Role of Molecular Layering in the Enhanced Mechanical Properties of Stable Glasses

Abstract

The density, degree of molecular orientation, and molecular layering of vapor-deposited stable glasses (SGs) vary with substrate temperature (T dep ) below the glass-transition temperature (T g ). Density and orientation have been suggested to be factors influencing the mechanical properties of SGs. We perform nanoindentation on two molecules which differ by only a single substituent, allowing one molecule to adopt an in-plane orientation at low T dep . The reduced elastic modulus and hardness of both molecules show similar T dep dependences, with enhancements of 15−20% in reduced modulus and 30−45% in hardness at T dep ≈ 0.8T g , where the density of vapor-deposited films is enhanced by ∼1.4% compared to that of the liquid-quenched glass. At T dep < 0.8T g , one of the molecules produces highly unstable glasses with in-plane orientation. However, both systems show enhanced mechanics. Both the modulus and hardness correlate with the degree of layering, which is similar in both systems despite their variable stability. We suggest that nanoindentation performed normal to the film's surface is influenced by the tighter packing of the molecules in this direction.

High Density Two-Component Glasses of Organic Semiconductors Prepared by Physical Vapor Deposition

“…Physical vapor deposition (PVD) can prepare glassy films with a range of characteristics depending upon deposition conditions. ,, In particular, PVD can produce ultrastable and also high density glasses. ,,− Previous studies on vapor-deposited glasses have shown high kinetic stability, ,,,− low enthalpy, ,,, high mechanical moduli, − high chemical stability and high photostability . These properties can be explained by the surface equilibration mechanism, which is based on high molecular mobility at the free surface relative to the bulk. ,,, Consistent with this mechanism, the density of a vapor-deposited low molecular weight system, ethylbenzene, was shown to approach the density of the ideal glass .…”

Unusual Transformation of Mixed Isomer Decahydroisoquinoline Stable Glasses

“…Physical vapor deposition (PVD) is a well-established technique to prepare glassy thin films of a wide array of materials including small organic molecules, − chalcogenides, , metallic glasses, , and polymers. − Glasses produced via PVD can have remarkable properties that cannot be achieved using other preparation techniques, including reduced enthalpy, ,− increased density, ,− multiple amorphous structures, suppressed secondary relaxations, − and anisotropic molecular orientation. , In addition, PVD glasses can exhibit high kinetic stability, which is the tendency to resist devitrification upon heating a glass above its glass transition temperature, T g . Significant progress has been made to understand how deposition conditions and molecular shape , control the structure and properties of the resulting glass.…”

Depletion of two-level systems in highly stable glasses with different molecular ordering

“…A signi cant linear coe cient on the speci c heat at the lowest temperatures, ascribed to the "universal" presence of TLS in glasses, was found to persist invariant in geologically hyperaged glasses of amber (a typical polymer glass) [30−32], when comparing the pristine samples (up to 110 My aged) to the rejuvenated samples. One possible difference to take into account between standard glasses and PVD glasses is that the former are essentially homogeneous and isotropic, whereas the latter typically exhibit anisotropic packing and layered structures [33][34][35][36] that can play a role in different properties [37][38][39][40][41][42]. In fact, the anisotropy of IMC ultrastable glasses was argued in Ref.…”