Molecular dynamics simulation of interactions between a sodium dodecyl sulfate micelle and a poly(ethylene oxide) polymer

We have performed atomistic molecular dynamics simulations of an anionic sodium dodecyl sulfate (SDS) micelle and a nonionic poly(ethylene oxide) (PEO) polymer in aqueous solution. The micelle consisted of 60 surfactant molecules, and the polymer chain lengths varied from 20 to 40 monomers. The force field parameters for PEO were adjusted by using 1,2-dimethoxymethane (DME) as a model compound and matching its hydration enthalpy and conformational behavior to experiment. Excellent agreement with previous experimental and simulation work was obtained through these modifications. The simulated scaling behavior of the PEO radius of gyration was also in close agreement with experimental results. The SDS-PEO simulations show that the polymer resides on the micelle surface and at the hydrocarbon-water interface, leading to a selective reduction in the hydrophobic contribution to the solvent-accessible surface area of the micelle. The association is mainly driven by hydrophobic interactions between the polymer and surfactant tails, while the interaction between the polymer and sulfate headgroups on the micelle surface is weak. The 40-monomer chain is mostly wrapped around the micelle, and nearly 90% of the monomers are adsorbed at low PEO concentration. Simulations were also performed with multiple 20-monomer chains, and gradual addition of polymer indicates that about 120 monomers are required to saturate the micelle surface. The stoichiometry of the resulting complex is in close agreement with experimental results, and the commonly accepted "beaded necklace" structure of the SDS-PEO complex is recovered by our simulations.     

Attractions, water structure, and thermodynamics of hydrophobic polymer collapse

We explore the prospects of a perturbation approach for predicting how weak attractive interactions affect collapse thermodynamics of hydrophobic polymers in water. Specifically, using molecular dynamics simulations of model polymers in explicit water, we show that the hydration structure is sensitive to the strength of the van der Waals attractions but that the hydration contribution to the potential of mean force for collapse is not. We discuss how perturbation theory ideas developed for small spherical apolar solutes need to be modified in order to account for the effect of attractions on the conformational equilibria of polymers.     

Discontinuous molecular dynamics simulation study of polymer collapse

Discontinuous molecular dynamics simulations were used to study the coil-globule transition of a polymer in an explicit solvent. Two different versions of the model were employed, which are differentiated by the nature of monomer-solvent, solvent-solvent, and nonbonded monomer-monomer interactions. For each case, a model parameter lambda determines the degree of hydrophobicity of the monomers by controlling the degree of energy mismatch between the monomers and solvent particles. We consider a lambda-driven coil-globule transition at constant temperature. The simulations are used to calculate average static structure factors, which are then used to determine the scaling exponents of the system in order to determine the theta-point values lambda(theta) separating the coil from the globule state. For each model we construct coil-globule phase diagrams in terms of lambda and the particle density rho. Additionally, we explore for each model the effects of varying the range of the attractive interactions on the phase boundary separating the coil and globule phases. The results are analyzed in terms of a simple Flory-type theory of the collapse transition.     

Shear thinning in dilute polymer solutions

We use bead-spring models for a polymer coupled to a solvent described by multiparticle collision dynamics to investigate shear thinning effects in dilute polymer solutions. First, we consider the polymer motion and configuration in a shear flow. For flexible polymer models we find a sharp increase in the polymer radius of gyration and the fluctuations in the radius of gyration at a Weissenberg number approximately 1. We then consider the polymer viscosity and the effect of solvent quality, excluded volume, hydrodynamic coupling between the beads, and finite extensibility of the polymer bonds. We conclude that the excluded volume effect is the major cause of shear thinning in polymer solutions. Comparing the behavior of semiflexible chains, we find that the fluctuations in the radius of gyration are suppressed when compared to the flexible case. The shear thinning is greater and, as the rigidity is increased, the viscosity measurements tend to those for a multibead rod.     

