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Our recent continuum theory for stress-gradient-induced migration of polymers in confined solutions, including the depletion from the solid boundaries [Hajizadeh, E., and R. G. Larson, Soft Matter, 13, 5942–5949 (2017)], is applied to a two-dimensional rotational shearing flow in the gap between eccentric cylinders. Analytical results for the steady-state distribution of polymer dumbbells in the limit of dilute polymer solution c/c∗≪1 (c∗ is the chain overlap concentration) and in the absence of hydrodynamic interactions are obtained. The effects of eccentricity e and of three perturbation variables, namely, Weissenberg number Wi, gradient number Gd(which defines the level of polymer chain confinement), and Peclet number Pe on the polymer concentration pattern, are investigated. The stress-gradient-induced migration results in polymer migration toward the inner cylinder, while wall-depletion-induced migration results in near-zero polymer concentration close to flow boundaries, which couples to a stress-gradient-induced migration effect. In the presence of wall-depletion, we obtain first order concentration variation proportional to Wi. However, in the absence of wall-depletion, there is no first order contribution and, therefore, the lowest-order concentration variation is proportional to Wi2. An upper limit of Wi=1.6 exists, beyond which the numerical solution demands an excessive under-relaxation to converge. In addition, for a high degree of polymer chain confinement, i.e., for Gd greater than 0.5, the continuum theory fails to be accurate and mesoscopic simulations that track individual polymer molecules are needed.

The linear viscoelasticity of coacervates formed by oppositely charged polyelectrolytes in the salt solution is reviewed, with a focus on time-temperature, time-salt, time-pH, and time-hydration superpositions, and on fundamental relaxation mechanisms. A variety of polyelectrolyte pairs are covered, showing the frequent, but not universal, success of the time-salt superposition. Master curves in many cases are similar to those for neutral polymers, including Rouse and reptation theories. However, in some cases, solidlike, as opposed to fluidlike, response is observed at low frequencies, especially at low salt concentrations. Some coacervates seem to fit “sticky diffusion” theory reasonably well, wherein relaxation is controlled by the breakage rate of ion pairs; the dependence of the “sticker” lifetime on salt concentration has been explored but is not well understood as yet. It is also possible that local relaxation is not controlled by breakage of ion pairs but by cooperative, “glassy,” relaxation of monomers, salt ions, and water molecules. Compilation and comparison of different datasets and suggested formulas for rheological time constants are presented, and some suggestions are given for future directions.

We develop a method for efficient prediction of linear and nonlinear rheology of polydisperse polymers by judicious selection of a small number of representative polymer molecules from the large ensemble of chains comprising the molecular weight and branching distribution. Specifically, we use a numerical inversion of the double reptation model to select five or six representative molecular species to optimally fit the linear rheology of commercial polymers and then use the regressed parameters to compute the nonlinear rheology via the Rolie-double-poly (RDP) model. The method is tested for several model systems, namely, two polydisperse linear polystyrene (PS) samples described in Shivokhin et al. [Polym. Eng. Sci. 56, 1012-1020 (2016)] and Münstedt [J. Rheol. 24, 847-867 (1980)], a commercial linear low density polyethylene (LLDPE), and a low density polyethylene (LDPE) whose rheological properties are measured in the current study. For the linear polymers including the PS samples and the LLDPE, the predictions of the "representative"RDP model of the start-up extensional rheology are comparable to those of the "full"RDP model based on all the species drawn from the gas permeation chromatography characterization. The method then successfully predicts the linear rheology of blends of LLDPE and LDPE using the same representative molecules found by fitting each of the pure polymers. Further fitting the extensional rheology of the LDPE requires using the "priorities"q i and stretch relaxation times τ s, i of the representative molecules as adjustable parameters, whose values are then held fixed when predicting the extensional rheology of blends of the LDPE with the LLDPE roughly as successfully as does branch-on-branch model. The reduction in the number of representative polymer species offers new opportunities for faster simulations of flowing polymers, as well as for the prediction of segmental orientation to be used in the modeling of flow-induced crystallization.

We perform united atom molecular dynamics (MD) simulations to quantify the role of flow-induced nematic order in polyethylene (PE) crystal nucleation. Blends of periodic stretched and nonperiodic u...

We extend the single-chain slip-spring model developed by Likhtman [Macromolecules 38, 6128 (2005)] to describe the dynamics and rheology of entangled polymers to wormlike micellar solutions by incorporating chain breakage and rejoining, which are the key additional dynamics present in wormlike micellar solutions. We show that the linear rheological properties obtained from this micelle slip-spring model are in good agreement with mesoscopic simulations using the “pointer algorithm” [W. Zou and R. G. Larson, J. Rheol. 58, 681 (2014)] and can be fit to experimental results after an adjustment to correct for the too-high flexibility of the micelles assumed in the slip-spring model. Finally, we use this model to predict the nonlinear rheological properties of entangled wormlike micelles, which are the first predictions that include the effects of entanglements, breakage and rejoining, Rouse modes, and stretch of bead-spring micellar chains with Hookean springs.

The time-dependent rates of growth of thickness of wax deposits, as well as the spatial distributions of wax components within the deposits, formed on a cold finger from stirred baths of n-dodecane...

