Noureddine Metatla

Interest is growing in the radiation science of supercritical water (SCW), yet how radiation energy is deposited still remains a question not well understood. To unveil the molecular structures of SCW, we carried out molecular dynamics simulations at 400 °C and different densities (0.17, 0.31, and 0.55 g cm⁻³). The results clearly show the heterogeneous character of these structures with coexisting high- and low-density regions. This water “clustering” behavior is consistent with observations in previous studies. It also supports the proposed role played by pre-existing polymeric clusters in the localization and hydration mechanisms of excess electrons in SCW. This underlying molecularity of SCW should be taken into account in track-structure chemistry models of SCW radiolysis.

The effect of shearing on Nafion/water systems was investigated using Dissipative Particle Dynamics (DPD). This simulation approach has been shown to accurately reveal the morphology of such systems in the steady state. We first confirmed that the length of the Nafion chain, 5 and 20 DPD units, has no influence on the overall morphology for four different concentrations, 10, 20, 30, and 40%, of water. Shearing effects with 0.05 and 0.2 rates were then studied according to the Nafion chain length, 5 and 20 DPD units, and water content, 10 and 30%. It was shown that low water contents and long chain length lead to the formation of water-rich tubes, aligned with the shear direction. The formation of such channels can ultimately lead to an increase in the proton conductivity at low humidity. The size of these water tubes thus results from a combined influence of shear strain, chain relaxation time, interfacial tension and water content.

Polymer nanocrystals have attracted considerable attention because of their potential applications in future technology and their fascinating properties which differ from those of corresponding bulk materials. The essential influence of the nanointerface in nanocrystals is apparent in the linear dependence of the melting temperature with the inverse sheet thickness, i.e. the Gibbs–Thomson behaviour. Yet, few experimental and theoretical works have been attempted to highlight the influence of nanointerfaces on the thermal properties of nanocrystals. In this work, simulations were used to evaluate the melting temperature of crystalline polymer nanosheets. Ensuing results were compared favourably to experimental melting temperatures stemming from alkane chains and functional polyolefins, thus validating our simulation approach. Both experimental and simulated results followed Gibbs–Thomson behaviour and a procedure was devised to extract the heat of melting as well as the surface energy from these results. Thus, surface energy of various nanocrystals was found to be widely different for various experimental systems, demonstrating the significance of the environment on thermal properties of nanocrystals.

The Kohlrausch-Williams-Watt (KWW) function, or stretched exponential function, is usually employed to reveal the time dependence of the polymer backbone relaxation process, the so-called α relaxation, at different temperatures. In order to gain insight into polymer dynamics at temperatures higher than the glass transition temperature T _g , the behavior of the Kohlrausch exponent, which is a component of the KWW function, is studied for a series of vinylic polymers, using an all-atomistic simulation approach. Our data show very good agreement with published experimental results and can be described by existing phenomenological models. The Kohlrausch exponent exhibits a linear dependence with temperature until it reaches a constant value of 0.44, at 1.26T _g , revealing the existence of two regimes. These results suggest that, as the temperature increases, the dynamics progressively change until it reaches a plateau. The non-exponential character then describes subdiffusive motion characteristic of polymer melts.

The effect of the molar volume on the mechanical properties of three polymers is computed using molecular mechanics. It is demonstrated that elastic properties significantly depend on the molar volume. To be reproducible they must be determined for a series of “stable” glassy configurations for which the potential energy is minimized not only according to the coordinates but also to the molar volume. If these conditions are not fulfilled, a great disparity in values between experiment and simulation is observed. The procedure is first applied to PVC. It is then argued that coherent results stemming from the application of this approach to both tacticities of PMMA justifies its application toward other polymers. magnified image

The aim of molecular modeling is to mimic reality by considering approximations appropriate to the scale at which the simulation is carried out. At the atomic level, forcefields that represent average atomic interactions are used. However, the phase space has to be adequately explored in order to compare successfully computed and experimental properties. The procedure exposed in this article considers an initial selection of relevant configurations on which a simulated annealing process is applied using the first generation forcefield OPLS, followed by a uniform hydrostatic compression using the second generation forcefield COMPASS©. The resulting data are fitted by an equation of state, from which density and bulk modulus are determined. The glass transition is then simulated and T _gs are computed. Our approach is tested using a series of vinylic polymers, which differ from each other by small variations in atomic interaction combinations. The excellent agreement with experimental data shows the validity of the procedure exposed. Moreover, a clear linear relationship between simulated and experimental T _gs is revealed.