Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session N18: Single-Molecule Characterization of Polymers and Soft Matter I: Heterogeneous and Crowded EnvironmentsFocus Recordings Available
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Sponsoring Units: DPOLY Chair: Danielle Mai, Stanford University Room: McCormick Place W-184D |
Wednesday, March 16, 2022 11:30AM - 11:42AM |
N18.00001: Single particle tracking of nanoparticle probes in homogenous and heterogenous polymer hydrogels Katie A Rose, Daeyeon Lee, Russell J Composto Hydrogels are hydrophilic polymeric networks found extensively in nature and in industrial applications. Many applications of hydrogels depend on understanding the dynamics and the underlying transport mechanisms of probes at the nanoscale in which there are often spatial heterogeneities on the length scale of the diffusing probe. Single particle tracking is a method to investigate particle dynamics at the nanoscale which retains the spatiotemporal characteristics of the diffusing probe, enabling the analysis of nanoscale heterogeneities. In our work, we use sub-10 nm quantum dots to investigate the effect of hydrogel microstructure, gelation and crosslinking density on nanoparticle dynamics. In particular, we find that pH-mediated interactions dominate probe dynamics in the hydrogel nanocomposites and rods diffuse faster than comparable spheres during the gelation of tetra-poly(ethylene glycol). This work offers important insights for predicting and controlling nanoparticle dynamics in gels, applicable to drug delivery and nanoscale filtration. |
Wednesday, March 16, 2022 11:42AM - 11:54AM |
N18.00002: Observing Polymer Clustering in Concentrated Solutions Using Single Particle Tracking Harrison Landfield, Muzhou Wang Non-covalent interactions governed by factors such as hydrophobicity and dipole interactions allow associating polymers to self-assemble into structures that assist in applications such as contaminant removal and drug delivery. Using single particle tracking (SPT) diffusion studies, we show that Poly(poly(ethylene glycol) Methacrylate) is a hydrophobically associating polymer, where polymer chains may diffuse individually or in clusters. Aided by the single-molecule resolution of SPT, we observe the mean square displacements of polymer chains in solution show a clear two-population division, with each population individually analyzable. This bifurcation is also seen in the distributions of final polymer displacements and the Van Hove distributions for the system. While the population of unclustered polymers shows Fickian diffusion, subdiffusive, inhibited diffusion is observed in the clustered population. Clustering behavior can be tuned by altering concentration and molecular weight of polymer, as well as the polarity of the solvent. Combined with Dynamic Light Scattering, which confirms polymer clustering as well as supplies diffusion data, and 1H NMR Diffusion-Ordered Spectroscopy, diffusion regimes up to semidilute entangled regime are experimentally observed. |
Wednesday, March 16, 2022 11:54AM - 12:06PM |
N18.00003: Single Particle Imaging of the Assembly of Elongated Nanoparticles at a Liquid Interface Satyam Srivastava, Alexander E Ribbe, David Hoagland, Thomas P Russell Despite conceptual and technological importance, the attachment and subsequent ordering of anisotropic particles at liquid interfaces remains poorly understood. Here, in situ scanning electron microscopy provided insights into the near two-dimensional interfacial assembly of model PEG-grafted anisotropic nanoparticles (NPs) of both ellipsoidal and sphero-cylindrical shape; aspects ratios varied from 2 to 7. Due to PEG's lower surface energy, bulk NPs segregated to the vacuum-liquid interface, jamming at high interfacial areal density after hours or days. The positions and orientations of individual NPs were unambiguously visualized throughout, enabling real-time characterization of nematic and smectic order parameters along with orientational correlation functions. The NPs formed long, persistent side-by-side stacks, as presumably driven by capillary forces. PEG length, as well as solvent surface tension, altered NP interactions, with stronger attractions kinetically trapping co-existing regions of orientational order and disorder. Ellipsoids and rods behaved similarly. |
Wednesday, March 16, 2022 12:06PM - 12:42PM |
N18.00004: Molecular Electron Videography of Heterogeneous Polymer Networks and Protein Transformation Invited Speaker: Qian Chen I will discuss my group’s recent efforts on “molecular electron videography” integrating liquid-phase transmission electron microscopy (TEM) imaging and molecular dynamics (MD) simulation. Together they can reveal molecular level structural features and dynamics of macromolecular systems in a native liquid environment via direct real-space and real-space imaging at nanometer resolution. I will cover two examples. The first is solvated polymer networked nanoparticles, whose spatial heterogeneity is mapped to reveal characteristic sizes of “blobs” of a few nanometers and tens of nanometer. We back map these blobs to MD simulated atomic coordinates to reveal the substructural motifs corresponding to the blobs and show these structural features affect the conformation change of the nanoparticles, which impact their applications in delivery and biomedicine. The second system is on membrane protein nanodiscs, where we are able to measure the nanodiscs mechanics from real-space fluctuations and showcase a few rare events of the protein breathing out of its most stable confirmation. We foresee the platform of molecular electron videography to provide molecular insights into solution-phase dynamics at the nanoscale. |
