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

Abstract

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.

Invariant Fast Diffusion on the Surfaces of Ultrastable and Aged Molecular Glasses

“…1 and Fig. S4[24] show that PVD glasses have lower fictive temperatures and higher stabilities compared to aged or liquid-quenched glasses, with the most stable glass deposited around 0.85 T g , consistent with previous studies of PVD glasses [16][17][18][19][26][27][28].…”

“…The fact that D s remains the same on glasses of various stabilities ensures that SME can proceed efficiently on the top surface layer regardless of the bulk dynamics. If the motion of molecules at the free surface slowed down significantly with the relaxation dynamics of previously formed layers, SME process would have been self-limiting and depends on film thickness, which is not the case [16,19,21]. However, the measurements presented here do not provide a full picture.…”

“…Organic glasses prepared by physical vapor deposition (PVD) at substrate temperatures below bulk glass transition temperature, T g , are found to have exceptional thermodynamic and kinetic stabilities [16][17][18][19]. Glasses are out of equilibrium materials, therefore one can produce glasses with varying structures and properties by varying their states in the corresponding energy landscape.…”

“…However, these changes in properties are typically small and take exponentially long times at lower temperatures. In contrast, within deposition time of minutes to hours, PVD can access states in the energy landscape that may take a liquid-quenched glass millions of years of aging to access [16,[19][20][21]. It has been proposed that due to the fast diffusion during PVD, molecules at the top surface have enough time to achieve a better configuration and thus a lower energy state before being buried by the next layer.…”

Long-range correlated dynamics in ultra-thin molecular glass films

“…However, indirect evidence suggests that the properties of these films may be strongly thickness dependent [18]. Thick PVD films have been shown to form exceptionally stable glasses upon deposition at temperatures just below T g [19][20][21][22]. While the detailed mechanisms of the formation of stable PVD glasses are still under investigation, most studies [19,20,23,24] indicate that surfacemediated equilibration (SME) is critical to their production.…”