Session G42: Focus Session: Physics of Glasses and Viscous Liquids I

Glass Transition of Polymers near Their Free Surface

Recent experiments have indicated that the relaxation dynamics near a free polymer surface may become fundamentally different from bulk $\alpha$ relaxation times. The dynamics lose many of the characteristics commonly associated with glasses. The dynamic properties lose their typical Vogel-Fulcher-Tammann (VFT) temperature dependence and take on an Arrhenius dependence. There is also evidence that the dynamic properties become more homogeneous near the free surface. Such direct measurements of the relaxation dynamics are rare and extremely difficult to perform on a wide range of polymeric systems. It has been shown that cooling rate-dependent glass transition temperature ($\mathrm{T_g}$) measurements can be used as an effective and simple method to estimate the relaxation dynamics of the free surface. The cooling rate is inversely proportional to the relaxation time of the film at the temperature at which the system falls out of equilibrium, $\mathrm{T_g}$. In thin polymer films, as the film thickness is decreased the dynamics of the film are affected more strongly by surface dynamics and therefore they provide a lower bound to the surface relaxation times. In thin polystyrene films measurements of $\mathrm{T_g}$ as a function of cooling rate indicate a clear onset of deviations from bulk properties at a temperature a few degrees above the bulk $\mathrm{T_g}$. We hypothesize that this could be due to either a new mode of relaxation that is exclusively available near the surface, or typical glassy dynamics that have faster time scales near the surface. In this study we investigate the effect of molecular weight and the polymer structure on the value of the onset temperature to verify whether the properties are consistent with one of these hypotheses. It is also observed that under certain conditions, where the dynamics of the free surface and the bulk relaxation dynamics are many decades apart, the system exhibits two distinct $\mathrm{T_g}$s associated with either bulk or surface relaxations. This data can be used to estimate the length scale of the surface dynamics and the length scale over which the effects penetrate into the bulk of the film.   

Anharmonicity and Fragility of Protic Ionic Liquids

Supercooled liquids are often characterized by their fragility which is associated with physicochemical properties. However, the origin of fragility is still controversial. Superfragile liquid, decahydroisoquinoline (DHiQ) is chosen as a parent molecule to systematically investigate the relationship between anharmonicity and fragility of supercooled liquids. Earlier study by Ueno et al. (J. Phys. Chem. B 2012, 116) demonstrated that the protonation of DHiQ by different Bronsted acids results in the loss of superfragility. To understand the nature of fragile liquids, we conducted inelastic/quasielastic (IE/QE) neutron scattering measurements to examine low frequency vibrational dynamics (boson peak) and the relaxation behavior of DHiQ (high fragility) and DHiQ-based ionic liquids with intermediate (formate, Fm) and low (trifluoromethansulfonimide, TFSI) fragilities. With the protonation, molecular acids will be hydrogen-deficient and the scattering will be dominated by cation, [DHiQ$^{+}$]. This strategy simplifies our interpretation in terms of understanding the fitting result from IENS/QENS spectra. By protonating DHiQ with stronger acids, large shift in low frequency vibrational modes and smaller mean square displacements were examined at temperatures higher than Tg. We illustrate how the degree of protonation and ionicity plays a role in the loss in superfragility of DHiQ.   

Acoustic properties in glycerol glass-former: Molecular dynamics simulation

Study of high-frequency collective dynamics around TeraHertz region in glass former has been a subject of intense investigations and debates over the past decade. In particular, the presence of the Boson peak characteristic of glassy material and its relation to other glass anomalies. Recently, experiments and simulations have underlined possible relation between Boson peak and transverse acoustic modes in glassy materials. In particular, simulations of simple Lennard Jones glass former have shown a relation between Ioffe-Regel criterion in transverse modes and Boson peak. We present here molecular dynamics simulation on high frequency dynamics of glycerol. In order to study mesoscopic order (0.5-5nm$^{-1}$), we made use of large simulation box containing 80000 atoms. Analysis of collective longitudinal and transverse acoustic modes shows striking similarities in comparison with simulation of Lennard-Jones particles. In particular, it seems that a connection may exist between Ioffe-Regel criterion for transverse modes and Bose Peak frequency. However,in our case we show that this connection may be related with structural correlation arising from molecular clusters.   

Fast Scanning Calorimetry study of non-equilibrium relaxation in fragile organic liquids

Fast scanning calorimetry (FSC), capable of heating rates in excess of 1000000 K/s, was combined with vapor deposition technique to investigate non-equilibrium relaxation in micrometer thick viscous liquid films of several organic compounds (e.g.2-ethyl-1-hexanol, Toluene, and 1-propanol) under high vacuum conditions. Rapid heating of samples, vapor deposited at temperatures above their standard glass softening transition (Tg), resulted in observable endotherms which onset temperatures were strongly dependent on heating rate and the deposition temperature. Furthermore, all of the studied compounds were characterized by distinct critical deposition temperatures at which observation of endotherm became impossible. Based on the results of these studies, we have developed a simple model which makes it possible to infer the equilibrium enthalpy relaxation times for liquids from FSC data. We will discuss implications of these studies for contemporary models of non-equilibrium relaxation in glasses and supercooled liquids.   

