Syamal S. Tallury

Molecular dynamics simulations of flexible polymer chains wrapping single-walled carbon nanotubes.

The goal of this study is to explore the interface between single-walled carbon nanotubes (SWCNTs) and polymer chains with flexible backbones in vacuo via molecular dynamics (MD) simulations. These simulations investigate whether the polymers prefer to wrap the SWCNT, what the molecular details of that interface are, and how the interfacial interaction is affected by the chemical composition and structure of the polymer. The simulations indicate that polymers with flexible backbones tend to wrap around the SWCNT, although not in any distinct conformation; no helical conformations were observed. PAN with the cyano side group showed a preference for transversing the length of the SWCNT rather than wrapping around its diameter, and the cyano group prefers to align parallel to the SWCNT surface. Flexible backbone polymers with bulky and aromatic side groups such as PS and PMMA prefer intrachain coiling rather than wrapping the SWCNT. Moment of inertia plots as a function of time quantify the interplay between intrachain coiling and adsorption to the SWCNT surface.

Molecular Dynamics Simulations of Polymers with Stiff Backbones Interacting with Single-Walled Carbon Nanotubes

The goal of this study is to explore the interface between single-walled carbon nanotubes (SWCNTs) and polymer chains with semiflexible and stiff backbones in vacuum via molecular dynamics (MD) simulations, which complements our previous work with flexible backbone polymers. These simulations investigate the structural and dynamical features of interactions with the SWCNT, such as how the polymers prefer to interface with the SWCNT and how the interfacial interaction is affected by the chemical composition and structure of the polymer. The simulations indicate that polymers with stiff and semiflexible backbones tend to wrap around the SWCNT with more distinct conformations than those with flexible backbones. Aromatic moieties along the backbone appear to dictate the adsorption conformation, which is likely due to the preference for optimizing pi-pi interactions, although the presence of bulky aliphatic side chains can hinder those interactions. Moment of inertia plots as a function of time indicate that the adsorption of polymers with stiff backbones tends to be a two-step process, in contrast to flexible backbones.

Dissipative particle dynamics of triblock copolymer melts: a midblock conformational study at moderate segregation.

As thermoplastic elastomers, triblock copolymers constitute an immensely important class of shape-memory soft materials due to their unique ability to form molecular networks stabilized by physical, rather than chemical, cross-links. The extent to which such networks develop in triblock and higher-order multiblock copolymers is sensitive to the formation of midblock bridges, which serve to connect neighboring microdomains. In addition to bridges, copolymer molecules can likewise form loops and dangling ends upon microphase separation or they can remain unsegregated. While prior theoretical and simulation studies have elucidated the midblock bridging fraction in triblock copolymer melts, most have only considered strongly segregated systems wherein dangling ends and unsegregated chains become relatively insignificant. In this study, simulations based on dissipative particle dynamics are performed to examine the self-assembly and networkability of moderately segregated triblock copolymers. Utilizing a density-based cluster-recognition algorithm, we demonstrate how the simulations can be analyzed to extract information about microdomain formation and permit explicit quantitation of the midblock bridging, looping, dangling, and unsegregated fractions for linear triblock copolymers varying in chain length, molecular composition, and segregation level. We show that midblock conformations can be sensitive to variations in chain length, molecular composition, and bead repulsion, and that a systematic investigation can be used to identify the onset of strong segregation where the presence of dangling and unsegregated fractions are minimal. In addition, because this clustering approach is robust, it can be used with any particle-based simulation method to quantify network formation of different morphologies for a wide range of triblock and higher-order multiblock copolymer systems.

Communication: Molecular-level insights into asymmetric triblock copolymers: Network and phase development

Molecularly asymmetric triblock copolymers progressively grown from a parent diblock copolymer can be used to elucidate the phase and property transformation from diblock to network-forming triblock copolymer. In this study, we use several theoretical formalisms and simulation methods to examine the molecular-level characteristics accompanying this transformation, and show that reported macroscopic-level transitions correspond to the onset of an equilibrium network. Midblock conformational fractions and copolymer morphologies are provided as functions of copolymer composition and temperature.

