Influence of Hydrogen Bonding on the Kinetic Stability of Vapor-Deposited Glasses of Triazine Derivatives

Abstract

It has recently been established that physical vapor deposition (PVD) can produce organic glasses with enhanced kinetic stability, high density, and anisotropic packing, with the substrate temperature during deposition (T) as the key control parameter. The influence of hydrogen bonding on the formation of PVD glasses has not been fully explored. Herein, we use a high-throughput preparation method to vapor-deposit three triazine derivatives over a wide range of T, from 0.69 to 1.08T, where T is the glass transition temperature. These model systems are structural analogues containing a functional group with different H-bonding capability at the 2-position of a triazine ring: (1) 2-methylamino-4,6-bis(3,5-dimethyl-phenylamino)-1,3,5-triazine (NHMe) (H-bond donor), (2) 2-methoxy-4,6-bis(3,5-dimethyl-phenylamino)-1,3,5-triazine (OMe) (H-bond acceptor), and (3) 2-ethyl-4,6-bis(3,5-dimethyl-phenylamino)-1,3,5-triazine (Et) (none). Using spectroscopic ellipsometry, we find that the Et and OMe compounds form PVD glasses with relatively high kinetic stability, with the transformation time (scaled by the α-relaxation time) on the order of 10, comparable to other highly stable glasses formed by PVD. In contrast, PVD glasses of NHMe are only slightly more stable than the corresponding liquid-cooled glass. Using IR spectroscopy, we find that both the supercooled liquid and the PVD glasses of the NHMe derivative show a higher average number of bonded NH per molecule than that in the other two compounds. These results suggest that H-bonds hinder the formation of stable glasses, perhaps by limiting the surface mobility. Interestingly, despite this difference in kinetic stability, all three compounds show properties typically observed in highly stable glasses prepared by PVD, including a higher density and anisotropic molecular packing (as characterized by IR and wide-angle X-ray scattering).

Polyamorphism of vapor-deposited amorphous selenium in response to light

“…However, after the oligomers have combined to make larger chains (equilibrium chain length ∼200 atoms per chain) within 10 −6 to 10 −4 s (33), the barrier for relaxation is increased. The interaction between the oligomers can potentially slow the surface mobility and reduce the depth of the mobile region, limiting the packing efficiency during SME, analogous to what has been observed in molecular SGs with strong intermolecular interactions (5,16,35).…”

“…P hysical vapor deposition (PVD) is broadly used to prepare amorphous thin films. Over the past decade, a variety of systems have been shown to produce stable glasses (SGs) during PVD, when the deposition temperature (T dep ) is held below their glass transition temperature (Tg ) (1)(2)(3)(4)(5)(6)(7)(8). SGs have improved thermal (1,2,6,9) and kinetic stability (3,4,7,8,10) compared to liquid quenched (LQ) glasses, resembling glasses aged for hundreds or millions of years.…”

“…It is understood that SG formation is due to enhanced surface mobility (5,8,12,(14)(15)(16)(17). When T dep < Tg , at slow deposition rates (∼0.2 nm/s for most organic glasses), the surface region has sufficient mobility (18) to reach more energetically favored states before being buried into dynamically arrested states.…”

“…However, surface mobility in these systems is less pronounced (8,14,24); thus, lower deposition rates are required to achieve a similar extent of stability as organic SGs (8). In all SG systems, besides T dep and deposition rate, few factors are available to control the degree of enhanced surface mobility and its penetration depth (5). a-Se is an exception, where above-bandgap light can strongly affect its surface structure and dynamics.…”

Distinguishing different classes of secondary relaxations from vapour deposited ultrastable glasses

“…19,20 Choosing the appropriate deposition conditions, it is possible to obtain over short times the so-called ultrastable glass (USG), with a density around 1.5% higher than that of ordinary glasses (OG) prepared by cooling from the liquid. [21][22][23] This remarkable increase in density and equivalent age of the glass implies a significant alteration of the properties of the secondary relaxation, 8,18 accompanying the dramatic slowing down of the structural a-relaxation, 16,24,25 and the immense shift of the a-relaxation to lower frequencies. Here, we have chosen six different materials and analysed some of their secondary relaxations in the USG state produced by vapour deposition, as well as in the OG state of the same samples obtained by cooling below the nominal T g .…”