Solutions Of Rodlike Polymers Research Articles

Orientational order and topological defects in a dilute solutions of rodlike polymers at low Reynolds number.

The relationship between the polymer orientation and the chaotic flow, in a dilute solution of rigid rodlike polymers at low Reynolds number, is investigated by means of direct numerical simulations. It is found that the rods tend to align with the velocity field in order to minimize the friction with the solvent fluid, while regions of rotational disorder are related to strong vorticity gradients, and therefore to the chaotic flow. The "turbulent-like" behavior of the system is therefore associated with the emergence and interaction of topological defects of the mean director field, similarly to active nematic turbulence. The analysis has been carried out in both two and three spatial dimensions.

Nematic-isotopic phase separations induced by hydrogen bonding between a rodlike polymer and solvent molecules

ABSTRACT We theoretically study nematic-isotropic phase transitions and phase separations in aqueous rodlike polymer solutions, such as rigid DNA (or rodlike polymer) in water. By taking into the hydrogen bonding between a rodlike polymer and solvent molecules, we predict novel phase diagrams, such as overhang-type coexistence curves with isotropic-nematic ( I + N ) phase separations and re-entrant I + N separations. We demonstrate that the hydrogen bonding between molecules of different components stabilises the nematic phase at lower polymer concentrations and temperatures.

Enhancement of drag and mixing in a dilute solution of rodlike polymers at low Reynolds numbers

We study the dynamics of a dilute solution of rigid rodlike polymers in a viscous fluid at low Reynolds number by means of numerical simulations of a simple rheological model. We show that the rotational dynamics of polymers destabilizes the laminar flow and causes the emergence of a turbulent-like chaotic flow with a wide range of active scales. This regime displays an increased flow resistance, corresponding to a reduced mean flow at fixed external forcing, as well as an increased mixing efficiency. The latter effect is quantified by measuring the decay of the variance of a scalar field transported by the flow. By comparing the results of numerical simulations of the model in two- and three-dimensions, we show that the phenomena observed are qualitatively independent on the dimensionality of the space.

Capillary-Induced Phase Separation in Ultrathin Jets of Rigid-Chain Polymer Solutions

In the present paper, we study the terminal stage of the capillary thinning of a polymer solution jet formed with rod-like molecules. On long scales exceeding the rod length a uniform jet gets unstable according to the classical Plateau-Rayleigh pinching mechanism. We show, however, that a qualitatively different faster process, which can prevent the jet from breaking-up, generically comes into play once the jet radius becomes smaller than the rod length. Namely, the solvent drains out onto the jet surface forming annular droplets there, while the rods stay trapped inside the jet polymer core. As a result, the jet core becomes more concentrated and can solidify eventually. This process can provide a universal mechanism of the capillary-induced solvent/polymer phase separation leading to fiber formation (fiber spinning) from rod-like polymer solution jets.

Global Well-Posedness of Strong Solutions of Doi Model with Large Viscous Stress

We study models of dilute rigid rod-like polymer solutions. We establish the global well-posedness of the Doi model for large data and for arbitrarily large viscous stress parameter. The main ingredient in the proof is the fact that the viscous stress adds dissipation to high derivatives of velocity.

Enhancement of mixing by rodlike polymers.

We study the mixing of a passive scalar field dispersed in a solution of rodlike polymers in two dimensions, by means of numerical simulations of a rheological model for the polymer solution. The flow is driven by a parallel sinusoidal force (Kolmogorov flow). Although the Reynolds number is lower than the critical value for inertial instabilities, the rotational dynamics of the polymers generates a chaotic flow similar to the so-called elastic-turbulence regime observed in extensible polymer solutions. The temporal decay of the variance of the scalar field and its gradients shows that this chaotic flow strongly enhances mixing.

