Session Z16: Molecular and Polymeric Glasses

Glass forming ability of polyzwitterions

The glass forming ability of a series of specially synthesized polyzwitterions was studied using fast scanning calorimetry (FSC). Polyzwitterions include those based on the sulfobetaine moiety: sulfobetaine acrylate (SBA), ethyl sulfobetaine methacrylate (ESBMA), sulfobetaine vinylimidazole (SBVI), sulfobetaine 4-vinylpyridine (SB4VP), sulfobetaine methacrylate (SBMA), and sulfobetaine methacrylamide (SBMAm). FSC was used to investigate the dynamic fragility over a large range of cooling rates, 10 K/s - 4000 K/s, minimizing thermal degradation of the polyzwitterions. The rate dependence of the limiting fictive temperatures (T_f) was measured and fit to the Williams-Landel-Ferry model, from which the polyzwitterion dynamic fragility was determined for the first time. Dynamic fragility was low, ranging from 41 to 110 depending on the underlying chemical structure, which allows classification of this series of polyzwitterions as moderate to relatively strong polymeric glass formers. Their high glass transition temperatures combined with low fragilities indicates that polyzwitterions are unique among polymeric glass formers. This behavior arises from formation of inter- and intrachain dipole-dipole crosslinks which causes more dense molecular packing and cohesion.   

Illuminating the Rigid Amorphous Fraction of Semiconducting Polymers, and its Pivotal Influence on Optoelectronic and Mechanical Performance

Nearly all semicrystalline polymers possess two types of amorphous domains, the mobile amorphous fraction (MAF) and the rigid amorphous fraction (RAF), which exhibit distinct glass transition phenomena and govern material performance. Yet, for semiconducting polymers, there is little information about the morphological landscape surrounding these transitions at device relevant thickness and hence their identity and the role they play in device performance is obscured and of great debate. Here, we not only elucidated the identity of these transitions (backbone T_g and RAF T_g) but also the mechanism by which they control material performance in four representative semiconducting polymers (P3HT, DPPT-C8C10, N2200, and PFFBT-4T). This was first achieved through temperature dependent in-situ ellipsometry, whereby, the thermal expansion, optical profile, and conformation were all assessed.  The contribution of RAF to the thermal expansion was observed to be 20% for P3HT and 75% for the remaining high-performance polymers. This was attributed to their high rigidity and paracrystalline disorder which promote crystalline-amorphous connectivity associated with RAF. The conformation and optoelectronic behavior were further assessed utilizing temperature dependent DFT simulation, GIWAXS, solid-state NMR, and OFET charge mobility, thus providing the 1^st holistic picture of the influence of RAF on the performance of semiconducting polymers. Our work demonstrates that the RAF is a critical component governing polymer connectivity and subsequently optoelectronic and mechanical behavior.   

Characterization of the interfacial orientation and molecular conformation in a glass-forming organic semiconductor

Vapor-deposition enables precise control over the nanostructure in molecular glasses which are an important class of materials used in organic electronics. Extensive characterization of film properties has identified bulk molecular orientation as a key structural motif that determines optoelectronic performance. However, few studies have investigated how molecules orient near buried interfaces, despite its importance in electronics. Here, we use polarized resonant soft X-ray reflectivity to measure nanometer-resolved molecular orientation depth profiles of vapor-deposited thin films of an organic semiconductor, TCTA. Our approach characterizes the molecular orientation throughout the film and reveals how molecules near the substrate and free surface have a nearly isotropic orientation compared to an anisotropic bulk. Pairing our results with NEXAFS enables a determination of the molecular conformation. We support our results with molecular dynamic simulations that provide insight into the mechanisms of film formation that occur at interfaces between orientationally distinct layers. Applications to multicomponent glasses will be discussed and the potential for future studies to develop critical structure-function relationships.   

Polarized Resonant Soft X-ray scattering reveals interfacial molecular orientation in phase-separated vapor-deposited glass.

Molecular orientation at interfaces in multi-component organic films can significantly affect the material’s function, but it can be difficult to quantify. The interaction between oriented chemical bonds and polarized resonant soft X-rays results in scattering (p-RSoXS) that is sensitive to interfacial molecular orientation, in addition to chemical composition. We use p-RSoXS to measure a glass prepared by co-vapor depositing two immiscible glass-forming molecules. When vapor-deposited, the two molecules phase separate to an extent that is determined by the substrate temperature during deposition. Both p-RSoXS and AFM reveal that, as the substrate temperature is increased, laterally segregated (in-the-plane of the film) domains become larger and more well-organized, likely due to increased molecular mobility at the surface. We use a forward-scattering GPU-accelerated simulation based on high-quality real space imaging to identify a real-space model that produces a combination of compositional, orientational, and vacuum scattering contrast that is consistent with experiment. This approach allows us to measure the direction, spatial extent, and magnitude of orientation in molecules at interfaces between laterally segregated glasses for the first time.   

