Real-space analysis of branch point motion in architecturally complex polymers

By means of large-scale molecular dynamics simulations, we investigate branch point motion in pure branched polymers and in mixtures of stars and linear chains. We perform a purely geometrical density-based cluster analysis of the branch point trajectories and identify regions of strong localization (traps). Our results demonstrate that the branch point motion can be described as the motion over a network of traps at the time scales corresponding to the reptation regime. Residence times within the traps are broadly distributed, even extending to times much longer than the side-arm relaxation time. The distributions of distances between consecutively visited traps are very similar for all the investigated branched polymers, even though tube dilation is much stronger in the star/linear mixtures than in the pure branched systems. Our analysis suggests that the diffusivity of the branch point introduced by hierarchical models must be understood as a parameter to account for the effective friction associated with the relaxed side arm, more than the description of a hopping process with a precise time scale.We acknowledge support from projects FP7-PEOPLE-2007-1-1-ITN (DYNACOP, EU), MAT2012-31088 (Spain), and IT654-13 (GV, Spain). We acknowledge the programs PRACE, HPC-Europa2 and ESMI (EU), and ICTS (Spain) for generous allocation of CPU time at GENCI (France), HLRS and FZJ-JSC (Germany), and CESGA (Spain).Peer Reviewe

Proposed sets of critical exponents for randomly branched polymers, using a known string theory model

The critical exponent for randomly branched polymers with dimensionality d equal to 3, is known exactly as 1/2. Here, we invoke an already available string theory model to predict the remaining static critical exponents. Utilizing results of Hsu et al. (Comput Phys Commun. 2005;169:114-116), results are added for d = 8. Experiment plus simulation would now be important to confirm, or if necessary to refine, the proposed values.N.H. March wishes to acknowledge that his contribution to this study was brought to fruition during a visit to DIPC in 2015.Peer Reviewe

Is there a higher-order mode coupling transition in polymer blends?

arXiv:cond-mat/0511181v3We present simulations on a binary blend of bead-spring polymer chains. The introduction of monomer size disparity yields very different relaxation times for each component of the blend. Competition between two different arrest mechanisms, namely, bulklike dynamics and confinement, leads to an anomalous relaxation scenario for the fast component, characterized by sublinear time dependence for mean squared displacements, or logarithmic decay and convex-to-concave crossover for density-density correlators. These anomalous dynamic features, which are observed over time intervals extending up to 4 decades, strongly resemble predictions of mode coupling theory for nearby higher-order transitions. Chain connectivity extends anomalous relaxation over a wide range of blend compositions.Support from the projects NMP3-CT-2004-502235 (SoftComp), MAT2004-01017 (Spain), and 206.215-13568∕2001 (GV-UPV∕EHU Spain) is acknowledged.Peer reviewe

From caging to Rouse dynamics in polymer melts with intramolecular barriers: A critical test of the mode coupling theory

12 páginas, 12 figuras.-- et al.By means of computer simulations and solution of the equations of the mode coupling theory (MCT), we investigate the role of the intramolecular barriers on several dynamic aspects of nonentangled polymers. The investigated dynamic range extends from the caging regime characteristic of glass-formers to the relaxation of the chain Rouse modes. We review our recent work on this question, provide new results, and critically discuss the limitations of the theory. Solutions of the MCT for the structural relaxation reproduce qualitative trends of simulations for weak and moderate barriers. However, a progressive discrepancy is revealed as the limit of stiff chains is approached. This disagreement does not seem related with dynamic heterogeneities, which indeed are not enhanced by increasing barrier strength. It is not connected either with the breakdown of the convolution approximation for three-point static correlations, which retains its validity for stiff chains. These findings suggest the need of an improvement of the MCT equations for polymer melts. Concerning the relaxation of the chain degrees of freedom, MCT provides a microscopic basis for time scales from chain reorientation down to the caging regime. It rationalizes, from first principles, the observed deviations from the Rouse model on increasing the barrier strength. These include anomalous scaling of relaxation times, long-time plateaux, and nonmonotonous wavelength dependence of the mode correlators.We acknowledge financial support from projects FP7-PEOPLE-2007-1-1-ITN (DYNACOP, EU), MAT2007-63681 (Spain), IT-436-07 (GV, Spain), ERC-226207-PATCHYCOLLOIDS (EU), and ITN- 234810-COMPLOIDS (EU).Peer reviewe

