Session B32: Molecular Glasses: Structure and Stability

The Structural Change Associated with Amorphous Solidification: A General Order Parameter Description

A central problem posed by the transformation of liquid into glass on cooling is to develop a general atomic level description of amorphous solidification. The answer proposed in this paper is to measure a configuration’s capacity to restrain the motion of the constituent atoms. Here we show that the instantaneous normal modes can be used to define a measure of atomic restraint that accounts for the difference between fragile and strong liquids and the collective length scale of the supercooled liquid. This measure provides a microscopic account of solidification that is independent of the rate of cooling. We show that a related measure can be obtained experimentally from the analysis of the temperature dependence of speckle scattering. These results represent a significant simplification of the description of amorphous solidification and a potentially powerful systematic treatment of the impact of thermal and mechanical history, atomic interactions, molecular shape and flexibility and surface effects on the structural transformation from supercooled liquid to amorphous solid.      

Description of short-range order in glass formers

Unlike crystals, liquids and amorphous solids do not possess a long-range order. This does not mean that a local arrangement of molecules is completely random. MD simulation of several molecular systems reveals existence of preferred shapes of clusters formed by neighboring molecules. The distribution of shapes of angstrom size clusters exhibits strong dependence on temperature as the glass transition is approached. The temperature dependence vanishes for clusters of the size of nanometer and larger, where the average shape of the cluster becomes approximately spherical. It is observed that the cluster shapes distribution differs in the metastable vs equilibrated state at the same temperature. The systems investigated include: thermal – Lennard-Jones dumbbells and model coarse-grained ortho-terphenyl and athermal – polydisperse hard spheres. Implications for the search of the order parameter controlling the glass transition will be discussed.   

Probing the ultimate density and stability of disordered films

The density and stability of experimental vapor deposited films increase as they are equilibrated to lower temperatures; equilibration become difficult below the glass transition temperature, Tg. Creating such films in silico, using size-dispersed spheres to avoid crystallization and atom swapping in a near-surface region to mimic the experimental surface mobility allows probing further. With this system, surface mobility is independent of temperature, and full equilibration can be obtained to ~0.75 Tg, with ρ(T=0) increasing to ~ 0.78, stability ~1000 times. We find that such equilibration creates icosahedral neighborhoods (densest local sphere packing), whose concentration increase linearly with log(swap intensity) to c_icos(T=0) ~5 %. The density rise slows substantially as the icosahedron concentration increases, suggesting close approach to limiting density and structural values.    

General behavior of ultrastability and anisotropic molecular packing in co-deposited organic semiconductor glass mixtures

Vapor-deposited glass mixtures of organic semiconductors serve as active layers in organic electronic devices. Here, we study the stability and anisotropic molecular packing of six co-deposited 50:50 organic semiconductor glass mixtures. The results show that all six binary systems exhibit highly enhanced kinetic stability and significantly reduced enthalpy when deposited at 0.78-0.88T_g,mixture. Furthermore, for a given binary system, the birefringence of the PVD glass mixture can be predicted from the temperature-dependent birefringence of the pure compounds. Our findings provide guidance for producing organic semiconductor glass mixtures with enhanced stability and desired structure, which has important implications for design of novel electronic devices with longer device lifetime and higher operational efficiencies.   

High-density stable glasses formed on soft substrates

Physical vapor deposition can create high-density stable glasses comparable to liquid-quenched glasses aged for millions of years, enabled by surface-mediated equilibration. Deposition is often performed on rigid substrates, at various rates and temperatures, to control glass properties. Here, we demonstrate that on soft, rubbery substrates, surface-mediated equilibration is enhanced up to 170 nm away from the interface, forming stable molecular glasses with densities up to 2.5% higher than liquid-quenched glasses, within 2.5 hours of deposition. To gain these properties on rigid substrates requires 10 million times slower deposition, taking ∼3000 years. Controlling the modulus of the rubbery substrate provides a large degree of control over the glass structure and density at a constant deposition condition. When heated to above the glass transition temperature, the stable glasses on soft substrates transform into supercooled liquid via two growth fronts, the one initiated from the soft interface propagates with a faster initial velocity, persisting up to 100 nm away from the interface, before slowing down to the same velocity as the one initiated from the free surface. This behavior also exhibits a high sensitivity to the modulus of the rubbery substrates. These results underscore the significance of substrate elasticity as a novel factor in manipulating the long-range dynamic perturbations, and thus the structure and properties of vapor-deposited glasses, allowing access to deeper states of the energy landscape, without the need for prohibitively slow deposition rates.   

