Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session A44: Friction, Adhesion, and Fracture of Polymers |
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Sponsoring Units: DPOLY Chair: Al Crosby, University of Massachusetts--Amherst Room: A309 |
Monday, March 21, 2011 8:00AM - 8:12AM |
A44.00001: Contact Adhesion of Wrinkled Surfaces Chelsea Davis, Alfred Crosby Inspired by examples in nature, recent research advances have demonstrated the ability to use topographic surface patterns rather than chemical modifications to control surface properties such as adhesion, wettability, and friction. Although most synthetic efforts have focused on the use of complicated lithographically-fabricated fibrillar structures, the use of spontaneously formed structures, such as surface wrinkles, have also proven advantageous. Wrinkles present many attributes, such as discretized length scales, which play an important role in adhesion control, yet the exact mechanisms for this control are not fully understood. We present a systematic study of the contact adhesion mechanics between a flat, rigid surface and a soft wrinkled surface. The wrinkles are fabricated using a technique that allows the effects of residual surface stresses and wrinkle topography to be decoupled in the context of adhesion control. We find that the maximum separation force for the wrinkled-flat interfaces increases with decreasing values of wrinkle wavelength and amplitude. These trends can be understood through the development of a simple scaling relationship, which links wrinkle geometry and materials properties to the maximum separation force. [Preview Abstract] |
Monday, March 21, 2011 8:12AM - 8:24AM |
A44.00002: Structural Effects on the Friction of Tethered PDMS Networks Claude Cohen, Lucas Landherr, Lynden Archer The interfacial properties of dry, surface-tethered end-linked polydimethylsiloxane (PDMS) films on silicon are examined. Thin network films (approximately 10 microns thick) were synthesized over a self-assembled monolayer supported on a silicon wafer. By systematically increasing the concentration of mono-functional PDMS chains in a mixture with telechelic precursor chains during cross-linking, structures ranging from near model elastic networks to very poorly cross-linked networks dominated by a preponderance of dangling/pendent chains were synthesized. Lateral force microscopy (LFM) employing a PE bead probe was used to quantify the effect of network structure and the role of viscoelasticity on the interfacial friction coefficient. [Preview Abstract] |
Monday, March 21, 2011 8:24AM - 8:36AM |
A44.00003: Lithography-Free Microchannel Fabrication in PDMS Jeyantt S. Sankaran, Wintana T. Kahsai, Uyen H.T. Pham, Samir M. Iqbal We report a novel method for the fabrication of microchannels that could potentially be used for pervaporation experiments, cell adhesion and cell movement studies and detection of selective protein bio-markers. PDMS can sustain high temperatures, has a high young's modulus and it is biologically inert. Hydrophobic-hydrophilic interactions at gel point of PDMS form the basis of the presented technique. The repulsion of hydrophilic particles by the hydrophobic polymer matrix, stemming from the reduction of entropy and free energy variations during polymerization, provides an elegant lithography-independent approach for the fabrication of self-aligned microchannels. [Preview Abstract] |
Monday, March 21, 2011 8:36AM - 8:48AM |
A44.00004: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 8:48AM - 9:00AM |
A44.00005: Polymer Brushes that Mimic Repulsive Properties of the Boundary Lubricant Glycoprotein Lubricin Jahn Torres, Gregory Jay, Qian Ni, David Bello, Geoffrey Bothun, Kyung-Suk Kim This is a report on the design of tailored functional groups which mimic the repulsive forces at work in the natural-joint boundary lubricant known as \textit{lubricin}. \textit{Lubricin}, an amphiphilic polyelectrolyte biomolecule, decreases friction and \textit{cellular adhesion} by exhibiting surface force fields based on \textit{steric hindrance}, \textit{Debye electrostatic double layer repulsion} and \textit{hydration repulsive forces}. We have identified a physically and chemically stable candidate polymers for anti-fouling coatings that will mimic lubricin's repulsive properties. Synthetic polymer brushes mimicking lubricin have been produced using these polymers grafted onto a glass surfaces. The average adhesive forces for the polymer brushes measured through atomic force microscopy are as low (56.796 $\pm $ 0.796 mN/m), similar to those exhibited by lubricin coated surfaces and on the same order of magnitude as superhydrophobic surfaces. [Preview Abstract] |
Monday, March 21, 2011 9:00AM - 9:12AM |
