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

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.