Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session P32: Molecular and Polymer Glass DynamicsFocus
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Sponsoring Units: DPOLY DSOFT DCP DMP Chair: Mark Ediger, University of Wisconsin - Madison Room: 504 |
Wednesday, March 4, 2020 2:30PM - 3:06PM |
P32.00001: Evidence for heterogeneous bulk melting dominating the transition of organic stable glasses Invited Speaker: Marta Gonzalez-Silveira For over a century, scientists have tried to understand the mechanisms behind the glass transition and the dynamics of glassy systems. Part of the difficulty arises from the time scales involved in these processes. The low stability of conventional glasses, obtained by cooling down a melted material, have determined for years how we study glass dynamics, focused basically on the aging regime, i.e., performing isothermal treatments below the limiting fictive temperature (Tf) of the glass, allowing the glass to evolve towards a more stable configuration. But the evolution of glasses above their Tf in isothermal regime, what we could call softening of rejuvenation as the glass progressively loses stability, has been barely explored at temperatures not far above the conventional glass transition temperature of the material, Tg. The time scales in this case are extremely short for conventional glasses, and in order to study rejuvenation one would need either glasses of high stability (to increase the measuring time) or advanced instrumentation (able to work in very short times). This scenario has changed with the ability to prepare glasses of a wide range of stabilities by means of PVD and the development of fast scanning nanocalorimetry. By performing isothermal treatments at temperatures spanning more than 30 K above the conventional Tg of the material we show that the rejuvenation process in stable glasses takes place via two different mechanisms: i) a homogeneous softening of the glass and ii) the formation of liquid patches in the bulk of the glass, which grow transforming the glass straight into the liquid, dominating the pace of the transformation and resembling the nucleation and growth process characteristic of the melting of crystals. Surprisingly, these two mechanisms are present even in glasses with stabilities close to those of the conventional glass, although clear differences can be found in the number of initial liquid patches and the transformation rate. |
Wednesday, March 4, 2020 3:06PM - 3:18PM |
P32.00002: Solvent vapor annealing of stable glasses Shivajee Govind, Haoqiang Zhao, Patrick Walsh, Zahra Fakhraai Stable glasses (SG) are highly dense, low energy glasses that are made by physical vapor deposition (PVD). Because of their desirable properties and wide range of applications, they are extensively studied glasses. However, measuring their dynamics can be difficult since they are produced directly into a SG state on a substrate during the process of PVD. We have introduced a novel and indirect method of measuring the dynamics of SG using solvent vapor annealing (SVA). Using this technique, together with in-situ ellipsometry, we have shown that even at low solvent vapor pressure, the surface of SG swells more quickly, in an analogous Fickian type diffusion, compared to that of the bulk. Above a certain threshold of vapor pressure, the bulk of the film swells linearly with time, which demonstrates a Case II type of diffusion. This shows the diffusion of the solvent molecule through the film is limited by the relaxation of the molecules such that the solvent uptake is governed by a two-layer model; one swollen with solvent and the other the dry medium. By monitoring the solvent front moving across the film, we compare the dynamics of the SG films of different stability produced at different deposition temperatures. |
Wednesday, March 4, 2020 3:18PM - 3:30PM |
P32.00003: Secondary Dynamics in Ultrastable Polystyrene Thin Films Studied by β-NMR Iain McKenzie, Danaan Cordoni-Jordan, Derek Fujimoto, Victoria Karner, Robert F Kiefl, Philip C. P. Levy, W Andrew MacFarlane, Ryan M. L. McFadden, Gerald Morris, Matt Pearson, Adam Raegen, John Ticknor, James A Forrest Ultrastable, highly mono-disperse polystyrene (PS) thin films can be produced by physical vapor deposition. These films can exhibit properties similar to those of a normal glass that has been aged for thousands of years. β-detected nuclear magnetic resonance (β-NMR) of implanted 8Li+ probes can be used to study dynamics in thin polymer films, which is not possible with conventional magnetic resonance techniques [I. McKenzie et al. Soft Matter 14, 7324 (2018)]. We have used β-NMR to study the temperature dependence of the secondary γ-relaxation process in an ultrastable PS film and a normal PS film that was produced by rejuvenating an ultrastable film by heating to Tg+25 K for 2 minutes. The γ-relaxation is ~38% slower at 295 K, and the activation energy is ~20% larger in the ultrastable glass compared with the normal glass. Our interpretation of the results is that the denser packing in the ultrastable glass hinders motion of the phenyl rings. |
