Through coarse-grained molecular dynamics simulation, we construct a novel kind of end-linked polymer network by employing dual end-functionalized polymer chains that chemically attach to the surface of nanoparticles (NPs), so that the NPs act as large cross-linkers. We examine the effects of the length and flexibility of polymer chains on the dispersion of NPs, and the effect of the chain length on the stress–strain behavior and the segment orientation during the deformation process. We find that the stress upturn becomes more prominent with the decrease of the chain length, attributed to the limited extensibility of the chain strand connecting two neighboring NPs. In addition, this end-linked polymer nanocomposite (PNC) is shown to have a temperature-dependent stress–strain behavior that is contrary to traditional physically mixed PNCs, whose mechanical properties deteriorate with increasing temperature. This is due to the stability of the dispersion of NPs and higher entropic elasticity at higher temperature for the former, while the latter has poorer interfacial interaction at higher temperature, leading to less reinforcing efficiency. By imposing a dynamic oscillatory shear deformation, we obtain a dynamic hysteresis loop for end-linked and physically mixed dispersions. Interestingly, the end-linked system possesses a much smaller hysteresis loss than does the physically mixed system, with the latter exhibiting a more prominent decrease with increasing temperature, due to less interfacial contact. Our results demonstrate that end-linked PNCs combine attractive static and dynamic mechanical properties and exhibit an unusual response to temperature, which could find potential applications in the future.

We perform molecular dynamics computer simulations of free-standing films of symmetric binary linear-cyclic polymer mixtures at equimolar composition. We show that the density of linear polymers at the free-surface is enhanced with respect to the bulk value for short chains, but it is depleted for long chains. Our findings suggest that the enhancement of the composition of linear chains reported for polystyrene polymer blends might hold in a more extended composition range than the one considered in previous experiments at the free surface. Focusing on the equimolar regime of the two polymer species with the identical chain length allows us to safely conclude that the observed surface deviations from the bulk are driven by the topological differences between the two polymer species, and rule out any role played by the different concentration of the two polymer species.

The complexity of induced ordering for tactic poly(methyl methacrylate) (PMMA) thin films in contact with water is examined through all-atom molecular dynamics with validated potentials. We observe that for the water molecules that are hydrogen bonded to the PMMA surface, the isotactic and atactic PMMA show a 33% longer relaxation time compared to syndiotactic PMMA. Almost 94% of hydrogen bonds are with the carbonyl groups of PMMA, irrespective of temperature and tacticity. The stability in re-orientation and nature of hydrogen bond participation for the carbonyl groups as well as about 20% higher interaction energies of carbonyl group hydrogen bonded with water for atactic form indicates existence of cooperative effects. Quantifying the dynamics of hydrogen bond at the tactic interface is important in understanding the role tacticity plays in controlling adhesion and biocompatibility, a design choice that has been gaining ground in the soft material science community.

Molecular dynamics simulations of a coarse-grained bead–spring model have been used to study the effects of molecular crowding on the accumulation of tension in the backbone of bottle-brush polymers tethered to a flat substrate. The number of bottle-brushes per unit surface area, Σ, as well as the lengths of the bottle-brush backbones Nbb (50 ≤ Nbb ≤ 200) and side chains Nsc (50 ≤ Nsc ≤ 200) were varied to determine how the dimensions and degree of crowding of bottle-brushes give rise to bond tension amplification along the backbone, especially near the substrate. From these simulations, we have identified three separate regimes of tension. For low Σ, the tension is due solely to intramolecular interactions and is dominated by the side chain repulsion that governs the lateral brush dimensions. With increasing Σ, the interactions between bottle-brush polymers induce compression of the side chains, transmitting increasing tension to the backbone. For large Σ, intermolecular side chain repulsion increases, forcing side chain extension and reorientation in the direction normal to the surface and transmitting considerable tension to the backbone.

