Influence of molecular structure on the nematic-nematic transition in polyethers based on 1-(4-hydroxyphenyl)-2-(2-R-4-hydroxyphenyl)ethane where R=CH₃ and Cl, and flexible spacers with an odd number of methylene units

In a previous paper (reference 1b) we reported that flexible main chain liquid crystalline copolymers based on various molar ratios between the flexible mesogenic groups 1-(4-hydroxyphenyl)-2-(2-R-4-hydroxyphenyl)ethane (RBPE) where R=F, Cl, CH₃ and several different molar ratios between flexible spacers containing odd numbers of methylenic units, i.e. 5, 7, and 9 display two enantiotropic uniaxial nematic mesophases. The homopolymers RBPE-X (where R=-CH₃ and-Cl) and X=number of methylenic units in the flexible spacer, i.e. X=5, 7, 9, 11, and 13) display also two nematic (n₁, n₂) mesophases which are however, monotropic or virtual. This paper will discuss the dependence of the phase transition temperatures between these two nematic mesophases (Tn₁n₂), the isotropization temperature (Tin₁), and of their associated thermodynamic parameters of the MBPE-X (M=CH₃) and C1BPE-X homopolyethers as a function of spacer length X. These results will be compared with the predictions of the only available theory which predicts the existence of two uniaxial nematic mesophases in flexible main chain liquid crystalline polymers (reference 6).