Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session B20: Invited Session: Physics of Glass-Forming Liquids: Challenges and Surprises I |
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Sponsoring Units: DPOLY Chair: Gregory McKenna, Texas Tech University Room: Ballroom B |
Monday, March 2, 2015 11:15AM - 11:51AM |
B20.00001: Glass-formers vs. Assemblers Invited Speaker: Sharon Glotzer In most instances, the formation of a glass signifies an inability of the constituents of a system to self-organize into a well-defined, thermodynamically preferred ordered structure. Thus good "assemblers" may make poor glass-formers, and good glass-formers tend to be poor assemblers. How good or bad a system is in assembling or vitrifying/jamming depends on many features of the constituent building blocks, including shape and interactions. In many cases, building blocks whose shapes make them good glass-formers can, through almost imperceptible perturbations, become good assemblers, and vice versa. We examine these issues through consideration of several model systems, including colloidal "rocks" and foldable nets. *with E.R. Chen, P. Damasceno, P. Dodd, M. Engel, A.S. Keys, D. Klotsa, E. Teich, and G. van Anders [Preview Abstract] |
Monday, March 2, 2015 11:51AM - 12:27PM |
B20.00002: Tuning polymer glass formation with additives and ions Invited Speaker: David Simmons A polymer's glass transition and associated dynamic and mechanical properties are among the most important factors determining its performance in engineering applications. For this reason, decades of research have aimed to establish methods of tuning polymers' glass formation behavior. Here I describe molecular simulations providing new insight into two approaches to altering a polymer's glass formation behavior: introduction of small-molecule diluents; and introduction of charged moieties to form an ionomer. In the first case, we explore how diluent molecular properties control modifications to the host polymer's glass transition and mechanical response. Results indicate that diluents can induce a rich array of effects, necessitating development of an expanded classification beyond the usual plasticizer/antiplasticizer dichotomy. In the second case, simulations indicate that ionomer glass formation is indistinguishable from that in polymer nanocomposites, in contrast to the longstanding assumption that covalent grafting of chains to ionic aggregates in these systems leads to a qualitatively distinct effect. Taken together, these results provide new guidance towards the rational understanding and control of polymer glass-formation in a range of materials.\\[4pt] In collaboration with Jayachandra Hari Mangalara and Dihui Ruan, The University of Akron. [Preview Abstract] |
Monday, March 2, 2015 12:27PM - 1:03PM |
B20.00003: The Significance of Incorporating Nanoscale Fluctuations in a Constitutive Description of Glassy Polymers Invited Speaker: James Caruthers The current picture of the glass involves dynamic heterogeneity, where nanoscopic regions of the glass have order-of-magnitude differences in local mobility that evolve with time. Dynamic heterogeneity provides a critical challenge to the traditional nonlinear continuum models, where both temporal and spatial fluctuations are averaged as a result of the continuum postulate. In order to acknowledge dynamic heterogeneity, a Stochastic Constitutive Model (SCM) has been developed to describe the nonlinear viscoelastic behavior of polymeric glasses, where (i) temporal fluctuations are explicitly included and (ii) the \textit{local} mobility depends upon the \textit{local} state of the material (e.g. local stress and local entropy) vs. traditional viscoelastic/viscoelastic models where macroscopic mobility depends upon the macroscopic state. The SCM is able to describe a number of nonlinear relaxation phenomena that cannot be predicted by traditional nonlinear viscoelastic/viscoplastic models, including (i) post-yield stress softening and its dependence on annealing time, (ii) the inversion of the strain dependence of nonlinear stress relaxation with the loading rate, (iii) stress memory and (iv) tertiary creep and creep-recovery. This paper will argue that incorporation of nanoscopic fluctuations is a necessary component for a description of the thermomechanical behavior of polymeric glasses. [Preview Abstract] |
Monday, March 2, 2015 1:03PM - 1:39PM |
B20.00004: Atomic motion and physical aging in structural glasses revealed by coherent X-rays Invited Speaker: Beatrice Ruta Glasses are essential materials in present day science and technology. Nevertheless, many of their properties remain the subject of numerous studies, since their intrinsic non-equilibrium nature poses formidable problems both at the technological and fundamental level. Although their physical aging has practical implication for material science, a microscopic understanding is still missing since experiments that study the dynamics at the microscopic level are extremely challenging [1]. Here, we will report on the first experiments that follow the evolution of the structural relaxation process in glasses at the atomic length scale. Measurements on metallic glasses have revealed the existence of microscopic structural rearrangements, contrary to the common expectation of a completely arrested state [2,3]. In these systems, the dynamics evolves from a diffusive atomic motion in the supercooled liquid phase to a stress-dominated dynamics in the glass, characterized by a complex hierarchy of aging regimes. These finding present many similarities with the dynamics of various complex soft materials, like emulsions, gels and glassy colloidal suspensions [4] suggesting the existence of a common physical mechanism. Albeit this apparent universal out-of-equilibrium dynamics, an even more complex scenario emerges when the investigation is enlarged to other glasses. Measurements on sodium-silicate glasses show a surprising fast atomic motion, even hundreds degrees below the glass transition temperature [5]. In addition no aging of the dynamics is observed on experimental time scales of several hours, not even in the glass transition regions, in marked disagreement with macroscopic studies. This surprising stationary dynamics has been observed also in the case of metallic glasses but only for very large annealing times [2,3] and suggests the existence of a very peculiar relaxation dynamics at the atomic level, unaccounted for in previous experimental and theoretical works [1]. [1] L. Berthier and G. Biroli, Rev. Mod. Phys. 83, 587 (2011). [2] B. Ruta \textit{et al.} Phys. Rev. Lett. 109, 165701 (2012). [3] B. Ruta \textit{et al.} J. Chem. Phys. 138, 054508 (2013). [4] L. Cipelletti et al. Faraday Discuss. 123, 237, (2003). [5] B. Ruta \textit{et al.} Nature Commun. 5, 3939 (2014). [Preview Abstract] |
Monday, March 2, 2015 1:39PM - 2:15PM |
B20.00005: Inelastic Neutron Scattering Studies of the Dynamics of Glass-Forming Materials in Confinement Invited Speaker: Reiner Zorn The study of the dynamics of glass-forming liquids in nanoscopic confinement may contribute to the understanding of the glass transition. Especially, the question of a cooperativity length scale may be addressed. In this presentation, results obtained by inelastic neutron scattering are presented. The first experiments were done to study the $\alpha $ relaxation of glass-forming liquids and polymers in nanoporous silica. Neutron scattering is a suitable method to study such composite materials because the scattering of the liquid component can be emphasized by proper choice of isotopes. By combining time-of-flight spectroscopy and backscattering spectroscopy it is possible to cover the large dynamical range spanned by the dynamics of glass-forming materials. The experiments demonstrated a broadening of the spectrum of relaxation times with faster as well as slower components compared to the bulk. In later experiments `soft' confinement in a microemulsion was used to reduce surface effects. In this system a definite acceleration of the dynamics was observed. In all cases the glass-specific fast vibrational dynamics (boson peak) was also studied, revealing a characteristic confinement dependence which allows conclusions on its nature. Finally, studies were carried out on polymers by neutron spin echo spectroscopy with the aim of observing the confinement effect on polymer specific dynamics (Rouse motion). These studies showed that a comparatively simple model is able to explain the deviation from bulk behavior. [Preview Abstract] |
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