Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session B18: Focus Session: Mechanical Properties of Polymers: Fracture and Adhesion |
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Sponsoring Units: DPOLY FIAP Chair: Theresa Hermel-Davidock, Dow Chemical Company Room: Morial Convention Center 210 |
Monday, March 10, 2008 11:15AM - 11:51AM |
B18.00001: High Strain Deformation and Fracture of Self-Assembled Polymer Gels Invited Speaker: Triblock copolymers with poly(methyl methacrylate (PMMA) endblocks and a poly(n-butyl acrylate) (PnBA) midblock form thermoreversible gels in a variety of alcohols. The mechanical response of the gels is determined by the nature of the PMMA aggregates that are bridged by PnBA midblocks. The close proximity of the ordering temperature to the glass transition of the PMMA aggregates gives rise to a remarkably strong temperature dependence of the relaxation time for the polymer gels. At low temperatures, where the relaxation times are very large, the gels can be deformed to very large strains prior to solid-like fracture. At intermediate temperatures the materials flow, but strain localization leads to a melt fracture phenomenon, and at higher temperatures the materials behave as viscous polymer solutions. We have used these gels as a model material for studying rate effects in the high strain deformation and fracture of soft solids. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B18.00002: Reinforcement of Epoxies Using Single Walled Carbon Nanotubes Ramanan Krishnamoorti, Jitendra Sharma, Tirtha Chatterjee The reinforcement of bisphenol-A and bisphenol-F epoxies using single walled carbon nanotubes has been approached experimentally by understanding the nature of interactions between the matrices and nanotubes. Unassisted dispersions of single walled carbon nanotubes in epoxies were studied by a combination of radiation scattering (elastic small angle scattering and inelastic scattering), DSC based glass transition determination, melt rheology and solid-state mechanical testing in order to understand and correlate changes in local and global dynamics to the tailoring of composite mechanical properties. Significant changes in the glass transition temperature of the matrix can successfully account for changes in the viscoelastic properties of the epoxy dispersions for concentrations below the percolation threshold, while above the percolation threshold the network superstructure formed by the nanotubes controls the viscoelastic properties. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B18.00003: Mechanical and Electrical Properties of Organogels with Multiwall Carbon Nanotubes Mohammad Moniruzzaman, Karen Winey Organogels are fascinating thermally reversible viscoelastic materials that are comprised of an organic liquid and low concentrations (typically $<$2 wt {\%}) of low molecular mass organic gelators. We have fabricated the first organogel/carbon nanotube composites using 12-hydroxystearic acid (HSA) as the gelator molecule and pristine and carboxylated multi-wall carbon nanotubes as the nanofillers and 1,2-dichlorobenzene as the organic solvent. We have achieved significant improvements in the mechanical and electrical properties of organogels by incorporating these carbon nanotubes. For example, the linear viscoelastic regime of the HSA organogel, an indicator of the strength of the gel, extends by a factor of 4 with the incorporation of 0.2 wt{\%} of the carboxylated nanotubes. Also, the carbon nanotubes (specially the pristine tubes) improve the electrical conductivity of the organogels, e.g. six orders of magnitude enhancement in electrical conductivity with 0.2 wt{\%} of pristine tubes. Differential scanning calorimetry experiments indicate that the nanotubes do not affect the thermoreversibility of the organogels. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B18.00004: Identification of key deformation mechanisms of polyethylene materials via in-situ x-ray scattering Theresa Hermel-Davidock, Brian Landes, Mehmet Demirors 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 which exhibit very similar morphology often exhibit very different Dart and Elmendorf tear values which cannot be differentiated by standard tensile test methods. 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 performance for both intrinsic and blown film samples. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 1:03PM |
