Dynamics Of Polymer Solutions Research Articles

Assessment of modified chitosan composite in acidic reservoirs through pilot and field-scale simulation studies

Chemical flooding through biopolymers acquires higher attention, especially in acidic reservoirs. This research focuses on the application of biopolymers in chemical flooding for enhanced oil recovery in acidic reservoirs, with a particular emphasis on modified chitosan. The modification process involved combining chitosan with vinyl/silane monomers via emulsion polymerization, followed by an assessment of its rheological behavior under simulated reservoir conditions, including salinity, temperature, pressure, and medium pH. Laboratory-scale flooding experiments were carried out using both the original and modified chitosan at conditions of 2200 psi, 135,000 ppm salinity, and 196° temperature. The study evaluated the impact of pressure on the rheological properties of both chitosan forms, finding that the modified composite was better suited to acidic environments, showing enhanced resistance to pressure effects with a significant increase in viscosity and an 11% improvement in oil recovery over the 5% achieved with the unmodified chitosan. Advanced modeling and simulation techniques, particularly using the tNavigator Simulator on the Bahariya formations in the Western Desert, were employed to further understand the polymer solution dynamics in reservoir contexts and to predict key petroleum engineering metrics. The simulation results underscored the effectiveness of the chitosan composite in increasing oil recovery rates, with the composite outperforming both its native counterpart and traditional water flooding, achieving a recovery factor of 48%, compared to 39% and 37% for native chitosan and water flooding, thereby demonstrating the potential benefits of chitosan composites in enhancing oil recovery operations.

Coarsening dynamics of ternary polymer solutions with mobility and viscosity contrasts.

Using phase-field simulations, we investigate the bulk coarsening dynamics of ternary polymer solutions undergoing a glass transition for two models of phase separation: diffusion only and with hydrodynamics. The glass transition is incorporated in both models by imposing mobility and viscosity contrasts between the polymer-rich and polymer-poor phases of the evolving microstructure. For microstructures composed of polymer-poor clusters in a polymer-rich matrix, the mobility and viscosity contrasts significantly hinder coarsening, effectively leading to structural arrest. For microstructures composed of polymer-rich clusters in a polymer-poor matrix, the mobility and viscosity contrasts do not impede domain growth; rather, they change the transient concentration of the polymer-rich phase, altering the shape of the discrete domains. This effect introduces several complexities to the coarsening process, including percolation inversion of the polymer-rich and polymer-poor phases-a phenomenon normally attributed to viscoelastic phase separation.

Rheology mediates transition of vortex evolution patterns in microcavity flow of polymer solutions

Vortex instability in cavity flow is a fundamental component of microfluidic applications such as flow mixing, nanoparticle synthesis, and cell/particle manipulation. In contrast to Newtonian fluids, non-Newtonian fluids exhibit significantly different flow behaviors due to their non-linear flow dynamics. This study experimentally investigates the flow dynamics of polymer solutions with distinct rheological properties through a microcavity and quantifies the influence of the rheological degree on the evolution dynamics of vortices. We find three typical vortex evolution patterns in the cavity flow of polymer solutions and show that the rheological degree mediates the transitions among these patterns. The vortex evolution in the cavity flow of all polymer solutions tested in this study shifts from a basic increasing logistic function to one of three typical patterns as the polymer concentration increases. It is clarified that the pattern transition is related to the elasticity number and shear-thinning index of the fluids, and the phase difference between identical patterns is due to differences in the viscosity and elasticity of the fluids. These results extend our understanding of the vortex dynamics of complex fluids in cavity flow and provide theoretical guidance for enhancing the working efficiency of cavity-structured microfluidic applications using polymer solutions. The results of this study may also inspire developments in the flow regulation of drug delivery in blood through the vascular system.

