Peter F. Erhardt

Normal Stress Effects in Cholesteric Mesophases

Normal forces have been found in rheological studies on cholesteryl chloride‐cholesteryl oleyl carbonate liquid crystal mixtures over the composition range 0–27 wt.% cholesteryl chloride. These normal forces appear to be associated with the homeotropic texture and are evident in the transition region for the shear induced focal conic to homeotropic transition. The Kotaka model for normal stresses arising from hydrodynamic interactions of rotating rods has been applied. The results are consistent with a model of the homeotropic texture of the cholesteric mesophase comprised of rodlike helical arrays of molecules rotating in an applied shear field. The calculated rod lengths are less than the characteristic pitch of the material.

The mechanism of shear induced structural changes in liquid crystals—cholesteric‐polymer solutions

The reflectance spectra and rheological properties for a cholesteric liquid crystal‐polyisobutylene solution, have been obtained as a function of mechanical shear. The optical data are explained in terms of a layered structure consisting of tilted and untilted helical Grandjean texture and a dynamic focal‐conic texture. At low shear rates < 1.72 sec−1, this system behaves optically and rheologically as a normal mixed cholesteric system. At greater shear rates, the structural reinforcing properties of the polyisobutylene cause optical and rheological behavior different from that observed in mixed cholesteric and cholesteric‐nematic mixtures. The most striking effect of the polyisobutylene is its facility to disrupt the shear induced focal‐conic component, causing relaxation to the Grandjean texture in a way that is similar to raising the temperature or releasing the surface constraints on the system.

Synthesis and Thermal Transition Properties of Styrene — Ethylene Oxide Block Copolymers

Block copolymers are composed of two or more monomers which are segregated into blocks along the polymer chain. The copolymers described in this paper have one block composed of polystyrene (PS) and one or two blocks composed of poly (ethylene oxide) (PEO). These materials were first synthesized by Richards and Szwarc (1) and their investigation of the collodial properties of the copolymers in solution generated considerable interest in the PS-PEO system. Subsequent studies by Skoulios et.al. (2–5) showed that phase separation of the copolymer blocks occurred when concentrated solutions of the copolymers were prepared in a solvent preferential for one of the components. The mesomorphous phases were cylindrical, lamellar and spherical in structure and were interconvertible by varying the concentration of the polymer solution. Sadron (6,7) was able to preserve the various mesophases indefinitely by using polymerizable monomers as the preferential solvents and polymerizing the solvent. A recent study by Kovacs and Lotz (8,9) demonstrated that single crystals of the block copolymers can be grown from dilute solution when the solvent is preferential for PS and that they are composed of PEO folded chain lamellae sandwiched between surface layers of amorphous PS.

The Effects of Preferential Solvents on the Phase Separation Morphology of Styrene-Ethylene Oxide Block Copolymers

Phase separation in block copolymer systems exhibits rather striking effects on resultant physical properties (1,2). Skoulios et al (3–6) have studied the morphology of block copolymers of poly(ethylene oxide) (PEO) and polystyrene (PS) as well as poly (propylene oxide) and PS, in concentrated solutions using small angle x-ray techniques. By varying both concentration and solvent type, they were able to create several interesting gel structures. As each component in their block systems had markedly different solubilities they were able to explore the effect of dispersing these polymers in preferential solvents. As an example, the more polar PEO was soluble primarily in polar solvents (nitromethane, water, etc.) while the non polar PS segment was soluble primarily in non polar media (ethylbenzene, p-xylene, etc.). By using these preferential solvents, several structures, namely the sphere, the cylinder and the lamella, were identified from small angle x-ray data. More recently, Kovacs et al (7–9) have studied single crystals of PS-PEO AB type blocks and have shown that PEO crystallizes with little interference from the glass forming PS in dilute solution experiments. They also indicate that the resultant PEO crystal lattices are identical to that of the homopolymer.

On the Mechanism of Shear Induced Texture Changes in Cholesterics-Electric Field Effects

Abstract Cholesteric systems are being used for display devices because they form several textures each having different optical properties. These textures are: Grandjean (highly reflective); focal conic (highly scattering); and homeotropic (practically transparent). In earlier papers, we have shown that shear induced transitions between these textures can be produced. These transitions are accompanied by color changes in the Grandjean texture and normal forces generated during shear. These effects can be explained in terms of the response of a correlated helical structure in the cholesteric mesophase to external stimuli. The shear induced transition associated with normal force generation is between the focal conic and homeotropic texture. In an effort to understand the responses of the cholesteric system to multiple stimuli, studies have been extended to include electric field effects. This work reports on the effects of an electric field on the dynamic focal conic to homeotropic transition examined as ...

Temperature Dependence and Rheological Behavior of the Shear-Induced Grandjean to Focal Conic Transition in the Cholesteric Mesophase

This report is a study of the shear induced Grandjean to dynamic focal conic transition by rheological and thermal measurements. Previous structural postulations of the sheared choles-teric mesophases are consistent with the observed data. For a room temperature cholesteric mixture of Cholesteryl Oleyl Carbonate (COC) and Cholesteryl Chloride (CCl) (23% by wt. CCl) activation energies for viscous flow are identical for the Grandjean and dynamic focal conic textures and equal to 15 Kcal/mole. Extrapolation of the dynamic focal conic viscosity region into the isotropic region is continuous. Transient rheological phenomena in the transition region are temperature dependent and are shown to be associated with non-equilibrium tilting of the cholesteric helices and breakdown of the normal Grandjean structure. The activation energy for the shear induced conversion of Grandjean to dynamic focal conic texture is 25 Kcal/mole.

Rheological Properties of Styrene-Ethylene Oxide Block Copolymers: Transition and Melt Flow Behavior

The block copolymers described herein are long linear sequences of styrene (non-polar, oleophilic, amorphous) units connected by primary chemical bonds to long linear sequences of ethylene-oxide (polar, hydrophilic, crystallizable) units. Because of the marked chemical dissimilarity of styrene and ethylene oxide, and the polymeric, long chain nature of the segmental units, the components are incompatible and phase separation in the melt is expected and observed. However, because of the primary chemical bonds tieing together the incompatible chain segments, phase separation is not complete and the rheological behavior of one phase is strongly affected by that of the other phase. Block copolymers are unique heterophase systems in this respect and form a distinct class of materials, the study of which should extend our knowledge of the manner of flow of mixed phases. The distinguishing feature of the ethylene oxide/styrene block copolymer system is the sensitivity of the ethylene oxide block to shear. This allows ethylene oxide blocks to be used as sensitive rheological probes.