Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session A25: Focus Session: Mechanical Properties, Fracture & Adhesion |
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Sponsoring Units: DPOLY Chair: Alfred Crosby, University of Massachusetts-Amherst Room: Colorado Convention Center 203 |
Monday, March 5, 2007 8:00AM - 8:36AM |
A25.00001: Mechanics of polymer interfaces Invited Speaker: Although size-dependent effects of constraint and deformation volumes on elastoplastic mechanical behavior of metallic and ceramic structures are increasingly well-studied, relatively little is known about how the deformation of polymers depends on microstructural and physical length scales. In particular, it is not yet clear how the structural and mechanical properties of amorphous (glassy) polymers differ at free surfaces, at rigid interfaces, and within the bulk. Such understanding is important in that free surface and interface properties dominate the mechanical behavior of (bio)polymeric thin film and nanocomposite applications. Recent experiments have demonstrated as much as a 50{\%} depression in the glass transition temperature $T_{g}$ within $\sim $100 nm of the free surface in amorphous polystyrene (PS) and poly(methyl methacrylate) (PMMA) thin films [1-3]. This indicates possible differences in the amorphous topology and/or macromolecular mobility that induce a mechanical response quite different from that indicated via bulk or $\mu $m-scale testing, even at room temperature, within 100 nm of the free surface. Here, we employ spherical nanoindentation experiments and analytical models to determine the indentation elastic moduli $E_{i}$ of three well-characterized, amorphous polymer surfaces (PS, PMMA, and polycarbonate or PC) for maximum contact depths ranging from 5 nm to 250 nm. Over this range, we observe a 200{\%} increase in $E_{i }$with respect to the bulk $E_{i}$. We demonstrate that this apparent stiffening of the polymeric surfaces cannot be attributed to experimental artifacts such as surface roughness, assumptions of indenter contact area, or loading rates. Further, we consider this effect as a function of monomer structure for a given molecular weight, molecular weight for a given monomer structure, processing routes (injection and compression molded, and spin coating), and physical environment (temperature and humidity). We propose a model for the physical basis of and length scale of this surface stiffening with respect to the structural length scales of these macromolecules, and discuss the implications of this effect in terms of mechanical performance for synthetic and biological polymeric nanocomposites [Preview Abstract] |
Monday, March 5, 2007 8:36AM - 8:48AM |
A25.00002: ABSTRACT WITHDRAWN |
Monday, March 5, 2007 8:48AM - 9:00AM |
A25.00003: A novel approach to friction measurements using dewetted polymer droplets Andrew B. Croll, Kari Dalnoki-Veress Friction is still quite difficult to accurately measure. The current state-of-the-art in friction measurement is the surface forces apparatus (SFA) or lateral force microscopy (LFM). While both are very useful tools, they suffer from a complicated distribution of pressure between substrate and slider. Here we present an experiment that overcomes that obstacle and also bridges the gap between point (LFM) and large area (SFA) contact. In these experiments we measure directly the constitutive law, which is not hindered by surface curvature effects because the dewetted polymer droplet acts as a slider with a perfectly flat interface. In the case of friction between a polystyrene droplet and an ultra-thin layer of poly(dimethyl siloxane), we obtain a power law dependence of friction on slider speed. Interestingly, the exponent of the power law is related to the normal force applied in a simple way. [Preview Abstract] |
Monday, March 5, 2007 9:00AM - 9:12AM |