Chain rotational dynamics in dilute polymer solutions and melts under shear flow

We present detailed scaling characteristics of the chain rotation dynamics in shear flow for melt and dilute linear polymer systems using extensive atomistic and coarse-grained Brownian dynamics (BD) molecular dynamics simulations, respectively. We find that the dilute systems with hydrodynamic interaction (HI) and excluded volume (EV) effects generally exhibit two distinct scaling behavior of the chain rotation time τrot between the intermediate and strong flow regimes, whereas the free-draining dilute polymer systems display a relatively simple single-scaling behavior for τrot with respect to the shear rate throughout the intermediate-to-strong flow regimes. This dynamic feature correlates with the scaling characteristics of average chain deformation and orientation of the system with respect to the flow strength. Notably, the combined HI and EV effects generally weaken the structural change of polymer chains in response to the applied flow and enhance the overall chain rotation dynamics by disturbing the chain stretch and alignment to the flow direction. Accordingly, due to the variation in the HI and EV effects (in association with chain deformation) with the flow strength, the dilute systems with HI and EV turn out to reveal relatively lower and higher scaling behaviors of τrot in the intermediate and strong flow regimes, respectively, in comparison to the corresponding free-draining systems. Despite the absence of HI and EV effects, the melt systems also exhibit two distinct scaling behaviors of τrot between the intermediate and strong flow regimes. This feature arises from the intermolecular interactions between chains, resulting in a significant influence of interchain entanglement and interchain dynamic collision on the chain rotation dynamics. It is further observed that the overall τrot scaling behavior for each system is predominantly determined by the corresponding scaling behavior of the kinematically stable (diffusive) angular regions near the flow direction, where the flow force is relatively weak. Additionally, polymer chains in kinematically unstable (convective) angular regions exhibit nearly affine chain rotation dynamics conforming to the imposed flow field, independent of the polymer concentration and chain length. This is considered to be a universal feature for general polymeric liquids in shear flow.

Solidification dynamics of polymer membrane by solvent extraction: Spontaneous stratification

Nonsolvent-induced phase separation is widely used to create polymer membranes, but its demixing process is generally understood by liquid-liquid separation. In this work, the solidification dynamics of polymer solutions for membrane formation are explored through dissipative particle dynamics simulations. The shrinkage of the symmetric film of polymer solution is monitored, and the evolutions of the concentration profiles of polymer, solvent, and nonsolvent are analyzed. Additionally, the evolution of the degree of crystallinity (local alignment) and the morphology of the developing membrane are also studied. Three regions can be identified: (i) interfacial region, (ii) dense layer, and (iii) middle region. The demixing process associated with liquid-solid separation is found to be caused by the extraction of solvent and concentration of polymers rather than nonsolvent-induced separation. Our results provide valuable insights into directional solidification and spontaneous stratification of the membrane, driven by solvent loss and oversaturation (beyond the maximum solubility of polymer).

Persistence Length and Encounter Frequency Determination from Fluorescence Studies of Pyrene-Labeled Poly(oligo(ethylene glycol) methyl ether methacrylate)s

A series of nine pyrene-labeled poly(oligo(ethylene glycol) methyl ether methacrylate)s (Py-PEGnMAs) with n equal to 0–5, 9, 16, and 19 were prepared by random radical copolymerization of 1-pyrenebutyl methacrylate and nine different EGnMA monomers. The process of pyrene excimer formation (PEF) in the Py-PEGnMA samples was characterized in acetone, THF, toluene, N,N-dimethylformamide (DMF), dioxane, and dimethyl sulfoxide (DMSO). The fluorescence decays of all Py-PEGnMA samples were acquired in the six solvents and analyzed with the fluorescence blob model (FBM) to yield the number Nblob of structural units (SU) in the subvolume of the polymer coil probed by an excited pyrene and referred to as a blob, and the rate constant kblob describing the encounters between two SU bearing an excited and a ground-state pyrenyl label located inside a same blob. Nblob and kblob remained constant with pyrene content for a same series of Py-PEGnMA samples. After averaging Nblob and kblob over all pyrene contents for a same Py-PEGnMA series, ⟨Nblob⟩ and the product ⟨kblob × Nblob⟩ were found to decrease with increasing side chain length reflecting a progressive stiffening and decrease in the internal dynamics of the polymethacrylate backbone, respectively. ⟨Nblob⟩ and ⟨kblob × Nblob⟩ could be parametrized as a function of the molecular weight of an SU and the solvent viscosity. The parametrized form of ⟨Nblob⟩ was applied to determine the persistence length (lp) of the PEGnMA samples using the Kratky–Porod equation. lp was found to increase linearly with the square of the side chain length of the PEGnMA samples, as expected theoretically. The parametrized form of ⟨kblob × Nblob⟩ was used as a calibration curve against which the internal dynamics of several polypeptides and poly(methyl acrylate) could be compared in DMSO. This study illustrates the ability of PEF measurements to determine the persistence length and quantify the internal dynamics of polymers in solution, two important parameters in the characterization of macromolecules.

