Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session C01: 2020 Dillon Medal SymposiumFocus Live Prize/Award
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Sponsoring Units: DPOLY Chair: Richard Register, Princeton University |
Monday, March 15, 2021 3:00PM - 3:36PM Live |
C01.00001: John H. Dillon Medal (2020): Thermorheological Complexity at Polymer Surfaces Invited Speaker: Rodney Priestley A polymer glass surface is not so glassy, as there exists a layer with enhanced molecular mobility. Here, we present our results from combined experiments and simulations of creep at a polymer free surface. We reveal a distinct mode of molecular dynamics at a polymer free surface. This unique mode of surface dynamics causes chains near the free surface that are unentangled in the bulk to exhibit a mechanical response characteristic of bulk entangled polymers. In chains that are already entangled in bulk, this effect leads to prolonging of the entangled response. Moreover, in both cases, the breadth of the entangled response grows on cooling, leading to a breakdown in time-temperature-superposition at the free surface. |
Monday, March 15, 2021 3:36PM - 3:48PM Live |
C01.00002: Exploiting supramolecular associations in interpenetrating networks and elastomers LaShanda Korley, Chase Thompson Supramolecular interactions may hold the key to the development of network systems with tunable mechanics and modulated architecture, such as observed in the muscle protein titin. It is the dynamic nature of these physical associations that we have exploited in the design of tough supramolecular materials that super-impose covalent and non-covalent interactions to tailor tensile response. We have developed supramolecular elastomers and interpenetrating network (IPN) systems that probe the interplay of non-covalent and covalent interactions in structural organization and mechanical response. By tailoring physical associations via control of self-assembly and composition, we have demonstrated enhanced supramolecular dynamics driven by architecture and toughness enhancements due to phase behavior. Recently, non-covalent interaction strength, network regularity, and chemoresponsiveness have been utilized as handles to derive gradient materials and to induce actuation behavior. |
Monday, March 15, 2021 3:48PM - 4:00PM Live |
C01.00003: Droplet aggregates as model systems for connecting granular systems to continuum mechanics: how few is too few? Kari Dalnoki-Veress, Jean-Christophe Ono-dit-Biot, Johnathan Hoggarth In recent years we have developed a method to produce microscopic monodisperse oil droplets in an aqueous environment. With an attractive interaction between the droplets, monodisperse droplets form perfect crystalline aggregates, while a blend of small and large droplets allows us to prepare a disordered glass. By carefully tuning the adhesion forces between the droplets, the aggregates provide model systems for studying various physical phenomena that are not accessible by investigating molecular systems. Here I will provide a brief overview of experiments we have carried out to address two fundamental questions. How does a system transition from crystal to glass, when blending large and small droplets? And secondly, how does a system transition from a few particles, to many particles, where continuum models are valid. These experiments enable us to study the transition from few-to-many, and crystal-to-glass. |
Monday, March 15, 2021 4:00PM - 4:12PM Live |
C01.00004: Hybridization of a Bimodal Distribution of Copolymer Micelles Timothy Lodge, Dan Zhao The hybridization of two diblock micelles was studied by dynamic light scattering and SAXS/SANS. Two PMMA-b-PnBMA copolymers, where PnBMA and PMMA are the core- and corona-forming blocks, respectively, were employed in mixed ionic liquid solvents. Time-resolved SANS quantified the unimer exchange time for each copolymer as a function of solvent composition. The two micelle solutions in a given solvent were then mixed, and the structural evolution of the blended micelles was monitored. In more selective solvents, the apparent weight-average molecular weight (Mw) of the micelles initially increases with time. These observations reflect a net transfer of shorter chains from smaller to larger micelles, a but in a sense takes the system further away from equilibrium. The long-time evolution shows that hybridization depends greatly on solvent selectivity. In more selective solvents, Mw continues increasing even after months, while for less selective solvents, the micelle size eventually decreases and approaches equilibrium. However, this process must involve other relaxation mechanisms, e.g., micelle fusion/fragmentation or micelle creation/annihilation, as the total number of micelles also needs to be adjusted. |
Monday, March 15, 2021 4:12PM - 4:24PM Live |