疏水改性聚丙烯酰胺溶液的分子模拟

设计了几种不同的非离子型改性聚丙烯酰胺(HM-PAM)和阴离子型改性聚丙烯酰胺(HM-HPAM). 通过分子动力学模拟(MD)方法研究了在聚合物链上加入不同疏水改性单体对提高聚丙烯酰胺耐盐性的影响, 考察了盐浓度对疏水改性聚丙烯酰胺的回旋半径(R_g)、 特性黏数([η])、 径向分布函数(RDF)和均方位移(MSD)的影响以及聚合物的微观结构与特性黏数之间的关系. 研究结果表明, 引入疏水改性单体后, 改性聚丙烯酰胺具有较好的耐盐性. 通过研究非键作用与氢键相互作用可知, 体系中溶质和溶剂间的相互作用及氢键作用越弱, 溶液的特性黏数越大. O-H原子对的RDF结果表明, 聚合物链的伸展与聚合物链及官能团间的相互作用有关. 当RDF峰较弱时, 聚合物链与水的作用越弱, 越有利于聚合物链保持舒展状态, 溶液的特性黏数也就越大. 另外, 从聚合物链的MSD曲线发现, 聚合链的移动性与特性黏数呈负相关.     

Dynamics of liquid–liquid demixing: mesoscopic simulations of polymer solutions

In this article, the mesoscopic simulation method dissipative particle dynamics (DPD) is applied to study the dynamics of polymer–solvent liquid–liquid phase separation. It will be shown that the degree of branching has a pronounced effect on the radius of gyration and the centre of mass diffusion of the polymer. Based on the simulation results it can be concluded that the difference in chemical potential between the mixed and the demixed state is the main driving force behind the centre of mass diffusion (and thus phase separation), rather than the reduced radius of gyration due to to polymer chain collapse.     

良溶剂中环形高分子单链的结构与动力学性质的模拟研究

本文采用多粒子碰撞动力学与分子动力学耦合的模拟方法研究了环形高分子单链在良溶剂中的静态与动态性质,并与线形分子进行了对比.研究发现,环形高分子链内粒子之间的平均距离小于线形链,即粒子排列得更加紧密;相应的均方回转半径也小于线形链,线形链与环形链的均方回转半径的比值为1.77;同时,环形链扩散的速度也比线形链快,两者比值为1.10.模拟结果揭示了扩散行为是排斥体积作用和流体力学相互作用耦合的结果,在扩散过程中,流体力学相互作用消减了排斥体积作用对扩散行为的贡献.此外,通过对有和没有流体力学相互作用的多粒子碰撞动力学得到的结果作对比,研究了流体力学相互作用对高分子静态和动态行为的影响,结果表明,流体力学相互作用使高分子链在极稀溶液中的扩散速度变快.     

Coarse-grained molecular dynamics simulation of self-assembly of polyacrylamide and sodium dodecylsulfate in aqueous solution

Coarse-grained molecular dynamics simulations have been performed to study the self-assembly of polymer, polyacrylamide (PAM) and surfactant, sodium dodecylsulfate (SDS) in aqueous solution. Our simulations revealed that PAM curled into clusters in the absence of SDS, while it was stretched if SDS was added. For the SDS-PAM complexes, the aggregate formation process can be divided into three stages: firstly, PAM quickly absorbs some SDS monomers until the radius of gyration (Rg) of polymer reaches a minimum; then, PAM stretches and the Rg of PAM increases due to more and more adsorbed SDS; ultimately, the commonly accepted "necklace" structure is formed with PAM located at the interface of the hydrophobic and hydrophilic regions of the SDS micelle. The main driving force for the association was hydrophobic interactions between the polymer backbone and the surfactant hydrophobic tails. As the concentration of SDS increased, the Rg of PAM increased up to a maximum, indicating the polymer was saturated with surfactant.     

Semiflexible amphiphilic polymers: cylindrical-shaped, collagenlike, and toroidal structures

A coarse-grained model is used to study the conformational properties of semiflexible polymers with amphiphilic monomer units containing both hydrophilic and hydrophobic interaction sites. The hydrophobically driven conformational transitions are studied using molecular dynamics simulations for the chains of varying stiffness, as characterized by intrinsic Kuhn segment lengths that vary over a decade. It is shown that the energy of hydrophobic attraction required for the realization of the coil-to-globule transition increases with increasing chain stiffness. For rather stiff backbone, the coil-to-globule transition corresponds to a first order phase transition. We find that depending on the chain stiffness, a variety of thermodynamically stable anisometric chain morphologies are possible in a solvent selectively poor for hydrophobic sites of amphiphilic monomer units. For flexible chains, the amphiphilic polymer forms a cylindrical globule having blob structure with nearly spherical blobs. With increasing stiffness, the number of blobs composing the globule decreases and the shape of blobs transforms into elongated cylinder. Further increase in stiffness leads to compaction of macromolecules into a collagenlike structure when the chain folds itself several times and different strands wind round each other. In this state, the collagenlike structures coexist with toroidal globules, both conformations having approximately equal energies.     

