Session M33: Dynamics of Polymers and Polyelectrolytes I

Oral: Elucidating nanoparticle reinforcing effects through low-volume chemical coupling as explored by coarse-grained molecular dynamics

The addition of linker molecules with varying topology and stoichiometry into a carbon nanotube (CN)-reinforced polymer composites exhibits pronounced morphological differences and resulting rheological properties compared to CN-reinforced only controls. The underlying mechanism of action is complex because the mesoscale properties evolve over a broad range of interrelated lengths and time scales. These originate at the local structure of the crosslinking junction, extending into the segmental motion in the Kuhn length regime and finally influencing the large-scale diffusive motions proportional to the radius of gyration. In this study, we present an easily scalable approach to elucidate the effect of linker topology and stoichiometry on the segmental dynamics and bulk properties in a covalently bonded CN polymer composite. We performed a series of coarse-grained molecular dynamics (CGMD) simulations to investigate the morphological and rheological performance of polymers in response to linker topology and crosslink reactions. Our CGMD results indicate that the degree of matrix phase separation is positively correlated with polymer segmental diffusivity, which can be limited through increasing linker rigidity and cross-linking. The CGMD results, which agree well with experimental measurement, can provide guidelines for the a priori design of CN-reinforced composite materials.   

Electrophoretic translocation of star-shaped polymers in single solid-state nanopore

The translocation of linear polymers in nanopores has been extensively studied, while the investigation of polymers with diverse architectures, particularly star-shaped polymers, has remained relatively less explored. One of the main challenges lies in the complexity of sample preparation. In this study, we employ multi-armed poly(ethylene glycol) (PEG) to address this challenge in the investigation of electrophoretic translocation behavior of star polymers. The influences of nanopore size (d), applied voltage (V_m), and the number of arms (f) on the dwell time (τ), polymer capture rate, and the conformation of polymer during translocation are examined. The results revealed a linear correlation between capture rate and fV_m, while τ demonstrates a linear relationship with f/V_m. Intriguingly, star-shaped polymers with f_in > f_out exhibit potential for successful translocation, contrary to previous findings where f_in < f_out, where f_in and f_out correspond to the number of arms inside and outside the nanopore when the polymer chain being captured. Notably, the optimal f_in increased from 0.25f to 0.5f as the pore size increased, where the degree of confinement (R_h/d) decreased. These findings offer new insights into the translocation behavior of star-shaped polymers in confined environments and highlight PEG-based polymers as an effective model for studying polyelectrolyte dynamics.   

Role of the Brush Sizes and Sidechain architecture on Linear Polymer Bottlebrushes: an atomistic simulation.

Bottlebrush polymers (BBPs) are unique and versatile macromolecules used in material science, electronics, battery materials, self-healing, and more. In this paper, using all atom molecular dynamics simulations, we studied the structural and hydration properties of Poly (methyl methacrylate) -g- poly(2-ethyl-2-oxazoline) (PMMA-g-PEtOx) BBP, as well as the BBP supported water properties as a function of backbone monomers (N_BB) and sidechain monomers (N_SC). The radius of gyration follows a scaling of Rg~N_SC^0.38 for smaller grafts and Rg~N_SC^0.5-0.63 for longer grafts. The overall shape (quantified by anisotropy) of the bottlebrush goes from rod to sphere shape with increasing N_SC. Both hydration per sidechain monomer and hydrogen bonds per oxygen and nitrogen decrease with increasing N_SC, with water-oxygen HBs being much higher than water-nitrogen HBs. The BBP-supported water molecules had two ordered domains, one more structured and one less structured, with the former disappearing with an increase in NSC due to the localization of the sidechain in the interior of the BBPs. Finally, despite the highly negative partial charges of the oxygen and nitrogen atoms (of the sidechain monomers), water molecule dipole orientation distributions around these atoms show a neutral environment. Overall, we anticipate that our research will generate a substantial future interest in probing the various widely explored BBPs systems in greater atomic detail.   

Nanoscale dynamics of self-assembled lipid nanoparticles via SANS

The nano-scale dynamics of lipid nanoparticles (LNPs) for drug delivery play a large role in their function. Limited molecular motion is important for avoiding degradation prior to entering cells. Intracellularly, enhanced molecular motion is then vital for effective endosomal escape. As a self-assembled structure of four distinct lipid-based components and a nucleic acid cargo driven by both electrostatic and hydrophobic interactions, the system complexities make analysis of dynamics difficult. We experimentally quantify LNP nano-scale dynamics through time-resolved small-angle neutron scattering (TR-SANS). This technique, previously leveraged only for simpler systems, provides unprecedented insight to molecular behavior of LNPs. We find, using TR-SANS in combination with traditional SANS and small-angle x-ray scattering (SAXS), that as pH drops, internal structure and molecular exchange of cholesterol change. The results provide a quantifiable metric by which to compare formulations. Successful analysis of this complex system also expands the opportunities for using TR-SANS to characterize molecular exchange in other multi-component macromolecular systems.   