Biaxiality of cholesteric phases in rod-like polymer solutions

A mean field theory for the biaxiality of a cholesteric phase in rod-like polymer solutions is presented. By taking into account both excluded volume and attractive interactions between rods, we calculate the biaxiality of the cholesteric phase as a function of temperature and concentration. At low concentrations of rods, the biaxiality increases with increasing temperature. At high concentrations above , the biaxiality becomes the maximum as a function of temperature. We also calculate phase diagrams on the temperature–concentration plane and study phase separations between an isotropic phase and a biaxial cholesteric phase.

Polydomain growth at isotropic-nematic transitions in liquid crystalline polymers

We studied the dynamics of isotropic-nematic transitions in liquid crystalline polymers by integrating time-dependent Ginzburg-Landau equations. In a concentrated solution of rodlike polymers, the rotational diffusion constant D(r) of the polymer is severely suppressed by the geometrical constraints of the surrounding polymers so that the rodlike molecules diffuse only along their rod directions. In the early stage of phase transition, the rodlike polymers with nearly parallel orientations assemble to form a nematic polydomain. This polydomain pattern, with characteristic length ℓ, grows with self-similarity in three dimensions over time with an ℓ~t(1/4) scaling law. In the late stage, the rotational diffusion becomes significant, leading to a crossover of the growth exponent from 1/4 to 1/2. This crossover time is estimated to be on the order of t~D(r)(-1). We also examined the time evolution of a pair of disclinations placed in a confined system by solving the same time-dependent Ginzburg-Landau equations in two dimensions. If the initial distance between the disclinations is shorter than some critical length, they approach and annihilate each other; however, at larger initial separations, they are stabilized.

Micro-macro numerical simulation of rod-like polymeric solutions

利用微宏观耦合方法模拟了棒状分子聚合物溶液在平板Couette流动中的复杂流变行为，其中微宏观模型通过非均匀Doi理论来描述.数值模拟中，应用有限体积方法耦合求解了介观尺度上的Smoluchowski方程和宏观尺度上的流场守恒方程.数值结果不仅得到了若干种典型的流动类型，而且还预测了另外两种新的复合瑕疵结构.数值试验表明：棒状分子聚合物的流变结构主要依赖于De数、分子相对尺度以及溶液浓度常数的取值；并且De数对分子指向矢的翻滚周期、随流取向角等微观特性也均有明显影响.

Colloid dynamics in semiflexible polymer solutions

We investigate the dynamics of monodisperse colloidal polystyrene particles suspended in solutions of the semiflexible polymer filamentous actin, over a range of filament lengths that either exceed or are substantially less than the particle radius. The filament length is controlled by the capping protein gelsolin, and particle surface chemistries that minimize the adsorption of filaments are used. The particle dynamics are measured on short time scales using diffusing wave spectroscopy. A sharp transition in the initial particle diffusivity marks the expected shift from a dilute to a tightly entangled polymer network as the filament average length increases. In both the dilute and entangled regimes, the measured particle dynamics are compared with the theories of rodlike and semiflexible polymer solution rheology using the generalized Stokes-Einstein relationship. In the dilute limit, the particle dynamics are in good agreement with theory. However, in the tightly entangled regime, the particle response is consistent with polymer depleted near the surfaces of the particles. The magnitude of the depletion layer thickness depends strongly on particle size and weakly on filament length. This behavior is in agreement with nonlocal entropic repulsions and the loss of conformational entropy associated with rodlike molecules near impenetrable particles. These results illustrate the use of microrheology as a method to investigate local structure and dynamics in colloid-polymer solutions.

Batchelor’s theory extended to elongated cylindrical or ellipsoidal particles

Extensions to Batchelor’s theory have been derived to take into account different shaped particles while relating extensional viscosity enhancement to three parameters — shape, volume fraction and particle aspect ratio. The extended theory now allows calculation of the extensional viscosity enhancement, at a given volume fraction of particles, for either ellipsoidal or cylindrical particles. The formula improves the predictive capability of Batchelor’s theory when compared with measurements found in the literature for different rod-like polymer solutions.

Self-consistent mode-coupling theory for the viscosity of rodlike polyelectrolyte solutions.