Wednesday, March 16, 2022 12:42PM - 12:54PM |
N18.00005: Experimental determination of the boundary between the topologically frustrated dynamical state and reptation regime Kuo Chen, Murugappan Muthukumar The movement of a polymer chain trapped in spatially restricted media is usually described by the classical Ogston model, entropic barrier model, and reptation model sequentially as the degree of confinement is increased. The newly discovered topologically frustrated dynamical state emerges at intermediate confinements before the onset of reptation regime, where the relaxation time of the whole polymer chain far exceeds the practical time scales of experimentation. The dynamically localized topologically frustrated state at intermediate confinements has recently been shown to arise from the collective behavior of multiple entropic traps acting on a single chain. Particle tracking in the presence of an external electric field has revealed that the net entropic barrier is on the orders of tens of kBT. Under what conditions does this entropic barrier become small so as to enable entanglement effects to dominate? To address this crossover between the topologically frustrated dynamical state and the entangled (reptation) regime, we have experimentally measured the entropic barrier of λ-DNA trapped in the polyacrylamide-co-sodium acrylate (PAM-co-NaAc) gel matrix with a wide range of cross-link densities based on single-molecule electrophoresis. We report that the dependence of the entropic barrier on cross-link density is nonmonotonic with a maximum at an intermediate cross-link density. The entropic barrier becomes weak (comparable to kBT) at a higher cross-link density which marks the boundary between the topologically frustrated dynamical state and the reptation regime. |
Wednesday, March 16, 2022 12:54PM - 1:06PM |
N18.00006: Transverse fluctuations and conformations of a semi-flexible chain in a crowded environment Jacob Bair We study how the transverse fluctuations of a semi-flexible chain are affected by the presence of other excluded volume (EV) particles as a function of the polymer chain length L, the peristence length lp , and the density of the EV particles. In absence of the EV particles, previously we have established universal scaling relations for the scaled end-to-end distance 〈RN2〉/2Llp and the scaled transverse fluctuations 〈l⊥2〉1/2/L respectively, as a function of the scaled distance L/lp (A. Huang, A. Bhattacharya, and K. Binder, J. Chem. 140, 214902 (2014)). We explore the validity of these universal scaling relations as a function of the density of the EV particles. Our preliminary results indicate that the crowding density only weakly affects the universal scaling relations. We discuss the implication of this result in living matter. |
Wednesday, March 16, 2022 1:06PM - 1:18PM |
N18.00007: Effects of self-assembly on the conformational chirality of block copolymers Poornima Padmanabhan, Natalie Buchanan Advances in polymer synthesis have led to the development of chiral polymer molecules. When incorporated into a block copolymer architecture, these molecules are capable of exhibiting chirality transfer. That is, chirality at the molecular scale is transferred to the conformational length scale, and ultimately to the mesostructural length scale. Thermodynamic descriptions of the mechanisms of chirality transfer are not fully understood. In this work, we employ particle-based simulations to develop a versatile and tunable model for chiral polymers. A wide range of conformations, ranging from a random-coil to a perfect helix, are obtained for a single chain by tuning suitable interaction parameters. We quantify experimentally measurable conformational characteristics for the single chiral polymer molecule. Subsequently, we incorporate the helical block in a block copolymer and study the self-assembly of a melt of thousands of molecules in the lamellar morphology to examine the impact of the morphology on the conformations of the helical blocks. Surprisingly, we find that the morphological constraints result in quantifiably lower helical nature of the molecules in the melt, compared to the isolated single chain. This suggests a tradeoff occurs, where enthalpic contacts between unlike monomers are reduced, but chain conformations do not minimize their internal energy. |
Wednesday, March 16, 2022 1:18PM - 1:30PM |
