The simulated glass transition temperature (Tg) is usually determined by the dilatometry procedure. However, we recently showed that a greater glass transition domain is uncovered where specific ...volume, coefficient of thermal expansion and heat capacity vary. This domain extends over 150 K, definitively higher than the experimental 3–5 K, raising the question of the identification of Tg. This article thus presents the computation of the bulk modulus and the Grüneisen parameter of two polymers, polyethylene and polystyrene. Fit of the Murnaghan equation when the system is in mechanical equilibrium, and the volume fluctuation method are used. The first method leads to important standard deviation for values higher than Tg. With the latter procedure, the computing values data are found consistent with experimental data for temperatures higher and lower than the glass transition domain. This study confirms the enlargement of the glass transition domain, and the difficulty to associate Tg with a specific change in the molecular behavior.
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•We confirm that molecular dynamics simulation leads to a glass transition domain spread over 150 K.•The bulk modulus computed using a dynamics approach agrees very well with experimental data.•We show that bulk modulus computed at the mechanical equilibrium is relatively steady until the glass transition domain.•The bulk modulus of polymers is in order of 2 GPa if no glass transition would occur.•The temperature behavior of the Grüneisen parameter extracted from simulation agrees with experimental data.
Macroscopic nonlinear optical (NLO) properties can arise due to the presence of a polar axis in chiral smectic C phases. A specific molecular design, based on a multi-scale approach, is suggested to ...induce valuable NLO responses. The design actually proceeds through two principal steps. Firstly, at the empirical design stage, one needs to propose a series of molecules with structures which fit the Boulder model. The second step, the rational design, corresponds to the multi-scale approach. ab initio and atomistic simulations are thus used to avoid difficulties in computing the distribution functions required to compare molecular and macroscopic NLO properties.
Protecting from solar radiation remains a very stimulating field of research and development. Improvement of performances of UV/IR radiation shielding devices is thus constantly pursued. With ...increase of computer strength and code efficiency, simulation becomes a real asset in this investigation. In this study, we report an efficient multiscale simulation protocol to design and optimize such photonic devices based on polymers. The simulation approach combines atomistic-level techniques, i.e. classical molecular mechanics, and linear scaling-DFT in order to get full description of the optical properties of polymers over a large wavelength range. At each step of the procedure, validation with experimental data is carried out, confirming the accuracy of the approach. The resulting optical constants enable designing of a multilayer photonic heterostructure purposely optimized for UV/NIR-radiations protection. The ensuing simulated device reveals very promising optical performance. It exhibits a transparency higher than 90% in the visible range and shows high UV absorptance. Moreover, a strong NIR-shielding ability of 96% is achieved.
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•A protocol to design and optimize polymers-based photonic devices is proposed.•The approach combines atomistic-level techniques and device-level simulation.•The ensuing simulated device reveals very promising optical performance.•Transparency higher than 90% and high UV absorptance are achieved.•A strong NIR-shielding ability of 96% is obtained.
An extensive understanding in the molecular motions that occur in Nafion® should lead to important development of improved proton exchange membrane for use in fuel cells (PEMFC). As water molecules ...are added in the system, changes within the Nafion® chain definitely take place. To visualize such a process, molecular dynamics is especially useful. Can information gained at this level of details be useful to propose new molecules, with ultimately better physical properties, such as higher proton conductivity? For this purpose, we first computed non‐bond parameters stemming from the study of the trifluorosufonic acid. They are inserted in the pcff force field. We then applied the procedure developed in our lab to extract the glass transition temperature of Nafion® with different water uptakes. The plasticization effect is first confirmed, fostering a molecular analysis. The particular behavior of the sulfur‐sulfur distance is revealed, guiding the design of new monomers.
The unique physical properties of auxetic compounds make them very attractive materials. Nevertheless, no synthesised materials known to exhibit negative Poisson's ratio at the molecular level have ...been made. One way to explore such compounds is to predict potential candidates prior to their synthesis. To achieve it, multi-atom simulation is a powerful predicting tool. However, the lack of existing systems means that the crucial step of validation cannot be carried out. This paper aims to provide a procedure to predict the auxeticity of proposed molecular systems. The strategy is based on first revealing the determinant step in reaching the mechanical equilibrium of existing isotropic compounds such as polymers, or organic glasses, to compute efficiently mechanical properties. The Poisson's ratio is found in good agreement with experimental data. The procedure is thus modified to be applied to anisotropic compounds, liquid crystals. The agreement with experimental behaviour allowed us to extrapolate the procedure to potentially auxetic compounds, thus offering great opportunities to reveal auxetic properties prior to the synthesis of the molecules.
