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Charge transport in conjugated polymers may be governed not only by the static microstructure but also fluctuations of backbone segments. Using molecular dynamics simulations, we predict the role of side chains in the backbone dynamics for regiorandom poly(3-alkylthiophene-2,5-diyl)s (P3ATs). We show that the backbone of poly(3-dodecylthiophene-2-5-diyl) (P3DDT) moves faster than that of poly(3-hexylthiophene-2,5-diyl) (P3HT) as a result of the faster motion of the longer side chains. To verify our predictions, we investigated the structures and dynamics of regiorandom P3ATs with neutron scattering and solid state NMR. Measurements of spin-lattice relaxations (T1) using NMR support our prediction of faster motion for side chain atoms that are farther away from the backbone. Using small-angle neutron scattering (SANS), we confirmed that regiorandom P3ATs are amorphous at about 300 K, although microphase separation between the side chains and backbones is apparent. Furthermore, quasi-elastic neutron scattering (QENS) reveals that thiophene backbone motion is enhanced as the side chain length increases from hexyl to dodecyl. The faster motion of longer side chains leads to faster backbone dynamics, which in turn may affect charge transport for conjugated polymers. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 1193–1202.

We explore the crystal structure and ionic conductivity of unfilled and cellulose nanowhisker‐filled polymer electrolytes, based on high molecular weight PEO and LiClO4 at a concentration EO:Li = 6:1 (EO = ether oxygen). At this concentration, the polymer electrolyte forms crystalline PEO6 (PEOy—the crystal structure co‐crystallizes "y" PEO ether oxygens to one Li‐anion pair). While the diffraction patterns for PEO3 and PEO6 with LiClO4 are available, their structures are not. Thus, we optimize these structures using density functional theory and show agreement between simulated and experimental diffraction patterns. The PEO6 crystal shows insufficient Li‐ion conductivity due to a lack of percolated conduction paths, and the crystal structure is unstable following 2 months after annealing. Cellulose nanowhiskers, with their patterned surface OH groups, extend the length of PEO6 tunnels, stabilize these tunnels for as long as a year, and result in the percolation of crystals. Although the EO:Li = 6:1 stoichiometry should not favor PEO3 formation, we observe this crystal phase in some unfilled samples. But, with the addition of nanowhiskers, the patterned surface templates PEO6 formation rather than PEO3. Despite these advantages, nanowhiskers do not improve ionic conductivity as hypothesized. The tools developed in this work allow further mechanistic exploration of conduction through crystalline domains. [ABSTRACT FROM AUTHOR]

Ionomers are polymers in which an ionic group is covalently bonded to the polymer backbone. Ion aggregates in ionomers have morphologies that allow for the packing of the attached polymer backbone. Using ion-only coarse-grained molecular dynamics, we observe that string-like ion aggregates become flat and sheet-like at lower dielectric constants. A consequence of the changing morphology is that the sheet-like aggregates self-assemble to form ordered, lamellar structures. We use a simple thermodynamic model to demonstrate that depletion attraction mediated by small aggregates can explain the observed order. Our results suggest that depletion attraction can drive ions to form structures that have the size scale suggested by direct visualization, produce the commonly observed experimental correlation peak from X-ray and neutron scattering, and satisfy chain-packing constraints that have been demonstrated to be important in simulations. [ABSTRACT FROM AUTHOR]

Coarse-grained models decrease the number of force sites and thus reduce computational requirements for molecular simulation. While these models are successful in describing structural properties, dynamic evolution is faster than the corresponding atomistic simulations or experiments. We consider coarse-grained models for four polymers and one polymer mixture, where accurate dynamics are obtained by scaling to match the mean-squared displacements of the corresponding atomistic descriptions. We show that the required scaling is dictated by local friction and that this scaling is only valid after the onset of continuous motion. [ABSTRACT FROM AUTHOR]

We examined three united atom models in light of their description of polyolefin dynamics and investigated the relative influence of various potentials on the resulting dynamics. Results were compared with a collection of experimental data on polyethylene, poly(ethylene-alt-propylene), polypropylene, and head-to-head polypropylene, including quasielastic neutron scattering measurements that we report for two of these materials. For materials with branching, differences between force fields are apparent at low temperature, with the NERD force field most accurate. Differences between NERD and the others are the strength of nonbonded interactions and the height of torsional barriers. We artificially raised each, both of which leads to a slow down in dynamics similar to that observed when lowering temperature. Increasing nonbonded interaction strength slows dynamics with the Vogel dependence of the α-relaxation, while raising torsional barriers, slows dynamics with the Arrhenius dependence of the β-relaxation. [ABSTRACT FROM AUTHOR]

