Solutions Of Flexible Polymers Research Articles

Comparison of viscoelastic flows in two- and three-dimensional serpentine channels.

Polymer solutions in the dilute regime play a significant role in industrial applications. Due to the intricate rheological properties of these highly viscoelastic fluids, especially in complex flow geometries, a thorough numerical analysis of their flow dynamics is imperative. In this research, we present a numerical investigation of purely elastic instability occurring in two- and three-dimensional serpentine channels under conditions where fluid inertia is negligible and across a broad spectrum of polymer relaxation times. Our findings reveal a strong qualitative agreement between the existing experimental results obtained from dilute solutions of flexible polymers in microfluidic devices and the numerical simulations conducted in two and three dimensions using the Oldroyd-B model. Spatial flow observations and statistical analysis of temporal flow features indicate that this purely elastic turbulent flow exhibits nonhomogeneous, non-Gaussian, and anisotropic characteristics across all scales. Additionally, our comparison of two- and three-dimensional simulations demonstrates that the elastic instability is primarily driven by the curvature of the streamlines induced by the flow geometry, rather than the weak secondary flow in the azimuthal direction. Therefore, our two-dimensional numerical simulations successfully replicate, at least qualitatively, the features observed in three-dimensional experiments. Furthermore, spectral analysis suggests that, in comparison to elastic turbulence in the dilute regime, the range of scales for the excited fluctuations is narrower.

Polymer and surfactant flows through a periodically constricted tube

The flow of three non-Newtonian fluids, comprising polymer and surfactant additives, in a periodically constricted tube (PCT) are experimentally compared. The radius of the tube walls is sinusoidal with respect to the streamwise direction. The three fluids are aqueous solutions of flexible polymers, rigid biopolymers and surfactants, which are typically used for drag-reduction in turbulent flows. Steady shear viscosity measurements demonstrate that rigid and flexible polymer solutions are shear-thinning, while surfactant solutions have a Newtonian and water-like shear viscosity. Capillary driven extensional rheology demonstrates that only flexible polymer solutions produce elastocapillary thinning. Particle shadow velocimetry is used to measure the velocity of each flow within the PCT at five Reynolds numbers spanning roughly 0.5 to 300. Relative to the Newtonian flows, rigid polymer solutions exhibit a blunt velocity profile. Flexible polymer solutions demonstrate a distinct chevron-shaped velocity contour and zones of opposing vorticity when the Deborah number exceeds 0.1. Using the vorticity transport equation, it is revealed that the opposing vorticity zones are coupled with a non-Newtonian torque. The PCT reveals that the surfactant solutions have similar non-Newtonian features as flexible polymer solutions – those being a chevron velocity pattern, opposing vorticity and a finite non-Newtonian torque. This observation is of practical importance since conventional shear and extensional rheometric measurements are not capable of demonstrating non-Newtonian features of the surfactant solutions. The investigation demonstrates that the PCT serves as a viable geometry for showing the non-Newtonian traits of dilute surfactant solutions.

Statistics and spectral analysis of turbulent duct flows with flexible and rigid polymer solutions

Statistics and spectral analysis of turbulent duct flows with flexible and rigid polymer solutions

Microrheological study of single chain dynamics in semidilute entangled flexible polymer solutions: Crossover from Rouse to Zimm modes

The dynamics of polymer chains in entangled semidilute solution have been of theoretical and experimental interest. Among a number of characteristic lengths and times of the polymer in solution, those of the correlation blob are the key to understand the applicability of the Rouse and Zimm models to rheology of the semidilute solution. Direct rheological measurements of Rouse and Zimm modes are limited as the corresponding time scale is out of the range of classical rheological techniques. We investigated the single chain dynamics of entangled poly(ethylene oxide) in semidilute aqueous solutions by high-frequency micro-rheology based on diffusing-wave spectroscopy compared to classical shear macro-rheology. Concentration dependence of the three characteristic times of the entangled polymer chains, reptation time, entanglement time, and correlation time, was studied with the help of the time-concentration superposition. At the low frequency range, dynamic moduli measured by macro-rheology and micro-rheology showed a good agreement without adjustable parameters. At the higher frequency range, we found the Rouse regime in the mean square displacement of the probe particles and in the magnitude of the complex specific viscosity of the solution. We propose a simple method to estimate the boundary of the Rouse regime. Finally, at the high frequency range, we demonstrate that the contribution of the solvent to the solution viscosity needs to be subtracted to observe the power-law behavior of the Zimm mode.