The physical origin of the large differences between the glass transition temperatures (T g) exhibited by various stereoregular PMMA is not yet fully understood. Currently available theories on the ...glass transition phenomena are not able to predict the observed behavior. In this paper, results obtained from molecular dynamics (MD) simulations are presented. Actually, MD is a very powerful tool to investigate and differentiate the local dynamics of molecular groups embedded in either the backbone or the side chain. From the calculated WLF parameters it is found that the dynamics of the backbone is independent of the PMMA tacticity whereas the dynamics of the side chain strongly depends on the PMMA stereoregularity. However, since the frequency of side-chain rotations is more important in isotactic PMMA compared to syndiotactic, the overall mobility of the isotactic chains becomes higher. Consequently, according to the free volume concept, the T g of isotactic PMMA is lower than the T g of the syndiotactic isomer.
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 1.26
T
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.
Two series of liquid crystals containing a tolane-core surrounded by an ester and a sulfinic ester, with different paraffinic side chains, were synthesized and characterized. The rigid core includes ...an internal ester and a terminal sulfinic ester. The two series differ in the orientation of the internal ester, being either parallel or antiparallel to the sulfinic ester. It is shown that the orientation of the central ester, which modulates dipole–dipole interactions, and the length of the flexible side chains greatly influence their mesogenic properties. Binary mixtures of the two series have been prepared and phase diagrams are reported. The results provide insight into the role of these structural features which extend the thermal stability of the smectic C phase. Behavior of these series is compared to previous results on homolog series containing two esters, in order to better understand the role of the central ester orientation on the liquid crystal polymorphism.
•Influence of the internal ester and a terminal ester group on the liquid crystal polymorphism•SmC phase is shown to be promoted by longer aliphatic chains.•Difference in thermal stability is due to the different crystallization temperatures.•Capacity to stabilize the SmC phase is not the main factor for thermal stability.
Nafion, an amphiphilic polymer based on fluorocarbon backbones and acid groups, is probably the most widely used fuel cell membranes. According to its hydration level it self-organizes leading to ...nano-cavities through which water diffuse. The diffusion of water that controls the protonic transport is then central in the conversion of chemical energy to the electrical one. Recently, a sub-diffusive and heterogeneous dynamics of water were experimentally evidenced paving the way for more efficient fuel cell membranes. Fundamentally, this dynamics which occurs at the nanoscale needs to be microscopically understood. Molecular dynamics simulations are thus performed to locally examine the water dynamics and its relation with the water and Nafion structure. The sub-diffusive regime is numerically corroborated and two sub-diffusive to diffusive transitions are found. The first is time dependent whereas the second is rather water uptake dependent. The sub-diffusive regime is ascribed to the water molecules strongly anchored close to the acid groups. We show that the sub-diffusive to diffusive time transition is the result of the elapsed time before to escape from the attractive interactions of the acid groups. The diffusive regime is recovered far from the acid groups in a homogeneous water phase that is the result of the percolation of the hydrogen bonding network. The progressive transition between sub-diffusive to diffusive regime as a function of the hydration level is due to the respective weight of diffusive dynamics that increases with respect to the sub-diffusive regime given the increase in diffusive pathways as the expense of the localized dynamics. Close to the fluorocarbon backbones the dynamics of water is also sub-diffusive but faster whereas time dynamical transition is not observed. Furthermore we highlight the existence of water corridors based on the hydrogen bonds between molecules forming single file structure in line with a sub-diffusive dynamics. These water corridors are thus possible conducting pathways of protons in a Grotthuss mechanism. Structurally we depict an interdigitated structure where the sulfonate are interleaved. Eventually, at high water uptake, we exhibit the self-organizing of Nafion leading to a phase separation between water and the Nafion membrane. We establish a specific interfacial organization of the hydrophilic sulfonate groups involving a water/Nafion interface.
•Water sub-diffusion at the nanoscale in the Nafion fuel cell membrane.•Time and water-uptake sub-diffusive to diffusive cross-over.•Molecular origin of the anomalous dynamics of water nano-confined into the Nafion.•Correlation between Nafion structure and the dynamics of water.•Self-assembly, interdigitated organization and water/Nafion interface.
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