Molecular dynamics of poly(ATRIF) homopolymer and poly(AN-co-ATRIF) copolymer investigated by dielectric relaxation spectroscopy

Aiming to develop new dielectric polymers containing CN and F groups with strong dipole moments, a novel copolymer of acrylonitrile (AN) and 2,2,2-trifluoroethyl acrylate (ATRIF) was synthesized in acetonitrile by free radical process as well as the respective homopolymer (poly(ATRIF)). The copolymer’s composition and microstructure were analyzed by FTIR, ¹H and ¹³C NMR spectroscopy and SEC. The molar incorporation of AN determined in the copolymer by NMR was 58 mol%. Thermogravimetric analysis of poly(AN-co-ATRIF) copolymer showed good thermal stability comparatively to the fluorinated homopolymer.Both copolymer, poly(AN-co-ATRIF), and homopolymer, poly(ATRIF), were dielectrically characterized over a frequency range from 10⁻¹ to 10⁶ Hz, and in a temperature range from 223 to 393 K. The dominating relaxation process detected in both materials is the α-relaxation, associated with the dynamic glass transition. A VFTH temperature dependence of the relaxation times (τ) was found for both materials, as characteristic of cooperative processes, from which the respective glass transition temperatures (T_g(τ = 100 s)) were estimated, which differ ∼40 K, the one of the copolymer being higher (307 K) in accordance to the calorimetric analysis. This effect was attributed to a higher stiffness of the backbone in the copolymer originated by the inclusion of the acrylonitrile groups. Both relaxation functions have the same breath of relaxation times allowing constructing a single master curve, indicating similar non-exponential character. A less fragile behavior was found for the copolymer. This was rationalized in a more straightforward way by the free volume approach instead from a correlation between fragility and intermolecular coupling. It was found that in the copolymer the free volume increases at a lower rate with the temperature increase. It was inferred from the VFTH temperature dependence of the dc conductivity and low values of the decoupling index that ion motion is significantly influenced by the dynamics of the α-process.     

Molecular dynamics of an epoxy resin modified with an epoxidized poly(styrene-butadiene) linear block copolymer during cure and microphase separation processes

Molecular dynamics of diglycidyl ether of bisphenol A (DGEBA) epoxy resin modified with an epoxidized poly(styrene-b-butadiene) (SepB) linear block copolymer has been monitored during cure and microphase separation process by dielectric relaxation spectroscopy (DRS) for wide frequency and temperature ranges. Different primary and secondary relaxation processes have been analyzed for neat components and ternary mixture. Relaxational behaviour has been modelled with Havriliak-Negami, Vogel-Fulcher-Tammann and Arrhenius equations and fitting parameters and their evolution have been obtained. The retention of the epoxidized poly(butadiene) (PepB) block in the epoxy-rich phase during all the polymerization process, previously detected by our group with atomic force and transmission electron microscopies, has been confirmed by dielectric relaxation spectroscopy. The evolution of molecular dynamics during the polymerization process of the epoxy resin in the ternary system indicates a change in the trend of the main relaxation at times that agree with phase separation detected by rheology.     

Molecular dynamics simulations of a poly(p-phenylene) oligomer

We have studied the conformation and coefficient of thermal expansion in the poly(p-phenylene) oligomer p-sexiphenyl (C₃₆H₂₆) by molecular dynamics simulations. Studies of the backbone phenyl–phenyl torsion angle in a simulated p-sexiphenyl crystal at room temperature indicate the presence of torsional librations of approximately ±20°. Further analysis of the phenyl–phenyl backbone torsion angle in less closely packed regions of the simulated crystal (crystal ends) indicate the presence of 180° phenyl ring flips, in agreement with solid-state deuterium NMR data on poly(p-phenylene oligomers). The linear coefficient of thermal expansion was also calculated and found to be negative, in qualitative agreement with experimental data on rigid-rod compounds. © 1993 John Wiley & Sons, Inc.     

Effect of molecular structure of curing agents on cyclic creep in highly cross-linked epoxy polymers

Ratcheting behavior of highly–cross-linked epoxy polymers was investigated considering the effect of molecular structure of curing agents by molecular dynamics simulations. Cyclic loading–unloading simulations at two different frequencies were conducted using atomistic models for epoxies cured by aliphatic and aromatic curing agents, triethylenetetramine (TETA) and diethyltoluenediamine (DETDA), respectively. Different ratcheting strain evolutions, dihedral angle stress accumulations, and stiffness variations were observed during the cyclic deformation simulations depending on the molecular structure of curing agents. The epoxy cured by DETDA exhibited a more rapid increase of ratcheting strain and a decrease of the stiffness toward the loading direction. Structural analyses were carried out by observing the orientation order parameter of the monomers, radius of gyration, and free volume evolution to understand the ratcheting strain behaviors and stiffness variations at atomistic scale. The structural analyses revealed that irreversible dihedral angle transitions near the benzene ring of the curing agent DETDA were responsible for low ratcheting resistance and stiffness degradation during the cyclic deformations. Whereas, the aliphatic curing agent TETA, which does not exhibit any stress possession by the irreversible dihedral angle change, was revealed to be advantageous for the ratcheting resistance and stiffness variation of the epoxy polymers.     