Conformation and Structure of Hydroxyethyl Cellulose Ether with a Wide Range of Average Molar Masses in Aqueous Solutions

The solution properties of a water-soluble chemically modified cellulose ether, hydroxyethyl cellulose (HeC), were examined using static light scattering (SLS), dynamic light scattering (DLS), small-to-wide-angle neutron scattering (S-WANS), small-to-wide-angle X-ray scattering (S-WAXS) and viscometric techniques at 25 °C. The examined HeC samples had average molar substitution numbers ranging from 2.36 to 2.41 and weight average molar masses (Mw) that fell within a wide range from 87 to 1500 kg mol-1. Although the relationship between the determined radius of gyration (Rg) and Mw was described as Rg ∝ Mw~0.6, as is observed usually in flexible polymer solutions in good solvents, the observed scattering vector (q) dependencies of excess Rayleigh ratios were well interpreted using a rigid rod particle model, even in high-Mw samples. Moreover, the ratios of the formed particle length (L) evaluated assuming the model for rigid rods to the determined Rg showed the relationship LRg-1 ~ 3.5 irrespective of Mw and were close to those theoretically predicted for rigid rod particle systems, i.e., LRg-1 = 12. The observed SLS behavior suggested that HeC molecules behave just like rigid rods in aqueous solution. As the L values were not simply proportional to the average molecular contour length calculated from the Mw, the chain conformation or structure of the formed particles by HeC molecules in aqueous solution changed with increasing Mw. The q dependencies of excess scattering intensities observed using the S-WANS and S-WAXS experiments demonstrated that HeC molecules with Mw less than 200 kg mol-1 have a diameter of ~1.4 nm and possess an extended rigid rod-like local structure, the size of which increases gradually with increasing Mw. The observed Mw dependencies of the translational and rotational diffusion coefficients and the intrinsic viscosity of the particle suspensions strongly support the idea that the HeC molecules behave as rigid rod particles irrespective of their Mw.

An Imaging Technique For Characterizing Viscoelasticity Of Drag-Reducing Solutions

Existing rheometric measurement techniques that use load and displacement sensors are unable to resolve the non-Newtonian features of dilute drag-reducing solutions. In the present investigation back-lit particle image velocimetry, or particle shadow velocimetry, was used to quantify the complex dynamics of these fluids in a novel flow geometry. The flow of three non-Newtonian solutions were investigated in a periodically constricted tube (PCT). The radius of the tube walls was sinusoidal with respect to the streamwise direction. The three fluids under consideration were aqueous solutions of a flexible polymer, rigid polymer and surfactant, all of which were previously shown to instill drag reduction in a high Reynolds number, turbulent channel flow. Three solutions with mass concentrations of 0.01%, 0.03% and 0.05% were considered for each additive. Steady shear viscosity measurements demonstrated that all rigid and flexible polymer solutions were noticeably shear-thinning, while the surfactant solutions had a water-like shear viscosity. Each solution was measured at five Reynolds numbers between approximately 1 and 100 within the PCT. Relative to Newtonian fluids within the PCT, the rigid polymer solutions produced a plug-like flow with a blunted velocity and attenuated vorticity at radial coordinate further from the tube centerline. The flexible polymer solution, known to have appreciable amounts of elasticity, demonstrated a distinct chevron shaped velocity contour, coupled with a negative vorticity pattern within the contraction regions of the PCT. Despite having a seemingly Newtonian shear viscosity, the surfactant solutions produced velocity and vorticity patterns reminiscent of the flexible polymer. The general implication is that surfactants share similar elastic traits as flexible polymers. The vorticity transport equation was used to derive distributions of the non-Newtonian torque, more commonly called the `polymer torque' in other literature. It was revealed that the non-Newtonian torque was the source of vorticity field disruption in the flows of flexible polymer and surfactant solutions.