Theory of Aging in Soft Viscoelastic Glasses

Almost all glass-forming materials age; i.e., their rheology and dynamical properties depend on the amount of time spent waiting before a measurement is taken. Thus, there is a fundamental interest in understanding aging in soft glassy materials, which also applies to biological matter such as biocondensates. In this talk, we present a theory of aging for viscoelastic glasses that examines the nonequilibrium, protocol-dependent properties of the system.  Using random first-order transition theory, we consider the local fluctuations in the driving force to account for the glassy material's heterogeneity and non-exponential relaxation rates. Our model demonstrates how the stress response and other mechanical properties vary with age. We also discuss general insights into the physics of aging.   

A new thin film scanning calorimeter to measure high-T thermodynamics in amorphous Ge-Te

A new thin-film calorimeter design suitable for measuring thin films on the heating curve of the glass transition using ultra-rapid differential scanning calorimetry (DSC) is presented. This new design displays a more uniform thermal profile over the sample area and has increased sensitivity compared to previous similar devices. The capacity to measure on the heating part of the curve will allow us to probe films grown using physical vapor deposition (PVD), allowing us to access states in the energy landscape that are lower than those typically available to traditional quenched glasses. Amorphous Ge-Te films are grown at various atomic compositions around the eutectic point and at various growth temperatures using PVD to show correlations between growth conditions, fragility, ultrastability, and high-T thermodynamics, including the glass transition. These results will be compared to low-T results for the same glasses to establish correlations between high-T properties and tunneling two level system (TLS) properties.   

Exploring the energy landscapes of soft glassy systems

Soft-glassy systems have been studied for a long time with wide-ranging applications in everyday life. The slow non-equilibrium dynamics, though, restricts the ability to study them computationally or experimentally using conventional tools, particularly at low temperatures or high volume fractions. The properties of low energy states are of particular importance to answering many fundamental questions surrounding the glass transition and in connecting simulation results to experiments. In this study, we present an algorithm to sample the energy landscape and probe low-energy inherent structures(IS) of the conventionally studied Kob-Anderson(KA) glass. Our algorithm is inspired by metadynamics, though rather than biasing on a small number of reaction coordinates, we bias on the system's high-dimensional configuration space. For certain bias dimensions in energy and space, the algorithm samples low energy ISs in a computationally efficient way, as compared to other conventional techniques. We further use algorithm-generated ISs to thermalize and generate equilibrium-like ensembles close to predicted Kauzmann temperatures. Lastly we note that the ISs reveal interesting fractal and low-dimensional characteristics of the underlying energy basins of the KA system.   

The Fragile-to-Strong Crossover in Simulated Supercooled Water

Glass-forming liquids usually exhibit either "fragile" or "strong" behavior approaching the glass transition. In contrast, experimental and computational evidence indicates that supercooled water exhibits a novel "fragile-to-strong crossover" on cooling at low pressure, which is related to an expected liquid-liquid transition at higher pressure. We utilize molecular dynamics simulations to investigate how the fragile-to-strong crossover alters the nature of collective molecular rearrangements in water. We compare the results from two different models, TIP4P/2005 and ST2, to identify which results are likely to be general, and not an artifact of the model. We show that the crossover can be identified with the onset of landscape-dominated dynamics. We find that the heterogeneity of molecular displacements diminishes approaching the crossover to strong behavior, which roughly coincides with the structure reaching an energetically stable random tetrahedral network. We examine how motion catalyzed by defects of the network interplays with collective motions involving larger groups of water molecules. We further consider how the changes in the heterogeneity of dynamics affect the breakdown of the Stokes-Einstein relation, which is believed to be a consequence of dynamic heterogeneity.   

Glassy behavior of intrinsically disordered proteins

Intrinsically disordered proteins keep being a puzzle regarding their behavior and biological function. In this work, we follow the relaxational dynamics of several common intrinsically disordered proteins from 10^-12 to 10^-5 seconds. To this end, we combine extensive Molecular Dynamics simulations (using realistic force fields recently tuned with experiments), with large-scale temperature replica-exchange MD simulations, to access the equilibrium dynamics. We show that the dynamics of intrinsically disordered proteins at room temperatures is far more complex than what previously thought. In particular, they display aging effects, an important dependence of the characteristic times with temperature, and collective relaxations. In summary, their microscopic dynamics resemble very much to that of dense glass forming liquids.    

