Kinetic stability and heat capacity of vapor-deposited glasses of o-terphenyl.

The reversing heat capacity of vapor-deposited o-terphenyl glasses was determined by in situ alternating current nanocalorimetry. Glasses were deposited at substrate temperatures ranging from 0.39 Tg to Tg, where Tg is the glass transition temperature. Glasses deposited near 0.85 Tg exhibited very high kinetic stability; a 460 nm film required ∼10(4.8) times the structural relaxation time of the equilibrium supercooled liquid to transform into the liquid state. For the most stable o-terphenyl glasses, the heat capacity was lower than that of the ordinary liquid-cooled glass by (1 ± 0.4)%; this decrease represents half of the difference in heat capacity between the ordinary glass and crystal. Vapor-deposited o-terphenyl glasses exhibit greater kinetic stability than vapor-deposited glasses of indomethacin, in qualitative agreement with recent surface diffusion measurements indicating faster surface diffusion on o-terphenyl glasses. The stable glass to supercooled liquid transformation was thickness-dependent, consistent with transformation via a propagating front initiated at the free surface.

Highly stable glasses of cis-decalin and cis/trans-decalin mixtures.

In situ AC nanocalorimetry was used to measure the reversing heat capacity of vapor-deposited glasses of decahydronaphthalene (decalin). Glasses with low heat capacity and high kinetic stability, as compared to the corresponding liquid-cooled glass, were prepared from cis-decalin and from several cis/trans-decalin mixtures. This is the first report of highly stable glass formation for molecular mixtures. The 50/50 cis/trans-decalin mixture is the highest fragility material reported to produce an ultrastable glass. The 50/50 mixture exhibited high kinetic stability, with a ∼500 nm film deposited at 116 K (0.86 Tg) displaying a transformation time equivalent to 10(4.4) times the structural relaxation time of the supercooled liquid at the annealing temperature. cis-Decalin and the decalin mixture formed stable glasses that had heat capacities as much as 4.5% lower than the liquid-cooled glass.

Vapor-deposited α,α,ß-tris-naphthylbenzene glasses with low heat capacity and high kinetic stability.

The reversing heat capacity of vapor-deposited glasses of α,α,ß-tris-naphthylbenzene (ααß-TNB) was measured using alternating current (AC) nanocalorimetry. Glasses deposited at 0.85 T(g), where T(g) is the glass transition temperature, have a 4 ± 1% lower heat capacity than the ordinary glass prepared by cooling from the liquid. This is a result of efficient packing and is consistent with the higher density of the vapor-deposited glass. Isothermal experiments show that vapor-deposited ααß-TNB glasses also have enhanced kinetic stability with respect to transformation into the supercooled liquid, as expected from previous work, with transformation times approaching 10(5) times the structural relaxation time of the liquid. Films thinner than 1 µm exhibit a thickness dependence to their transformation times that is consistent with transformation to the supercooled liquid via a surface-initiated growth front.

Enabling the synthesis of perfluoroalkyl bicyclobutanes via 1,3 γ-silyl elimination.

Two new bicyclobutanes were prepared from cyclobutyl systems by a novel, solvolytic, carbocation-based methodology. An electron-withdrawing perfluoroalkyl group at the incipient cationic center enhances neighboring-group participation of the γ-silyl group, inducing facile, remarkably selective 1,3-elimination yielding only bicyclobutanes. The method unlocks potential access to a host of EWG-substituted strained rings and a potential new method for the synthesis of trifluoromethylcyclopropanes.

Observation of low heat capacities for vapor-deposited glasses of indomethacin as determined by AC nanocalorimetry.

Highly stable glass films of indomethacin (IMC) with thicknesses ranging from 75 to 2900 nm were prepared by physical vapor deposition. Alternating current (AC) nanocalorimetry was used to evaluate the heat capacity and kinetic stability of the glasses as a function of thickness. Glasses deposited at a substrate temperature of 0.84T(g) displayed heat capacities that were approximately 19 J/(mol K) (4.5%) lower than glasses deposited at T(g) (315 K) or the ordinary glass prepared by cooling the liquid. This difference in heat capacity was observed over the entire thickness range and is significantly larger than the approximately 2 J/(mol K) (0.3%) difference previously observed between aged and ordinary glasses. The vapor-deposited glasses were isothermally transformed into the supercooled liquid above T(g). Glasses with low heat capacities exhibited high kinetic stability. The transformation time increased by an order of magnitude as the film thickness increased from 75 to 600 nm and was independent of film thickness for the thickest films. We interpret these results to indicate that the transformation of stable glass into supercooled liquid can occur by either a surface-initiated or bulk mechanism. In these experiments, the structural relaxation time of the IMC supercooled liquid was observed to be nearly independent of sample thickness.