We describe results of an experimental investigation into the orientation state of liquid crystalline solutions of poly(benzyl glutamate) under shear flow and how the microscopic structure relates to the macroscopic mechanical rheological behavior. The technique of flow birefringence was used to study the degree of molecular orientation. A spectrographic flow birefringence apparatus is described that eliminates ambiguities associated with multiple orders of retardation in birefringence measurements. The birefringence observed in textured solutions under shear flow is always less than that measured in quiescent, defect-free monodomains of the solutions. At low shear rates, the birefringence is roughly constant and in the range of 53-63% of that observed in a monodomain; there is no evidence of a low-orientation, "piled polydomain" structure. At high shear rates, the birefringence is again roughly constant and around 90% of the monodomain value. The transition between lowand high-orientation states as a function of shear rate is closely correlated with changes in sign of the first normal stress difference of these solutions, leading us to identify it as a manifestation of a transition between regimes of director tumbling at low shear rates and flow alignment at high shear rates. These observations are compared qualitatively and quantatively with predictions of the nonlinear Doi molecular model for textureless samples [Larson, R. G. Macromolecules 1990,23, 3983] and the linear Larson and Doi tumbling polydomain model for textured samples [Larson, R. G.; Doi, M. J. Rheol. 1991, 35, 539]. An accompanying paper considers transient flow phenomena at low shear rates.
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.
Flow birefringence is used to study the degree of molecular orientation in transient shear flows of liquid crystalline solutions of poly(benzy1 glutamate) (PBG). Experiments are confiied to the low shear rate regime in which it is known that such solutions exhibit director tumbling. Our optical results exhibit characteristics common to PBG solutions, including oscillatory responses that scale with shear strain and relaxation processes that scale inversely with the previously applied shear rate. The measured degree of orientation is compared with the transient predictions of the linear Larson and Doi tumbling polydomain model [Larson, R. G.; Doi, M. J. Rheol. 1991,35,539], which is known to qualitatively reproduce transient streas behavior well. Our results reveal some ~hortcomings of the model regarding its predictions of molecular orientation. Our ma& startling observation is a significant increase in molecular orientation upon cessation of shear flow. This increase in orientation provides a straightforward explanation for the gradual decrease in dynamic moduli that is observed following cessation of shear flow. This result is in direct contradiction with conclusions drawn by Asada and co-workers on similar PBG solutions [Aaada, T.; On&, S.; Yanm, H. Polym. &ng. Sci. 1984,24,355]. We attribute this discrepancy to ambiguity in birefringence measurements resulting from multiple retardation orders; we have employed a spectrographic birefringence technique that eliminates this source of uncertainty. This work is an extension of an accompanying paper on steady-state flow behavior.
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