Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session H59: Dillon Medal SymposiumFocus Prize/Award
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Sponsoring Units: DPOLY Chair: Nitash Balsara, Univ of California - Berkeley Room: LACC Petree Hall D |
Tuesday, March 6, 2018 2:30PM - 3:06PM |
H59.00001: John H. Dillon Medal Talk: Skipping Polymer Physics Invited Speaker: Bradley Olsen Associative polymer networks are one of the most pervasive categories of materials in polymer physics; however, self-diffusion in these systems has not been widely studied. Recent experimental measurements using forced Rayleigh scattering (FRS) from our lab have shown an apparent super-diffusive regime that is unexpected based on current theories but is well modeled by a simple empirical model for exchange of the polymers between fast and slow diffusing states. Brownian dynamics simulations suggest that these fast and slow states may be free molecules and physically attached molecules, suggesting that associative polymers may diffuse in a combination of hops and walking steps analogous to locomotion by skipping. Both the empirical two-state model and the simulations show mean square displacements that are linear in time; the superdiffusive regime observed by FRS results from the fact that there are fast and slow components to the diffusion. Beyond the obvious implications for systems such as self-healing elastomers and injectable biomedical gels where relevant properties are strongly influenced by the rate of self-diffusion, these results provide a potential pathway to a completely new mechanism of filtration in polymeric media. Inspired by a family of proteins called nucleoporins that regulate transport into the nucleus, we have developed a theoretical framework and demonstration materials that suggest how physical associations with a stationary medium may accelerate overall flux. The key hypothesis is that even though skipping is slower than free diffusion, an enhanced concentration of skipping molecules can lead to a large increase in flux. This insight suggests molecular designs for replicating the properties of nucleoporins in synthetic polymer gels. |
Tuesday, March 6, 2018 3:06PM - 3:18PM |
H59.00002: Impact of Helical Chain Shape on Polypeptoid Block Copolymer Self-Assembly Rachel Segalman, Emily Davidson, Anastasia Patterson, Adrianne Rosales, Beihang Yu, Ronald Zuckermann Controlling the self-assembly of block copolymers with variable chain shape and stiffness is important for both driving the self-assembly of functional materials containing non-ideal chains, as well as for developing materials with new mesostructures. In sequence-defined polypeptoids the backbone secondary structure can be controlled by the choice and tacticity of the side chains. Bulky, homochiral side chains are introduced at specific locations along the chain to form a helical secondary structure, or a racemic mixture of side chains forms a disordered chain shape. In a series of poly(peptoid-b-(n-butyl acrylate) polymers, block copolymers with a helical block display larger domain spacings than those with a Gaussian coil block. The increases in domain spacing are similar expectations from introducing a stiffer polymer chain that fills a larger conformational volume and has larger penalties for chain stretching. However, while the polypeptoid helix has a persistence length longer than that of the unstructured polymer chain, it is actually less space-filling. We attribute the changes in domain structure to a combination of packing frustrations and increased penalties for stretching the helix relative to the disordered chain. |
Tuesday, March 6, 2018 3:18PM - 3:30PM |
H59.00003: Fatty Acid-Based Triblock Copolymers with a Transient Network Megan Robertson The self-assembly and physical properties of triblock copolymers containing long-chain polyacrylate midblocks have been explored. Long-chain polyacrylates, which may be derived from fatty acids found in vegetable oils, are attractive as sustainable materials with biorenewable origins. The long alkyl side-chains of these polyacrylates provide a route to tunable physical behavior, yet also hinder their application due to lack of entanglements. Incorporation of a transient network was explored as a method to improve mechanical properties in triblock copolymers containing a midblock composed of a random copolymer of lauryl acrylate, a derivative of lauric acid, and acrylamide, which undergoes hydrogen bonding. Poly(styrene-b-(lauryl acrylate-co-acrylamide)-b-styrene) triblock copolymers exhibited spherical morphologies and elastomeric behavior. The polymers underwent a high extent of hydrogen bonding, which greatly impacted their domain size, order-disorder transition temperature, and rheological and mechanical behavior. Importantly, triblock copolymers with hydrogen bonding in the matrix exhibited significantly higher modulus, strain at break, and tensile strength as compared to polymers in the absence of hydrogen bonding. |
Tuesday, March 6, 2018 3:30PM - 3:42PM |