Wetting transitions of polymer solutions: Effects of chain length and chain stiffness.

Wetting and drying phenomena are studied for flexible and semiflexible polymer solutions via coarse-grained molecular dynamics simulations and density functional theory calculations. This study is based on the use of Young's equation for the contact angle, determining all relevant surface tensions from the anisotropy of the pressure tensor. The solvent quality (or effective temperature, equivalently) is varied systematically, while all other interactions remain unaltered. For flexible polymers, the wetting transition temperature Tw increases monotonically with chain length N, while the contact angle at temperatures far below Tw is independent of N. For semiflexible polymer solutions, Tw varies non-monotonically with the persistence length: Initially, Tw increases with increasing chain stiffness and reaches a maximum, but then a sudden drop of Tw is observed, which is associated with the isotropic-nematic transition of the system.

DNA Conformation Dictates Strength and Flocculation in DNA-Microtubule Composites.

Polymer topology has been shown to play a key role in tuning the dynamics of complex fluids and gels. At the same time, polymer composites, ubiquitous in everyday life, have been shown to exhibit emergent desirable mechanical properties not attainable in single-species systems. Yet, how topology impacts the dynamics and structure of polymer composites remains poorly understood. Here, we create composites of rigid rods (microtubules) polymerized within entangled solutions of flexible linear and ring polymers (DNA) of equal length. We couple optical tweezers microrheology with confocal microscopy and scaled particle theory to show that composites with linear DNA exhibit a strongly nonmonotonic dependence of elasticity and stiffness on microtubule concentration due to depletion-driven polymerization and flocculation of microtubules. In contrast, composites containing ring DNA show a much more modest monotonic increase in elastic strength with microtubule concentration, which we demonstrate arises from the decreased conformational size and increased miscibility of rings.

Viscoelastic fluid flow in microporous media

This paper describes an experimental investigation of the flow of flexible polymer solutions through a distinctive micro-porous structure (typical pore sizes of 500 microns) with the primary focus on the interaction of the viscoelasticity of the working fluid with the micro-porous structure. In particular, we relate the bulk flow properties to measurable rheological parameters, demonstrating that a key parameter in estimating the pressure drop through the porous media (for a variety of polymer types, concentrations, solvents, molecular weights, and states of degradation) is the extensional relaxation time (or, more correctly, a characteristic time for extensional stress growth). A Weissenberg number (Wi) is calculated as a product of this extensional relaxation time and the nominal shear rate in the flow. Results, suitably normalised with Newtonian pressure-drop data, show a critical Wi of roughly 0.01 where all working fluids in two different pore structures reveal the onset of elastic dominance over viscous forces as the flowrate increases. Such a low critical value of Wi is due to the estimate of a nominal shear rate based on pore size, which severely underestimates the maximum shear rates within the complex pore structure. Significant deviation from the universal behaviour is observed for high concentrations of the polyacrylamide, which is thought to be a result of shear-thinning for these systems. Systematic degradation of the polyethylene oxide solutions caused an exponential decay in elasticity, which is reflected in the pressure-drop measurements. These imply that, although the flow through porous media is known to be a complex combination of both shear and extensional flow, the extensional effects are of primary importance in this context (or that transient stretching in the media is well represented by this relaxation time) of viscoelasticity. The findings highlight the significance of the transient extensional flow through the neck regions between the pores of the media, which are regions of contraction/expansion.

A direct comparison of turbulence in drag-reduced flows of polymers and surfactants

Abstract

Dielectric dispersion in solutions of flexible polymers

Dielectric dispersion in solutions of flexible polymers

The relationship between viscoelasticity and elasticity.

Soft materials that are subjected to large deformations exhibit an extremely rich phenomenology, with properties lying in between those of simple fluids and those of elastic solids. In the continuum description of these systems, one typically follows either the route of solid mechanics (Lagrangian description) or the route of fluid mechanics (Eulerian description). The purpose of this review is to highlight the relationship between the theories of viscoelasticity and of elasticity, and to leverage this connection in contemporary soft matter problems. We review the principles governing models for viscoelastic liquids, for example solutions of flexible polymers. Such materials are characterized by a relaxation time λ, over which stresses relax. We recall the kinematics and elastic response of large deformations, and show which polymer models do (and which do not) correspond to a nonlinear elastic solid in the limit λ → ∞. With this insight, we split the work done by elastic stresses into reversible and dissipative parts, and establish the general form of the conservation law for the total energy. The elastic correspondence can offer an insightful tool for a broad class of problems; as an illustration, we show how the presence or absence of an elastic limit determines the fate of an elastic thread during capillary instability.