Structure and dynamics of self-assembled comb copolymers: Comparison between simulations of a generic model and neutron scattering experiments

12 páginas, 15 figuras, 1 tabla.-- El pdf del artículo es la versión post-print.We have performed extensive molecular dynamic simulations on a simple bead−spring model for copolymers with comblike architecture. Monomers located at the main chain and at the arms are respectively denoted as S (“slow”) and F (“fast”). The model parameters are selected in order to induce segregation in domains rich in one component and poor in the other. In particular, we investigate the case in which the linear homopolymer of F-monomers exhibits much faster intrinsic dynamics than the S-counterpart. As a consequence, a strong dynamic asymmetry between both components is still present in the self-assembled copolymer system. We investigate static and dynamic properties as a function of arm length and temperature. The fast component exhibits decoupling of self- and collective dynamics as well as strongly stretched relaxation. Stretching is an intrinsic feature and is not necessarily related to gradients of mobility. The observed qualitative trends are fully consistent with recent neutron scattering experiments on poly(n-alkyl methacrylates), suggesting that they are generic in comb copolymers with strong dynamic asymmetry.We acknowledge financial support from the projects FP7-PEOPLE-2007-1-1-ITN (DYNACOP, EU), MAT2007-63681 (Spain), and IT-436-07 (GV, Spain).Peer reviewe

Cluster crystals in confinement

El pdf del artículo es la versión pre-print: arXiv:0808.1363v1A large class of fluids of particles interacting via ultrasoft, repulsive pair potentials crystallize into cluster crystals. Here, we employ density functional theory and computer simulations to study the behavior of a system of particles that repel each other with a exp(-r^8)-potential [A. J. Moreno and C. N. Likos, Phys. Rev. Lett., 2007, 99, 107801] under planar confinement. We compare the behavior for purely repulsive to that for attractive slit walls. In particular, we present the phase diagram and we show that for repulsive walls the system freezes from the middle, whereas for attractive ones crystallization sets in at the walls and proceeds to the middle. For large wall-wall-separations we find continuous growth of a fluid or crystalline layer on the wall, depending on the wall-particle interaction, which is interrupted by capillary melting or freezing close to the bulk crystallization transition. An asymptotic scaling analysis of the width of the liquid or crystalline films growing at the walls indicate complete wetting in both cases.This work has been supported by the DFG within the SFB TR6, project section C3 and by the EU Network of Excellence "Softcomp". CNL wishes to thank the Erwin Schrödinger International Institute for Mathematical Physics (ESI, Vienna), where parts of this work have been carried out, for a Senior Research Fellowship and for its hospitality. AJM acknowledges support from 2007-60I201 (CSIC, Spain).Peer reviewe

An anisotropic effective model for the simulation of semiflexible ring polymers

This is an open access article published under a Creative Commons Attribution (CC-BY) License.We derive and introduce anisotropic effective pair potentials to coarse-grain solutions of semiflexible ring polymers of various lengths. The system has been recently investigated by means of full monomer-resolved computer simulations, revealing a host of unusual features and structure formation, which, however, cannot be captured by a rotationally averaged effective pair potential between the rings' centers of mass [ Bernabei, M.; Soft Matter 2013, 9, 1287 ]. Our new coarse-graining strategy is to picture each ring as a soft, penetrable disk. We demonstrate that for the short- and intermediate-length rings the new model is quite capable of capturing the physics in a quantitative fashion, whereas for the largest rings, which resemble flexible ones, it fails at high densities. Our work opens the way for the physical justification of general, anisotropic penetrable interaction potentials.This work has been supported by the Austrian Science Fund (FWF), Grant 23400-N16.Peer Reviewe

