Spectroscopic characterization of trans-gauche isomerization in liquid crystal polymers with two nematic states

Abstract

Nuclear magnetic resonance (n.m.r.) spectroscopy and Fourier transform infra-red ( FTi.r.) spectroscopy are used to characterize the trans-gauche isomerization of the methylene groups in a new class of main-chain liquid crystal polymers. This new series of liquid crystalline copolymers is based on the 1-(4-hydroxyphenyl)-2-(2-R-4-hydroxyphenyl)ethane mesogen, where R is F, Cl or CH 3 and flexible spacers containing an odd number of menthylene units. These copolymers are particularly interesting because they show two nematic states. Trans-gauche isomerization is characterized by FTi.r. spectroscopy through measurements of the absorbances of characteristically trans bands, and characteristically gauche bands. Trans-gauche isomerization is characterized by 13C n.m.r. spectroscopy through measurements of the 13C chemical shifts in the solid state. FTi.r. spectroscopy shows that an increase in temperature results in an increase in the percentage of gauche isomers for the methylene units in both the spacer and the mesogen. N.m.r. spectroscopy shows 13% more gauche isomers in the high-temperature nematic state, relative to the low-temperature nematic state, for rotation about the C 28C 29 bond axis in the spacer.