The Water-Polymer Method of Hydroperforation of Oil and Gas wells

The aim of the study is to develop an efficient, innovative method of perforating oil and gas well casing strings with a high-velocity stream of polymer solution that conserves resources and energy and meets the requirements for environ- mentally friendly hydrocarbon production. New knowledge about the dynamics of polymer solutions in the model conditions of flow through the nozzles of the hydro perforator, which form a high-velocity jet, became the main scientific basis for substantiating the water-polymer method of hydro perforation of oil and gas wells. The study of the reaction of polymer solutions to tension hydrodynamic influence allowed to formulate a structural concept, the "common denominator" of which is the strong deformation effect of tension hydrodynamic field on high molecular weight coils, which, according to nonequilibrium thermodynamics, should cause a kind of high elasticity. Peculiarities of hydrodynamic behavior of aqueous polymer solutions in model conditions of flow through jet-forming nozzles are studied. The possibility of transition to a highly developed state of the tangle of polyethylene oxide macromolecules in the conditions of the inlet section of the nozzle of the hydro perforator has been experimentally proved. The mechanism of perforation of oil and gas wells by a high-velocity jet of an aqueous polymer solution has been elucidated. It is proved that the mechanism of the high cutting ability of the high-velocity jet of aqueous polymer solution is the destructive effect of the dynamic pressure of water-polymer jet "reinforced" by highly developed macromolecular chains in the inlet nozzles of the hydro perforator nozzles under the action of tensile leakage. Experimental and industrial testing of the method of perforation of wells by a high-speed jet of an aqueous solution of polyethylene oxide confirmed the practical and economic feasibility of its use.

Non-affine motion and selection of slip coefficient in constitutive modeling of polymeric solutions using a mixed derivative

Constitutive models for the dynamics of polymer solutions traditionally rely on closure relations for the extra stress or related microstructural variables (e.g., conformation tensor) linking them to flow history. In this work, we study the eigendynamics of the conformation tensor within the GENERIC framework in mesoscopic computer simulations of polymer solutions to separate the effects of nonaffine motion from other sources of non-Newtonian behavior. We observe that nonaffine motion or slip increases with both the polymer concentration and the polymer chain length. Our analysis allows to uniquely calibrate a mixed derivative of the Gordon–Schowalter type in macroscopic models based on a micro-macromapping of the dynamics of the polymeric system. The presented approach paves the way for better polymer constitutive modeling in multiscale simulations of polymer solutions, where different sources of non-Newtonian behavior are modelled independently.

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.

Interactions between polymers and flows in macro and micro length scales.

