Motivation. Poly(methyl methacrylate), PMMA, offers a particular regard to the study of the glass... more Motivation. Poly(methyl methacrylate), PMMA, offers a particular regard to the study of the glass transition since according to the tacticity of its chain, a different glass temperature transition, Tg, is exhibited. Molecular modeling is thus a perfect tool to study such a variation since changes that occur during these transitions can be regarded as differences in molecular characteristics only. The selection of a forcefield that correctly depicts this difference is thus of primary importance. In this article, two forcefields, AMBER and AMBER/OPLS, have been used to simulate the glass transition of the stereoregular PMMAs. Results stemming from these simulations are compared with those, already published, coming from a second generation force field, pcff. Method. Molecular dynamics simulations are used to study the glass transition of stereoregular polymers, and
An entry from the Cambridge Structural Database, the world's repository for small molecule cr... more An entry from the Cambridge Structural Database, the world's repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
Abstract The simulated glass transition temperature (Tg) is usually determined by the dilatometry... more Abstract 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 Gruneisen 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.
Abstract Protecting from solar radiation remains a very stimulating field of research and develop... more Abstract 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.
Abstract The unique physical properties of auxetic compounds make them very attractive materials.... more Abstract 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.
An extensive understanding in the molecular motions that occur in Nafion® should lead to importan... more 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 difference in the glass transition temperatures, T,, of the iso-and syndiotactic forms of PMM... more The difference in the glass transition temperatures, T,, of the iso-and syndiotactic forms of PMMA has been investigated through molecular dynamics simulations of short duration (110 ps) using an accurate force field, pcfl To correlate with the Gibbs-DiMarzio theory, which specifies that conformational energy is a primary factor in determining T , an energetic analysis has been camed out. It is found that syndiotactic PMMA possesses lower intramolecular energy where the governing contribution is brought by the angle bending term; whereas the isotactic PMMA exhibits lower intermolecular energy. Such a behavior suggests that the chain packing greatly determines the difference in the T,.
The idtared absorption spectra of poly(tetrafluoroethylene), PTFE, and poly(chlorotrifluoroethyle... more The idtared absorption spectra of poly(tetrafluoroethylene), PTFE, and poly(chlorotrifluoroethylene), PCTFE, are simulated using a method based on a combination of classisal molecular simulations of the amorphous polymer phase with Kramers-Kronig relations. The differences and the analogies between experimental and calculated spectra of the non-stereoregular fluoride polymer, PTFE, are first reported. The isotactic and syndiotactic configurations of PCTFE are then investigated. The comparison between experimental and simulated spectra is established on a vibrational analysis. It reveals the preferred syndiotactic configuration adopted by the PCTFE chain.
The difference in the glass transition temperatures, T g , of the iso-and syndiotactic forms of P... more The difference in the glass transition temperatures, T g , of the iso-and syndiotactic forms of PMMA has been investigated through molecular dynamics simulations of short duration (110 ps) using an accurate force field, pcff. To correlate with the Gibbs-DiMarzio theory, which specifies that conformational energy is a primary factor in determining T g , an energetic analysis has been carried out. It is found that syndiotactic PMMA possesses lower intramolecular energy where the governing contribution is brought by the angle bending term; whereas the isotactic PMMA exhibits lower intermolecular energy. Such a behavior suggests that the chain packing greatly determines the difference in the T g .
We have measured, the thickness dependence of the glass transition temperature Tg(h), using ellip... more We have measured, the thickness dependence of the glass transition temperature Tg(h), using ellipsometry at variable temperature, for poly(methyl-methacrylate) (PMMA) of various tacticity in confined geometry. We report that several factors significantly affect Tg(h): i) polymer microstructure (stereoregularity of PMMA) related to local dynamics; ii) interfacial interactions; iii) conformation of the polymer chains. These results raise many fundamental questions on the origin of the thickness-dependent glass transition. Why and how do the interactions with the substrate significantly affect Tg(h)? Does Tg(h) depend on the modifications of conformational parameters of the chains (their entropy)? What is the correlation between local dynamics and Tg(h) in thin films? The aim of this paper is to summarise these open questions, which should stimulate further investigations in the thin polymer film scientific community.
