A. A. Darinskii

Interpolyelectrolyte Complexes between Starlike and Linear Macromolecules: A Structural Model for Nonviral Gene Vectors

Molecular dynamics simulations are used to probe the structural organization of nonstoichiometric interpolyelectrolyte complexes (IPECs) formed by oppositely charged starlike and linear polyelectrolytes (PEs) in dilute aqueous solution. We demonstrate that undercompensated star-IPEC consists of a denser coacervate core and a charged starlike corona. Two distinctive populations of star branches completely embedded in a coacervate core and stretched in a lyophilizing corona are found. The scaling arguments support the stability of IPEC with partitioned star branches.

Conformational mobility of carbosilane dendrimer: Molecular dynamics simulation

Molecular dynamic simulations were carried out for carbosilane dendrimers of the 5th generation immersed in CCl4 solvent at different temperatures. The calculations were accomplished by using the AMBER force field in the united atom approximation. Lennard-Jones particles were considered as the solvent molecules with potential parameters corresponding to CCl4. There was one molecule of a dendrimer in each calculation cell, and the cells size was large enough to exclude any interaction between dendrimers. The internal structure of the dendrimer (density distributions for both dendrimer and solvent atoms) as well as dynamics of trans–gauche transitions of single bonds and fluctuations of branching points were analysed. It was shown that the one barrier mechanism of conformational transitions observed earlier in linear polymers occurs to be valid also for the conformational rearrangements in dendrimers with the hindered rotation around chain bonds. The contribution of rotational restrictions is essential only for branching points, which are close to the core.

Local orientational mobility in dendrimers. Theory and computer-aided simulation

The orientational mobility of segments in dendrimers are studied by the method of Brownian dynamics, and the results are analyzed in terms of an earlier proposed analytical theory. The orientational autocorrelation function for the cosine of an angle of segmental rotation in dendrimers of a given generation P 1(t) is controlled by three relaxation processes with the corresponding relaxation times. Characteristic times and the contribution from the above processes to P 1(t) are calculated. The first process refers to the local mobility of a selected segment; the second process, to the rotations of a dendrimer branch, which originates from the selected segment of a given generation; and the third process, to the rotation of a dendrimer macro-molecule as a whole. The proposed approach makes it possible to estimate the relaxation spectrum of a dendrimer by studying the orientational mobility of segments in different generations. The relaxation times can be used to describe various relaxation processes observed by different experimental methods, such as dielectric relaxation, NMR, dynamic birefringence, and polarized luminescence.

Nano-patterned structures in cylindrical polyelectrolyte brushes assembled with oppositely charged polyions

We demonstrate that the electrostatically driven association of cylindrical polyelectrolyte brushes (CPBs) with oppositely charged linear polyelectrolytes (PEs) in dilute aqueous solution gives rise to well-defined and colloidaly stable (not undergoing secondary aggregation with time) polymeric nano-assemblies representing novel water-soluble interpolyelectrolyte complexes (IPECs). Each complex particle comprises a single “host” CPB whose charge is undercompensated by the “guest” PE chains. Molecular dynamics (MD) simulations were used to probe the structural organization of these nano-assemblies. We find that they spontaneously adopt the shape of a necklace of complex coacervate pearls which comprise charged monomer units of CPB and those of the guest PEs in approximately stoichiometric (1:1) ratio. Each complex coacervate pearl is decorated by a star-like PE corona formed by the branches of the CPB not involved in the interpolyelectrolyte complexation. Repulsive interactions between these coronas stabilize the periodic intra-molecular structure and assure aggregative stability of the IPEC derived from the single CPB. AFM images of the complex particles deposited on mica unambiguously support their pearl-necklace structural organization. Our work theoretically predicts and experimentally confirms a possibility to tune the periodicity of one-dimensional intramolecular nano-patterned structures at will by a variation of the base-molar ratio between the oppositely charged macromolecular building blocks incorporated in the polymeric nano-assembly, that is, by a variation of the stoichiometry of the IPECs.

Conformations of Molecular Brushes Based on Polyimide and Poly(methyl methacrylate) in Selective Solvents: Experiment and Computer Simulation

The effect of solvent quality with respect to main and side chains on the conformations of molecular brushes is investigated by the methods of molecular hydrodynamics and optics as well as by computer simulation. Copolymers with a polyimide backbone and poly(methyl methacrylate) side chains are studied in solvents featuring strongly different thermodynamic qualities (chloroform, ethyl acetate, and 3-heptanone). The studied samples have close total molecular masses and backbone lengths but differ in grafting densities and lengths of side chains: a brush with densely grafted and relatively short side chains versus a brush with loosely grafted but very long side chains. For both types of brushes, similar changes in hydrodynamic behavior with improvement in the solvent quality are found experimentally. Computer simulation shows that these changes have different origins. In the former case, macromolecules are elongated, and their volumes grow simultaneously, while in the latter case, the shape of the macromolecules remains close to spherical and the changes in hydrodynamic parameters are mainly due to an increase in the volume of macromolecules.