Macromolecular properties and interactions of proteins and polysaccharides determine the rheology of real and vegan food formulations

Proteins and polysaccharides frequently coexist in biological systems, and many food formulations that substitute animal-based proteins with vegan, plant-sourced alternatives incorporate polysaccharides as rheology modifiers. The extensional rheology response of these formulations is known to be highly associated with mouthfeel and consumer perceptions of texture, thickness, stringiness, and stickiness. Thus, assessing the roles played by macromolecular phenomena (e.g., complexation, aggregation, adsorption) as well as pH/salt in determining the response to stretching flows is critical in the design of sustainable food formulations. However, quantitative measurements of responses to streamwise velocity gradients for these complex systems has been a longstanding challenge. In this talk, we select proteins and polysaccharides used in several vegan food foams and emulsions, and characterize their pinching dynamics and extensional rheology using dripping-onto-substrate (DoS) rheometry. We elucidate how complexation transforms the perceived stringiness and both shear and extensional rheology response.   

Unlocking the Dynamics of a Novel Natural Polymer-Based Material for Multiple Applications

We have developed a unique polymer system using a natural polymer, which is a byproduct of the biorefinery industry. This material exhibits unusual viscoelastic behaviors, making it well-suited for multifunctional applications, including shape memory, self-healing, and thermally adaptive mechanical properties. These distinctive characteristics result from the careful tailoring of the macromolecular dynamics of the natural polymer, ranging from the nano to micro and macrostructure levels. A deep understanding of the dynamics of this complex polymer system is essential for controlling both its physical and functional properties. In this talk, I will discuss the potential of employing various characterization techniques, such as rheology in combination with neutron reflectivity, quasi-elastic neutron scattering, small-angle neutron scattering, and wide-angle X-ray diffraction, to investigate the dynamics of complex polymer systems at different length scales, both within bulk and interface systems.   

Direct Visualization of Flow-induced Scission of DNA

Scission of DNA into smaller fragments is an important step in next-generation sequencing, with hydrodynamic breakage being a preferred method. Previous studies on flow-induced scission of DNA (and polymers in general) have focused on observing the distribution of scission products to infer the breakage mechanism and kinetics. Few studies have directly visualized the breakage of individual molecules in a flow of controlled type. Here, we employ fluorescence microscopy to image the breakage of labelled DNA molecules in a microfluidic four-roll mill, which can generate different flow types. We focus on extensional flow with a stagnation point, where we observe the breakage dynamics of DNA and measure its breakage probability versus the local extensional strain rate. We compare these results with simple shear and examine how the rotational component of the flow affects breakage. Our findings shed light onto the flow-induced breakage dynamics of polymer molecules and are helpful in guiding the design of microfluidic devices for tunable fragment lengths.   

Behavior of two knotted DNA molecule under nanochannel confinement

In confined spaces like nanochannels, DNA molecules can form knots. While extant studies have predominantly focused on a single knot, DNA molecules are capable of hosting multiple knots. Herein, we use Langevin dynamics simulations to probe the behaviors of two knotted DNA molecules under nanochannel confinement of different channel sizes. Two distinct knot behaviors were observed: the knots either remained (i) separate or (ii) intertwined to form a prime knot that persisted for a long time duration. The intertwined state is commonly observed in most channel sizes and occurs spontaneously, with an energy preference above the separated state of approximately 0.5 k_BT. Additionally, the study explores the dynamic evolution between the knots, shedding light on the underlying knot passing mechanisms. Specifically, we found that the knot intertwining/passing mechanism is closely related to the knot expansion, where a knot enlarges to allow another knot to move along it. Moreover, we found that knot intertwining/passing is inhibited by small channel size, at which the knot expansion is also restricted.   

Topology-Driven Dynamics and Randomness Control in Prime Knots

Knots are entangled structures that cannot be untangled without a cut. Knots appear ubiquitously across length scales and play a key role in understanding and controlling the behaviors of complex systems. It has been known that entanglement structure alters polymer material's properties such as relaxation time, fragility, and viscosity. However, the precise link between structure and dynamics has not been well established. To help elucidate this link, we present a detailed analysis of the relation between the dynamics, topology, and complexity (crossing number) of polymer knots, focusing on a subset of closed knots called prime knots. We first identify three main motions of knots—orthogonal, aligned, and mixed motions—whose different compositions create unique dynamics for each knot. As knot complexity increases, we observe a gradual fading of dynamics, showing for the first time that connectivity alone can lead to topology-driven dynamical arrest in knots of high complexity. As knot size shrinks, knots undergo a transition from nearly stochastic motions to either non-random or "quasiperiodic" dynamics before culminating in dynamical arrest. Together, these findings demonstrate a clear link between structure and dynamics and present applications to biomedicine and nanotechnology.   