Molecular dynamics simulations of interactions between polyanilines in their inclusion complexes with β-cyclodextrins.

Conductive polymers have several applications such as in flexible displays, solar cells, and biomedical sensors. An inclusion complex of a conductive polymer and cyclodextrin is desired for some applications such as for molecular wires. In this study, different orientations of β-cyclodextrin rings on a single polyaniline (PANI) chain in an alternating emeraldine form were simulated using molecular dynamics. The simulations were performed in an implicit solvent environment that corresponds to experimental conditions. When the larger opening of the β-cyclodextrin toroids face the same direction, the cyclodextrins tend to repel each other. Alternating the orientation of the β-cyclodextrins on the chain causes the β-cyclodextrin rings to be more attractive to one another and form pairs or stacks of rings. These simulations explain how the β-cyclodextrins can be used to shield the polyaniline from outside chemical action by analyzing the PANI/cyclodextrin interactions from a molecular perspective.

Physical Microfabrication of Shape-Memory Polymer Systems via Bicomponent Fiber Spinning.

As emerging technologies continue to require diverse materials capable of exhibiting tunable stimuli-responsiveness, shape-memory materials are of considerable significance because they can change size and/or shape in controllable fashion upon environmental stimulation. Of particular interest, shape-memory polymers (SMPs) have secured a central role in the ongoing development of relatively lightweight and remotely deployable devices that can be further designed with specific surface properties. In the case of thermally-activated SMPs, two functional chemical species must be present to provide (i) an elastic network capable of restoring the SMP to a previous strain state and (ii) switching elements that either lock-in or release a temporary strain at a well-defined thermal transition. While these species are chemically combined into a single macromolecule in most commercially available SMPs, this work establishes that, even though they are physically separated across one or more polymer/polymer interfaces, their shape-memory properties are retained in melt-spun bicomponent fibers. In the present study, we investigate the effects of fiber composition and cross-sectional geometry on both conventional and cold-draw shape memory, and report surprisingly high levels of strain fixity and recovery that generally improve upon strain cycling.

Visualization of the Molecular Dynamics of Polymers and Carbon Nanotubes

Research domains that deal with complex molecular systems often employ computer-based thermodynamics simulations to study molecular interactions and investigate phenomena at the nanoscale. Many visual and analytic methods have proven useful for analyzing the results of molecular simulations; however, these methods have not been fully explored in many emerging domains. In this paper we explore visual-analytics methods to supplement existing standard methods for studying the spatial-temporal dynamics of polymer-nanotube interface. Our methods are our first steps towards the overall goal of understanding macroscopic properties of the composites by investigating dynamics and chemical properties of the interface. We discuss a standard computational approach for comparing polymer conformations using numerical measures of similarities and present matrix- and graph-based representations of the similarity relationships for some polymer structures.

Exploration of polymer conformational similarities in polymer-carbon nanotube interfaces

We are using molecular simulations to investigate the interface between the polymer matrix and the carbon nanotube reinforcement, which is the key aspect of the bulk properties of nanocomposites. These simulations are typically analyzed with standard techniques like graphs and animations; however, existing methods are limited for certain exploratory tasks for analyzing the interfacial domains. We present a supplemental exploratory approach that employs standard effective visual-analytical techniques to analyze spatial and temporal properties of the polymer-carbon nanotube interfaces. Our approach is based on a computational method that uses a numerical measure of similarity to compare multiple molecular conformations. We discuss some numerical measures for exploring the behavior of polymer molecules in interfacial domains and present a matrix-based visualization to display and explore local and global similarity relationships of the polymer structures, including dynamical aspects. These methods constitute our initial efforts for using visual-analytical tools to relate the interfacial dynamics to macroscopic properties of the nanocomposite interfaces.