Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session H42: Dillon Medal SymposiumFocus
|
Hide Abstracts |
Sponsoring Units: DPOLY Chair: Mark Ediger, University of Wisconsin - Madison Room: BCEC 210A |
Tuesday, March 5, 2019 2:30PM - 3:06PM |
H42.00001: John H. Dillon Medal Talk: Probing Glass Physics Through Measurements of Polymer Dynamics in Thin films and in Strongly Confined Systems Invited Speaker: Zahra Fakhraai Extensive research in the past two decades has shown that the free surfaces of glasses have dramatically faster dynamics compared to the bulk dynamics with much lower activation energies for rearrangement. In ultrathin films of polymers and molecular glasses, the enhanced surface mobility results in large changes in the value of the glass transition temperature (Tg), enhanced overall dynamics, as well as other property changes such as elasticity, mechanical response, and aging rate. |
Tuesday, March 5, 2019 3:06PM - 3:18PM |
H42.00002: The glass transition in polydisperse polymers: contribution of mixing entropy Valentin Ruffine, Adam Raegen, James Forrest All polymerization processes give rise to polydisperse samples where the molecules exhibit a distribution of polymerization index, N. Due to the strong dependence of glass transition temperature, Tg, on N for small N, such polydispersity can potentially have a strong impact on the measured Tg values. Gibbs and DiMarzio 60 years ago (J. Chem. Phys.28, 373 1958) suggested that the Tg value of polydisperse samples is determined by the number average N of the sample “except for an ordinarily small correction” due to the excess mixing entropy. By considering mixtures of near oligomeric polystyrene with different compositions but the same number average molecular weight we can separate the entropic contribution due to chain ends from that of the chain mixing. We find that the measured Tg values of these mixtures has a significant composition dependence. We quantify this difference by considering the extra mixing entropy of the small chains. This analysis is also able to explain previously measured anomalies in bimodal mixtures of PS. |
Tuesday, March 5, 2019 3:18PM - 3:30PM |
H42.00003: Investigation of deformation mechanism of impact polypropylene Yue Zhang, Li Qian, Ying Lu, Yingying Sun, Yongfeng Men Polypropylene (PP) is one of the most important plastics widely used due to its heat resistance, high stiffness and good processability. However, the application of PP is often limited by its poor impact resistance. In industry, in reactor copolymerization of ethylene-propylene (EP) following PP homo polymerization is an economic and efficient way of toughening PP where the product is impact copolymer of polypropylene (ICP). Yet the mechanism of PP toughening is still elusive which may involve several energy adsorbing mechanism upon deformation. Here we investigate the cavitation formation process upon deformation in ICP using ultra-small angle X-ray scattering (USAXS), where cavity size and shape evolution were monitored upon deformation. Different ICP systems with varying EP fraction and compositions were studied, aiming to correlate the micro-structure developed upon deformation with the molecular attributes of different ICP systems. |
Tuesday, March 5, 2019 3:30PM - 3:42PM |
H42.00004: Fragility and the Glass Transition of Geometrically Confined Polynorbornenes Laura Gray, Rodney Priestley Over the past twenty years many studies have shown a reduction in the glass transition temperature (Tg) of thin polymer films confined on the nanoscale when supported on non-attractive substrates or free-standing. These results have been explained by the propagation of enhanced mobility from the free-surface into the polymer film. Many questions remain as to the varying length scale over which Tg decreases, the magnitude of the Tg perturbation observed for different polymers as well as the role cooperatively rearranging regions play in Tg. In this study we measure the dynamic fragility, cooperative length scale, and confined Tg for a series of chemically similar polynorbornenes. We do not see a clear trend in bulk fragility, cooperative length scale, and the magnitude of the Tg perturbation over a wide range of fragilities. Utilizing the unique capabilities of flash differential scanning calorimetry we are also able to quantify the influence of confinement on the fragility and cooperative motion of polynorbornenes. |
Tuesday, March 5, 2019 3:42PM - 3:54PM |
H42.00005: Polymerization Thermodynamics under Nanoconfinement Qian Tian, Haoyu Zhao, Sindee Simon The behavior of materials confined at the nanoscale has been of considerable interest over the past several decades, especially changes in the glass transition temperature (Tg) and/or melting point (Tm). Less well studied are the effects of nanoconfinement on polymerization kinetics and thermodynamics. Our recent focus has been on understanding how nanoconfinement influences the polymer/monomer equilibrium in the free radical reaction of poly(alkyl methacrylates). We find that nanoconfinement shifts the monomer/polymer equilibrium back towards monomer, and this effect can be exploited to determine the entropy loss on confining a chain. We find that the entropy loss is as high as 10 J/mol/K, approximately 10 % of the change going from monomer to polymer. The results seem to indicate that as the n-alkyl group increases from methyl to ethyl to butyl, the entropy of confinement decreases. Interestingly, the magnitude of the Tg depression in ultrathin films of poly(n-alkyl methacrylate)s also decreases as the length of the alkyl group increases in work by Vogt et al.; whether the origin is linked to the change in chain confinement entropy is an open question. |