Motivation. To describe the glass transition phenomenon using molecular modeling, poly(methyl met... more Motivation. To describe the glass transition phenomenon using molecular modeling, poly(methyl methacrylate), PMMA, offers a noteworthy regard since according to the tacticity of its chain, a different T g is displayed. Accordingly, if a force field accurately describes such variation, insights can be gained in the understanding of the reasons that give rise to this difference, and consequently, in the glass transition phenomenon itself. Several studies have been carried out using a second category force field, pcff. In this article, two first class force fields, AMBER and AMBER/OPLS, have been envisioned. It is found that, although T g s are obtained at higher temperatures, the difference between the two T g s is better mimicked using the AMBER/OPLS force field. The presence of the cross terms in the force fields is thus not a determining factor in obtaining the T g s variation. Method. Molecular dynamics simulations have been employed to compare glass transition of stereoregular polymers, and their energetic differences, with respect to the use of two force fields of different generation.. Results. A first generation force field could deal with the representation of the difference in T g s between stereoregular polymers. Conclusions. Accordingly to the fact that a first generation force field can deal with the difference in T g s difference, a greater amount of phase space could be regarded. Moreover, studies of specific interaction, such as PMMA behavior with surfaces, can be undertaken. Availability. Abbreviations and notations Collect here in alphabetical order all abbreviations and notations used in the paper MD, molecular dynamics PMMA, poly(methyl metha crylate) i-PMMA, isotactic PMMA s-PMMA, syndiotactic PMMAi
New mesogens are always a source of interest, especially when they possess a non-conventional arc... more New mesogens are always a source of interest, especially when they possess a non-conventional architecture. In this article are presented the synthesis and polymorphism of a series of compounds possessing a 1,4-diaryl-1-buten-3-yne moiety as the rigid core with an alkoxy chain on each side. Such a core is termed an enyne core. The alkoxy chain is lengthened on each side of the enyne core according to two different fashions: symmetrically and asymmetrically. In this way a rich polymorphism is achieved in some compounds. At lower chain length, the compounds exhibit smectic H and nematic phases where cybotactic groups are observed in X-ray diffraction patterns. As the alkoxy chains extend, smectic C and smectic F phases appear. The non-cylindrical shape of these compounds involves a molecular packing that is preserved throughout the polymorphism. A comparison between symmetric and asymmetric compounds, from X-ray diffraction pattern analysis of their smectic H phases, reveals a parallel molecular stacking. It also discloses the importance of the moiety that is lengthened since different polymorphisms are obtained.
The aim of molecular modeling is to mimic reality by considering approximations appropriate to th... more 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 COMPASSq. 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 g s 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 g s is revealed.
With experimental investigations and current theories, molecular modeling became an inevitable te... more With experimental investigations and current theories, molecular modeling became an inevitable technique to study the perplexing phenomenon of glass transition. Among polymers, small variations in atomic interactions yield different values of the glass transition temperature, T g. To reveal the influence of differences in the atomic functionality on the value of T g , and thus to probe the molecular mechanisms responsible for this transition, atomistic simulations have to be undertaken. However, such simulations are argued not to accurately represent physically the glass transition due to the long relaxation times involved. Here we show the univer-sality of the well-known Williams-Landel-Ferry equation for the experimental thermal dependence of polymer viscosities as demonstrated with atomistic simulations. Consequently, atomic aspects could be explicitly revealed. The contribution of atomistic simulation to the study of glass transition is thus confirmed. However, it has to be used complementarily with experiments and coarse-grained simulation to reveal the atomic aspects of current theories.