Multi-blob coarse graining for ring polymer solutions

We present a multi-scale molecular modeling of concentrated solutions of unknotted and non-concatenated ring polymers under good solvent conditions. The approach is based on a multi-blob representation of each ring polymer, which is capable of overcoming the shortcomings of single-blob approaches that lose their validity at concentrations exceeding the overlap density of the solution [A. Narros, A. J. Moreno, and C. N. Likos, Soft Matter, 2010, 6, 2435]. By means of a first principles coarse-graining strategy based on analytically determined effective pair potentials between the blobs, computed at zero density, we quantitatively reproduce the single molecule and solution properties of a system with well-defined topological constraints. Detailed comparisons with the underlying, monomer-resolved model demonstrate the validity of our approach, which employs fully transferable pair potentials between connected and unconnected blobs. We demonstrate that the pair structure between the centers of mass of the rings is accurately reproduced by the multi-blob approach, thus opening the way for simulation of arbitrarily long polymers. Finally, we show the importance of the topological constraint of non-concatenation on the structure of the concentrated solution and in particular on the size of the correlation hole and the shrinkage of the rings as melt concentrations are approached.This work has been supported by the Austrian Science Fund (FWF), Grant 23400-N16.Peer Reviewe

Dynamic heterogeneity in random and gradient copolymers: A computational investigation

By means of molecular dynamics simulations, we investigate the structural relaxation in disordered random copolymers and lamellar phases of gradient copolymers, containing chemical species of very different mobilities. Two models have been investigated: a generic bead-spring system and a MARTINI coarse-grained model of a polyester resin. The lamellar phase of the gradient copolymer is formed by domains rich in one species and poor in the other one, which are separated by broad interfaces. Unlike in strongly segregated block copolymers, there is a finite probability of finding monomers of a given species at any position within the domains rich in the other species. A direct consequence of this feature is that monomers can probe very different chemical environments, and because of the strong dynamic asymmetry between the two components, their relaxation are characterized by an extreme dynamic heterogeneity. This is confirmed by an analysis of dynamic correlators as a function of the distance to the interface. In the case of random copolymers long-range ordering is not possible, and local microsegregation results in a much weaker dynamic heterogeneity. The former features are consistent with the experimental observation of narrow glass transitions in random copolymers but extremely broad ones in lamellar gradient copolymers. © 2013 American Chemical Society.Part of this work was carried out under the HPC-Europa2 project (project number: 228398) with the support of the European Commission Capacities Area-Research Infrastructures Initiative. We also acknowledge support from Projects MAT2007-63681 (Spain), MAT2012-31088 (Spain), and IT-436-07 (GV, Spain).Peer Reviewe

Simulation guided design of globular single-chain nanoparticles by tuning the solvent quality

The control of primary and further structures of individual folded/collapsed synthetic polymers has received significant attention in recent years. However, the synthesis of single-chain nanoparticles (SCNPs) showing a compact, globular conformation in solution has turned out so far to be highly elusive. By means of simulations, we propose two methods for obtaining globular SCNPs in solution. The first synthesis route is performed in the bad solvent, with the precursor anchored to a surface. In the second route we use a random copolymer precursor with unreactive solvophilic and reactive solvophobic units, which form a single core-shell structure. Both protocols prevent intermolecular cross-linking. After recovering good solvent conditions, the swollen nanoparticles maintain their globular character. The proposed methods are experimentally realizable and do not require specific sequence control of the precursors. Our results pave the way for the synthesis via solvent-assisted design of a new generation of globular soft nanoparticles mimicking global conformations of native proteins in solution.We acknowledge financial support from the Projects MAT2012-31088 (MINECO) and T-654-13 (GV).Peer Reviewe