Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session R32: Molecular and Polymer Glass StructureFocus
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Sponsoring Units: DPOLY DSOFT DCP DMP Chair: James Caruthers, Purdue University Room: 504 |
Thursday, March 5, 2020 8:00AM - 8:12AM |
R32.00001: On the Allowable or Forbidden Nature of Vapor-Deposited Glasses Mathieu Bauchy Vapor deposition can yield glasses that are more stable than those obtained by the traditional melt-quenching route. However, it remains unclear whether vapor-deposited glasses are "allowable" or "forbidden," that is, if they are equivalent to glasses formed by cooling extremely slowly a liquid or if they differ in nature from melt-quenched glasses. Here, based on reactive molecular dynamics simulation (MD) of silica glasses, we show that, under certain conditions, vapor-deposited glasses can indeed be more stable than melt-quenched glasses. Importantly, we demonstrate that the allowable or forbidden nature of vapor-deposited glasses depends on the temperature of the substrate and, in turn, is found to be encoded in their medium-range order structure. |
Thursday, March 5, 2020 8:12AM - 8:24AM |
R32.00002: Stable polystyrene glass films through PVD and UV radiation Junjie Yin, Adam Raegen, James A Forrest Stable glasses have proved to be an extraordinary type of material with enhanced density and exceptional kinetic stability compared to normal glasses. So far stable glasses prepared through physical vapor deposition (PVD) have been generally limited to organic small molecules such as indomethacin. We have recently employed PVD to make stable oligomeric glasses. In this study, we extend our capacity to make high molecular weight polystyrene stable glass by using oligomeric stable glass as a starting point. We do this by crosslinking the stable oligomeric glasses by dehydrogenation reaction under ultraviolet (UV) radiation. Depending on the degree of crosslinking, the resulting stable glass can have significantly higher molecular weight and kinetic stability. The sample molecular weight distribution is characterized by MALDI (matrix-assisted laser desorption/ionization), and the stable glass properties are characterized by ellipsometry. |
Thursday, March 5, 2020 8:24AM - 8:36AM |
R32.00003: Physical vapor deposition of a polyamorphic system Benjamin Kasting, Madeleine Beasley, Megan Tracy, Mark Ediger In a few recently investigated systems, physical vapor deposition (PVD) has been shown to produce glasses that transform into liquids with properties that differ from the liquid produced by melting the crystal. Here we use PVD to investigate triphenyl phosphite (TPP), a known polyamorphic system where the second amorphous state (glacial phase) can be obtained by annealing the ordinary liquid (liquid 1) for hours slightly above its glass transition temperature (205 K). The properties of PVD glasses of TPP and the liquids formed from these glasses are studied by alternating current nanocalorimetry and interdigitated electrode dielectric spectroscopy. Deposition between 0.75 and 0.95 Tg results in glasses with very high kinetic stability. Independent of substrate temperature, the liquid that results from transforming the deposited glasses displays the properties of liquid 1, the ordinary liquid. Interestingly, films deposited above Tg also grow as liquid 1 even at temperatures previously used to transform liquid 1 into the glacial phase. The PVD mechanism allows the bulk of the film to inherit the structure at the free surface. These results suggest the preferred surface structure of TPP is similar to the structure of liquid 1, irrespective of deposition temperature. |
Thursday, March 5, 2020 8:36AM - 9:12AM |
R32.00004: Physical properties of ultrastable computer-generated glasses Invited Speaker: Ludovic Berthier Computer simulations give unique insights into the microscopic behavior of amorphous materials. It became recently possible to generate ultrastable glass configurations using a simple Monte Carlo algorithm for a broad variety of model glass-formers. In this talk, we show that this discovery has allowed a deeper understanding of the rheological, thermodynamic and thermal properties of amorphous solids. We discuss in particular recent work regarding the brittle yielding of glassy materials, the Kauzmann transition, and low-temperature excitations of amorphous solids. |
Thursday, March 5, 2020 9:12AM - 9:24AM |
R32.00005: Effects of internal degrees of freedom on simulated vapor deposited glass films Alex Moore, Patrick Walsh, Zahra Fakhraai, Robert Riggleman Recently, amorphous films of small molecules formed via physical vapor deposition (PVD) have been shown to exhibit remarkable thermodynamic and kinetic properties, equivalent to those that have been aged on the order of hundreds of years. Previous results suggest that the stability differences observed in these PVD glasses are the result of enhanced mobility near the free surface, such that molecules are allowed to sample a greater number of configurations before being trapped by subsequent deposited layers. However, much remains to be learned about the effects of properties like molecular shape and rotational freedom on the stability and morphology of these PVD amorphous packings. By using molecular dynamics simulations to mimic the PVD process of coarse-grained small molecules, we can make fine-tuned changes to these properties and then closely observe changes in films, both during and after the deposition process. Notably, we can then potentially make a connection between changes in internal degrees of freedom and the corresponding molecular entropy and changes in PVD film properties. Thus far, we suggest that molecules exhibiting greater entropy in their supercooled liquid state create more stable PVD glass films. |
