Atomistic Simulations of Polydisperse Lignin Melts Using Simple Polydisperse Residue Input Generator.

Understanding the physics of lignin will help rationalize its function in plant cell walls as well as aiding practical applications such as deriving biofuels and bioproducts. Here, we present SPRIG (Simple Polydisperse Residue Input Generator), a program for generating atomic-detail models of random polydisperse lignin copolymer melts i.e., the state most commonly found in nature. Using these models, we use all-atom molecular dynamics (MD) simulations to investigate the conformational and dynamic properties of polydisperse melts representative of switchgrass (Panicum virgatum L.) lignin. Polydispersity, branching and monolignol sequence are found to not affect the calculated glass transition temperature, Tg. The Flory-Huggins scaling parameter for the segmental radius of gyration is 0.42 ± 0.02, indicating that the chains exhibit statistics that lie between a globular chain and an ideal Gaussian chain. Below Tg the atomic mean squared displacements are independent of molecular weight. In contrast, above Tg, they decrease with increasing molecular weight. Therefore, a monodisperse lignin melt is a good approximation to this polydisperse lignin when only static properties are probed, whereas the molecular weight distribution needs to be considered while analyzing lignin dynamics.

Influence of charge sequence on the adsorption of polyelectrolytes to oppositely-charged polyelectrolyte brushes.

When a solution of polyanionic chains is placed in contact with a polycationic brush, the polyanions adsorb into the brush. We investigate the influence of the charge sequences of the free and bound species on the thermodynamics of polyelectrolyte adsorption. As model systems, we consider free and brush polyelectrolytes with either block or alternating charge sequences, and study the adsorption process using coarse-grained Langevin dynamics with implicit solvent, explicit counterions, and excess salt. Free energy, internal energy, and entropy of adsorption are computed using umbrella sampling methods. When the number of polyanions exceed the number of polycations, the brush becomes overcharged. Free chains adsorb most strongly when both free and tethered chains have a block charge sequence, and most weakly when both species have an alternating sequence. Adsorption is stronger when the free polyanion has a block sequence and the tethered polycation is alternating than in the reverse case of an alternating free polymer and a tethered block copolymer. Sequence-dependent effects are shown to be largely energetic, rather than entropic, in origin.

Ion transport mechanisms in lamellar phases of salt-doped PS-PEO block copolymer electrolytes.

We use a multiscale simulation strategy to elucidate, at an atomistic level, the mechanisms underlying ion transport in the lamellar phase of polystyrene-polyethylene oxide (PS-PEO) block copolymer (BCP) electrolytes doped with LiPF6 salts. Explicitly, we compare the results obtained for ion transport in the microphase separated block copolymer melts to those for salt-doped PEO homopolymer melts. In addition, we also present results for dynamics of the ions individually in the PEO and PS domains of the BCP melt, and locally as a function of the distance from the lamellar interfaces. When compared to the PEO homopolymer melt, ions were found to exhibit slower dynamics in both the block copolymer (overall) and in the PEO phase of the BCP melt. Such results are shown to arise from the effects of slower polymer segmental dynamics in the BCP melt and the coordination characteristics of the ions. Polymer backbone-ion residence times analyzed as a function of distance from the interface indicate that ions have a larger residence time near the interface compared to that near the bulk of lamella, and demonstrates the influence of the glassy PS blocks and microphase segregation on the ion transport properties. Ion transport mechanisms in BCP melts reveal that there exist five distinct mechanisms for ion transport along the backbone of the chain and exhibit qualitative differences from the behavior in homopolymer melts. We also present results as a function of salt concentration which show that the mean-squared displacements of the ions decrease with increasing salt concentration, and that the ion residence times near the polymer backbone increase with increasing salt concentration.

On the relationship between the local segmental dynamics and the tagged monomer dynamics in lamellar phases of diblock copolymers.

In this brief article, we present results from coarse-grained molecular dynamics simulations which probed the relationship between the local segmental dynamics and the tagged monomer dynamics in lamellar phases of diblock copolymers. Our results demonstrate that monomer relaxation times do not provide directly a quantitatively accurate measure of the spatial variations in segmental dynamics. However, a convolution of the monomer density distributions with their corresponding relaxation times is shown to provide an approximate, but accurate, quantitative characterization of the average local segmental dynamics.

Segmental dynamics in lamellar phases of tapered copolymers.

