1,721,102 research outputs found
Application of the Christensen-Lo model to the reinforcement of elastomers by fractal fillers
No full textSEPARAZIONE DI CARICA IN MISCELE DI MATERIALI ORGANICI: VALE LA LEGGE DI COULOMB?
Full text linkDopo una breve introduzione ai materiali fotovoltaici organici, viene affrontato il problema della separazione di carica in questi sistemi. Nei dispositivi migliori, le eccitazioni prodotte dall’assorbimento di un fotone possono essere convertire in portatori di carica positivi e negativi (rispettivamente buche ed elettroni) con una efficienza vicina al 100%. Queste efficienze sono apparentemente in contrasto con la legge di Coulomb, che porterebbe le cariche a ricombinarsi a causa della loro reciproca attrazione elettrostatica. Un modello teorico sviluppato dal mio gruppo consente di superare questa apparente contraddizione, tendendo conto di effetti quantomeccanici quali la delocalizzazione dei portatori di carica.After a brief introduction to organic photovoltaic materials, I discuss the problem of charge separation in these systems. In the best devices, the excitations produced by absorption of a photon can be converted into positive and negative charge carriers (holes and electrons, respectively) with near-100% efficiency. Such efficiencies are apparently at odds with Coulomb’s l aw, which would lead to change recombination due to their mutual electrostatic attraction. A theoretical model developed within my group overcomes this apparent contradiction, by taking into account quantum mechanical effects such as the delocalization of the charge carrier
A Cluster of Chains Can Be Smaller Than a Single Chain: New Interpretation of Kinetics of Collapse Experiments
No full textImpact of Interaction Strength and Surface Heterogeneity on the Dynamics of Adsorbed Polymers
Full text linkTuning Adhesion and Energy Dissipation in Polymer Films between Solid Surfaces via Grafting and Cross-Linking
No full textPolymer adhesion is underpinned by a network of chain molecules forming both adhesive bonds with the surfaces and cohesive bonds among themselves. Understanding how this network propagates mechanical stress and dissipates energy under tension is a challenging but essential task for improving polymer adhesives. To this end, we present a series of coarse-grained molecular dynamics simulations of polymer films joining two parallel flat surfaces. The polymer molecules are modeled as chains of beads that can covalently bind to the surfaces and to each other via a Morse potential, allowing bond breakage and chain scission. The mechanical response of the polymer film is studied under both small oscillatory strains and large deformations, leading to complete breakup. Starting from a melt of ungrafted polymer chains and introducing one feature at a time, we find that polymer cross-linking enhances adhesion more than covalent surface grafting. Cross-linking and grafting may also act in synergy, provided the grafting density is homogeneous. Moreover, surface heterogeneity at the nanometer scale affects the viscoelastic response at small strains. The simple model used in our simulations provides a valuable platform to translate molecular-level features of the polymer network, such as chain connectivity and bond strength, into measurable quantities, such as adhesion forces and energy dissipation
Chain Interactions in Poor-Solvent Polymer Solutions: Equilibrium and Nonequilibrium Aspects
No full textFree energies of molecular crystal surfaces by computer simulations: application to tetrathiophene
No full textGlassy dynamics of a polymer monolayer on a heterogeneous disordered substrate
Full text linkWe present molecular dynamics simulations of a polymer monolayer on randomly functionalized
surfaces that are characterized by different fractions of weakly and strongly attractive sites. We show
that the dynamics slow-down upon cooling resembles that of a strong glass-forming liquid. Indeed, the
mean-square displacements show an increasingly lasting subdiffusive behaviour before the diffusive
regime, with signs of Fickian yet not Gaussian diffusion, and the dynamic correlation functions exhibit a
stretched exponential decay. The glassy dynamics of this relatively dilute system is dominated by the
interaction of the polymer with the substrate and becomes more marked when the substrate composition is
heterogeneous. Accordingly, the estimated glass transition temperature shows a non-monotic dependence
on surface composition, in agreement with previous results for the activation energy and with an analysis
of the potential energy landscape experienced by the polymer beads. Our findings are relevant to the
description of polymer–surface adhesion and friction and the development of polymer nanocomposites with
tailored structural and mechanical properties
Molecular modeling of crystalline oligothiophenes: testing and development of improved force fields
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