Large Mismatch in Tg Values Does Not Impede Formation of Stable Co-deposited Glasses

It has been well-established that the right combination of substrate temperature and deposition rate allows for the formation of a vapor-deposited glass with enhanced stability for a variety of materials. The stability of vapor-deposited glasses of more than one component, which are particularly important for technological applications such as OLED screens, is only just starting to be explored. In this work, we study the co-deposited glasses of methyl-m-toluate (MMT) and methyl acetate (MeAc), both small molecules that form stable glasses through vapor deposition. These particular molecules are very similar structurally, consisting of the same ester group with bonded to either a methyl group (MeAc) or a larger phenyl ring (MMT); however, their T_g values are different by ~50%, meaning that they have very different surface mobilities at the same substrate temperature. In this work, we observe the formation of stable vapor-deposited glasses of MMT/MeAc mixtures using dielectric spectroscopy, noting a similar dependence of the stability on the substrate temperature to the pure components relative to the T_g of the mixture. This result suggests that very large differences in T_g are not an impediment to the formation of stable co-deposited glasses.   

Probing Bond Angle Orientations in CuZr Glass using Fluctuation X-ray Scattering and Angular Cross-Correlation

Local orientational order is important for the thermodynamics of amorphous and liquid materials.  Most experiments characterize amorphous materials using the structure factor S(q), the Fourier Transform of the two-point density-density correlation function. Combining Fluctuation X-ray Scattering (FXS) and Angular Cross Correlation Analysis (ACCA) provides a quantitative framework for characterizing angular correlations. This study probes the bond angle orientations in a Cu₅₀Zr₅₀ glass. Diffraction patterns from the Cu₅₀Zr₅₀ glass samples were collected at the nano-focus beam line, ID13, at ESRF. The diffraction patterns were processed with an angular cross-correlation method based on the Martin et al. 2017, Pair Angle Distribution Function (PADF) analysis, enabling extraction of angular correlation functions in real space. The PADF analysis revealed significant correlations between bond angles, indicating preferred arrangements of atomic bonds and local structural motifs that are indicative of non-uniform structural configurations. These results highlight the power of combining advanced X ray scattering techniques and innovative data analysis methods to provide a more comprehensive description of the structural properties of amorphous materials and liquids.    

Effect of fragility on the stability and thermodynamic properties of amorphous Te-Ge films

Amorphous materials exhibit great a range in their properties due to their ability to possess different local structures. Depending on the local structure, amorphous materials can achieve a wide range of stabilities, both above and below the traditional fast-cooled materials. We measured this stability primarily by examining the thermodynamic properties of the material, including using fast-scanning calorimetry to measure the glass transition onset and magnitude. In this work, we show changes in the stability of amorphous as-prepared TeGe films prepared with different growth parameters. By varying these conditions we are able to achieve understable, normally stable, and ultrastable films, even at the same composition. We further compare the results of different TeGe compositions, and therefore fragility, on the ability to form glasses of various stabilities.   

Dense co-deposited glasses of organic semiconductors have enhanced thermal stability

Physical Vapor Deposition (PVD) is utilized for production of organic semiconductor devices due to their favorable properties which cannot be accessible by other methods. Although layers in the device are usually composed of two or more components, the fundamental characteristics of co-vapor deposited glasses are elusive yet. Here, spectroscopic ellipsometry (SE) is employed to characterize optical properties, thermal stabilities and densities of co-deposited NPD&TPD glasses with varied compositions and deposition temperatures. We find that, regardless of the composition, highly stable glasses can be prepared. A strong correlation between thermal stability and density is observed, as expected from the surface equilibration mechanism. In addition, the transformation of the stable glass into the liquid occurs via a growth front that initiated at the free surface.   