A self-consistent mode-coupling theory is presented for the viscosity of solutions of charged rodlike polymers. The static structure factor used in the theory is obtained from polymer integral equation theory; the Debye-Huckel approximation is inadequate even at low concentrations. The theory predicts a nonmonotonic dependence of the reduced excess viscosity eta(R) on concentration from the behavior of the static structure factor in polyelectrolyte solutions. The theory predicts that the peak in eta(R) occurs at concentrations slightly lower than the overlap threshold concentration, c*. The peak height increases dramatically with increasing molecular weight and decreases with increased concentrations of added salt. The position of the peak, as a function of concentration divided by c*, is independent of salt concentration or molecular weight. The predictions can be tested experimentally.

Osmotic pressure of isotropic solutions of rodlike polymers

The osmotic pressure of isotropic solutions of athermal rodlike polymers is studied using Monte Carlo simulations. The simulation results show that scaling theory is in qualitative error for the dependence of pressure on concentration. The virial expansion truncated after the second virial coefficient, however, is in quantitative agreement with the simulation results.

Studies on the texture of nematic solutions of rodlike polymers. 3. Rheo-optical and rheological behavior in shear

The texture in shear flow is discussed for nematic solutions of rodlike poly(1,4-phenylene terephthalamide), reported here, and poly(1,4-phenylene-2,6-benzobisthiazole), reported elsewhere. These are similar, with a sequence of scattering and visual features that develop with time, terminating at a level characteristic of the shear rate in steady state. With increasing shear rate in torsion between parallel disks, steady-state flow reveals sequentially a diffuse scattering with a turbid appearance, diffuse scattering with an opalescent appearance, a broad elliptical scattering along the radial direction with poorly-defined striations, a sharp scattering along the radial direction with well-defined striations along the flow, and a clear, highly birefringent sample, with little scattering. These same features are seen, in the same sequence in time, in transient behavior leading to steady state. This behavior is attributed to the development of striations with twist distortions alternating in angle relative to the flow, and essentially in the sample plane in the fast steady-state flow, reflecting the lower free energy of that distortion in comparison with bend and splay distortions, especially near the parallel plates of the rheometer. The deterioration of this texture at slower flows is associated with increasing distortions out of the sample plane, leading to an essentially chaotic spatial distribution of the order tensor at a slow flow.

Viscoelastic Behavior of a Supramolecular Polymeric System Consisting of Tri-3,7-dimethyloctyl cis-1,3,5-cyclohexanetricarboxamide and n-Decane

Viscoelastic behavior of a supramolecular polymeric system, so-called an organogel, consisting of tri-3,7-dimethyloctyl cis-1,3,5-cyclohexanetricarboxamide (DO3CH) and n-decane (C10) was examined varying the concentration of DO3CH (c) and temperature ranged from 20 to 50°C. The obtained storage and loss moduli for the system were well described with the sum of two Maxwell models possessing two sets of a relaxation time and strength: τf and Gf, and τs (>τf) and Gs, whereas the viscoelastic behavior of organogels consisting of N, N', N"-tris(3,7-dimethyloctyl)benzene-1,3,5-tricarboxamide (DO3B) and n-alkanes is well described with only one Maxwell model. The value of Gf is proportional to c2 as observed in entangled flexible polymer systems and the organogels of DO3B. On the contrary, the value of Gs is approximately proportional to c1.3 similarly to the behavior predicted for rigid rodlike polymer solutions. The value of τf is essentially independent of c, while that of τs is kept at a constant and is followed by increasing above c = 10 gL−1. The activation energies of both the relaxation times are less than that for the viscosity of C10. Supramolecular polymeric structure bearing 2-type, rigid rodlike and flexible, portions are generated in the system due to intermolecular hydrogen bonding. The fast relaxation mode is attributed to the entanglement release between the flexible portions as observed in the organogels of DO3B, and the other is attributed to rotational relaxation of the rigid rodlike portion.

Flow-Induced Molecular Orientation in Coating Flow of Concentrated Solution of Rod-Like Polymers and Its Application to Alignment Layer for Liquid Crystals.

