55 research outputs found

    Tension enhancement in branched macromolecules upon adhesion on a solid substrate

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    The effect of self-generated tension in the backbone of a bottle-brush (BB) macromolecule, adsorbed on an attractive surface, is studied by means of Molecular Dynamics simulations of a coarse-grained bead-spring model in the good solvent regime. The BB-molecule is modeled as a backbone chain of LL beads, connected by breakable bonds and with side chains, tethered pairwise to each monomer of the backbone. Our investigation is focused on several key questions that determine the bond scission mechanism and the ensuing degradation kinetics: how are frequency of bond scission and self-induced tension distributed along the BB-backbone at different grafting density σg\sigma_g of the side chains? How does tension ff depend on the length of the side chains NN, and on the strength of surface adhesion ϵs\epsilon_s? We examine the monomer density distribution profiles across the BB-backbone at different ϵs\epsilon_s and relate it to adsorption-induced morphological changes of the macromolecule whereby side chains partially desorb while the remaining chains spread better on the surface. Our simulation data are found to be in qualitative agreement with experimental results and recent theoretical predictions. Yet we demonstrate that the interval of parameter values where these predictions hold is limited in NN. Thus, at high values of ϵs\epsilon_s, too long side chains mutually block each other and freeze effectively the bottle-brush molecule.Comment: 6 pages, 8 figure

    Star Polymers Confined in a Nanoslit: A Simulation Test of Scaling and Self-Consistent Field Theories

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    The free energy cost of confining a star polymer where ff flexible polymer chains containing NN monomeric units are tethered to a central unit in a slit with two parallel repulsive walls a distance DD apart is considered, for good solvent conditions. Also the parallel and perpendicular components of the gyration radius of the star polymer, and the monomer density profile across the slit are obtained. Theoretical descriptions via Flory theory and scaling treatments are outlined, and compared to numerical self-consistent field calculations (applying the Scheutjens-Fleer lattice theory) and to Molecular Dynamics results for a bead-spring model. It is shown that Flory theory and self-consistent field (SCF) theory yield the correct scaling of the parallel linear dimension of the star with NN, ff and DD, but cannot be used for estimating the free energy cost reliably. We demonstrate that the same problem occurs already for the confinement of chains in cylindrical tubes. We also briefly discuss the problem of a free or grafted star polymer interacting with a single wall, and show that the dependence of confining force on the functionality of the star is different for a star confined in a nanoslit and a star interacting with a single wall, which is due to the absence of a symmetry plane in the latter case.Comment: 15 pages, 9 figures, LaTeX, to appear in Soft Matte

    Unexpected crossover dynamics of single polymer in a corrugated tube

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    We present molecular dynamics study of a generic (coarse-grained) model for single-polymer diffusion confined in a corrugated cylinder. For a narrow tube, i.e., diameter of the cylinder δ<2.3\delta < 2.3, the axial diffusion coefficient DD_{||} scales as DN3/2D_{||} \propto N^{-3/2}, with chain length NN, up to N100N \approx 100 then crosses over to Rouse scaling for the larger NN values. The N3/2N^{-3/2} scaling is due to the large fluctuation of the polymer chain along its fully stretched equilibrium conformation. The stronger scaling, namely N3/2N^{-3/2}, is not observed for an atomistically smooth tube and/or for a cylinder with larger diameter.Comment: 10 pages, 3 figures, LaTeX, version accepted by J. Chem. Phy

    Detachment of semiflexible polymer chains from a substrate - a Molecular Dynamics investigation

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    Using Molecular Dynamics simulations, we study the force-induced detachment of a coarse-grained model polymer chain from an adhesive substrate. One of the chain ends is thereby pulled at constant speed off the attractive substrate and the resulting saw-tooth profile of the measured mean force vs height $D$ of the end-segment over the plane is analyzed for a broad variety of parameters. It is shown that the observed characteristic oscillations in the $< f >$-$D$ profile depend on the bending and not on the torsional stiffness of the detached chains. Allowing for the presence of hydrodynamic interactions (HI) in a setup with explicit solvent and DPD-thermostat, rather than the case of Langevin thermostat, one finds that HI have little effect on the -DD profile. Also the change of substrate affinity with respect to the solvent from solvophilic to solvophobic is found to play negligible role in the desorption process. In contrast, a changing ratio ϵsA/ϵsB\epsilon_s^A / \epsilon_s^B of the binding energies of AA- and BB-segments in the detachment of an ABAB-copolymer from adhesive surface strongly changes the -DD profile whereby the BB-spikes vanish when ϵsA/ϵsB<0.15\epsilon_s^A / \epsilon_s^B < 0.15. Eventually, performing an atomistic simulation of a (bio)-polymer {\it polyglycine}, we demonstrate that the simulation results, derived from our coarse-grained model, comply favorably with those from the all-atom simulation.Comment: Latex, 12 pages, 8 figures, to appear in JC

    Controlling the Interactions between Soft Colloids via Surface Adsorption

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    By employing monomer-resolved computer simulations and analytical considerations based on polymer scaling theory, we analyze the conformations and interactions of multiarm star polymers strongly adsorbed on a smooth, two-dimensional plane. We find a stronger stretching of the arms as well as a stronger repulsive, effective interaction than in the three dimensional case. In particular, the star size scales with the number of arms ff as f1/4\sim f^{1/4} and the effective interaction as f2\sim f^{2}, as opposed to f1/5\sim f^{1/5} and f3/2\sim f^{3/2}, respectively, in three dimensions. Our results demonstrate the dramatic effect that geometric confinement can have on the effective interactions and the subsequent correlations of soft colloids in general, for which the conformation can be altered as a result of geometrical constraints imposed on them.Comment: 17 pages, LaTeX, 5 figures, to appear in Macromolecule

    Polymer chain scission at constant tension - an example of force-induced collective behaviour

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    The breakage of a polymer chain of segments, coupled by anharmonic bonds with applied constant external tensile force is studied by means of Molecular Dynamics simulation. We show that the mean life time of the chain becomes progressively independent of the number of bonds as the pulling force grows. The latter affects also the rupture rates of individual bonds along the polymer backbone manifesting the essential role of inertial effects in the fragmentation process. The role of local defects, temperature and friction in the scission kinetics is also examined.Comment: 6 pages, 7 page