Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session J56: Invited Session: Dillon Medal Symposium |
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Sponsoring Units: DPOLY Chair: Thomas Russell, University of Massachusetts Amherst Room: Four Seasons Ballroom 4 |
Tuesday, March 4, 2014 2:30PM - 3:06PM |
J56.00001: John H. Dillon Medal Lecture: Buckling Instabilities of Polymer Multilayers Invited Speaker: Ryan Hayward Soft polymer networks, such as gels and elastomers, can undergo a wide variety of geometry-dependent mechanical shape instabilities when subjected to compressive stress, providing opportunities to tailor the structure and properties of stimuli-responsive materials. These include global buckling modes of unsupported sheets as well as local surface modes such as wrinkling and creasing. The introduction of two or more elastic layers provides a multi-dimensional parameter space in terms of the contrasts in stiffness, geometry, and pre-strain between the layers, yielding a rich landscape of behaviors. Our group has recently focused on two examples. In the first case, we consider the role of mismatch strain in creasing and post-wrinkling bifurcations, which allows for fine control over both the types of surface modes that appear and their hysteretic behavior. In the second case, we consider the buckling of unsupported elastic trilayers as a route to define self-folding and responsive three-dimensional objects. [Preview Abstract] |
Tuesday, March 4, 2014 3:06PM - 3:18PM |
J56.00002: Light-induced sequential self-folding of pre-strained polymer sheets Jan Genzer, Ying Liu, Brandi Shaw, Michael Dickey Self-folding is a self-assembly process that causes a predefined 2D template to fold into a desired 3D structure with high fidelity.$^{\, \, }$We have developed a simple method of self-folding that uses predefined ink ``hinges'' printed onto pre-strained polymer sheets via a desk top printer. The ink absorbs external light, causing the area underneath the hinge to heat up and relax the strain in the hinge regions gradually across the sheet thickness. This process results in folding the sheet at the hinge region. We will demonstrate that sequential folding of multiple hinges on the same sample can be programmed by changing the light source and ink color of the hinge. We have successfully employed this strategy to produce complex origami shapes. [Preview Abstract] |
Tuesday, March 4, 2014 3:18PM - 3:30PM |
J56.00003: Snap-through instabilities of curved folds on curved, polymer shells Christian Santangelo, Nakul Bende, Sarah Innes-Gold, Art Evans, Jesse Silverberg, Itai Cohen, Ryan Hayward Snap-through instabilities are commonly associated with the catastrophic failure of an elastic structure; yet in nature, snap-through instabilities are also used to execute fast motions. Inspired by origami, the ancient art of paper folding, we show that ``folds'' can be introduced on shells by the local thinning of material. These folds can either be activated continuously or can snap-through to a geometrically determined angle, depending on the delicate interplay between the curvature of the shell and the shape of the fold. We describe how geometry can (and sometimes cannot) be used to control the dynamics of foldable shells. [Preview Abstract] |
Tuesday, March 4, 2014 3:30PM - 3:42PM |
J56.00004: Capillary leveling of stepped films with inhomogeneous molecular mobility Kari Dalnoki-Veress, Joshua D. McGraw, Thomas Salez, Oliver B\"{a}umchen, \'{E}lie Rapha\"{e}l The simple geometry of a polymer film on a substrate with a step at the free surface is unfavourable due to the excess interface induced by the step. Above the glass transition Laplace pressure gradients will drive flow, thus providing an excellent probe for nano-rheology. Here we recap some of our recent progress on the capillary leveling of stepped films. In particular, we present new studies on polymeric samples with precisely controlled, spatially inhomogeneous molecular weight distributions. [Preview Abstract] |
Tuesday, March 4, 2014 3:42PM - 3:54PM |
J56.00005: Wrinkling vs. scarring: Stress collapse in surface-confined assemblies Gregory Grason Confining assemblies to surfaces possessing Gaussian curvature frustrates the microscopic order of the packing, thus introducing mechanical costs for assemblies in such diverse contexts as viral capsids and particle-coated drops. The structure and stability of these systems is complicated by the non-trivial competition between distinct modes of stress relaxation, including ``elastic" shape deformation of the surface-bound assembly; defect-mediated ``plastic" reorganization of packing. We consider the interplay between these shape-deformation and defect-relaxation for a model of crystalline patch bound to an adhesive and deformable sphere, where the distinct patterns of relaxation become, respectively, radial chains of dislocations, or ``scars", and radial wrinkles. Analysis of highly-wrinkled and defect-riddle states reveals remarkably that both modes achieve the {\it identical} mechanical state in the limits of vanishing thickness and lattice spacing, and further, that the degeneracy between these modes is lifted only by the microscopic and sub-dominant energetics that select their optimal symmetry. We present a structural relaxation phase diagram that predicts a wrinkle-to-scar transition driven both by increasing substrate stiffness and substrate curvature. [Preview Abstract] |