The authors report the rheology and shear flow velocity profiles of pseudo-ternary “lamellar gel networks” commonly found in cosmetic emulsions such as creams, lotions, and hair conditioners, consisting of the cationic surfactant behentrimonium methosulfate, a fatty alcohol mixture of cetyl and stearyl alcohol, and water at surfactant/fatty alcohol mole fractions ranging from 0.1 to 0.6 and water mass fractions of 85%–97%. The high shear-thinning viscosity shows hysteresis in up- and down-ramps of the shear rate, with the gel “remembering” its highest shear for around 12 h before it heals and recovers its original viscosity. These materials exhibit rheological signatures characteristic of a soft glassy material (as described by the soft glassy rheology model) such as shear hysteresis, inhomogeneous velocity gradient, a solidlike creep response, power-law stress decay in step strain experiments, and high storage modulus relative to loss modulus over a wide range of frequencies, confirming its nearly elastic response for small deformations. Using particle image velocimetry (PIV) in steady shear experiments, a transition from a linear velocity profile to a pluglike profile was observed at shear rates above around 0.01 s−1. PIV also revealed macroscopic inhomogeneities including the development of fracture planes and shear bands, with rheology becoming gap-dependent and displaying some characteristics of solidlike friction.

Using both coarse-grained (CG) and fine-grained (FG) simulations we show how strain hardening in polymeric glasses under uniaxial extension arises from highly stretched strands that form as the pol...

We blend newly synthesized nearly monodisperse four-arm star 1,4-polybutadienes with various well-entangled linear polymers, confirming the conclusions in Desai et al. [ Macromolecules201649 (13)49...

We combine the self-consistent field theory and the Derjaguin approximation to predict the polymer-induced colloidal interactions and the nonuniform distributions of the loops and bridges of telech...

We review thixotropy, its attributes, and accompanying rheological phenomena, such as yielding, hysteresis in shear-rate ramps, the influence of rest time and viscosity bifurcation, and the prevalence and importance of thixotropy in common fluids. While older work is reviewed in brief, the major emphasis is on recent developments, including nonmonotonic responses of stress to changes in strain rate, viscosity bifurcation, shear banding, and kinematic hardening. The major categories of phenomenological constitutive models are reviewed; these can include viscoelasticity and aging, plasticity, kinematic hardening, and thixotropy; and distinctions between these phenomena and thixotropy are discussed. A few available microstructural models are also reviewed, including population balance models and mesoscopic simulations. We end by highlighting important future work that is needed, including further development of microscopic models and their connection to phenomenological constitutive equations, detailed measurements of microstructures and flow fields with bands, and the investigation of flows other than simple shear.

A new transport model is proposed for paraffin wax deposition onto a cold finger from flowing wax-containing oils. The model solves transient energy and mass balances simultaneously for a reversible first-order kinetic rate for precipitation of pseudo-single-component wax, and the effects of yield stress using a critical solid wax concentration to withstand flow-induced stress at the deposit-fluid interface, Cpi. The model can predict the time evolution of the deposit thickness, and the spatial and temporal evolution of temperature and wax concentration and was validated using experiments involving a cylindrical cold finger. We found that for oils with Cpi close to zero, the deposit thickness growth is dominated by heat transfer. However, mass transfer cannot be neglected as diffusion of wax into the deposit continues to take place even after the deposit has stopped growing. For oils with non-zero Cpi, the deposit growth is slow and accompanied by occasional sloughing.

We compute desorption rates for isolated polymers adsorbed to a solid wall with a rare event sampling technique called multilevel splitting, also known as forward flux sampling. We interpret computed rates with theories based on the conjecture that the product tdesDRg2 of the desorption time tdes and diffusivity D divided by squared radius of gyration Rg scales with exp(h/Rg) where h is the equilibrium ratio of adsorbed surface concentration of polymer &Gamma, to bulk concentration of polymer c. As the polymer&ndash, wall interaction energy is increased, the slope of lntdesDRg2 vs. NVMFkBT nearly approaches unity, as expected for strongly-adsorbing chains, where N is the degree of polymerization and VMF is the height-averaged monomer&ndash, wall interaction energy for a strongly adsorbed chain. However, we also find that this scaling law is only accurate when adsorption strength per monomer exceeds a threshold value on the order of 0.3&ndash, 0.5 kBT for a freely jointed chain without or with excluded volume effects. Below the critical value, we observe that tdesDRg2 becomes nearly constant with N, so that tdes&prop, N&alpha, with &alpha, &asymp, 2. This suggests a crossover from &ldquo, strong&rdquo, detachment-controlled to a &ldquo, weak&rdquo, diffusion-controlled desorption rate as VMF/kBT drops below some threshold. These results may partially explain experimental data, that in some cases show &ldquo, exponential dependence of desorption time on chain length, while in others a &ldquo, power-law dependence is found. However, in the &ldquo, adsorption case, our results suggest much longer desorption times than those measured, while the reverse is true in the weak adsorption limit. We discuss possible reasons for these discrepancies.

