Recent studies of molecular orientation in sheared
liquid crystalline polymers have often
yielded contradictory results. To check the self-consistency of
methods for quantitative measurements
of molecular orientation, liquid crystalline solutions of
(hydroxypropyl)cellulose [HPC] and poly(benzyl
glutamate) [PBG] have been studied using flow birefringence, X-ray
scattering, and neutron scattering.
HPC X-ray scattering patterns show an arclike pattern with a
distinct peak as a function of scattering
vector, while PBG patterns show a more diffuse equitorial streak.
These differences are attributed to
more strongly correlated lateral packing in HPC solutions due to their
higher concentration. Measurements of orientation in steady shear flow agree well among the three
techniques. Lyotropic HPC and
PBG solutions differ in orientation at low shear rates. HPC
solutions exhibit near zero orientation at
low rates, while X-ray and neutron scattering measurements confirm
previous birefringence data showing
a low shear rate plateau of moderate orientation in PBG.
Differences with recent neutron scattering
measurements on PBG solutions that show low orientation at low shear
rate are attributed to choice of
solvent, rather than choice of technique. X-ray and optical data
are consistent in showing decreasing
orientation in HPC solutions during relaxation, but discrepancies are
found in relaxation of PBG solutions.
Large increases in flow birefringence suggest substantial
orientation enhancement. X-ray data on one
PBG solution confirm increasing orientation, but X-ray and neutron
scattering data on a more concentrated
solution show only modest changes in orientation. It is suggested
that flow birefringence fails in this
case due to texture coarsening to the point where there is no longer
effective averaging over the distribution
of director orientations along the light path.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.