Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Y43: Molecules, Solutions, Networks, & Gels |
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Sponsoring Units: DPOLY Chair: Daniel Savin, University of Southern Mississippi Room: A306/307 |
Friday, March 25, 2011 8:00AM - 8:12AM |
Y43.00001: Distinct Tensile Response of Model Semi-flexible Elastomer Networks Bernardo M. Aguilera-Mercado, Claude Cohen, Fernando A. Escobedo Through coarse-grained molecular modeling, we study how the elastic response strongly depends upon nanostructural heterogeneities in model networks made of semi-flexible chains exhibiting both regular and realistic connectivity. Idealized regular polymer networks have been shown to display a peculiar elastic response similar to that of super-tough natural materials (e.g., organic adhesives inside abalone shells). We investigate the impact of chain stiffness, and the effect of including tri-block copolymer chains, on the network's topology and elastic response. We find in some systems a dual tensile response: a liquid-like behavior at small deformations, and a distinct saw-tooth shaped stress-strain curve at moderate to large deformations. Additionally, stiffer regular networks exhibit a marked hysteresis over loading-unloading cycles that can be deleted by heating-cooling cycles or by performing deformations along different axes. Furthermore, small variations of chain stiffness may entirely change the nature of the network's tensile response from an entropic to an enthalpic elastic regime, and micro-phase separation of different blocks within elastomer networks may significantly enhance their mechanical strength. [Preview Abstract] |
Friday, March 25, 2011 8:12AM - 8:24AM |
Y43.00002: Elastically tuned defect mode in cholesteric elastomers Juan Adrian Reyes, Laura Olivia Palomares We consider an axially elongated cholesteric elastomer having a twist defect. We show that its localized mode can be mechanically tuned, and the scaling of the inverse relative line width can be largely enhanced when the values of the deformation and shape anisotropy are near the pseudo isotropic curve. This choice causes a tremendous variation in the behavior of the photon dwell time in the defect mode, which then grows linearly versus the sample thickness. The shift of the defect wavelength, the reflection band width, and the angle between the electric and magnetic fields are also calculated. [Preview Abstract] |
Friday, March 25, 2011 8:24AM - 8:36AM |
Y43.00003: Glassy structure and thermal fluctuations of amorphous nematogenic solids Fangfu Ye, Bing Lu, Xiangjun Xing, Paul Goldbart Amorphous nematogenic solids (ANS) are media comprising rod-like nematogens that have been randomly linked to form macroscopic, elastically deformable networks. Classes of ANS include chemical nematogen gels (i.e., networks of small molecules) and liquid crystalline elastomers (built from crosslinked nematogen-containing macromolecules), as well as biophysical networks, such as those composed of actin filaments. One common feature of these systems is that the linking process introduces into them a new type of random field, consisting of a conventional static part along with a new, thermal-fluctuation-induced, dynamic part. We develop a phenomenological model of ANS which shows how this composite random field, together with the coupling between the orientational and positional fluctuation that nematogens exhibit, leads to the occurrence of decaying but also oscillatory correlations of the thermal fluctuation, and also shows how these correlations influence the glassy structure of ANS. [Preview Abstract] |
Friday, March 25, 2011 8:36AM - 8:48AM |
Y43.00004: Thermal and Mechanical Properties of Sequential and Simultaneous Thiol-Ene-Isocyanate Networks Olivia McNair, Davis Brent, Daniel Savin Ternary networks containing having stoichiometrically balanced thiol /(ene+isocyanate) ranging from 0 to 20 mol% isocyanate were synthesized via sequential or simultaneous thiol/ene and thiol/isocyanate click reactions. The effects of cross-link density were studied using three thiols, GDMP (difunctional), 3T (trifunctional) and 4T (tetrafunctional) respectively. TEA catalyzes the isocyanate-thiol coupling and chain extension, while the photoinitiator DMPA initiates a radical thiol-ene crosslinking process. Real-time FTIR was used to study kinetics of both light and dark reactions utilizing thiol, ene and isocyanate peaks which appear independently. It was found that difunctional thiols and isocyanates reacted initially, forming chain extended prepolymers end-capped with thiol functionalities. Upon UV irradiation, thiol functionalized prepolymers reacted with TTT, a trifunctional ene, forming networks containing incorporated thiourethane linkages. Initial DSC results indicated higher Tgs for higher cross-linked networks; however, isocyanate content has significant effects on each system. Films were also be thermally characterized via DMA and mechanical properties measured using MTS. [Preview Abstract] |
Friday, March 25, 2011 8:48AM - 9:00AM |