A44.00006: Activation-deactivation of self-healing in supramolecular rubbers Laurent Corte, Florine Maes, Damien Montarnal, Sabine Cantournet, Francois Tournilhac, Ludwik Leibler Self-healing materials have the ability to restore autonomously their structural integrity after damage. Such a remarkable property was obtained recently in supramolecular rubbers formed by a network of small molecules associated via hydrogen bonds [1]. Here we explore this self-healing through an original tack experiment where two parts of supramolecular rubber are brought into contact and then separated. These experiments reveal that a strong self-healing ability is activated by damage even though the surfaces of a molded part are weakly self-adhesive. In our testing conditions, a five minute contact between crack faces is sufficient to recover most mechanical properties of the bulk while days are required to obtain such adhesion levels with melt-pressed surfaces. We show that the deactivation of this self-healing ability seems unexpectedly slow as compared to the predicted dynamics of supramolecular networks. Fracture faces stored apart at room temperature still self-heal after days but are fully deactivated within hours by annealing. Combining these results with microstructural observations gives us a deeper insight into the mechanisms involved in this self-healing process. [1] P. Cordier, F. Tournilhac, C. Soulie-Ziakovic {\&} L. Leibler, Nature, 451, 2008. [Preview Abstract] |
Monday, March 21, 2011 9:12AM - 9:24AM |
A44.00007: Self-Healing of Polyethylene Oxide Dorina Magdalena Chipara, Maritza Flores, Nancy Puente, Karen Lozano Autonomic self-healing is expected to enhance the lifetime of polymeric materials, resins, and composites subjected to long term mechanical stresses. The self-healing process is initiated by the rupture of some polyurea-formaldehyde microcapsules filled with monomer. The self-healing polymer is actually a compound containing microcapsules filled with monomer and catalyst particles. The monomer released from these broken microcapsules is diffusing within the polymer, reacting with the catalyst and starting a polymerization reaction. This new polymer, growing within the propagating crack, stops the mechanical failure. While the process is pretty slow (timescale of the order of 10 to 100 s), there are many important technological applications that would benefit from the availability of self-healing polymers. We report about the addition of self-healing capabilities to polyethylene oxide by using polyurea formaldehyde microcapsules filled with dicyclopentadiene and first generation Grubbs catalysts. Details regarding the physical and chemical steps used to add self-healing capabilities to polyethylene oxide will be presented. Self-healing efficiency was assessed by fatigues tests. [Preview Abstract] |
Monday, March 21, 2011 9:24AM - 9:36AM |
A44.00008: Modeling the Nano-indentation of Self-healing Materials Solomon F. Duki, German V. Kolmakov, Victor V. Yashin, Tomasz Kowalewski, Krzysztof Matyjaszewski, Anna C. Balazs We use computational modeling to determine the mechanical response of crosslinked nanogels to an atomic force microscope (AFM) tip that is moved through the sample. We focus on two-dimensional systems where the nanogels are interconnected by both strong and labile bonds. We model each nanogel as a deformable particle using the modified lattice spring model that is applicable to a broad range of elastic materials.We utilize the Bell model to describe the bonds between these nanogel particles, and subsequently, simulate the rupturing of bonds due the force exerted by the moving indenter. The ruptured labile bonds can readily reform and thus, can effectively mend the cavities formed by the moving AFM tip. We determine how the fraction of labile bonds, the nanogel stiffness, and the size and velocity of the moving tip affect the self-healing behavior of the material. We find that samples containing just 10$\%$ of labile bonds can heal to approximately 90$\%$ of their original, undeformed morphology. [Preview Abstract] |
Monday, March 21, 2011 9:36AM - 9:48AM |
A44.00009: Viscoelastic solid glue produced by orb-weaving spiders Vasav Sahni, Todd Blackledge, Ali Dhinojwala Modern orb-weaving spiders have evolved well-designed adhesives to capture preys. This adhesive is laid on a pair of soft and highly extensible axial silk fibers as micron-sized glue droplets that are composed of an aqueous coat of salts surrounding the nodules made of glycoproteins. Understanding the adhesion mechanism of these glue droplets has been challenging because both the glue droplets and the axial fibers contribute to the adhesive forces required to detach a thread from a surface. Here, we have decoupled these contributions by developing a novel experimental method to probe individual glue droplets and an energy model to separate the strain energy of the axial silk fibers from the adhesion energy required to peel the glue droplets. We observe that the glue droplets behave as a viscoelastic solid and are strongly affected by humidity and the rate of peeling. Knowledge of the adhesion and the mechanics of the glue will aid in developing bioinspired adhesives in the future. [Preview Abstract] |
Monday, March 21, 2011 9:48AM - 10:00AM |
A44.00010: Hydrophobic Interactions on a Protein-Polymer Functionalized Surface of Varying Hydrophilicity Kristen Keller, Pinar Akcora We have developed a novel heterogeneous surface consisting of streptavidin and poly(methyl methacrylate) (PMMA) grafted to silicon substrates. Such a system has been fabricated using the ever-growing click chemistry approach. Functionalities found at the surface of the substrates are characterized through FTIR while the hydrophobic effects arising from the interactions between the grafted components of differing hydrophilicities are investigated through AFM. Adhesive properties of such a heterogeneous surface are calculated using data acquired from force-distance measurements. Furthermore, changes in these properties resulting from variations in streptavidin surface coverage and PMMA chain length are similarly studied. [Preview Abstract] |