Correction to Unified Entanglement Scaling for Flexible, Semiflexible, and Stiff Polymer Melts and Solutions

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionNEXTORIGINAL ARTICLEThis notice is a correctionCorrection to Unified Entanglement Scaling for Flexible, Semiflexible, and Stiff Polymer Melts and SolutionsScott T. Milner*Scott T. MilnerMore by Scott T. MilnerCite this: Macromolecules 2020, 53, 20, 9152Publication Date (Web):October 15, 2020Publication History Published online15 October 2020Published inissue 27 October 2020https://doi.org/10.1021/acs.macromol.0c02231Copyright © 2020 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views534Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (420 KB) Get e-Alerts Get e-Alerts

Ring-Patterned Perovskite Single Crystals Fabricated by the Combination of Rigid and Flexible Templates.

Regular microstructures can improve the electrical and optical characteristics of perovskite single crystals because of the removal of defects and grain boundaries. Microstructured single crystals are commonly fabricated by either rigid or flexible templates. However, rigid templates usually need surface treatment before crystal fabrication to create an antiadhesion layer, while flexible templates encounter difficulties in achieving a large area of uniform single crystals without any deformation. In this work, we present a facile and robust method to fabricate perovskite single crystals using rigid silicon pillars coated with flexible polymer solutions, in which surface treatment is avoided in the preparation process, and deformation is absent in the formed crystals. The method realized the fabrication of colorful concentric-ring patterns composed of nanoscale single crystals for the first time. In order to concisely control the preparation of the template, the Newton's ring phenomenon was used to value the droplet height because the number of rings changed with the optical path difference. A related digital simulation was performed to find the correlation of the Newton's ring pattern with the shape of the droplets. The simulated results were consistent with the experimental observations generally, indicating that the pattern could be controlled mechanically. Concomitantly, the resulting perovskite nanoscale single crystals formed a regular colorful concentric-ring pattern. By changing the design of the rigid templates, the parameters of the fabrication process, or the selection of the coating polymer solution, different ring-patterned single crystals were successfully prepared without surface treatment and deformation. The crystals have potential applications in lasers or photodetectors.

Unified Entanglement Scaling for Flexible, Semiflexible, and Stiff Polymer Melts and Solutions

The entanglement length Ne is a key parameter for all entangled polymer fluids for which no comprehensive scaling theory yet exists. We have pieces of a theory; the Lin-Noolandi (LN) argument predicts Ne scaling for flexible chains that agrees with data on polymer melts. There are arguments for how Ne should depend on polymer concentration, but which are not obviously consistent with LN. Morse scaling describes entanglement for solutions of stiff chains, consistent with data. Everaers proposed an ansatz that Ne depends only on the arclength concentration, as if chains were uncrossable threads of vanishing thickness. This ansatz is consistent with simulations of bead-spring chains, but not with LN, as it has no role for packing length, the central parameter in LN scaling. We propose a comprehensive scaling theory that includes LN in one limit, thread ansatz in another, and reduces to Morse scaling for stiff chains. One new ingredient is that the typical distance of closest approach between two chains is governed by the packing length or chain diameter, whichever is larger. If a chain is sufficiently flexible and bulky, the packing length is relevant; but for stiffened bead-spring chains without side groups, the packing length is smaller than the chain diameter, so thread scaling applies. Our approach presents a consistent physical picture of entanglements in all regimes as close encounters between two chains. For solutions, we determine the entanglement probability between chain segments, and consistently describe the crossover between the Edwards and semidilute regimes.

