Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session J43: Focus Session: Manipulating Glasses: Mechanics |
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Sponsoring Units: DPOLY GSOFT Chair: Daniel Sussman, University of Pennsylvania Room: 214C |
Tuesday, March 3, 2015 2:30PM - 3:06PM |
J43.00001: BREAK |
Tuesday, March 3, 2015 3:06PM - 3:18PM |
J43.00002: Nano confinement effects on dynamic and viscoelastic properties of Selenium Films Heedong Yoon, Gregory McKenna In current study, we use a novel nano bubble inflation technique to study nano confinement effects on the dynamic and viscoelastic properties of physical vapor deposited Selenium films. Film thicknesses ranged from 60 to 260 nm. Creep experiments were performed for the temperatures ranging from T$_{\mathrm{g,macroscopic}}$-14 $^{\circ}$C to T$_{\mathrm{g,\thinspace macroscopic}}+$19 $^{\circ}$C. Time temperature superposition and time thickness superposition were applied to create reduced creep curves, and those were compared with macroscopic data [J. Non-Cryst. Solids. 2002, 307, 790-801]. The results showed that the time temperature superposition was applicable in the glassy relaxation regime to the steady-state plateau regime. However in the long time response of the creep compliance, time thickness superposition failed due to the thickness dependence on the steady-state plateau. It was observed that the steady state compliance increased with film thickness. The thickness dependence on the plateau stiffening followed a power law of D$_{\mathrm{Plateau}}\propto $ h$^{2.46}$, which is greater than observed in organic polymers where the exponents observed range from 0.83 to 2.0 [Macromolecules. 2012, 45 (5), 2453-2459]. [Preview Abstract] |
Tuesday, March 3, 2015 3:18PM - 3:30PM |
J43.00003: Effect of temperature on segmental mobility is reduced, but not eliminated during constant strain rate deformation of poly(methyl methacrylate) glasses Kelly Hebert, Benjamin Bending, Josh Ricci, M.D. Ediger Deformation of polymer glasses is typically nonlinear and not understood at a molecular level. During deformation, segmental motion in polymer glasses can be accelerated by over a factor of 1000. While temperature has a big impact on the segmental motion of polymer glasses in the absence of deformation, some workers suggest that segmental mobility in polymer glasses undergoing deformation should be independent of temperature. We have measured segmental mobility in poly(methyl methacrylate) glasses during constant strain rate deformation at four different temperatures using a probe reorientation method. We find that during deformation, the dependence of segmental mobility on temperature is significantly reduced, though not eliminated. This is in qualitative agreement with the work of Chen and Schweizer. We also find that the KWW $\beta $ parameter increases during deformation, indicating a narrower distribution of segmental relaxation times. At a given strain rate, this increase of the KWW $\beta $ parameter is larger at lower temperature. [Preview Abstract] |
Tuesday, March 3, 2015 3:30PM - 3:42PM |
J43.00004: Thermally Induced Deformation in Metallic Glass: the Activations and Relaxations Yue Fan, Takuya Iwashita, Takeshi Egami Thermally induced deformation in metallic glasses was investigated by sampling the potential energy landscape (PEL) and probing the changes in the atomic properties (e.g. energy, displacement, stress). The complete deformation processes consist of two stages: the activation (i.e. trigger, from initial minima to nearby saddle states on PEL), and relaxation (i.e. from saddle states to final minima on PEL). We show that the activation stages are triggered by local rearrangements of a small number of atoms, typically 5 atoms in average. Surprisingly, the individual triggers are invariant of the cooling history or elastic structure of the system. However, the organizations between different trigger centers can be varied and are related to the overall stability of the system. On the other hand, relaxation stages consist of two branches, a localized branch, and a cascade branch. While the localized branch is insensitive to the cooling history the system, the cascade branch is highly related with the processing conditions. In particular, for a faster quenched system, the cascade relaxation is found more prominent than in a slowly quenched system. [Preview Abstract] |
Tuesday, March 3, 2015 3:42PM - 3:54PM |
J43.00005: An energy landscape description of the mechanical response of model glassy materials Minglei Wang, Kai Zhang, Meng Fan, Yanhui Liu, Jan Schroers, Mark Shattuck, Corey O'Hern We perform molecular dynamics simulations of binary Lennard-Jones glasses to determine their mechanical response over a range of cooling rates spanning more than three orders of magnitude. To quantify the mechanical response, we measure the shear and bulk moduli using pure shear and compression deformation modes. To correlate the mechanical response to properties of the energy landscape, we also perform zero-temperature quasistatic pure shear simulations and measure the energy per particle as a function of strain. We show that glasses quenched at slower rates possess more brittle response since they exist in deeper energy minima with wider basins. In contrast, rapidly quenched glasses possess ductile response since they exist in shallow, narrow energy minima, which are easily overcome through applied shear. [Preview Abstract] |
Tuesday, March 3, 2015 3:54PM - 4:06PM |