Polymers are used in a wide range of applications that involve chemical and physical processes taking place at surfaces or interfaces which influence the interaction between the polymer material and the substance that comes into contact with it. Polymer surfaces are usually modified either chemically or physically for specific applications such as facilitating wetting, reducing friction, and enhancing adhesion. The variety and complexity of surface and interfacial processes requires a molecular-level understanding of the structural and dynamical properties of the surface/interface layer to help in the design of materials with desired functional properties. Using molecular dynamics (MD) simulations, we investigate the structure and dynamics at the surface of polymer films. We find that the density profiles of the films as a function of distance relative to an instantaneous surface have a structure indicative of a layering at the polymer/vapor interface similar to the typical layered structure observed at the polymer/substrate interface. However, the interfacial molecules at the polymer/vapor interface have a higher mobility compared to that in the bulk while the mobility of the molecules is lower at the polymer/substrate interface. Time correlation of the instantaneous polymer/vapor interface shows that surface fluctuations are strongly temperature dependent and are directly related to the mobility of polymer chains near the interface.

We have used all-atom molecular dynamics (MD) simulations to calculate the surface tension of melt poly(methyl methacrylate) (PMMA) as a function of tacticity. Computation of surface tension using the Kirkwood-Buff approach required hundreds of nanoseconds of equilibration. The computed slopes of surface tension versus temperature are in very good agreement with reported experimental values. Using a rigorous treatment of the true interface, which takes into account the molecular roughness, we find that isotactic PMMA, in comparison to syndiotactic and atactic PMMA, shows a larger surface concentration of polar ester-methyl and carbonyl groups on the surface versus nonpolar α-methyl groups. A mechanistic hypothesis based on the helical nature of the isotactic PMMA chains, their relative flexibility, and their reported conformational energies is proposed to explain the trends in composition near the surface. We highlight here how surface composition and surface tension are controlled by both polarity and steric constraints imposed by tacticity.

•Structural and electronic properties of PCPDT-BT derivatives are investigated.•PDTP-DFBT has lower energy gaps compared to PCPDT-BT and PCPDT-DFBT.•PDTP-DFBT has the highest change in the ground to excited state dipole moment.•PDTP-DFBT has the highest torsional barrier.Heteroatom-containing conjugated polymers are promising candidates for designing efficient polymer solar cells. However, fundamental understanding of the role of heteroatoms on structure-property relationships of these polymers is not yet fully understood. This work, based on first-principles calculations at the molecular level, uncovers how fluorine and oxygen introduction on poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDT-BT) affect structural and electronic properties. Systematic computations of torsional defects, energy gaps, molecular electrostatic potential surfaces and dipole moments are carrier out for PCPDT-BT and its fluorine and oxygen derivatives. We found that oxygen derivative favors lowest energy planar conformation, low energy gaps and high ground to excited state dipole differences. The present results further suggest that oxygenation might increase charge dissociation and reduce charge recombination in the excited state, supporting the recent experimental findings.

•Conformational and electronic properties of DTBT and DTBT/C70 are investigated.•Trans-planar conformation is found to have the lowest energy.•The lowest-energy conformation is found to be stabilized by hydrogen bonding.•Equilibrium separation between DTBT and C70 is found to be ∼3.2 Å.Understanding the bulk heterojunction (BHJ) morphology of dithienyl benzothiadiazole (DTBT)-based conjugated polymers, the most widely used third-generation electron-donors in BHJ-based Organic photovoltaic (OPV) devices, is the current focus of the OPV community. However, there are still debates [J. Am. Chem. Soc. 2013, 135, 1806–1815 and J. Am. Chem. Soc. 2012, 134, 3498–3507] on the most stable conformation of DTBT and is the main focus of the present study. Herein, we report the conformational and electronic properties of DTBT performing detailed first-principle calculations at the molecular level. We found that the energy difference between the two debated DTBT conformations is about 1.3 kT, regardless of methyl or hexyl substituted thiophenes. This energy difference is mainly due to the extent of intramolecular hydrogen bonding. We further report that the low-energy DTBT conformation has a low energy gap, low equilibrium separation (∼3.2 Å) with C70 and proper orbital energy offset, thereby suggesting DTBT-based polymers to be efficient electron donors for OPV devices.

