Transformation of Highly Stable Two-Component Glasses with Large T_g Contrast

Physical vapor deposition (PVD) is a method of glass formation in which molecules utilize enhanced mobility at the free surface to reach highly equilibrated amorphous states. Codeposited glasses, made by simultaneously depositing more than one type of molecule onto the same substrate, are of technological and fundamental interest. Here, we use PVD to codeposit glasses of methyl-m-toluate (T_g = 170.0 K) and methyl acetate (T_g = 113.5 K), two molecules with extremely high contrast regarding their glass transition temperatures, T_g. For all compositions, we observe a delayed return to the equilibrium liquid when codeposited glasses are heated above the T_g of the mixture, as quantified by the onset temperature for the glass transition. When compared using normalized onset temperatures, the codeposited glasses have high kinetic stabilities that are only slightly lower than those of PVD glasses of the pure components. These results are readily interpreted if we assume that the surface mobility of the two components is similar during codeposition, despite the large ratio of T_g values for the pure components. Additionally, we deposit bilayer samples and measure the rate at which the lower T_g component dissolves glasses of the high T_g component for both highly stable and liquid-cooled glasses. Under these conditions, glass stability has little impact on the rate of dissolution.