Many efforts in experimental and theory are dedicated to study the puzzling problem of glass tran... more Many efforts in experimental and theory are dedicated to study the puzzling problem of glass transition. With the involvement of molecular modeling, a fresh look at this intricate phenomenon was taken. Nevertheless, the difficulty to accurately probe all the domains of times necessary to describe this process still remains. Moreover, using a full-atomic description to account for equilibrated systems has been called into question. However, such depiction offers a special regard since it could deal with small modifications in the polymer architecture. From an appropriate selection of phase space, it was shown that atomistic simulation was able to link simulated and experimental glass transition temperatures of vinylic polymers using the established Williams-Landel-Ferry equation. Consequently, atomic insight into the glass transition phenomenon can be gained from a comparison of simulated data with actual models and experimental data. In this paper, the different parameters intervening in the Vogel-Fulcher-Tamman equation stemming from the local dynamics of a series of vinylic polymers are thus compared. Their behavior yields an atomistic viewpoint of the Adam-Gibbs model.
Two series of mesogenic molecules have been simulated using molecular dynamics with a full atomis... more Two series of mesogenic molecules have been simulated using molecular dynamics with a full atomistic representation. The two series differ from each other only with the orientation of an ester linker inside the rigid core. Experimentally, this small difference leads to drastically different polymorphisms: one displays the smectic A phase while the other exhibits the smectic C and the smectic A phase. From molecular dynamics trajectories, intermolecular potentials are extracted and analyzed. It is then found that the minor difference inside the rigid core can be related to these various interactions. Finally, experimental and simulation results are integrated in a coherent analysis. The correlation between both types of data offers interesting insights in the role of the Coulomb and van der Waals interactions in the molecular origin of the smectic C phase.
Two series of mesogenic molecules of variable terminal chain lengths have been synthesised and ch... more Two series of mesogenic molecules of variable terminal chain lengths have been synthesised and characterised. The molecules feature a tolane-based rigid core surrounded by two esters groups. The two series differ from each other only by the orientation of an ester linker. Their mesogenic properties have been characterised and are reported. It is found that the orientation of the ester linker and the length of the terminal chain greatly influence their polymorphism. Binary mixtures of the two series have been prepared and their phase diagrams are also reported. They offer some insight at the role of the side chains in the formation of the S C phase.
The purpose of this study is to consider the effect of polydispersity in triblock copolymer on it... more The purpose of this study is to consider the effect of polydispersity in triblock copolymer on its order-to-disorder phase transition and morphology, by supplementing continuous description of polymer indices carried out using random phase approximation by a discrete sampling of the polydispersity performed at the mesoscale level. Both methods confirm that the increase in polydispersity involves a greater stability of the ordered state, in agreement with published data on the effect of polydispersity on diblock copolymers. The morphology simulations reveal that longer blocks are phase segregated and shorter blocks are more or less uniformly distributed throughout the polymer. It is proposed that avoiding very short polymer chains will increase the degree of phase segregation, thus improve the mechanical and conductive properties of proton exchange membrane more efficiently.
The domains of time and space generally covered by full atomistic simulation (AS) to represent th... more The domains of time and space generally covered by full atomistic simulation (AS) to represent the glass transition temperature, T g , are very small. Physical interpretations of the phenomena occurring at this transition are inevitably limited. To specifically address such limitation, behavior of the heat capacity that accounts for the freezing of the degrees of freedom as temperature is decreased, is investigated. The selected polymer is poly(methyl methacrylate) since it offers the opportunity to exhibit a different T g according to the tacticity of its chain. AS and experimental data are thus compared to a theoretical model that takes into account three contributions to the leap in the heat capacity occurring at T g. The comparison discloses that an excellent agreement is obtained between simulated and experimental contributions from vibrations and free volume. However, from an AS viewpoint changes in the confor-mation weakly contribute to this leap. Despite this discrepancy local contributions to the glass transition as predicted by atomistic simulation, are sufficient to determine T g .