Elementary excitations and flow in the liquid

A new mode of excitation is introduced to elucidate the dynamics in simple liquids at the atomic scale. Some properties of liquid defy easy explanations. For instance, in liquids phonons are overdamped with a very short lifetime. Nevertheless the Dulong-Petit law (C$_{\mathrm{V}}$ $\sim$ 3k$_{\mathrm{B}})$ is widely observed at high temperatures. As temperature is reduced the specific heat markedly increases in the supercooled state, only to drop down sharply at the glass transition. Viscosity shows an Arrhenian behavior at high temperatures, but increases rapidly toward the glass transition in the supercooled state. We suggest that these perplexing observations can be naturally explained in terms of the local configurational excitations (LCE's) which locally change the atomic connectivity by an atom losing or gaining one nearest neighbor. We show that the lifetime of LCE, $\tau_{LC}$, is equal to the Maxwell relaxation time, $\tau_{M}$, at temperatures above the crossover temperature, $T_{A}$. Above T$_{\mathrm{A}}$ the phonon mean-free path, $\xi = c_{T}\tau_{LC}$, where $c_{T}$ is the transverse sound velocity, becomes shorter than the interatomic distance, resulting in phonon localization. Therefore LCE's are the elementary excitations in the liquid. They are independent of each other above $T_{A}$, but below $T_{A}$ LCE's interact through phonon exchange, resulting in the rapid increase in $\tau _{M}$, culminating in the glass transition. LCE's are also the mechanism of flow at low temperature under strong shear stress. In this case, however, losing and gaining of the neighbors are strongly coupled, so that $\tau _{M} = \tau_{LC}$ /2 [1]. We also discuss dynamic heterogeneity in terms of LCE interactions. \\[4pt] [1] T. Iwashita and T. Egami, \textit{Phys. Rev. Lett.}, \textbf{108,} 196001 (2012).   

Ab-initio atomic level stresses in Cu-Zr crystal, liquid and glass phases

The Cu-Zr system provides interesting playground for the study of glass structure, stability, and formability and liquid dynamics. Glasses form over a wide range of concentrations while they compete against various intermetallic compounds. We have calculated from first-principles the atomic level stresses, a new tool to characterize materials, within the local approximation to Density Functional Theory (DFT) for Cu-Zr glasses and compounds from low temperature to 4500K. Comparisons between ordered crystalline compounds and liquids and glasses allow us to relate atomic level stress to relaxation of chemical short-range order and structural relaxation. The results are counter-intuitive at times; a smaller atom is under higher compressive pressure, whereas geometrically they should be under tension. Ab-initio calculations were done using Vienna Ab-initio Simulation Package (VASP) and Locally Self-consistent Multiple Scattering (LSMS) codes.   

Computer Simulations of Non-Equilibrium Dynamics in Silica

We present results from molecular dynamics computer simulations of aging silica (modeled by the BKS model). The system is equilibrated at $T_{\rm i}=5000$~K and quenched instantaneously to $T_{\rm f}=2500$~K. After a waiting time $t_{\rm w}$ we investigate the dynamics of the Si- and O-atoms as the system evolves over time t. Our simulations run long enough in order to observe the transition from out-of-equilibrium to equilibrium dynamics. We determine for our system the generalized incoherent intermediate scattering function $C(q,t_{\rm w},t_{\rm w}+t)$ and the dynamic susceptibility $\chi_4(q,t_{\rm w},t_{\rm w}+t)$ where $q$ corresponds to the wavevector. Curves corresponding to different waiting times $t_{\rm w}$ collapse on the scaling plot $\chi_4(q,t_{\rm w},t_{\rm w}+t)/\chi_4^{\rm max}(q,t_{\rm w})$ as a function of $\large (1-C(q,t_{\rm w},t_{\rm w}+t) \large )$, which agrees with a prediction from spin glass theory.   

Persistent medium-range order and anomalous liquid properties of Al$_{1-x}$Cu$_{x}$ alloys

The development of short-to-medium-range order in atomic arrangements--that is, the aggregation or packing of short-range order (SRO) atomic clusters--has generally been observed in noncrystalline solid systems such as metallic glasses. Whether such medium-range order (MRO) can exist in materials at well above their melting or glass-transition temperature, manifesting itself in some observable property such as a liquid--liquid transition, has been a long-standing important scientific challenge. Here, using \emph{ab initio} molecular dynamics simulations, we show that a novel, persistent MRO exists in liquid Al-Cu alloys, both in the nano- and bulk phases, near the composition of CuAl$_{3}$. In a sense, the MRO liquid lies in between glasses and normal liquids, and thus it exhibits anomalous liquid properties. Our \emph{ab initio} calculations provide a detailed atomistic description of the MRO as well as a microscopic explanation for its formation via a percolation-like transition. Interestingly, we find that the appearance of MRO in the liquid phase manifests itself in a substantially enhanced viscosity that is consistent with a previously unexplained experimental observation of a peak in the viscosity of Al-Cu alloys.   

Transient Networks and Dense Colloidal Suspensions: From Viscous Flow to Elastic Instabilities

In order to analyze the mechanical response of viscoelastic materials in highly non-linear regimes, we have designed a new kind of Hele-Shaw cell where both viscous liquids and soft elastic solids can be tested at a controlled loading rate. We first consider model Maxwell liquids -- characterized by a single relaxation time -- with the project of benchmarking the response of complex, glassy systems. We use several solutions of microemulsions connected by telechelic polymers. We show that these materials undergo instability in a broad range of loading rates. At low rates, this instability is shown to be of the viscous Saffman-Taylor type. At high rates, we observe a purely elastic bulk instability discovered recently in the context of soft elastomers. A microfluidic version of our cell makes it possible to study the response of colloidal suspensions. We use more or less concentrated PNIPA aqueous solutions for which temperature controls the volume fraction. Observations are interpreted in the light of our understanding of their viscoelastic properties.