Search results for " mechanics."
showing 10 items of 5002 documents
Phase transitions of single polymer chains and of polymer solutions: insights from Monte Carlo simulations
2008
The statistical mechanics of flexible and semiflexible macromolecules is distinct from that of small molecule systems, since the thermodynamic limit can also be approached when the number of (effective) monomers of a single chain (realizable by a polymer solution in the dilute limit) is approaching infinity. One can introduce effective attractive interactions into a simulation model for a single chain such that a swollen coil contracts when the temperature is reduced, until excluded volume interactions are effectively canceled by attractive forces, and the chain conformation becomes almost Gaussian at the theta point. This state corresponds to a tricritical point, as the renormalization gro…
Competition between liquid-crystalline ordering and glassy freezing in melts of semiflexible polymers: A monte carlo simulation
1999
We present results of a Monte Carlo simulation of dense melts of semiflexible polymers using the bond-fluctuation model. The chosen Hamiltonian increases the chain stiffness upon cooling which in turn leads to glass-transition like freezing of the polymer mobility. Employing an efficient simulation algorithm, which is able to equilibrate the simulated systems to lower temperature than the Rouse-type algorithm showing the glassy freezing, we are able to observe an isotropic-nematic phase transition. This transition lies above the glass transition temperature one would extrapolate from the observed freezing behavior.
Monte Carlo simulations of the polymer glass transition: From the test of theories to material modeling
1997
We present results on the glass transition in polymer melts using Monte Carlo simulations of the bond fluctuation lattice model. There are two questions we address in this work. What is the temperature dependence of the entropy density in such a model polymer melt and how well is it described by theories like the Gibbs-DiMarzio theory of the glass transition? And to what degree is one able to map the Hamiltonian of such an abstract lattice model onto a specific polymer material and use it to model the large scale and long time properties of a realistic polymer melt?
Frictional Drag Mechanisms between Polymer-Bearing Surfaces
2001
The fundamental features of friction between two polymer-bearing surfaces in relative sliding motion are investigated by molecular dynamics simulation. Adsorbed and grafted polymers are considered in good and bad solutions. The solvent is not treated explicitly but indirectly in terms of a Langevin thermostat. In both systems, we observe shear thinning that is attributed to an orientation of the radius of gyration along the sliding direction. This effect is particularly strong for surfaces bearing polymer brushes. In this case, the shear stresses are mainly determined by the degree of the interpenetration of brushes.
Electronic structure of polysilanes: influence of substitution and conformation
1993
Abstract The valence effective Hamiltonian (VEH) quantum-chemical approach is used to investigate the electronic properties of polysilane. The valence band structure calculated for this fully saturated polymer is analyzed in terms of orbital contributions and compared to that of the closely related carbon polymer, polyethylene. The effects of alkyl substitution and silicon backbone conformation are studied by elucidating the modifications that these structural changes induce on the electronic valence band structure of all-trans unsubstituted polysilane. The VEH results predict a decrease of the band gap upon alkyl substitution and on going from helical to all-trans conformations.
Hydrodynamic mechanisms of spinodal decomposition in confined colloid-polymer mixtures: A multiparticle collision dynamics study
2013
A multiscale model for a colloid-polymer mixture is developed. The colloids are described as point particles interacting with each other and with the polymers with strongly repulsive potentials, while polymers interact with each other with a softer potential. The fluid in the suspension is taken into account by the multiparticle collision dynamics method (MPC). Considering a slit geometry where the suspension is confined between parallel repulsive walls, different possibilities for the hydrodynamic boundary conditions (b.c.) at the walls (slip versus stick) are treated. Quenching experiments are considered, where the system volume is suddenly reduced (keeping the density of the solvent flui…
Investigation of Multicomponent Sorption in Polymers from Fluid Mixtures at Supercritical Conditions: The Case of the Carbon Dioxide/Vinylidenefluor…
2008
The simultaneous sorption of carbon dioxide and vinylidene fluoride (VDF) in poly(vinylidenefluoride) from their supercritical (sc) mixtures was studied using an experimental method, already described in a previous publication, based on the gas-chromatographic determination of the equilibrium composition of the fluid phase in contact with the polymer. Argon was added to the system as a nonabsorbable molecular probe in the polymer to take into account the effect of the volume swelling on the measurement. Sorption behavior has been studied at 50 °C by changing the composition and the density of the supercritical phase. We have found that VDF dissolves in its polymer with concentrations much l…
Steel Fibres: Effective Way to Prevent Failure of the Concrete Bonded with FRP Sheets
2016
Although the efficiency of steel fibres for improving mechanical properties (cracking resistance and failure toughness) of the concrete has been broadly discussed in the literature, the number of studies dedicated to the fibre effect on structural behaviour of the externally bonded elements is limited. This experimental study investigates the influence of steel fibres on the failure character of concrete elements strengthened with external carbon fibre reinforced polymer sheets. The elements were subjected to different loading conditions. The test data of four ties and eight beams are presented. Different materials were used for the internal bar reinforcement: in addition to the conventiona…
Toward Understanding the Photochemistry of Photoactive Yellow Protein: A CASPT2/CASSCF and Quantum Theory of Atoms in Molecules Combined Study of a M…
2009
Photochemical processes that take place in biological molecules have become an increasingly important research topic for both experimentalists and theoreticians. In this work, we report the reaction mechanism of a model of the photoactive yellow protein (PYP) chromophore in vacuo. The results obtained here, using a strategy based on the simultaneous use of the minimum energy path concept and the quantum theory of atoms in molecules applied to this excited state process, suggest a possible way in which the protein could increase the efficiency of the reaction. The role played by other electronic states of the same and different spin multiplicities in the reaction process is also analyzed, wi…
Advanced descriptors for long-range noncovalent interactions between SARS-CoV-2 spikes and polymer surfaces.
2021
The recent pandemic triggered numerous societal efforts aimed to control and limit the spread of SARS-CoV-2. One of these aspects is related on how the virion interacts with inanimate surfaces, which might be the source of secondary infection. Although recent works address the adsorption of the spike protein on surfaces, there is no information concerning the long-range interactions between spike and surfaces, experimented by the virion when is dispersed in the droplet before its possible adsorption. Some descriptors, namely the interaction potentials per single protein and global potentials, were calculated in this work. These descriptors, evaluated for the closed and open states of the sp…