Polymer Consistent Force Field Research Articles

Uncertainties in the predictions of thermo-physical properties of thermoplastic polymers via molecular dynamics

We quantify the effect of various sources of uncertainties in the prediction of thermo-physical properties of polymers using molecular dynamics simulations. We quantify how the choice of polymer builder, force field, molecular weight and data analysis affect predicted values of the glass transition temperature (Tg), room temperature density and coefficient of thermal expansion of poly(methyl-methacrylate) (PMMA) and polystyrene (PS). Interestingly, we find that the data analysis introduces significant uncertainties in Tg (approximately 5%) while the other properties are insensitive to it. The force field is the only variable that significantly affects the predictions of density. Polymer-consistent force field (PCFF) resulted in a higher density for PMMA than Dreiding and the opposite trend was observed in PS; in all cases the difference in density was less than 2%. Strongly correlated with density, we find that PCFF leads to a higher Tg than Dreiding for PMMA and both force fields predict similar Tg values for PS. The trends in Tg can be explained by differences in segmental mobility of the melts predicted by the two force fields. We find that the choice of amorphous polymer builder results in uncertainties in predictions comparable to those associated with the force field due to subtle, but persistent, differences in molecular structure. The results presented here provide insight into the physics behind molecular simulations of polymers and quantitative levels of uncertainties associated with individual sources that can help practitioners of molecular simulations interested in using their results in engineering applications.

Effect of temperature on elastic properties of CNT-polyethylene nanocomposite and its interface using MD simulations.

This paper investigates the effect of temperature on the elastic modulus of carbon nanotube-polyethylene (CNT-PE) nanocomposite and its interface using molecular dynamics (MD) simulations, by utilizing the second-generation polymer consistent force field (PCFF). Two CNTs-armchair and zigzag-were selected as reinforcing nano-fillers, and amorphous PE was used as the polymer matrix. For atomistic modelling of the nanocomposite, the commercially available code Materials Studio 8.0 was used and all other MD simulations were subsequently performed using the open source code Large-Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). To obtain the elastic modulus of the nanocomposite, stress-strain curves were drawn at different temperatures by performing uniaxial deformation tests on the nanocomposite material, whereas the curvatures of the interfacial interaction energy vs. strain curves were utilized to obtain Young's modulus of the interface. In addition, the glass transition temperatures of the polymer matrix and nanocomposites were also evaluated using density-temperature curves. Based on the results, it is concluded that, irrespective of temperature condition, a nanocomposite reinforced with CNT of larger chirality (i.e., armchair) yields a higher value of Young's modulus of the nanocomposite and its interface. It was also found that, at the phase transition (from a glassy to a rubbery state) temperature (i.e., glass transition temperature), Young's moduli of the polymer matrix, nanocomposite, and its interface drop suddenly. The results obtained from MD simulations were verified with results obtained from continuum-based rule-of-mixtures.

Selection of Optimal Polymerization Degree and Force Field in the Molecular Dynamics Simulation of Insulating Paper Cellulose

To study the microscopic thermal aging mechanism of insulating paper cellulose through molecular dynamics simulation, it is important to select suitable DP (Degree of Polymerization) and force field for the cellulose model to shorten the simulation time and obtain correct and objective simulation results. Here, the variation of the mechanical properties and solubility parameters of models with different polymerization degrees and force fields were analyzed. Numerous cellulose models with different polymerization degrees were constructed to determine the relative optimal force field from the perspectives of the similarity of the density of cellulose models in equilibrium to the actual cellulose density, and the volatility and repeatability of the mechanical properties of the models through the selection of a stable polymerization degree using the two force fields. The results showed that when the polymerization degree was more than or equal to 10, the mechanical properties and solubility of cellulose models with the COMPASS (Condensed-phase Optimized Molecular Potential for Atomistic Simulation Studies) and PCFF (Polymer Consistent Force Field) force fields were in steady states. The steady-state density of the cellulose model using the COMPASS force field was closer to the actual density of cellulose. Thus, the COMPASS force field is favorable for molecular dynamics simulation of amorphous cellulose.