Computational Study of the Morphology of Benzimidazolium and Imidazolium Anion Exchange Membranes

Finding suitable alternatives to unsustainable energy sources is a pressing matter considering the challenge of climate change. Hydrogen fuel cell technologies are a promising option due to their high efficiencies and energy densities. However, the traditional implementations of fuel cells have been plagued by scalability issues due to reliance on fluorine-based materials and to high costs of noble metals such as platinum. Hydrocarbon-based anion-exchange membranes (AEMs) are being developed to address these issues. We have been studying several AEM candidates using both computational and experimental techniques with a focus on ionenes, where the charge is along the polymer backbone. In this work we compare the structure and ion dynamics in membranes based on a series of benzimidazolium and imidizolum backbones using techniques such as molecular dynamics simulations and clustering analyses in order to improve our understanding of the differences between candidate membranes.   

A Comprehensive Study of Azole Containing Systems with High Proton Conductivity under Anhydrous Conditions

As a device that is capable of transferring the chemical energy of hydrogen to electrical energy, the fuel cell is critical for achieving the net zero emissions goal. Among the components in a fuel cell system, the electrolyte is responsible for proton conduction, and its conductivity plays a central role in the overall performance of a fuel cell system. Currently, most proton conduction membranes work under aqueous conditions as water can promote proton conductions. However, this requirement not only inhibits the high-temperature application of fuel cells but also requires the fuel cell to be equipped with an internal humidifier. As a proton donor and acceptor, azole molecules are proposed to facilitate proton conduction through proton hopping. Thus, in this research, we studied 1,2,3-triazole and imidazole and their blends with phosphonic acid as potential candidates for high-conductivity proton conductors under anhydrous conditions. A high proton conductivity in the range of 100 mS/cm was obtained for one of the samples, which is on par with the conductivity of phosphoric acid-doped polybenzimidazole (PBI-PA) under anhydrous conditions. By combining viscosity and diffusion coefficient with conductivity, we find evidence for the proton hopping mechanism in these systems.   

Molecular Dynamics study on the dielectric relaxation of low relative permittivity and dissipation factor polymers

Microelectronic devices are typically supported on polymeric materials that must adapt to diverse working and environmental conditions. The suitability of the material for a specific application depends on its mechanical, thermal, electric, and chemical properties. In this work, we explore potential polymers for printed circuit board (PCB) substrates for next generation communication devices. Using Molecular Dynamic simulations, we perform dielectric spectroscopy, an experimental technique employed to characterize polymers under electromagnetic radiation. The polarization fluctuations in the materials are assessed by the thermal fluctuations of the individual dipoles, which allows us to calculate the complex permittivity response in a wide range of frequencies. We observe a low relative permittivity and dielectric loss for maileimides and siloxane structures with incorporated phenyl functional groups. Moreover, we found that the static permittivity and relaxation mechanism of the polymers are affected by the number and distribution of the aromatic groups along the backbone.   

Enhanced ion conduction by decoupling ion transport from polymer segmental relaxation in single-ion-conducting, polymer blend electrolytes

The sluggish polymer segmental relaxation in solid-state polymer electrolytes (SPEs) is known to constrain conductivity enhancements and limit overall performance. Improvements in conductivity can be realized through decoupling the ion transport from polymer segmental dynamics. In this work, we blended a glassy single-ion-conducting (SIC) polymer, poly[lithium sulfonyl(trifluoromethane sulfonyl)imide methacrylate] (PLiMTFSI), with a flexible polymer, poly(oligo-oxyethylene methyl ether methacrylate), at various compositions. We connected the ion transport mechanism to the packing efficiency of polymer chains and investigated the decoupled ion transport as a function of PLiMTFSI molecular weight and ion concentration via differential scanning calorimetry, alternating current impedance spectroscopy, and dynamic mechanical analysis. High ionic conductivities approaching 1 × 10^-2 S/cm were realized as a result of this decoupled ion transport. Additionally, immobilized TFSI^- resulted in high Li⁺ selectivity (Li⁺ transference number = 0.9), electrochemical stability (up to 4.7 V against Li⁺/Li, and limiting current density (1.8 mA/cm²), which exceeds many solvent-plasticized SIC polymer electrolytes. This electrolyte compared favorably to the benchmark SPE – polyethylene oxide, with desirable features that could support battery operation at higher voltages using energy-dense Li metal anodes.