Y43.00005: Computational and Experimental Investigation of Morphology of Polymer Gels Yelena Sliozberg, Kenneth Strawhecker, Jan Andzelm, Joseph Lenhart Thermo-reversible polymer gels based on block copolymers represent a remarkable class of materials for a wide range of applications. An efficient approach to control and modify the properties of these gels is to use multicomponent mixtures of self-assembling block copolymers differing in architecture, length and chemical nature. As a result of microphase separation, ``mixed'' or ``pure'' micelles, containing block copolymers of the same or different types, are developed. Here, we present a dissipative particle dynamics (DPD) study of the morphology of a binary mixture of AB/ABA block copolymers differing in length of the insoluble blocks in B-selective solvent. We have observed numerous morphologies of AB/ABA blends, which are characterized by formation of pure and mixed micelles of various compositions, structures and sizes. We have discovered that changing the copolymer ratio and processing conditions impacts morphology of these blends. Finally, we have established factors that affect an intermicellar distance and a bridge fraction which ultimately determines the mechanical properties of the gels. Results of our computations were compared with our experimental findings based on atomic force microscopy and the other experimental and theoretical studies and demonstrated a good agreement. [Preview Abstract] |
Friday, March 25, 2011 9:00AM - 9:12AM |
Y43.00006: Theory of volume phase transitions of polyelectrolyte gels Jing Hua, Mithun Mitra, Murugappan Muthukumar We will present theoretical results for the effect of charge regularization accompanying volume phase transitions of polyelectrolyte gels. Our theoretical formulation of the cascade effect that couples the effective degree of ionization and the polymer density leads to significant deviations from the classical Flory-type theories. [Preview Abstract] |
Friday, March 25, 2011 9:12AM - 9:24AM |
Y43.00007: Plasticity and slow relaxation phenomena in reinforced rubbers Paul Sotta, Stephane Dupres, Didier Long Elastomers reinforced with nanometric solid particles or aggregates exhibit remarkable properties: temperature dependent reinforcement of the modulus in the linear regime, non linear effects, irreversibility and hysteretic effects. Important progress has been achieved recently in modeling these properties, based on glassy layers around filler particles. In some cases, reinforcement as a function of temperature and filler volume fraction was explained quantitatively. We shall focus here on the plasticity and related slow relaxation phenomena which occur in these systems. We show that the amplitude of plasticity is correlated to the reinforcement amplitude, and that plasticity relaxes with a very broad distribution of relaxation times (similar to an ageing phenomenon), in the same way as the stress relaxes at high strain amplitude. These experimental observations of long time evolution are well described by the mesoscale model for reinforced rubbers that we have proposed. [Preview Abstract] |
Friday, March 25, 2011 9:24AM - 9:36AM |
Y43.00008: Migration of chemical additives in a rubber under UV irradiation Ludovic Devant, Roland Benoit, Marie-Louise Saboungi, Emmanuel Gomez, Benoit Le Rossignol The evolution of the chemical composition of a rubber, in particular that of its surface, is governed by several factors including temperature, oxidation and migration of additives. Oxidation mechanisms alone do not account for all the phenomena observed, for example the appearance of deposits on the surface. We have studied the effects of temperature and photo-oxidation on the migration of chemical additives on a rubber surface. The morphological and chemical evolution were followed by AFM and XPS, respectively. 3D reconstruction with time-of-flight secondary-ion mass spectrometry (TOF-SIMS) combined with the AFM and XPS results enabled us to establish a relation between the oxidizing degradation of the rubber and the surface migration of the additives. This finding is supported by a kinetic study of the surface evolution. [Preview Abstract] |
Friday, March 25, 2011 9:36AM - 9:48AM |
Y43.00009: Probing the sliding interactions between bundled actin filaments Andy Ward, Zvonimir Dogic Assemblies of filamentous biopolymers are hierarchical materials in which the properties of the overall assemblage are determined by structure and interactions between constituent particles at all hierarchical levels. For example, the overall bending rigidity of a two bundled filaments greatly depends on the bending rigidity of, and the adhesion strength between individual filaments. However, another property of importance is the ability for the filaments to slide freely against one another. Everyday experience indicates that it is much easier to bend a stack of papers in which individual sheets freely slide past each other than the same stack of papers in which all the sheets are irreversibly glued together. Similarly, in filamentous structures the ability for local re-arrangement is of the utmost importance in determining the properties of the structures observed. In order to study this phenomenon we create bundles of biopolymers by inducing attractive interactions between actin filaments via the depletion mechanism. We find that bundles of actin filaments to do not slide freely across one another. In order to characterize these sliding interactions, we perform active experiments using laser tweezers to pull one filament across the other at constant velocity. [Preview Abstract] |
Friday, March 25, 2011 9:48AM - 10:00AM |