We evaluate explicit atom, united atom, and coarse-grained force fields for molecular dynamics simulation of poly(methyl methacrylate) (PMMA) by comparison to structural and dynamic neutron scattering data. The coarse-grained force field is assigned based on output of the united atom simulation, for which we use an existing force field. The atomic structure of PMMA requires the use of two types of coarse-grained beads, one representing the backbone part of the repeat unit and the other representing the side group. The explicit atom description more closely resembles dynamic experimental data than the united atom description, although the latter provides a reasonable approximation. The coarse-grained description provides structural and dynamic properties in agreement with the united atom description on which it is based, while allowing extension of the time trajectory of the simulation. [ABSTRACT FROM AUTHOR]

We test a coarse-grained model assigned based on united atom simulations of C50 polyethylene to seven chain lengths ranging from C76 to C300. The prior model accurately reproduced static and dynamic properties. For the dynamics, the coarse-grained time evolution was scaled by a constant value [t=αtCG] predictable based on the difference in intermolecular interactions. In this contribution, we show that both static and dynamic observables have continued accuracy when using the C50 coarse-grained force field for chains representing up to 300 united atoms. Pair distribution functions for the longer chain systems are unaltered, and the chain dimensions present the expected N0.5 scaling. To assess dynamic properties, we compare diffusion coefficients to experimental values and united atom simulations, assign the entanglement length using various methods, examine the applicability of the Rouse model as a function of N, and compare tube diameters extracted using a primitive path analysis to experimental values. These results show that the coarse-grained model accurately reproduces dynamic properties over a range of chain lengths, including systems that are entangled. [ABSTRACT FROM AUTHOR]

The influence of composition on component dynamics and relevant static properties in a miscible polymer blend is investigated using molecular dynamics simulation. Emphasis is placed on dynamics in the single chain dilution limit, as this limit isolates the role of inherent component mobility in the polymer’s dynamic behavior when placed in a blend. For our systems, a biased local concentration affecting dynamics must arise primarily from chain connectivity, which is quantified by the self-concentration, because concentration fluctuations are minimized due to restraints on chain lengths arising from simulation considerations. The polyolefins simulated [poly(ethylene-propylene) (PEP) and poly(ethylene-butene) (PEB)] have similar structures and glass transition temperatures, and all interactions are dispersive in nature. We find that the dependence of dynamics upon composition differs between the two materials. Specifically, PEB (slower component) is more influenced by the environment than PEP. This is linked to a smaller self-concentration for PEB than PEP. We examine the accuracy of the Lodge-McLeish model (which is based on chain connectivity acting over the Kuhn segment length) in predicting simulation results for effective concentration. The model predicts the simulation results with high accuracy when the model’s single parameter, the self-concentration, is calculated from simulation data. However, when utilizing the theoretical prediction of the self-concentration the model is not quantitatively accurate. The ability of the model to link the simulated self-concentration with biased local compositions at the Kuhn segment length provides strong support for the claim that chain connectivity is the leading cause of distinct mobility in polymer blends. Additionally, the direct link between the willingness of a polymer to be influenced by the environment and the value of the self-concentration emphasizes the importance of the chain connectivity. Furthermore, these findings are evidence that the Kuhn segment length is the relevant length scale controlling segmental dynamics. [ABSTRACT FROM AUTHOR]

We compare static and dynamic properties obtained from three levels of modeling for molecular dynamics simulation of poly(ethylene oxide) (PEO). Neutron scattering data are used as a test of each model’s accuracy. The three simulation models are an explicit atom (EA) model (all the hydrogens are taken into account explicitly), a united atom (UA) model (CH2 and CH3 groups are considered as a single unit), and a coarse-grained (CG) model (six united atoms are taken as one bead). All three models accurately describe the PEO static structure factor as measured by neutron diffraction. Dynamics are assessed by comparison to neutron time of flight data, which follow self-motion of protons. Hydrogen atom motion from the EA model and carbon/oxygen atom motion from the UA model closely follow the experimental hydrogen motion, while hydrogen atoms reinserted in the UA model are too fast. The EA and UA models provide a good description of the orientation properties of C–H vectors measured by nuclear magnetic resonance experiments. Although dynamic observables in the CG model are in excellent agreement with their united atom counterparts, they cannot be compared to neutron data because the time after which the CG model is valid is greater than the neutron decay times. [ABSTRACT FROM AUTHOR]

Coarse-grained models that preserve atomistic detail display faster dynamics than atomistic systems alone. We show that this “ indirect speed up” is robust: coarse-grained dynamic observables computed with time scaled by a constant factor are in excellent agreement with their underlying atomistic counterparts. Borrowing from accelerated dynamics methods used in the field of rare events, we predict the scaling factor within 7%, based on reduced intermolecular attraction yielding faster neighbor cage escapes. [ABSTRACT FROM AUTHOR]