We determine experimentally the “dilution exponent” α for entangled polymers from the scaling of terminal crossover frequency with entanglement density from the linear rheology of three 1,4-polybutadiene star polymers that are blended with low-molecular-weight, unentangled linear 1,4-polybutadiene at various star volume fractions, ϕs. Assuming that the rheology of monodisperse stars depends solely on the plateau modulus GN(ϕs) ∝ ϕs1+α, the number of entanglements per chain Me(ϕs) ∝ ϕs–α, and the tube-segment frictional Rouse time τe(ϕs) ∝ ϕs–2α, we show that only an α = 1 scaling superposes the Me(ϕs) dependence of the terminal crossover frequency ωx,t of the blends with those of pure stars, not α = 4/3. This is the first determination of α for star polymers that does not rely on any particular tube model implementation. We also show that a generalized tube model, the “Hierarchical model”, using the “Das” parameter set with α = 1 reasonably predicts the rheological data of the melts and blends featured in...

We study the rheological responses and shear-rate inhomogeneities and shear banding behaviors of a thixotropic fumed silica suspension in shear startup tests and flow reversal tests. We find that this suspension under transient shear exhibits not only viscoelasticity, yielding, kinematic hardening, and thixotropy, but also time-dependent shear inhomogeneities including bands when the apparent shear rate is below a critical value between 0.1 and 0.25 s-1. Through multiple shear startup tests and flow reversal tests, we find that thixotropy promotes flow heterogeneity while kinematic hardening suppresses it. We propose a simple thixo-plastic constitutive equation that can qualitatively predict the important features of the rheological response and banding dynamics in shear startup tests and flow reversal tests.

We combine mechanical rheometry, diffusing wave spectroscopy (DWS), and small angle neutron scattering (SANS) with a simulation model, the "pointer algorithm", to obtain characteristic lengths and time constants for wormlike micelle (WLM) solutions over a range of salt concentrations encompassing the transition from unentangled to entangled solutions. The solutions contain sodium lauryl ethylene glycol sulfate (SLE1S), cocamidopropyl betaine (CAPB), and NaCl. The pointer algorithm is extended to include relaxation of unentangled micelles, allowing micelle parameters to be extracted from the rheology of partially entangled solutions. DWS provides the data at high frequency needed to determine micelle persistence length accurately. From pointer algorithm fits to rheology, we observe a salt-induced rapid change in micellar length as the solution enters the well-entangled regime and a weaker growth with surfactant concentration consistent with mean-field theory. At a lower surfactant concentration, micelle length and persistence length from SANS are roughly consistent with values from rheology once the lower surfactant concentration used in SANS is accounted for. This is, to our knowledge, the first time that quantitative comparisons of structural features including micelle length are made between rheology and SANS. Finally, scaling laws for micelle diffusion and recombination times indicate that micelle kinetics are reaction controlled leading to mean-field recombination with surrounding micelles over the entire range of concentration of interest except at very low and very high surfactant concentrations where either short micelles or branched micelle clusters are dominant.

We infer the swelling/de-swelling behavior of weakly ionizable poly(acrylic acid) (PAA) brushes of 2-39 kDa molar mass in the presence of KCl concentrations from 0.1-1000 mM, pH = 3, 7, and 9, and grafting densities σ = 0.12-2.15 chains per nm2 using a Quartz Crystal Microbalance with Dissipation (QCM-D), confirming and extending the work of Wu et al. to multiple chain lengths. At pH 7 and 9 (above the pKa ∼ 5), the brush initially swells at low KCl ionic strength (10 mM) in the "osmotic brush" regime, and de-swells at higher salt concentrations, in the "salted brush" regime, and is relatively unaffected at pH 3, below the pKa, as expected. At pH 7, at low and moderate grafting densities, our results in the high-salt "salted brush" regime (Cs10 mM salt) agree with the predicted scaling H ∼ Nσ+1/3Cs-1/3 of brush height H, while in the low-salt "osmotic brush" regime (Cs10 mM salt), we find H ∼ Nσ+1/3Cs+0.28-0.38, whose dependence on Cs agrees with scaling theory for this regime, but the dependence on σ strongly disagrees with it. The predicted linearity in the degree of polymerization N is confirmed. The new results partially confirm scaling theory and clarify where improved theories and additional data are needed.

We use an analytical mean-field theory and all-atom molecular dynamics (MD) simulations to predict that external tension, together with the nematic coupling interactions, can drive phase separation of long chains from short ones in bidisperse homopolymer melts. The nematic coupling parameter α for polyethylene (PE) oligomers under applied tension is extracted from the MD simulations and used in the mean-field free energy to predict the phase boundary for bidisperse melts in which the longer chains are stretched by uniaxial tension. The predicted phase diagram is validated by direct MD simulations. We also show that extensional flow, and possibly even shear flow, may lead to nematic phase separation in molten PE oligomers, because the flow can impose a stronger tension on the longer chains than the short ones., Using theories and simulations, we show nematic interactions and unequal tension imposed by flow or other fields on chains of different lengths can drive phase separation in homopolymer melts.

We use coarse-grained (CG) molecular dynamics simulations to determine the effect of uniaxial strain on the stress, scission stress, and scission energy of solutions of wormlike micelles of cetyltrimethylammonium chloride/sodium salicylate (NaSal). We find that the breaking stress, stretch modulus, and scission energy of the charged micelles are nonmonotonic functions of oppositely charged hydrotrope (NaSal) concentration. While the stretch modulus shows a peak at a value of surfactant-to-hydrotrope concentration ratio ( R) close to unity as expected due to neutralization of head-group charge at R = 1, the breaking stress and scission energy produce a peak at R < 1.0 because of thinning of the micelle diameter with increased R. The breaking stress from the simulations depends on the rate of deformation and roughly agrees with the experimental values of Rothstein ( J. Rheol. 2003 , 47 , 1227 ) after extrapolation to the much lower experimental rates. The method and results can be used to predict the effects of flow and mechanical stress on rates of micellar breakage, which is important in the rheology of wormlike micellar solutions.