•Orbital energy modeling of twelve representative conjugated molecules is benchmarked.•Range-separation parameters (w) of range-separated (RS) DFT functionals are tuned.•For RS functionals, optimal values of w are found between 0.1 and 0.15 Bohr−1.•w is found to play a significant role on the accuracy of orbital energies.Density functional theory (DFT) calculations with range-separated (RS) functionals are important for orbital energy modeling of conjugated molecules that involve charge transfer excitation. However, the accuracy of the computed results depends on the range-separation parameter (w), and the optimal values of w for a wide range of conjugated systems has hitherto been missing. Herein, orbital energy modeling of twelve representative conjugated molecules, that are promising electron-donors in bulk heterojunction-based organic photovoltaic devices, are benchmarked using DFT and time-dependent DFT (TD-DFT) with three RS functionals (LC-BLYP, wB97XD and CAM-B3LYP). The results using the RS functionals under consideration with default values of w deviate largely from the experimental values (mean signed error (MSE) on HOMO are 2.1 eV, 0.93 eV and 1.5 eV, and MSE on vertical excitation energies are 0.47 eV, 0.55 eV and 0.82 eV, respectively). Computation of orbital energies using tuned range-separation parameter for these RS functionals in the range of 0.05 ⩽ w ⩾ 0.5 Bohr−1 indicates that w plays a significant role on the accuracy of the ground and the excited state energies. We found that the accurate orbital energies of conjugated systems can be predicted using values of w between 0.1 and 0.15 Bohr−1, much smaller than the default values of 0.47, 0.33 and 0.20 Bohr−1 used in LC-BLYP, CAM-B3LYP and wB97XD, respectively.

Short spacer length and high end-group coordination lead to the top network acting as a template for the buried sulfur–gold interface of n-alkanethiols (SH–(CH2)n–OH or SH–(CH2)n–CH3) on gold {111}. Annealing and templating both drive toward a higher sampling of the spatially favorable bridge adsorption sites. The hydrogen-bonded network increases in strength by increasing the number of hydrogens participating per oxygen, from 1.75 to 1.98 for n = 14–30. Higher n leads to better packing (five times for hydroxyl-terminated and seven times for methyl-terminated for n = 14–30) and stability of monolayers, while lower n results in better epitaxial transfer (transfer coefficient ratio = 13.5 for {SH–(CH2)14–OH}/{SH–(CH2)30–CH3}) and actuation. Odd values of n for the hydroxyl-terminated n-alkanethiols lead to lattice spacing of an average of 0.04 ± 0.01 Å higher than even values. There is a structural transition in properties around spacer length n = 24–27. Characterization of monolayer assembly through correlation between adatom and network layers provides recursive design principles for actuation and sensing applications.

Fluorination of conjugated polymers is a popular way of designing new electron donors for the bulk heterojunction (BHJ) based organic solar cells (OSCs). However, not all fluorinated polymers observed experimentally enhance the power conversion efficiency of OSCs, and the fundamental understanding of the effect of fluorination is not yet fully uncovered. Herein, we report the effect of fluorine substitution on the electronic properties of polythienothiophene-co-benzodithiophenes as well as their complexes with fullerene, using density functional theory (DFT) and time-dependent DFT methods at the molecular level. Systematic computations of energy gaps (Egopt and Eghl), ionization potentials (IP), electron affinities (EA), molecular electrostatic potential (MEP) surfaces, and dipole moments (μ) are carried out for these systems. We found that the fluorination of the thienothiophene unit favors lower Egopt, Eghl, IP, and EA as well as stronger μ compared to the fluorination of the benzodithiophene unit, suggesting that efficient exciton dissociation and charge carriers formation may take place efficiently for the former case. These results support recent experimental findings that the performance of polythienothiophene-co-benzodithiophene-based organic solar cells enhances when thienothiophene unit is fluorinated. The present results highlight that more efficient conjugated polymers for OSC can be designed if the gap engineering is carried out by focusing on the low IP, low EA, and high dipole moment.Keywords: band gap; dipole moment; first-principles calculations; fluorinated copolymers; molecular electrostatic potential surface; organic solar cell

