Stanislav G. Falkovich

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.

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.

Mathematical simulation of lysine dendrimers: Temperature dependences

The mathematical simulation of second- and fourth-generation lysine dendrimers is performed via the molecular-dynamics method. Temperature dependences of primary structural characteristics are obtained. It is shown that the sizes and atomic distributions of these dendrimers are weakly temperature-dependent. Together with the structural properties, the local mobility of CH2 groups in the dendrimers is investigated via the molecular-dynamics method and NMR spectroscopy. It is shown that the orientational mobility of internal groups of the lysine dendrimers is lower than that of terminal groups, in agreement with the data available for flexible-chain dendrimers. Changes in correlation times with temperature are well described by the Arrhenius dependence. At the same time, the orientational mobility of internal groups in the lysine dendrimers depends on the generation number. This behavior is different from that of flexible-chain dendrimers, in which the mobility of internal groups is the same for dendrimers of different generations.

Mechanism of shear deformation of a coiled myosin coil: Computer simulation

Polymer coiled coils are composed of entangled linear chains in a helical conformation. Their mechanical characteristics are interesting because these structures are involved in the composition of natural fibrillar structures. The method of molecular dynamics is used for the simulation of stretching at a constant rate for a superhelical fragment of myosin protein composed of two identical α helices containing 126 amino acid residues in each helix. The case of shear deformation of a molecule is considered (the load is applied to the N terminus of one chain and to the C terminus of another chain). In this case of loading, slippage of chains with respect to each other can occur. Deformation of a molecule proceeds in several stages. At the initial stage, the superhelix is unfolded and there is a gradual unfolding of end fragments of individual α helices; this process is accompanied by their displacement with respect to the helical fragment of the neighboring chain. In this case, the reaction force increases. At the second stage of stretching, the process passes to the mechanism of deformation when, in the central part of the molecule, α-helical fragments of both chains unfold. In this region, the reaction-force-extension curve shows a plateau region. Between unfolded fragments, new hydrophobic contacts and hydrogen bonds are formed, and fragments of the β structure emerge. Once all turns of α helices in the central parts of the molecule are unfolded, the mechanism of deformation changes and further extension of a molecule proceeds via straightening of previously unfolded central fragments, a process that is accompanied by an increase in the reaction force. When chains achieve their limiting extension, slippage commences with an accompanying decrease in the reaction force.

Computational Modeling of Polylactide and Its Cellulose-Reinforced Nanocomposites

Computer simulations based on all-atom models can provide significant assistance to researchers in understanding molecular mechanisms that lead to the appearance of new properties in composites different than those of the individual components. Recently, this simulation of polymer nanocomposites has become more popular, since it provides answers that cannot be obtained experimentally. To date, there have been only a few publications devoted to the computer simulations of polymer nanocomposites based on cellulose, even though these materials are very promising and attract great attention because of their renewable character. One of the main biodegradable polymers, obtained from natural sources and used for developing such biocomposites, is polylactide (PLA). This chapter focuses on computer simulations of PLA and nanocellulose, as well as on composite materials based on these materials.