Experiments have shown that relaxation of oil/water interfacial tension by adsorption of alkyl ethoxylate surfactants from water onto an oil droplet is delayed relative to diffusion-controlled adsorption. We examine possible causes of this delay, and we show that several are implausible. We find that redissolution of the surfactant in the oil droplet cannot explain the apparent interfacial resistance at short times because the interface will preferentially fill before any such redissolution occurs. We also perform umbrella sampling with molecular dynamics simulation and do not find any evidence of a free-energy barrier or low-diffusivity zone near the interface. Nor do we find evidence from the simulation that premicellar aggregation slows diffusion enough to cause the observed resistance to interfacial adsorption. We are therefore unable to pinpoint the cause of the resistance, but we suggest that "dead time" associated with the experimental method could be responsible-specifically a local depletion of surfactant by the ejected droplet when creating the fresh interface between the oil and water.

We employ Brownian dynamics simulations combined with forward flux sampling and theoretical first-passage time analyses to describe the rates of transitions between loops and bridges of telechelic polymers between solid surfaces representing e.g. latex colloids in the limit that the telechelic stickers bind strongly enough to the surfaces to make free chains very rare. It is shown that the bridge formation rate can be expressed by combining times for two processes, namely, the escape of one end sticker from the narrow-but-deep association well near the colloidal surface and the longer-range motion of the chain end to the other surface inhibited by stretching free energy. We find that when using multibead chains to represent the telechelic polymers, the longer-range motion requires use of a multidimensional first passage time analysis that we borrow from the work of Likhtman and co-workers, which was originally developed to describe polymer end fluctuations in a one-dimensional reptation tube. From these i...

We build a Brownian dynamics simulation model to study the linear rheology of latex particles interacting with telechelic polymers (for example, so-called “HEUR” polymers), with the latter modeled as Finitely Extensible Nonlinear Elastic (FENE) dumbbells with ends that stick to Brownian colloidal spheres. We identify from this model four relaxation modes, including (1) a single chain relaxation time, τ′FENE, which is closely related to τFENE, the relaxation time of a free chain predicted using the FENE model, but is around 1.4 times longer because of surface constraints on polymer configuration; (2) a loop translational-rotational relaxation time, τloop, produced by migration of the chain over the particle surface, allowing it to further relax its orientational conformation while still trapped on the particle surface; (3) a bridge formation/breakage relaxation time, τbridge; and (4) one or more colloidal particle cluster relaxation times. We show that τloop scales as the square of particle radius. τbridge...

We present a scheme to calculate wormlike micelle scission free energies from a potential of mean force (PMF) derived from a weighted histogram analysis method (WHAM) applied to coarse grained dissipative particle dynamics (DPD) simulations. In contrast to previous related work, we use a specially chosen external potential based on a reaction coordinate that reversibly drives surfactants out of the nascent scission location. For the application to a model body wash formulation, we predict how addition of NaCl and small molecules such as perfume raw materials (PRMs) affect scission energies. The results show qualitative agreement and correct trends compared to recently determined scission energies for the same system; however, a more rigorous parametrization of the underlying DPD potential is required for quantitative agreement.

To predict the complex transient rheology of thixotropic elasto-viscoplastic (TEVP) fluids, we generalize a previous scalar thixotropic-only “multilambda” (ML) model [Wei et al., J. Rheol. 60(6), 1301–1315 (2016)] and combine it with the isotropic kinematic hardening (IKH) model [C. J. Dimitriou and G. H. Mckinley, Soft Matter 10(35), 6619–6644 (2014)]. This new constitutive equation, which we call ML-IKH model, has the following features: (1) Multiple thixotropic structure parameters that collectively exhibit a stretch-exponential thixotropic relaxation in step tests; (2) nonlinear thixotropic kinetic equations in both the shear rate or stress and the structure parameters; (3) incorporation of the Armstrong-Frederick kinematic hardening rule [C. O. Frederick and P. Armstrong, Mater. High Temp. 24(1), 1–26 (2007)] for the evolution of yield stress; and (4) viscoelasticity. We evaluate this 12-parameter model, discuss its four key features, and compare its predictions with those of the ML, IKH, and modified Delaware thixotropic models [Armstrong et al., J. Rheol. 60(3), 433–450 (2016)] for two sets of experimental data [Wei et al., J. Rheol. 60(6), 1301–1315 (2016); Armstrong et al., J. Rheol. 60(3), 433–450 (2016)] of a TEVP fumed silica suspension. The shear-rate histories include steady state, step shear rate, step stress, intermittent shear, flow reversal, and large amplitude oscillatory shear (LAOS). We show that in step tests the thixotropic and viscoelastic evolutions are dominant, while intermittent shear tests the multiple thixotropic timescales, and flow reversal tests the viscoelastic and plastic evolutions. The rheological responses in LAOS tests are more complex and involve all aspects of TEVP rheology. The four features quantitatively capture different aspects of TEVP rheology. We also provide a tensorial formulation of the ML-IKH model that is frame-invariant, obeys the second law of thermodynamics, and can reproduce the predictions of the scalar version.To predict the complex transient rheology of thixotropic elasto-viscoplastic (TEVP) fluids, we generalize a previous scalar thixotropic-only “multilambda” (ML) model [Wei et al., J. Rheol. 60(6), 1301–1315 (2016)] and combine it with the isotropic kinematic hardening (IKH) model [C. J. Dimitriou and G. H. Mckinley, Soft Matter 10(35), 6619–6644 (2014)]. This new constitutive equation, which we call ML-IKH model, has the following features: (1) Multiple thixotropic structure parameters that collectively exhibit a stretch-exponential thixotropic relaxation in step tests; (2) nonlinear thixotropic kinetic equations in both the shear rate or stress and the structure parameters; (3) incorporation of the Armstrong-Frederick kinematic hardening rule [C. O. Frederick and P. Armstrong, Mater. High Temp. 24(1), 1–26 (2007)] for the evolution of yield stress; and (4) viscoelasticity. We evaluate this 12-parameter model, discuss its four key features, and compare its predictions with those of the ML, IKH, and modifie...

