Yeocheol Yoon

Existence of highly ordered smectic structures in a series of main-chain liquid-crystalline polyethers

Abstract A series of polyethers has been synthesized from 1-(4-hydroxy-4′-biphenyl)-2-(4-hydroxyphenyl) propane and α,ω-dibromoalkanes having both odd and even numbers of methylene units [TPP( n = odd)s and TPP( n = even)s]. These polymers exhibit multiple thermodynamic first-order transitions during cooling and heating which demonstrate little supercooling dependence at different rates. The transition thermodynamic properties (enthalpy and entropy changes at the transitions) are studied by means of differential scanning calorimetry experiments. The contributions of the mesogenic groups and methylene units to each ordering process can be separated and indicate the characteristics of these processes, thereby providing estimations of the transition types. To construct the phase diagrams for both TPP( n = odd)s and TPP( n = even)s, identification of each phase structure can be obtained from wide-angle X-ray diffraction powder and fiber patterns at different temperatures as well as from electron diffraction experiments. In addition to the nematic liquid-crystalline phase, highly ordered smectic F, smectic crystal G and H phases have been identified. These phase assignments are also supported by morphological observations in the liquid crystals under polarized light and transmission electron microscopy. The concepts of the liquid-crystalline phases defined for small-molecule liquid crystals are applicable to main-chain liquid-crystalline polymers.

Liquid-crystalline polyethers based on conformational isomerism. Part 33.–Thermotropic polyethers based on a mesogenic group containing rigid and flexible units: 1-(4′-hydroxybiphenyl-4-yl)-2-(4-hydroxyphenyl)propane

The synthesis of a mesogenic group containing rigid and flexible units: 1-(4′-hydroxybiphenyl-4-yl)-2-(4-hydroxy-phenyl)propane (TPP) is presented. TPP was polyetherified under phase-transfer catalysed conditions with α,ω-dibromoalkanes containing 4–20 methylenic units. The resulting polyethers, TPP-x(where x= number of methylenic groups in the spacer), were characterized by a combination of techniques consisting of differential scanning calorimetry, thermal optical polarized microscopy and small- and wide-angle X-ray scattering experiments. All TPP-x polyethers displayed multiple crystalline phases whose nature was determined by the spacer length. TPP-x with x of less than 9 exhibited crystalline phases in which the mesogenic and spacer were intermeshed. Polymers with longer spacers displayed crystalline phases in which the mesogen and spacer were separated in separate layers of different electron densities. TPP-x with x= 5, 7, 9 and 11 also exhibited a nematic mesophase.

Thermodynamic transition properties of highly ordered smectic phases

A series of polyethers have been synthesized from 1-(4-hydroxy-4′-biphenyl)-2-(4-hydroxyphenyl)propane and α, Ω-dibromoalkanes having different numbers of methylene units [TPPs]. Both odd- and even-numbered TPPs [TPP(n=odd)s and TPP(n=even)s) exhibit multiple transitions during cooling and heating and they show little supercooling dependence, indicating close-to-equilibrium nature of these transitions. Combining the structural characterization obtainedvia wide angle X-ray diffraction powder and fiber patterns at different temperatures and the morphological observations from microscopy techniques, not only the nematic liquid crystalline phase but also highly ordered smecticF, smectic crystalG andH phases have been identified. The phase diagrams for both TPP(n=odd)s and TPP(n=even)s have been constructed [1–3]. Thermodynamic properties (enthalpy and entropy changes) during these transitions are studied based on differential scanning calorimetry experiments. The contributions of the mesogenic groups and methylene units to each ordering process can be separated and they indicate the characteristics of these processes thereby providing estimations of the transition types.