Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session Y16: Molecular GlassesFocus Recordings Available
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Sponsoring Units: DPOLY DSOFT DCP DMP Chair: Rodney Priestley, Princeton University Room: McCormick Place W-184A |
Friday, March 18, 2022 8:00AM - 8:36AM |
Y16.00001: Fast equilibration mechanisms in disordered materials mediated by slow liquid dynamics Invited Speaker: Simone Napolitano The rate at which a nonequilibrium system decreases its free energy is commonly ascribed to molecular relaxation processes, arising from spontaneous rearrangements at the microscopic scale. While equilibration of liquids usually requires density fluctuations at timescales quickly diverging upon cooling, growing experimental evidence indicates the presence of a different, alternative pathway of weaker temperature dependence. Such equilibration processes exhibit a temperature-invariant activation energy, on the order of 100 kJ mol-1. We identify this molecular process with a slow relaxation mechanism, universally observed in the liquid dynamics of thin films. By adequately processing our samples, we have been able to enhance the nonequilibrium character and to detect a molecular process whose activation energy matches that of the equilibration rate. Our results show that measurements of liquid dynamics can be used to predict the equilibration rate in the glassy state |
Friday, March 18, 2022 8:36AM - 8:48AM |
Y16.00002: 4D-STEM orientation mapping in anisotropic molecular glasses Mark D Ediger, Debaditya Chatterjee, Shuoyuan Huang, Kaichen Gu, Junguang Yu, Lian Yu, Paul Voyles, Harald Bock We have used four dimensional – scanning transmission electron microscopy (4D – STEM) to study the molecular orientation domain structure of vapor-deposited thin film glasses of a phenanthroperylene ester, a discotic mesogen known to form an equilibrium columnar hexagonal phase. Previously, grazing-incidence wide-angle X-ray diffraction and ellipsometry experiments have shown that substrate temperature and deposition rate control the average orientation of the molecules and the hexagonal packing order at different growth conditions. 4D STEM mapping of diffraction from the intracolumnar disc stacking provides an image of the local molecular orientation with sub-nanometer spatial resolution. We find that the in-plane columnar orientation persists within domains whose length scale varies from 10s of nanometers for films deposited at 12 K below the glass transition temperature Tg to several microns for films annealed at Tg + 65 K. Our analysis allows us to detect domain boundaries via automated identification of shifts in column orientations across regions. Defects in the orientation maps are also visible. These results probe the structure of the films directly at the length scales relevant to intracolumn charge transport in optoelectronic devices. |
Friday, March 18, 2022 8:48AM - 9:00AM |
Y16.00003: Development of Coarse-Grained Models for Glass Forming Liquids Jack Yungbluth, Grigori A Medvedev, Brett M Savoie, James M Caruthers A long-standing problem in condensed matter physics is the establishment of a mechanism for super-Arrhenian mobility behavior of glass forming materials, where molecular simulations can provide significant insight. All atom potentials are useful, but coarse-grained potentials offer the possibility of both faster simulations and the ability to create new molecules that can probe various postulates that underlie the cause of super-Arrhenian behavior. Coarse-grained potentials have been prepared using the iterated Boltzmann inversion technique, where real glass forming molecules are used as the target system. These potentials produce systems which stabilize the crystalline, liquid, and supercooled liquid phase, which are necessary components the excess quantities that have been postulated to control the mobility in the supercooled region. Determination of thermodynamic quantities in the crystal can be polluted by pre-melting; thus, a new method of extracting and extrapolating the crystalline energy has been developed. Using the coarse-grained potentials along with the improved methods of determining excess quantities, the connection between mobility in the super-cooled region and relevant excess quantities is investigated. |
Friday, March 18, 2022 9:00AM - 9:12AM |
Y16.00004: Suppression of beta relaxation in vapor deposited glass of poly (methyl methacrylate) Zhaohui Cai, Michael F Thees, Junjie Yin, James A Forrest Dielectric relaxation is used to probe the beta relaxation in vapor deposited films of poly (Methyl methacrylate). The dielectric storage and loss at 1 kHz are measured during heating and cooling for as prepared vapor deposited glasses and for glasses that have been annealed or rejuvenated to a supercooled liquid and back into a glass. Suppression of the beta relaxation is observed in the vapor deposited glasses. We discuss our results by comparing the dielectric loss and capacitance measurements for different deposition temperatures and their resulting stabilities in PMMA. |
