Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session V30: Focus Session: Mechanical Properties: Microscale Deformation and Failure |
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Sponsoring Units: DPOLY Chair: Jae Hyun Kim, National Institute of Standards and Technology Room: Baltimore Convention Center 327 |
Thursday, March 16, 2006 11:15AM - 11:51AM |
V30.00001: Fundamental Fracture Behavior of Polymer Nanocomposites Invited Speaker: |
Thursday, March 16, 2006 11:51AM - 12:03PM |
V30.00002: Adhesion at Entangled Polymer Interfaces: A Unified Approach.. Richard Wool A unified theory of fracture of polymer interfaces was developed which was based on the Rigidity Percolation model of fracture [R.P. Wool, J.Polym.Sci. Part A: Polym Phys., 43,168(2005)]. The polymer fractured critically when the normalized entanglement density p, approached the percolation threshold p$_{c}$. The fracture energy was found to be G$_{1c} \quad \sim $ [p-p$_{c}$]. When applied to interfaces of width X, containing an areal density $\Sigma $ of chains, each contributing L chain entanglements, the percolation term p $\sim \quad \Sigma $L/X and the percolation threshold was related to $\Sigma _{c}$, L$_{c}$, or X$_{c}$. For welding of A/A symmetric interfaces, p = $\Sigma $L/X, and p$_{c} \quad \approx $ L$_{c}$/M $\approx $ 0, such that when $\Sigma $/X $\sim $1/M for randomly distributed chain ends, p$\sim $L $\sim $ (t/M)$^{1/2}$, G/G* = (t/$\tau $*)$^{1/2}$, where the weld time $\tau $* $\sim $ M. When the chain ends are segregated to the surface, $\Sigma $ is constant with time and G/G* = [t/$\tau $*]$^{1/4}$. For sub-T$_{g}$ welding, there exists a surface mobile layer (due to the critical Lindemann Atom fraction) of depth X $\sim $ 1/$\Delta $T$^{\nu }$ such that G $\sim $ $\Delta $T$^{-2\nu }$, where the critical exponent v = 0.8. For incompatible A/B interfaces of Helfand width d, normalized width w = d/R$_{ge}$, and entanglement density N$_{ent} \quad \sim $ d/L$_{e}$, p $\sim $ d such that, G$_{1c} \quad \sim $ [d-d$_{c}$], G$_{1c} \quad \sim $ [w-1], and G $\sim $ [N$_{ent}$-N$_{c}$]. For incompatible A/B interfaces reinforced by an areal density $\Sigma $ of compatibilizer chains, L and X are constant, p $\sim $ $\Sigma $, p$_{c} \quad \sim \Sigma _{c}$, such that G$_{1c} \quad \sim $ [$\Sigma -\Sigma _{c}$], which is in excellent agreement with experimental data. [Preview Abstract] |
Thursday, March 16, 2006 12:03PM - 12:15PM |
V30.00003: Effect of entanglement density on mechanical properties of glassy polymers Robert Hoy, Mark Robbins The mechanical properties of model glassy polymers are studied using computer simulations. Using primitive path analysis, we relate the macroscopic stress-strain behavior of polymer glasses to the microscopic evolution of the underlying entanglement network. The network plays little role in phenomena such as the initial yield and strain softening, but dominates the mechanical response at larger strains. Comparison of results for systems with a wide range of entanglement densities allows study of the onset of network- dominated behavior. The density of entanglements is varied by changing the bending stiffness and diluting with shorter chains. [Preview Abstract] |
Thursday, March 16, 2006 12:15PM - 12:27PM |
V30.00004: Entanglement Effects in Plastic Deformation of Filled Linear Polymer Glasses Cameron Abrams, David Richardson We have performed coarse-grained molecular dynamics simulations of linear polymer glasses with low weight fractions of stiff rodlike particles. This is relevant to understanding the molecular-scale origins of toughness in nanotube-filled polymer composites. Linear polymer glasses are tougher than classical brittle glasses because chains are force to slide along each other under tensile load to undo entanglements. We focus here on the role of (a) the concentration and (b) orientational distibution of the rod-like inclusion particles on the entanglement network. We observed a substantial (10\%) increase in the amount of work required to deform a particular glass plastically to a given strain (2.0) when equal weight fractions of rods were oriented randomly vs. parallel to the pulling axis. Using the method of primitive path analysis [Everaers, et al., Science 303:823 (2004)], we identify entanglements and particle-particle bridging interactions which are operative for a given pulling direction in a simulated tensile test in order to predict this enhancement from chain conformations in the initial unstrained sample. We apply this method to many samples with different weight fractions of rods, rod orientations, and rod-polymer interaction strengths. [Preview Abstract] |