Tuesday, March 5, 2019 3:54PM - 4:06PM |
H42.00006: Shear modulus and shear-stress relaxation in simulated free-standing polymer films Jorg Baschnagel, Geevarghese George, Ivan Kriuchevskyi, Hendrik Meyer, Joachim Wittmer Using molecular dynamics simulations of a coarse-grained model for polymer glasses we examine viscoelastic properties of free-standing (nonentangled) polymer films. We focus on the (in-plane) shear relaxation modulus G(t) and the (in-plane) shear modulus μ, which we analyze as a function of film thickness (h) for temperatures (T) above and below the glass transition temperature Tg(h). The shear modulus is determined via the stress-fluctuation formalism and found to depend, in addition to h and T, also on the time window (Δt) employed for data sampling. This dependence on Δt can related quantitatively to the time dependence of G(t), provided time-translational invariance holds. Therefore, G(t) is the important underlying quantity, which obeys, in good approximation, a time-temperature superposition principle. We determine the viscosity of the films. For a given T the viscosity decreases with film thickness. This effect can be mainly traced back to the decrease of Tg with decreasing h. The presence of the free interfaces also weakens the shear rigidity of the polymer glass relative to the bulk, which can be understood via the relation between μ and G(t) mentioned before. |
Tuesday, March 5, 2019 4:06PM - 4:18PM |
H42.00007: Mobility Gradients in Supported Glass-Forming Polymer Films Do Not Imply Gradients in Cooperative Motion Jack Douglas, Wengang Zhang, Francis Starr We investigate the extent of collective motion in the interfacial regions of a thin supported polymer film and within the film interior by molecular dynamics simulations to understand the role of collective motion in the often large changes in interfacial molecular mobility observed in polymer films. Contrary to commonly stated expectations, we find that the extent of collective motion determining the temperature dependence of the structural relaxation time does not vary significantly within the film, a finding consistent with Adam-Gibbs proposal that the extent of collective motion in glass-forming liquids is related to the configurational entropy, a thermodynamic property that cannot vary with position within the film. |
Tuesday, March 5, 2019 4:18PM - 4:30PM |
H42.00008: Dynamic Phase Transitions in Confined Polymer Glasses Robert Riggleman, Robert Ivancic Despite more than two decades of effort, several qualitative features regarding the segmental dynamics of thin films of glass-forming materials remainly poorly understood. This challenge is of paramount importance in thin film membranes, organic electronic devices, and critical steps of semiconductor manufacturing. Recent simulations and experiments have provided compelling evidence that the dynamics near surfaces are qualitatively different from those of a bulk supercooled liquid, and in this talk I will describe our efforts at characterizing those dynamics by investigating a non-equilibrium phase transition associated with glass-forming materials. This transition is associated with a supercooled liquid that undergoes a sharp transition from a mobile dynamic phase to a phase with reduced mobility, and it provides a clear signal in the dynamics of the glass-forming polymer under confinement. I will describe our characterization of this transition in free-standing thin films of supercooled liquids, where we find that only the bulk of the film is able to undergo this transition. This suggests that the dynamics at the free surface are not glassy in nature, at least not at the temperatures accessible to our simulations. |
Tuesday, March 5, 2019 4:30PM - 4:42PM |
H42.00009: Coarse-Grained Models for Predicting Structure and Thermodynamics in Polymer Systems with Specific and Directional Intermolecular Interactions Arthi Jayaraman Recent work in my research group has been aimed at developing predictive coarse-grained (CG) models for investigating structure and dynamics in soft materials with chemistries that have specific and directional molecular interactions. Although computational studies have been tremendously useful in understanding molecular phenomena and guiding synthesis and engineering of new macromolecular soft materials for a wide variety of applications, the inability to capture small scale specific and directional interactions (e.g., hydrogen bonds) alongside macromolecular length and time scales represents a key limitation of most studies to date. We address this limitation by developing coarse-grained models that capture the anisotropic, directional and specific interactions (e.g., hydrogen bonding interaction) governing the structure and thermodynamics in many polymer systems of interest. We have been using molecular dynamics simulations with these new coarse-grained models for studies of biomaterials and polymer nanocomposites. In this talk, I will discuss one example of how the development of these CG models is enabled by synergistic feedback from concurrent/past experiments which I will highlight alongside the computational results from my group. |