Single-parameter aging in molecular glasses

Physcial aging is the study of how the properties of a system change over time as it relaxes toward equilibrium. Physical aging can be observed in glasses just below the glass transition, where relaxation is slow, but still fast enough to be observed. Physical aging, the glass transition and the puzzles of supercooled liquids are thus closely linked phenomena.

In a series of papers we have shown how physical aging can be described by a single-parameter scenario through precise measurements and careful temperature protocols. Single-parameter aging is a reformulation of the Tool-Narayanaswamy idea of a fictive temperature controlling the state of the system - as well as the property monitored - during aging.

In addition, our measurements show that: 1) the high-frequency shear elastic modulus governs the relaxation rate during aging for DC704 (tetra-phenyl tetra-methyl trisiloxane), thus suggesting that in equilibrium the temperature dependence of could explain the non-Arrhenius behavior of the relaxation time (the shoving model); 2) a terminal relaxation rate exists in nonlinear relaxation curves of squalane, which is difficult to resolve in linear spectroscopic measurements; 3) non-linear aging can be predicted from equilibrium fluctuations in the molecular liquids VEC (4-vinyl-1,3-dioxolan-2-one) and NMEC (N-methyl-ε-caprolactam), as well as in computer simulations of a binary Lennard-Jones liquid.

These findings may be general for supercooled liquids.   

Glassy dynamics and electrical conductivity of Ionic liquid Crystals: Dependence of the cation n-alkyl side chain length

We investigate the molecular dynamics of a homologous series of linear-shaped guanidinium based ionic liquid crystals (ILCs) that vary in alkyl chain length, R = 8, 10, 12, 14, 16,  by employing broadband dielectric spectroscopy (BDS), Fast Scanning Calorimetry (FSC), and temperature modulated FSC (TMFSC).  Besides conductivity, the dielectric dispersion reveals two relaxation modes:  a fast γ and a slow α₁ relaxation. The former is assigned to localized fluctuations while the latter is due to segmental dynamics of the alkyl chains.  Calorimetric investigation reveals one process, the α₂ process, for ILC12,14 and 16, and two processes, α₂ and α_3, for ILC8 and 10. The relaxation rates of the α₂ process have a similar temperature dependence as that of the α₁ relaxation, which indicates that both BDS and FSC probe the segmental dynamics of alkyl side chains. We interpret the α₃ process of ILC8 and 10 as the segmental dynamics of the cation core. For all ILCs, the absolute values of DC conductivity increase by 4 orders of magnitude at the transition from the plastic crystalline to hexagonal columnar phase. The α₂ process shifts to higher temperatures with increasing alkyl chain length. Conversely, the DC conductivity drops by 3 orders of magnitude from ILC8 to ILC16.    

Measuring slow relaxation and stability in vapor deposited stable polymer glasses

Recent work in our lab allows us to create stable polymer vapor deposited glasses of polystyrene with N up to ~12, with kinetic stability observed down to deposition temperatures of ~0.84*T_g.  These stable vapor deposited polymer glasses have fictive temperatures as low as T_g - 20 K relative to the supercooled liquid line for glasses of polystyrene of the same N.  Previous measurements on stable molecular glasses suggests that annealing above the deposition temperature rejuvenates the glass, at least partially, forming a less stable glass apparently similar to a conventional glass formed by cooling an equilibrium liquid.  This rejuvenation behavior of the glasses may be interpreted as the glass moving up through a metabasin of the potential energy landscape.  Physical aging measured by ellipsometry has been used as a direct measurement of stability in conventional polymer glasses, but not stable polymer glasses.  We explore here the physical aging behavior of stable vapor deposited, aged, and rejuvenated polymer glasses (N>9 and a T_g = 308 K).  Slow relaxation dynamics are measured and compared with measurements of stability before and after rejuvenation in vapor deposited PS and PMMA films.   

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Light absorption, emission, and propagation through structured media give rise to a myriad of fascinating optical phenomena, including the generation of various modes of light polarization in support of numerous photonic, medical, and optoelectronic technologies.  Natural light is largely unpolarized which can be considered to consists of equal amounts of right- and left-handed circularly polarized components. Of particular interest is the generation of pure circularly polarized light through monodomain Ch-GLC films. The underlying optical process is amenable to the analysis by Good-Karali theory for light absorption and reflection mediated by a dichroic dye oriented in the cholesteric film, resulting in a composite of selective absorption and reflection. This complex process culminates in preferential transmission and reflection of circularly polarized light of opposite handedness. A rich framework is identified for unravelling the effects on the transmission / reflection ratio by the Ch-GLC film thickness, the dichroic dye concentration, and spectral separation between the host film's stopband and the dye's absorption spectrum.