Temperature effects on spreading of water nano‐droplet on poly(methyl methacrylate): A molecular dynamics simulation study

In this article, the effect of temperature on the spreading behavior of a water nano‐droplet on poly(methyl methacrylate) substrate is investigated. The contact angle analysis illustrates that the spreading process occurs in a stage‐like manner and the increase in temperature causes a regime change from partial to total wetting. The interaction energy distributions show that there exist sites on the surface which could trap water molecules and provide a better path for other molecules to overcome the asperities. Estimations of the coefficients of self‐diffusivity suggest that temperature has a major effect in the reorientation stage, which results in the formation of the interfacial layer. In the second stage of spreading, temperature affects the process by providing sufficient energy for water molecules to overcome the interactions with the substrate. Therefore, this stage is controlled by the movement of water molecules on the surface and is highly influenced by their interaction with the surface asperities, strong interaction sites, and the carbonyl groups. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1532–1541     

Molecular dynamics simulation of poly(3‐hexylthiophene) helical structure In Vacuo and in amorphous polymer surrounding

The stability of poly(3‐hexylthiophene) (P3HT) helical structure has been investigated in vacuo and in amorphous polymer surrounding via molecular dynamics‐based simulations at temperatures below and above the P3HT melting point. The results show that the helical chain remains stable at room temperature both in vacuo and in amorphous surrounding, and promptly loses its structure at elevated temperatures. However, the amorphous surrounding inhibits the destruction of the helix at higher temperatures. In addition, it is shown that the electrostatic interactions do not significantly affect the stability of the helical structure. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2448–2456     

Water and polymer dynamics in poly(hydroxyl ethyl acrylate-co-ethyl acrylate) copolymer hydrogels

Water and polymer dynamics in hydrogels based on random copolymers of hydrophilic poly(hydroxyl ethyl acrylate) (PHEA) and hydrophobic poly(ethyl acrylate) (PEA), in wide ranges of composition, were investigated by means of two dielectric techniques, thermally stimulated depolarization currents (TSDC) and, mainly, broadband dielectric relaxation spectroscopy (DRS) at several levels of relative humidity/water content. Water sorption of the hydrogels was studied by equilibrium sorption isotherms (ESI). Two secondary relaxations (γ and β_sw) and the primary (segmental) α relaxation associated with the glass transition of the copolymer matrix were followed and analyzed against copolymer composition and water content. The results show that the copolymers are homogeneous at nm scale, except at very high PEA content. Correlations were observed between results on the organization of water in the hydrogels and on water effects on polymer dynamics. Distinct changes in the dielectric response, in particular in the time scale and the dielectric strength of the β_sw relaxation, at the water content of the completion of the first hydration layer indicate that water molecules themselves contribute to the dielectric response at higher water contents. Proton conductivity of the hydrogels at various levels of water content was also studied and correlation to segmental dynamics (decoupling) was analyzed.     

Molecular dynamics simulations of the hydration of poly(vinyl methyl ether):Hydrogen bonds and quasi-hydrogen bonds

Atomistic detailed hydration structures of poly(vinyl methyl ether)(PVME) have been investigated by molecular dynamics simulations under 300 K at various concentrations. Both radial distribution functions and the distance distributions between donors and acceptors in hydrogen bonds show that the hydrogen bonds between the polymer and water are shorter by 0.005 nm than those between water molecules. The Quasi-hydrogen bonds take only 7.2% of the van der Waals interaction pairs. It was found the hydrogen bonds are not evenly distributed along the polymer chain,and there still exists a significant amount(10%) of ether oxygen atoms that are not hydrogen bonded to water at a concentration as low as 3.3%. This shows that in polymer solutions close contacts occur not only between polymer chains but also between chain segments within the polymer,which leads to inefficient contacts between ether oxygen atoms and water molecules. Variation of the quasi-hydrogen bonds with the concentration is similar to that of hydrogen bonds,but the ratio of the repeat units forming quasi-hydrogen bonds to those forming hydrogen bonds approaches 0.2. A transition was found in the demixing enthalpy at around 30% measured by dynamic testing differential scanning calorimetry(DTDSC) for aqueous solutions of a mono-dispersed low molecular weight PVME,which can be related to the transition of the fractions of hydrogen bonds and quasi-hydrogen bonds at ～27%. The transition of the fractions of hydrogen bonds and quasi-hydrogen bonds at ～27% can be used to explain the demixing enthalpy transition at 30% at a molecular scale. In addition,at the concentration of 86%,each ether oxygen atom bonded with water is assigned 1.56 water molecules on average,and 'free' water molecules emerge at the concentration of around 54%.     