Thursday, March 5, 2020 9:24AM - 9:36AM |
R32.00006: Anisotropy of extremely monodisperse polymer stable glass thin films Adam Raegen, Junjie Yin, Qi Zhou, James A Forrest In the twelve years since their introduction, ultrastable molecular glasses have become a topic of great interest. Thin films prepared by vapour deposition at substrate temperatures slightly below T_g can exhibit increased densities and stabilities similar to those expected for isotropic glassy films aged hundreds or thousands of years. However, these ultrastable glasses may not truly present the same state as aged glasses. In many ultrastable glasses, the samples display anisotropy, unless steps are taken to make films only in a narrow range of production parameters such as substrate temperature and deposition rate. Simulations have suggested (Lin et. al. J. Chem. Phys. 140, 204504 (2014)) that evaporated oligomeric samples may provide a way to avoid this anisotropy and produce truly isotropic stable glass. In this case, the length of oligomeric samples can greatly influence anisotropy. Using our recently demonstrated technique for making polymer stable glasses, we present an experimental study of extremely monodisperse, ultrastable polymeric/oligomeric thin films. We use spectroscopic ellipsometry to measure birefringence caused by anisotropy in such samples as a function of the stability (fictive temperature) and the oligomeric size. |
Thursday, March 5, 2020 9:36AM - 9:48AM |
R32.00007: Stability dependence of local elastic inhomogeneities of amorphous solids Alireza Shakerpoor, Elijah Flenner, Grzegorz Szamel There is a rich history of studies that suggest the vibrational and thermal anomalies in amorphous solids are linked to the local fluctuations in the elastic matrix of the amorphous medium. More recently numerical and experimental studies have shown a correlation between the vibrational properties and the stability of the medium. In this work, we studied the interplay between the stability and the fluctuations in the local elastic constants for a wide range of stabilities. We found that the variance of the local elastic constants becomes smaller with increasing stability. We also investigated the spatial correlations of the local elastic constants and determined that these constants have only very short range correlations. |
Thursday, March 5, 2020 9:48AM - 10:00AM |
R32.00008: Strong elasticity anisotropy in molecular glasses Zuyuan Wang, Yu Cang, George Fytas, Camille Bishop, Mark Ediger Although glass is often considered isotropic, anisotropic glasses can be prepared by physical vapor deposition of rod-shaped organic molecules. Those glasses feature tunable molecular orientations and anisotropic properties that are beneficial for applications such as organic semiconductors. While the deposition-condition-dependent molecular orientation and the associated optical birefringence of molecular glasses have been investigated, little is known about their mechanical anisotropy, which, however, is needed for understanding their strength upon mechanical loading in packaging and application. In this work, we use micro-Brillouin light spectroscopy to determine the elastic stiffness tensors of three glass films of itraconazole vapor-deposited at substrate temperatures (Tsub) of 330, 315, and 290 K, respectively. The five independent elastic constants in each tensor demonstrate the strong influence of the molecular orientation on the elastic anisotropy. The in- and out-of-plane Young’s moduli of the high Tsub sample, which features a vertical molecular orientation, exhibit a record high anisotropy ratio of 2.1 among molecular systems. |
Thursday, March 5, 2020 10:00AM - 10:12AM |
R32.00009: High Stability of Ultrathin Vapor Deposited Molecular Glasses Yi Jin, Yue Zhang, Sarah Wolf, Aixi Zhang, Shivajee Govind, Connor N Woods, Subarna Samanta, Mikhail Zhernenkov, Guillaume Freychet, Zahra Fakhraai Previous studies demonstrate that a mobility gradient exists within organic glass films, where molecules at or close to the free surface are more mobile than those in the bulk. The stability of glasses formed via vapor deposition relies on the equilibration of as-deposited molecules at the surface. An ultrathin stable glass (SG) film (~50 nm) has a higher proportion of molecules affected by the free surface dynamics, therefore likely has enhanced stability compared to a bulk SG (>200 nm). Here, we show that ultrathin SG films of TPD (N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine) have higher stability, indicated by a higher density increase and a lower fictive temperature than bulk SG films of TPD. Surprisingly, at optimal deposition conditions, density increases higher than the limiting super-cooled liquid states are achieved. Anisotropy indicated by synchrotron scattering measurements suggest a possibility that there exists an amorphous as-deposited state of the molecule, volumetrically denser than its supercooled liquid counterpart, that is more isotropic than bulk SG. The potential origins of this phenomenon will be discussed in this presentation. |
Thursday, March 5, 2020 10:12AM - 10:24AM |
