1,720,984 research outputs found
Constrained reaction coordinate dynamics for systems with constraints
No full textIn the context of molecular dynamics simulations of rare events, the application of constraints on a suitable reaction coordinate has often been found useful for sampling of the free energy barrier. The efficiency of these calculations is hampered by geometrical difficulties, related to the metric factor and inertial forces. Some years ago Mulders et al. [1996, J. chem. Phys., 104, 4869] suggested a way to simplify the approach. Their idea was demonstrated shortly afterwards by Sprik and Ciccotti [1998, J. chem. Phys., 109, 7737]. The present paper extends these results to vector reaction coordinate and molecular system
Rescaling of structural length scales for “soft effective segment” representations of polymers in good solvent
No full textIt is shown by simple scaling arguments that the structural length scales of semi-dilute polymer solutions, calculated from coarse-grained "soft effective segment" representations of long polymers, need to be rescaled by a single, well-defined scaling factor to match the corresponding properties of the underlying microscopic polymer model. The validity of this rescaling is illustrated by extensive Monte Carlo simulations of the density profiles and average height of stretched polymer brushes. PACS numbers: 68.47.Mn, 68.47.Pe, 61.25.Hq, 82.35.Lr © 2011 The Royal Society of Chemistry
Competing micellar and cylindrical phases in semi-dilute diblock copolymer solutions
No full textWe develop a "first principles" coarse-graining procedure based on a soft effective segment representation of an athermal AB diblock copolymer model in selective solvent, to map out the self-assembly phase diagram for several asymmetry ratios f using Monte Carlo free energy calculations. We find that the free energy per unit volume is surprisingly insensitive to the aggregation number of monodisperse cubic and cylindrical phases for a given polymer volume fraction. The cylindrical phase is found to pre-empt the cubic micellar phases for nearly symmetric (f ≃ 0.6) copolymers. © 2010 The Royal Society of Chemistry
A systematic coarse-graining strategy for semi-dilute copolymer solutions: from monomers to micelles
No full textA systematic coarse-graining procedure is proposed for the description and simulation of AB diblock copolymers in selective solvents. Each block is represented by a small number, nA or nB, of effective segments or blobs, containing a large number of microscopic monomers. n A and nB are unequivocally determined by imposing that blobs do not, on average, overlap, even if complete copolymer coils interpenetrate (semi-dilute regime). Ultra-soft effective interactions between blobs are determined by a rigorous inversion procedure in the low concentration limit. The methodology is applied to an athermal copolymer model where A blocks are ideal (theta solvent), B blocks self-avoiding (good solvent), while A and B blocks are mutually avoiding. The model leads to aggregation into polydisperse spherical micelles beyond a critical micellar concentration determined by Monte Carlo simulations for several size ratios f of the two blocks. The simulations also provide accurate estimates of the osmotic pressure and of the free energy of the copolymer solutions over a wide range of concentrations. The mean micellar aggregation numbers are found to be significantly lower than those predicted by an earlier, minimal two-blob representation (Capone et al 2009 J.Phys.Chem.B113 3629). © 2011 IOP Publishing Ltd
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Sequence controlled self-knotting colloidal patchy polymers
No full textKnotted chains are a promising class of polymers with many applications for materials science and drug delivery. Here we introduce an experimentally realizable model for the design of chains with controllable topological properties. Recently, we have developed a systematic methodology to construct self-assembling chains of simple particles, with final structures fully controlled by the sequence of particles along the chain. The individual particles forming the chain are colloids decorated with mutually interacting patches, which can be manufactured in the laboratory with current technology. Our methodology is applied to the design of sequences folding into self-knotting chains, in which the end monomers are by construction always close together in space. The knotted structure can then be externally locked simply by controlling the interaction between the end monomers, paving the way to applications in the design and synthesis of active materials and novel carriers for drugs delivery. © 2013 American Physical Society
Telechelic star polymers as self-assembling units from the molecular to the macroscopic scale
No full textBy means of multiscale molecular simulations, we show that telechelic-star polymers are a simple, robust, and tunable system, which hierarchically self-assembles into soft-patchy particles and mechanically stabilizes selected, open crystalline structures. The self-aggregating patchy behavior can be fully controlled by the number of arms per star and by the fraction of attractive monomeric units at the free ends of the arms. Such self-assembled soft-patchy particles while forming, upon augmenting density, gel-like percolating networks, preserve properties as particle size, number, and arrangement of patches per particle. In particular, we demonstrate that the flexibility inherent in the soft-patchy particles brings forward a novel mechanism that leads to the mechanical stability of diamond and simple cubic crystals over a wide range of densities, and for molecular sizes ranging from about 10nm up to the micrometer scale. © 2012 American Physical Society
Hierarchical self-assembly of telechelic star polymers: From soft patchy particles to gels and diamond crystals
No full textThe design of self-assembling materials in the nanometer scale focuses on the fabrication of a class of organic and inorganic subcomponents that can be reliably produced on a large scale and tailored according to their vast applications for, e.g. electronics, therapeutic vectors and diagnostic imaging agent carriers, or photonics. In a recent publication (Capone et al 2012 Phys. Rev. Lett. 109 238301), diblock copolymer stars have been shown to be a novel system, which is able to hierarchically self-assemble first into soft patchy particles and thereafter into more complex structures, such as the diamond and cubic crystal. The self-aggregating single star patchy behavior is preserved from extremely low up to high densities. Its main control parameters are related to the architecture of the building blocks, which are the number of arms (functionality) and the fraction of attractive end-monomers. By employing a variety of computational and theoretical tools, ranging from the microscopic to the mesoscopic, coarse-grained level in a systematic fashion, we investigate the crossover between the formation of microstructure versus macroscopic phase separation, as well as the formation of gels and networks in these systems. We finally show that telechelic star polymers can be used as building blocks for the fabrication of open crystal structures, such as the diamond or the simple-cubic lattice, taking advantage of the strong correlation between single-particle patchiness and lattice coordination at finite densities. © IOP Publishing and Deutsche Physikalische Gesellschaft
Design and folding of colloidal patchy polymers
No full textThe creation of functional nanoscale materials with complex 3D structures has been achieved by biological systems e.g. proteins, but remains a daunting challenge in materials science. Recent progress in this direction has been made with patchy particles, which can be made to self-assemble into specific structures by fine tuning the numbers, locations and interactions of the patches. Here, we present a different, bio-inspired approach to create 3D objects from chains of patchy particles that fold into structures determined by the particle sequence along the chain. The particles linked in the chains are spherical with homogeneous weak repulsive or attractive potentials and symmetry-breaking patches that provide attractive directional interactions. We show, using computer simulations, that particle sequences along the string can be designed to steer the folding into specific target structures. Moreover, we introduce a scheme to discriminate configurations that present a golf-hole like free energy landscape, which inhibits folding, from target structures that are easy to design. © 2013 The Royal Society of Chemistry
- …
