Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Z42: Biopolymers: Molecules, Solutions, Networks, and Gels |
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Sponsoring Units: DPOLY Chair: Jan-Michael Carrillo, University of Connecticut Room: A302/303 |
Friday, March 25, 2011 11:15AM - 11:51AM |
Z42.00001: How do polymers degrade? Invited Speaker: Materials derived from agricultural products such as cellulose, starch, polylactide, etc. are more sustainable and environmentally benign than those derived from petroleum. However, applications of these polymers are limited by their processing properties, chemical and thermal stabilities. For example, polyethylene terephthalate fabrics last for many years under normal use conditions, but polylactide fabrics cannot due to chemical degradation. There are two primary mechanisms through which these polymers degrade: via hydrolysis and via oxidation. Both of these two mechanisms are related to combined factors such as monomer chemistry, chain configuration, chain mobility, crystallinity, and permeation to water and oxygen, and product geometry. In this talk, we will discuss how these materials degrade and how the degradation depends on these factors under application conditions. Both experimental studies and mathematical modeling will be presented. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z42.00002: Stretching semiflexible filaments with quenched disorder Panayotis Benetatos, Eugene M. Terentjev Many biopolymers, such as DNA, are characterized by sequence heterogeneity. At large scales, this heterogeneity may behave as a quenched random variable. We consider a wormlike chain with uncorrelated quenched disorder in its arc-length dependent spontaneous curvature. In the weakly bending approximation, we obtain analytic results for the elastic response to a stretching force applied at its end-points. We show that the effect of quenched disorder does not always reduce to a simple renormalization of the bending stiffness of the pure system. We also discuss a formally similar disordered system where a stretched wormlike chain is subject to random uncorrelated transverse forces. [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z42.00003: Chains Are More Flexible Under Tension Andrey Dobrynin, Jan-Michael Carrillo, Michael Rubinstein The mechanical response of networks, gels, and brush layers is a manifestation of the elastic properties of the individual macromolecules. The two main classes of models describing chain elasticity include the worm-like and freely jointed chain models. The selection between these two classes of models is based on the assumptions about chain flexibility. We are proposing a unified chain deformation model that describes the force deformation curve in terms of the chain bending constant, $K$, and bond length, $b$. This model demonstrates that the worm-like and freely jointed chain models correspond to two different regimes of polymer deformation, and the crossover between these two regimes depends on the chain bending rigidity and the magnitude of the applied force. Polymer chains with bending constant $K>1$ behave as a worm-like chain under tension in the interval of the applied forces $f\le KkT/b$ and as a freely jointed chain for $f\ge KkT/b$. ($k$ is the Boltzmann constant and $T$ is the absolute temperature.) The proposed crossover expression for chain deformation is in excellent agreement with the results of the molecular dynamics simulations of chain deformation and single molecule deformation experiments of biological and synthetic macromolecules. [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z42.00004: Statistical Mechanics of Helical Wormlike Model Ya Liu, Toni Perez, Wei Li, James Gunton, Amanda Green The bending and torsional elasticities are crucial in determining the static and dynamic properties of ~biopolymers such as dsDNA and sickle hemoglobin. We investigate the statistical mechanics of stiff polymers ~described by the helical wormlike model. We provide a numerical method to solve the model using a transfer matrix formulation. The correlation functions have been calculated and display rich profiles which are sensitive to the combination of the temperature and the equilibrium torsion. The asymptotic behavior at low temperature has been investigated theoretically and the predictions fit the numerical results very well. Our analysis could be used to understand the statics of dsDNA and other chiral polymers. