Effects of Spacer Length on Thermal Properties of Dimeric Model Compounds for Main Chain Liquid Crystalline Polymers

Abstract

ABSTRACT:The thermal properties of a series of o:,w-bis[4-(4-biphenyloxycarbonyl)phenyl]alkanedioates (BPA-n) dimeric liquid crystals containing even-number methylene spacer from n = 8 to n = 20 were studied. All synthesized compounds showed the same crystal form at room temperature. They were all enantiotropic nematic except BPA-20 which was monotropic. The temperature range over which the nematic phase exists becomes smaller with increasing spacer length, and finally at n = 20 it no longer shows liquid crystal on heating. Thermodynamic quantities were classified as contributions from mesogens and polymethylene spacers. The overall entropies LlSK1 of transition from crystals at room temperature to the isotropic liquid were shown as a linear equation of the number, n, of methylene units as follows, LlSK1 =8.50n +95.41 K 1 1 . The coefficient (8.501 K 1 1 ), which means the increase in LlSK, value per methylene, corresponds well to melting entropy per methylene of n-alkanes. The entropies of transition from nematic to isotropic phase were almost constant (8.3-8.9 J K 1 1 ), independent of the number of methylene units in the flexible spacer.KEY WORDS Dimer I Liquid Crystal I Nematic Phase I Model Compounds I Spacer Length Effect I Thermotropic Polyester I Long Spacer I Mesogens I Thermal Properties I Dimeric liquid crystals formed by a sequence of are low molecules from the standpoint of the degree of polymerization, though these nematic-isotropic transition temperatures and heats of transition depend on odd or even numbers of methylene units in the spacer, very much the same as main chain liquid crystalline polymers with repeated units of mesogen-spacers. 1 -3 This is a quite different character from low molecular mass liquid crystals which have small odd-even oscillation. Therefore dimeric liquid crystals are considered as a proper model of liquid crystalline polymer. There are many reports stating how the thermodynamic quantity of dimeric liquid crystals depends on spacer length, 4 • 5 but there is no study that treats such thermodynamic quantity quantitatively with separation of contribution of mesogens and contribution of polymethylene spacer parts respectively.If spacer length becomes certain large and crystal structures of compounds show same form, then we consider that contribution caused by mesogens and polymethylene spacer would work almost independently for thermodynamic quantity of phase transitions, and then handle dimeric compounds with considerably long polymethylene chain.In this work, we synthesized and studied dimeric liquid crystals having long polymethylene chain, as shown below:We designated these dimers BPA-n, which consist of both a flexible even-number polymethylene spacer of varying length (n= 8, 10, 12, 14, 16, 18, 20) and two terminal mesogens. Since the nematic-isotropic transition temperatures of the dimers with even numbers less 326 than n = 8 were so high that the precise thermodynamic quantities could not be obtained, they were not dealt with here. The dimers with odd numbers wil...

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

“…9 The results of elemental analysis have been found to be in good agreement with the calculated values shown in Table I. The samples were obtained by slow crystallization from toluene solution.…”

“…The data of even-numbered dimers are reported in previous paper. 9 Both transition temperatures, TKN• and TN 1 , were found to decrease with an increase inn. TKN shows slightly odd--even effect, but TN1 shows a distinctive odd--even effect.…”