Victor M. Ugaz

Transient molecular orientation and rheology in flow aligning thermotropic liquid crystalline polymers

Quantitative measurements of molecular orientation and rheology are reported for various transient shear flows of a nematic semiflexible copolyether. Unlike the case of lyotropic liquid crystalline polymers (LCPs), whose structure and rheology in shear are dominated by director tumbling, this material exhibits flow aligning behavior. The observed behavior is quite similar to that seen in a copolyester that we have recently studied [Ugaz and Burghardt (1998)], suggesting that flow aligning dynamics may predominate in main-chain thermotropes that incorporate significant chain flexibility. Since the flow aligning regime has received little attention in previous attempts to model the rheology of textured, polydomain LCPs, we attempt to determine whether available models are capable of predicting the orientation and stress response of this class of LCP. We first examine the predictions of the polydomain Ericksen model, an adaptation of Ericksen’s transversely isotropic fluid model which accounts for the polydo...

Transient 1-2 plane small-angle x-ray scattering measurements of micellar orientation in aligning and tumbling nematic surfactant solutions

A new x-ray shear cell capable of probing complex fluid structure in the flow-gradient (“1–2”) plane is used to study orientational dynamics of nematic surfactants in transient shear flows. Two surfactant systems are studied, one of which (SDS/decanol) apparently exhibits shear aligning dynamics and the other of which (CPyCl/hexanol) exhibits director tumbling. Data on the SDS/decanol system exhibit what appear to be common characteristics of aligning nematics: constant orientation state in steady shear; a single undershoot of long duration in average orientation upon flow reversal; and no significant orientation change upon step increase or decrease in shear rate or upon flow cessation. These data are discussed in light of similar measurements on shear aligning thermotropic liquid-crystalline polymers, and polydomain model predictions of transient orientation for aligning nematics. Shear reversal data on CPyCl/hexanol show an oscillatory response typical of tumbling materials, but in better qualitative a...

X-RAY SCATTERING INVESTIGATION OF HIGHLY CONCENTRATED POLY(BENZYL GLUTAMATE) SOLUTIONS UNDER SHEAR FLOW

In situ x-ray scattering techniques are used to measure molecular orientation and investigate the coexistence of hexagonal and nematic phases in highly concentrated PBG/cresol solutions under shear flow. Beyond a threshold concentration, the diffuse nematic lobe in x-ray scattering patterns is accompanied by sharp reflections indicating lateral packing of molecules in a hexagonal arrangement. The threshold concentration for formation of this hexagonal phase depends on molecular weight, but occurs near 35 wt % PBG and corresponds directly to the onset of region I shear thinning. Applied shear acts to reduce the amount of hexagonal phase and ultimately, at high enough shear rates, return the solution to a fully nematic state. Relaxation experiments at high shear rates indicate that the final state of the solution is highly sensitive to the continued presence of the hexagonal phase during shear. Specifically, a much higher final orientation results if the hexagonal phase—even a very small amount—persists dur...