聚乙烯在石墨表面结晶的分子动力学模拟

采用分子动力学方法模拟了聚乙烯在石墨(001)表面的吸附和结晶过程;直观的给出了聚乙烯链被石墨(001)面吸附并诱导形成有序的片层晶体的过程;发现结晶温度对得到的有序结构中的聚乙烯链相对石墨表面的特定取向有影响(300 K和600 K时的取向方向不同);表面覆盖率影响聚乙烯吸附层的厚度,对取向的方向无影响.     

Molecular dynamics of amorphous/crystalline polymer blends studied by broadband dielectric spectroscopy

The molecular dynamics of poly(vinyl acetate), PVAc, and poly(hydroxy butyrate), PHB, as an amorphous/crystalline polymer blend has been investigated using broadband dielectric spectroscopy over wide ranges of frequency (10⁻² to 10⁵ Hz), temperature, and blend composition. Two dielectric relaxation processes were detected for pure PHB at high and low frequency ranges at a given constant temperature above the T_g. These two relaxation peaks are related to the α and α′ of the amorphous and rigid amorphous regions in the sample, respectively. The α′-relaxation process was found to be temperature and composition dependent and related to the constrained amorphous region located between adjacent lamellae inside the lamellar stacks. In addition, the α′-relaxation process behaves as a typical glass relaxation process, i.e., originated from the micro-Brownian cooperative reorientation of highly constraints polymeric segments. The α-relaxation process is related to the amorphous regions located between the lamellar crystals stacks. In the PHB/PVAc blends, only one α-relaxation process has been observed for all measured blends located in the temperature ranges between the T_g’s of the pure components. This last finding suggested that the relaxation processes of the two components are coupled together due to the small difference in the T_g’s (ΔT_g = 35 °C) and the favorable thermodynamics interaction between the two polymer components and consequently less dynamic heterogeneity in the blends. The T_g’s of the blends measured by DSC were followed a linear behavior with composition indicating that the two components are miscible over the entire range of composition. The α′-relaxation process was also observed in the blends of rich PHB content up to 30 wt% PHB. The molecular dynamics of α and α′-relaxation processes were found to be greatly influenced by blending, i.e., the dielectric strength, the peak broadness, and the dielectric loss peak maximum were found to be composition dependent. The dielectric measurements also confirmed the slowing down of the crystallization process of PHB in the blends.     

Molecular dynamics of hydrophilic poly(propylene imine) dendrimers in aqueous solutions by 1H NMR relaxation

The local dynamics of three poly(propylene imine) dendrimers with hydrophilic triethylenoxy methyl ether terminal groups were studied in D₂O by the measurement of the ¹H NMR relaxation times, which were treated with the Lipari–Szabo model‐free approach. The results showed that the overall mobility increased with temperature and decreased with increasing dendrimer size. An Arrhenius trend was observed for both overall and local motions. The activation energy of overall tumbling increased from 11.3 to 17.5 kJ/mol with the dendrimer size. The local mobility decreased from the outer part to the inner part of the dendrimer and with the dendrimer size. The spatial restriction of local motions decreased with increasing temperature up to 55 °C and remained constant above 55 °C. Local motions were more restricted when the dendrimer size increased. The results showed that the hydrophilic end groups of the dendrimers were located preferentially at the periphery of the molecules and were extended in the aqueous environment. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2969–2975, 2003     

Molecular dynamics simulation study of glass transition in hydrated Nafion

The thermal transition of Nafion is studied using a molecular dynamics simulation through a chemically realistic model. Static and dynamic properties of polymer melts with different water contents are investigated over a wide range of temperatures to obtain viscometric and calorimetric glass transition temperatures. The effect of cooling rate of the simulation on the glass transition of the hydrated polymer is also examined within the well‐known Williams–Landel–Ferry (WLF) equation. Variation of relaxation times versus temperature shows a fragile‐to‐strong transition. The hydration level has a significant impact on the static and dynamic properties of the polymer chains and water molecules confined in nanometric spaces between polymer chains. The results of this study are useful to predict the behavior of Nafion for various applications including fuel cells, sensors, actuators, and shape memory devices at different temperatures. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 907–915     

溴化钾团簇相变的分子动力学研究

众所周知，团簇的尺寸介于原子或分子与大块物质之间。由于它有较大的表面／体积比而有独特的物理化学性质。团簇在成核，晶体生长，材料科学和纳米器件加工等领域起了至关重要的作用。近年来，计算机模拟已成为物理，化学，生物，天文，地质等领域的重要研究工具。计算机模拟不仅可以     