Using fully atomistic and coarse-grained (CG) molecular dynamics (MD) simulations, we report, for the first time, the self-assembly of initially randomly dispersed dodecyl-β-d-maltoside (DDM) surfactants into a striped cylindrical micelle (SCM) with lamellae of surfactant heads and tails alternating along the cylindrical axis, with both heads and tails in contact with the water. By changing the interaction strength of the head group with water relative to itself, we find that such micelles are most likely for head groups with marginal solubility in the water solvent. Unlike the surfactants in a regular cylindrical micelle, whose tails are in the fluid micelle interior, the diffusion of DDM surfactants along the micelle body is blocked by the lamellar patterning. As a consequence, branches cannot slide along the micelle body and surfactant molecules cannot exchange between the micelle body and the branch, which should have a significant impact on the rheological properties of these micelles.

We developed a novel hybrid population balance-Brownian dynamics (Pop-BD) simulation technique to investigate the phase behavior and linear viscoelastic response of suspensions of colloids bridged reversibly by telechelic polymers such as latex particles bridged by polyethylene oxide urethanes. The Pop-BD method couples the solution of a set of population balance equations of the bridge-to-loop exchange kinetics of the telechelic polymer chains on the latex particles with Brownian dynamics simulations governing the dynamics of the latex particles. Two relaxation times are identified, corresponding to bridge association/dissociation from the latex particle and relaxation of the transient network of bridged particles. The relaxation modulus computed from the Pop-BD equations matches that for fully explicit BD simulations of both colloids and polymers at time scales equal to or greater than the polymer bridge relaxation time.

We determine the thermal conductivities of α, β, and γ graphyne nanotubes (GNTs) as well as of carbon nanotubes (CNTs) using molecular dynamics simulations and the Green-Kubo relationship over the temperature range 50–400 K. We find that GNTs demonstrate considerably lower thermal conductivity than CNTs with the same diameter and length. Among α, β, and γ-GNTs, γ-GNT has the highest thermal conductivity at all temperatures. By comparing the phonon transport properties of GNTs with CNTs, we find that as the fraction of acetylene bonds in the atomic network increases, the population of high-energy optical phonons increases. This enhances phonon-phonon scattering, and reduces the mean free path, adversely affecting the thermal conductivity of GNTs relative to CNTs. Also reducing the thermal conductivity of GNTs relative to CNTs is the considerably lower acoustic phonon group velocities for the former as well as the lower volumetric heat capacity of GNTs. Optical phonons in α-GNT are high in energy (0.26 eV) with a high population number, making them more energetic than the electronic direct band gap and significantly more energetic than the thermal energy at room temperature. Therefore, we suggest α-GNT as a potential candidate for phonovoltaic energy conversion applications.

By measuring stress relaxation following a step strain, we find that strain hardening is prevalent over a temperature range of 15–25 °C for a solution of cetyl trimethylammonium bromide (CTAB) with the added hydrotrope sodium salicylate (NaSal) at hydrotrope-to-surfactant concentration ratios ( CS/CD) between 0.5 and 3.0. The extent of strain hardening upon nonlinear deformation varies nonmonotonically as a function of salt-to-surfactant ratio for different temperatures. As the strain amplitude is progressively increased, a transition from strain hardening to softening or linear response is observed at strains that are dependent on temperature and concentration. Strain hardening was also found in CTAB when using sodium 3-Hydroxy-2-naphthoate as a hydrotrope with CTAB, but solutions of anionic sodium lauryl sulfate surfactants in salt with no hydrotrope showed no strain hardening, indicating that the hydrotrope is critical to obtaining strain hardening in step strains. The results indicate a stress relaxat...