The effect of inhomogeneous stickiness on polymer aggregation

The aggregation of polymers is important in the formation of marine aggregates and the vertical transport of material in the ocean. A polymer may be inhomogeneous along its length, with associating groups at some points along its length where bonds are more likely to form. In this paper we investigate the effects of inhomogeneous 'stickiness' along the polymer length. We describe the results of three-dimensional off-lattice simulations of polymer-polymer aggregation for four different types of polymer: polymers which are sticky along their entire length, polymers which are sticky at the ends only and two types of polymer which are slightly sticky along their entire length. We examine the mean radius of gyration and the fractal dimension of the resulting aggregates and the dynamics of aggregation. The slightly sticky polymers and the polymers which are sticky only at the ends form aggregates with a higher fractal dimension than the polymers which are sticky along their entire length. However, the mean radius of gyration of the aggregates formed by polymers which are sticky only at the ends is significantly larger than that of the aggregates formed from slightly sticky polymers. The aggregation dynamics are also different for the polymers which are sticky only at the ends compared to the slightly sticky polymers. A single 'stickiness value' is therefore likely to be inadequate to describe a polymer. We also examine the effect of polymer rigidity; it seems that the effect of inhomogeneous stickiness is greater for almost-straight polymers than for coiled chains.     

高分子链坍塌转变动力学过程的动态蒙特卡罗模拟

基于键长涨落格子模型和动态蒙特卡罗模拟方法,引入疏水相互作用,对均聚高分子链坍塌转变动力学过程进行了模拟研究.模拟发现,其坍塌时间呈高斯分布,而平均坍塌时间随淬火深度的变化类似于蛋白质折叠,但难以发现局部最小;另外,平均坍塌时间随链长呈指数形式变化.在其坍塌动力学过程中,高分子链构象先由橄榄球状演变为项链状,进而演变为香肠状,最后形成近球状的熔融球;基于团簇数目、团簇大小和非球面参数等参量,对前人提出的动力学过程四阶段划分进行了更为清晰的界定.     

线形、梳形和星形高分子静态和动态性质的模拟

以梳形高分子为纽带,基于粗粒化分子动力学模拟方法,研究了线形、梳形和星形拓扑结构高分子的静态和动态性质,以揭示稀溶液中高分子链行为与链拓扑结构依赖关系的一般性规律.研究结果表明,随着线形-梳形-星形的链拓扑结构转变,回转半径的标度关系由仅依赖分子聚合度转变为同时依赖链聚合度与臂数或侧链数.分析了星形高分子和梳形高分子的静态和动态性质的特征规律.星形高分子的臂数增加使其尺寸迅速减小,形状则由长椭球形转变为类球形,且扩散系数也随之增加;其均方回转半径(〈R_g〉)和扩散系数(D)与分子聚合度(N)及臂数(f)的标度规律为〈R_g〉~N~(0.581)f~(-0.402),D~N~(-0.763)f~(0.227).梳形高分子的静态与动态性质与分子聚合度及侧链数的依赖关系为〈R_g〉~N~(0.597)f~(-0.212)(每个支化点只有一条侧链)和〈R_g〉~N~(0.599)f~(-0.316)(每个支化点有多条侧链).     