C01.00005: Non-linear Deformation of Polymer Grafted Nanoparticles Ramanan Krishnamoorti The linear viscoelastic properties of polymer-grafted nanoparticles indicate that they behave like elastic solids with elastic moduli that scale (non-linearly) with concentration of the nanoparticles (or grafting density) and with the molecular weight of the polymer chains grafted to the particles. We studied the non-linear viscoelastic properties of such polymer grafted nanoparticle melts through a systematic examination of step strain measurements, steady shear behavior and the recovery after cessation of steady shear flow. These materials exhibit a yield-strain that is grafting-density and molecular-weight independent. The local re-arrangement of particles and the formation of slip-planes and the regeneration of local order upon cessation of flow dominate the flow and recovery properties of these nanocomposites. |
Monday, March 15, 2021 4:24PM - 4:36PM Live |
C01.00006: Programming surface energy driven Marangoni convection to pattern polymer films Christopher Ellison The Marangoni effect describes how fluid flows in response to gradients in surface energy. We recently developed a method for photochemically preprograming surface energy patterns in glassy polystyrene (PS) thin films. Patterned UV irradiation through a mask selectively dehydrogenates PS, increasing surface energy in the UV exposed regions compared to the unexposed regions. After heating the film to the liquid state, transport of polymer occurs from regions of low surface energy to regions of high surface energy. This method can be harnessed to rapidly manufacture polymer films possessing prescribed three-dimensional topographies reflective of the original light exposure pattern without solvent washes or etching procedures. To better understand this phenomenon, a theoretical model will be presented that reveals the physics of this process, its limits and ways to apply it efficiently for various target metrics. |
Monday, March 15, 2021 4:36PM - 4:48PM Live |
C01.00007: Influence of Pore Morphology on the Diffusion of Water in Triblock Copolymer Membranes Dipak Aryal, Michael Howard, Rituparna Samanta, segolene antoine, Rachel Segalman, Thomas M Truskett, Venkatraghavan Ganesan Understanding the transport properties of water in self-assembled block copolymer morphologies is important for furthering the use of such materials as water-purifying membranes. In this study, we used coarse-grained dissipative-particle-dynamics (DPD) simulations to clarify the influence of pore morphology on the self-diffusion of water in linear-triblock-copolymer membranes. We considered representative lamellar, cylindrical, and gyroid morphologies and present results for both the global and local diffusivities, as well as the structural characteristics of water in the pores. Our results suggest that the diffusivity of water in the confined, polymer-coated pores differs from that in the unconfined bulk. Explicitly, in confinement, the mobility of water is reduced by the hydrodynamic friction arising from the hydrophilic blocks coating the pore walls. We demonstrate that in lamella and cylindrical morphologies, the latter effects can be rendered as a universal function of the pore size relative to the brush height of the hydrophilic blocks. |
Monday, March 15, 2021 4:48PM - 5:00PM Live |
C01.00008: Enhanced Conductivity via Homopolymer-Rich Pathways in Block Polymer Composite Electrolytes Thomas Epps The BP electrolyte polystyrene-block-poly(oligo-oxyethylene methacrylate) [PS-b-POEM] was blended with POEM homopolymers of varying molecular weights to explore the impact of polymer additives on ion conductivity. The incorporation of a higher molecular weight homopolymer additive promoted a ‘dry brush-like’ homopolymer distribution within the BP self-assembly and led to higher lithium salt concentrations in the more mobile homopolymer-rich region, increasing overall ionic conductivity relative to the ‘wet brush-like’ and unblended composites. Furthermore, using 7Li solid-state nuclear magnetic resonance spectroscopy, we found a temperature corresponding to a transition in lithium mobility (TLi mobility) that was a function of blend-type. TLi mobility was found to be 39 °C above Tg in all cases. Interestingly, the ionic conductivity of the blended BPs was highest in the ‘dry brush-like’ composites even though these composites had higher Tgs than the ‘wet brush-like’ composites, suggesting that homopolymer-rich conducting pathways formed in the ‘dry brush-like’ assemblies had a larger influence on conductivity than the greater lithium ion mobility in the ‘wet brush-like’ blends. |
Monday, March 15, 2021 5:00PM - 5:12PM Live |
C01.00009: Overaging in PMMA glasses? The effect of cyclic loading/unloading on the segmental dynamics Mark Ediger, Trevor Bennin, Enran Xing, Josh Ricci The acceleration of structural relaxation or physical aging by deformation, known as overaging, has been reported in experiments and simulations of polymer and colloid glasses, and correctly accounting for overaging is important for the prediction of the long-term behavior of polymer glasses in engineering applications. Here the effects of cyclic loading/unloading on the segmental dynamics and mechanical properties of PMMA glasses are investigated using a probe reorientation technique and time-aging time superposition of the mechanical response, respectively. Sets of 5000 tensile loading/unloading cycles were performed at temperatures between Tg – 10 K and Tg – 25 K with cycle extension strains ranging from 0.003 to 0.007. After cycling, the segmental dynamics measured with the probe reorientation technique either remained unchanged or were faster relative to an undeformed sample. No evidence of overaging was observed in the optical or mechanical measurements as a result of these cyclic loading/unloading experiments. The relationship between these results and others in the literature will be discussed. |
