Grafting-Induced Structural Ordering of Lactide Chains

The structure of a grafted layer of lactide chains in the “dry brush” regime immersed in a melt of chemically similar polymer was examined while varying graft lengths. To this end, microsecond atomistic molecular dynamics simulations were performed. Almost no influence of graft length on the fraction of the grafted chains backfolded to the grafting surface was found. However, a structural ordering was unexpectedly observed in the system when the length of the grafted lactide chains was close to approximately 10 Kuhn segments. This ordering of the grafts is characterized by the formation of helical fragments whose structure is in good agreement with the experimental data for the α crystal of the lactide chains. Both the backfolding and the structural ordering may be viewed as the initial stage of the crystallization of the layer of grafted lactide chains. In contrast to the known behavior for conventional polymer brushes in the “dry brush” regime, the structure of the grafted lactide chains can be either amorphous or ordered, depending on the graft length N and the grafting density σ when their product Nσ is fixed

Cooling-Rate Computer Simulations for the Description of Crystallization of Organic Phase-Change Materials

A molecular-level insight into phase transformations is in great demand for many molecular systems. It can be gained through computer simulations in which cooling is applied to a system at a constant rate. However, the impact of the cooling rate on the crystallization process is largely unknown. To this end, here we performed atomic-scale molecular dynamics simulations of organic phase-change materials (paraffins), in which the cooling rate was varied over four orders of magnitude. Our computational results clearly show that a certain threshold (1.2 × 1011 K/min) in the values of cooling rates exists. When cooling is slower than the threshold, the simulations qualitatively reproduce an experimentally observed abrupt change in the temperature dependence of the density, enthalpy, and thermal conductivity of paraffins upon crystallization. Beyond this threshold, when cooling is too fast, the paraffin’s properties in simulations start to deviate considerably from experimental data: the faster the cooling, the larger part of the system is trapped in the supercooled liquid state. Thus, a proper choice of a cooling rate is of tremendous importance in computer simulations of organic phase-change materials, which are of great promise for use in domestic heat storage devices

Toward predictive molecular dynamics simulations of asphaltenes in toluene and heptane

\u3cp\u3eThe conventional definition of asphaltenes is based on their solubility in toluene and their insolubility in heptane. We have utilized this definition to study the influence of partial charge parametrization on the aggregation behavior of asphaltenes using classical atomistic molecular dynamics simulations performed on the microsecond time scale. Under consideration here are toluene- and heptane-based systems with different partial charges parametrized using the general AMBER force field (GAFF). Systems with standard GAFF partial charges calculated by the AM1-BCC and HF/6-31G*(RESP) methods were simulated alongside systems without partial charges. The partial charges implemented differ in terms of the resulting electrical negativity of the asphaltene polyaromatic core, with the AM1-BCC method giving the greatest magnitude of the total core charge. Based on our analysis of the molecular relaxation and orientation, and on the aggregation behavior of asphaltenes in toluene and heptane, we proposed to use the partial charges obtained by the AM1-BCC method for the study of asphaltene aggregates. A good agreement with available experimental data was observed on the sizes of the aggregates, their fractal dimensions, and the solvent entrainment for the model asphaltenes in toluene and heptane. From the results obtained, we conclude that for a better predictive ability, simulation parameters must be carefully chosen, with particular attention paid to the partial charges owing to their influence on the electrical negativity of the asphaltene core and on the asphaltenes aggregation.\u3c/p\u3

Toward predictive molecular dynamics simulations of asphaltenes in toluene and heptane

The conventional definition of asphaltenes is based on their solubility in toluene and their insolubility in heptane. We have utilized this definition to study the influence of partial charge parametrization on the aggregation behavior of asphaltenes using classical atomistic molecular dynamics simulations performed on the microsecond time scale. Under consideration here are toluene- and heptane-based systems with different partial charges parametrized using the general AMBER force field (GAFF). Systems with standard GAFF partial charges calculated by the AM1-BCC and HF/6-31G*(RESP) methods were simulated alongside systems without partial charges. The partial charges implemented differ in terms of the resulting electrical negativity of the asphaltene polyaromatic core, with the AM1-BCC method giving the greatest magnitude of the total core charge. Based on our analysis of the molecular relaxation and orientation, and on the aggregation behavior of asphaltenes in toluene and heptane, we proposed to use the partial charges obtained by the AM1-BCC method for the study of asphaltene aggregates. A good agreement with available experimental data was observed on the sizes of the aggregates, their fractal dimensions, and the solvent entrainment for the model asphaltenes in toluene and heptane. From the results obtained, we conclude that for a better predictive ability, simulation parameters must be carefully chosen, with particular attention paid to the partial charges owing to their influence on the electrical negativity of the asphaltene core and on the asphaltenes aggregation

Asphaltenes as novel thermal conductivity enhancers for liquid paraffin: Insight from in silico modeling

