LIQUID-CRYSTALLINE POLYETHERS BASED ON CONFORMATIONAL ISOMERISM .20. NEMATIC NEMATIC TRANSITION IN POLYETHERS AND COPOLYETHERS BASED ON 1-(4-HYDROXYPHENYL)-2-(2-R-4-HYDROXYPHENYL)ETHANE WITH R = FLUORO, CHLORO, AND METHYL AND FLEXIBLE SPACERS CONTAINING AN ODD NUMBER OF METHYLENE UNITS

A new type of thermotropic phase transition occurring between two uniaxial nematic phases in a class of main-chain liquid crystalline polyethers and copolyethers is reported. The polymers exhibiting this phase transition are based on 1-(4-hydroxyphenyl)-2-(2-R-4-hydroxyphenyl)ethane where R = F, Cl, and CH3 and flexible spacers containing an odd number of methylene units (RBPE-X where X is the number of methylene units in the flexible spacer). In homopolymers and copolymers based on a single mesogenic unit and three spacers containing 5, 7, and 9 methylene units in the molar ratio 33/33/33, the low-temperature nematic phase is monotropic. In copolymers based on two and three different mesogens (in the molar ratio A/B/C) and three different spacers containing 5, 7, and 9 methylenic units in the molar ratio 33/33/33 [i.e., FBPE/ClBPE/MBPE(A/B/C)-5/7/9(33/33/33)] this mesophase is enantiotropic. The possible relationship between the two presently observed nematic phases and those theoretically predicted by Vasilenko et al.11 is briefly considered.