The mobility of vitreous boron oxide is studied by molecular dynamics simulation. A polarization model that incorporates induced dipoles arising both from charges and from other induced dipoles on atoms with nonzero polarizability is used to simulate boron oxide glass at various temperatures above the glass transition temperature. Particle mobility is investigated through the calculation of the self-intermediate scattering function and the mean-squared displacement. The calculations clearly reveal a two-step relaxation with a plateau at intermediate times for all investigated temperatures. With respect to atomic species, boron atoms are less mobile than oxygen atoms at all temperatures within the plateau region. Through analyzing particle trajectories, it is revealed that BO3 groups move as one unit and follow each other in a stringlike manner. Three connected BO3 groups comprise a six-membered boroxol ring, which is shown to move in a collective manner, requiring the simultaneous movement of all ring atoms. The boroxol ring is observed to be confined, or caged, during the plateau region, and jumps to a new location at longer times. This observation is linked to the concept of strong versus fragile glass formers and the potential energy landscape. In addition to the caging feature, an overshoot or dip occurs in the plateau regions of the mean-squared displacement and self-intermediate scattering functions respectively. These features are followed by a ringing pattern, previously associated with finite size effects in other strong glass formers, which persist for the duration of the plateau region. Both features are shown to be consistent with the bending of atomic “cages” from the plane of the boroxol ring, and arise due to the displacement of atoms from local minimum energy configurations. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Molecular simulation is used to characterize the spatial dependence of collective motion in four saturated hydrocarbon polymers. The observable is the distinct intermediate scattering function, as measured in coherent quasielastic neutron scattering experiments. Ranges of 0.01–1000 ps in time and 2–14 Å in spatial scale are covered. In this time range, a two-step relaxation, consisting of a fast exponential decay and a slower stretched decay, is observed for all spatial scales. The relaxation times for the fast process are very similar to those obtained by following self motion, with a small modulation of relaxation times near the peak in the static structure factor which is well described by the narrowing picture suggested by de Gennes. For the slow process, self and collective relaxation times have larger numerical differences and follow different scaling with spatial scale. The modulation of slow relaxation times is larger than that observed for the fast process, but is overestimated by the de Gennes prediction, which only works qualitatively. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations are used to investigate the spatial dependence of dynamics in a series of polyolefins. The dynamic indicator used is the self-intermediate scattering function, which parallels the observable in an incoherent quasielastic neutron scattering experiment such as time of flight or backscattering. As with neutron time of flight experiments, two processes are evident. The fast process is a single exponential, and has relaxation times that scale as q[sup -2], where q is the momentum transfer. The slow process is the stretched exponential decay usually associated with the motion underlying the glass transition. The stretching exponent is a function of spatial scale, with the minimum values occurring near the spatial scale of interchain packing. Relaxation times for the slow process scale as q[sup -2/β] for all materials investigated. The relative contribution of the two processes is a function of spatial scale, with the crossover from fast to slow dynamics at the location of closest possible interchain contacts, which is approximately three times the cage size. These observations apply equally well to the four materials considered. We consider the relative ordering of relaxation times of the series in light of their local chain architecture. This ordering varies depending on the observable calculated. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We investigate the effect of densification on intramolecular and intermolecular chain packing and structural relaxation of four saturated hydrocarbon polymers: poly(ethylene propylene) (PEP), poly(ethylene butene), atactic polypropylene, and head-to-head polypropylene. Within this series, the local chain architecture, specifically the frequency and size of pendant groups, varies. Density changes of ρ[sub 0]±5%, where ρ[sub 0] is the ambient density, are considered at a temperature of 400 K, which is well above the glass transition temperatures of all four polymers. Intramolecular chain packing remains nearly unchanged over this density range, and intermolecular packing shows a tendency towards closer and more efficient packing at higher densities. The effect of density on dynamics depends on the spatial scale considered. Self-intermediate scattering functions, as measured in incoherent quasielastic neutron scattering experiments, are calculated for spatial scales between 2.5 and 20 Å. At length scales less than the closest intermolecular spacing, densification has little effect on mobility. The largest effects are observed in the vicinity of the nearest chain spacing distance, where mobility is decreased and dynamic heterogeneity, as evidenced by the stretching parameter in stretched exponential fits, is increased with densification. At length scales approaching R[sub g], both effects level off and are slightly less than at spatial scales of interchain packing. The most flexible of the four materials, PEP, is least affected by density changes. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Monte Carlo simulations of polymer chains on a discretized lattice, where each atom occupies multiple lattice sites, have been shown to provide accuracies comparable with off-lattice simulations in one-tenth the computation time. For polyatomic molecules, the main requirement for the level of discretization required is set by the intramolecular geometry of the molecule. The current work provides a method to determine this level of discretization without running full off lattice simulations for comparison of results. Chains are generated off-lattice, placed on lattices of various discretizations, and the resulting errors are analyzed. We apply the method to three cases, united atom polyethylene, united atom polypropylene, and atomistic polyethylene. The atomic diameter to lattice size ratios required are 12 for the united atom cases, and 17 for atomistic polyethylene. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]