Monday, March 15, 2021 5:12PM - 5:24PM Live |
C01.00010: Effects of nanoscale confinement on dye diffusivity in polymer films depend on polymer molecular weight: Relationship to fragility-confinement effects John Torkelson, Tong Wei, Tian Lan We studied the translational diffusivity of 9,10-bis(phenylethynyl)anthracene (Ddye) in supported polystyrene (PS) films. Relative to bulk films and near the Tg, Ddye is reduced by 80-90% in 100-nm-thick, high molecular weight (MW) PS (400 kg/mol) films. These results are associated with fragility-confinement effects, with fragility decreasing with decreasing film thickness below ~ 200 nm. Fragility reflects the breadth of the cooperative segmental relaxation distribution: that breadth narrows with confinement. The thickness dependence of Ddye reflects the time scales associated with the fast-relaxation tail of that distribution. At thickness below ~ 200 nm, the distribution narrows and the shortest relaxation times shift to longer times, resulting in a reduction inDdye. We also studied Ddye in low MW PS (6 kg/mol). Because 6 kg/mol PS exhibits much lower bulk fragility than high MW PS, confinement has a much-reduced effect on low MW PS fragility: both fragility and Ddye in 100-nm-thick, low MW PS films are unchanged from bulk. Thus, for thicknesses where fragility-confinement effects are observed (high MW PS), Ddye also exhibits confinement effects. By suppressing fragility-confinement effects by using very low MW PS, confinement effects on Ddye are also suppressed. |
Monday, March 15, 2021 5:24PM - 5:36PM Live |
C01.00011: Polymer solution droplet-to-capsule drying in levitation and microfluidics Joao Cabral Motivated by ubiquitous spray drying approaches in the manufacturing of polymer particles and composites, we experimentally investigate the mechanism and kinetics of particle formation by controlled solvent extraction and evaporation of polymer solution droplets. We employ microfluidics and acoustic levitation, and couple our experimental platform with small angle neutron scattering (SANS) to elucidate chain conformation under equilibrium conditions and along the particle formation pathway, supplemented by optical and electron microscopies that characterise overall external and internal particle morphologies. We examine semi-flexible polyelectrolyte sodium carboxymethyl cellulose, amorphous polystyrene, and high Tg semicrystalline poly(2, 6-diphenyl-p-phenylene oxide). We investigate the roles of composition, molecular mass, viscosity, and salt addition collapse (for NaCMC) in particle and capsule formation. Equipped with this knowledge, we then predictively design and fabricate polymer particles and capsules with prescribed dimensions, shape, porosity, microstructure and dissolution profile, and discuss complementarity with ‘flash nanoprecipitation’ methods. |
Monday, March 15, 2021 5:36PM - 5:48PM Live |
C01.00012: Aging Effects in Nanoparticle Composites Sanat Kumar We have studied the aging behavior of compatible mixtures of nanoparticles and polymers, and in a parallel study polymer grafted nanoparticles cast from solvent. Small angle x-ray scattering and x-ray photon correlation spectrocopy show that these materials restructure through NP motion, even though the initial state corresponds to a random dispersion of the NPs. We conjecture that this reordering is driven by the charge on the NPs which leads to a long range repulsion. We thus propose that mixtures of nanoparticles and polymers are frequently out of equilibrium and that the preperation method may sometimes critically affect the properties that result. |
Monday, March 15, 2021 5:48PM - 6:00PM On Demand |
C01.00013: Harnessing Nanoparticle Vibrations to Probe Surface Mobility and Glass Transition Malgorzata Bartkiewicz, Bartlomiej Graczykowski, George Fytas, Katelyn Randazzo, Rodney Priestley Advances in polymer nanoparticle synthesis and assembly techniques have enabled new applications, from drug delivery carriers to coatings. However, the polymer dynamics, especially at the nanoparticle surfaces of architected colloids, must be understood in order to realize their potential. Brillouin light spectroscopy, as a direct probe of the particle surface mobility via nanoparticle vibrations, reveals the correlation between the glass transition behavior and surface dynamics, and the presence of a low frequency mode extremely sensitive to particle-particle interactions. These are enabled by the surface mobility which can be engineered by different shell architecture layers. We demonstrate that irreversibly adsorbed layer atop soft and hard nanoparticles is able to eliminate the effect of enhanced particle surface mobility and drastically modify the structure of nanoparticle assembly. Furthermore, this surface mobility can be strongly affected by the application of gas pressure enabling a facile soldering of the colloidal film. |
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