The practical use of paraffin and other organic phase-change materials for heat storage is largely limited by their low thermal conductivity. In this paper we employed 60 microsecond-long atomic-scale computer simulations to explore for the first time whether the asphaltenes, natural polycyclic aromatic hydrocarbons, can be used as thermal conductivity enhancers for paraffin. We focused on a simple model molecule of asphaltene (a polycyclic aromatic core decorated with the peripheral alkane chains) and showed that the asphaltenes of such molecular architecture are not able to improve the thermal conductivity of paraffin. This is most likely due to the steric constraints imposed by the peripheral alkane groups, which prevent formation of the extended ordered asphaltene aggregates. To overcome this, we proposed a possible chemical modification of the asphaltene molecules through removing the peripheral alkane groups from their aromatic cores; this could be achieved e.g. by thermal cracking (dealkylation) of asphaltenes. It turns out that such a chemical modification drastically changes the situation: the modified asphaltenes form extended columnar aggregates which can serve as thermal conduction paths, considerably enhancing the thermal conductivity of a liquid composite sample. This effect, however, vanishes upon cooling because the columnar extended stacks of chemically modified asphaltenes transform into the helical twisted structures, which reduces the overlap of adjacent asphaltenes in aggregates. Importantly, all the simulations have been carried out with two different all-atom force fields. We have demonstrated that both computational models give qualitatively similar results. Overall, our findings clearly show that chemically modified asphaltene molecules can be considered as promising carbon-based thermal conductivity enhancers for liquid paraffin; this result can be used for optimizing the paraffin-based thermal energy storage systems

Evaluation of thermal conductivity of organic phase-change materials from equilibrium and non-equilibrium computer simulations: Paraffin as a test case

An accurate in silico evaluation of the thermal conductivity is critical for improving the thermal properties of organic phase-change materials on a rational basis. To explore the impact of a theoretical model on the computed thermal conductivity, here we employed the equilibrium and the non-equilibrium molecular dynamics (MD) simulations to study paraffin (n-eicosane) bulk samples, in both crystalline and liquid states, with the use of 10 atomistic force fields, both all-atom and united-atom ones. Overall, we found that the equilibrium MD method is preferable for computing the thermal conductivity of n-eicosane samples (at least for a 10-nm-size simulation box). For the n-eicosane crystals, the all-atom models provide larger thermal conductivity coefficients than their united-atom counterparts and, correspondingly, a better match with the experimental data. This is most likely because the crystalline lattice of the models with explicit hydrogen atoms is additionally stabilized by the electrostatic interactions. In contrast, in the liquid state, most all-atom models overestimate the experimental data for n-eicosane, providing thereby worse performance as compared to the united-atom force fields. However, when it comes to the experimentally observed increase in the thermal conductivity of n-eicosane samples upon crystallization, only all-atom models are able to reproduce quantitatively the experimental data. Each force field of n-eicosane was also characterized by an overall score which accumulated the deviations of the computed thermal conductivity coefficients from the experimental values, for both crystalline and liquid samples. It turns out that the best performance among 10 atomistic models of n-eicosane is observed for the all-atom GAFF force field. All in all, our study clearly demonstrates that a proper choice of the model for computing the thermal conductivity is a non-trivial task: even for such relatively simple compounds as paraffins (n-alkanes), different models perform quite differently, in equilibrium and in non-equilibrium MD simulations, as well as in crystalline and liquid phases

Scale-dependent miscibility of polylactide and polyhydroxybutyrate:molecular dynamics simulations

\u3cp\u3eMiscibility of polylactide (PLA) and polyhydroxybutyrate (PHB) is studied by the microsecond atomistic molecular dynamics (MD) simulations for the first time. The model and the simulation protocol were confirmed through comparison of the glass transition temperature (T\u3csub\u3eg\u3c/sub\u3e) with experimental data. It was established that PLA and PHB are miscible on the basis of the Flory-Huggins theory. Analysis of the mobilities of PLA and PHB subchains revealed that the blends have two transitions to a glassy state at the length scale of a few Kuhn segments, which is in line with the predictions of the self-concentration model. At the same time at the larger length scale a single transition to a glassy state was observed, suggesting scale dependence of PLA and PHB miscibility. This scale dependence was confirmed through the evaluation of the interchain pair correlation functions.\u3c/p\u3

Scale-dependent miscibility of polylactide and polyhydroxybutyrate:molecular dynamics simulations

\u3cp\u3eMiscibility of polylactide (PLA) and polyhydroxybutyrate (PHB) is studied by the microsecond atomistic molecular dynamics (MD) simulations for the first time. The model and the simulation protocol were confirmed through comparison of the glass transition temperature (T\u3csub\u3eg\u3c/sub\u3e) with experimental data. It was established that PLA and PHB are miscible on the basis of the Flory-Huggins theory. Analysis of the mobilities of PLA and PHB subchains revealed that the blends have two transitions to a glassy state at the length scale of a few Kuhn segments, which is in line with the predictions of the self-concentration model. At the same time at the larger length scale a single transition to a glassy state was observed, suggesting scale dependence of PLA and PHB miscibility. This scale dependence was confirmed through the evaluation of the interchain pair correlation functions.\u3c/p\u3

Computer simulation of asphaltenes

The review describes theoretical approaches based on computer simulations at various levels of details (from quantum chemical calculations to atomistic and coarse-grained models) to study asphaltenes and systems containing asphaltenes. The used methods are described, their advantages and disadvantages are discussed in terms of computational costs and time- and spatial-scales available for simulations. The results of studies of the asphaltenes interactions with each other and their aggregation behavior in low-molecular solvents are presented. The most promising approaches of computer simulations of asphaltenes-based systems are determined