Search for: All records

X-ray scattering has been used to characterize the columnar packing and the π stacking in a glass-forming discotic liquid crystal. In the equilibrium liquid state, the intensities of the scattering peaks for π stacking and columnar packing are proportional to each other, indicating concurrent development of the two orders. Upon cooling into the glassy state, the π–π distance shows a kinetic arrest with a change in the thermal expansion coefficient (TEC) from 321 to 109 ppm/K, while the intercolumnar spacing exhibits a constant TEC of 113 ppm/K. By changing the cooling rate, it is possible to prepare glasses with a wide range of columnar and π stacking orders, including zero order. For each glass, the columnar order and the π stacking order correspond to a much hotter liquid than its enthalpy and π–π distance, with the difference between the two internal (fictive) temperatures exceeding 100 K. By comparison with the relaxation map obtained by dielectric spectroscopy, we find that the δ mode (disk tumbling within a column) controls the columnar order and the π stacking order trapped in the glass, while the α mode (disk spinning about its axis) controls the enthalpy and the π–π spacing. Our finding is relevant for controlling the different structural features of a molecular glass to optimize its properties. 

Liquid crystals (LCs) undergo fast phase transitions, almost without hysteresis, leading to the notion that it is difficult to bypass LC transitions. However, recent work on itraconazole has shown that a nematic-to-smectic phase transition can be frustrated or avoided at moderate cooling rates. At each cooling rate, the highest smectic order obtained is determined by the kinetic arrest of the end-over-end molecular rotation. We report that the same phenomenon occurs in the system saperconazole, an analog of itraconazole where each of the two Cl atoms is replaced by F. Saperconazole has a wider temperature range over which smectic order can develop before kinetic arrest, providing a stronger test of the previous conclusion. Together these results indicate a general principle for controlling LC order in organic glasses for electronic applications. 

X-ray scattering has been used to characterize glassy itraconazole (ITZ) prepared by cooling at different rates. Faster cooling produces ITZ glasses with lower (or zero) smectic order with more sinusoidal density modulation, larger molecular spacing, and shorter lateral correlation between the rod-like molecules. We find that each glass is characterized by not one, but two fictive temperatures T_f(the temperature at which a chosen order parameter is frozen in the equilibrium liquid). The higher T_fis associated with the regularity of smectic layers and lateral packing, while the lower T_fwith the molecular spacings between and within smectic layers. This indicates that different structural features are frozen on different timescales. The two timescales for ITZ correspond to its two relaxation modes observed by dielectric spectroscopy: the slower δ mode (end-over-end rotation) is associated with the freezing of the regularity of molecular packing and the faster α mode (rotation about the long axis) with the freezing of the spacing between molecules. Our finding suggests a way to selectively control the structural features of glasses.