Dynamics of complexation of a charged dendrimer by linear polyelectrolyte: Computer modelling

Brownian-dynamics simulations have been performed for complexes formed by a charged dendrimer and a long oppositely charged linear polyelectrolyte when overcharging phenomenon is always observed. After a complex formation the orientational mobility of the individual dendrimer bonds, the fluctuations of the dendrimer size, and the dendrimer rotational diffusion have been simulated. Corresponding relaxation times do not depend on the linear-chain length in a complex and are close to those for a single neutral dendrimer. At the same time fluctuations of the size of a complex are completely defined by the corresponding fluctuations of a linear polyelectrolyte size. Adsorbed polyelectrolyte practically does not feel the rotation of a dendrimer; simulated complexes may be considered as nuts with light core (dendrimer) and heavy shell (adsorbed linear polymer); the electrostatic contacts between dendrimer and oppositely charged linear polymer are easily broken due to the very fast dendrimer-size fluctuations

The effect of perception anisotropy on particle systems describing pedestrian flows in corridors

We consider a microscopic model (a system of self-propelled particles) to study the behaviour of a large group of pedestrians walking in a corridor. Our point of interest is the effect of anisotropic interactions on the global behaviour of the crowd. The anisotropy we have in mind reflects the fact that people do not perceive (i.e. see, hear, feel or smell) their environment equally well in all directions. The dynamics of the individuals in our model follow from a system of Newton-like equations in the overdamped limit. The instantaneous velocity is modelled in such a way that it accounts for the angle under which an individual perceives another individual. We investigate the effects of this perception anisotropy by means of simulations, very much in the spirit of molecular dynamics. We define a number of characteristic quantifiers (including the polarization index and Morisita index) that serve as measures for e.g. organization and clustering, and we use these indices to investigate the influence of anisotropy on the global behaviour of the crowd. The goal of the paper is to investigate the potentiality of this model; extensive statistical analysis of simulation data, or reproducing any specific real-life situation are beyond its scope.Comment: 24 page

Time scales and mechanisms of relaxation in the energy landscape of polymer glass under deformation: direct atomistic modeling

Mol.-dynamics simulation is used to explore the influence of thermal and mech. history of typical glassy polymers on their deformation. Polymer stress-strain and energy-strain developments have been followed for different deformation velocities, also in closed extension-recompression loops. The latter simulate for the first time the exptl. obsd. mech. rejuvenation and overaging of polymers, and energy partitioning reveals essential differences between mech. and thermal rejuvenation. All results can be qual. interpreted by considering the ratios of the relevant time scales: for cooling down, for deformation, and for segmental relaxation. [on SciFinder (R)

Direct atomistic modelling of deformed polymer glasses

We use molecular-dynamics computer simulations to explore the influence of thermal and mechanical history of typical glassy polymers, atactic polystyrene (PS) and (bis)phenol A polycarbonate (PC), on their deformation. Polymer stress-strain and energy-strain developments have been followed for different deformation velocities, also in closed extension-recompression loops. The latter simulate for the first time the experimentally observed mechanical rejuvenation and overaging of polymers. Energy partitioning reveals essential differences between mechanical and thermal rejuvenation. All results are qualitatively interpreted by considering the ratio's of relevant timescales: for cooling down, for deformation, and for intrinsic segmental relaxation

Conformational effects in non-stoichiometric complexes of two hyperbranched molecules with a linear polyelectrolyte

We report results from Brownian dynamics computer simulations of systems comprised by two terminally charged hyperbranched molecules preferentially branched in the periphery, with an oppositely charged linear chain of varying length. Comparison of the findings from the present study to stoichiometric counterparts and to analogous dendrimer-based complexes, reveal that the presence of the second hyperbranched molecule incurs significant changes in the conformational characteristics of both components of the complex. Instead of step-like changes in the average size and shape of the hyperbranched component that were noted in the previously studied stoichiometric systems, a rather smooth change is observed upon increase of the length of the linear component. In addition, a markedly different behavior is also noticed in the conformational characteristics of the linear chain when compared to that in similar dendrimer-based systems. The above findings are consistent with the higher degree of deformability of the peripherally branched molecules which allow appropriate rearrangements in shape in order to accommodate the favorable Coulombic interactions between the two components of the complex. This behavior offers new insight towards the design of more efficient hyperbranched-based systems which can take advantage of the multifunctionality and the structural properties of the highly branched polymer components