Motivation. Poly(methyl methacrylate), PMMA, offers a particular regard to the study of the glass... more Motivation. Poly(methyl methacrylate), PMMA, offers a particular regard to the study of the glass transition since according to the tacticity of its chain, a different glass temperature transition, Tg, is exhibited. Molecular modeling is thus a perfect tool to study such a variation since changes that occur during these transitions can be regarded as differences in molecular characteristics only. The selection of a forcefield that correctly depicts this difference is thus of primary importance. In this article, two forcefields, AMBER and AMBER/OPLS, have been used to simulate the glass transition of the stereoregular PMMAs. Results stemming from these simulations are compared with those, already published, coming from a second generation force field, pcff. Method. Molecular dynamics simulations are used to study the glass transition of stereoregular polymers, and
An entry from the Cambridge Structural Database, the world's repository for small molecule cr... more An entry from the Cambridge Structural Database, the world's repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
Abstract The simulated glass transition temperature (Tg) is usually determined by the dilatometry... more Abstract 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 Gruneisen 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.
Abstract Protecting from solar radiation remains a very stimulating field of research and develop... more Abstract 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.
Abstract The unique physical properties of auxetic compounds make them very attractive materials.... more Abstract 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.
An extensive understanding in the molecular motions that occur in Nafion® should lead to importan... more 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 difference in the glass transition temperatures, T,, of the iso-and syndiotactic forms of PMM... more The difference in the glass transition temperatures, T,, of the iso-and syndiotactic forms of PMMA has been investigated through molecular dynamics simulations of short duration (110 ps) using an accurate force field, pcfl To correlate with the Gibbs-DiMarzio theory, which specifies that conformational energy is a primary factor in determining T , an energetic analysis has been camed out. It is found that syndiotactic PMMA possesses lower intramolecular energy where the governing contribution is brought by the angle bending term; whereas the isotactic PMMA exhibits lower intermolecular energy. Such a behavior suggests that the chain packing greatly determines the difference in the T,.
The idtared absorption spectra of poly(tetrafluoroethylene), PTFE, and poly(chlorotrifluoroethyle... more The idtared absorption spectra of poly(tetrafluoroethylene), PTFE, and poly(chlorotrifluoroethylene), PCTFE, are simulated using a method based on a combination of classisal molecular simulations of the amorphous polymer phase with Kramers-Kronig relations. The differences and the analogies between experimental and calculated spectra of the non-stereoregular fluoride polymer, PTFE, are first reported. The isotactic and syndiotactic configurations of PCTFE are then investigated. The comparison between experimental and simulated spectra is established on a vibrational analysis. It reveals the preferred syndiotactic configuration adopted by the PCTFE chain.
The difference in the glass transition temperatures, T g , of the iso-and syndiotactic forms of P... more The difference in the glass transition temperatures, T g , of the iso-and syndiotactic forms of PMMA has been investigated through molecular dynamics simulations of short duration (110 ps) using an accurate force field, pcff. To correlate with the Gibbs-DiMarzio theory, which specifies that conformational energy is a primary factor in determining T g , an energetic analysis has been carried out. It is found that syndiotactic PMMA possesses lower intramolecular energy where the governing contribution is brought by the angle bending term; whereas the isotactic PMMA exhibits lower intermolecular energy. Such a behavior suggests that the chain packing greatly determines the difference in the T g .
We have measured, the thickness dependence of the glass transition temperature Tg(h), using ellip... more We have measured, the thickness dependence of the glass transition temperature Tg(h), using ellipsometry at variable temperature, for poly(methyl-methacrylate) (PMMA) of various tacticity in confined geometry. We report that several factors significantly affect Tg(h): i) polymer microstructure (stereoregularity of PMMA) related to local dynamics; ii) interfacial interactions; iii) conformation of the polymer chains. These results raise many fundamental questions on the origin of the thickness-dependent glass transition. Why and how do the interactions with the substrate significantly affect Tg(h)? Does Tg(h) depend on the modifications of conformational parameters of the chains (their entropy)? What is the correlation between local dynamics and Tg(h) in thin films? The aim of this paper is to summarise these open questions, which should stimulate further investigations in the thin polymer film scientific community.
