Thermotropic liquid-crystalline polymers having five-membered heterocycles as mesogens, 3. New semi-rigid thermotropic liquid-crystalline polyesters based on a twin biphenyl analogue of 1,3,4-thiadiazole

Yoshinaga

Koide

2000

Polym J

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ABSTRACT:Semi-rigid polyesters based on twin biphenyl analogues of 1,3,4-thiadiazole (2-phenyl-1,3,4-thiadiazoles) with hepta-and octamethylene chains in the central part were synthesized by melt polycondensation of bismethyl ester derivatives of twin 2-phenyl-1,3,4-thiadiazole with a ,w-alkylenediols and dioxydialcohol derivatives of bi phenyl. The relationship between polymer structure and thermotropic liquid-crystallinity (LC) was investigated by differential scanning calorimetry (DSC), polarizing microscope observations, miscibility tests and temperature-dependent X-ray analysis. All the polymers derived from a ,w-alkylenediols appeared to form enantiotropic or monotropic nematic phase independent of the lengths of aliphatic chains. Among the polyesters containing the bi phenyl unit, most polymers with octamethylene segment between the twin 2-phenyl-1,3,4-thiadiazoles showed the nematic phase, but polymers composed of the twin bi phenyl analogue having the central heptamethylene chain had no LC melts.KEY WORDS Semi-Rigid Polyester/ 2-Phenyl-1,3,4-thiadiazole / Melt Polycondensation / Bismethyl Ester Derivative/ Liquid-Crystallinity / Nematic Phase/ Differential Scanning Calorimetry Measurement/ A terphenyl analogue of 1,3,4-thiadiazole (2,5-diphenyl-1,3,4-thiadiazole), a five-membered heterocyclic unit containing sulfur and nitrogen atoms, is a calamitic mesogen. 1 Derivatives composed of mesogen with flexible chains on both sides, twin terphenyl analogues of the 1,3,4-thiadiazole having alkylenedioxy segments in the center 2 -5 and main-6 -8 or side-chain 9 polymers containing the mesogen form nematic and smectic mesophases in spite of its bent molecular structure. 2 · 10 -12 Our previous articles described that twin 2,5-diphenyl-1,3,4-thiadiazole derivatives linking an octamethylenedioxy chain to the center 5 have stable nematic and smectic phases. Side-chain polymers with the 2,5-diphenyl-1,3,4-thiadiazole as a mesogen represent cholesteric and chiral smectic C * phases.9 Enantiotropic thermotropic liquid-crystalline (LC) phases emerge in semi-rigid polyesters derived from diacylchloride of the 2,5-diphenyl-1,3,4-thiadiazole and aliphatic diols, 8 although wholly aromatic polyesters consisting of calamitic mesogen have no LC melts. 13Biphenyl analogues of 1,3,4-thiadiazole (2-phenyl-1,3,4-thiadiazoles) have the monotropic smectic A phase, 1 · 14 · 15 although LC temperature range was narrow. 2 In recent years, Tschierske et al. found that homoand heterotrimeric or tetrameric derivatives with the 2-phenyl-1,3,4-thiadiazole ring linked to central flexible chains show the smectic C phase. 14 · 15 Thermotropic LC phases appear in semi-rigid LC polyesters based on twin biphenyl analogue of 1,3,4-thiadiazole having decamethylene chain in the central part. 16 The polymer with octamethylene spacer displayed the monotropic nematic phase and those having decamethylene and dodecamethylene chains showed enantiotropic smectic phases, but LC temperature range was narrow.The present work describes the synthesis of vario...