Dissipative particle dynamics modeling of polyelectrolyte membrane-water interfaces

Previous experiments of water vapor penetration into polyelectrolyte membrane (PEM) thin films have indicated the influence of the water concentration gradient and polymer chemistry on the interface evolution, which will eventually affect the efficiency of the fuel cell operation. Moreover, PEMs of different side chains have shown differences in water cluster structure and diffusion. The evolution of the interface between water and polyelectrolyte membranes (PEMs), which are used in fuel cells and flow batteries, of three different side-chain lengths has been studied using dissipative particle dynamics (DPD) simulations. Higher and faster water uptake is usually beneficial in the operation of fuel cells and flow batteries. The simulated water uptake increased with the increasing side chain length. In addition, the water uptake was rapid initially and slowed down afterwards, which is in agreement with the experimental observations. The water cluster formation rate was also found to increase with the increasing side-chain length, whereas the water cluster shapes were unaffected. Water diffusion in the membranes, which affects proton mobility in the PEMs, increased with the side-chain length at all distances from the interface. In conclusion, side-chain length was found to have a strong influence on the interface water structure and water penetration rates, which can be harnessed for the better design of PEMs. Since the PEM can undergo cycles of dehydration and rehydration, faster water uptake increases the efficiency of these devices. We show that the longer side chains with backbone structure similar to Nafion should be more suitable for fuel cell/flow battery usage.</p

Evolution of the deformation profile of a horizontal thin ethanol layer when heated locally

Thermocapillary breakdown of thin horizontal layer of ethanol when heated from a localized heat source was studied experimentally. The influence of layer depth on the breakdown process was investigated. Evolution of the layer thickness in the heating point and deformation profile were being monitored and the critical thickness of the layer was evaluated using confocal technique. Pulsations of layer thickness over the heating area before the breakdown have been found

Molecular Dynamics Study of Charged Dendrimers in Salt-Free Solution: Effect of Counterions

Polyamidoamine (PAMAM) dendrimers, being protonated under physiological conditions, represent a promising class of nonviral, nano-sized vectors for drug and gene delivery. We performed extensive molecular dynamics simulations of a generic model dendrimer in a salt-free solution with dendrimer's terminal beads positively charged. Solvent molecules as well as counterions were explicitly included as interacting beads. We find that the size of the charged dendrimer depends non-monotonically on the strength of electrostatic interactions demonstrating a maximum when the Bjerrum length equals the diameter of a bead. Many other structural and dynamic characteristics of charged dendrimers are also found to follow this pattern. We address such a behavior to the interplay between repulsive interactions of the charged terminal beads and their attractive interactions with oppositely charged counterions. The former favors swelling at small Bjerrum lengths and the latter promotes counterion condensation. Thus, counterions can have a dramatic effect on the structure and dynamics of charged dendrimers and, under certain conditions, cannot be treated implicitly

Orientational mobility and relaxation spectra of dendrimers : theory and computer simulation

The developed theory of the orientational mobility of individual segments of a perfectly branched dendrimer is used to calculate the relaxation spectrum of a dendrimer. Frequency dependences of NMR relaxation 1/T1 and of the nuclear Overhauser effect have been theoretically calculated from the Brownian dynamics simulation data. The dendrimer segmental orientational mobility is governed by three main relaxation processes: (i) the rotation of the dendrimer as a whole, (ii) the rotation of the dendrimer's branch originated from a given segment, and (iii) the local reorientation of the segment. The internal orientational mobility of an individual dendrimer segment depends only on the topological distance between this segment and the terminal shell of the dendrimer. Characteristic relaxation times of all processes and their contributions to the segmental mobility have been calculated. The influence of the number of generations and the number of the generation shell on the relaxation times has been studied. The correlation between the characteristic times and the calculated relaxation spectrum of the dendrimer has been established