Alexey V. Lyulin

Computer simulations of hyperbranched polymers: The influence of the Wiener index on the intrinsic viscosity and radius of gyration

The influence of the Wiener index on solution properties of trifunctional hyperbranched polymers has been investigated using Brownian dynamics simulations with excluded volume and hydrodynamic interactions. A range of degrees of polymerization (N) and degrees of branching (DB) were used. For each DB and N, several molecules with different Wiener indices (W) were simulated, where W depends on the arrangement of branch points. The intrinsic viscosity and the radius of gyration (Rg) of HPs were both observed to scale with W at a constant N via a power law relationship, as found in the literature. Through their relationships to W, an expression relating intrinsic viscosity to Rg was obtained. This relationship is found to fall centrally between the predictions of Flory and Fox for linear polymers and that of Zimm and Kilb for branched polymers. Molecular shape in solution is also found to depend on W and N, as observed through the W dependence of the ratio of Rg to the hydrodynamic radius, Rh.

Molecular-dynamics simulation of polyimide matrix pre-crystallization near the surface of a single-walled carbon nanotube

Polyimide-based composite materials with a single-walled carbon nanotube as filler were studied by means of extensive fully-atomistic molecular-dynamics simulations. Polyimides (PI) were considered based on 1,3-bis-(3′,4-dicarboxyphenoxy)-benzene (dianhydride R) and various types of diamines: 4,4′-bis-(4′′-aminophenoxy)-diphenylsulfone (diamine BAPS) and 4,4′-bis-(4′′-aminophenoxy)-diphenyl (diamine BAPB). The influence of the chemical structure of the polyimides on the microstructure of the composite matrix near the filler surface and away from it was investigated. The formation of subsurface layers close to the nanotube surface was found for all composites considered. In the case of R–BAPB-based composites, the formation of an organized structure was shown that could be the initial stage of the matrix crystallization process observed experimentally. Similar structural features were not observed in the R–BAPS composites. Carbon nanotubes induce the elongation of R–BAPB chains in composites whereas R–BAPS chains become more compact similar to what is observed for EXTEM™ polyimide. It was shown that electrostatic interactions do not influence the microstructure of composites but slow down significantly the dynamics of PI chains in composites.

Brownian dynamics simulation of linear polymers under elongational flow: Bead–rod model with hydrodynamic interactions

Computer simulations of perfectly branched dendrimers up to the sixth generation have been performed under the influence of uniaxial elongational flow for the first time for a model with explicit dendritic topology. The Brownian dynamics simulation technique has been applied to a freely jointed bead−rod model with excluded volume both with and without hydrodynamic interactions. The dependence of conformational properties and the intrinsic elongational viscosity on the flow rate were obtained. The coil−stretch transition was observed for dendrimers of all generations with it being less pronounced than the same type of transition observed for a linear polymer chain. Hydrodynamic interactions shift the onset of this transition to higher elongational rates. The transition is observed to occur in two stages as it was for a linear polymer. The dendrimer first orients at low flow rate as a whole along the flow axis without significant deformation and local orientation. Increasing flow rate leads to local orienta...

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/T(1) 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 dendrimers 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.

Linker Formation in an Overcharged Complex of Two Dendrimers and Linear Polyelectrolyte

The complexes formed by two dendrimers with charged terminal groups and oppositely charged long linear polyelectrolyte (LPE) have been studied using Brownian dynamics simulations. The structural properties of the complexes and their dependence on the LPE chain length were investigated. It was observed that dendrimers in the considered complexes are sufficiently overcharged; i.e., the number of adsorbed LPE monomers is larger than required for the neutralization. The degree of overcharging increases with the increase of the LPE length and is accompanied by the linker appearance until saturation in overcharging is reached. Nonmonotonic dependence of the linker size on the LPE length was observed. To describe the structural properties of the complexes formed by two macroions and a polyelectrolyte chain, the correlation theory has been developed.

Molecular-dynamics simulations of thin polyisoprene films confined between amorphous silica substrates

Constant temperature-constant pressure (NpT) molecular-dynamics computer simulations have been carried out for the united-atom model of a non-crosslinked (1,4) cis-polyisoprene (PI) melt confined between two amorphous, fully coordinated silica surfaces. The Lennard-Jones 12-6 potential was implemented to describe the polymer-silica interactions. The thickness H of the produced PI-silica film has been varied in a wide range, 1 < H/R(g) < 8, where R(g) is the individual PI chain radius of gyration measured under the imposed confinement. After a thorough equilibration, the PI film stratified structure and polymer segmental dynamics have been studied. The chain structure in the middle of the films resembles that in a corresponding bulk, but the polymer-density profile shows a pronounced ordering of the polymer segments in the vicinity of silica surfaces; this ordering disappears toward the film middles. Tremendous slowing down of the polymer segmental dynamics has been observed in the film surface layers, with the segmental relaxation more than 150 times slower as compared to that in a PI bulk. This effect increases with decreasing the polymer-film thickness. The segmental relaxation in the PI film middles shows additional relaxation process which is absent in a PI bulk. Even though there are fast relaxation processes in the film middle, its overall relaxation is slower as compared to that in a bulk sample. The interpretation of the results in terms of polymer glassy bridges has been discussed.