We explore a rich phase space of crystals self-assembled from colloidal “polyhedral sphere clusters (PSCs),” each of which consists of equal-sized “halo” spheres placed at the vertices of a polyhedron such that they just touch along each edge. Such clusters, created experimentally by fusing spheres, can facilitate assembly of useful colloidal crystal symmetries not attainable by unclustered spheres. While not crucial for their self-assembly, the center of the PSC can contain a “core” particle that can be used as a scaffold to build the PSC. Using Brownian dynamics simulations, we show the self-assembly of eight distinct crystalline phases from PSCs that correspond to the five Platonic polyhedra, and that are made of spheres with purely repulsive interactions. Strong crystalline order is seen in the centers of mass of the PSCs, or equivalently the core particles. The halo particles also may organize into crystal structures, usually with weaker crystalline order than the core particles. Notably, however, in crystals assembled from the octahedral and icosahedral PSCs, the halo particles are also well ordered, nesting within the crystals formed by the cores. Interestingly, despite the rounded nature of the PSCs, in some cases we obtain structures similar to those of the corresponding faceted polyhedra interacting only via excluded volume. Only the tetrahedral PSCs fail to self-assemble into a crystal, but we demonstrate that a pre-assembled crystal – whose halo particles sit on a close-packed face-centered cubic lattice, and whose core particles form a diamond structure – is stable at high density and melts into a hexagonal phase at lower density.

Using PYS, TraPPE, OPLS-L, and Flexible-Williams (FW) force field models, atomistic simulations at temperatures ranging from 450 K to 600 K are performed to predict the melt density ρ, the persistence length Np, the nematic coupling constant α, and crystallization dynamics for pentacontane (C50). The coupling constant α arises from packing entropy of rodlike Kuhn segments and increases with increasing ρ and Np. Together with a self-consistent field theory, Np and α are then used to predict the isotropic-to-nematic (IN) transition temperature for polyethylene (PE) oligomers as a function of chain length. The nematic phase is found to be metastable since the IN transition temperature lies below the crystal melting temperatures for C50 in simulations using different force fields. Finally, isothermal simulations of crystallization for PE C50 oligomers and C1000 polymers show that crystal nucleation may be much accelerated by quenching below the IN transition temperature, where chains in the isotropic state first rapidly form nematic ordered domains, within which crystalline order then grows. We also find that the PYS, TraPPE, and FW models overpredict the melting temperature for C50 by around 50 K, while the most flexible OPLS-L model gives a melting temperature within around 10 K of the experimental value. Although giving a more accurate melting temperature, the slow crystallization kinetics of the OPLS-L model may limit its application in direct simulations of PE crystallization.

We use a quartz crystal microbalance with dissipation (QCM-D) to investigate the swelling/de-swelling and hysteresis in brushes of weakly ionizable polyanion poly(acrylic acid) (PAA) brushes and bilayers containing a PAA brush and a poly(ethylene imine) (PEI) overlayer. We show that for a long PAA chain (Mw = 39 kDa), at low grafting density (σ < 0.05 chains/nm2) and at a pH value (=4.2) at which it is partially charged, in the low-salt “osmotic brush” regime, the brush height no longer increases with increased grafting density as is seen for shorter brushes and denser grafting, but shows a slight decrease in height, in qualitative agreement with predictions of scaling theory. In a cycle of stepped pH changes, we also show that at a low grafting density of σ = 0.023 chains/nm2 and Mw = 39 kDa, there is hysteresis in swelling over timescales of many minutes. For higher grafting densities σ = 0.87 chains/nm2 and shorter chains (2 kDa), we see little or no measurable hysteresis, and, at intermediate chain length 14 kDa and grafting density σ = 0.06 chains/nm2, hysteresis is observed at short timescales but is greatly reduced at longer timescales. These results are similarly observed when bilayers are made by adsorbing onto the PAA brush a layer of the polycation PEI. In addition, we also note hysteresis in swelling upon changes of salt concentration when pH is fixed. These results show the rich thermodynamics and kinetics of even monolayers and bilayers of polyelectrolyte films.

All-atom molecular dynamic simulations (AA-MD) are performed for aqueous solutions of hydrophobic drug molecules (phenytoin) with model polymer excipients, namely, (1) N-isopropylacrylamide, (pNIPAAm), (2) pNIPAAm-co-acrylamide (Am), and (3) pNIPAAm-co-dimethylacrylamide (DMA). After validating the force field parameters using the well-known lower critical solution behavior of pNIPAAm, we simulate the polymer-drug complex in water and its behavior at temperatures below (295 K) and above the LCST (310 K). Using radial distribution functions, we find that there is an optimum comonomer molar fraction of around 20-30% DMA at which interaction with phenytoin drug molecules is strongest, consistent with recent experimental findings. The results provide evidence that molecular simulations are able to provide guidance in the optimization of novel polymer excipients for drug release.

We propose a rheological model for ideal thixotropy, defined as a reversible time-dependent shear-thinning viscous response, in both shear-rate-controlled (RC Model) and stress-controlled (SC Model) forms. The model introduces a spectrum of structure parameters that collectively relaxes as a stretched exponential. It retains time-invariance symmetry with only a stretching exponent β as an additional model parameter relative to conventional single-structure-parameter models. The kinetic equations are nonlinear in both the structure parameters and the flow parameters of strain rate or stress. We demonstrate that introducing multiple structure parameters can successfully capture the stretched-exponential and nonmonotonic evolution of stress or shear rate in general step tests; and using nonlinear kinetic equations can explain the relaxation time's dependence on both the initial and final values of shear rate or stress in step tests. We present rheological data for a fumed silica dispersion in a number of shear histories including steady state, step shear rate, step stress, shear-rate ramp, and stress ramp. A systematic way to parameterize the models is provided. Both models fit experimental data well although the SC Model provides better agreement with the measurements. The ideal thixotropic models can be combined with existing methods that incorporate viscoelasticity so as to extend their validity into the region of low shear rates.