R32.00010: Over what length-scale can the substrate perturb the structure of a vapor-deposited organic glass? Kushal Bagchi, Chuting Deng, Camille Bishop, Yuhui Li, Nicholas Jackson, Michael Toney, Lian Yu, Juan De Pablo, Mark Ediger Ultrathin (<30 nm) vapor-deposited layers are often utilized in OLED (organic light emitting diodes) devices. However, the vast majority of structural studies of vapor-deposited glasses have focused on films that are 100 nm or thicker. As a result these studies have not been sensitive to glass structure at the buried interface, which is of significance for OLED performance. Here we study, with grazing incidence x-ray scattering, the structure of vapor-deposited glasses of molecular glass former DSA-Ph as a function of film thickness. We span a thickness range of 10-600 nm. For films deposited on a Si/SiO2 substrate at ~ 0.8 Tg, we estimate that the region near the buried interface with qualitatively different packing is less than 4 nm. A coarse-grained Lennard-Jones model of DSA-Ph qualitatively reproduces the short interfacial length-scale observed experimentally, implying our results are broadly applicable to molecular glasses that form continuous films on deposition. |
Thursday, March 5, 2020 10:24AM - 10:36AM |
R32.00011: Molecular Orientation Depth Profiles from Resonant Soft X-ray Reflectivity Jacob Thelen, Kushal Bagchi, Camille Bishop, Subhrangsu Mukherjee, Eliot H Gann, R. Joseph Kline, Mark Ediger, Dean DeLongchamp Orientation within amorphous molecular glass films can impart unique optoelectronic properties that provide enhanced performance in devices such as organic light emitting diodes (OLEDs). Bulk molecular orientation in vapor-deposited glass films can be tuned through processing conditions; however, little is known about the molecular orientation at the film surface or at the buried interface(s), where anisotropic optoelectronic properties can have a large impact on device performance. We use vapor-deposited films of tris(4-carbazoyl-9-ylphenyl)amine (TCTA) as a model system to develop soft X-ray techniques to extract molecular orientation from organic thin films. Variable incident-angle near edge X-ray absorption fine structure (NEXAFS) spectroscopy is used to probe the average molecular orientation within the top few nanometers of the TCTA films, as well as to determine the energy-dependence of orientation sensitivity for TCTA. Informed by the NEXAFS spectra, polarized resonant soft X-ray reflectivity (pRSoXR) measurements are performed at selected X-ray energies to probe molecular orientation through the depth of the film. Using the combined NEXAFS and pRSoXR data, we demonstrate a methodology to extract molecular orientation depth profiles with (sub)nanometer-level resolution. |
Thursday, March 5, 2020 10:36AM - 10:48AM |
R32.00012: Molecular dynamics simulation for investigation of Boson peak in a polymer system Akira Koyama, Koji Fukao, Takashi Yamamoto We performed a molecular dynamics simulation to investigate Boson peak (BP) in a united atom polyethylene system. We computed vibrational density of states (DOS), dynamic structure factor (DSF) and wave-vector-dependent dynamic susceptibility (WDS) in a wide range of the angular frequency ω and the wave vector q. The BP was successfully observed in the profiles of DOS and DSF at the temperatures near and below the glass transition point. We determined the peak position of the BP (ωBP/2π=0.25 THz) from the fitting results of DOS; the peak position does not change with changing temperature. The BP was also recognized as an isolated peak on the 3D graph of DSF; the peak position along the q axis (qBP) on the line of ω=ωBP shifts toward high q with decreasing temperature T, where qBP~T-0.5. Using a simple harmonic oscillation model, we estimated the number of monomers nBP giving rise to the BP; nBP=3.2. In the 3D graph of the real part of WDS, 2 distinct steps intersect on the line of ω=ωBP; and in that of the imaginary part, a ridge laying diagonally from high q and ω region to low q and ω region terminates on the same line. From these results, we conclude that the BP in this simulation originates from the localized oscillation of the monomer groups consisting of 3.2 monomers. |
Thursday, March 5, 2020 10:48AM - 11:00AM |
R32.00013: The effect of molecular architecture on the physical properties of supercooled liquids studied by MD simulations. Density scaling and its relation to the equation of state Kajetan Koperwas, Grzybowski Andrzej, Marian Paluch Theoretical concepts in condensed matter physics are typically verified and also developed by exploiting computer simulations mostly in simple models. However, the simplicity of the simple-liquids makes that predictions based on the mentioned model systems are often at odds with measurement results obtained for real materials. One of the examples is an intriguing problem within the density scaling idea (which has attracted attention in recent decades due to its hallmarks of universality), i.e., the fact that difference between density scaling exponent and exponent of the equation of state is observed for real materials, whereas it has not been reported for the model system. Given this problem into account, we project new model systems, which in contrast to the simple-liquids exhibit structural anisotropy, and we examine the effect of molecular anisotropy on the dynamic and thermodynamic properties of the material. We identify the applicable range of intermolecular interactions for a given physical process, and then we explain the reason for observed differences between the behavior of the model and real systems [J.Chem.Phys.150,014501(2019)]. |
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