Motivation. To describe the glass transition phenomenon using molecular modeling, poly(methyl met... more Motivation. To describe the glass transition phenomenon using molecular modeling, poly(methyl methacrylate), PMMA, offers a noteworthy regard since according to the tacticity of its chain, a different T g is displayed. Accordingly, if a force field accurately describes such variation, insights can be gained in the understanding of the reasons that give rise to this difference, and consequently, in the glass transition phenomenon itself. Several studies have been carried out using a second category force field, pcff. In this article, two first class force fields, AMBER and AMBER/OPLS, have been envisioned. It is found that, although T g s are obtained at higher temperatures, the difference between the two T g s is better mimicked using the AMBER/OPLS force field. The presence of the cross terms in the force fields is thus not a determining factor in obtaining the T g s variation. Method. Molecular dynamics simulations have been employed to compare glass transition of stereoregular polymers, and their energetic differences, with respect to the use of two force fields of different generation.. Results. A first generation force field could deal with the representation of the difference in T g s between stereoregular polymers. Conclusions. Accordingly to the fact that a first generation force field can deal with the difference in T g s difference, a greater amount of phase space could be regarded. Moreover, studies of specific interaction, such as PMMA behavior with surfaces, can be undertaken. Availability. Abbreviations and notations Collect here in alphabetical order all abbreviations and notations used in the paper MD, molecular dynamics PMMA, poly(methyl metha crylate) i-PMMA, isotactic PMMA s-PMMA, syndiotactic PMMAi
New mesogens are always a source of interest, especially when they possess a non-conventional arc... more New mesogens are always a source of interest, especially when they possess a non-conventional architecture. In this article are presented the synthesis and polymorphism of a series of compounds possessing a 1,4-diaryl-1-buten-3-yne moiety as the rigid core with an alkoxy chain on each side. Such a core is termed an enyne core. The alkoxy chain is lengthened on each side of the enyne core according to two different fashions: symmetrically and asymmetrically. In this way a rich polymorphism is achieved in some compounds. At lower chain length, the compounds exhibit smectic H and nematic phases where cybotactic groups are observed in X-ray diffraction patterns. As the alkoxy chains extend, smectic C and smectic F phases appear. The non-cylindrical shape of these compounds involves a molecular packing that is preserved throughout the polymorphism. A comparison between symmetric and asymmetric compounds, from X-ray diffraction pattern analysis of their smectic H phases, reveals a parallel molecular stacking. It also discloses the importance of the moiety that is lengthened since different polymorphisms are obtained.
The aim of molecular modeling is to mimic reality by considering approximations appropriate to th... more 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 COMPASSq. 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 g s 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 g s is revealed.
With experimental investigations and current theories, molecular modeling became an inevitable te... more With experimental investigations and current theories, molecular modeling became an inevitable technique to study the perplexing phenomenon of glass transition. Among polymers, small variations in atomic interactions yield different values of the glass transition temperature, T g. To reveal the influence of differences in the atomic functionality on the value of T g , and thus to probe the molecular mechanisms responsible for this transition, atomistic simulations have to be undertaken. However, such simulations are argued not to accurately represent physically the glass transition due to the long relaxation times involved. Here we show the univer-sality of the well-known Williams-Landel-Ferry equation for the experimental thermal dependence of polymer viscosities as demonstrated with atomistic simulations. Consequently, atomic aspects could be explicitly revealed. The contribution of atomistic simulation to the study of glass transition is thus confirmed. However, it has to be used complementarily with experiments and coarse-grained simulation to reveal the atomic aspects of current theories.