The morphology and chain packing structures in block copolymers strongly impact their mechanical response; therefore, to design and develop high performance materials that utilize block copolymers, it is imperative to have an understanding of their self-assembly behavior. In this research, we utilize coarse-grained (CG) molecular dynamics to study the effects of peptidic volume fraction and secondary structure on the morphological development and chain assembly of the triblocks poly(γ-benzyl-l-glutamate)-b-poly(dimethylsiloxane)-b-poly(γ-benzyl-l-glutamate) (GSG) and poly(dimethylsiloxane)-b-poly(γ-benzyl-l-glutamate)-b-poly(dimethylsiloxane) (SGS). This necessitated developing a complete coarse-grained parameter set for poly(dimethylsiloxane) that closely captures the radial pair distribution of a united atom model and the experimental density at 300 K. These parameters are combined with the MARTINI amino acid CG force field and validated against prior reported values of domain spacing and peptide chain packing for GSG. The combined CG parameter set is then used to model SGS, a triblock currently in development for nature-inspired mechanically enhanced hybrid materials. The results reveal that the peptide side chain strongly influences the final morphology. For instance, lamellar or hexagonally packed cylindrical domain formation can result from the variation in side-chain interactions, namely, side-chain sterics preventing curved interface formation by increasing interfacial free volume. Ultimately, this research lays the foundation for future studies involving systems with dispersity, mixtures of secondary structures, and larger multiblock copolymers, such as polyurethanes and polyureas.

Exciton binding energies (Eb) in promising organic photovoltaic (OPV) materials, specifically in poly(3-hexylthiophene) (P3HT) and thieno[3,4-b]thiophene-alt-benzodithiophene copolymer (PTB7) are investigated using density functional theory (DFT) and time-dependent DFT methods at the molecular level. A systematic investigation of the dependence of Eb on backbone torsion and chain length in P3HT and PTB7 is carried out by calculating the fundamental gap (Egfund), HOMO–LUMO gap (Eghl) and optical gap (Egopt) of these systems. We found that PTB7 has lower Egfund,Eghl,Egopt and Eb that are less-dependent on torsional angle compared to P3HT. These findings indicate that excitons are easily dissociated to produce charge carriers in PTB7 than in P3HT and hence supports recent experimental findings that PTB7-based OPV devices with improved active layer morphology have much higher power conversion efficiency than the most investigated P3HT-based OPV devices.

Utilizing all-atom molecular dynamics (MD), we have analyzed the effect of tacticity and temperature on the surface structure of poly(methyl methacrylate) (PMMA) at the polymer–vacuum interface. We quantify these effects primarily through orientation, measured as the tilt with respect to the surface normal, and the surface number densities of the α-methyl, ester-methyl, carbonyl, and backbone methylene groups. Molecular structure on the surface is a complex interplay between orientation and number densities and is challenging to capture through sum frequency generation (SFG) spectroscopy alone. Independent quantification of the number density and orientation of chemical groups through all-atom MD presents a comprehensive model of stereoregular PMMA on the surface. SFG analysis presented in part I of this joint publication measures the orientation of molecules that are in agreement with MD results. We observe the ester-methyl groups as preferentially oriented, irrespective of tacticity, followed by the α-methyl and carbonyl groups. SFG spectroscopy also points to ester-methyl being dominant on the surface. The backbone methylene groups show a very broad angular distribution, centered along the surface plane. The surface number density ratios of ester-methyl to α-methyl groups show syndiotactic PMMA having the lowest value. Isotactic PMMA has the highest ratios of ester- to α-methyl. These subtle trends in the relative angular orientation and number densities that influence the variation of surface structure with tacticity are highlighted in this article. A more planar conformation of the syndiotactic PMMA along the surface (x–y plane) can be visualized through the trajectories from all-atom MD. Results from conformation tensor calculations for chains with any of their segments contributing to the surface validate the visual observation.