Drag reduction in turbulent flows by diutan gum: A very stable natural drag reducer

We study the ability of diutan gum (DG), a polysaccharide that belongs to a class called sphingans, to improve drag reduction (DR) in turbulent flows. This new drag reducer is very stable. Its efficiency in terms of DR does not significantly fall, which means that the polymer degradation or de-aggregation is not important at all. The onset of DR is independent of Reynolds number and concentration, typical from rigid materials for which the DR mechanism is called Type-B. The efficiency of DG solutions was compared with those of xanthan gum (XG) (rigid polymer) and polyethilene oxide (PEO) (flexible polymer) solutions.

Macromolecular crowding effects in flexible polymer solutions

Biological environments are often “crowded” due to high concentrations (300–400[Formula: see text]g/L) of macromolecules. Computational modeling approaches like Molecular Dynamics (MD), rigid-body Brownian Dynamics and Monte Carlo simulations have recently emerged, which allow to study the effects macromolecular crowding at a microscopic level and to provide complementary information to experiments. Here, we use a recently introduced multiple-conformation Monte Carlo (mcMC) approach in order to study the influence of intermolecular interactions on the structural equilibrium of flexible polyethylene glycol (PEG) polymers under self-crowding conditions. The large conformational space accessible to PEG polymers allows us to evaluate the general applicability of the mcMC approach, which describes the intramolecular degrees of freedom by a finite-size ensemble of discrete conformations. Despite the simplicity of the approach, we show that influences of intermolecular interactions on the intramolecular free energy surface can be described qualitatively using mcMC. By varying the magnitude of distinct terms in the intermolecular potential, we can further study the compensating effects of repulsive and nonspecific attractive intermolecular interactions, which favor compact and extended polymer states, respectively. We use our simulation results to derive an analytical model that describes the effects of intermolecular interactions on the stability of PEG polymer conformations as a function of the radius of gyration and the corresponding solvent accessible surface. We use this model to confirm the role of molecular surfaces for attractive interactions that can counteract excluded volume effects. Extrapolation of the model further allows for the analysis of scenarios that are not easily accessible to direct simulations as described here.

Effect of stretching-induced changes in hydrodynamic screening on coil-stretch hysteresis of unentangled polymer solutions

Extensional rheometry and Brownian Dynamics simulations of flexible polymer solutions confirm predictions based on blob concepts that coil--stretch hysteresis in extensional flows increases with concentration, reaching a maximum at the critical overlap concentration $c^\ast$ before progressively vanishing in the semidilute regime. These observations demonstrate that chain stretching strengthens intermolecular hydrodynamic screening in dilute solutions, but weakens it in semidilute solutions. Flow can thus strongly modify the concentration dependence of viscoelastic properties of polymer solutions.

Influence of shear on globule formation in dilute solutions of flexible polymers.

Polyelectrolytes, polymers in poor solvents, polymers mixed with particles, and other systems with attractions and repulsions show formation of globules/structures in equilibrium or in flow. To study the flow behavior of such systems, we developed a simple coarse-grained model with short ranged attractions and repulsions. Polymers are represented as charged bead-spring chains and they interact with oppositely charged colloids. Neglecting hydrodynamic interactions, we study the formation of compact polymer structures called globules. Under certain conditions, increase in shear rate decreases the mean first passage time to form a globule. At other conditions, shear flow causes the globules to breakup, similar to the globule-stretch transition of polymers in poor solvents.

Formation of nanofibers in Poly(9,9-dioctylfluorene) toluene solutions during aging

We report the formation of β-phase nanofibers of poly(9,9-dioctylfluorene) (PF8) in aged toluene solution at low temperature and link their photophysical properties and rheological behaviors with the solution structures. By cooling the PF8 toluene solution and subsequently aging, the PF8 chains develop into nanofiber morphology and the effects of aging time, temperature, and solution concentration on the nanofiber formation are investigated. The formation and development process of PF8 β-phase in aged solution captured by TEM has not been reported in previous studies. Further increasing the solution concentration leads to the gelation of PF8 after aging at low temperature. It is interesting to show that a scaling law correlating specific viscosity with the polymer concentration and intrinsic viscosity originally proposed for flexible polymer solutions also exists for this semiflexible PF8 polymer solution. This study may offer an effective way to improve the understanding of the formation mechanism of PF8 β-phase and facilitate the ongoing exploration of using such nanofiber networks in electronic devices. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 633–639