Many efforts in experimental and theory are dedicated to study the puzzling problem of glass tran... more Many efforts in experimental and theory are dedicated to study the puzzling problem of glass transition. With the involvement of molecular modeling, a fresh look at this intricate phenomenon was taken. Nevertheless, the difficulty to accurately probe all the domains of times necessary to describe this process still remains. Moreover, using a full-atomic description to account for equilibrated systems has been called into question. However, such depiction offers a special regard since it could deal with small modifications in the polymer architecture. From an appropriate selection of phase space, it was shown that atomistic simulation was able to link simulated and experimental glass transition temperatures of vinylic polymers using the established Williams-Landel-Ferry equation. Consequently, atomic insight into the glass transition phenomenon can be gained from a comparison of simulated data with actual models and experimental data. In this paper, the different parameters intervening in the Vogel-Fulcher-Tamman equation stemming from the local dynamics of a series of vinylic polymers are thus compared. Their behavior yields an atomistic viewpoint of the Adam-Gibbs model.
Two series of mesogenic molecules have been simulated using molecular dynamics with a full atomis... more Two series of mesogenic molecules have been simulated using molecular dynamics with a full atomistic representation. The two series differ from each other only with the orientation of an ester linker inside the rigid core. Experimentally, this small difference leads to drastically different polymorphisms: one displays the smectic A phase while the other exhibits the smectic C and the smectic A phase. From molecular dynamics trajectories, intermolecular potentials are extracted and analyzed. It is then found that the minor difference inside the rigid core can be related to these various interactions. Finally, experimental and simulation results are integrated in a coherent analysis. The correlation between both types of data offers interesting insights in the role of the Coulomb and van der Waals interactions in the molecular origin of the smectic C phase.
Two series of mesogenic molecules of variable terminal chain lengths have been synthesised and ch... more Two series of mesogenic molecules of variable terminal chain lengths have been synthesised and characterised. The molecules feature a tolane-based rigid core surrounded by two esters groups. The two series differ from each other only by the orientation of an ester linker. Their mesogenic properties have been characterised and are reported. It is found that the orientation of the ester linker and the length of the terminal chain greatly influence their polymorphism. Binary mixtures of the two series have been prepared and their phase diagrams are also reported. They offer some insight at the role of the side chains in the formation of the S C phase.
The purpose of this study is to consider the effect of polydispersity in triblock copolymer on it... more The purpose of this study is to consider the effect of polydispersity in triblock copolymer on its order-to-disorder phase transition and morphology, by supplementing continuous description of polymer indices carried out using random phase approximation by a discrete sampling of the polydispersity performed at the mesoscale level. Both methods confirm that the increase in polydispersity involves a greater stability of the ordered state, in agreement with published data on the effect of polydispersity on diblock copolymers. The morphology simulations reveal that longer blocks are phase segregated and shorter blocks are more or less uniformly distributed throughout the polymer. It is proposed that avoiding very short polymer chains will increase the degree of phase segregation, thus improve the mechanical and conductive properties of proton exchange membrane more efficiently.
The domains of time and space generally covered by full atomistic simulation (AS) to represent th... more The domains of time and space generally covered by full atomistic simulation (AS) to represent the glass transition temperature, T g , are very small. Physical interpretations of the phenomena occurring at this transition are inevitably limited. To specifically address such limitation, behavior of the heat capacity that accounts for the freezing of the degrees of freedom as temperature is decreased, is investigated. The selected polymer is poly(methyl methacrylate) since it offers the opportunity to exhibit a different T g according to the tacticity of its chain. AS and experimental data are thus compared to a theoretical model that takes into account three contributions to the leap in the heat capacity occurring at T g. The comparison discloses that an excellent agreement is obtained between simulated and experimental contributions from vibrations and free volume. However, from an AS viewpoint changes in the confor-mation weakly contribute to this leap. Despite this discrepancy local contributions to the glass transition as predicted by atomistic simulation, are sufficient to determine T g .
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Papers by Armand Soldera