Investigating the Kinetic stability of Highly Confined Molecular Glasses Using in-situ Solvent Vapor Annealing

Nanocomposite films containing a substantial load of nanoparticles (NPs) hold great promise for various applications, such as structural coatings. Recently, capillary rise infiltration (CaRI) has emerged as a method to generate highly loaded nanocomposites. This process involves the penetration of interstitial gaps in densely packed NP films by polymers or molecular glasses, driven by capillarity. As NP diameter is decreased, increasing confinement leads to a significant rise in the glass transition temperature (Tg) and thermal resilience of the material. Here, we investigate the influence of nanoconfinement on the mechanisms and rate of solvent diffusion and uptake in Molecular Nanocomposites (MN) through in-situ solvent vapor annealing coupled with spectroscopic ellipsometry. N, N’-bis(3-methylphenyl)-N, N’-diphenylbenzidine (TPD) are infiltrated in silica NPs of diverse diameters as a model system, with toluene serving as a favorable solvent. Our findings reveal that reducing the pore size reduces the rate of solvent diffusion. Additionally, the diffusion process transitions from Case II, where solvent uptake occurs at a constant speed in the glassy material, to Fickian, even at elevated solvent vapor pressures, these results offer insight into the dynamical changes of glass in confinement and its role in decelerating the motion of solutes in the glassy matrix.   

Influencing Crystal Nucleation by Glass Preparation Method

Amorphous solids exist in a metastable state and tend to change over time, with driving forces towards more thermodynamically stable states, such as the crystalline form. Efforts exist to find tools to kinetically prevent crystallization of glassy materials, for applications including pharmaceuticals. Pharmaceutical glasses may be utilized in an amorphous form due to improvements in solubility and bioavailability, but storage of such forms may lead to crystallization. In exploring crystallization of a typical amorphous pharmaceutical, we find that, even as crystal growth rates remain constant, a thickness-dependence of crystal nucleation is observed in surprisingly thick films (~200 nm). Exploration of this apparent "confinement" effect leads to intriguing influences of preparation on both spin-coated and melt-quenched molecular glasses. This presentation seeks to explain the relationships between preparation conditions and crystal nucleation rates.   

Self-healing in Glasses with a Little Push

If glasses are liquids, they can flow. If they can flow, mechanical fractures can repair spontaneously over time, formally enabling self-healing in glassy materials. Besides increasing the temperature, mechanical deformations are another way to circumvent the small hiccup of age-of-the-universe-long relaxation times. Previous work has shown that shear deformations can accelerate the molecular mobility of glass-forming liquids, and further anneal glasses down their energy landscape. 

Based on these ideas and using molecular dynamics simulations, we show that segmental mobility in deeply quenched glasses is accelerated by oscillatory shear deformations, a result that strongly depends on different regimes of deformation amplitudes related to the local or global breaking of molecular cages. We use this principle to induce healing of mechanical fractures in a regime of oscillatory deformations which maintains the bulk system below the glass transition and enhances the mobility around the crack, where surface effects speed up the segmental dynamics.   

Tuning Thermal Properties of Copolymers via Monomer Sequence and Interactions

Controlling the glass transition temperature (T_g) is a primary objective in developing functional polymeric materials, and copolymerization is a common strategy. The Fox equation is often employed to predict T_g, but it does not consider the influence of monomer sequence or interactions between dissimilar monomers. To understand experimental deviations from the Fox equation, we prepared and analyzed polymers with identical compositions but different sequences. Initially, we synthesized poly(methyl methacrylate-co-4-tert-butylstyrene) (PMMA-PtBS) with varied compositions through free radical polymerization at low conversion. The T_g of each copolymer, as measured by DSC, was consistently lower than the value predicted by the Fox equation, with a maximum deviation of 10 °C at 29 wt% MMA. We also prepared PMMA-PtBS using atom transfer radical polymerization, yielding a down-chain gradient in composition, which exhibited an even more pronounced negative deviation from the Fox equation. We have also prepared and analyzed PMMA-b-PtBS and PMMA-b-(PMMA-PtBS) block copolymers. For the study of monomer interaction effects, poly(methyl methacrylate-co-9-vinylcarbazole) (PMMA-P9VC) was also investigated. When the content of 9VC was relatively low, a positive deviation from the Fox equation was observed, but at high 9VC content, a negative deviation was observed.