Effects of mussel-inspired co-deposition of 2-hydroxymethyl methacrylate and poly (2-methoxyethyl acrylate) on the hydrophilicity and binding tendency of common hemodialysis membranes: Molecular dynamics simulations and molecular docking studies

Despite advances in the field, hemoincompatibility remains a critical issue for hemodialysis (HD) as interactions between various human blood constituents and the polymeric structure of HD membranes results in complications such as activation of immune system cascades. Adding hydrophilic polymer structures to the membranes is one modification approach that can decrease the extent of protein adsorption. This study conducted molecular dynamics (MD) simulations to understand the interactions between three human serum proteins (fibrinogen [FB], human serum albumin, and transferrin) and common HD membranes in untreated and modified forms. Poly(aryl ether sulfone) (PAES) and cellulose triacetate were used as the common dialyzer polymers, and membrane modifications were performed with 2-hydroxymethyl methacrylate (HEMA) and poly (2-methoxyethyl acrylate) (PMEA), using polydopamine-assisted co-deposition. The MD simulations were used as the framework for binding energy simulations, and molecular docking simulations were also performed to conduct molecular-level investigations between the two modifying polymers (HEMA and PMEA) and FB. Each of the three proteins acted differently with the membranes due to their unique nature and surface chemistry. The simulations show PMEA binds less intensively to FB with a higher number of hydrogen bonds, which reflects PMEA's superior performance compared to HEMA. The simulations suggest PAES membranes could be used in modified forms for blood-contact applications as they reflect the lowest binding energy to blood proteins.     

The conformation of poly(3-dodecyl thiophene) within methacrylic polymer matrix

Blends of poly(3-dodecyl thiophene) (PDDT) with poly(methyl methacrylate), poly(butyl methacrylate) (PBMA), and poly(methyl methacrylate-co-butyl methacrylate) (PMMA/PBMA) were studied by polarization optical microscopy, atomic-force microscopy, and absorption spectroscopy and were modeled using molecular dynamics (MD) simulations. The observed thermochromic transitions are shown to be host-matrix dependent, with PDDT/PBMA absorption spectra differing substantially from pristine PDDT. The dispersion of PDDT within PBMA matrix is observed to be greater than in the other host polymers. MD calculations of both individual PDDT molecules and molecular aggregates suggest that the distribution of dihedral angles present in the PDDT backbone is the narrowest for aggregates of PDDT embedded within a polymer matrix. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2909–2917, 1999     

Molecular Dynamics Simulation of Glass Transition Behavior of Polyimide Ensemble

The prediction of glass transition temperature from chemical structure has a great significance to select and design new high-properties materials. However, for the estimation and correlation methods, the deficiency of parameters for newer groups will lead to invalidity of Tg prediction or greater deviation from experiment. In the present work, we predicted Tg for a polyimide (PI) ensemble with rigid moieties, and analyzed structural factor that regards to the rotation barrier of the bridging…     

Effect of cation exchange capacity on the structure and dynamics of poly(ethylene oxide) in Li+ montmorillonite nanocomposites

Molecular dynamics computer simulations are used to study the structure and dynamics of 1-nm wide films of poly(ethylene oxide) (PEO) confined between mica-type layered silicates of different cation exchange capacities (CEC). The simulation setup mimics experimental systems formed by intercalation of PEO in montmorillonite alumino-silicates with varied inherent charges. It is shown that the presence and population of lithium has a significant influence on the behavior of the system, in addition to the confinement-induced effects caused by the extreme spatial restriction. The structural features of the confined PEO are strongly altered with the number of Li⁺, which determines the polymer/inorganic interactions. The combination of the nanoconfinement and the presence of lithium preclude regular ordered arrangements of PEO, similar to those observed in the bulk unconfined polymer. The segmental dynamics of PEO in confinement are also greatly influenced by the presence of lithium, because of the strong interaction between Li⁺ and the oxygen of the PEO backbone. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3460–3477, 2005     

Molecular dynamics simulations of hydration shell on montmorillonite (001) in water

Molecular dynamics simulations (MDS) of montmorillonite (001)/water interface system were used for studying the hydration shell on the montmorillonite surface in this work. The study was performed on the simulation of concentration profile and self‐diffusion coefficients. The results have shown that there was a hydration shell on the surface with the thickness of approximately 1.74 nm, which was composed of six ordered water molecule layers, including ordered layers and transition layers. The water molecules in the shell were closely and orderly arranged than those in bulk water, leading to a higher concentration of water molecules. Copyright © 2016 John Wiley & Sons, Ltd.