Phase Transitions of Dimeric Liquid Crystals Containing Long Odd-Numbered Methylene Spacers

Abstract

ABSTRACT:A series of o:,w-bis{4-(4-biphenyloxycarbonyl)phenyl} alkanedioates (BPA-n) dimeric liquid crystals having long odd-numbered methylene spacers was prepared. The number of methylene groups in the flexible alkyl spacer has been varied from 9 to 21. Long period L of crystalline BPA-n obtained by X-ray powder diffraction were shown as a linear equation of the number n of methylene units, as follows, L= l.I6n + 35.2 (A). BPA-21 showed no liquid crystalline phase. BPA-19 is monotropic nematic. The other BPA-n showed an enantiotropic nematic phase. The overall entropies ilSK1 of the transition from the crystals at room temperature to the isotropic liquid were also shown as a linear equation of n, as follows, ilSK 1 = 8.54n + 94.8 (1 K-1 mol-1 ). The crystal-nematic (TKN) transition temperature shows a weak odd-even oscillation, but the nematic-isotropic (TN1 ) transition temperature showed a distinctive odd--even effect. The odd-even effect of a TN, decreases with an increase in n. The nematic-isotropic (ilSN1 ) entropy change is almost constant with an increase in n with a strong odd-even oscillation, higher values being detected for the homologues with even-numbered spacers.KEY WORDS Thermotropic Liquid Crystalline Polymer I Dimeric Liquid Crystals I Odd-Numbered Methylene Spacers I Thermal Properties I Odd-Even Effects I Main chain liquid crystalline polymers with repeated units of mesogen spacers have a characteristic that the odd-even effect of spacer length is more distinctive than the low molecular liquid crystals. 1 This odd--even effect is observed for the nematic-isotropic phase transition temperature and associated entropy change. As most of the dimeric liquid crystals formed by a sequence of mesogen-spacer-mesogen showed very similar odd-even effect to the liquid crystalline polymers, they have been extensively studied as models for polymers. 2 -5 The recent interest of this dimers stems from their quite different properties to conventional low molecular liquid cryatals. 6 • 7 There is an approach to understand the thermal properties of flexible molecules. It is to separate thermodynamic quantities the contributions of methylene units and terminal groups. For example, thermal properties for n-alkanes have been treated on the supposition that both the enthalpy and entropy of melting may be adequately represented by linear expressions, each comprising a term proportional to the number of methylene units and an additive constant for contributions associated with the two methyl groups. 8 There have been a number of studies of the thermal properties of dimers, but there had been no reports that treats such thermodynamic quantity quantitatively with separation of contribution of mesogens and contribution of polymethylene spacer parts, respectively. In a previous paper, we reported a new series of dimeric liquid crystals with an even-number long methylene spacers from n = 8 to n = 20, as shown below 9 :t To whom correspondence should be addressed.We found that the overall entropies LlSK 1 transition fr...

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