Y43.00010: Apparent Yield Stress and Interfacial Viscoelasticity of Globular Protein Solutions Vivek Sharma, Aditya Jaishankar, Ying-Chih Wang, Gareth H. McKinley Globular proteins influence the dynamics, phase behavior and transport of biomolecules and drugs in the mammalian body. In conventional rheological studies conducted on torsional rheometers, protein solutions are commonly reported to have a solid-like response at concentrations as low as 0.03{\%} by weight. In this study, we probe the bulk and interfacial viscoelasticity of bovine serum albumin (BSA) solutions as a canonical example of a globular protein system. Using a stress-controlled rotational rheometer, augmented by microfluidic rheometry and interfacial rheometry, we demonstrate that the origin of the yield-like response reported in bulk viscometric flows lies in the formation of a film of adsorbed protein, formed spontaneously at the solution/gas interface. We directly measure the concentration-dependent interfacial viscoelasticity of the adsorbed protein and we describe a coherent means of extracting the interfacial contribution from bulk viscosity measurements. Finally, we demonstrate how the presence of surfactants changes both the interfacial and bulk rheology of pharmaceutical formulations based on protein solutions. [Preview Abstract] |
Friday, March 25, 2011 10:00AM - 10:12AM |
Y43.00011: The enhanced bubble formation in short dsDNA loops O-chul Lee, Wokyung Sung Recent experiments have shown the dsDNAs readily bend and loop over the nanometer scale much shorter than its persistence length (50 nm). Motivated by this, we study possibility of enhanced bubble formation in short dsDNA loops by evaluating free energy of bubble formation analytically, and also by simulating the breathing DNA model. We analyze the bubble size distribution and the average bubble size as a function of the loop length, which are compared with those of the linear DNA of the same length. 1) T. E. Cloutier and J Widom, Mol. Cell \textbf{14}, 355 (2004). 2) P. A. Wiggins \textit{et al}., Nat. Nanotechnol. 1, 137 (2006). 3) O. Lee, J. H. Jeon and W. Sung, Phys. Rev. E 81, 021906 (2010) [Preview Abstract] |
Friday, March 25, 2011 10:12AM - 10:24AM |
Y43.00012: Universality in the timescales of internal loop formation in unfolded proteins and single-stranded oligonucleotides Ryan Cheng, Takanori Uzawa, Kevin Plaxco, Dmitrii Makarov Understanding the rate at which various parts of a molecular chain come together to facilitate the folding of a biopolymer (e.g., a protein) into its functional form remains an elusive goal. Here we use experiments, simulations, and theory to study the kinetics of internal loop closure in disordered biopolymers such as single-stranded DNA and unfolded proteins. We present theoretical arguments and computer simulation data to show that the relationship between the timescale of internal loop formation and the positions of the monomers enclosing the loop can be recast in a form of a universal master dependence. Our measurements of the loop closure times in single-stranded oligonucleotides, as well the internal loop closure kinetics in unfolded proteins reported by others, are all well described by this theoretically predicted dependence. Experimental deviations from the master dependence can then be used as a sensitive probe of dynamical and structural order in unfolded proteins and other biopolymers. [Preview Abstract] |
Friday, March 25, 2011 10:24AM - 10:36AM |
Y43.00013: Topological interactions between ring polymers Dieter Heermann The detailed topological and entropic forces between loops still remain elusive. We have quantitatively determined the potential of mean force between the centers of mass of two ring polymers, i.e. loops. We find that the transition from a linear to a ring polymer induces a strong increase in the entropic repulsion between these two polymers. On top, topological interactions such as the non-catenation constraint further reduce the number of accessible conformations of close-by ring polymers by about 50 percent, resulting in an additional effective repulsion. [Preview Abstract] |
Friday, March 25, 2011 10:36AM - 10:48AM |
Y43.00014: Modulation of DNA condensation by cation valence Preethi Chandran, Emilios Dimitriadis, Candida Silva, Peter Basser, Ferenc Horkay Aggrecan is a negatively charged bottlebrush-shaped proteoglycan in the extracellular matrix, with unique polyelectrolyte properties. Aggrecan-hyaluronic acid aggregates are responsible for the compressive resilience of articular cartilage. Unlike linear polyelectrolytes such as DNA, aggrecan is insensitive to the presence of multivalent counterions (e.g., calcium ions) and self-assembles into micro-gels in near-physiological salt solutions. These features are preserved by aggrecan adsorbed on mica surfaces. To probe both the nature of aggrecan assemblies in solution and their surface interactions, we image the aggrecan assemblies adsorbed on mica surface using Atomic Force Microscopy The effect of counterion valence on the hydration-dehydration properties of the aggrecan assemblies will be discussed. [Preview Abstract] |
Friday, March 25, 2011 10:48AM - 11:00AM |
Y43.00015: Combining DNA Nanotechnology and Fluorescence Polarization Microscopy to Determine the Orientation of DNA-bound Fluorophores Hunter Banks, Christoph Schneider, Deborah Fygenson We describe a technique to measure the axis of the transition dipole moment of a fluorophore bound to dsDNA and compare results with existing techniques. We use DNA nanotubes to present the dsDNA in a known orientation and query a variety of intercalating (e.g., YO-YO, TO-TO), groove-binding (e.g. DAPI) or covalently linked (e.g., Fluorescein, Cy3, Cy5) dyes. A de S\'{e}narmont prism in front of the camera generates simultaneous images of fluorescence polarized perpendicular and parallel to the DNA nanotube axis, allowing for ratio measurements that are insensitive to bleaching. We suggest the use of technique to detect helical supertwist, and possibly other nanoscale structural features, of DNA nanostructures. [Preview Abstract] |
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