The numerical simulations for the model coating flow of concentrated solutions of hydroxypropylcellulose (HPC) are carried out using the Doi model for rod-like polymers. The high degree of molecular orientation is induced by an elongational flow at the die lip and the molecules align parallel to the free surface. This molecular orientation near the free surface in coating flow is discussed in connection with the surface alignment of liquid crystals (LCs) on the HPC films. In the experiment on surface alignment of 5CB on the HPC film formed by coating, parallel surface alignment of 5CB is obtained in the conditions of large coating speed, thin coating thickness and high concentration of HPC. This tendency is well explained from the simulation results. From both the results of the numerical simulation and the experiment, it is confirmed that the surface alignment of LCs on the HPC films formed by coating results from the interaction of LC molecules with HPC molecules oriented parallel to the free surface.

The Effects of Concentration, Pressure, and Temperature on the Diffusion Coefficient and Correlation Length of SDS Micelles

The effects of sodium dodecyl sulfate (SDS) concentration (≤ 10 mass %), temperature (35 and 50 °C), and pressure (0.1 to 100 MPa) on the cooperative diffusion coefficient and static correlation length of micelles in solutions in 1 M NaCl were studied using static and dynamic light scattering. These data are interpreted in terms of models of dilute and semidilute solutions of rod-like polymers. The results indicate that the effects of pressure are most significant near the crossover concentration between dilute and semidilute solution behavior. This concentration is a function of temperature and pressure because both of these thermodynamic parameters affect the mean micellar size.

On the determination of the average molecular weight, radius of gyration, and mass/length ratio of polydisperse solutions of polymerizing rod-like macromolecular monomers by multi-angle static light scattering

The angular dependence of the light scattered from polydisperse solutions of rod-like or worm-like linear polymers formed by the polymerization of rod-like macromolecular monomers was studied with the aid of computer simulations. Except at very low conversion degrees, these ensembles are characterized by curved Zimm-like plots. An appraisal of the use of polynomial fittings for the derivation of the weight-average molecular weight (Mw) and of the z-average square radius of gyration ( z) from such plots is presented and discussed. Depending on the average size and shape distribution, the use of polynomial fittings allows the applicability of the Zimm method well beyond the standard condition q2 z<<1, q being the scattering wavevector. In addition, the derivation of the w/z-average mass/length ratio ML of the polymers from only partially linear Casassa plots, from which large errors in the derived ML values can be made, is analyzed. By combining the Casassa method with the complementary Holtzer plots, it is usually possible to assess the reliability of the results and give an estimate of their accuracy. However, it was also found that apparently linear Casassa plots, allowing a good estimate of the ML value, may arise in particular situations even when the Holtzer plot would indicate otherwise.

2015 棒状高分子濃厚溶液のコーティング流れ

Mechanism of surface alignment of liquid crystals (LCs) on the films formed by coating with concentrated solutions of hydroxypropylcellulose (HPC) is studied using a model coating flow. The numerical simulations for the model coating flow are carried out using the Doi model for concentrated solutions of rod-like polymers. The development of molecular orientation on the free surface is revealed in the simulation. Furthermore, parallel surface alignment is obtained under the conditions of high coating speed and thin coating thickness in the experiment of the coating film. The experimental results are discussed in connection with the simulation ones.

Studies on the Texture of Nematic Solutions of Rodlike Polymers. 2. Development and Relaxation of Loop Disclinations

Features of the development and relaxation of loop defects in the texture of nematic solutions of two rodlike polymers are discussed. Examples given for solutions of poly(1,4-phenylene terephthalamide) and poly(1,4-phenylene-2,6-benzobisthiazole) show the overall similarity of the behavior for these two materials. The development of the mottled texture observed shortly after the solution is extruded into a rectangular capillary, to form a tangle of defect lines which then evolves to form individual loops, is discussed. The differing relaxation dynamics of individual loops observed in the final stages of coarsening near the cell center and close to a surface are presented, along with novel effects observed in the interaction of such loops with an external magnetic field.