Wednesday, March 4, 2020 3:30PM - 3:42PM |
P32.00004: Uncovering β-relaxations in amorphous phase-change materials Shuai Wei, Si-Xu Peng, Yudong Cheng, Julian Pries, Hai-Bin Yu, Matthias Wuttig Relaxation processes are decisive for many relevant physical properties of amorphous materials. For amorphous phase-change materials (PCMs) employed in non-volatile memories, relaxation processes are, however, difficult to characterize due to the lack of bulk samples. Here, instead of bulk samples, we use powder mechanical spectroscopy for powder samples to detect the prominent excess wings – a characteristic feature of β-relaxations – in a series of amorphous PCMs at temperatures below the glass transition. By contrast, β-relaxations are vanishingly small in amorphous chalcogenides of similar composition, which lack the characteristic features of phase-change materials. This conclusion is corroborated upon crossing the border from PCMs to non-PCMs, where β-relaxations drop significantly. Such a distinction implies that amorphous PCMs belong to a special kind of covalent glasses whose locally fast atomic motions are preserved even below the glass transitions. These findings also suggest a correlation between β-relaxation and crystallization kinetics of PCMs, which may have technological implications for phase-change memory functionalities. |
Wednesday, March 4, 2020 3:42PM - 3:54PM |
P32.00005: A direct static scattering evidence on the dynamic nature of glass formation process in polystyrene He Cheng, Guisheng Jiao, Taisen Zuo, Changli Ma, Zehua Han, Junrong Zhang, Junpeng Zhao, Charles C Han The combination of neutron total scattering, high concentration labelling and molecular dynamics (MD) simulation enable us to visualize the temperature dependence most-probable all-atom positions in atactic polystyrene (PS), and find out the dynamic nature of its glass forming. The scattering profiles from MD simulation are in agreement with all of the neutron scattering curves, as well as the X-ray scattering data in literatures at different temperatures. Two main scattering peaks at ~0.6 Å-1 and 1.4 Å-1 can thus be allocated. The peak at ~0.6 Å-1 is from the segment-segment interaction, which is a combination of backbone-backbone and backbone-phenyl interactions. And the peak at 1.4 Å-1 is mainly from phenyl-phenyl interaction. Below glass formation temperature, both peaks keep almost constant. Their heights jump during glass formation, revealing the melting of the configuration variation and development of dynamic heterogeneity. |
Wednesday, March 4, 2020 3:54PM - 4:06PM |
P32.00006: Effects of aromatic side-group structure on thermal transitions of polyzwitterions Andrew Clark, Yajnaseni Biswas, Ayse Asatekin, Morgan E Taylor, Matthew J Panzer, Christoph Schick, Peggy Cebe Aromatic structures can have a pronounced effect on the glass forming properties of ionic liquids and polymers. Polyzwitterions are polymers with side groups that contain a covalently linked anion and cation. Depending upon side chain structure, this can lead to high glass transition temperatures, occurring near the onset of degradation. In this study we are using fast scanning calorimetry to investigate how aromatic structures in the side-group alters the glass formation of polyzwitterions. Poly(sulfobetaine vinylimidazole) (PSBVI) and poly(sulfobetaine-4-vinylpyridne) (PSB4VP) were synthesized and cast into films. Both polymers contain identical sulfonate anions allowing for a good comparison of the effect of aromatic structure variations in the side-group. The glass transition could not be observed prior to degradation in either polymer using conventional slow scan DSC. Using fast scanning calorimetry, degradation was avoided by heating and cooling at 2000 K/s which allowed the first measurement of the glass transition process in these materials. We found that PSBVI has a Tg of 250 oC while PSB4VP has Tg at 215 oC. Analysis of the fragility reveals that PSBVI is a stronger glass former than PSB4VP. |