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z42.00005: DNA walks one step at a time in electrophoresis Juan Guan, Bo Wang, Steve Granick Testing the classical view that in DNA gel electrophoresis, long polymer chains navigate through their gel environment via reptation, we reach a different conclusion: this driven motion proceeds by stick-slip. Our single-molecule experiments visualize fluorescent-labeled lambda-DNA, whose intramolecular conformations are resolved with 30 ms resolution using home-written software. Combining hundreds to thousands of trajectories under amplitudes of electric field ranging from zero to large, we quantify the full statistical distribution of motion with unprecedented statistics. Pauses are seen between steps of driven motion, probably reflecting that the chain is trapped inside the gel matrix. The pausing time is exponentially distributed and decreases with increasing electric field strength, suggesting that the jerky behavior is an activated process, facilitated by electric field. We propose a stretch-assisted mechanism: that the energy barrier to move through the gel environment is first overcome by a leading segment, the ensuing intramolecular stress from stretching causing lagging segments to recoil and follow along. [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z42.00006: The effects of end-interactions on semiflexible polymers looping Jaeoh Shin, Wokyung Sung Biopolymer looping is a ubiquitous dynamic process that occurs in cell, such as gene regulation and protein folding. We study the dynamics of looping for a variety of chain contour and persistence lengths via simulation and analytical theory. To speed up the looping time in simulation, which is very long for the short, rigid chains, we use the path integral hyperdynamics method. We analyze the effects of static and hydrodynamic interactions between the end beads on the looping time. [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z42.00007: Modeling of biomimetic peptoid polymers Dina Mirijanian, Steve Whitelam Peptoids are sequence-specific, oligo-N-substituted glycine polymers that can mimic the structural motifs and functionalities of proteins. Recently, novel sheet-like nanostructured materials have been self-assembled from peptoids under physiological conditions. These structures are biocompatible and may be selectively functionalized. We have constructed atomistic models of peptoids using high level ab initio calculations to guide the parameterization of a classical force field based on the CHARMM22 peptide force field. Atomistic molecular dynamics simulations show the accessible configurations of peptoids in water to be markedly different from those of peptides. We have also used our parameterized force field to study the molecular structure of peptoid sheet-like nanostructures. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z42.00008: Periciliary Layer as a Protective Barrier of Human Airways Liheng Cai, Brian Button, Richard Boucher, Michael Rubinstein The human airway surface layer consists of an overlaying gel-like mucus layer and a lower periciliary layer (PCL) protecting epithelial surface from mucus and the pathogens it contains. We investigated the permeability of the PCL using polymers that can readily penetrate through mucus. We found that in dilute solutions dextran larger than $\sim $30 nm are excluded from the PCL, whereas dextran smaller than that can penetrate the PCL. The penetration depth increases (distance of dextran from epithelial surface decreases) as the dextran size decreases. We also found that the PCL can be compressed by semidilute solutions of dextran larger than 50 nm with concentration above a certain value, at which the solution correlation length (osmotic pressure) is about 30 nm (300 Pa). Above this concentration the height of the PCL decreases with the increasing concentration (osmotic pressure). The dependence of the PCL height on correlation length for semidilute solutions that compresses it is similar to the dependence of distance from cell surface on dextran size for the case of the PCL penetration by smaller polymers from dilute solutions. Our results suggest that the PCL protects the airways by limiting the penetration of inhaled infectious particles. [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z42.00009: Cellulose aerogel from ionic liquid solution dried by silylation Dmitry Rein, Yachin Cohen Aerogels are a class of materials characterised by a highly porous structure with low solids content. There is much interest in cellulose aerogel (aerocellulose) as a biodegradable and sustainable material. Cellulose lyogel can be fabricated from its solution in ionic liquids (IL) by coagulation with a nonsolvent such as water. However, subsequent drying capillary forces in the gel pores that result in severe shrinkage and pore closure. The use of supercritical fluids for drying or freeze-drying entails high equipment and energy requirements. We describe the fabrication and structure of aerocellulose fabricated from IL solution with a simple novel drying process: Addition of a compatible reactive agent (trimethylchlorosilane) and its diffusion into the water-swollen cellulose hydrogel pores results in a reaction with water as well as the pore surface hydroxyl groups. The remaining hydrophobic compound (hexamethyldisiloxane-HMDS), which fills the initially hydrophilic cellulose hydrogel pores, has a low intrinsic surface tension in the pores allowing easy drying with minimal shrinkage. Furthermore it allows modification of the pore surface and even fabrication of cellulose-polysiloxane composites. Relations between aerocellolose processing conditions and the resulting structural features will be discussed. [Preview Abstract] |
Friday, March 25, 2011 1:27PM - 1:39PM |