Thursday, March 16, 2006 12:27PM - 12:39PM |
V30.00005: Nanoparticles alignment, exclusion and entrapment during failure of Glassy Polymer Nanocomposite Jong-Young Lee, Qingling Zhang, Todd Emrick, Alfred Crosby Crazing is a polymer deformation process during which dense arrays of nanoscale fibrils grow prior to the propagation of a crack. In the presence of nanoscale inorganic fillers, the mechanisms of craze formation and propagation are altered significantly. We use a model material of polystyrene blended with surface modified CdSe nanoparticles to investigate the interaction between polymer molecules and nanoparticles during the process of crazing. We demonstrate that nanoparticles in the presence of a craze undergo three stages of rearrangement: 1) Alignment along the precraze (fluid-like region), 2) Expulsion from nanoscale craze fibrils, and 3) Assembly into clusters trapped between craze fibrils. These results not only provide direct evidence for the physical mechanisms that control the mechanical properties of polymer nanocomposites, but they give fundamental insight into the behavior of polymers and nanoparticles in the presence of a directed field, which will impact a wide array of advanced material applications. [Preview Abstract] |
Thursday, March 16, 2006 12:39PM - 12:51PM |
V30.00006: Crazing in Glassy Polymer Nanocomposites Jong-Young Lee, Qingling Zhang, Todd Emrick, Alfred Crosby Crazing is a polymer deformation process in which dense arrays of nanoscale fibrils grow prior to the propagation of a crack. Here, we discuss experimental results on the impact of two nanostructured materials on the crazing process: 1) ordered glassy block copolymers and 2) homopolymer/nanoparticle composites. We not only find that an ordered lamellar microstructure leads a lower craze growth rate compared with polystyrene homopolymer, but also nanoscale, surface terraces significantly decrease the failure strain of these advanced materials. For homopolymer/nanoparticle composites, we discover significant alterations in the crazing process. Specifically, nanoparticles in the presence of a craze undergo three stages of rearrangement: 1) Alignment along the precraze (fluid-like region), 2) Expulsion from nanoscale craze fibrils, and 3) Assembly into clusters trapped between craze fibrils. Although nanoparticles have no effect on the initiation strain, fibril breakdown strain, and craze growth rate, the composite failure strain can be increased significantly by nearly 100{\%} compared to neat homopolymer films. These results provide direct evidence for the physical mechanisms that control the mechanical properties of polymer nanocomposites. [Preview Abstract] |
Thursday, March 16, 2006 12:51PM - 1:03PM |
V30.00007: Tensile Modulus Measurements of Carbon Nanotube Incorporated Electrospun Polymer Fibers Yavuz Ozturk, JaeMin Kim, Kwanwoo Shin Electrospinning has become a popular method for producing continuous polymer fibers with diameters in sub-micron scale. By this technique uniaxially aligned fibers can also be obtained, by using two separate parallel strips as conductive collectors. Uniaxial alignment of polymer fibers gives us the chance to well-characterize their structural properties via tensile modulus measurements. Here we report a simple and new technique for tensile testing of polymer fibers which employs a computerized spring-balance/step-motor setup. The key point in our technique is the production of fibers directly on the tensile tester by using two vertical strips as collectors. By this way, even fibers of very brittle nature can be tested without handling them. Calculation of total cross-sectional areas - which is crucial for determining stress values - was done by using scanning electron and optical microscope images for each sample. In this study we have investigated mechanical properties of Polystyrene (PS), Polymethylmethacrylate (PMMA) and PS/PMMA blend fibers; as well as Carbon Nanotube (CNT) incorporated PS, PMMA and PS/PMMA blend fibers. It is expected that the extraordinary mechanical properties of CNTs can be transferred into polymer matrix, by their incorporation into confined space within electrospun fibers. Here we analyzed the influence of CNT on polymer fibers as function of CNT amounts. [Preview Abstract] |
