The effect of chemical structure on the stability of physical vapor deposited glasses of 1,3,5-triarylbenzene.

We detail the formation and properties associated with stable glasses (SG) formed by a series of structural analogues of 1,3-bis(1-naphthyl)-5-(2-naphthyl)benzene (α,α,β-TNB), a well-studied SG former. Five compounds with similar structural properties were synthesized and physical vapor-deposited with a constant deposition rate at various substrate temperatures (Tdep) in the range between 0.73 Tg and 0.96 Tg. These molecules include α,α,β-TNB, 3,5-di(naphthalen-1-yl)-1-phenylbenzene (α,α-P), 9-(3,5-di(naphthalen-1-yl)phenyl)anthracene (α,α-A), 9,9'-(5-(naphthalen-2-yl)-1,3-phenylene)dianthracene (β-AA), and 3,3',5,5'-tetra(naphthalen-1-yl)-1,1'-biphenyl (α,α,α,α-TNBP). Ellipsometry was used to study the transformations from the as-deposited glasses into ordinary glasses (OG). The stability of each film was evaluated by measuring the fictive temperature (Tf) and density difference between the as-deposited glass and OG. It is demonstrated that all five molecules can form SGs upon vapor deposition in this temperature range. In-depth studies on the dependence of the stability of as-deposited glasses upon Tdep were performed with three molecules, α,α,β-TNB, α,α-P, and α,α-A. The general trends of stability were comparable at the same Tdep/Tg for these three compounds. Similar to previous studies on α,α,β-TNB, vapor-deposited glasses of α,α-P and α,α-A formed the most stable structures around Tdep = 0.8-0.85 Tg. The most stable glass of each molecule showed the lowest thermal expansion coefficient compared to OG and a positive optical birefringence. However, the SGs of α,α-A were less stable compared to α,α-P and α,α,β-TNB at the relative Tdep/Tg. Based on Arrhenius extrapolation of the aging time, as a measure of stability, the most stable α,α-A glass was only aged for a few years as opposed to hundreds or thousands of years for other glasses. We hypothesize that the reduced stability is due to slower mobility at the free surface of α,α-A glass compared to the other two molecules.