Photoisomers of Azobenzene Star with a Flat Core: Theoretical Insights into Multiple States from DFT and MD Perspective.

This study focuses on comparing physical properties of photoisomers of an azobenzene star with benzene-1,3,5-tricarboxamide core. Three azobenzene arms of the molecule undergo a reversible trans-cis isomerization upon UV-vis light illumination giving rise to multiple states from the planar all-trans one, via two mixed states to the kinked all-cis isomer. Employing density functional theory, we characterize the structural and photophysical properties of each state indicating a role the planar core plays in the coupling between azobenzene chromophores. To characterize the light-triggered switching of solvophilicity/solvophobicity of the star, the difference in solvation free energy is calculated for the transfer of an azobenzene star from its gas phase to implicit or explicit solvents. For the latter case, classical all-atom molecular dynamics simulations of aqueous solutions of azobenzene star are performed employing the polymer consistent force field to shed light on the thermodynamics of explicit hydration as a function of the isomerization state and on the structuring of water around the star. From the analysis of two contributions to the free energy of hydration, the nonpolar van der Waals and the electrostatic terms, it is concluded that isomerization specificity largely determines the polarity of the molecule and the solute-solvent electrostatic interactions. This convertible hydrophilicity/hydrophobicity together with readjustable occupied volume and the surface area accessible to water, affects the self-assembly/disassembly of the azobenzene star with a flat core triggered by light.

Molecular modeling to predict peptide accessibility for peptide-functionalized hydrogels.

Peptide-functionalized (PF) hydrogels are being widely investigated by the tissue engineering and regenerative medicine communities for a broad range of applications because of their unique potential to mimic the natural extracellular matrix and promote tissue regeneration. In order for these complex material systems to perform their intended bioactive function (e.g., cell signaling), the peptides that are tethered to the hydrogel matrix must be accessible at the hydrogel surface for cell-receptor binding. The factors influencing the surface accessibility of the tethered peptide mainly include the length of the tethers, the loading (i.e., concentration) of the peptide, and the association between the tethered peptide and the hydrogel matrix. In the present work, the authors developed coarse-grained molecular models based on the all-atom polymer consistent force field for a type of poly(ethylene glycol)-based PF hydrogel and conducted molecular simulations to investigate the distribution of the peptide within the hydrogel and its surface accessibility as a function of tether length and peptide concentration. The calculated results of the effects of these design parameters on the surface accessibility of the peptide agree very well with corresponding experimental measurements in which peptide accessibility was quantified by the number of cells attached to the hydrogel surface per unit area. The developed modeling methods are able to provide unique insights into the molecular behavior of PF hydrogels and the distribution of the tethered peptides, which can serve as a guide for hydrogel design optimization.

Molecular dynamics simulation on pressure and thickness dependent density of squalane film

Molecular dynamics (MD) simulations using the polymer consistent force field (PCFF) were adopted to investigate the pressure and thickness dependent density of squalane film in a nanogap at 373 K, with three different initial film thicknesses, and for a wide range of pressures. The equivalent densities predicted by MD simulations were compared with the empirical data. Results show that the squalane atoms tend to form layers parallel to the confining substrates but the orientations of squalane molecules are irregular throughout the film. In addition, distinct excluded volumes are not found at the interfaces of the film and substrates. Furthermore, with the same initial film thickness h0, the film thickness h and compressibility decrease with increasing pressure, but the compressibility is similar for films with different initial film thicknesses. The equivalent densities predicted by MD simulations with the maximum initial film thickness (9.44 nm) are accurate to the values of Tait equation. The MD simulation with adequate initial film thickness can accurately and conveniently predict the bulk densities of lubricants.

Multiscale approach for the construction of equilibrated all-atom models of a poly(ethylene glycol)-based hydrogel.