N18.00008: Automated Synthesis for Single-Molecule Electronics Edward R Jira, Songsong Li, Nicholas H Angello, Jialing Li, Jeffrey S Moore, Martin D Burke, Charles M Schroeder The development of next-generation organic electronic materials critically relies on understanding structure-function relationships in conjugated polymers. However, unlocking the full potential of organic materials requires access to their vast chemical space while efficiently managing the large synthetic workload to survey new materials. In this talk, we report the use of a new automated synthesis platform to prepare large libraries of conjugated oligomers for molecular electronic applications. Molecular libraries are designed with systematically varied backbone and side chain composition. Single-molecule charge transport measurements reveal that molecular junctions with long alkyl side chains exhibit a concentration-dependent bimodal conductance with an unexpectedly high conductance state that arises due to surface adsorption and backbone planarization. These results are supported by a series of control experiments using asymmetric, planarized, and sterically hindered molecules. Density functional theory simulations and additional control experiments using different anchors and alkoxy side chains highlight the role of side chain chemistry on charge transport. Overall, this work opens new avenues for using automated synthesis for the development of organic electronic materials. |
Wednesday, March 16, 2022 1:30PM - 1:42PM |
N18.00009: Investigating Sequence-Conformation Relationship Using Polypeptoids: A Single-Molecule Force Study Hoang P Truong, Rachel A Segalman, Omar A Saleh The effects of sequence on the structure and dynamics of a polymer chain are of interest to many research fields, such as block copolymer self-assembly or protein physics. Single-molecule force experiments offer a unique and unobstructed approach to studying these effects by extracting key polymer physics parameters of intramolecular electrostatic and hydrophobic interactions that influence the global chain structure. Here, we present the use of polypeptoids, an analog of polypeptides, as a model to isolate and investigate the effect of charge sequence on polymer conformations by designing simple sequences with only two types of side chains: neutral hydrophilic and charged. Using high-resolution single-molecule magnetic tweezers that can access subpicoNewton forces, we found a clear trend in the effect of net charge and charge spacing on the conformational behaviors of the polypeptoids. Overall, this work elucidates the electrostatic sequence-conformation relationship of polymers, which better understanding of the physical principles of biopolymers' in vivo behaviors and guide the design of new polymeric materials. |
Wednesday, March 16, 2022 1:42PM - 1:54PM |
N18.00010: Chain Tension Dependence of the Kuhn Length Liel Sapir, Danyang Chen, Michael Rubinstein Flexible polymers are characterized by their Kuhn length, the statistical segment of the chain. The Kuhn length is typically determined from the chain size measured in a theta solvent by scattering experiments, bscat. Another method is single-molecule force spectroscopy, with force-extension curves fitted by a model in which the Kuhn length, bpull, is a fitting parameter. We explain the long-standing discrepancy that the Kuhn lengths obtained by these two methods typically differ by factor of two: bscat/bpull~2. The reason is that flexible polymers in theta solvents are quasi-ideal with long-range correlations resulting in n-dependence of the Kuhn length of n-mers, approaching its infinite-chain value as ~n-1/2. These correlations are cutoff at the size of the tension blob in chains under tension, resulting in the effective Kuhn length, beff, decreasing with increasing tension. We present a theory for the dependence of the Kuhn length on the applied force, use it to correct the modified Freely Jointed Chain (mFJC) model, and verify this dependence by molecular dynamics simulations. |
Wednesday, March 16, 2022 1:54PM - 2:06PM |
N18.00011: Inter-DNA attraction mediated by monovalent ions Rajesh Sharma, Ishita Agrawal, Liang Dai, Patrick Doyle, Slaven Garaj The compaction of negatively charged DNA is, in some cases, driven by crowding neutral polymers, while monovalent ions purportedly only act to screen inter-DNA repulsion. On the other hand, multivalent ions govern DNA condensation through strong electrostatic inter-DNA attraction. In the past, tantalizing hints implying monovalent ions induced DNA attraction was never proven due to the lack of a desirable technique. Here we unveil the existence of monovalent ions mediated effective attraction interactions between DNA molecules at high salt concentrations. We employ a new nanopore-based sensing technique to precisely quantify the knotting probability of long DNA chains, which is a very sensitive measure to inter-DNA interactions. Modelling effective DNA width from the knotting data, we demonstrate that the DNA-DNA interaction evolves from repulsive to attractive with increasing salt concentration, with the onset of attraction at the critical ionic concentration ranging between C?????? = 1.5?? − 2??. The C?????? corresponds to mean ionic distance approaching the hydration ion radius and scales well with the hydration radius of different salts indicating ion-induced correlations as the driving force for attraction. Our results provide valuable insights into behavior of DNA in the physiological environment and will be useful for the development of synthetic biological machinery in high salt monovalent ionic environments. |
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