A25.00004: Capillary wrinkling of thin floating films Jiangshui Huang, Wim H. de Jeu, Narayanan Menon, Thomas P. Russell We study the wrinkling instability induced in freely-floating polystyrene films, tens of nanometers in thickness, by the interfacial tension of tiny drops of water placed on their surface. The wrinkling pattern is characterized by the number, N, and length, L, of the wrinkles. The dependence of N on the elastic properties of the sheet and on the capillary force exerted by the drop provides a detailed experimental test of recent theoretical predictions on wavelength selection in the wrinkling instability. A scaling relation is developed for the length of the wrinkles. These scaling relations for the number and length of the wrinkles demonstrate the basis for a metrology of the elasticity and thickness of extremely thin films that relies on no more than a dish of fluid and a low-magnification microscope. [Preview Abstract] |
Monday, March 5, 2007 9:12AM - 9:24AM |
A25.00005: Nanometer voids prevent crack growth in polymer thin films Hideaki Yokoyama, Cedric Dutriez, Kotaro Satoh, Masami Kamigaito Macroscopic voids initiate cracks and cause catastrophic failure in brittle materials. The effect of micrometer voids in the mechanical properties of polymeric materials was studied in 1980's and 90's with the expectation that such small voids may initiate crazing, the toughening mechanism in polymer solids, similar to dispersed rubber particles widely used in industry. However, the micrometer voids showed only limited resistance against crack growth, and it was concluded that much smaller voids are necessary for the drastic change in mechanical properties. We have recently succeeded the nondestructive introduction of nanometer voids (30--70 nm) in polymeric materials using block copolymer template and carbon dioxide (CO$_2$) by partitioning CO$_2$ in CO$_2$-philic nanodomains of block copolymers. The reduction of Young's modulus with such nanometer voids was minimal (2 to 1 GPa) due to the (short-range) ordered spherical voids. While the unprocessed copolymer films failed in brittle manner at around 2 \% of tensile strain, the processed copolymer films with nanometer voids did not break up to at least 60 \%. A microscopic observation under strain of the crack tip revealed that the nanometer voids were deformed under strain and directly converted into the networked fibrils near the crack tip similar to crazing and thus prevented the crack growth. [Preview Abstract] |
Monday, March 5, 2007 9:24AM - 9:36AM |
A25.00006: Aging with Applied Strain of a Black-Filled Natural Rubber Vulcanizate: Intrinsic Flaw Sizes Crittenden Ohlemacher, Gary Hamed Black-filled natural rubber, with an inefficient sulfur cure, was aged at 90$^{\circ}$C and 110$^{\circ}$C under nitrogen, with and without applied strain. Samples aged under strain became ``double networks'' and retained a residual extension ratio. Intrinsic flaws are distributed randomly throughout the rubber, and influence its breaking strength. Intrinsic flaw sizes of single networks aged under nitrogen purge were larger than those of the unaged networks, except for the most severe aging. The increase is attributed to annealing of the network; however, as aging becomes more severe, this is offset by: 1) decreased ability of the rubber to dissipate energy, and 2) increased oxidation damage to the network. For samples strained to $\lambda_{i}$ = 2.0 for 48 hours at room temperature, the intrinsic flaw size increased by a factor of 1.7. Perhaps aging under strain promotes healing of network defects. For double networks, perpendicular specimens generally had intrinsic flaw sizes similar to single networks. Parallel specimens generally had smaller intrinsic flaw sizes than single networks, similarly aged. The limited extensibility of the oriented chains dominates the effects of microvoid healing and annealing. [Preview Abstract] |
Monday, March 5, 2007 9:36AM - 9:48AM |
A25.00007: Physical Aging And Non-Exponentiality In A Crosslinked Coating Subjected To Degradative Weathering. B.M.D. Fernando, X. Shi, S.G. Croll Polymeric coatings provide protection and aesthetics for many materials and equipment, and, in service, they must fulfill their roles for extended periods in a predictable manner. Molecular relaxation in a polymeric coating that is degrading during weathering is affected both by the ambient conditions and concurrent chemical degradation by ultraviolet radiation or other aggressive species. Purely physical aging was contrasted with the effect of concurrent chemical degradation by measuring non-exponentiality which showed some differences according to whether it was determined from `enthalpy recovery' or stress relaxation measurements. Less directly determined parameters, such as `non-linearity' and the size of `co-operatively relaxing regions', also changed. Changes in fictive temperature at each level of degradation demonstrated that physical aging was in competition with the effect of chemical degradation in the crosslinked network. Relaxation times measured in this coating extended longer than cycle periods typical of accelerated weathering tests, suggesting that frequency effects might be important when resolving differences in outcome between natural and laboratory, accelerated weathering cycles. [Preview Abstract] |