The ferrite-martensite interfacial energy and equilibrium interfacial length as a function of martensite carbon content are assessed using first-principles atomistic simulations. The weight percent of carbon in the martensite phase was implicitly varied from 0.6 to 1.8 wt percent by modifying the lattice constant of body-centered tetragonal (BCT) martensite according to Kurdjumov and Kaminsky’s empirical expressions. With increasing carbon content, a decrease is found in both the interfacial energy and in the equilibrium distance between ferrite and martensite interfaces. Moreover, the Morse interatomic potentials between the atoms in the ferrite-martensite interface for four different martensite carbon contents are calculated, and the parameters of the Morse potential are correlated linearly with the martensite carbon content. In addition, the dissociation local strains during uniaxial loading in a direction normal to the interfacial plane are calculated from the interatomic potentials. The local strain at the interface needed for ferrite-martensite interface separation increases with increase in martensite carbon content. The fitted expressions can be used to predict the ferrite-martensite interfacial energy, equilibrium interfacial distance, dissociation local strain at the interface, and the Morse parameters as functions of martensite carbon content within the range of 0.6–1.8 wt percent. Furthermore, the introduced implicit method can potentially be used to study the mechanical properties of other materials with dopant impurities such as n-type and p-type semiconductors.

We compare predictions of two of the most advanced versions of the tube model, namely the “Hierarchical model” by Wang et al. [J. Rheol. 2010, 54, 223] and the BoB (branch-on-branch) model by Das et al. [J. Rheol. 2006, 50, 207], against linear viscoelastic G′ and G″ data of binary blends of nearly monodisperse 1,4-polybutadiene 4-arm star polymer of arm molar mass 24 000 g/mol with a monodisperse linear 1,4-polybutadiene of molar mass 58 000 g/mol. The star was carefully synthesized and characterized by temperature gradient interaction chromatography and by linear rheology over a wide frequency region through time–temperature superposition. We found large failures of both the Hierarchical and BoB models to predict the terminal relaxation behavior of the star/linear blends, despite their success in predicting the rheology of the pure star and pure linear polymers. This failure occurred regardless of the choices made concerning constraint release, such as assuming arm retraction in “fat” or “skinny” tubes....

We develop a microstructure-based model to characterize and model failure initiation in DP steels using an extended finite element method (XFEM) to simulate martensite cracking on the mesoscale combined with representative volume element (RVE) modeling. A mini tensile test with digital image correlation (DIC) analysis is linked to local SEM analysis to identify the local strain at which failure is initiated. In-situ bending tests in SEM with electron backscatter diffraction (EBSD) measurements before and after the test are carried out to validate that the crack initiates in the martensite islands. Empirical equations for XFEM parameters as functions of local carbon content in martensite are fit to experimental results for laboratory-annealed DP600 steels with varying martensite content. The equations are then shown to predict successfully failure initiation in industrially produced DP steels with various chemistries, strengths and martensite fractions.

After the start of the flow in a home-built flow-loop, the thickness of a waxy layer deposited on a stainless steel (SS) test section was found to increase immediately, while deposition was delayed for around 4 h on a polyethylene (PE) test section. The radial thermal gradient, wax precipitation rate, radial diffusivity of wax, and shear stresses at the surface of the deposit layer were calculated, eliminating these factors as sources of different deposition behavior. However, the contact angles of oils containing precipitated wax crystals on PE surfaces were found to increase more with increasing wax content than those on SS surfaces. Consistent with this, while the wax contents of deposits were found to be around 18 wt % for both PE and SS pipes, the adhesion of the deposit on the PE surface was found to be much weaker than on the SS surface, as evidenced by the easier of removal of the deposit from PE than from SS pipes when heated in vertical orientation under gravity. These results point to pipe wall wettability as a significant factor in both initial growth of wax deposit, and ease of removal.

We develop a systematic coarse-grained (CG) model for methylcellulose polymers, including random copolymers with compositions representative of modeling commercial METHOCEL A polymer, using one CG bead per monomer. We parametrize our CG model using the RDFs from atomistic simulations of short methylcellulose oligomers, extrapolating the results to long chains. Using a LJ 9–6 potential, the CG model captures the effect of monomer substitution type and temperature observed in detailed atomistic simulations. We use dissociation free energy to validate our CG model against the atomistic model. We then use this CG model to simulate single chains up to 1000 monomers long, and we calculate persistence lengths for a selection of homogeneous and heterogeneous methylcellulose chains, which show good agreement with experimental results. Interestingly, simulations of 600-mer heterogeneous chains show a collapse transition at 50 °C and form a stable ring structure with outer diameter around 14 nm. This structure appea...

