Monte Carlo simulations of copolymers at homopolymer interfaces: Interfacial structure as a function of the copolymer density

By means of extensive Monte Carlo simulations of the bond fluctuation model, we study the effect of adding AB diblock copolymers on the properties of an interface between demixed homopolymer phases. The parameters are chosen such that the homopolymers are strongly segregated, and the whole range of copolymer concentrations in the two phase coexistence region is scanned. We compare the ``mushroom'' regime, in which copolymers are diluted and do not interact with each other, with the ``wet brush'' regime, where copolymers overlap and stretch, but are still swollen by the homopolymers. A ``dry brush'' regime is never entered for our choice of chain lengths. ``Intrinsic'' profiles are calculated using a block analysis method introduced by us in earlier work. We discuss density profiles, orientational profiles and contact number profiles. In general, the features of the profiles are similar at all copolymer concentrations, however, the profiles in the concentrated regime are much broader than in the dilute regime. The results compare well with self-consistent field calculations.Comment: to appear in J. Chem. Phy

Symmetric Diblock Copolymers in Thin Films (I): Phase stability in Self-Consistent Field Calculations and Monte Carlo Simulations

We investigate the phase behavior of symmetric AB diblock copolymers confined into a thin film. The film boundaries are parallel, impenetrable and attract the A component of the diblock copolymer. Using a self-consistent field technique [M.W. Matsen, J.Chem.Phys. {\bf 106}, 7781 (1997)], we study the ordered phases as a function of incompatibility $\chi$ and film thickness in the framework of the Gaussian chain model. For large film thickness and small incompatibility, we find first order transitions between phases with different number of lamellae which are parallel oriented to the film boundaries. At high incompatibility or small film thickness, transitions between parallel oriented and perpendicular oriented lamellae occur. We compare the self-consistent field calculations to Monte Carlo simulations of the bond fluctuation model for chain length N=32. In the simulations we quench several systems from $\chi N=0$ to $\chi N=30$ and monitor the morphology into which the diblock copolymers assemble. Three film thicknesses are investigated, corresponding to parallel oriented lamellae with 2 and 4 interfaces and a perpendicular oriented morphology. Good agreement between self-consistent field calculations and Monte Carlo simulations is found.Comment: to appear in J.Chem.Phy

Equilibrium properties and force-driven unfolding pathways of RNA molecules

The mechanical unfolding of a simple RNA hairpin and of a 236--bases portion of the Tetrahymena thermophila ribozyme is studied by means of an Ising--like model. Phase diagrams and free energy landscapes are computed exactly and suggest a simple two--state behaviour for the hairpin and the presence of intermediate states for the ribozyme. Nonequilibrium simulations give the possible unfolding pathways for the ribozyme, and the dominant pathway corresponds to the experimentally observed one.Comment: Main text + appendix, to appear in Phys. Rev. Let

Interfaces between highly incompatible polymers of different stiffness: Monte Carlo simulations and self-consistent field calculations

We investigate interfacial properties between two highly incompatible polymers of different stiffness. The extensive Monte Carlo simulations of the binary polymer melt yield detailed interfacial profiles and the interfacial tension via an analysis of capillary fluctuations. We extract an effective Flory-Huggins parameter from the simulations, which is used in self-consistent field calculations. These take due account of the chain architecture via a partial enumeration of the single chain partition function, using chain conformations obtained by Monte Carlo simulations of the pure phases. The agreement between the simulations and self-consistent field calculations is almost quantitative, however we find deviations from the predictions of the Gaussian chain model for high incompatibilities or large stiffness. The interfacial width at very high incompatibilities is smaller than the prediction of the Gaussian chain model, and decreases upon increasing the statistical segment length of the semi-flexible component.Comment: to appear in J.Chem.Phy

Visual adaptation to convexity in macaque area V4

Aftereffects are perceptual illusions caused by visual adaptation to one or more stimulus attribute, such as orientation, motion, or shape. Neurophysiological studies seeking to understand the basis of visual adaptation have observed firing rate reduction and changes in tuning of stimulus-selective neurons following periods of prolonged visual stimulation. In the domain of shape, recent psychophysical work has shown that adaptation to a convex pattern induces a subsequently seen rectangle to appear slightly concave. In the present study, we investigate the possible contribution of V4 neurons of rhesus monkeys, which are thought to be involved in the coding of convexity, to shape-specific adaptation. Visually responsive neurons were monitored during the brief presentation of simple shapes varying in their convexity level. Each test presentation was preceded by either a blank period or several seconds of adaptation to a convex or concave stimulus, presented in two different sizes. Adaptation consistently shifted the tuning of neurons away from the convex or concave adapter, including shifting response to the neutral rectangle in the direction of the opposite convexity. This repulsive shift resembled the known perceptual distortion associated with adaptation to such stimuli. In addition, adaptation caused a nonspecific response decrease, as well as a specific decrease for repeated stimuli. The latter effects were observed whether or not the adapting and test stimuli matched closely in their size. Taken together, these results provide evidence for shape-specific adaptation of neurons in area V4, which may contribute to the perception of the convexity aftereffect

Gauge-variant propagators and the running coupling from lattice QCD

On the occasion of the 70th birthday of Prof. Adriano Di Giacomo we report on recent numerical computations of the Landau gauge gluon and ghost propagators as well as of a non-symmetric MOM-scheme ghost-gluon vertex in quenched and full lattice QCD. Special emphasis is paid to the Gribov copy problem and to the unquenching effect. The corresponding running coupling \alpha_s(q^2) is found and shown to decrease for q^2 \le 0.3 GeV^2 in the infrared limit. No indication for a non-trivial infrared fixed point is seen in agreement with findings from truncated systems of Dyson-Schwinger equations treated on a four-dimensional torus.Comment: contribution to "Sense of Beauty in Physics", Festschrift in honor of Adriano Di Giacomo's 70-th birthda

Simulations of the Magneto-rotational Instability in Core-Collapse Supernovae

We assess the importance of the magneto-rotational instability in core-collapse supernovae by an analysis of the growth rates of unstable modes in typical post-collapse systems and by numerical simulations of simplified models. The interplay of differential rotation and thermal stratification defines different instability regimes which we confirm in our simulations. We investigate the termination of the growth of the MRI by parasitic instabilities, establish scaling laws characterising the termination amplitude, and study the long-term evolution of the saturated turbulent state.Comment: 6 pages, 1 figure. To appear in Proceedings of 4th International Conference on Numerical Modeling of Space Plasma Flows (Chamonix 2009

Signaling Without Common Prior: An Experiment

The common prior assumption is pervasive in game-theoretic models with incomplete information. This paper investigates experimentally the importance of inducing a common prior in a two-person signaling game. For a specific probability distribution of the sender’s type, the long-run behavior without an induced common prior is shown to be different from the behavior when a common prior is induced, while for other distributions behavior is similar under both regimes. We also present a learning model that allows players to learn about the other players’ strategies and the prior distribution of the sender’s type. We show that this learning model accurately accounts for all main features of the data.common prior;signaling;experiment;learning