Friday, March 18, 2022 9:12AM - 9:24AM |
Y16.00005: Unifying description of the vibrational anomalies of amorphous materials Shivam Mahajan The vibrational density of states of solids controls their thermal and transport properties. Amorphous materials' vibrational modes differ from crystals' for the following reasons: (i) there is an excess of modes over Debye's prediction or Boson peak; (ii) there exist quasi-localized low-frequency vibrational modes; and (iii) low-frequency phonons attenuate via Rayleigh scattering. It is unclear if any connection exists between these vibrational features, as they occur in different frequency regimes. |
Friday, March 18, 2022 9:24AM - 9:36AM |
Y16.00006: Toward understanding the depletion of two-level systems in glasses Wencheng Ji The density of Two-level systems (TLS) controls the low-temperature thermal properties in glasses. It is recently found that TLS are almost depleted in ultrastable glasses. Although it is thought to have a close relationship with the dramatic decrease of quasi-localized modes (QLMs), it is not yet clearly formalized. Based on the soft-potential model and the tunneling TLS model, we argue that the TLS correspond to the QLMs with being a typical frequency ω0. Their density n0 is proportional to the density of QLMs DL(ω0) and to the fraction of equal double-wells f(ω0) at ω0. We numerically estimate ω0 and n0 in computer glasses at different stabilities. We find that ω0 is about 10% to 20% of the Debye frequency. n0 decreases by a factor of 500 in ultrastable glasses than poorly prepared glasses with both DL(ω0) and f(ω0) playing a significant role. Remarkably, the estimations of n0 are consistent with the order of magnitude of the change in n0 found in experiment in amorphous silicon. |
Friday, March 18, 2022 9:36AM - 9:48AM |
Y16.00007: Rigorous foundation of the Adam-Gibbs relation Takashi Odagaki The Adam-Gibbs relation which relates a transition rate to a configurational entropy has been used frequently in the analysis of relaxation in non-equilibrium systems, though its foundation is sloppy. In this presentation, I give a rigorous proof of the Adam-Gibbs relation on the basis of the free energy landscape (FEL) approach to non-equilibrium systems [1]. The FEL consists of many basins in a 3N dimensional space, and the elementary process of structural relaxations occurs between two adjacent basins. I define a cooperatively rearranging region (CRR) by the area of atoms forming two adjacent basins and tessellate the entire FEL into a set of CRR's. When the system contains N particles and the number of CRR’s is L, the average size of CRR is given by <????RR>=N/L. Denoting the probability of finding the k-th CRR of size ???? by ????, I find that the configurational entropy is given by ????= <−????lnΠ??=1??????> =??????∗=????∗??∕<????RR> with ????∗ =????ln2. The transition rate within the k-th CRR can be written as ????=??0exp (−Δ?? ????/??????), where Δ?? is the activation free energy per particle and ??0 is the attempt frequency. Then, the average transition rate is given by W(T)= (Πk=1??W??)1/??=??0exp (−Δ?? Σ??=1??????/??k????).Therefore, the transition rate is given by W(T)= ??0exp [−Δ?? ??????∗ /??????????(??)], which is the Adam-Gibbs relation. With the proper definition of CRR and the configurational entropy, I assess various estimations of the size of CRR reported before. |
Friday, March 18, 2022 9:48AM - 10:00AM |
Y16.00008: Computational Insights into the Molecular Origins of the Chain Length Dependence of Polymers' Glass Transition William F Drayer, David S Simmons The question of the precise molecular origin of the profound molecular weight dependence of the glass transition temperature (Tg) remains inconclusive despite the central importance of this phenomenon to polymer science. |
Friday, March 18, 2022 10:00AM - 10:12AM |
Y16.00009: Excitations Provide a Thermodynamic Order Parameter for Liquid Dynamics Marcus T Cicerone Recent evidence from simulation and model systems suggests that locally excited structures (excitations) play an important role in the dynamics of supercooled liquids and glasses. Here, for the first time, we measure excitations in real liquids and show that they obey Bose-Einstein statistics with enthalpy and entropy of creation closely related to that of melting. We also show that excitations - particularly their population in the first solvent shell - serve as an order parameter for driving transformations between distinct mechanistic regimes of liquid relaxation, and specifically for the appearance of dynamic heterogeneity. The connection between excitations and mechanistic changes in relaxation suggests a simple and compelling explanation for previously cryptic aspects of these changes and provides a new connection between the dynamics and thermodynamics of liquids. The thermodynamic underpinning of the excitations suggests a path for connecting the full range of complex liquid dynamics with easily measurable quantities. |