B18.00005: Brittle-tough transitions during crack growth in toughened adhesives Invited Speaker: The use of structural adhesives in automotive applications relies on an effective understanding of their performance under crash conditions. In particular, there is considerable potential for mechanics-based modeling of the interaction between an adhesive layer and the adherends, to replace current empirical approaches to design. Since energy dissipation during a crash, mediated by plastic deformation of the structure, is a primary consideration for automotive applications, traditional approaches of fracture mechanics are not appropriate. Cohesive-zone models that use two fracture parameters - cohesive strength and toughness - have been shown to provide a method for quantitative mechanics analysis. Combined numerical and experimental techniques have been developed to deduce the toughness and strength parameters of adhesive layers, allowing qualitative modeling of the performance of adhesive joints. These techniques have been used to study the failure of joints, formed from a toughened adhesive and sheet metal, over a wide range of loading rates. Two fracture modes are observed: quasi-static crack growth and dynamic crack growth. The quasi-static crack growth is associated with a toughened mode of failure; the dynamic crack growth is associated with a more brittle mode of failure. The results of the experiments and analyses indicate that the fracture parameters for quasi-static crack growth in this toughened system are essentially rate independent, and that quasi-static crack growth can occur even at the highest crack velocities. Effects of rate appear to be limited to the ease with which a transition to dynamic fracture could be triggered. This transition appears to be stochastic in nature, and it does not appear to be associated with the attainment of any critical value for crack velocity or loading rate. Fracture-mechanics models exist in the literature for brittle-ductile transitions in rate-dependent polymers, which rely on rate dependent values of toughness with unstable branches. The present observations do not appear to follow the type of behavior that would be expected from these models, but are consistent with prior observations of fracture instabilities in bulk model rubber-toughened epoxies. Some alternative models for the transitions in fracture mode will be discussed. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B18.00006: Controlling polymer adhesion with surface wrinkles Edwin Chan, Erica Smith, Ryan Hayward, Alfred Crosby One of nature's solutions in controlling adhesion is through the intelligent design of patterned interfaces. For example, the feet of geckos and some insects are decorated with fibrillar structures designed specifically for locomotion, i.e. enhanced control of adhesion and release. Recently, there have been significant efforts in mimicking these materials to develop synthetic analogs to tailor polymer adhesion. However, challenges such as reusability and fabrication scalability limit the succesful applications of these materials. In this work, we present an alternative approach to the design of a patterned adhesive that utilizes surface wrinkles to contol the adhesion of a poly($n$-butyl acrylate) (P$n$BA) elastomer. Using a probe-type contact adhesion test, we experimentally show that surface wrinkles enhance adhesion of the P$n$BA elastomer based on a mechanism termed contact line splitting. We demonstrate that the efficiency of this contact line splitting mechanism is coupled with the wavelength of the wrinkles. Furthermore, the geometry of the surface wrinkles facilitates a repeatable interfacial response necessary to function as a reuseable adhesive. Our approach provides a simple and scaleable strategy to the design of patterned adhesives that is amenable to a variety of polymeric materials while facilitating enhanced control of interfacial response. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B18.00007: Soft-soft nanocomposite adhesives made from colloidal particles Costantino Creton, Fanny Deplace, Michael Rabjohns, Andrew Foster, Peter Lovell, Chunghong Lei, Joseph Keddie, Keltoum Ouzineb, Jeanne Marchal Nanocomposites are often made with a dispersed hard phase in a soft matrix. In some cases however it might be interesting to obtain the stiffness from a continuous honeycomb structure and the toughness from the inside of the cells. Colloidal latex particles provide some unique opportunities to design nanocomposites. We have synthesized core-shell particles of acrylic copolymers where the outside shell contains a small amount of crosslinkable groups. These groups can then be used to introduce crosslinking points in and between the shells during the drying of the latex dispersion into a homogeneous film. The resulting films have very different large strain properties while the small and intermediate strain properties remain controlled by the viscoelastic properties of the inside of each particle. We show that films made in that way can have a very pronounced strain softening at intermediate strains (characteristic of viscoelastic liquids) while retaining a pronounced hardening at large strains (characteristic of viscoelastic solids'). This particular balance of properties results in a strong adherence to low energy surfaces while retaining a good resistance to creep. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B18.00008: Dangling chain effect on the modulus of polyurethane networks Bruno Fayolle, Julie Diani, Pierre Gilormini While the theory of elasticity has been well verified for the non ideal structures accounting for contribution of entanglements, some effort is needed to investigate the contribution of dangling chains. In order to establish a quantitative contribution of dangling chains, networks with a controlled architecture, i.e. where architecture is determined by synthesis of well-characterized reactants, are required. In this work, polyurethane networks based on poly(propylether) and poly(tetramethyl adipate) crosslinked by triisocyanate were prepared and studied. Different polyether molar masses are chosen from 430 g/mol up to 4000 g/mol. By varying the stoechiometric ratio r = [NCO]/[OH] between 0.4 and 1, dangling chains are introduced, provided that the reaction between NCO groups is negligible. After synthesis, the Young's modulus (E) of the networks has been measured from tensile tests according to the neo-Hookean law. The molar mass of elastically active network chains (ENAC) is determined from E. Since this molar mass is close to the molar mass of each diol used for synthesis at r=1 (``ideal'' network), a correction taking into account the number of dangling chains (b) calculated from r is proposed. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B18.00009: Morphological Determinants of Yield Stress for Semicrystalline Ethylene / Methacrylic Acid Copolymers Robert Scogna, Richard Register Reducing the crystal thickness of ethylene/$\alpha$-olefin copolymers typically results in a decrease in the measured yield stress. However, statistical incorporation of methacrylic acid, also a noncrystallizable comonomer, actually increases the yield stress at room temperature. The yield stress for ethylene/methacrylic acid (E/MAA) copolymers as a function of temperature and test rate is described using a model which accounts for polyethylene crystal plasticity through thermal nucleation of screw dislocations in addition to the effects of incomplete relaxation of the amorphous fraction at the strain rate employed. This is possible using a small number of physically reasonable best-fit parameters. Yield stress master curves can be constructed for any material that obeys the model; such curves have been constructed for a low-density polyethylene and five copolymers of varying MAA content from data taken at various strain rates and temperatures. The master curves clearly show that this unusual behavior of the yield stress is caused by the increase in $\beta$ relaxation temperature with increasing MAA content, as seen via dynamic mechanical testing. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B18.00010: On the statistics of Gaussian two and three-dimensional networks: Fluctuations of junctions and collapse driven by structure. Michael Lang, Sergey Panyukov, Michael Rubinstein, Jens-Uwe Sommer We investigate ideal Gaussian networks both analytically and with computer simulations using the Bond Fluctuation model with and without excluded volume interactions. The focus of this study is on fluctuations and the collapse of networks with different connectivity and dimensionality. We show that the size of a perfect square 2D network made from $n^2$ Gaussian chains with $N$ monomers each is $R_{g} \sim N^{1/2}log(n)$. Thus fluctuations in two-dimensional networks diverge logarithmically with the size of these films while fluctuations of three-dimensional networks $\sim N^{1/2}$ and do not increase with their size. We study the cross-over between two and three-dimensional networks by following the dependence of junction fluctuations on the thickness of films. The results of model systems are compared with more realistic networks in order to understand the effect of disorder on the properties of the network and fluctuations of network junctions. [Preview Abstract] |
Monday, March 10, 2008 2:03PM - 2:15PM |
B18.00011: Physical understanding of the bulk modulus of polyisoprene by molecular dynamics simulations Julie Diani, Bruno Fayolle, Pierre Gilormini The temperature dependence of the Young's modulus of amorphous polymers has been explained well in the literature, however, the moderate drop of the bulk modulus of a factor 2 or 3 is still not fully understood and the question of a possible entropic contribution to the bulk deformation is still open. In this work, we propose to address the question of the physical source of the bulk modulus of the polyisoprene cis1-4, using molecular dynamics simulations. Using Material Studio MD simulations software, we built several periodic virtual polyisoprene. The materials were submitted to volume contractions at various temperatures. During the simulations, we monitored among other parameters, the volume, the pressure, the temperature, the bonded, nonbonded and kinetic energies of the cell. These data were used in a classical thermodynamics analysis to assess the physical source of the bulk modulus below and above the glass transition temperature. The results showed that the Van der Waals interactions control the bulk modulus below the glass transition. Above the glass transition the internal energy contribution to the bulk modulus is driven by the Van der Waals interaction but a non negligible entropic contribution appeared which at a first order may be considered as independent on the temperature. [Preview Abstract] |
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