Tuesday, March 4, 2014 3:54PM - 4:06PM |
J56.00006: Using theory and simulation to link molecular features of nanoscale fillers to morphology in polymer nanocomposites Arthi Jayaraman, Tyler Martin Polymer nanocomposites are a class of materials that consist of a polymer matrix embedded with nanoscale fillers or additives that enhance the inherent properties of the matrix polymer. To engineer polymer nanocomposites for specific applications with target macroscopic properties (e.g. photovoltaics, photonics, automobile parts) it is important to have design rules that relate molecular features to equilibrium morphology of the composite. In the first part of the talk I will present our recent theory and simulation work on composites containing polymer grafted nanoparticles, showing how polydispersity in graft and matrix polymers (physical heterogeneity) can be used to stabilize dispersion of the nanoparticles within a polymer matrix. In the second part of the talk I will present our recent work linking block-copolymer functionalization to the nanoparticle location in a polymer matrix consisting of homopolymer blends. [Preview Abstract] |
Tuesday, March 4, 2014 4:06PM - 4:18PM |
J56.00007: Entropically-Driven Destabilization of Nanoparticle Crystals Sanat Kumar Detailed computer simulations show that polymer-induced depletion forces can cause nanoparticles (NPs) in the athermal limit to crystallize, but only for short polymers at melt-like densities. For long chains, this depletion-induced attraction is in competition with the entropy loss associated with confining a polymer chain within the cavities in the NP crystal. For chains larger than these voids, these crystals are unstable and the NPs form an aggregated, but non-crystalline structure. The experimental results of Mackay et al. [\textit{Science }311:1740, 2006], who find immiscibility for chains smaller than the NP radii but miscibility for larger chains, thus probably reflect this physics, which we show to only be important when the osmotic pressure driving the polymer chains into the NP crystal is relatively large. [Preview Abstract] |
Tuesday, March 4, 2014 4:18PM - 4:30PM |
J56.00008: Anomalous Kinetics in Reactive Polymer Glasses Gila Stein Image formation in modern lithographic processes is based on the acid-catalyzed deprotection of glassy polymer films. It is well-established that slow acid diffusion controls the reaction kinetics, but models based on Fickian transport coupled with a first-order reaction cannot describe experimental data. We studied the acid-catalyzed deprotection of glassy poly(hydroxystyrene-co-tertbutyl acrylate) films using infrared absorbance spectroscopy and stochastic simulations. Experimental data were interpreted with a model that explicitly accounts for acid transport, where heterogeneities at local length scales are introduced through a non-exponential distribution of waiting times between successive hopping events. Subdiffusive behavior predicts key attributes of the observed deprotection rates, such as fast reaction at short times, slow reaction at long times, and a non-linear dependence on acid loading. This transport model is consistent with other literature studies of probe diffusion in inert glasses. We highlight the complex behavior in photoresists by changing the size of acid-counterion pairs, incorporating plasticizer, and reducing film thickness. These data can facilitate the development of predictive lithography models that reflect the behavior of confined polymers. [Preview Abstract] |
Tuesday, March 4, 2014 4:30PM - 4:42PM |