Polymer solution with large molecular weights show scale dependent nonlinear behavior. The dynamics of such polymer solution in a meter length scale are very different from those in a micrometer length scale. To clarify this scale dependent behavior, we have conducted several experiments that focus on interactions between polymers and flows in multiple length scales [1Hidema R. Suzuki H. Hisamatsu S. Komoda Y. Furukawa H. Effects of the extensional rate on two-dimensional turbulence of semi-dilute polymer solution flows.Rheol. Acta. 2013; 52: 949https://doi.org/10.1007/s00397-013-0733-3Crossref Scopus (13) Google Scholar, 2Hidema R. Suzuki H. Hisamatsu S. Komoda Y. Characteristic scales of two-dimensional turbulence in polymer solutions.AICHE J. 2014; 60: 1854https://doi.org/10.1002/aic.14364Crossref Scopus (15) Google Scholar, 3Hidema R. Murao I. Komoda Y. Suzuki H. Effects of the extensional rheological properties of polymer solutions on vortex shedding and turbulence characteristics in a two-dimensional turbulent flow.J. Non-Newtonian Fluid Mech. 2018; 254: 1https://doi.org/10.1016/j.jnnfm.2018.02.001Crossref Scopus (12) Google Scholar, 4Hidema R. Fukushima K. Yoshida R. Suzuki H. Vortex deformation and turbulent energy of polymer solution in a two-dimensional turbulent flow.J. Non-Newtonian Fluid Mech. 2020; 285104385https://doi.org/10.1016/j.jnnfm.2020.104385Crossref Scopus (3) Google Scholar, 5Fukushima K. Kishi H. Suzuki H. Hidema R. Modification of turbulence caused by cationic surfactant wormlike micellar structures in two-dimensional turbulent flow.J. Fluid Mech. 2022; 933: A9https://doi.org/10.1017/jfm.2021.1058Crossref Scopus (1) Google Scholar, 6Hidema R. Hayashi S. Suzuki Hiroshi Drag force of polyethyleneglycol in flow measured by a scanning probe microscope.Phys. Rev. Fluids. 2019; 4074201https://doi.org/10.1103/PhysRevFluids.4.074201Crossref Scopus (2) Google Scholar, 7Hidema Ruri Fujito Ken-ya Suzuki Hiroshi Drag force of polyethyleneglycol in flows of polymer solutions measured by a scanning probe microscope.Soft Matter. 2022; 18: 455https://doi.org/10.1039/D1SM01305JCrossref PubMed Google Scholar].In the first part of this talk, I will highlight characteristic vortex deformation and energy transfer caused by polymers in a two-dimensional turbulent flow [1Hidema R. Suzuki H. Hisamatsu S. Komoda Y. Furukawa H. Effects of the extensional rate on two-dimensional turbulence of semi-dilute polymer solution flows.Rheol. Acta. 2013; 52: 949https://doi.org/10.1007/s00397-013-0733-3Crossref Scopus (13) Google Scholar, 2Hidema R. Suzuki H. Hisamatsu S. Komoda Y. Characteristic scales of two-dimensional turbulence in polymer solutions.AICHE J. 2014; 60: 1854https://doi.org/10.1002/aic.14364Crossref Scopus (15) Google Scholar, 3Hidema R. Murao I. Komoda Y. Suzuki H. Effects of the extensional rheological properties of polymer solutions on vortex shedding and turbulence characteristics in a two-dimensional turbulent flow.J. Non-Newtonian Fluid Mech. 2018; 254: 1https://doi.org/10.1016/j.jnnfm.2018.02.001Crossref Scopus (12) Google Scholar, 4Hidema R. Fukushima K. Yoshida R. Suzuki H. Vortex deformation and turbulent energy of polymer solution in a two-dimensional turbulent flow.J. Non-Newtonian Fluid Mech. 2020; 285104385https://doi.org/10.1016/j.jnnfm.2020.104385Crossref Scopus (3) Google Scholar, 5Fukushima K. Kishi H. Suzuki H. Hidema R. Modification of turbulence caused by cationic surfactant wormlike micellar structures in two-dimensional turbulent flow.J. Fluid Mech. 2022; 933: A9https://doi.org/10.1017/jfm.2021.1058Crossref Scopus (1) Google Scholar]. The effects of the extensional rheological properties of polyethylene oxide solutions were focused to understand the vortex deformation in turbulent flow and turbulent statistics of the polymer solution. In the polymer solution, although the turbulent energy production was virtually zero, a characteristic peak appeared in turbulent energy. The results imply the extension and relaxation of polymers affect the energy transfer.In the second part, I will introduce a method to measure the drag force due to synthetic polymers in flowing fluids by using a scanning probe microscope [[6]Hidema R. Hayashi S. Suzuki Hiroshi Drag force of polyethyleneglycol in flow measured by a scanning probe microscope.Phys. Rev. Fluids. 2019; 4074201https://doi.org/10.1103/PhysRevFluids.4.074201Crossref Scopus (2) Google Scholar,[7]Hidema Ruri Fujito Ken-ya Suzuki Hiroshi Drag force of polyethyleneglycol in flows of polymer solutions measured by a scanning probe microscope.Soft Matter. 2022; 18: 455https://doi.org/10.1039/D1SM01305JCrossref PubMed Google Scholar]. The conformation of polymers attached to the cantilever probe was predicted, and the drag force due to the deformed polymers in a flow was calculated. The drag forces obtained by model calculations were compared to the force detected by experiments, and found to be reasonably close.

Perforacja odwiertów ropnych i gazowych za pomocą strumienia roztworu polimeru o dużej prędkości

