The Berry phase and the pump flux in stochastic chemical kinetics

We study a classical two-state stochastic system in a sea of substrates and products (absorbing states), which can be interpreted as a single Michaelis-Menten catalyzing enzyme or as a channel on a cell surface. We introduce a novel general method and use it to derive the expression for the full counting statistics of transitions among the absorbing states. For the evolution of the system under a periodic perturbation of the kinetic rates, the latter contains a term with a purely geometrical (the Berry phase) interpretation. This term gives rise to a pump current between the absorbing states, which is due entirely to the stochastic nature of the system. We calculate the first two cumulants of this current, and we argue that it is observable experimentally

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

Out of equilibrium dynamics of poly(vinyl methyl ether) segments in miscible poly(styrene)-poly(vinyl methyl ether) blends

The local dynamics of the low-Tg component in a polymer blend, dynamically asymmetric poly(styrene)-poly(vinyl methyl ether) (PS-PVME), is studied below the glass transition, via dielectric relaxation spectroscopy. A particular attention has been paid to blends with a high PS content (PS weight fraction higher than 50%). A relaxation process, slower than the localized motions inducing the PVME secondary relaxations, is detected. Even though these blends fall out of equilibrium in this temperature regime, the structural recovery process is not efficient on the time scale of this PVME motional process. This relaxation is attributed to rather localized, weakly cooperative PVME motions resulting from the topological constraints imposed by the frozen PS chains.The authors acknowledge the University of the Basque Country (Project No. 9/UPV00206.215-13568/2001) and the Spanish Ministry of Science and Technology (Project No. MAT 2001/0070) for their support.Peer reviewe

Polymer chain dynamics: Evidence of nonexponential mode relaxation using thermally stimulated depolarization current techniques

The slowest (p=1) mode relaxation of several polyisoprenes has been experimentally isolated by thermally stimulated depolarization current techniques. Close to the glass transition the p=1 mode deviates from the exponential behavior assumed by Rouse and tube-reptation theories. This effect is found to be a consequence of the closeness of τp=1 and α-relaxation time scales. The scenario resembles that of broadened fast component dynamics in polymer blends with high dynamic asymmetry and suggests a possible general interpretation in terms of the effect of local density fluctuations (α relaxation) on chain dynamics.We acknowledge the support from the following research projects: MAT2012-31088, supported by the Spanish Ministry “Ministerio de Economía y Competitividad,” and IT-654-13, supported by the Basque Government.Peer Reviewe

A synergetic combination of neutron scattering, atomistic simulations and theoretical modelling

This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0.Trabajo presentado a: QENS/WINS 2014 - 11th International Conference on Quasielastic Neutron Scattering and 6th International Workshop on Inelastic Neutron SpectrometersMotivated by the proposition of a new theoretical ansatz [V.N. Novikov, K.S. Schweizer, A.P. Sokolov, J. Chem. Phys. 138, 164508 (2013)], we have revisited the question of the characterization of the collective response of polyisobutylene at intermediate length scales observed by neutron spin echo (NSE) experiments. The model, generalized for sublinear diffusion –as it is the case of glass-forming polymers– has been successfully applied by using the information on the total self-motions available from MD-simulations properly validated by direct comparison with experimental results. From the fits of the coherent NSE data, the collective time at Q → 0 has been extracted that agrees very well with compiled results from different experimental techniques directly accessing such relaxation time. We show that a unique temperature dependence governs both, the Q → 0 and Q →∞ asymptotic characteristic times. The generalized model also gives account for the modulation of the apparent activation energy of the collective times with the static structure factor. It mainly results from changes of the short-range order at inter-molecular length scales.We thank support from the projects IT-654-13 (GV) and MAT2012-31088.Peer Reviewe

Dynamics of cyclic polyethers by broadband dielectric spectroscopy

Resumen del trabajo presentado al International Chemical Congress of Pacific Basin Societies, celebrado en Honolulu, Hawai (USA) del 15 al 20 de diciembre de 2015.Peer Reviewe

Chain dynamics on crossing the glass transition: Nonequilibrium effects and recovery of the temperature dependence of the structural relaxation

In this paper we report thermally stimulated depolarization current results on the chain and segmental dynamics of two monodisperse polyisoprenes accessing both dynamics at ultralow frequency range and exploring the relationship between segmental and chain time scales when crossing the glass transition. In this range, we have recorded experimental evidence of nonequilibrium effects on the slowest chain mode dynamics. The nonequilibrium effects seem to occur simultaneously for both chain and α-relaxation. Moreover, detailed analysis strongly indicates the recovery of an even T-dependence for the chain and α-relaxation dynamics on crossing glass transition and in the glassy state. The obtained results can be understood taking into account the different temperature dependences of the length scales involved in the segmental and chain relaxations.We acknowledge the support of the following research projects MAT2012-31088 supported by the Spanish Ministry "Ministerio de Economía y Competitividad" and IT-654-13 supported by the Basque Government.Peer Reviewe

Dielectric relaxation of 2-ethyl-1-hexanol around the glass transition by thermally stimulated depolarization currents

We explore new routes for characterizing the Debye-like and α relaxation in 2-ethyl-1-hexanol (2E1H) monoalcohol by using low frequency dielectric techniques including thermally stimulated depolarization current (TSDC) techniques and isothermal depolarization current methods. In this way, we have improved the resolution of the overlapped processes making it possible the analysis of the data in terms of a mode composition as expected for a chain-like response. Furthermore the explored ultralow frequencies enabled to study dynamics at relatively low temperatures close to the glass transition (Tg ). Results show, on the one hand, that Debye-like and α relaxation timescales dramatically approach to each other upon decreasing temperature to Tg . On the other hand, the analysis of partial polarization TSDC data confirms the single exponential character of the Debye-like relaxation in 2E1H and rules out the presence of Rouse type modes in the scenario of a chain-like response. Finally, on crossing the glass transition, the Debye-like relaxation shows non-equilibrium effects which are further emphasized by aging treatment and would presumably emerge as a result of the arrest of the structural relaxation below Tg.We acknowledge the support of the following research projects MAT2012-31088 supported by the Spanish Ministry “Ministerio de Economía y Competitividad” and IT-654-13 supported by the Basque Government.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

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