A multiscale modeling approach is presented for the efficient construction of an equilibrated all-atom model of a cross-linked poly(ethylene glycol) (PEG)-based hydrogel using the all-atom polymer consistent force field (PCFF). The final equilibrated all-atom model was built with a systematic simulation toolset consisting of three consecutive parts: (1) building a global cross-linked PEG-chain network at experimentally determined cross-link density using an on-lattice Monte Carlo method based on the bond fluctuation model, (2) recovering the local molecular structure of the network by transitioning from the lattice model to an off-lattice coarse-grained (CG) model parameterized from PCFF, followed by equilibration using high performance molecular dynamics methods, and (3) recovering the atomistic structure of the network by reverse mapping from the equilibrated CG structure, hydrating the structure with explicitly represented water, followed by final equilibration using PCFF parameterization. The developed three-stage modeling approach has application to a wide range of other complex macromolecular hydrogel systems, including the integration of peptide, protein, and/or drug molecules as side-chains within the hydrogel network for the incorporation of bioactivity for tissue engineering, regenerative medicine, and drug delivery applications.

Additives Effects on Crystal Morphology of Dihydroxylammonium 5,5ʹ-Bistetrazole-1,1ʹ-diolate by Molecular Dynamics Simulations

Dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate (TKX-50) is a newly synthesized explosive with excellent comprehensive properties: high energy storage, low impact sensitivity, and low toxicity. To understand and improve the crystal morphology of TKX-50, we reported the polymer consistent force field to simulate the crystal morphology of TKX-50 by growth morphology (GM) method. We then used this force field in molecular dynamics (MD) simulations to predict the influences of additives on crystal facets of TKX-50. The calculated results indicate that ethanol, ethylene glycol, and acetic acid are more favorable to the spheroidization of TKX-50, which provides a theoretical support for the additive selection of crystalline system. Furthermore, we added the selected additives in the recrystallization system of TKX-50. The recrystallized samples possessed a small aspect ratio and were close to spherical in shape, which indicates that the experimental results are consistent with the simulated results.

Atomistic investigation of the interfacial mechanical characteristics of carbon nanotube reinforced epoxy composite

In this work, we investigate the interfacial mechanical characteristics of carbon nanotube (CNT) reinforced epoxy composite using molecular dynamics (MD) simulations. The MD simulations were carried out by developing two pull-out models; namely, the displacement-load model and the velocity-load model. The second-generation force field – polymer consistent force field (PCFF) – is used in the current MD simulations. The effects of various parameters, such as epoxy density, length and diameter of a CNT, the CNT–epoxy interfacial thickness, LJ cut-off distance and capping conditions of a CNT on the interfacial mechanical characteristics have also been investigated and discussed. The results from the present pull-out model, which are validated with earlier studies, reveal that (i) the interfacial shear strength (ISS) of the CNT-reinforced epoxy composite is improved with the increase in the epoxy density, (ii) the ISS of the CNT-reinforced epoxy composite decreases with the increase in the values of length and diameter of a CNT, and the thickness of CNT–epoxy matrix interface, (iii) the LJ cut-off distance has marginal effect on the ISS and the pull-out force of CNT, and (iv) incorporation of an end cap in the simulations results high initial pull-out peaks, which well correlate with the experimental findings.

Molecular dynamics and atomistic based continuum studies of the interfacial behavior of nanoreinforced epoxy

In this study, we investigate the interfacial mechanical characteristics of carbon nanotube (CNT) reinforced epoxy composite using molecular dynamics (MD) simulations. The second-generation polymer consistent force field (PCFF) is used in the MD simulations. In particular, we compare MD results with those obtained by atomistic-based continuum (ABC) multiscale modeling technique, which makes use of the appropriate constitutive relations derived solely from interatomic potentials. The results of our comparative investigation suggest that (i) the ABC multiscale model and MD simulation provides almost identical predictions for the interfacial properties of the nanocomposite for smaller diameter of CNTs, (ii) the ABC model slightly over predict the interfacial properties of the nanocomposite for larger diameter of CNTs, and (iii) the MD simulations represents the real nanocomposite structure with the minimum assumptions compared to that of the ABC multiscale model but with much greater computer requirements and limited length scale.