Monday, March 21, 2011 10:00AM - 10:12AM |
A44.00011: Dissipative mechanisms of the lateral friction in contact-mode atomic force microscopy of flexible alkane molecule films F.Y. Hansen, P. Soza, H. Taub, M. Kiwi, U. Volkmann Molecular dynamics simulations are used to investigate lateral friction in contact-mode Atomic Force Microscopy of tetracosane ($n$-C$_{24}$H$_{50}$) films. We find larger friction coefficients on the surface of monolayer and bilayer films in which the long axis of the molecules is parallel to the interface than on a surface of molecules with the long axis perpendicular to the surface, in agreement with experimental results. The simulations reveal that the strength of the attractive film-tip interaction is an important factor in energy dissipation and that molecular flexibility provides a major dissipative mechanism as manifested by torsional motion about the carbon-carbon bonds of the molecules. [Preview Abstract] |
Monday, March 21, 2011 10:12AM - 10:24AM |
A44.00012: Stress-Induced Slip at Polymer-Polymer Boundaries Andrew Gustafson The phenomena of stress-induced tangential slip at polymer-polymer interfaces is studied by simulation and analytic theory. Simulations combine a slip-link model of entanglement with a self-consistent field description of the interface. We consider how the slip velocity depends upon shear stress, interfacial entanglement density, and polymer chain length. Our analysis assumes that the strongly non-linear shear thinning of the interface observed in experiment is a result of convective release of interfacial entanglements. [Preview Abstract] |
Monday, March 21, 2011 10:24AM - 10:36AM |
A44.00013: Optical Properties of Isolated MEH-PPV polymers in Developing Crazes Meng-Kuan Wang, Chiau-Heng Tsau, Wei-Cheng Li, Arnold Chang-Mou Yang, Jui-Hung Hsu, Jonathon D. White Potential applications in light emitting devices and solar cells have led to extensive research into optimizing the optical properties of Luminescent Conjugated Polymers. Straining MEH- PPV/polystyrene thin films has been observed to result in craze formation and an enhancement of photoluminescence (PL). Using confocal microscopy, the optical properties of these crazes were investigated. Emission from developed crazes was found to be highly polarized while a variety of effects were found for developing crazes. The survival time of polymers in the crazed regions was increased by over 30\% relative to the bulk. This suggests a stretch induced alignment of emitting segments in MEH-PPV as well as an increased resistance to photobleaching. [Preview Abstract] |
Monday, March 21, 2011 10:36AM - 10:48AM |
A44.00014: Quantitative surface parameter maps using Intermodulation Atomic Force Microscopy Daniel Forchheimer, Daniel Platz, Erik Thol\'en, Carsten Hutter, David Haviland It is well known that the phase image in amplitude modulation atomic force microscopy (AM-AFM) is sensitive to material properties of the surface. However that information is not enough to fully quantify the tip-surface interaction. We have developed Intermodulation AFM, based on a spectral analysis of the cantilever's {\em nonlinear} dynamics, which increases the amount of information obtained without increasing scan time.\footnote{D. Platz, E. A. Tholen, D. Pesen, and D. B. Haviland, Appl. Phys. Lett., 92, 153106 (2008)} We show how it is possible to extract quantitative material properties of the surface from this additional information. The method works under the assumption of a tip-surface force model, such as the DMT model, fitting the model parameters to the measured spectral data. The parameters are obtained at each pixel of the AFM image and form surface property maps which can be displayed together with topography. We demonstrate this on different surfaces such as polymer blends, extracting stiffness and adhesive properties. [Preview Abstract] |
Monday, March 21, 2011 10:48AM - 11:00AM |
A44.00015: Direct Measurement of Acid-Base Interaction Energy at Polymer-Solid Interfaces Anish Kurian, Shishir Prasad, Ali Dhinojwala We have studied acid-base interactions at solid-liquid and solid- solid interfaces using interface-sensitive sum frequency generation (SFG) spectroscopy. The shift of the sapphire hydroxyl peak in contact with several polar and non-polar liquids and polymers was used to determine the interaction energy. The trend in the interaction energies cannot be explained by only measuring water contact angles. Molecular rearrangements at the sapphire interface, to maximize the interaction of the acid-base groups, play a dominant role and these effects are not accounted for in the current theoretical models. These results provide important insights in understanding adhesion, friction, and wetting on solid interfaces. In addition, we will present the consequences of the acid-base interactions on understanding surface segregation in polymer blends and copolymers. [Preview Abstract] |
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