Recent experiments have reported that the lamellar phase of salt-doped tapered copolymers exhibit higher ionic conductivity compared to those seen in similar morphologies of diblock copolymers. Such observations were in turn rationalized by invoking the corresponding glass transition temperature of the segregated copolymers. In this work we report the results of coarse-grained molecular dynamics simulations to identify the mechanisms underlying such characteristics. Explicitly, we probe the combined influences of the degree of segregation and the disparity in mobilities of the segments of the two blocks, upon the local relaxation dynamics of tapered copolymers segregated in lamellar phases. Our results show that the local dynamics of tapered copolymers depend on two independent factors, viz., the degree of segregation of such copolymers relative to their order-disorder transition temperature, and the relative mobilities (glass transition temperatures) of the two blocks. In qualitative correspondence with experiments, we find that for appropriate combinations of mobility ratios and degree of segregation, the lamellar phases of tapered copolymers can exhibit faster local segmental dynamics compared to diblock copolymers.

Influence of nanoparticle-ion and nanoparticle-polymer interactions on ion transport and viscoelastic properties of polymer electrolytes.

We use atomistic simulations to probe the ion conductivities and mechanical properties of polyethylene oxide electrolytes containing Al2O3nanoparticles. We specifically study the influence of repulsive polymer-nanoparticle and ion-nanoparticle interactions and compare the results with those reported for electrolytes containing the polymorph ß-Al2O3nanoparticles. We observe that incorporating repulsive nanoparticle interactions generally results in increased ionic mobilities and decreased elastic moduli for the electrolyte. Our results indicate that both ion transport and mechanical properties are influenced by the polymer segmental dynamics in the interfacial zones of the nanoparticle in the ion-doped systems. Such effects were seen to be determined by an interplay between the nanoparticle-polymer,nanoparticle-ion, and ion-polymer interactions. In addition, such interactions were also observed to influence the number of dissociated ions and the resulting conductivities. Within the perspective of the influence of nanoparticles on the polymer relaxation times in ion-doped systems, our results in the context of viscoelastic properties were consistent with the ionic mobilities. Overall, our results serve to highlight some issues that confront the efforts to use nanoparticle dispersions to simultaneously enhance the conductivity and the mechanical strength of polymer electrolyte.

Coarse-graining in simulations of multicomponent polymer systems.

We investigate the mapping required between the interaction parameters of two different coarse-grained simulation models to ensure a match of the long-range structural characteristics of multicomponent polymeric system. The basis for our studies is the recent work of Morse and workers, which demonstrated the existence of a mapping between the interaction parameters of different coarse-grained simulation models which allow for a matching of the peak of the disordered state structure factor in symmetric diblock copolymers. We investigate the extensibility of their results to other polymeric systems by studying a variety of systems, including, asymmetric diblock copolymers, symmetric triblock copolymers, and diblock copolymer-solvent mixtures. By using the mapping deduced in the context of symmetric diblock copolymers, we observe excellent agreement for peak in the inverse structure between both two popular coarse grained models for all sets of polymeric melt systems investigated, thus showing that the mapping function proposed for diblock copolymer melts is transferable to other polymer melts irrespective of the blockiness or overall composition. Interestingly, for the limited parameter range of polymer-solvent systems investigated in this article, the mapping functions developed for polymer melts are shown to be equally effective in mapping the structure factor of the coarse-grained simulation models. We use our findings to propose a methodology to create ordered morphologies in simulations involving hard repulsive potentials in a computationally efficient manner. We demonstrate the outcomes of methodology by creating lamellar and cylindrical phases of diblock copolymers of long chains in the popularly used Kremer-Grest simulation model.

Influence of topographically patterned angled guidelines on directed self-assembly of block copolymers.

Single chain in mean-field Monte Carlo simulations were employed to study the self-assembly of block copolymers (BCP) in thin films that use trapezoidal guidelines to direct the orientation and alignment of lamellar patterns. The present study explored the influence of sidewall interactions and geometry of the trapezoidal guidelines on the self-assembly of perpendicularly oriented lamellar morphologies. When both the sidewall and the top surface exhibit preferential interactions to the same block of the BCP, trapezoidal guidelines with intermediate taper angles were found to result in less defective perpendicularly orientated morphologies. Similarly, when the sidewall and top surface are preferential to distinct blocks of the BCP, intermediate tapering angles were found to be optimal in promoting defect free structures. Such results are rationalized based on the energetics arising in the formation of perpendicularly oriented lamella on patterned substrates.