A study on the relationship between polycarbonate microstructure and performance as determined by a combined experimental and molecular dynamics simulation method

AbstractThe influence of the microstructure of polycarbonate (PC) on performance was systematically investigated by both experimental method and molecular simulation. Yield stress, impact strength, molecular weight, and transmittance were used to distinguish the degradation processes between different PCs, and thermal degradation kinetics was studied to obtain the activation energy. At the molecular level, through 13C nuclear magnetic resonance (NMR) spectroscopy, it was observed that PCs have a more polar group of benzene rings, resulting in the high density, dielectric constant, and tensile modulus. Meanwhile, molecular dynamics (MD) simulation was employed under a polymer consistent force field force field. Specific volume and mechanical property were analyzed to investigate the thermodynamic property. The molecular dynamics simulation and experimental results on half decomposition temperature (T1/2), refraction index, flow activation energy, average density, cohesive energy density, glass transition temperature (Tg), and elastic modulus had good agreement. Therefore, it was indicated that the molecular simulation could successfully study the characteristics and properties. The fundamental studies would be expected to supply useful information for designing materials and optimizing processing technology.

Effect of Methanol/Water Mixtures on the Lower Critical Solution Temperature of Poly(N-isopropylacrylamide)

ABSTRACTPoly(N-isopropylacrylamide) (PNIPAM) is a thermo-sensitive polymer that exhibits a lower critical solution temperature (LCST) around 305 K. Below the LCST, PNIPAM is soluble in water and above this temperature polymer chains collapse prior to aggregation. In the presence of methanol, electron paramagnetic resonance (EPR) spectroscopy suggests that, LCST of PNIPAM is depressed up to certain mole fraction of methanol (0.35 mole fraction) and it is speculated that addition of methanol affects the PNIPAM-water interactions. Above 0.35 mole fraction of methanol, LCST gets elevated to temperatures above ∼305 K (32°C) and cannot be detected up to 373 K (100 °C). The atomistic origin of this co-solvency effect on the LCST behavior is not completely understood. In the present study, we have used molecular dynamics (MD) simulations to investigate the effect of methanol-water mixtures on conformational transitions and the LCST of PNIPAM. We employ two different force fields i.e. polymer consistent force-field (PCFF) and CHARMM to study solvation dynamics and the PNIPAM LCST phase transition in various methanol-water mixture compositions (0.018, 0.09, 0.27, 0.5, and 0.98 mole fractions). Simulations are conducted at fully atomistic level for three different temperatures (260, 278, and 310 K) and radius of gyration (Rg) of PNIPAM chains was computed for determination of LCST behavior of PNIPAM.

The role of graphene in enhancing the stiffness of polymeric material: A molecular modeling approach

Amorphous epoxy is considered for investigating the role of graphene in enhancing elastic stiffness of polymers. Graphene is incorporated in the amorphous epoxy in order to develop graphene-epoxy systems. The mechanical properties of crosslinked graphene-epoxy (G-Ep) nanocomposites have been investigated using molecular mechanics (MM) and molecular dynamics (MD) simulations. The influences of graphene nanoplatelet weight concentrations, aspect ratios, and dispersion on elastic constants were studied. Both randomly oriented and stacked graphene-epoxy nanocomposites were considered. A polymer consistent force field (pcff) was used in the analysis. The G-Ep nanocomposites system underwent MD equilibration followed by uniform deformation. The stress-strain responses were evaluated in order to determine Young's modulus. MM simulation was also used to calculate the Young's modulus and shear modulus at 0 K. The results from MD and MM simulation showed reasonable improvement in Young's modulus and shear modulus for G-Ep system in comparison to neat epoxy resin. The graphene concentrations in the range of 1%-3% and graphene with high aspect ratio are seen to improve the Young's modulus by 82% approximately. The results from the simulations were compared with the results from micromechanics based analysis and nanoindentation tests. It was observed from both the atomistic scale simulation and nanoindentation tests that incorporation of graphene in neat epoxy at low weight concentration improves the elastic properties. Using similar MD scheme, it was also seen that the dispersed graphene-epoxy system possesses enhanced in-plane elastic modulus compared to the agglomerated graphene-epoxy system.