ABSTRACT: The work is devoted to the development of a technological process for perforating oil and gas well casing strings by a high-velocity jet of a polymer solution. The proposed method of well perforation refers to methods for the secondary opening of productive deposits in the well by hydrojet perforation of the casing strings, annulus cement ring (stone) and rock. The new knowledge about the dynamics of polymer solutions under the conditions of flow through the jet-forming nozzles of a hydroperfora-tor, which create a high-velocity jet, became the main scientific basis for this method of hydroperforation of oil and gas well casings. The study of the reaction of polymer solutions to the hydrodynamic effect with stretching led to the formulation of a structural con-cept, the “common denominator” of which is a strong deformation effect of the hydrodynamic field on macromolecular coils, which in terms of nonequilibrium thermodynamics generates a kind of rubber-like high elasticity. The peculiarities of the hydrodynamic behaviour of aqueous solutions of polyethylene oxide (PEO) during flow under the conditions of various nozzle jets were modelled, and the regularities of the influence of the resulting dynamic structures on the efficiency of hydrojet water–polymer perforation were established. The mechanism of hydrojet water–polymer perforation of casing columns in oil and gas wells was clarified. The mecha-nism of the large destructive capacity of a high-velocity polymer solution jet is not due to the reduction of turbulent friction by small polymer additives (the Toms effect), but consists in the destructive action of the dynamic pressure of the water–polymer jet “rein-forced” by highly developed macromolecular coils and the dynamic structures formed under the action of extended flow in the inlet section of the hydroperforator nozzles. The method of perforating oil and gas well casings comprises the exact determination of the perforation zone in lowering on production tubing a hydroperforator with 2–4 jet flow-forming nozzles for directing hydrojet to the zone of perforation, sealing the inside cavity of tubing pipes and the jet operators, actuating a ball valve at the bottom of the jet operators, sealing the annulus with a self-sealing gland and supplying the working cutting fluid to the tubing – which differs in that the aqueous solution of PEO used as a working cutting fluid has a molecular weight of 6 · 106 and a concentration 0.003–0.007% by weight and a working pressure of 100–300 MPa. The PEO additives are very environmentally friendly because this polymer is not harmful to humans or the environment. Experimental and industrial testing of this method of well perforation, which was carried out during the secondary opening of a reservoir at a well in the Carpathian oil- and gas-bearing region, confirmed the practical and eco-nomic feasibility of its use.

Polymer solution structure and dynamics within pores of hexagonally close-packed nanoparticles.

Using coarse-grained molecular dynamics simulations, we examine structure and dynamics of polymer solutions under confinement within the pores of a hexagonally close-packed (HCP) nanoparticle system with nanoparticle diameter fifty times that of the polymer Kuhn segment size. We model a condition where the polymer chain is in a good solvent (i.e., polymer-polymer interaction is purely repulsive and polymer-solvent and solvent-solvent interactions are attractive) and the polymer-nanoparticle and solvent-nanoparticle interactions are purely repulsive. We probe three polymer lengths (N = 10, 114, and 228 Kuhn segments) and three solution concentrations (1, 10, and 25%v) to understand how the polymer chain conformations and chain center-of-mass diffusion change under confinement within the pores of the HCP nanoparticle structure from those seen in bulk. The known trend of bulk polymer Rg2 decreasing with increasing concentration no longer holds when confined in the pores of HCP nanoparticle structure; for example, for the 114-mer, the HCP 〈Rg2〉 at 1%v concentration is lower than HCP 〈Rg2〉 at 10%v concentration. The 〈Rg2〉 of the 114-mer and 228-mer exhibit the largest percent decline going from bulk to HCP at the 1%v concentration and the smallest percent decline at the 25%v concentration. We also provide insight into how the confinement ratio (CR) of polymer chain size to pore size within tetrahedral and octahedral pores in the HCP arrangement of nanoparticles affects the chain conformation and diffusion at various concentrations. At the same concentration, the N = 114 has significantly more movement between pores than the N = 228 chains. For the N = 114 polymer, the diffusion between pores (i.e., inter-pore diffusion) accelerates the overall diffusion rate for the confined HCP system while for the N = 228 polymer, the polymer diffusion in the entire HCP is dominated by the diffusion within the tetrahedral or octahedral pores with minor contributions from inter-pore diffusion. These findings augment the fundamental understanding of macromolecular diffusion through large, densely packed nanoparticle assemblies and are relevant to research focused on fabrication of polymer composite materials for chemical separations, storage, optics, and photonics. We perform coarse-grained molecular dynamics simulations to understand structure and dynamics of polymer solutions under confinement within hexagonal close packed nanoparticles with radii much larger than the polymer chain's bulk radius of gyration.

Passive and active tracer dynamics in polymer solutions with isotropic-to-nematic phase transition.