J43.00006: Yield and plastic behavior of glassy polymers Luca Conca, Alain Dequidt, Didier Long, Fran\c{c}ois Lequeux, Paul Sotta, Jean-Yves Delannoy We extend here a model for the dynamics of non-polar polymers close to the glass transition, based on percolation of slow subunits. This model is solved in 3D by numerical simulations, in order to describe and calculate the mechanical properties of glassy polymers, from the linear regime up to the plastic regime of deformation, with a spatial resolution of 3-5 nm. We propose that the applied stress results in an acceleration of the dynamics of the subunits. Our simulations describe the onset of plastic behavior and the reorganization at the scale of dynamical heterogeneities. They allow for calculating how the relaxation time distribution is modified under applied stress. We show that deformation is localized in shear bands on the scale of about 10 nm at yield. Our simulation allow also for calculating the elastic and dissipative moduli as functions of strain amplitude and as functions of temperature. We show that $G'$ decreases by several orders of magnitude at large deformation amplitudes as compared to the linear regime, whereas $G''$ decreases by a factor of about 3 only. [Preview Abstract] |
Tuesday, March 3, 2015 4:06PM - 4:18PM |
J43.00007: What drives polymer glasses ductile? Shi-Qing Wang, Shiwang Cheng, Xiaoxiao Li, Panpan Lin, Jianning Liu The presentation aims to explore an important question in polymer physics: what makes polymer glasses of high molecular weight ductile? Upon deformation, load-bearing strands (LBS) emerge along the direction of the stress. Chain tension builds up in LBS due to the displacement of LBS. Segments that do not belong to LBS sequentially get activated by the LBS in the order of their proximity to LBS. If the chain network breaks down before driving the glass into a state of global plasticity, crazing and brittle fracture takes place instead of yielding and macroscopic plastic deformation. Fast deformation assures that chain pullout does not have time to materialize. Global plasticity also takes time to develop. Thus, the outcome is dictated by which process takes place first. In light of the recently proposed molecular model [1] for yielding and brittle-ductile transition of polymer glasses, we present different examples from experiment to illustrate the importance to understand the interplay between short-ranged intersegmental interactions and long-ranged intrachain networking. [1] S.Q. Wang \textit{et al. J. Chem. Phys. }\textbf{141}, 094905 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 4:18PM - 4:30PM |
J43.00008: Plastic deformation of a model glass induced by a local shear transformation Nikolai Priezjev The effect of a local shear transformation on plastic deformation of a three-dimensional amorphous solid is studied using molecular dynamics simulations. We consider a spherical inclusion, which is gradually transformed into an ellipsoid of the same volume and then converted back into the sphere. It is shown that at sufficiently large strain amplitudes, the deformation of the material involves localized plastic events that were identified based on the relative displacement of atoms before and after the shear transformation. We found that the density profiles of cage jumps decay away from the inclusion, which correlates well with the radial dependence of the local deformation of the material. At the same strain amplitude, the plastic deformation becomes more pronounced in the cases of weakly damped dynamics or large time scales of the shear transformation. We showed that the density profiles can be characterized by the universal function of the radial distance multiplied by a dimensionless factor that depends on the friction coefficient and the time scale of the shear event. [Preview Abstract] |
Tuesday, March 3, 2015 4:30PM - 4:42PM |
J43.00009: What deformation does and does not do in ductile polymer glasses Jianning Liu, Shi-Qing Wang Entangled polymeric liquids have so far only shown strain softening, signified by stress overshoot upon startup shear. We show for the first time that entangled solutions of polymers with high glass transition temperature undergoes non-Gaussian chain stretching, losing its ability to yield through chain disentanglement. The stronger than linear increase of the shear stress with the strain ends with a sharp decline, forming a cusp. In situ visualization by particle-tracking velocimetry confirms that the solution undergoes abrupt rupture at a shearing plate in the sample interior. The rapid sample recoils elastically, producing the observed stress drop. [Preview Abstract] |
Tuesday, March 3, 2015 4:42PM - 5:18PM |
J43.00010: Heterogeneous Glasses and Sustainable Cement Invited Speaker: Emanuela del Gado Calcium-silicate hydrate (C-S-H) is the main binding agent in cement and concrete. It forms at the beginning of cement hydration, it progressively densifies as cement hardens and is ultimately responsible for the performances of concrete. This hydration product is a cohesive nano-scale heterogeneous glass, whose structure and mechanics are still poorly understood, in spite of its practical importance. I will review some of the open questions for this fascinating material and discuss a statistical physics approach recently developed, which allows us to investigate the structural arrest and solidification under the out-of-equilibrium conditions typical of cement hydration and the role of the nano-scale structure in C-S-H mechanics upon hardening. Our approach unveils how some distinctive features of the kinetics of cement hydration can be related to changes in the morphology of this glassy material and elucidates the role of nano-scale mechanical heterogeneities in the hardened C-S-H. [Preview Abstract] |
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