Thursday, March 16, 2006 1:03PM - 1:15PM |
V30.00008: Rate-dependent Mechanical Deformation Behavior of POSS-filled and Plasticized Poly(vinyl chloride). Sharon Soong, Robert Cohen, Mary Boyce In different temperatures or strain-rate regimes, the rate sensitivities of polymers change as various primary and secondary molecular mobility mechanisms are accessed. Incorporating nanoparticles into the polymer matrix can potentially alter the molecular level structure and offers an opportunity to tailor the rate-dependent mechanical deformation behavior of the polymer. In this study, methacryl-POSS (C56H88O28Si8) and dioctyl phthalate (DOP) were incorporated into poly(vinyl chloride) (PVC) through melt blending. Dynamic Mechanical Analysis revealed that the incorporation of POSS in PVC plasticizes PVC and reduces both the alpha and beta transition temperatures. As for the PVC/DOP blends, while the alpha-transition temperatures were reduced, beta-motions vanishes with high DOP contents. The rate dependent yield behavior is characterized in compression testing. Zwick Mechine is used for low to moderate strain rate (0.0001 to 0.1/s) and Split Hopkinson Bar is used for high strain rate (500 to 2000/s). It was found that PVC with POSS shows a delay in yield strength rate-sensitivity transition. For PVC with higher DOP contents which show restricted beta-motions, the rate-sensitivity transition in yield stress faded away. Constitutive model used was able to predict the rate-sensitivity transitions in the compression yield behavior of PVC compounds. [Preview Abstract] |
Thursday, March 16, 2006 1:15PM - 1:27PM |
V30.00009: Tensile Properties and Hysteresis Behavior of Graft Copolymers with Complex Molecular Architecture R. Weidisch, U. Staudinger, Y. Zhu, S.P. Gido, D. Uhrig, J.W. Mays, N. Hadjichristidis, H. Iatrou PS-PI multigraft copolymers with tri- tetra- and hexafunctional polystyrene branch points have been studied to investigate the influence of molecular architecture on tensile properties and hysteresis behaviour. It was found that mechanical properties are mainly controlled by functionality of the graft copolymers and the number of branch points per molecule. Tetrafunctional multigraft copolymers show surprising high strain at break values up to 1550 {\%}. With increasing number of branch points strain at break and tensile strength increase, where a linear dependence of mechanical properties on the number of branch points is obvious. Hysteresis experiments have proved excellent elasticity of tetra and hexafunctional multigrafts far exceeding that of commercial elastomers like Styroflex. A tetrafunctional multigraft copolymer can be deformed till 1400 {\%} showing a residual strain of only 40 {\%} demonstrating super elastic property profile. [Preview Abstract] |
Thursday, March 16, 2006 1:27PM - 1:39PM |
V30.00010: Structure and Tensile mechanical properties of poly(ester urethane) materials. Marilyn Hawley, Robert Houlton, Philip Rae, E. Bruce Orler, Debra Wrobleski Scanning probe microscopy (SPM) techniques, primarily phase imaging, and Stress/Strain Tensile Measurements were used to study the properties of model poly (ester urethane) and 23{\%} Estane{\texttrademark} compression molded samples. SPM was used to characterize the surface microstructure and the corresponding distribution of hard and soft segments within these samples. The model samples contained a range of nitroplasticizer (NP) content from 0 to 15{\%}. SPM phase imaging yielded maps of variations in local mechanical properties at the nanometer scale. Tensile stress/strain measurements were carried out at 0.1 mm/min. Sequential tensile stress/strain measurements to increasing strains, sequential tensile stress/strain measurements to the sample strain, and at different temperatures allow us to understand the deformation mechanism in this complex material. Significant elastic behavior was seen below the glass transition. [Preview Abstract] |