J56.00009: Regular and Irregular Mixing in Hydrocarbon Block Copolymers Richard Register, Bryan Beckingham Since hydrocarbon polymers interact through relatively simple (dispersive) interactions, one might expect them to be described by simple models of mixing energetics, such as regular mixing. However, the pioneering work of Graessley on saturated hydrocarbon polymer blends showed that while regular mixing is obeyed in some cases, both positive and negative deviations (in the magnitude of the mixing enthalpy) from regular mixing are observed in other cases. Here, we describe the mixing energetics for two series of hydrocarbon polymers wherein the interaction strengths may be continuously tuned, and which can be readily incorporated into block copolymers. Random copolymers of styrene and medium-vinyl isoprene, in which either the isoprene or both the isoprene and styrene units have been saturated, obey regular mixing over the entire composition range and for both hydrogenated derivatives. Well-defined block copolymers with arbitrarily small interblock interaction strengths can be constructed from these units, permitting the interdomain spacing to be made arbitrarily large while holding the order-disorder transition temperature constant. However, block copolymers of hydrogenated polybutadiene with such random copolymers show very strong positive deviations from regular mixing when the styrene aromaticity is preserved, and sizable negative deviations when the styrene units are saturated to vinylcyclohexane. Both of these cases can be quantitatively described by a ternary mixing model. [Preview Abstract] |
Tuesday, March 4, 2014 4:42PM - 4:54PM |
J56.00010: Thermally Switchable Aligned Nanopores by Magnetic-Field Directed Self-Assembly of Block Copolymers Chinedum Osuji Magnetic fields provide a facile approach to direct the self-assembly of magnetically anisotropic block copolymer nanostructures in a scalable manner. Here we combine such field-based processing with materials design to enable the fabrication of polymer films with highly aligned stimuli-responsive nanopores. Etch removal of a poly(D,L-lactide) (PLA) brush that is the minority component of a liquid crystalline block copolymer is used to produce nanopores of $\sim$ 8 nm diameter. The pores can be reversibly closed and opened while retaining their alignment by appropriate heating and cooling. We present TEM and temperture resolved scattering data during pore closure and re-opening to explore the mechanism and kinetics of pore collapse. [Preview Abstract] |
Tuesday, March 4, 2014 4:54PM - 5:06PM |
J56.00011: Tuning the theta temperature and critical micellization temperature of polymers in ionic liquids Timothy Lodge, Megan Hoarfrost Ionic liquids feature a combination of properties that make them very interesting solvents for polymers, but questions remain regarding the thermodynamics of polymer/ionic liquid solutions. In this work, the lower-critical-solution-temperature (LCST) phase behavior of poly($n$-butyl methacrylate) (PnBMA) in mixtures of the ionic liquids 1-butyl-3-methylimidazolium: bis(trifluoromethylsulfonyl)imide ([BMIm][TFSI]) and 1-ethyl-3-methylimidazolium:TFSI ([EMIm][TFSI]) is characterized by transmittance, light scattering, and small-angle neutron scattering measurements. Relevant thermodynamic parameters are readily tuned by varying the ionic liquid composition. In particular, the cloud point, spinodal, and theta temperatures are all found to increase linearly with [BMIm] content. The interaction parameters are determined as a function of temperature and concentration using three different methods, and the results from each method are compared. The theta temperatures are then compared quantitatively to the critical micellization temperatures (CMTs) for PnBMA-poly(ethylene oxide) diblocks, to test the proposition that the CMT corresponds to a fixed value of chi. [Preview Abstract] |
Tuesday, March 4, 2014 5:06PM - 5:18PM |
J56.00012: Design of P3HT-g-P2VP Graft Copolymers as Efficient Compatibilizers for Stable Operation of Polymer Solar Cells (PSCs) Bumjoon Kim, Hyeong-Jun Kim, Jin-Seong Kim, Hyun-Seung Yang Fabrication of ordered structures from block copolymers of conjugated polymers has been limited due to the strong rod$-$rod interactions between the conjugated blocks. In this work, we developed a molecular design of conjugated polymer-based graft copolymers to control the rigidity of the copolymers and to produce a variety of ordered nanostructures. A series of well-defined poly(3-hexylthiophene)-graft-poly(2-vinylpyridine) (P3HT-g-P2VP) copolymers in which the P2VP chains had different molecular weights (M$_{n})$ was prepared. As the M$_{n}$ of the grafted P2VP chains increased, the crystallinity of the P3HT block in the copolymers decreased. Therefore, we produced well-ordered, non-fibrillar nanostructures of P3HT-based copolymers. In addition, P3HT-g-P2VP polymers can be used as efficient compatibilizers in the active layer of PSCs. P3HT-g-P2VP polymers can modify the sharp interface between polymer donors and fullerenes, resulting in dramatic enhancement in the thermal stabilities and mechanical properties of PSCs. The effectiveness of the graft copolymers as compatibilizers will be demonstrated by comparing them with the P3HT-b-P2VP block copolymers. [Preview Abstract] |
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