Macromolecular crowding plays a crucial role in determining the dynamics in a living cell. We adopt Langevin dynamics simulations to investigate the anomalous diffusion dynamics of passive and active particles in a solution of polymer chains with tunable stiffness. The solution's anisotropic feature is modulated by changing both the polymer stiffness and volume fraction, where isotropic-to-nematic phase transition is involved. Our results demonstrate the significant impact of polymer flexibility on the dynamics of both passive and active probes. The distinct diffusion mechanism for an active particle is clarified by the interplay between polymer stiffness, crowdedness and activity. Polymer stiffness leads to a global inhibition effect on passive particle diffusion. The diffusion coefficient exhibits an intriguing non-monotonic variation at increasing polymer stiffness, which is due to the fact that the alignment of polymer chains is beneficial for diffusion along the nematic direction but unfavorable for that in the direction perpendicular to it. In sharp contrast, polymer stiffness plays a dominant role in facilitating active particle diffusion. Self-propulsion of the particle can utilize stiffness-induced elastic interactions more efficiently, which promotes its mobility in both directions. Meanwhile, an active particle might have a stronger ability to take advantage of the polymer alignment, contributing substantially enhanced diffusivity. In addition, the diffusion coefficient of an active particle is subject to a tendency of degeneration against varying volume fraction. This counter-intuitive behavior is due to the contrasting factors that increasing crowdedness induces a lower particle speed but a longer persistent motion time.

Multilevel Monte Carlo method for the Brownian configuration field of polymer fluids

Stochastic Brownian dynamics is an extremely powerful way to simulate the polymer dynamics in solutions and melts. Mathematically, these models are described by stochastic differential equations. The most challenging problems are the Monte Carlo algorithm, which simulates the motion of a large number of model particles and hence requires an enormous amount of computer time. It is therefore necessary to develop an efficient numerical method in operational emergency response applications. In this paper, we give an improved multilevel Monte Carlo (improved-MLMC) method based on equilibrium control variables at each level to calculate the propagation of polymers. The improved-MLMC method can be shown to result in asymptotically optimal random errors and reduce total cost when compared to the standard Monte Carlo and MLMC methods. Finally, the effect of the Wi number (dimensionless parameter) on the total cost of the presented MLMC method is also discussed in detail.

Flexibility, Extensibility, and Ratio of Kuhn Length to Packing Length Govern the Pinching Dynamics, Coil-Stretch Transition, and Rheology of Polymer Solutions

We elucidate the influence of chemical structure on macromolecular hydrodynamics, rheological response, and pinching dynamics underlying drop formation/liquid transfer using polyethylene oxide (PEO...

Heterogeneous Solvent Dielectric Relaxation in Polymer Solutions of Water and Alcohols

The dynamics of polymer solutions are heterogeneous. The different molecule sizes and structures cause different mobilities of polymers and solvents. Recently, regarding the heterogeneity of water, two dielectric relaxation processes of water were argued in several supercooled polymer–water mixtures. To investigate whether this is unique to water, we performed broadband dielectric spectroscopy measurements of poly(vinylpyrrolidone)–water (PVP–water), PVP–propylene glycol, PVP–ethylene glycol, and PVP–propanol mixtures with 65 wt% PVP in a temperature range of 123–298 K. For the PVP–water mixture, α-relaxation of PVP and two distinct relaxation processes appeared simultaneously: one was the primary relaxation process of water (fast water process) and the other was the relatively small relaxation process (slow process), at a frequency between that of the α-process of PVP and the fast water process. The strength ratio of the large fast water and small slow processes remained nearly constant in all the temperatures measured. For the PVP–alcohol mixtures, in addition to the α-process of PVP, two or three relaxation processes of alcohol appeared. The primary relaxation process of alcohol above 240 K changed to the small fast secondary process. The small slow processes, which can be recognized below 240 K (the glass transition temperature of the α-process of PVP, Tg,PVP) appears at intermediate frequency between that of the PVP α-process and the primary process of alcohol. The small slow process changed to the primary larger process. The properties of the multiple relaxation processes of solvents below Tg,PVP in PVP–alcohol mixtures are completely different from those in PVP–water mixture.

Effect of molecular architecture on ring polymer dynamics in semidilute linear polymer solutions

Understanding the dynamics of ring polymers is a particularly challenging yet interesting problem in soft materials. Despite recent progress, a complete understanding of the nonequilibrium behavior of ring polymers has not yet been achieved. In this work, we directly observe the flow dynamics of DNA-based rings in semidilute linear polymer solutions using single molecule techniques. Our results reveal strikingly large conformational fluctuations of rings in extensional flow long after the initial transient stretching process has terminated, which is observed even at extremely low concentrations (0.025 c*) of linear polymers in the background solution. The magnitudes and characteristic timescales of ring conformational fluctuations are determined as functions of flow strength and polymer concentration. Our results suggest that ring conformational fluctuations arise due to transient threading of linear polymers through open ring chains stretching in flow.