Molecular Dynamics Simulation and Characterization of Graphene–Cellulose Nanocomposites

The mechanical properties of graphene-cellulose (GC) nanocomposites are investigated using molecular dynamic (MD) simulations in this work. The influences of graphene concentrations, aspect ratios, and agglomeration on elastic constants and interfacial properties are reported. A polymer consistent force field (pcff) was used in the analysis. The GC nanocomposites system underwent NVT (constant number of atoms, volume, and temperature) and NPT (constant number of atoms, pressure, and temperature) ensemble with an applied uniform strain during the MD simulations. The stress-strain responses were evaluated for both randomly dispersed and stacked GC unit cell in order to study the effects of graphene concentrations, aspect ratio, and agglomeration on Young's modulus. The results indicate that Young's modulus of neat cellulose may be enhanced by incorporating graphene in the GC nanocomposites. It is observed that dispersed graphene shows a comparatively higher Young's modulus than the same with agglomerated graphene. The cohesive and pullout forces versus displacement data are reported under normal and shear modes. It is seen that both cohesive and pullout forces are enhanced for GC specimens with higher graphene aspect ratios due to enlarged surface/interfacial area. The MD simulation results show reasonable agreement with available experimental data.

Interaction between P3HT and Au/Ag/Cu/Al nanowires: A molecular dynamics study

The synthesis and characterization of polymer–nanowires (NWs) composites have recently received important attention because of the new and improved properties of these materials for a wide range of applications such as ultrastrong materials, solar cell technology, bio-environmental sensing, aerospace, catalysis, sensors, optical devices, ultra-low power electronic devices and high density memories. In this work, molecular dynamics simulations (MD) with polymer consistent force field (PCFF) were performed to study the interaction between Poly(3-hexythiophene) (P3HT) and gold, silver, copper and aluminum nanowires in vacuum, because the interfacial interaction between polymer and NW has main effect on mechanical and electrical properties. The influence of main factors such as NW radius, type of metal and temperature on the interfacial adhesion of NW–P3HT and radius of gyration of polymers (Rg) were studied. This study shows that the interaction energy decreases weakly with increasing temperature. We found that Au shows the strongest interaction energy, then Ag, Cu, and finally Al. In addition, the interaction energy increased with increasing NW radius, thus the NW with large radius is the best type for reinforcement. The influence of NW diameter and temperature on Rg was investigated. We found that Rg oscillated slowly with increasing of temperature and NW diameter and they have no any influences on the radius of gyration of polymers (Rg). We found that the interaction energy between P3HT and NW is stronger than interaction energy between P3HT and CNTs, thus NWs can replace instead of CNTs in ultrastrong materials.

Analysis of force fields and BET theory for polymers of intrinsic microporosity

A detailed force field analysis for polymers of intrinsic microporosity (PIMs) was carried out in this study. The generalised amber force field (GAFF) with united atom transferable potential for phase equilibria (TraPPE-UA), and the atomistic polymer consistent force field were evaluated. Analysis carried out with PIM-1 showed that the use of GAFF for bonded interactions and TraPPE-UA for non-bonded interactions yielded a simulated sample that compared best with available experimental data (wide-angle X-ray scattering and nitrogen adsorption at 77 K). In addition, Brunauer–Emmett–Teller surface areas, calculated from simulated nitrogen isotherms as pseudo-experimental data, showed that this common method failed to measure the geometric surface area of this type of material. These findings are expected to facilitate the predictive screening of different PIM functionalities.

Molecular Dynamic Simulation of Graphene Reinforced Nanocomposites for Evaluating Elastic Constants

The mechanical properties of graphene/cellulose (GC) nanocomposites have been investigated using molecular dynamics (MD) simulations. The influence of graphene concentrations and agglomeration on elastic constants is studied. A polymer consistent force field (pcff) is used in the analysis. An efficient time integration scheme was implied in calculating the force in each time step from the corresponding position and velocity of the atom. The GC nanocomposites system undergoes NVT (constant number of atoms, volume and temperature) and NPT (constant number of atoms, pressure and temperature) ensemble with an applied uniform strain during MD simulation. The stress-strain responses were evaluated for GC unit cell in order to study the effects of graphene concentrations and agglomeration on Young's modulus. The results show significant improvement in Young's modulus for the GC nanocomposites compared to neat cellulose. The MD simulation results show reasonable agreement with the micromechanics based model and available experimental data.