We present a hybrid model for polymeric glasses under deformation that combines a minimal model of segmental dynamics with a beads-and-springs model of a polymer, solved by Brownian dynamics (BD) simulations, whose relaxation is coupled to the segmental dynamics through the drag coefficient of the beads. This coarse-grained model allows simulations that are much faster than molecular dynamics and successfully capture the entire range of mechanical response including yielding, plastic flow, strain-hardening, and incomplete strain recovery. The beads-and-springs model improves upon the dumbbell model for glassy polymers proposed by Fielding et al. (Phys. Rev. Lett., 2012, 108, 048301) by capturing the small elastic recoil seen experimentally without the use of ad hoc adjustments of parameters required in the model of Fielding et al. With appropriate choice of parameters, predictions of creep, recovery, and segmental relaxation are found to be in good agreement with poly(methylmethacrylate) (PMMA) data of Lee et al. (Science, 2009, 323, 231-234). Our model shows dramatic differences in behavior of the segmental relaxation time between extensional creep and steady extension, and between extension and shear. The non-monotonic response of the segmental relaxation time to extensional creep and the small elastic recovery after removal of stress are shown to arise from sub-chains that are trapped between folds, and that become highly oriented and stretched at strains of order unity, connecting the behavior of glassy polymers under creep to that of dilute polymer solutions under fast extensional flows. We are also able to predict the effects of polymer pre-orientation in the parallel or orthogonal direction on the subsequent response to extensional deformation.

The phase separation behavior of poly(ethylene oxide) (PEO) in ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) was investigated by rheological, optical microscopy, FT-IR and DSC measurements. It is demonstrated that specific interactions, particularly the hydrogen bonding between PEO and the ionic liquids as evidenced by FT-IR, in which a subtle but apparent absorption peak shift near the phase transition appears, account for the unusual low critical solution temperature (LCST) phase separation. Unlike the typical trend in which the storage modulus G′ simply increases with temperature near the phase boundary for polymer blends without specific interaction, in our study, a novel “V-shaped” rheological response is observed, namely a dip in G′ followed by an upturn, especially at low PEO concentration (

We model the adsorption of hydrophobically ethoxylated urethane (HEUR) thickeners onto two hydrophobic surfaces separated by a 50 nm gallery using coarse-grained molecular dynamics (CG-MD) with implicit solvent and three-dimensional self-consistent field theory (SCFT) with explicit solvent. The CG-MD simulations can be readily extended to encompass very long HEUR chains (up to 540 EO groups) but cannot with current computer speed simulate adsorption of HEURs with hydrophobes longer than 12 carbons (C12). The SCFT method can readily simulate HEURs with longer, C16, hydrophobes but has a greater challenge simulating very long EO chains. For HEURs with 180 EO units and C8 and C12 hydrophobes, both methods can be applied, allowing a combination of the two methods to span much of the parameter space of interest to experimentalists. It is demonstrated that depending on the hydrophobe strength and the HEUR concentration, HEUR chains can adsorb to the surfaces directly or indirectly (as adsorbed micelles or admic...

We parametrize a molecular dynamics force field for poly(ethylene oxide) (PEO) within the Dry Martini coarse-grained (CG) implicit solvent model and show that it yields similar distributions of bond lengths, angles and torsional angles, and a similar dependence of radius of gyration on PEO chain length as those obtained from atomistic and explicit-solvent CG simulations. We apply these new parameters to simulate self-assembly of long-chain single-end-capped CnEm surfactants and double-end-capped telechelic polymers CnEmCn, where n is the number of alkane carbons and m the number of ethylene oxide monomers. We determine that the average equilibrium micelle aggregation number at high concentrations is around 23 hydrophobes for C12E100, C12E100C12, and C12E200C12, consistent with experimental/theoretical results. However, for EO block less than 200 monomers at 300 K, for example, for C12E45C12 and C12E136C12, telechelic polymers phase separate to form a polymer dense phase at low concentrations due to the fo...

We apply our recently developed mesoscopic simulation method for entangled wormlike micelle (WLM) solutions to extract multiple micellar characteristic lengths and time constants: i.e., average micelle length, breakage rate, and entanglement and persistence lengths, from linear rheological measurements on commercial surfactant solutions, one containing sodium lauryl one ether sulfate (SLE1S), and the other containing both SLE1S and cocamidopropyl betaine, as well as a perfume mixture, in both cases with a sample salt (NaCl) added. Measurements include both mechanical rheometry and diffusing wave spectroscopy, the latter providing the high-frequency data needed to determine micelle persistence length accurately. By fitting the experimental data ( G′ and G″) across the entire frequency range through our iteration procedure, the method is of practical use in predicting micellar parameters, which are difficult to obtain from other theoretical or experimental methods. The dependence of micellar parameters on a...

We review constitutive modeling of solutions and melts of linear polymers, focusing on changes in rheological behavior in shear and extensional flow as the concentration increases from unentangled dilute, to entangled, to dense melt. The rheological changes are captured by constitutive equations, prototypes of which are the FENE-P model for unentangled solutions and the DEMG model for entangled solutions and melts. From these equations, and supporting experimental data, for dilute solutions, the extensional viscosity increases with the strain rate from the low–strain rate to the high–strain rate asymptote, but in the densely entangled state, the high–strain rate viscosity is lower than the low–shear rate value, especially when orientation-dependent friction is accounted for. In shearing flow, shear thinning increases dramatically as the entanglement density increases, which can eventually lead to a shear-banding inhomogeneity. Recent improvements in constitutive modeling are paving the way for robust and accurate numerical simulations of polymer fluid mechanics and industrial processing of polymers.