Influence of the carbon nanofiller surface curvature on the initiation of crystallization in thermoplastic polymers

Experimental results have shown that graphitizated carbon nanofibers initiate crystallization in R-BAPB polyimides twice as fast as single-wall carbon nanotubes (CNT) leading to the hypothesis that nanofiller curvature influences polyimide crystallization. Therefore, atomistic molecular-dynamics simulations have been performed for R-BAPB in the presence of a flat graphene sheet and the results were compared with those obtained in the presence of a small-radius CNT. The polyimide chain segments tend to lie parallel to the nanofiller surface and this tendency is stronger and the segments are closer to the graphene surface than to the CNT one. Moreover, the density of the polyimide in the near-surface layer is higher for composites filled with graphene than with CNT. This confirms the assumption that the nanofiller surface curvature is indeed a factor influencing the polymer patterning structure, and that a smaller curvature (i.e. flat surface) provides an enhanced initiation of polymer ordering.

Roughness and ordering at the interface of oxidized polystyrene and water

For the first time, atomistically detailed molecular dynamics calculations revealed molecular ordering of the water-oxidized atactic polystyrene (aPS) interface. Both ordering of the water molecules and the phenyl rings occur. In addition, the natural roughness of the surface has been simulated and compared to experimental values. The composition of the simulated aPS films is based on spin-coated aPS films that have been oxidized and characterized experimentally. The aPS surfaces are oxidized with ultraviolet-ozone radiation and have been characterized by XPS, AFM, and water contact angle measurements. XPS measurements show that the oxygen content in the sample increases rapidly with exposure and reaches saturation near 24 at. % of oxygen. The molecular dynamics simulations show smoothening of an hydrophobic aPS surface upon transition from vacuum to water. The smoothening decreases with increasing hydrophilicity. The calculations reveal ordering of oxidized phenyl rings for aPS surfaces in water. The order increases with increasing hydrophilicity. Additionally, we investigated the water structure near the aPS-water interface as a function of the surface hydrophilicity. With increasing hydrophilicity, the density of water at the aPS-water interface increases. The water density profile is steeper in the presence of hydrophobic aPS. The water shows an ordered layer near both the hydrophobic and hydrophilic surfaces; the position of this layer shifts toward the interface with increasing hydrophilicity.

Glassy boundary layers vs enhanced mobility in capped polymer films

Molecular-dynamics simulations have been carried out for a coarse-grained model of a random AB-copolymer confined between two crystalline substrates. The strength of substrate-polymer interactions, and the distance between the two substrates have been varied in a wide range. For thick films the film-averaged segmental mobility decreases for intermediate adsorption strengths, but start to increase for very high substrate-polymer attraction strength. We saw that this non-monotonic behavior is caused by a very strong heterogeneity of the segmental dynamics above the glass-transition temperature: the segmental mobility slows down drastically close to adsorbing substrates, but strongly increases in the middle part of the film. This effect, and its sensitive dependence on film thickness, are explained by finite-size effects in confinement, in combination with glassy boundary layers. It is demonstrated that film-averaged mobility as often measured cannot be understood without resolving local mobility in space and time.

Local dynamics of isotactic and syndiotactic polypropylene in solution

The local dynamics of polypropylene (PP) in solution is studied by 13C NMR relaxation and by molecular dynamics (MD) simulation via the orientational autocorrelation function (OACF) of C-H bonds. The interpretation protocol of this function proposed by Dejean de la Bâtie, Laupretre and Monnerie (DLM) [R. Dejean de la Bâtie, F. Laupretre, and L. Monnerie, Macromolecules 21, 2045(1988)] is applied and tested on new NMR measurements of the various microstructures of polypropylene. This interpretation scheme of the OACF is supported by a detailed study employing simulated PP trajectories in an atomistic heat bath. MD simulations indicate that, quite generally, the correlation time for segmental motions, τ1, extracted from the DLM motional model is closely linked with the mean time between conformational jumps. Both experiments and simulations suggest a slightly higher mobility of meso sequences by comparison with racemic sequences. Our analysis of the microscopic aspects of the segmental dynamics and its mani...