H59.00004: Mechanisms Underlying Ion Transport in Polymerized Ionic Liquid-Ionic Liquid Blends Venkatraghavan Ganesan, Santhosh Mogurampally We use atomistic molecular dynamics simulations to study molecular mechanisms underlying ion transport in ionic liquid electrolytes with varying loading of their counterpart polymerized electrolytes. Our results reveal that in pure polymerized ionic liquid systems, anion transport occurs along the polymer backbone via the formation and breaking of the ion-pairs involving four polymerized cationic monomers of two different polymer chains. For such situations, we demonstrate that the anion mobilities are quantitatively correlated to the lifetime of anion-polymerized cation pairs. Surprisingly, even for the blend systems, we observe that the anions associated with the polymerized cation move by the same mechanism unearthed for the pure polymerized ionic liquids. For the blend systems the anion mobilities are shown to be correlated to the ion-pair relaxation time of the non-polymerized cation-anion pairs. |
Tuesday, March 6, 2018 3:42PM - 3:54PM |
H59.00005: Block Polymer Chiral Networks Edwin Thomas, Mujin Zhuo, Gregory Grason, Ishan Prasad Structures having nanoscale 3D geometries are valuable as multifunctional materials, where multi-continuous microphases can synergistically influence mechanical, optical, transport and other properties. Due to the ability to individually tailor the properties of the network(s) and matrix, for example, to create strong dielectric or impedance contrast, such spatially periodic structures are excellent for the interference of waves (electromagnetic for photonic applications and acoustic for phononic applications) that can lead to bandgaps and hence the control of wave propagation in the material. Quantitative morphological characterization of such complex geometric structures is quite challenging. Unit cell sizes range from 10-300nm with corresponding feature sizes on the 2-50nm scale. In order to understand optical properties for periodic chiral phases such as the single gyroid, we need to be able to measure the local features such as the set of dihedral angles between successive nodes in the network. 3D tomographic reconstructions can be done by slice and view scanning electron microscopy to allow access to the network connectivity and geometry. |
Tuesday, March 6, 2018 3:54PM - 4:06PM |
H59.00006: Quantifying Lithium Salt and Polymer Density Distributions in Ion-Conducting Block Polymers Thomas Epps, Thomas Gartner, Melody Morris, Cameron Shelton Block polymer (BP) electrolytes offer an enhanced balance of performance and stability for next-generation lithium-ion batteries, but to develop these technologies further, it is essential to understand both the overall self-assembly behavior and the distinct salt and polymer distributions within the specific nanoscale domains. To this end, we have quantitatively examined the lithium salt and polymer distributions in salt-doped polystyrene-b-poly(oligo-oxyethylene methacrylate) [PS-POEM] BPs and explored how the presence of salt affects BP chain conformations and thermodynamics. With respect to the lithium salt distribution, neutron reflectometry was leveraged to infer the salt distributions in POEM domains for a series of lithium salts. All salt-doped BP specimens exhibited lithium salt distributions that were strongly correlated with the POEM density profiles. Furthermore, using X-ray reflectometry in combination with strong-segregation theory, we estimated effective Flory-Huggins interaction parameters (χeff) and the POEM statistical segment lengths (bPOEM) as a function of salt concentration and rationalized the noted trends in χeff and bPOEM in terms of lithium counterion basicity. |
Tuesday, March 6, 2018 4:06PM - 4:18PM |
H59.00007: Curvilinear Dynamics of Topologically Constrained Polymers Zhen-Gang Wang, Ahmad Omar The current physical picture for topologically constrained polymer dynamics is that the motion of a chain is constrained to a curvilinear path dictated by the surrounding topology – with diffusion along this path proceeding via (unconstrained) Rouse dynamics. In this talk, we present results on a micromechanical version of an early model system for topologically constrained polymers – a two-dimensional Rouse chain in the presence of point-like obstacles – where the curvilinear path and unconstrained Rouse motion are defined exactly. In linear response, we find that the obstacles introduce an entropic barrier to curvilinear motion (on length scales comparable to the obstacle spacing), manifested as an increased timescale for curvilinear relaxation. While in linear-response one can interpret this topological “friction” as a shift-factor, non-Rouse physics emerges in both the transient and steady state response to weakly non-linear microrheological deformation. For example, upon dragging a chain-end at a constant velocity, we find an initial and prolonged elastic response that is in marked contrast to Rouse physics. We discuss the origin and possible consequences of these curvilinear topological effects. |
Tuesday, March 6, 2018 4:18PM - 4:30PM |
H59.00008: Mega-supramolecules Julie Kornfield, Simon Jones, jeremy wei Guided by the statistical mechanics of ring-chain equilibrium, we designed polymers |
Tuesday, March 6, 2018 4:30PM - 4:42PM |
H59.00009: Consequences of Grafting Density on Bottlebrush Rheology Frank Bates, Ingrid Haugan, Michael Maher, Alice Chang, Tzu-Pin Lin, Robert Grubbs, Marc Hillmyer The linear viscoelastic behavior of poly(norbornene dimethyl ester)-g-poly(DL-lactide) graft polymers was investigated as a function of grafting density, ranging from 0 to 100 percent, and backbone molecular weight at constant graft chain molecular weight. Master curves reveal that the zero-shear viscosity of these polymers display a sharp transition from Rouse to reptation scaling, demonstrating that grafting density strongly impacts the entanglement molecular mass. The scaling of the entanglement plateau modulus (Ge) as a function of the grafting density was compared to theoretical predictions. Polymers with high and low grafting density were found to be consistent with theoretical models for dense brushes (Ge ~ ng1.2) and loose combs (Ge ~ ng0). Graft polymers with intermediate grafting densities did not fit existing models for graft polymers (Ge ~ ng3.5), including loose brushes and dense combs. The strong dependency of Ge in the intermediate regime reflects an increase in flexibility, which is attributed to backbone “kinking” to space out the side chains as grafting density increases. |