Thursday, March 16, 2006 1:39PM - 1:51PM |
V30.00011: Identification of key deformation mechanisms of polyethylene materials via in-situ x-ray scattering Theresa Hermel-Davidock, Willem DeGroot, Mehmet Demirors, Brian Landes, Rajen Patel, Tracy Peltier Changes in the microstructure of ethylene based copolymers can be used to modify and enhance their mechanical performance. In this study, the effects of comonomer content, molecular weight, and molecular weight distribution on the mechanical performance of select polyethylene polymers were examined. Two key performance parameters for commercial polymer materials, especially in the area of blown film applications, are tear resistance and puncture resistance. However, polyethylene films exhibiting very different Dart and Elmendorf tear values cannot be differentiated by standard tensile test methods even though they exhibit very similar morphology. Alternative methods to link morphology and mechanical response need to be found. Wide-angle and small-angle x-ray scattering was collected during in-situ tensile testing to understand how compositional and structural differences affect the mechanical response of semicrystalline polyethylene polymers. Microstructural changes observed during the in-situ deformation process are correlated to Elmendorf tear and Dart impact performance for both intrinsic and blown film samples. [Preview Abstract] |
Thursday, March 16, 2006 1:51PM - 2:03PM |
V30.00012: Microindentation Studies in Polymers at Very Low Crystallinities Francisco Balta-Calleja, Araceli Flores Indentation with a sharp indenter, involving a deformation on a micron and submicron scale is known to be a convenient method to measure the mechanical properties of polymers [1-2]. The aim of the present talk is to offer an overview on recent advances concerning the microindentation hardness, H, of a series of very low crystallinity (below 7 percent) polyethylene materials relating to nanostructure. The H values for the ethylene-octene copolymers are found to be notably smaller than those of linear and commercial short-chain branched polyethylene. The microhardness of an ethylene based material having a nearly zero crystallinity value has been measured for the first time. Results are discussed on the basis of the surface free energy, dimensions of the nanocrystals and the energy required for plastic deformation. In these low crystallinity materials the deformation modes involve bond rotation of the molecules within the amorphous phase, and elastic compression, bending and slippage of the nanocrystals. According to these deformation mechanisms, which are mainly modulated by the viscosity of the amorphous phase, the average dimensions of the nanocrystals after deformation remain practically unaffected. [1] A Flores, FJ Balta Calleja, T Asano, J Appl Phys 90, 6006 (2001) [2] A Flores, VBF Mathot, GH Michler, R Adhikari, FJ Balta Calleja, Polymer (in press) [Preview Abstract] |
Thursday, March 16, 2006 2:03PM - 2:15PM |
V30.00013: Effect of crystalline organization on toughness. Laurent Corte, Ludwik Leibler Impact resistance of semi-crystalline polymers can be greatly improved by the incorporation of rubber or inorganic particles. We report that the crystalline organization of the polymer matrix is a key-parameter for toughening. Cutting test bars into injected plates of toughened polyamide allows to study the impact behaviour of a same sample volume under various impact directions. When impact is applied perpendicularly to the injection direction, these systems exhibit a ductile behaviour while they become dramatically brittle when impact is parallel to it. More generally, the impact properties of these toughened systems depend strongly on thermo-mechanical history and processing conditions. We show by X-ray and TEM observations that this behaviour is to be correlated to the crystalline organization and propose a theoretical model that links toughness and crystalline organization. [Preview Abstract] |
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