Metal–Organic Framework/Polymer Mixed-Matrix Membranes for H2/CO2 Separation: A Fully Atomistic Simulation Study

A fully atomistic simulation study is reported to investigate H2/CO2 separation in mixed-matrix membranes composed of polybenzimidazole (PBI) and zeolitic imidazolateframework-7 (ZIF-7). PBI is mimicked by the polymer-consistent force field and ZIF-7 by a new force field developed here. The predicted density and glass transition temperature of PBI are in fairly good agreement with experimental results. The simulated lattice constants, bond lengths and angles of ZIF-7 match well with measured data. PBI/ZIF-7 membranes have stronger mechanical strength than PBI as evidenced by the enhanced bulk modulus upon adding ZIF-7. With increased ZIF-7 loading, the fractional free volume and percentage of large voids (>3 A) become larger; the solubilities of H2 and CO2 increase because of stronger affinity with ZIF-7. H2 diffusion in PBI membrane is not discernably retarded by adding ZIF-7, in contrast to CO2 diffusion. The simulation results reveal that H2 and CO2 have higher permeabilities in PBI/ZIF-7 membranes tha...

Investigation of adsorption of polymers on metallic nanowires: A molecular dynamics study

Composite of polymer with a small content of strong material, such as carbon nanotube (CNT) and metallic nanowire (NW) has interesting mechanical, thermal, optical and electrical properties. For the first time, we used molecular dynamics simulations (MD) with polymer consistent force field (PCFF) to study adsorption of polymers involving Poly(3-hexythiophene) (P3HT) and Poly[[[(2ethylhexyl)oxy]methoxy-1,4-phenylene]-1,2-ethenediyl] (MEH-PPV) on metallic NW including silver and gold. The influence of main factors such as NW radius and temperature on the interfacial adhesion of NW–polymer and radius of gyration of polymers (Rg) were studied. We showed that the interaction energy decreases slowly with increasing temperature, thus the temperature influence is very weak. Our results showed that P3HT-Au has the strongest interaction energy, then MEH-PPV-Au, P3HT-Ag, and finally MEH-PPV-Ag. In addition, the interaction energy increased with increasing NW radius, thus the NW with large radius is the best type for reinforcement. We studied the influence of NW radius and temperature on the radius of gyration (Rg). We found that Rg oscillated slowly and no obvious trend was seen. In other words, NW radius and temperature had no influence on Rg value. We showed that the Rg value for P3HT was higher than MEH-PPV, thus P3HT expanded more than MEH-PPV on NW surface.

Investigation of molecular interaction between single‐walled carbon nanotubes and conjugated polymers

AbstractSingle‐walled carbon nanotubes (SWNTs) have unique properties such as high electrical conductivity and high tensile strength. Their composites with polymers have a great role in new sciences such as organic solar cells and ultrastrong lightweight materials. In this article, molecular dynamic simulations with polymer consistent force field are performed to study the interaction between SWNTs and conjugated polymers including poly(2‐methoxy‐5‐(3‐7‐dimethyloctyloxy)‐1,4‐phenylenevinylene) (MDMO‐PPV), poly(3‐hexythiophene) (P3HT), and poly[(9,9′‐dioctylfluorenyl‐2,7‐diyl)‐co‐bis(N,N′‐(4,butylphenyl))bis(N,N′‐phenyl‐1,4‐phenylene)diamine] (PFB). We computed the interaction energy and morphology of polymers adsorbed to the surface of SWNTs was studied by the radius of gyration (Rg). The influence of important factors such as SWNT radius, chirality, and the temperature on the interfacial adhesion of SWNT–polymer and Rg of polymers were studied. We found that the strongest interaction between the SWNTs and these polymers was, first observed for P3HT, then MDMO‐PPV, and finally PFB. Our results showed that the interaction energy is influenced by SWNT radius and the specific monomer structure of the polymers, but the effects of chirality and temperature are very weak. In addition, we found that the temperature, chirality, and radius have not any important effect on the radius of gyration. POLYM. COMPOS.,, 2012. © 2012 Society of Plastics Engineers