Monday, March 5, 2007 9:48AM - 10:00AM |
A25.00008: The Elastic Properties of Polymer Nanofibers: Influence of Confinement on Conformation State of Macromolecules and Supermolecular Structures Arkadi Arinstein, Michael Burman, Eyal Zussman This research deals with open problems concerning polymer materials with reduced size and dimension such as thin and ultra thin films, nanofibers, and nanotubes. Such materials exhibit exceptional mechanical properties compared to those of their macroscopic counterparts. In particular, abrupt increase in Young modulus of polymer nanofibers has been observed when their diameters became small enough. Such features are poorly understood, and lack of explanation of the observed phenomena, based on mechanical (macroscopic) concepts, requires detailed microscopic examination of systems in question. We hypothesize that the supermolecular structure is the dominant role in the deformation process of polymer nanofibers, more precisely, confinement of this supermolecular structures which is caused by shrinking of the transversal size of above objects. In this work we report on results of our studies in conformational statistics of polymer macromolecules under conditions of confinement, and supermolecular structure formation; and on experimental studies of the mechanical and structural properties of electrospun nanofibers. [Preview Abstract] |
Monday, March 5, 2007 10:00AM - 10:12AM |
A25.00009: Predicting Structure-Property Relationship in Segmented Polyurethanes Valeriy Ginzburg, Alan Schrock, Christopher Christenson, Jozef Bicerano, Alexander Patashinski We develop new theoretical framework to study the relationship between composition and mechanical properties in segmented polyurethanes (PU) and poly(urethane-ureas) (PUU). In particular, we analyze polymer mechanical properties (quasi-static Young's modulus, E, and temperature-dependent storage shear modulus, G') as function of the hard and soft segment chemistry, hard segment weight fraction, soft segment molecular weight, and temperature. It has been known for some time [1] that in many segmented PU and PUU polymers, the hard-soft microphase separation causes the formation of a ``percolated hard phase''. We propose a new formalism that enables one to predict the onset of the hard phase percolation as function of temperature, soft segment molecular weight, and chemical structures of hard and soft segments. Based on this formalism, we can build micromechanical models to estimate mechanical properties of segmented polyurethanes as function of temperature. We used this theoretical framework to simulate storage moduli of several model compounds, and found very reasonable qualitative agreement with experimental data. [1] See, e.g., A. J. Ryan et al., \textit{Macromolecules,} \textbf{24}, 2883 (1991); \textit{Polymer} \textbf{32}, 1426 (1991). [Preview Abstract] |
Monday, March 5, 2007 10:12AM - 10:24AM |
A25.00010: Compliance Effects of a Modern Rheometer Stephen Hutcheson, Gregory McKenna Instrument compliance effects caused by both the transducer and entire instrument itself can induce large errors on shear measurements of viscoelastic properties of materials [1,2]. This effect can also lead to an error in estimating the relaxation time and shape parameter in the Kohlrausch-Williams-Watts (KWW) function [3]. We present examples of instrument compliance effects on the measurement of the material properties of small molecular glass formers and a commercially available polydimethysiloxane (PDMS) rubber using a TA Instruments ARES Rheometer. The 2KFRT (Force Rebalanced Transducer) was replaced with a strain gage transducer (Honeywell-Sensotec). Stress relaxation and dynamic frequency sweep experiments were performed. We also present a technique to correct for compliance effects in stress relaxation experiments and dynamic frequency sweep experiments. Recommendations are made for both experimental and instrument design to avoid and/or minimize compliance effects. [1] M Gottlieb and C.W. Macosko, Rheol. Acta 1982 90-94. [2] M.E. Mackay and P.J. Halley, J. Rheol. 1991 1609-14. [3] R. Kohlrausch, Poggendorf's Ann. Phys. 91, 179 (1854).; G. Williams and D. C. Watts, Trans. Faraday Soc. 66, 80 (1970). [Preview Abstract] |
