufacture high-performance engineered poly(L-lactic acid)/poly(D-lactic acid)/thermoplastic polyurethane nanofiber composites on an industrial scale. The oriented tough TPU nanofibers (TNFs), rigid integrated PLA hybrid crystals and their good interface compatibility produce a strengthening-toughening bundle structure. When the PDLA content reached 3 wt%, the bundle structures interlinked with each other to form a strengthening-toughening 3D network. This resulted in a high-performance engineered 80L/5D/15T nanofiber composite with super toughness of 74.1 kJ/m2, high strength of 47.3 MPa and superior Young's modulus of 1378 MPa, which are 28.6 times, 85% and 92.7%, respectively, compared with neat PLLA. Compared with those published literatures, the 80L/5D/15T nanofiber composite not only possesses super toughness (more than 28 times) but also maintains good strength (maintain 85%), additionally good heat distortion resistance was also obtained. The performance of 80L/5D/15T nanofiber composite was significantly superior in comparison with biodegradable plastics as well as petroleum-based non-degradable general-purpose plastics and engineering plastics, demonstrating its enormous potential as a substitute structural material in harsh environments. It is believed that the novel and industrial scale methodology opens new perspectives for manufacturing other high-performance engineered biodegradable materials.

Elongational viscosity of poly(propylene carbonate) melts: tube-based modelling and primitive chain network simulations

In fast elongational flows, linear polymer melts exhibit a monotonic decrease of the viscosity with increasing strain rate, even beyond the contraction rate of the polymer defined by the Rouse time. We consider two possible explanations of this phenomenon: (a) the reduction of monomeric friction and (b) the reduction of the tube diameter with increasing deformation leading to an Enhanced Relaxation of Stretch (ERS) on smaller length scales. (Masubuchi et al. (2022) reported Primitive Chain Network (PCN) simulations using an empirical friction reduction model depending on segmental orientation and could reproduce the elongational viscosity data of three poly(propylene carbonate) melts and a polystyrene melt. Here, we show that the mesoscopic tube-based ESR model (Wagner and Narimissa 2021) provides quantitative agreement with the same data set based exclusively on the linear-viscoelastic characterization and the Rouse time. From the ERS model, a parameter-free universal relation of monomeric friction reduction as a function of segmental stretch can be derived. PCN simulations using this friction reduction relation are shown to reproduce quantitatively the experimental data even without any fitting parameter. The comparison with results of the earlier PCN simulation results with friction depending on segmental orientation demonstrates that the two friction relations examined work equally well which implies that the physical mechanisms of friction reduction are still open for discussion.

Control of graphene nanoplatelets at the interface of the co-continuous polypropylene/polyamides 6 blend under the elongational flow

Controlling the filler selective distribution at the interface of co-continuous polymer blend is an effective way to fabricate high performance composites. In this work, the graphene nanoplatelets (GNPs) were distributed at the interface of co-continuous polypropylene (PP)/polyamides 6(PA6) blend by melt blending under the elongational flow generated by convergent-divergent channels. And the formation of co-continuous structure and migration mechanism of GNPs in the PP/PA6 blend under the elongational flow were discussed based on the analysis of rheological measurement, scanning electron micrographs, selective etching experiment and thermal conductivity measurement. The results show that both PP and PA6 phase are elongated into fibrous phase structure under the elongational flow. This indeed enlarge the volume ratio range of composites forming co-continuous structure. And GNPs are oriented along the phase interface and stay at the interface during migration under the elongational flow, which can improve its in-plane thermal conductivity. The in-plane thermal conductivity of the PP/PA6/GNPs composites prepared under the elongational flow are all higher than those prepared under the shear flow. Particularly, the composite prepared under the elongational flow with the 65/35 vol ratio of PP/PA6 still kept a co-continuous structure and the GNPs distributed at the interface. Its in-plane thermal conductivity reached a maximum value of 1.13 W/m K.

A mixing device with phase transition for enhanced exfoliation and dispersion in composites fabrication

In this study, a novel mixing device under convergent–divergent elongational flow is developed, mainly contains vane conveying unit and wedge channel unit, generating multi-source pulsating pressure in the mixing procedure to induce phase transition. During the design process, some mathematical works and simulations are conducted to optimize the structural design. High-density polyethylene (HDPE)/expanded graphite (EG) composites are fabricated by phase transition assisted melting mixing. The composites feature lower melt resistance and improved tensile modulus. Furtherly, the prepared HDPE/EG composites exhibit a greater increment of dielectric constant when adding more EG, but always maintain low dielectric loss about 0.05 in low frequency. These results show the excellent exfoliation and dispersion effect, confirming that this new designed equipment and phase transition effect complement each other with fabulous effect in the preparation of filler reinforced polymer.

