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The paper focuses on the problem of electrostatic interactions in molecular dynamics simulations of thermal properties of heterocyclic polymers. The study focuses on three thermoplastic polyimides synthesized on the basis of 1,3-bis-(3 0 ,4-dicarboxyphenoxy)benzene (dianhydride R) and three diamines: 4,4 0 -bis-(4 00 -aminophenoxy) diphenylsulfone (diamine BAPS), 4,4 0 -bis-(4 00 -aminophenoxy) biphenyl (diamine BAPB), and 4,4 0 -bis-(4''-aminophenoxy) diphenyloxide (diamine BAPO). In the molecular dynamics simulations these polyimides were described by the Gromos53a5 force field. To parameterize the electrostatic interactions four methods of calculating the partial atomic charges were chosen: B3LYP/6-31G*(Mulliken), AM1(Mulliken), HF/6-31G*(Mulliken), and HF/6-31G*(ChelpG). As our parameterization is targeted to reproduce thermal properties of the thermoplastic polyimides, the choice of proper partial charges was finalized on a basis of the closest match between computational and experimental data for the thermal expansion coefficients of the polyimides below glass transition temperatures. Our finding clearly show that the best agreement with experimental data is achieved with the Mulliken partial atomic charges calculated by the Hartree-Fock method with 6-31G* basis set. Furthermore, in addition to the thermal expansion coefficients this set of partial atomic charges predicts an experimentally observed relationship between glass transition temperatures of the three polyimides under study:. A mechanism behind the change in thermal properties upon the change in the chemical structure in considered polyimides may be related to an additional spatial ordering of sulfone groups due to dipole-dipole interactions. Overall, the modified force-field is proved to be suitable for accurate prediction of thermal properties of thermoplastic polyimides and can serve as a basis for building up atomistic theoretical models for describing other heterocyclic polymers in bulk.

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Abstract. New polyimide nanocomposites containing organically modified montmorillonite (MMT), synthetic silicate (chrysotile) nanotubes (SNT), and zirconium dioxide (ZrO2) were prepared to investigate the influence of the nanoparticle morphology on the nanocomposite rheology and mechanical properties under selected conditions that the materials are likely to encounter during use. The efficiency of homogeneous dispersion of the nanoparticles in the polyimide matrix was studied by measuring the rheology of model oligoimides (OI) dispersions containing the desired amounts of the nanoparticles. The OI/nanoparticles dispersions showed significant increase in complex viscosity with increasing concentration of the nanoparticles that depended strongly on the nanoparticle morphology and aspect ratio. Polyimide nanocomposite films (PI-PM) prepared from the poly(amic acid) of poly(pyromellitic dianhydride-co-4,4′-oxydianiline) (PM) filled with the desired concentration of the nanoparticles showed an increase in tensile modulus with increasing nanoparticle concentration in the order MMT>SNT>ZrO2. In contrast to the PI-PM/MMT films, the PI-PM films filled with 10 vol% of SNT and ZrO2 showed higher sample failure strains, suggesting that the SNT and ZrO2 may be more effective in improving the ductility of the polyimide nanocomposites for applications where the relatively brittle polyimide/MMT nanocomposites films are not useable. Vol.2, No.7 (2008) [485][486][487][488][489][490][491][492][493] Available online at www.expresspolymlett.com DOI: 10.3144/expresspolymlett.2008.58 composition and particle size. For example, MMT minerals from different deposits might differ considerably in composition. This variation in composition of MMT significantly complicates the task of making functional nanocomposites with prescribed properties for targeted applications. Therefore, there is a need to develop synthetic nanofillers with prescribed particle composition, shape and size for use as fillers in polymer nanocomposites with well defined properties. In this area, synthetic nano-dimensional silicates may provide a number of opportunities in polymer nanocomposites that is relatively little studied and poorly understood relative to the well studied polymer nanocomposites filled with natural layered MMTs [5][6][7][8][9]. In contrast to the commonly used layered MMT compounds, it is envisaged that use of nanoparticles with different morphology (e.g., nanotubes and nanoparticles of isometric shape) might provide additional benefits in polymer nanocomposites not possible with MMT. Suitable techniques for hydrothermal synthesis of silicate nanotubes and particles of isometric form (e.g., zirconium dioxide) that gives the possibility to control the resulting nanoparticle's structure by varying the mode of hydrothermal synthesis has been reported [7][8][9]. Polyimide (PI)-based nanocomposites are of special interest because of the well known excellent heat resistance, chemical stability, and superior electric properties of polyimides [10]. A...

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