Connectivity effects in the segmental self- and cross-reorientation of unentangled polymer melts

The segmental (bond) rotational dynamics in a polymer melt of unentangled, linear bead-spring chains is studied by molecular dynamics simulations. To single out the connectivity effects, states with limited deviations from the Gaussian behavior of the linear displacement are considered. Both the self and the cross bond-bond correlations with rank ℓ=1,2 are studied in detail. For ℓ=1 the correlation functions are precisely described by expressions involving the correlation functions of the chain modes. Several approximations concerning both the self- and the cross-correlations with ℓ=1,2 are developed and assessed. It is found that the simplified description of the excluded volume static effects derived elsewhere [D. Molin et al., J. Phys.: Condens. Matter 18, 7543 (2006)] well accounts for the short time cross-correlations. It also allows a proper modification of the Rouse theory which provides quantitative account of the intermediate and the long time decay of the rotational correlations with ℓ=1

Statics, short time dynamics and relaxation in polymers and viscous liquids

By performing Molecular Dynamics (MD) simulations it is possible to explain the validity of a theoretical model in a fully controlled environment. Their use became a relevant part of the normal activity of all scientific researching groups. The MD potentialities extend beyond the simply theoretical validation: thanks to the great precision and low-cost computational power archived presently, they can be applied to better interpret experimental observation too. The work here presented tries to highlight indeed both of these aspects in the context of glass-formers and polymers. During my PhD I worked on several projects all connected by one "lait motif": the interest for mechanical relaxation processes inside complex liquids. This huge theme in the matter physics borders with chemistry and material engineering due to the practical and also, why not, futuristic applications (e.g. pharmaceutical, dental and internal prosthesis, food preservation and cryogenic suspension of living beings, new functional materials with fine tuned optical and mechanical properties). The hidden heart, feeding all this research fields on the same landscape, is the Glass Transition (GT). The knowledge of the physics controlling the relaxation arrest experimented by liquids under hight density and low temperature regimes is still unraveled, but it continues to intrigue despite the decades of interest. The GT theme is approached here from the numerical point of view. To study this complex phenomenology, the chosen prototype of viscous liquid is the simple beads and springs model for polymeric chain liquids. Polymers represent indeed a central class in the GT research because of their "natural" disorder. For this reason a polymer liquid, rather than crystallize in a regular lattice, reaches an amorphous state, the glassy state. This thesis is structured into two parts: in the first two chapter, the numerical model of a polymeric liquid, its statics (Chpt.1) and its dynamics (Chpt.2) is presented; then this model is utilized in the context of GT speculations (Chpt.3). To be a little more precise in Chpts.1 and 2 the polymer model is analyzed in terms of Rouse modes which, carefully mixed, help to formulate predictions on bond relaxation functions. These are regularly observable in dielectric spectroscopy experiment. Of course, the Rouse picture represents a first approximation, nevertheless just to pose mind on this imperfections is sufficient to increase our knowledge on the model. Indeed, the effort spent was repaid with interesting results by comparing the theory and simulations data. In Chpt.3 the GT is faced moving from a series of relevant observations on a huge corpus of simulated state points. A strong correlation, between structural relaxation time and the Debye-Waller factor (DW), is proved to exist and to be described by a parabolic extension of the Hall-Wolynes law \cite{HallWolynes87}. The extreme robustness on numerical data, also on other MD simulations from literature, gave us the amazing possibility to successfully breakthrough into the ``real'' world. Indeed, both the observables have their experimental equivalents and can be found in literature for a not negligible number of liquids. What we propose represents the most "universal" law in this field both for the wide range of fragility (20 < m < 191) and for the time scales spanned ( 10E-12 sec < t < 10E2 sec)

Scaling between structural relaxation and caged dynamics in Ca_0.4 K_0.6 (NO_3)_1.4 and glycerol: free volume, time scales and implications for the pressure-energy correlations

International audienceThe scaling of the slow structural relaxation with the fast caged dynamics is evidenced in the molten salt Ca${}_{\, 0.4}$K${}_{\, 0.6}$(NO${}_{\, 3}$)${}_{\,1.4}$ (CKN) over about thirteen decades of the structural relaxation time. Glycerol scaling was analyzed in detail. In glycerol, the short-time mean-square displacement $\langle u^2 \rangle$, a measure of the caged dynamics, is contributed by free-volume. It is seen that, in order to evidence the scaling, the observation time of the fast dynamics must be shorter than the time scales of the relaxation processes. Systems with both negligible (like CKN, glycerol and network glassformers) and high (like van der Waals liquids and polymers) pressure-energy correlations exhibit the scaling between the slow relaxation and the fast caged dynamics. According to the available experiments, an isomorph-invariant expression of the master curve of the scaled data is not distinguishable from a simpler not-invariant expression. Instead, the latter agrees better with the simulations on a wide class of model polymers

Proton transport in barium stannate: classical, semi-classical and quantum regimes

