A strategy is developed for generating equilibrated high molecular-weight polymer melts described with microscopic detail by sequentially backmapping coarse-grained (CG) configurations. The microscopic test model is generic but retains features like hard excluded volume interactions and realistic melt densities. The microscopic representation is mapped onto a model of soft spheres with fluctuating size, where each sphere represents a microscopic subchain with N b monomers. By varying N b a hierarchy of CG representations at different resolutions is obtained. Within this hierarchy, CG configurations equilibrated with Monte Carlo at low resolution are sequentially fine-grained into CG melts described with higher resolution. A Molecular Dynamics scheme is employed to slowly introduce the microscopic details into the latter. All backmapping steps involve only local polymer relaxation thus the computational efficiency of the scheme is independent of molecular weight, being just proportional to system size. To demonstrate the robustness of the approach, microscopic configurations containing up to n = 1000 chains with polymerization degrees N = 2000 are generated and equilibration is confirmed by monitoring key structural and conformational properties. The extension to much longer chains or branched polymers is straightforward.Studying equilibrium and rheological properties of melts of long polymer chains with computer simulations requires the preparation of equilibrated configurations described with microscopic detail. For this purpose, stochastic approaches have been proposed to circumvent the prohibitively large relaxation times in schemes with physically realistic dynamics, resulting from chain entanglements. Among methods addressing directly the microscopic scale, re-bridging (RB) algorithms 1 are the most advanced, modifying the chain connectivity while avoiding significant changes in local monomer packing. Even with their help, the longest melts currently addressed are those of linear polyethylene, corresponding to monodisperse samples with a few C 1000 chains. 1 Introducing polydispersity, increases the acceptance rate of RB moves and longer chains can be modeled. However, the system becomes less well-defined, e.g., for understanding rheological behavior and the samples remain rather small: the longest C 6000 (average length) melt 2 that was realized contained 32 chains. To prove equilibration these studies relied on the decay of conformational correlations. However, recent findings 3 demonstrate that the combination of chain connectivity and limited compressibility affects chain conformations. Since RB moves are largely decoupled from density fluctuations, such subtle effects suggest 3 that to verify unambiguously melt equilibration more sensitive descriptors of chain shape, such as internal distance plots, 3,4 should be considered.To overcome the limitations encountered when 1 arXiv:1610.07511v1 [cond-mat.soft]
Specific ion effects are ubiquitous in biological and colloidal systems. The addition of electrolytes to ionic surfactant solutions has pronounced effects on micellar properties, such as critical micelle concentration (cmc), micellar size, and shape. Ions play an important role in colloid stability and aggregation behavior of ionic surfactant solutions. Despite extensive experimental data, there is no well established molecular theory on specific ion effects. Published molecular thermodynamic theories for ionic surfactants do not properly account for ion-specific effects such as the inversion of the lyotropic series for the cmc of alkyl sulfates and carboxylates. In this work, we present a molecular thermodynamic theory for ionic surfactant solutions to take into account the headgroup-counterion specificity and address ion-specific effects on the cmc and aggregation number. We assume that the charged headgroup and the counterion at the Stern layer form solvent-shared ion pair with different degrees of cosphere overlap. The thickness of the Stern layer is estimated from molecular structures of hydrated surfactant heads and hydrated counterions, and from the knowledge of the qualitative strength of headgroup-counterion interaction in line with Collins' concept of matching water affinities. Our proposed thermodynamic model properly predicts the cmc of both anionic and cationic surfactants of various counterions, and the effect of different inorganic salts on micellization of ionic surfactants.
Topological constraints due to chain connectivity and uncrossability greatly impact the long time dynamics and rheology of high molecular weight polymer melts. Computer simulations to study properties of such melts are very advantageous, since perfect control of molecular conformation and melt morphology is available. We present a methodology to prepare well-equilibrated polymer melts which only requires local relaxation. The approach efficiently leads to equilibrated ensembles of bead-spring polymer melts of 1 000 chains of up to 2 000 beads, which correspond to 24 (fully flexible) and 45 entanglement lengths (semi-flexible chains). Entanglements are identified by a primitive path analysis and a master curve of the entanglement lengths for different chain and persistence lengths is presented.
The effect of adding an alcohol to surfactant systems depends much on the alcohol chain length. Investigations on the effect of alcohols in micellar systems point out that medium-chain alcohols are appreciably incorporated in the micellar phase whereas short-chain alcohols are localized mainly in the aqueous phase. Nonetheless, penetration of the hydrocarbon chain of alcohols in the micellar shell has been experimentally observed for the entire homologous series of linear 1-alcohols. We present a thermodynamic model in which the alcohol molecules play two roles: cosurfactant and cosolvent. The cosurfactant effect of the alcohols is included by assuming that the alcohol molecules are nonionic surfactants. The cosolvent effect is modeled by accounting for the changes in the free energy to relocate the surfactant tail from the solvent to the aggregate core. The effects of short-chain alcohols in the macroscopic interfacial tension and dielectric constant of the solvent medium are also taken into account. For short-chain alcohols the partition coefficient of the alcohols between water and liquid hydrocarbons provides knowledge of the fraction of the molecules that participate in each function. Our proposed thermodynamic model improves the modeling of the effect of short- and medium-chain alcohols in self-assembly of molecules that are of increasing importance in modern scientific research and technological processes.
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