An ab initio-based improved force field is reported for poly(3-hexylthiophene) (P3HT) in the solid state, deriving torsional parameters and partial atomic charges from ab initio molecular structure calculations with explicit treatment of the hexyl side chains. The force field is validated by molecular dynamics (MD) simulations of solid P3HT with different molecular weights including calculation of structural parameters, mass density, melting temperature, glass transition temperature, and surface tension. At 300 K, the P3HT crystalline structure features planar backbones with non-interdigitated all-trans hexyl side chains twisted ∼90° from the plane of the backbone. For crystalline P3HT with infinitely long chains, the calculated 300 K mass density (1.05 g cm–3), the melting temperature (490 K), and the 300 K surface tension (32 mN/m) are all in agreement with reported experimental values, as is the glass transition temperature (300 K) for amorphous 20-mers.

All-atom molecular dynamics simulations have been carried out to study the wetting of atactic polystyrene (aPS) thin films by water droplets. The effect of oxidation of the aPS surface on the contact angle has been studied as a function of oxygen concentration. Oxidation of aPS has been achieved by randomly replacing with oxygen the ortho and/or meta hydrogens on the aromatic rings within 1 nm of the aPS surface until the desired concentration of oxygen is reached. The simulated contact angle is found to decrease monotonically with increasing degree of oxidation, consistent with recent experimental results. The number of hydrogen bonds between water molecules and polystyrene at the interface is found to monotonically increase with oxygen concentration. By use of a modified Good–Girafalco–Fowkes–Young equation, the contribution of nondispersion interactions, γslP, to the interfacial energy at the aPS/water interface has been determined as a function of the degree of oxidation. The values of γslP extracted appear to follow a quadratic dependence on oxygen concentration of the aPS surface. The roughness of the polystyrene surface appears to be independent of oxygen concentration when the polystyrene is exposed to vacuum, and it appears to increase slightly when it is in contact with water. The orientational ordering of the phenyl rings at the polystyrene surface exhibits no dependence on oxygen concentration for polystyrene in vacuum. However, the ordering appears to decrease slightly with increasing oxygen concentration when the polystyrene is in contact with water.

This paper employs fully atomistic molecular dynamics simulations to characterize relationships between structural and elastic properties of thermosetting polymers both in glassy and rubbery state. The polymer system investigated consists of epoxy resin DGEBA and hardener DETDA. An effective cross-linking procedure that enables generation of thermoset structures containing up to 35000 atoms with realistic structural characteristics was used. A dynamic deformation approach has been used that takes into consideration both potential energy and thermal motions in the structure. Small uniaxial, volumetric and shear deformations were applied to the systems to obtain elastic moduli. A method to independently determine Poisson's ratio was proposed that reduces statistical errors and circumvents the time scale limitations of molecular dynamics simulations. The influence of variables such as extent of curing and length of epoxy strands on elastic response at various temperatures was explored. Expected trends in the dependence of the elastic constants on these practical process parameters were shown. The relationship between the four independently calculated elastic constants was seen to comply with those predicted by the classical theory of linear elasticity in an isotropic medium, which provides confidence in the validity of these simulations. Moreover, the elastic properties obtained are also in good agreement with experimental data reported in the literature. Close agreements between predicted elastic constants and experimentally measured values underscore the ability of the approaches used in this study to provide realistic predictions of the mechanical response of thermosetting polymers, both in glassy and rubbery states. These results show significant improvement over earlier studies based on a static approach which takes into account the potential energy contribution to the elastic response but ignores temperature effect.

The development of organic photovoltaic (OPV) solar cells has seeded a bright hope of achieving low-cost solar energy harvesting. Practical realization and successful commercialization require enhancing the efficiency of solar energy harvesting, which, in turn, relies on the core understanding of structure–property relationships in OPV materials. Here, we report the first large-scale density functional calculations of the nanoconformational and electronic properties of the thieno[3,4-b]thiophene-alt-benzodithiophene copolymer (PTB7), a high-efficiency OPV material. These first-principles results include the chain length dependence of the torsional potential, the nearest-neighbor torsional coupling, the band gap, and the electronic conjugation length. Importantly, PTB7 was found to have a torsional potential almost independent of chain length, very weak nearest-neighbor torsional coupling, a low band gap (∼1.8 eV), and a very long conjugation length (∼147 Å) compared to the other conjugated polymers like polythiophene and poly(3-alkylthiophene). These results suggest that PTB7 can be an efficient electron donor for OPV devices.