Topological Transition in Multicyclic Chains with Structural Symmetry Inducing Stress-Overshoot Phenomena in Multicyclic/Linear Blends under Biaxial Elongational Flow

Blends of multicyclic and linear polymers under biaxial elongational flow were analyzed using coarse-grained molecular dynamics simulations. The multicyclic/linear blends displayed overshoot in the normal stress difference at the start-up of biaxial elongational flow. This overshoot was steeper for multicyclic/linear blends than for our previously reported monocyclic/linear blends [T. Murashima, K. Hagita, and T. Kawakatsu, Macromolecules, 2021, 54, 7210]. Investigation of the origin of the overshoot in the multicyclic/linear blends revealed a different mechanism than that previously observed in our monocyclic/linear blends. Specifically, a "topological transition" mechanism comprising a morphological change from the open- to closed-ring state was observed in the multicyclic chains, but not in the monocyclic chains. This topological transition drastically changes the stress of the rings. Although the topological transition was also observed in asymmetric-multicyclic/linear blends, no stress overshoot appeared, owing to the asymmetry in the multicyclic chains. Therefore, we hypothesized that the structural symmetry in multicyclic chains is indispensable for overshoot behavior to occur. We determined that the topological transition in multicyclic chains with structural symmetry induces stress-overshoot behavior in multicyclic/linear blends under biaxial elongational flow.

Serpentine Micromixers Using Extensional Mixing Elements.

Computational fluid dynamics modeling was used to characterize the effect of the integration of constrictions defined by the vertices of hyperbolas on the flow structure in microfluidic serpentine channels. In the new topology, the Dean flows characteristic of the pressure-driven fluid motion along curved channels are combined with elongational flows and asymmetric longitudinal eddies that develop in the constriction region. The resulting complex flow structure is characterized by folding and stretching of the fluid volumes, which can promote enhanced mixing. Optimization of the geometrical parameters defining the constriction region allows for the development of an efficient micromixer topology that shows robust enhanced performance across a broad range of Reynolds numbers from Re = 1 to 100.

Threading Polystyrene Stars: Impact of Star to POM‐POM and Barbwire Topology on Melt Rheological and Foaming Properties

AbstractPolymer topology highly impacts rheological melt and foaming properties. Grafting sidechains onto a linear polymer typically results in shear thinning and strain hardening in elongation flow. To study the influence of an increasing number of covalently connected polystyrene (PS) starsswith a linear PS chain (Mw,PS= 90 kg mol−1), low‐disperse branched polymers withsranging from one to four are synthesized. Each star contains approximately 12 sidechains with a length ofMw,a≈ 25 kg mol−1. Oscillatory shear measurements indicated that the zero‐shear viscosityη0scales withη0 ≈ atTref= 180 °C. Moreover, uniaxial elongation rheology allows determining the strain hardening factorSHF, which varies betweenSHF= 2–135, with increasings. Foaming experiments revealed that combining viscosity reduction with the improvement of stretchability promotes higher volume expansion ratios (VER= 3.21–10.41) and the formation of larger cells (D= 4.8–14.8 µm).

Efficient fabrication of low-density polyethylene/polyethylene oxide/carbon nanotubes films with robust shape stability and photothermal property for thermal management and afterheat utilization

The smart energy absorption and storage properties make the phase change materials the ideal candidate for solar thermal conversion and afterheat utilization. Applying the vane extruder (VE) coupled with a single-layer blown film die, a fast and efficient one-step method is proposed for the successive and mass production of LDPE/PEO/CNTs (PPCs) nanocomposite films. Benefiting from the volumetric elongational flow field provided by the VE, the CNTs are uniformly dispersed in the matrix and a co-continuous network is formed, thereby the thermal conductivity and tensile strength of the PPCs film are effectively improved. The melting enthalpy of PPC0.02 film is 110 J g−1 and tensile strength is 3.308 MPa. Furthermore, the PPCs films maintain stable phase change temperature and robust chemical properties after 150 thermal cycles. The temperature of the simulated greenhouse can be controlled within the range that is conducive to crop growth when the PPCs film is applied to its roof. This can be ascribed to the sufficient light-to-thermal conversion capability and thermal conductivity of the PPCs film. The PPCs film with sufficient shape stability and leakage resistance can be an excellent candidate for harvesting and recycling solar and heat energy sources.

Advanced thermoplastic polyurethane/hexagonal boron nitride thermal conductors via elongational flow from the aspect of experiment and simulation

In this work, a novel melt blending approach for fabricating thermoplastic polyurethane (TPU)/hexagonal boron nitride (h-BN) composites using an elongational flow method was proposed. With 50 wt% of h-BN, the thermal conductivity (TC) of composite produced by elongation flow is 2.9 W/(m·K), compared with 2.41 W/(m·K) for the composite produced by conventional shear flow method, indicating a significant improvement was obtained. The peeling of h-BN, which is responsible for increased TC, was proved and the peeling mechanism was proposed. Meanwhile, the effect of changed h-BN geometrical dimension during the peeling process on TC was investigated by lapped simulation. The result shows that thinner sheet thickness brings a higher lapped rate and a longer heat conduction path at the same h-BN content. Additionally, the importance of maintaining lateral dimensions is proved by the simulation. This strategy paved a new way to manufacture high thermal conductivity 2D nanosheet composites with a green and efficient method.

Forming a composite model for non-Brownian suspensions

We propose a two-part composite model to describe the rheology of non-Brownian suspensions. The stress response is composed of the sum of a matrix part (Tm) described by a multi-mode Oldroyd-B model and a second component (To) which is assumed to be a Thompson–Souza Mendes model. We show how to determine the parameters to satisfy agreement with experiments in steady viscometric flows, uniaxial elongation flows, small to medium size sinusoidal strains, and reversing shear strain rates. Where possible, comparison is made with computations. Agreement with experiments and computations is reasonable, but more accurate computations and experiments would be welcome.