John C. Mastrangelo

NOVEL GLASS-FORMING LIQUID CRYSTALS. IV. EFFECTS OF CENTRAL CORE AND PENDANT GROUP ON VITRIFICATION AND MORPHOLOGICAL STABILITY

Abstract To unravel the effects of the volume-excluding central core and the mesogenic pendant group on both the glass-forming ability and morphological stability of the thermally quenched glass, nine model compounds were synthesized that contain various nematogenic and cholesteryl pendant groups. The glass-forming ability of the melt and morphological stability of the thermally quenched glass were assessed using the DSC, XRD, and hot-stage POM techniques. With cyanobiphenyl as the pendant group, the following descending order in morphological stability against thermally activated recrystallization was established: trans-cyclohexane < all-exo-bicyclo [2.2.2] oct-7-ene < cubane < cis-cyclohexane < benzene. While the cyclohexane compound containing three cyanoterphenyl groups showed a strong tendency to crystallize upon quenching, the chiral nematic system in which one of the cyanoterphenyl groups is substituted by a cholesteryl group showed superior glass-forming ability and morphological stability. Additi...

Thermotropic chiral nematic side-chain polymers and cyclic oligomers

Abstract For all the theoretical, experimental and computational research on chiral nematic systems over the last four decades, understanding of cholesteric mesomorphism from the molecular perspective is still rather limited at the present time. Nevertheless, the unique property of selective wavelength reflection accompanied by circular polarization has presented tremendous potential for various optical applications. Theories governing optical birefringence and selective reflection bandwidth are outlined to serve as a foundation for molecular design. Both polymeric and low molar mass glass-forming chiral nematics are surveyed in terms of chemical structure, thermotropic and optical properties. Characterized by relative ease of processing, the low molar mass approach is illustrated with our recent results in terms of design concept, materials synthesis and characterization. We also highlight outstanding issues that need to be addressed to help advance chiral nematics as a class of advanced optical materials.

Novel glass-forming liquid crystals V. Nematic and chiral-nematic systems with an elevated glass transition temperature

The formerly implemented molecular design concept behind glass-forming liquid crystals (gLCs) was generalized by increasing the volume of the non-mesogenic central core, with an attendant increase in the number of nematic pendants, using 5-hydroxyisophthalic acid as the bridging unit. New nematic gLCs were synthesized and characterized, showing an elevation in Tg by 30 to 40°C with no definite trend in Tc over the benzene, cis, cis-cyclohexane, and exo, endo-bicyclo[2.2.2]oct-7-ene base cores. The exo, exo-configured gLC showed a higher Tg and a higher T c than the exo, endo-counterpart. Morphological characterization with X-ray diffractometry revealed the non-crystalline nature of pristine samples and the morphological stability of thermally processed gLC films against recrystallization for six months. Nematic gLC films were prepared for characterization by FTIR linear dichroism, resulting in an orientational order parameter in the range 0.52 to 0.63. A chiral-nematic gLC derived from exo, exo-bicyclo[2.2.2.]oct-7-ene also showed an elevation in Tg by 10 to 20°C over the cyclohexane-based systems reported previously. With (S)-(-)-1-phenylethylamine as the chiral moiety, the left-handed, chiral-nematic gLC film yielded a selective reflection band centred around 375 nm. Tunability of selective reflection from the UV to visible region was demonstrated by mixing the chiral-nematic and nematic gLCs at varying ratios.

Crystallization upon thermal annealing of a glass‐forming liquid crystal in the nematic regime

As an example of a novel class of glass‐forming liquid crystals, compound (I) was synthesized and characterized to possess a nematic mesophase between Tg and Tc as the pristine crystal was heated beyond its Tm followed by quenching to below room temperature. Differential scanning calorimetry (DSC) and x‐ray diffraction techniques were employed to investigate its morphological stability. It was found that the nematic mesophase persists upon annealing for a period of up to 22 h without the appearance of new phases. However, after annealing in the nematic regime over a longer period of time, thermally activated phase transformations were observed, resulting in a new crystalline phase plus the pristine crystalline phase based on DSC thermal transition data and x‐ray diffraction patterns.

New materials technology for latching electro-optic devices

This paper presents the current status of a new class of liquid crystal material being developed for latching electrooptic applications. This new material has the unique property of being electrooptic and fully latching. That is, in one state, the material has the properties of a conventional liquid crystal, capable of being aligned with either an electric or magnetic field; in its other state, it is an optical quality solid that maintains the molecular alignment set while in the fluid state. Experiments have shown that current materials can be switched on the order of milliseconds, as is the case with conventional nematic liquid crystals. In the solid state, the electric field can be removed with no change to the previously set optical properties because the molecular alignment is frozen in place, which should last for an extended period of time. In addition, the material exhibits broad temperature stability in the solid state, enabling devices to be developed that operate from cryogenic temperatures to 80 degrees C without the use of a temperature controller. This new material is ideally suited for applications where the size and mechanical robustness of an electrooptic device is desired, along with the latching capability of optomechanical devices. This materials technology alone will currently not meet high-speed switch requirements. However, this technology can be integrated with other state-of-the-art high-speed materials to provide a high-speed latching device. Devices currently under investigation using this materials include optical switches, optical attenuators and tunable filters.

Vitrification and Morphological Stability of Liquid Crystals

Based on the extensive database collected for model glass-forming liquid crystals with various mesogenic groups attached to benzene, cubane, cis- and trans-cyclohexane, and all-exo bicyclo[ 2.2.2]oct-7-ene, it was found that the glass-forming ability and morphological stability are determined by a delicate structural balance between the volume-excluding core and the pendant group, and the stereochemical features of the hybrid system.

Novel Vitrified Liquid Crystals and Potential Applications

An overview is presented for a generic approach to low molar mass liquid crystalline materials capable of vitrification into morphologically stable, optically anisotropic thin films. Potential applications as polarization control devices, optical notch filters, and luminescent elements will be explored. Preliminary data for defect formation and annihilation will also be presented to illustrate an advantage of low molar mass materials over polymer analogs.