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

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

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

Counterion Penetration and Effective Electrostatic Interactions in Solutions of Polyelectrolyte Stars and Microgels

Counterion distributions and effective electrostatic interactions between spherical macroions in polyelectrolyte solutions are calculated via second-order perturbation (linear response) theory. By modelling the macroions as continuous charge distributions that are permeable to counterions, analytical expressions are obtained for counterion profiles and effective pair interactions in solutions of star-branched and microgel macroions. The counterions are found to penetrate stars more easily than microgels, with important implications for screening of bare macroion interactions. The effective pair interactions are Yukawa in form for separated macroions, but are softly repulsive and bounded for overlapping macroions. A one-body volume energy, which depends on the average macroion concentration, emerges naturally in the theory and contributes to the total free energy.Comment: 15 pages, 5 figure

Energetic and conformational aspects of dendrimer overcharging by linear polyelectrolytes

Extensive Brownian dynamics simulations of conformational changes accompanying the overcharging of a dendrimer by an oppositely charged long linear polyelectrolyte (LPE) have been carried out. The simulated results have been compared with the predictions of the Nguen and Shklovskii correlation theory [Physica A 293, 324 (2001)] for impenetrable charged spherical macroion. Dendrimer overcharging is caused by the spatial correlations between the "excess" of the LPE charges adsorbed onto its surface. The simulated LPE-length dependence of the corresponding "correlation" energy is in agreement with the theoretical predictions. Maximum of the LPE adsorption occurs at some critical LPE length N, and the first order phase transition from completely coiled conformation to the conformation with released tails takes place. The phase transition is accompanied by the drastic increase in the relative fluctuations of the polyelectrolyte size. Upon increasing the linear-chain length above N, the one-long-tail conformation becomes energetically preferable; the exchange time between the long-tail conformation and the short-tail conformation is very large

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

Brownian dynamics simulation of charged dendrimers: Statistical properties

Computer simulation by the method of Brownian dynamics is used to study the statistical properties of neutral and charged dendrimers up to the sixth generation (382 groups) in dilute solutions. Excluded-volume interactions are described by the Lennard-Jones repulsive potential corresponding to an athermal solvent. Electrostatic interactions are considered in the Debye-Hückel approximation. The resultant radial functions of monomer density distribution have a maximum in the core and decline toward the periphery. The average radius of gyration changes with increasing Debye radius rD to a minor extent. The swelling of a neutral dendrimer is adequately described by the Flory mean-field theory. The fractal dimension found for neutral dendrimers is df = 2.77; for charged dendrimers, it amounts to df = 2. 63 at rD = 0.8 and df = 2.42 at rD = 100. Terminal groups are distributed all over the dendrimer volume. The maximum in the distribution of terminal groups shifts toward the periphery as r D increases

Brownian dynamics simulation of charged dendrimers: Statistical properties

Computer simulation by the method of Brownian dynamics is used to study the statistical properties of neutral and charged dendrimers up to the sixth generation (382 groups) in dilute solutions. Excluded-volume interactions are described by the Lennard-Jones repulsive potential corresponding to an athermal solvent. Electrostatic interactions are considered in the Debye-Hückel approximation. The resultant radial functions of monomer density distribution have a maximum in the core and decline toward the periphery. The average radius of gyration changes with increasing Debye radius rD to a minor extent. The swelling of a neutral dendrimer is adequately described by the Flory mean-field theory. The fractal dimension found for neutral dendrimers is df = 2.77; for charged dendrimers, it amounts to df = 2. 63 at rD = 0.8 and df = 2.42 at rD = 100. Terminal groups are distributed all over the dendrimer volume. The maximum in the distribution of terminal groups shifts toward the periphery as r D increases