Tuesday, March 6, 2018 4:42PM - 4:54PM |
H59.00010: Double-Semidilute Liquid and Gel Coacervates formed by Oppositely Charged Polyelectrolytes Michael Rubinstein, Sergey Panyukov, Qi Liao We develop a scaling model for two qualitatively different classes of coacervates formed by oppositely charged polyelectrolytes. The weakly interacting coacervates are liquids with electrostatic interaction energy per charge less than thermal energy kT. The strongly interacting coacervates are gels with cross-links formed by ion pairs of opposite charges attracting each other with energy stronger than kT. The liquid coacervate is a double-semidilute solution with two correlation lengths and two qualitatively different types of conformations of weaker and stronger charged polyelectrolytes. Weaker charged chains form a screening “coat” around stronger charged chains. The conformations of weaker charged chains in this screening coat is analogous to a semidilute solution of uncharged polymers. The conformation of stronger charged polyelectrolytes in liquid coacervates is similar to their conformation in semidilute polyelectrolyte solutions. The strongly interacting coacervates form bottlebrush gels for longer polyelectrolytes with higher charge density and star-like gels for shorter higher charge desity chains. |
Tuesday, March 6, 2018 4:54PM - 5:06PM |
H59.00011: Undoped, Non-conjugated Radical Polymer Glasses with High Electrical Conductivity Values Bryan Boudouris Radical polymers have shown promise with respect to their application as transparent conducting macromolecules; however, their ultimate electronic performance has been stymied due to the disorder associated with their solid-state packing. In order to create an amorphous radical polymer with a glass transition temperature near room temperature poly(4-glycidyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl) (PTEO) was synthesized. In generating a redox-active macromolecule with a flow temperature far-removed from the degradation temperature of the radical sites within the polymer, annealing of the radical polymer thin film resulted in the formation of locally-ordered (yet non-crystalline), high-conductivity domains. Because of this local order in the glassy state, a greater than 1,000-fold increase is observed in the electrical conductivity of PTEO relative to all other reports of electrical conductivity in radical polymers. Moreover, the ultimate conductivity of ~20 S m-1 places this undoped polymer conductor in the same regime as many grades of commercially-available, chemically-doped conducting polymers. Thus, these data open new insights into the physical mechanism of charge transport in radical polymer thin films, and provide a means by which to probe local order in glassy polymers. |
Tuesday, March 6, 2018 5:06PM - 5:18PM |
H59.00012: Tuning the Persistence Length of Semiflexible Methylcellulose Chains by Grafting Timothy Lodge, Sveta Morozova Low molecular weight thiol-terminated poly(ethylene glycol) has been grafted onto a high molecular weight methylcellulose by a facile thiol-ene click reaction; graft densities varied from 0.7% to 33% (grafts per anhydroglucose unit). Static and dynamic light scattering reveals that the overall radius of the chain increases systematically with graft density, in a manner in excellent agreement with excluded volume theory. As the underlying contour length remains unchanged, it is apparent that grafting leads to an increase in the persistence length, by as much as a factor of four. These results represent the first experimental verification of the excluded volume theory at low grafting densities, and demonstrate a promising synthetic platform for systematically increasing the persistence length of a model water-soluble semiflexible polymer. |
Tuesday, March 6, 2018 5:18PM - 5:30PM |
H59.00013: Bio-inspired metal-coordination crosslinking: easy access to broad dynamics when engineering polymer gel mechanics Niels Holten-Andersen Efforts to engineer polymer material mechanics is increasingly coupled to the design of transient crosslink dynamics. We have sought to gain a deeper understanding of how polymer gel mechanical properties can be controlled over multiple hierarchical time-scales via design of bio-inspired metal-coordinate crosslink structure on multiple length-scales. By utilizing metal ion-coordination complexes and metal nanoparticle-coordination junctions as supra-molecular crosslink structures, we have gained unique access to network dynamics on the microscopic scale, and thereby opportunities to broadly shape the distribution of network stress relaxation on the macroscopic scale. Our findings offer deeper insights on how to engineer gel stress relaxation mechanics directly via design of supramolecular crosslink structure dynamics, and could help improve our understanding of spatio-temporal molecular hierarchy in loadbearing biological materials. |
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