Lattice-Boltzmann simulations of the dynamics of polymer solutions in periodic and confined geometries

A numerical method to simulate the dynamics of polymer solutions in confined geometries has been implemented and tested. The method combines a fluctuating lattice-Boltzmann model of the solvent [Ladd, Phys. Rev. Lett. 70, 1339 (1993)] with a point-particle model of the polymer chains. A friction term couples the monomers to the fluid [Ahlrichs and Dunweg, J. Chem. Phys. 111, 8225 (1999)], providing both the hydrodynamic interactions between the monomers and the correlated random forces. The coupled equations for particles and fluid are solved on an inertial time scale, which proves to be surprisingly simple and efficient, avoiding the costly linear algebra associated with Brownian dynamics. Complex confined geometries can be represented by a straightforward mapping of the boundary surfaces onto a regular three-dimensional grid. The hydrodynamic interactions between monomers are shown to compare well with solutions of the Stokes equations down to distances of the order of the grid spacing. Numerical results are presented for the radius of gyration, end-to-end distance, and diffusion coefficient of an isolated polymer chain, ranging from 16 to 1024 monomers in length. The simulations are in excellent agreement with renormalization group calculations for an excluded volume chain. We show that hydrodynamic interactions in large polymers can be systematically coarse-grained to substantially reduce the computational cost of the simulation. Finally, we examine the effects of confinement and flow on the polymer distribution and diffusion constant in a narrow channel. Our results support the qualitative conclusions of recent Brownian dynamics simulations of confined polymers [Jendrejack et al., J. Chem. Phys. 119, 1165 (2003) and Jendrejack et al., J. Chem. Phys. 120, 2513 (2004)].     

Simulation study of the coil-globule transition of a polymer in solvent

Molecular dynamics simulations are used to study the coil-globule transition for a system composed of a bead-spring polymer immersed in an explicitly modeled solvent. Two different versions of the model are used, which are differentiated by the nature of monomer-solvent, solvent-solvent, and nonbonded monomer-monomer interactions. For each case, a model parameter lambda determines the degree of hydrophobicity of the monomers by controlling the degree of energy mismatch between the monomers and solvent particles. We consider a lambda-driven coil-globule transition at constant temperature. The simulations are used to calculate average static structure factors, which are then used to determine the scaling exponents of the system in order to determine the theta-point values lambdatheta separating the coil from the globule states. For each model we construct coil-globule phase diagrams in terms of lambda and the particle density rho. The results are analyzed in terms of a simple Flory-type theory of the collapse transition. The ratio of lambdatheta for the two models converges in the high density limit exactly to the value predicted by the theory in the random mixing approximation. Generally, the predicted values of lambdatheta are in reasonable agreement with the measured values at high rho, though the accuracy improves if the average chain size is calculated using the full probability distribution associated with the polymer-solvent free energy, rather than merely using the value obtained from the minimum of the free energy.     

Pressure driven flow of polymer solutions in nanoscale slit pores

Polymer solutions subject to pressure driven flow and in nanoscale slit pores are systematically investigated using the dissipative particle dynamics approach. The authors investigated the effect of molecular weight, polymer concentration, and flow rate on the profiles across the channel of the fluid and polymer velocities, polymer density, and the three components of the polymers radius of gyration. They found that the mean streaming fluid velocity decreases as the polymer molecular weight and/or polymer concentration is increased, and that the deviation of the velocity profile from the parabolic profile is accentuated with increase in polymer molecular weight or concentration. They also found that the distribution of polymers conformation is highly anisotropic and nonuniform across the channel. The polymer density profile is also found to be nonuniform, exhibiting a local minimum in the center plane followed by two symmetric peaks. They found a migration of the polymer chains either from or toward the walls. For relatively long chains, as compared to the thickness of the slit, a migration toward the walls is observed. However, for relatively short chains, a migration away from the walls is observed.     

Molecular dynamics simulation study of solvent effects on conformation and dynamics of polyethylene oxide and polypropylene oxide chains in water and in common organic solvents

