LIQUID-CRYSTALLINE POLYETHERS BASED ON CONFORMATIONAL ISOMERISM .10. SYNTHESIS AND DETERMINATION OF THE VIRTUAL MESOPHASES OF POLYETHERS BASED ON 1-(4-HYDROXYPHENYL)-2-(2-METHYL-4-HYDROXYPHENYL)ETHANE AND ALPHA,OMEGA-DIBROMOALKANES CONTAINING FROM 17 TO 20 METHYLENE UNITS

The synthesis and characterization of the polyethers based on the flexible mesogen 1-(4-hydroxyphenyl)-2-(2-methyl-4-hydroxyphenyl)ethane (MBPE) and α,ω-dibromoalkanes containing 17,18,19, and 20 methylene units (MBPE-17, MBPE-18, MBPE-198 and MBPE-20) are described. All polymers present a virtual nematic mesophase. The virtual isotropic–nematic transition tempertures and thermodynamic parameters were determined from copolymerization experiments. Both virtual isotropic–nematic transition temperatures and thermodynamic parameters of the entire set of polyethers MBPE-X (X = number of methylene units in the flexible spacer) with X = 4-20 were plotted and lead to the following conclusions. Virtual isotropic–nematic thermal transition temperatures and their associated enthalpy and entropy changes follow an odd–even dependence on X. At large X, the odd–even dependence tends to vanish Extrapolation of the virtual isotropic–nematic temperatures to X values approaching infinity suggests the presence of a virtual mesophase in polyethylene. Previously, this mesophase was only theoretically predicted.22 The conformational entropic contribution per CH2 unit at the isotropic–nematic transition agrees very well for both the even and odd series with the data reported for thermotropic polyesters based on rigid rodlike mesogens and flexible spacers.4 The entropic contribution per MBPE unit is, however, larger than the one reported for the rigid rodlike mesogenic group 4,4’-dihydroxy-2,2’- dimethylazoxybenzene.4 This difference was explained by considering that in the case of the flexible MBPE this entropic factor represents a combination of orientational and conformational contributions, while in the case of the rigid rodlike mesogens this factor contains only an orientational contribution. Consequently, the entropy changes associated with the isotropization temperatures of polyethers based on flexible rodlike mesogens are higher than those of polyesters based on rigid rodlike mesogens most probably due to the lower entropy of the former in the isotropic phase and not due to a higher degree of order in the mesomorphic phase. © 1990, American Chemical Society. All rights reserved.