Nematic-like stable glasses without equilibrium liquid crystal phases

Abstract

We report the thermal and structural properties of glasses of posaconazole, a rod-like molecule, prepared using physical vapor deposition (PVD). PVD glasses of posaconazole can show substantial molecular orientation depending upon the choice of substrate temperature, T, during deposition. Ellipsometry and IR measurements indicate that glasses prepared at T very near the glass transition temperature (T) are highly ordered. For these posaconazole glasses, the orientation order parameter is similar to that observed in macroscopically aligned nematic liquid crystals, indicating that the molecules are mostly parallel to one another and perpendicular to the interface. To our knowledge, these are the most anisotropic glasses ever prepared by PVD from a molecule that does not form equilibrium liquid crystal phases. These results are consistent with a previously proposed mechanism in which molecular orientation in PVD glasses is inherited from the orientation present at the free surface of the equilibrium liquid. This mechanism suggests that molecular orientation at the surface of the equilibrium liquid of posaconazole is nematic-like. Posaconazole glasses can show very high kinetic stability; the isothermal transformation of a 400 nm glass into the supercooled liquid occurs via a propagating front that originates at the free surface and requires ∼10 times the structural relaxation time of the liquid (τ). We also studied the kinetic stability of PVD glasses of itraconazole, which is a structurally similar molecule with equilibrium liquid crystal phases. While itraconazole glasses can be even more anisotropic than posaconazole glasses, they exhibit lower kinetic stability.

Vapor deposition of a nonmesogen prepares highly structured organic glasses

“…1E. The high degree of orientational order as revealed by X-ray scattering is consistent with previous birefringence and infrared absorption data on vapor-deposited posaconazole (21). The correlation length from this broad peak, corrected for instrumental broadening, is 2.9 nm, which is equal to approximately 6 molecules arranged laterally in-plane; this small correlation length is consistent with the view that the vapor-deposited film is not a 3D crystal.…”

“…We infer that this highly vertical orientation of the long axis at the free surface is therefore responsible for the high degree of orientational order seen in the vapor-deposited glass. It was previously hypothesized that posaconazole may have a highly oriented surface structure similar to that observed in nematic liquid crystals (21,40); Fig. 2 confirms that hypothesis.…”

“…1A. Above its glass-transition temperature, T g , posaconazole is an isotropic liquid (21). When a glass is formed by cooling the equilibrium liquid to room temperature from 334 K (T g + 4 K), it inherits the structure of the isotropic equilibrium liquid, as shown in the GIWAXS pattern in Fig.…”

“…If the liquid crystal is aligned along a common direction before cooling by using alignment layers or an electromagnetic field (20), the solid that is formed will have strongly anisotropic local and macroscopic structure. Vapor deposition of molecules with liquid-crystalline phases has recently been identified as another method for producing highly anisotropic, macroscopically aligned glasses with both orientational (13,21) and translational (22,23) order. Glasses prepared with liquid-crystalline order have enhanced charge mobility (24,25) compared to their isotropic counterparts.…”

Extreme Elasticity Anisotropy in Molecular Glasses

“…For glasses of itraconazole, we find a strong correlation between molecular orientation and elastic anisotropy implying that deformation along the long molecular axis of itraconazole is subject to a higher modulus. It was previously shown that vapor‐deposited itraconazole glasses have kinetic stabilities equal to or exceeding a liquid‐cooled glass , [ 48 ] indicating that glasses can simultaneously be mechanically anisotropic and kinetically stable.…”