Computer simulation of kinetics of coil-stretched chain transition in elongational flow

Abstract The Brownian dynamics technique is used to simulate the behavior of a polymer chain in an elongational flow. A macromolecule is represented by dumbbell with conformation-dependent friction coefficient. Equilibrium and kinetic properties of the chain are studied. The effective potential approximation is shown to be quite appropriate for the description of the chain distribution function. The transition times through the effective potential barrier are in a good agreement with the prediction of the Kramers theory. The 5-shaped curve for the extension ratio of the dumbbell as a function of the velocity gradient is observed when simulation times are comparable with the transition times.

Molecular Dynamics of Lysine Dendrimers. Computer Simulation and NMR

In this chapter we describe application of the method of molecular dynamics (MD) for the computer simulation of the poly‐L‐lysine dendrimers of different generations and at different temperatures. These dendrimers differ from other dendrimers by asymmetric branching which could influence the size and shape of dendrimer as well as their dynamical properties. In particular we have shown that the linear spacers between neighboring branch points in all lysine dendrimers are strongly extended and the angles between short spacers are almost fixed. Due to these reasons the lysine dendrimers are rather rigid and their structural characteristics almost do not dependent on temperature. The dynamical characteristics of lysine dendrimers obtained by MD and NMR methods agree rather well. It was obtained that the local mobility of inner CH2 groups is smaller than mobility of corresponding terminal groups in agreement with our previous results for the flexible dendrimers. At the same time local orientation mobility of the internal groups depends on the number of generations in lysine dendrimer. This behavior differs from the behavior of flexible dendrimers in which the internal mobility is practically the same for the dendrimers of different generations.

Charge inversion of dendrimers in complexes with linear polyelectrolytes in the solutions with low pH

Complexes of fully ionized third-generation dendrimers with oppositely charged linear polyelectrolyte chains are studied by the Brownian dynamics method. A freely jointed model of a dendrimer and a linear chain is used. Electrostatic interactions are considered within the Debye-Huckel approximation with the Debye radius exceeding the dimensions of a dendrimer. In these systems, the phenomenon of charge inversion is observed, and the degree of “overcharging” is higher as compared with that taking place in analogous complexes formed by dendrimers in which only terminal groups are charged. The dependence of the amount of chain units adsorbed on a dendrimer on the polyelectrolyte chain length is nonmonotnic and agrees qualitatively with the predictions of the theory proposed by Nguyen and Shklovskii for a complex composed of a spherical macroion with an oppositely charged linear chain. This nonmonotonic character also manifests itself for certain other structural characteristics of the complexes. Upon the formation of a complex, a chain is shown to penetrate deeply into a dendrimer.

Molecular dynamics of a polymer chain with a crosslink

The method of molecular dynamics has been used to study the behaviour of a polymer chain made up of rigid interacting links with practically free internal rotation. A single slightly deformed crosslink, whose length is equal to the length of a link, is introduced into the chain. A condensed polymeric system at high temperature is discussed. It is shown that the translational and rotational mobility of the crosslink are appreciably impeded. The impeding effect of the crosslink spreads to a small number of adjacent chain elements (one link for the orientational mobility and 4–8 links for the translational mobility). The results of a numerical experimental are compared with an analytical calculation for a viscoelastic model of sub-chains with a crosslink and with experimental data obtained using polarization luminescence, the crosslink being labelled. The ways in which certain dynamic properties of crosslinked systems depend on the mole fraction of crosslinks have been calculated.

Structural behavior of hyperbranched polymers in solvents of various qualities: Brownian dynamics simulation

Structural characteristics of third- and fourth-generation dendrimers and irregular hyperbranched polymers of various topologies equal to regular dendrimers in terms of molecular mass in solutions of various topologies are studied via Brownian dynamics simulation. Terminal and inner groups of the studied molecules feature different sensitivities to the quality of a solvent. The mean-square of the radius of gyration, ; hydrodynamic radius ; and the radial density distribution functions of monomer units, ρ(r), are calculated. A change in the structural characteristics of molecules induced by worsening in the quality of the solvent is affected by molecular mass, the amount of terminal groups, the ratio between the amount of terminal groups and the total amount of monomer units, and the topology of hyperbranched macromolecules. A comparison of simulation results with the available experimental data makes it possible to use the computersimulation data for determination of the topology of polymers.