Yc. Chiang et al., Electro-rheological behavior of liquid crystal polymers (LCPs) dissolved in a nematic solvent: dependence on temperature and LCP structure, POLYMER, 41(11), 2000, pp. 4127-4135
The viscosity increments delta eta(on) and delta eta(off), upon dissolving
a liquid crystal polymer (LCP) in a thermotropic nematic solvent, were dete
rmined by rheometry in the presence and absence, respectively. of a saturat
ion electric field. Four species of molecular architectures were studied. a
rigid main-chain LCP, two types of flexible main chain LCP, one with mesog
ens oriented perpendicular to the backbone, the other with mesogens paralle
l to the backbone, and a flexible side-chain LCP with mesogens attached sid
e-on to the backbone. For all mixtures, delta eta(on) and delta eta(off) ex
hibit a similar Arrhenius dependence on temperature, determined by the visc
ous activation energy of the nematic solvent. However, the temperature depe
ndence of the ratio delta eta(on)/delta eta(off) and of the intrinsic visco
sities, [eta(on)] and [eta(off)], is different for each LCP, and is strongl
y dependent on the flexibility of the LCP. By equating eta(on) and eta(off)
to the Miesowicz viscosities, eta(c) and eta(b), respectively, these resul
ts can be interpreted in terms of the ratio of the LCP chain dimensions par
allel and perpendicular to the nematic director, R-parallel to and R-perpen
dicular to (since theory predicts delta eta(c)/delta eta(b) = R-parallel to
(4)/R-perpendicular to(4)). The results indicate that the conformation of e
ach LCP is strongly prolate, i.e. R-parallel to much greater than R-perpend
icular to. For the rigid main-chain LCP, R-parallel to/R-perpendicular to,
changes very little with temperature; for the flexible LCPs, R-parallel to/
R-perpendicular to, decreases strongly with increase of temperature, indica
tive that the conformation becomes more spherical. The behavior of R-parall
el to/R-perpendicular to, is compared with recent theoretical models of LCP
conformation. (C) 2000 Elsevier Science Ltd. All rights reserved.