Influence of polymer flexibility on nanoparticle dynamics in semidilute solutions.

The hierarchical structure and dynamics of polymer solutions control the transport of nanoparticles (NPs) through them. Here, we perform multi-particle collision dynamics simulations of solutions of semiflexible polymer chains with tunable persistence length lp to investigate the effect of chain stiffness on NP transport. The NPs exhibit two distinct dynamical regimes - subdiffusion on short time scales and diffusion on long time scales. The long-time NP diffusivities are compared with predictions from the Stokes-Einstein relation (SER), mode-coupling theory (MCT), and a recent polymer coupling theory (PCT). Increasing deviations from the SER as the polymer chains become more rigid (i.e. as lp increases) indicate that the NP motions become decoupled from the bulk viscosity of the polymer solution. Likewise, polymer stiffness leads to deviations from PCT, which was developed for fully flexible chains. Independent of lp, however, the long-time diffusion behavior is well-described by MCT, particularly at high polymer concentration. We also observed that the short-time subdiffusive dynamics are strongly dependent on polymer flexibility. As lp is increased, the NP dynamics become more subdiffusive and decouple from the dynamics of the polymer chain center-of-mass. We posit that these effects are due to differences in the segmental mobility of the semiflexible chains.

Bottom-up approach to represent dynamic properties in coarse-grained molecular simulations.

Several molecular coarse-graining methods have been proposed in recent years to derive chemical- and state-point transferable force fields. While these force fields describe structural and thermodynamic properties in good agreement with fine-grained models and experiments, dynamic properties are usually overestimated. Herein, we examine if the long-time dynamic properties of molecular coarse-grained (CG) systems can be correctly represented by employing a dissipative particle dynamics (DPD) thermostat, which is "bottom-up informed" by means of a variant of the Markovian Mori-Zwanzig (MZ) DPD coarse-graining method. We report single-site and multiple-site CG models for a monomer, dimer, and 24mer based on 2,2-dimethyl propane as a chemical repeat unit and report data obtained from MZ-DPD simulations of liquids, polymer solutions, and polymer melts. We find that despite incomplete time scale separation of the molecular CG model, MZ-DPD achieves quantitative accuracy in predicting diffusive dynamics in single-component liquids and polymer solutions (24mers in a dimer solvent). We also find that MZ-DPD simulations of molecular penetrant diffusion in polymer networks do not reach quantitative agreement with the fine-grained model. Modeling diffusion governed by the activated barrier crossing of small molecular penetrants in these dense systems requires an accurate description of energy barriers, presumably combined with the treatment of memory effects. The use of a MZ-DPD thermostat extends the scope and applicability of molecular CG models for multicomponent systems where a correct description of the relative diffusion rates of the different components is important.

Conformationally averaged iterative Brownian dynamics simulations of semidilute polymer solutions.

The dynamics of semidilute polymer solutions are important to many polymer solution processing techniques such as fiber spinning and solution printing. The out-of-equilibrium molecular conformations resulting from processing flows directly impact material properties. Brownian dynamics (BD) simulations are a standard technique for studying this connection between polymer conformations in solution and processing flows because they can capture molecular-level polymer dynamics. However, BD simulations of semidilute polymer solutions are computationally limited by the calculation of hydrodynamic interactions (HIs) via an Ewald summed diffusion tensor and stochastic Brownian displacements via the decomposition of the diffusion tensor. Techniques based on the Cholesky decomposition scale with the number of particles N as O(N 3) and approximations in the literature have reduced this scaling to as low as O(N). These methods still require continuous updating of the diffusion tensor and Brownian displacements, resulting in a significant constant per-time step cost. Previously, we introduced a method that avoids this cost for dilute polymer solutions by iterative conformational averaging (CA) of intramolecular HIs. In this work, we extend the CA method to semidilute solutions by introducing a grid-space average of intermolecular HIs and a pairwise approximation to the Brownian displacements based on the truncated expansion ansatz of Geyer and Winter. We evaluate our method by first comparing the computational cost with that of other simulation techniques. We verify our approximations by comparison with expected results for static and dynamic properties at equilibrium and use our method to demonstrate the concentration dependence of HI screening.