Friday, March 18, 2022 10:12AM - 10:24AM |
Y16.00010: Surface Relaxation of Vapor Deposited Polystyrene Glasses Junjie Yin, Michael F Thees, James A Forrest Our laboratory has recently reported the technique of preparing stable glass films of polymers through PVD and the exceptional properties of such products. This technique is in principle applicable to a wide range of polymers, and it has been demonstrated for polystyrene and poly(methyl methacrylate). Stable glasses are known to have higher density and enhanced kinetic stability compared to ordinary glasses, but less is known about their surface dynamics given the fact that polymer glasses have higher surface mobility compared to their bulk counterparts. In studies of surface dynamics, atomic force microscopy (AFM) is commonly used to characterize the relaxation of surface perturbations. Here we use AFM to probe the temporal evolution of surface structure of vapor deposited polystyrene stable glasses. By varying the stability of the glasses, we are able to better understand how surface dynamics depends on the glass stability. By controlling the temperature of relaxation and other parameters, we explore the mechanism under a wide range of experimental conditions. |
Friday, March 18, 2022 10:24AM - 10:36AM |
Y16.00011: Studying the kinetic stability of molecular glasses using solvent vapor annealing Shivajee Govind, Aixi Zhang, Yi Jin, Sarah Wolf, Peng Luo, Zahra Fakhraai Vapor deposition enables access to low-energy stable glasses (SGs). Upon heating above the onset temperature of transformation (well above nominal glass transition temperature), SGs transform via thermal transformation fronts, analogous to those seen in crystals, indicating their increased kinetic stability. However, it is difficult to directly observe these fronts in SGs with low stability or in liquid quenched glasses, where the glass can transform homogeneously. Here, we perform solvent vapor annealing (SVA) as an alternative approach and demonstrate that the mechanism by which the solvent penetrates a molecular glass film depends on the solvent vapor pressure. Above a certain threshold, the solvent front moves linearly into the film (Case II diffusion). By monitoring the solvent front's velocity, we can directly compare the kinetic stability for films deposited at a wide range of temperatures, deposition rates, and with various thicknesses. The results correlate well with thermal growth fronts, when comparisons are possible, but SVA can be used on a wider range of glasses with more accuracy, as it can avoid the bulk transformation mechanisms. We discuss the variations of kinetic stability when the thermal stability and density are varied across a range of film thicknesses. |
Friday, March 18, 2022 10:36AM - 10:48AM |
Y16.00012: Combined Description of Polymer PVT and Relaxation Data using a Dynamic "SL-TS2" Mean-Field Lattice Model Valeriy Ginzburg We develop a combined model to describe the pressure-volume-temperature (PVT) thermodynamics and the α- and β-relaxation time dynamics in glass-forming amorphous materials. The PVT results are described using a two-state modification of the Sanchez-Lacombe equation of state (SL-EoS), while the dielectric relaxation is captured using the TS2 mean-field model (Soft Matter 16, 810 [2020]). The application of simple first-order kinetic equations also allows us to study the volume recovery after so-called “down jumps” (when the material is rapidly cooled from liquid to glassy state). We applied the new framework to describe experimental data for polystyrene and poly(methylmethacrylate) and found a good qualitative and quantitative agreement. |
Friday, March 18, 2022 10:48AM - 11:00AM |
Y16.00013: In-situ ellipsometry characterization of vapor-deposited stable molecular glasses Peng Luo, Sarah Wolf, Shivajee Govind, Zahra Fakhraai Recent studies [1,2] in physical vapor-deposited (PVD) molecular glasses and liquid-quenched polymers indicate that thin films have higher density than bulk, can access unique phases at low deposition temperatures, and have gradients in their density, as measured through their refractive index. These effects motivate in-situ studies of the evolution of density and stability as a function of film thickness, which were performed using spectroscopic ellipsometry during vapor deposition of N,N’-bis(3-methylphenyl)-N,N’-diphenylbenzidine (TPD) glasses at various deposition rates and substrate temperatures. The in-plane and out-of-plane refractive indices show non-monotonic variations with thickness, starting from high initial values, indicative of high thin film density compared to thick films. The birefringence goes through a maximum at intermediate thicknesses, showing non-homogenous packing along the normal direction. Steady-state properties are achieved at a thickness ≥300 nm, independent of the deposition rate (0.025~1.5 nm/s). These results elucidate the long-range effect of the thin film geometry on the packing and properties of PVD molecular glasses. |
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