We present an analysis of heterogeneous dynamics in molecular dynamics simulations of a thin polymeric film near the glass transition. The film is supported by an absorbing structured surface. It is simulated using a coarse-grained bead-spring model, along with a specific criterion to select slow -"immobile"-beads. As it turns out, the immobile beads occur throughout the film, yet their distribution is inhomogeneous, with the probability of their occurrence decreasing with larger distance from the substrate. Still, enough immobile beads are located near the free surface to cause them to percolate in the direction perpendicular to substrate surface, at a temperature near the glass transition temperature. This result is in agreement with a recent theoretical model of glass transition. PACS: 64.70.Pf, 61.20.Ja, 64.60.Ak.Ultrathin films of polymers have received ample attention in recent literature, due to their relevance for a wide range of applications. They are used as masks in lithographic processes in the microelectronics industry and their response to stress is relevant to the overall dynamical properties of polymer-nanoparticle composites [1]. Experiments [2] and simulations [3,4] alike have shown that, depending on polymer-surface interaction energy, the glass transition temperature (Tg) of such thin films can be either substantially higher or lower than the corresponding Tg of the bulk polymers. Various theories, such as the layer model, have been proposed to explain this observed deviation of the Tg from its bulk value [5].Long and coworkers have recently proposed a phenomenological model based on thermally induced density fluctuations [6]. They hypothesize that the glass transition is governed by the percolation of small (size 2 nm) domains of high density. They argue that, for a bulk system, the glass transition is governed by threedimensional percolation of immobile domains. Since percolation in two dimensions requires a larger fraction of immobile domains than in three dimensions, the Tg of a thin film repelled by the substrate is below that of the bulk value, a result in agreement with experimental observations. On the other hand, with an attractive surface, glass transition is governed by percolation of immobile domains in the direction perpendicular to the film plane. Hence, the Tg of such a system is above that of the bulk value. Moreover, by assuming that slow domains correspond to high-density regions, the model explains the heterogeneous dynamics that is observed close to the glass transition in a number of experimental studies [7].The notion that the observed change in mechanical properties at a glass transition could be caused by the percolation of domains of high density throughout the system has been around for a few decades but is still an unresolved issue. Dynamic heterogeneities in glassforming systems have been widely discussed over the past decade. Whereas one might naively expect that mobility of particles or polymeric segments directly varies with the local density, recent studies [8,...

The generally observed bipolar electrical switching of Cu\CuTCNQ\metal memories (metal=Al,Yb,Ti) between two stable resistance states is shown to occur at the CuTCNQ\metal interface and not in the bulk of CuTCNQ. The switching is explained by a model involving electrochemical formation and dissolution of Cu filaments at the interface. In this mechanism, CuTCNQ acts as solid ionic conductor and source for the Cu+ cations. The model also explains earlier reported findings of bipolar switching in CuTCNQ devices, including the apparently contradictory observation that neutral TCNQ appears in the low-resistance state.

Resistive electrical switching of the organic semiconductor Cu-tetracyanoquinodimethane ͑CuTCNQ͒ was investigated between gold bottom and aluminum top contacts. Corresponding Au/ CuTCNQ/ Al crossbar memories achieved several thousand write/erase cycles. The switching process was further studied by current-time measurements, and temperature-dependent measurements of the on state conductivity.

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We investigate the network topologies of an ensemble of telechelic polymers. The telechelic polymers serve as "links" between "nodes", which consist of aggregates of their associating endgroups. Our analysis shows that the degree distribution of the systems is bimodal and consists of two Poissonian distributions with different average degrees. The number of nodes in each of them as well as the distribution of links depends on temperature. By comparing the eigenvalue spectra of the simulated network gels with those of reconstructed networks, the most likely topology at each temperature is determined. Topological changes occur at the jamming and gel transition temperatures reported in our previous studies [1]. The jammed state topology can be described by a robust bimodal network in which hubs or superpeer nodes are linked among each other and peer nodes are linked to superpeers.Reversible polymeric gels contain networks of physically associating polymers. These are copolymers incorporating soluble fragments along with a small fraction of insoluble chemical groups or linkers, which strongly attract each other. Different chemical units may be used as linkers depending on the solvent, e.g., hydrophobic fragments on water-soluble polymers in aqueous solutions, and ionic groups on ionomers in organic solvents. The linkers form stable aggregates, which serve as temporary junctions in the resulting network structure [2,3]. A transition from a fluid "sol" state to a glassy "gel" state is obtained by increasing the polymer concentration or decreasing the temperature. Transitions can also be triggered by applying a shear stress. It has been argued that the slowdown in the dynamics of the system when approaching the gel transition is not exclusively due to the longer lifetime of the aggregates. Structural changes in the network are believed to be a factor too [4,5]. In previous work [1] we have indeed shown that it is possible to define a jamming transition for gels similar to that defined for glassy systems, granular matter, and other amorphous systems. Such a jamming transition is believed to result from a self-organization of the system [6].In this letter, we use graph theory to quantify the topology of a simulated gel network (SGN). Details of the simulations can be found in [1]. We will show that the topology changes as a function of temperature and point out differences above and below the jamming transition. All simulations are carried out on a system of 1000 telechelic polymer chains. Each polymer contained 8 beads. Both end groups are linkers. Non-bonded interactions between the beads are modeled using a purely repulsive Lennard-Jones (LJ) potential. All quantities are expressed in terms of the parameters (σ,ε) of this potential. Beads connected by the chain structure interact through a FENE potential. Beads at chain ends can form junctions. They are modeled by a FENE potential with the same parameters. The positions of the beads are updated in Molecular Dynamics simulations using the aforementioned force-fields. At...