In this paper, the conformation and dynamics properties of polyethylene oxide (PEO) and polypropylene oxide (PPO) polymer chains at 298 K have been studied in the melt and at infinite dilution condition in water, methanol, chloroform, carbon tetrachloride, and n-heptane using molecular dynamics simulations. The calculated density of PEO melt with chain lengths of n = 2, 3, 4, 5 and, for PPO, n = 7 are in good agreement with the available experimental data. The conformational properties of PEO and PPO show an increasing gauche preference for the O-C-C-O dihedral in the following order water>methanol>chloroform>carbon tetrachloride = n-heptane. On the contrary, the preference for trans conformation has a maximum in carbon tetrachloride and n-heptane followed in the order by chloroform, methanol, and water. The PEO conformational preferences are in qualitative agreement with results of NMR studies. PEO chains formed different types of hydrogen bonds with polar solvent molecules. In particular, the occurrence of bifurcated hydrogen bonding in chloroform was also observed. Radii of gyration of PEO chains of length larger than n = 9 monomers showed a good agreement with light scattering data in water and in methanol. For the shorter chains the observed deviations are probably due to the enhanced hydrophobic effects caused by the terminal methyl groups. For PEO the fitting of end-to-end distance distributions with the semi-flexible chain model at 298 K provided persistence lengths of 0.375 and 0.387 nm in water and methanol, respectively. Finally, the radius of gyration of Pluronic P85 turned out to be 2.25 ± 0.4 nm at 293 K in water in agreement with experimental data.     

New insights into the molecular architecture of hardwood lignins by chemical degradative methods

Lignins are composed of phenylpropane units interconnected by labile and resistant bonds. A two-step degradation, thioacidolysis, provides detailed information on these network polymers. The first step involves lignin depolymerization with ethanethiol and BF₃ etherate. The determination of the recovered thioethylated monomers provides an estimation of lignin units only involved in labile ether bonds. The ligninderived dimers, representative of resistant interunit bonds, are determined after a further desulfurization step. Results obtained for native and industrial hardwood lignins underline their structural differences. Native hardwood lignins are typified by a high proportion of linear fragments linked bifunctionally by β-O-4 bonds.     

Molecular dynamics in thin (grafted) polymer layers

Broadband dielectric spectroscopy is employed to study the molecular dynamics in thin polymer layers of (spin-coated) cis-1, 4-polyisoprene (PI) and of (grafted and spin-coated) polydimethylsiloxane (PDMS). For the former, besides the segmental and the normal mode relaxation, a confinement-induced mode is observed, which is assigned to fluctuations of terminal subchains. It turns out that the uptake of water has a strong influence on the dynamics of this relaxation, which takes place in the immediate vicinity of the interface. Between grafted and spin-coated PDMS a pronounced difference in the dynamics is observed for layers that are comparable in thickness to the radius of gyration of the confined chain. This is attributed to the different conformation of grafted and spin-coated polymer layers.     

Structure and dynamics of dextran in binary mixtures of a good and a bad solvent

The structure and dynamics of the common polysaccharide dextran have been investigated in mixed solvents at two different temperatures using small-angle X-ray scattering (SAXS) and viscosity measurements. More specifically, binary mixtures of a good solvent (water, formamide, dimethylsulfoxide, ethanolamine) and the bad solvent ethanol as the minority component have been considered. The experimentally observed effects on the polymer conformation (intrinsic viscosity, coil radius, and radius of gyration) of the bad solvent addition are discussed in terms of hydrogen bonding density and are correlated with the Hansen solubility parameters and the surface tension of the solvent mixtures. Hydrogen bonding appears to be an important contributor to the solubility of dextran but is not sufficient to capture the dextran coil contraction in the mixtures of good+bad solvents.     

FTIR-ATR Monitoring and SEC/RI/MALLS Characterization of ATRP Synthesized Hyperbranched Polyacrylates

This work reports the synthesis at 1 L scale of hyperbranched polyacrylates based upon acrylate/diacrylate monomers such as n-butyl acrylate (nBA)/1,6-hexanediol diacrylate (HDDA) and using atom transfer radical polymerization (ATRP). A FTIR-ATR immersion probe was used to monitor the polymerization reaction. The dynamics of the build-up of polymer structure was studied by off-line analysis of samples at different reaction times by size exclusion chromatography (SEC) with detection of refractive index (RI) and multi-angle laser light scattering (MALLS) signals, leading to molecular weight distribution and z-average radius of gyration. Kinetic measurements and observed parameters of the molecular architecture are compared with theoretical predictions which can be used to design new synthesis strategies to improve the homogeneity of hyperbranched polymers. Another goal of this study was elucidating the impact on polymerization of secondary reactions such as intramolecular cyclizations. For comparison purposes, FRP (conventional radical polymerization) of the same monomers is also considered.