Role of Molecular Layering in the Enhanced Mechanical Propertiesof Stable Glasses

The density, degree of molecular orientation, andmolecular layering of vapor-deposited stable glasses (SGs) vary withsubstrate temperature (Tdep) below the glass-transition temperature(Tg). Density and orientation have been suggested to be factorsinfluencing the mechanical properties of SGs. We perform nano-indentation on two molecules which differ by only a single substituent,allowing one molecule to adopt an in-plane orientation at lowTdep.The reduced elastic modulus and hardness of both molecules showsimilarTdepdependences, with enhancements of 15-20% in reducedmodulus and 30-45% in hardness atTdep approximate to 0.8Tg, where the densityof vapor-depositedfilms is enhanced by similar to 1.4% compared to that ofthe liquid-quenched glass. AtTdep< 0.8Tg, one of the moleculesproduces 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 inboth systems despite their variable stability. We suggest that nanoindentation performed normal to thefilm's surface is influenced bythe tighter packing of the molecules in this direction