Section: Thermal and Mesogenic Properties Of Polymersmentioning

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Thermotropic Liquid-Crystalline Polymers Having Five-Membered Heterocycles as Mesogens V. Relationship between Polymer Structure and Thermotropic Liquid-Crystallinity in Semi-Rigid Polyesters Composed of Twin 2-Phenyl-1,3,4-thiadiazoles Having Central Aliphatic Chains

Yoshinaga

Koide

2000

Polym J

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Thermotropic liquid‐crystalline polymers containing five‐membered heterocyclic groups. X. Relationships between structures of semirigid polyesters based on three disubstituted 2,5‐diphenyl‐1,3,4‐thiadiazoles and thermotropic liquid‐crystalline and optical properties

J. Polym. Sci. A Polym. Chem.

Nakashima

Uemoto

2003

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Thermotropic liquid-crystalline (LC) semirigid polyesters based on three terphenyl analogues of 1,3,4-thiadiazole (2,5-diphenyl-1,3,4-thiadiazole)s (DPTD) linking undecamethyleneoxy chain at different substituted positions were synthesized from three disubstituted (4,4Ј-, 3,4Ј-, and 3,3Ј-) dioxydiundecanols of DPTD and four diesters, and the relationships between polymer structures and LC and optical properties were investigated. DSC measurements, texture observations, and wide-angle X-ray analyses revealed that the polymers composed of DPTD moiety having a more linear molecular structure and 1,4-phenylene unit or short aliphatic chain tend to exhibit LC smectic C and/or A phases. The following observations were made: (1) the emergence of smectic C and/or A phases in all the polymers on the basis of 4,4Ј-disubstituted DPTD, (2) formation of enantiotropic smectic C and/or A phases in the polymers containing a 1,4-phenylene unit in the main chain, (3) formation of a more stable smectic C phase in the polymers having a short aliphatic [(CH 2 ) 4 ] chain, and (4) a decrease of the mesomorphic property of the polyesters in the order of 4,4Ј-DPTD Ͼ 3,4Ј-DPTD Ͼ 3,3Ј-DPTD. Solution and solid-state ultraviolet-visible and photoluminescent spectra indicated that all the polyesters display maximum absorbances and blue emissions arising from the DPTD moiety, whose peak maxima were shifted to lower wavelengths in the order of 4,4Ј-DPTD Ͼ 3,4Ј-DPTD Ͼ 3,3Ј-DPTD as well as the aforementioned LC property.

Synthesis and optical and electrochemical properties of thermotropic, liquid‐crystalline, semirigid copolyesters based on 2,5‐diphenyl‐1,3,4‐thiadiazole units

J. Polym. Sci. A Polym. Chem.

Tada

Nakashima

et al. 2005

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Novel thermotropic liquid-crystalline (LC) copolyesters were prepared with three disubstituted (4,4Ј-, 3,4Ј-, and 3,3Ј-) dioxydiundecanol derivatives of terphenyl analogues of 1,3,4-thiadiazole [2,5-diphenyl-1,3,4-thiadiazole (DPTD)], and their optical and electrochemical properties were examined. Their structures were characterized with Fourier transform infrared, 1 H NMR spectroscopy, and elemental analyses. The thermal and mesomorphic properties of the copolyesters were investigated with differential scanning calorimetry measurements, polarized microscopy observations, and X-ray analyses; the data suggested that these copolymers formed LC smectic or nematic phases. The mesomorphic tendency decreased in the following order: 4,4Ј-DPTD and 3,4Ј-DPTD copolyesters Ͼ 4,4Ј-DPTD and 3,3Ј-DPTD copolyesters Ͼ 3,4Ј-DPTD and 3,3Ј-DPTD copolyesters. Solution and solid-state ultraviolet-visible (UV-vis) and photoluminescence spectra indicated that the copolyesters displayed maximum absorbances and blue emissions according to the DPTD unit; the peak maxima of absorption and emission spectra of the copolyesters shifted to lower wavelengths in the aforementioned order for the LC properties. Cyclic voltammetry measurements indicated that the electrochemical band gaps of the polyesters estimated from the onset of reduction and oxidation processes were almost the same as the optical band gaps determined from the solid-state UV-vis spectral data. The DPTD unit enhanced the hole-injection barrier and improved the charge-injection balance in these polyesters.