Wednesday, March 4, 2020 4:06PM - 4:18PM |
P32.00007: Rigorous analysis of the linear viscoelastic behavior of thermo-rheologically complex amorphous materials Grigori Medvedev, James M Caruthers It is well-known that linear viscoelastic response of amorphous materials, including both low molecular weight and polymeric, as probed in mechanical and dielectric relaxation experiments is thermo-rheologically complex. A common approach to analyzing viscoelastic data is to use a combination of phenomenological functions such as stretched exponential, Havriliak-Negami, etc. manifesting as distinct peaks in the relaxation spectrum. Unfortunately, not only the location on the time axis but also the shape and the spectral strength of these peaks is required to change with temperature rendering such representation merely a curve fit. A brute force Prony series expansion has also been tried, where the relaxation times are typically assumed to be evenly spaced on the logarithmic scale. The resulting set of Debye peaks of varying strength also lacks physical interpretation. We propose a different approach where all processes have the same strength, but the corresponding relaxation times are dictated by the data. When broad temperature interval comprising sub-Tg and above Tg regions is investigated a complete relaxation map emerges. Examples of the relaxation map will be shown for several materials, including epoxy systems of varying architecture. |
Wednesday, March 4, 2020 4:18PM - 4:30PM |
P32.00008: Relaxation processes in polymer glasses: a hierarchical approach Peter Olmsted, Daniel L Baker, Matthew Reynolds, Johan Mattsson, Robin Masurel The glass transition temperature Tg in polymers increases with increasing molecular weight M, but the detailed Tg(M) dependence in polymers is not well understood. Here, we present experimental results of the M-dependence of both the structural (alpha) relaxation process, which controls the glass transition, and faster (beta and gamma) secondary relaxation processes for a range of polymers of varying chain flexibility. Based on our results, we propose a hierarchical relaxation scenario that links these relaxations and controls Tg in polymers, where the fundamental metric is linked to ‘local’ flexibility. We identify regimes in M where intra and inter-molecular relaxation dynamics play different roles in defining the dynamics (and thus Tg). We argue that dihedral barriers play a crucial role in controlling the dynamics, which gives rise to distinct differences in behaviour from what is observed in glass-formers with simpler ‘rigid’ structures, or in simplified bead-spring models of polymers. |
Wednesday, March 4, 2020 4:30PM - 4:42PM |
P32.00009: Unusual Viscoelasticity in Polyrotaxane Glasses Karan Dikshit, Carson J Bruns This work characterizes the viscoelastic properties of glasses made from polyrotaxanes, a class of mechanically interlocked polymers possessing a bead-on-a-string structure. The rheology of polyrotaxane-based glasses in the devitrified state reveals an absence of elastic plateau over a large range of frequencies.The motivation for this investigation stems from a need to understand the processability of polyrotaxanes. These ”molecular necklaces”, without any modification, are crystalline due to the presence of strong hydrogen bonding between the cyclodextrin-based molecular beads. Similar to cellulose, which also exhibits strong hydrogen bonding, these materials do not flow when heated. Chemicalmodification of the cyclodextrins leads to the breakdown of hydrogen bonding which, in turn, reduces crystallinity |
Wednesday, March 4, 2020 4:42PM - 4:54PM |
P32.00010: Volume recovery and physical aging of pressure-densified glasses Daniel Fragiadakis, Adam Holt, Charles M. Roland When a glass is formed by cooling under high pressure, the resulting pressure densified glass exhibits a higher density and different thermal and mechanical properties than the corresponding glass prepared by conventional cooling at low pressure. Understanding, manipulating, and exploiting the full potential of pressure densified glasses offers the possibility of better properties. The effects of pressure densification on 1,3,5-tri(1-naphthyl)benzene (TNB) are assessed from volumetric and calorimetric measurements. The pressure densified glass (PDG) exhibits anomalous physical aging. Rather than evolving monotonically towards the equilibrium density, there is an overshoot to a lower density state. Only when the density of the PDG becomes equivalent to the corresponding CG does the density begin a slow approach towards equilibrium. Using molecular dynamics simulations of a simplified model of TNB, we show that the effects of vitrification pressure and subsequent volume recovery and aging of the glass are comparable to experimental results for real TNB. A two-parameter description of the underlying non-equilibrium structure, entailing a fictive temperature and fictive pressure, appears adequate to interpret the “anomalous” aging behavior of pressure-densified TNB. |