Monday, March 5, 2007 10:24AM - 10:36AM |
A25.00011: Entanglements of End Grafted Polymer Brushes in a Polymeric Matrix Gary S. Grest, Robert S. Hoy The entanglement of a polymer brush immersed in a melt of mobile polymer chains is studied by molecular dynamics simulations. A primitive path analysis (PPA) is carried out to identify the brush/brush, brush/melt and melt/melt entanglements as a function of distance from the substrate. The PPA characterizes the microscopic state of conformations of the polymer chain and is ideally suited to identify chain/chain entanglements. We use a new thin-chain PPA technique to eliminate spurious non-entangled inter chain contacts arising from excluded volume. As the grafting density of the brush increases we find that the entanglements of the brush with the melt decrease as the system crosses over from the wet to dry brush regime. Results are compared to brush/brush entanglements in an implicit solvent of varying solvent quality. Sandia is a multiprogram laboratory operated by Sandia Corp., a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000. [Preview Abstract] |
Monday, March 5, 2007 10:36AM - 10:48AM |
A25.00012: High Strength Development at Incompatible Semicrystalline Polymer-Polymer Interfaces C.H. Hong, Richard Wool For incompatible A/B interfaces, the strength G$_{1c}$ is related to the equilibrium width w (normalized to the tube diameter) of the interface by G$_{1c}$/G* = (w-1), where G* is the virgin strength [R.P. Wool, C.R, Chimie, 9 (2006) 25]. However, the interface strength is quite weak due to very limited interdiffusion. The mechanism of high strength development of a series of thermoplastic polyurethane elastomers (TPU) bonding with ethylene vinyl alcohol copolymers (EVOH) was investigated. During cool down of the A/B interface in the co-extruded melt, there exits a unique process window---the $\alpha -\beta $ window-which promotes considerable strength development. We used the differences in melting points and the volume contraction during asymmetric crystallization to generate influxes ($\Sigma $ nano-nails/unit area), where an influx occurs by the fluid being pulled into the crystallizing side. TPU samples with higher degree of crystallization typically exhibited higher peel strengths, due to the formation of both inter- and intra- spherulitic influxes of nano-dimension across the interface. The peel energy now behaves as G$_{1c} \quad \sim $ $\Sigma $L$^{2}$, where L is the length of the influx and L$>>$w. Annealing between the $\alpha $ and $\beta \quad {\rm t}$ relaxation temperatures of the EVOH generated additional influxes which provided significant connectivity and peel strength. [Preview Abstract] |
Monday, March 5, 2007 10:48AM - 11:00AM |
A25.00013: Elastic breakup in extensional flow of entangled melts. Yangyang Wang, Pouyan Boukany, Shi-Qing Wang In contrast to shear deformation, extensional flow behavior is more difficult to study because (a) the experimental system is always of finite dimensions and (b) the total strain in any given test is only finite. Despite uncertainties, there has been great deal of research carried out to explore various aspects of nonlinear behavior of entangled polymers. We applied our recently developed new theory [1] for flow of entangled polymers to predict the uniaxial stretching behavior of entangled melts. Specifically, we show that cohesive breakdown would take place, under the same conditions independent of molecular weight, during and after extension when the elastic force due to stretching overcomes a combination of the entanglement force and inter-chain frictional interactions. [1] \textit{Phys. Rev. Lett. }\textbf{97}, 187801 (2006); full manuscript to be submitted to \textit{J. Chem. Phys.}. [Preview Abstract] |
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