Z42.00010: Macroscopic structure and properties of aqueous methylcellulose gels Tirtha Chatterjee, Roland Adden, Meinolf Brackhagen, Alan I. Nakatani, David Redwine, Robert L. Sammler Cold semi-dilute aqueous methylcellulose (MC) solutions are known to undergo thermoreversible gelation when warmed. Here, studies on two MC materials, which contrast in thermal gelation performance (gel temperature, hot gel modulus etc.) even though they were prepared with similar methyl ether substitution levels and molecular weight distributions are presented. Small-angle neutron scattering (SANS)* measurements reveal differences in their gel structures which presumably are relevant to their thermal gelation performances. MC gel with higher gel temperature and lower hot gel modulus contains a single temperature invariant characteristic length ($\sim $ 1000 {\AA}). However, besides this length scale, an additional and distinct smaller structure is also observed for the material with the lower gel temperature and the higher hot gel modulus. Further, in this case, the characteristic length scale decreases as temperature rises where as, the other length scale (smaller in size) remain almost temperature-invariant. The smaller domain size of the gel structure leads to the higher hot gel modulus for these methylcellulose materials. *Performed at NG3 beamline, NCNR, NIST. [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 1:51PM |
Z42.00011: Responsive Gelation in Physical Double Network Hydrogels from Artificial Protein Polymers B.D. Olsen, M.J. Glassman Artificial protein polymers with responsively associating groups on two different length scales are engineered in order to form physical double networks with potential application as shear-thinning hydrogels that may be toughened after injection. Gel-forming molecules are prepared by conjugating poly(N-isopropylacrylamide) (PNIPAM) at both ends of an artificial protein polymer to form PNIPAM-protein-PNIPAM triblock copolymers. Aggregation of the polymer endblocks forms a longer length scale network, while associating coiled-coil groups within the protein midblock form a shorter length scale network. At low temperatures where the coiled-coil domains are physically crosslinked but the copolymer endblocks are soluble, the materials form soft shear-thinning hydrogels. Elevating the temperature results in self-assembly of the second network, as manifest by stiffening of the gels. The structure of the materials is characterized using light scattering, X-ray scattering, and microscopy. Kinetics of the second network formation are characterized by linear oscillatory shear rheology, and nonlinear rheology is used to characterize the effect of the second network on the yield stress in these gels. [Preview Abstract] |
Friday, March 25, 2011 1:51PM - 2:03PM |
Z42.00012: Universality in Nonlinear Elasticity of Biological and Polymeric Networks and Gels Jan-Michael Carrillo, Andrey Dobrynin Networks and gels are part of our everyday experience starting from automotive tires and rubber bands to biological tissues and cells. Biological and polymeric networks show remarkably high deformability at relatively small stresses and can sustain reversible deformations up to ten times of their initial size. A distinctive feature of these materials is highly nonlinear stress-strain curves leading to material hardening with increasing deformation. This differentiates networks and gels from conventional materials, such as metals and glasses, showing linear stress-strain relationship in the reversible deformation regime. Using theoretical analysis and molecular dynamics simulations we propose and test a model that describes nonlinear mechanical properties of a broad variety of biological and polymeric networks and gels by relating their macroscopic strain hardening behavior with molecular parameters of the network strands. This model provides a universal relationship between the strain-dependent network modulus and the network deformation and explains strain-hardening of natural rubber, synthetic polymeric networks, and biopolymer networks of actin, collagen, fibrin, vimentin and neurofilaments. [Preview Abstract] |
Friday, March 25, 2011 2:03PM - 2:15PM |
Z42.00013: Criticality and isostaticity in fiber networks Xiaoming Mao, Olaf Stenull, Tom C. Lubensky, Chase P. Broedersz, Fred C. Mackintosh We investigated the elastic response of model semiflexible networks based on diluted periodic lattices, using a new effective medium theory and numerical simulations. In this model, central forces link nearest neighbor sites and bending forces link second neighbor sites along fibers. We found that by turning on fiber bending rigidity, the central force rigidity critical point became unstable, and the lattices lose rigidity at a lower threshold that is independent of fiber bending rigidity. We calculated scaling relations and exponents at both critical points. In addition to the bending and stretching dominated regimes, we identified a novel bend-stretch coupled regime in the vicinity of the central force critical point, in which the shear modulus exhibits a fractional power-law dependence on both the fiber bending and stretching rigidities. [Preview Abstract] |
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