Wednesday, March 4, 2020 4:54PM - 5:06PM |
P32.00011: Understanding aging phenomena by the free-energy-landscape approach Takashi Odagaki Aging phenomena have been observed in many non-equilibrium systems such as polymers and glasses, where physical properties depend on the waiting time before observation. It is not known what causes the aging, nor what information can be extracted from them. Exploiting the free-energy-landscape (FEL) theory[1], which explains dynamic and thermodynamic properties of non-equilibrium systems in a unified frame work, I argue the physical origin of the aging phenomena for the first time. The FEL depends on temperature and, thus, will respond to a temperature modulation with delay. I consider physical quantities observed after a waiting time tw when the temperature is modulated at time t = 0 and show that the delayed response of the FEL manifests itself in an additional aging. As an example, I investigate aging phenomena of dielectric relaxation represented by a two-level model. I show that the relaxation function and the linear susceptibility show non-trivial dependence on tw from which one can deduce the relaxation time of the FEL. |
Wednesday, March 4, 2020 5:06PM - 5:18PM |
P32.00012: Lifetime of Rate Domains: Comparison of Simulations and Single-Molecule Experiments in o-Terphenyl Harveen Kaur, Keewook Paeng, Laura Kaufman, Mark Berg Supercooled liquids have domains with different relaxation rates. The lifetime of those domains is not well understood. The usefulness of single-molecule measurements has been limited by conventional, molecule-by-molecule analysis, which suffers from excessive noise. An alternative, ensemble-based analysis using multidimensional correlation functions is applied to single-molecule dichroism results in o-terphenyl near Tg (Tg+4.5–1.5 K, Trot = 3–15 s). It yields a detailed rate-exchange correlation function, which is biphasic. The slow, main component is 22 times slower than the alpha-relaxation time, implying that single-particle relaxation does not fully equilibrate the liquid. A small, fast phase of rate exchange is attributed to molecules on the boundaries of rate domains. The same analysis was applied to a ms-long, all-atom simulation of o-terphenyl from the D. E. Shaw group [J. Phys. Chem. B 117, 12898 (2013)]. It is at a higher temperature (272.5 K, Trot = 16 μs), but it is still well below the mode-coupling temperature (290 K). There are clear differences from the results near Tg, suggesting that time–temperature superposition does not hold for rate exchange. |
Wednesday, March 4, 2020 5:18PM - 5:30PM |
P32.00013: Unified description of the Arrhenian and super-Arrhenian behavior of OTP by the excess internal energy model James M Caruthers, Jack Yungbluth, Grigori Medvedev, Brett Savoie As previously reported (Caruthers, Medvedev 2018), the excess internal energy model, 1/Ux, unlike the excess entropy model of Adam and Gibbs, is able to describe the mobility of liquids in the super-cooled state as a function of temperature and pressure. The 1/Ux model prediction of the temperature effect on mobility was validated for 21 molecular glass formers and of the temperature and pressure – for ortho-terphenyl (OTP) – the only material for which there was sufficient data. In the current work MD simulations were performed on model OTP in the high temperature region that includes the onset of the super-Arrhenian behavior. The excess internal energy was obtained at pressures from 0.1 MPa to 5 GPa. The translational and rotation diffusivities were determined at these temperatures and pressures. The MD mobility data (including extremely long time 0.1 MPa simulations from Eastwood et al. 2013) were combined with experimental data, where it was shown that in both the high temperature Arrhenian region and the lower temperature super-Arrhenian region the mobility is a linear function of 1/Ux, albeit with different proportionality constants; the transition between the Arrhenian and super-Arrhenian behaviors is relatively sharp at a critical internal energy Ux,α. |
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