International audienceDensity-functional theory calculations are performed to investigate proton transport in BaSnO 3. Structural optimizations in the stable and saddle point configurations for transfer (hopping) and reorientation allow description of the high-temperature classical and semi-classical regimes, in which diffusion occurs by over-barrier motion. At lower temperature (typically below 300 K), we describe the thermally-assisted quantum regime, in which protonic motion is of quantum nature and occurs in ''coincidence'' configurations favored by thermal fluctuations of the surrounding atoms. Both the non-adiabatic and the adiabatic limits are examined. In the adiabatic limit, the protonic energy landscape in the coincidence configuration is very flat. Path-integral molecular dynamics simulations of the proton in the coincidence potential reveal, in the transfer case, that the density of probability of H + has its maximum at the saddle point, because the zero-point energy exceeds the coincidence barrier. Arguments are given that support the adiabatic picture for the transfer mechanism. In the case of reorientation, the time scales for the existence of the coincidence and for protonic motion, as estimated from the time-energy uncertainty principle by using a simple one-dimensional model, are of the same order of magnitude, suggesting that the adiabatic limit is not reached. Protonic transfer and reorientation in this oxide are therefore governed by different mechanisms below room temperature

Oxygen diffusion mechanism in the mixed ion-electron conductor NdBaCo2O5+x†

International audienceDouble perovskite cobaltites were recently presented as promising cathode materials for solid oxide fuel cells. While an atomistic mechanism was proposed for oxygen diffusion in this family of materials, no direct experimental proof has been presented so far. We report here the first study that directly compares experimental and theoretical diffusion pathways of oxygen in an oxide, namely in the double cobaltite compound, NdBaCo2O5+x. Model-free experimental nuclear density maps are obtained from the maximum entropy method combined with Rietveld refinement against high resolution neutron diffraction data collected at 1173 K. They are then compared to theoretical maps resulting from classical molecular dynamics calculations. The analysis of 3D maps of atomic densities allows identifying unambiguously the pathways and the mechanisms involved in the oxide ion diffusion. It is shown that oxygen diffusion occurs along a complex trajectory between Nd- and Co-containing a,b planes. The study also reveals that Ba-containing planes act as a barrier for oxygen diffusion. The diffusion mechanism is also supported through the oxygen sites occupancy analysis that confir ms the increase of oxygen vacancies in the cobalt-planes on heating. The use of such combined experimental and theoretical analysis should be considered as a very powerful approach for materials design

Design of dendritic core carbazole-based hole transporting materials for efficient and stable hybrid perovskite solar cells

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Assessing challenging intra‐ and inter‐molecular charge‐transfer excitations energies with double‐hybrid density functionals

We investigate the performance of a set of recently introduced range‐separated double‐hybrid functionals, namely ωB2‐PLYP, ωB2GP‐PLYP, RSX‐0DH, and RSX‐QIDH models for hard‐to‐calculate excitation energies. We compare with the parent (B2‐PLYP, B2GP‐PLYP, PBE0‐DH, and PBE‐QIDH) and other (DSD‐PBEP86) double‐hybrid models as well as with some of the most widely employed hybrid functionals (B3LYP, PBE0, M06‐2X, and ωB97X). For this purpose, we select a number of medium‐sized intra‐ and inter‐molecular charge‐transfer excitations, which are known to be challenging to calculate using time‐dependent density‐functional theory (TD‐DFT) and for which accurate reference values are available. We assess whether the high accuracy shown by the newest double‐hybrid models is also confirmed for those cases too. We find that asymptotically corrected double‐hybrid models yield a superior performance, especially for the inter‐molecular charge‐transfer excitation energies, as compared to standard double‐hybrid models. Overall, the PBE‐QIDH and its corresponding range‐separated RSX‐QIDH functional are recommended for general‐purpose TD‐DFT applications, depending on whether long‐range effects are expected to play a significant role.The work in Alicante is supported by project PID2019-106114GB-I00 (“Ministerio de Ciencia e Innovación”). E.B. thanks ANR (Agence Nationale de la Recherche) and CGI (Commissariat à l'Investissement d'Avenir) for their financial support to this work through Labex SEAM (Science and Engineering for Advanced Materials and devices), Grant Nos. ANR-10-LABX-096 and ANR-18-IDEX-0001. I.C. gratefully acknowledge support from the European Research Council (ERC) for grant agreement No. 648558 (STRIGES CoG). The authors thank the GENCI-CINES for HPC resources (Project Nos. A0060810359 and A0080810359)

Modeling Multi-Step Organic Reactions: Can Density Functional Theory Deliver Misleading Chemistry?

Many organic reactions are characterized by a complex mechanism with a variety of transition states and intermediates of different chemical natures. Their correct and accurate theoretical characterization critically depends on the accuracy of the computational method used. In this work, we study a complex ambimodal cycloaddition with five transition states, two intermediates, and three products, and we ask whether density functional theory (DFT) can provide a correct description of this type of complex and multifaceted reaction. Our work fills a gap in that most systematic benchmarks of DFT for chemical reactions have considered much simpler reactions. Our results show that many density functionals not only lead to seriously large errors but also differ from one another in predicting whether the reaction is ambimodal. Only a few of the available functionals provide a balanced description of the complex and multifaceted reactions. The parameters varied in the tested functionals are the ingredients, the treatment of medium-range and nonlocal correlation energy, and the inclusion of Hartree–Fock exchange. These results show a clear need for more benchmarks on the mechanisms of large molecules in complex reactions