Tuesday, March 5, 2019 4:42PM - 4:54PM |
H42.00010: Heterogeneous polymer degradation due to photothermal heating Laura Clarke, Honglu Huang, Gabriel Firestone, Russell E Gorga, Jason Bochinski We are interested in observing the effects of thermally-driven chemical reactions occurring in small volumes within a solid material, where diffusion of reactants and products is limited. Such experiments can be achieved by photothermally heating metal nanoparticles incorporated within the polymer which results in significant heat generation at the particles and an inhomogeneous steady state temperature distribution within the solid material, where regions far from any particle are cool, whereas those in a particle's immediate vicinity experience temperatures of 100-200 deg. C. Polyethylcyanoacrylate (PECA) degrades by depolymerizing and in confinement, the monomer will repolymerize to form oligomers. In principle, such a process might enable internal conversion of polymer to oligomer without significant loss of mechanical properties, and thus potentially address issues such as microfragmentation of plastics in the environment. We characterize degradation of PECA:starch:nanoparticle composites via GPC, dilute solution viscometry, electron and optical microscopy, and mechanical property measurements, and compare and contrast degradation obtained via photothermal heating with that obtained by conventional means. |
Tuesday, March 5, 2019 4:54PM - 5:06PM |
H42.00011: Charge Density- and Hydrophobicity-Dependent Dynamics of Polyelectrolyte Complex Coacervates Jennifer Laaser, Jun Huang We use a library of polyacrylamide-based polymers to investigate the roles of charge density and hydrophobicity in determining the phase behavior and viscoelasticity of poleyelectrolyte complex coacervates. The polymers, which are made by post-polymerization functionalization of poly(N-acryloxy succinimide), have charged monomer fractions between 60 and 100%, with the balance comprised of either hydrophilic (acrylamide) or hydrophobic (butyl acrylamide) comonomers. We identify the critical salt concentration of the coacervates by optical turbidity, and characterize their degree of swelling and viscoelasticity via thermogravimetric analysis and small amplitude oscillatory shear rheology, respectively. We find that as the charge density of the polymers is decreased, the critical salt concentration and volume fraction of polymer in the coacervates also decrease, and the relaxation dynamics speed up. Interestingly, these properties depend only on the charge density, and not on the hydrophobicity of the comonomer. This suggests that hydrophobic interactions are much weaker than the entropic forces driving coacervate formation, and may provide a new window into understanding the extent to which polymer concentration and electrostatic interactions determine coacervate dynamics. |
Tuesday, March 5, 2019 5:06PM - 5:18PM |
H42.00012: Rearrangement of 2D aggregates of droplets under compression: signatures of the energy landscape from crystal to glass Kari Dalnoki-Veress, Jean-Christophe Ono-dit-Biot, Pierre Soulard, Solomon Barkley, Eric Weeks, Thomas Salez, Elie Raphael We study a signature of the energy landscape through the crystal-to-glass transition by compressing 2D finite aggregates of emulsion droplets. Oil droplets of two distinct sizes are used to compose small aggregates in an aqueous environment. Aggregates range from perfectly ordered monodisperse crystals to disordered bidisperse glasses. The aggregates are compressed between two parallel boundaries; crucially, one of the boundaries acts as a force sensor. The compression forces provide a signature of the aggregate composition and give insight into the energy landscape. In particular, crystals dissipate all the stored energy through single catastrophic fracture events whereas the glassy aggregates break step-by-step. Remarkably, the yielding properties of the 2D aggregates are strongly impacted by even a small amount of disorder. |
Tuesday, March 5, 2019 5:18PM - 5:30PM |
H42.00013: Photo-Induced Order-Disorder Transitions in Block Copolymer Solutions Timothy Lodge, Cecilia C Hall, Cecelia Rivera, Claire Seitzinger, Yuki Hirose Photo-stimuli responsive materials are of interest for a broad range of applications, and take advantage of the inherently non-invasive, spatially resolved, and species-selective optical response. We demonstrate here a photo-reversible order-disorder transition in a block copolymer/ionic liquid system. The copolymer is a symmetric poly(methyl methacrylate)-b-poly(benzyl methacrylate-stat-azomethacrylate) diblock, with M ≈ 45 kDa, and 5% azomethacrylate in the second block. Poly(benzyl methacrylate) exhibits LCST behavior in the common ionic liquid ethyl methyl imidazolium bistrifluoromethylsulfonimide (EMI TFSI). Under UV irradiation, the trans to cis photoisomerization of azobenzene significantly increases the solubility of the thermoresponsive block, leading to an order-to-disorder transition at appropriate concentrations and temperatures. Under visible light, the cis to trans recovery causes the sample to re-order. The progress of the transition is monitored by in situ photo-rheology, and the morphologies are confirmed by SAXS. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700