Using all-atom molecular dynamics simulations, the multilayer adsorption of two single-carbon alkane analogues—methane (CH4) and chloromethane (CH3Cl)—in the liquid phase on a metallic substrate (molybdenum (100) surface) has been studied to elucidate differences in the adsorbed film structure as a function of the overall film thickness, system temperature, substrate mobility, and pairwise substrate interaction potential (Lennard-Jones 12-6 versus embedded atom method versus Lennard-Jones 9-3 “wall” potential, used as a basis for comparison). Simulations suggest a clear predilection to well-ordered packing arrangements dictated by the adsorbate molecular geometry when adsorbed on the flat, featureless Lennard-Jones wall, but adsorption on the atomistic molybdenum substrate exhibits a more complex structure and orientation influenced by the adsorbate molecular geometry but also dependent on the presence or lack of small variations in surface structure caused by substrate mobility and interatomic potential. The result illustrates differences in packing geometry and surface binding energy on very noticeable scales despite the magnitude of surface vibrations allowed.

Molecular dynamics and molecular mechanics simulations have been used to study thermo-mechanical response of highly cross-linked polymers composed of epoxy resin DGEBA and hardener DETDA. The effective cross-linking approach used in this work allowed construction of a set of stress-free molecular models with high conversion degree containing up to 35000 atoms. The generated structures were used to investigate the influence of model size, length of epoxy strands, and degree of cure on thermo-mechanical properties. The calculated densities, coefficients of thermal expansion, and glass transition temperatures of the systems are found to be in good agreement with experimental data. The computationally efficient static deformation approach we used to calculate elastic constants of the systems successfully compensated for the large scattering of the mechanical properties data due to nanoscopically small volume of simulation cells and allowed comparison of properties of similar polymeric networks having minor differences in structure or chemistry. However, some of the elastic constants obtained using this approach were found to be higher than in real macroscopic samples. This can be attributed to both finite-size effect and to the limitations of the static deformation approach to account for dynamic effects. The observed dependence of properties on system size, in this work, can be used to estimate the contribution of large-scale defects and relaxation events into macroscopic properties of the thermosetting materials.

Molecular dynamics simulations of multilayer adsorption of binary mixtures of two tetrasubstituted halomethanes (CF4 and CF3Br) on two very different substrates (graphite vs hydroxylated SiO2) were performed for three different bulk compositions (40%, 50%, and 60% CF4) and over a range of temperatures from 80 to 200 K. The goal of these simulations was to investigate in depth how these factors affect film structure, layer composition, lateral arrangement, and molecular orientation in the first adsorbed layer on each substrate. In line with a previous study of single-component adsorption on these surfaces, mixtures adsorbed on the hydroxylated SiO2 surface show stable number density profiles that are largely independent of temperature, up to 160 K. This level of stability is essentially absent in the case of adsorption on graphite, which show densities and surface populations that are largely dependent on overall film composition, molecular orientation, and adsorbate–substrate interactions, in addition to system temperature. Further, the composition of the first adsorbed layer at each solid surface appears to be influenced by the choice of substrate, with CF3Br the majority component at the graphite surface for all compositions and temperatures, while the first adsorbed layer on hydroxylated SiO2 more clearly mirrors the overall film composition at temperatures below 160 K.

Molecular dynamics (MD) simulations were used to study the structural and dynamic properties of multilayer adsorption of each of three halomethanes, CF4, CF3Cl, and CF3Br, adsorbed onto the (001) surface of either of two atomically flat but chemically and structurally different substrates (graphite and hydroxylated α-quartz) at temperatures ranging from 60 to 300 K. Analysis of the data shows a strong influence on the adsorption characteristics of these halomethane films due to the surface characteristics of the chosen substrate. In particular, the nature of the hydroxylation of α-quartz shows a striking ability to alter the affinity with which species adsorb onto its surface. This effect appears to be at least partly responsible for the differences in the orientation and packing of molecules in the first film layer as well as differences in the effect of temperature variation on phase behavior and dynamics.