When viewed with a polarizing-light microscope, liquid crystals (LCs) exhibit dazzling birefringent "textures" that are related to the long- range orientational order in this curious fluid state of matter. (Nematic phase of 4,4'(1,3,4- oxadiazole-2,5- diyl) di-p-heptyl- benzoate)

Nonlinear ("boomerang-" or "banana-shaped") LCs are of current interest because in stratified (smectic) phases these bent molecules can adopt ferroelectric and antiferroelectric supramolecular arrangements wherein the molecular sense respectively, is retained (a), or alternates direction (b), from layer to layer:

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(a) ferroelectric     (b) antiferroelectric

Ferroelectric phases in turn, exhibit rapid switching and are of technological interest for manufacturing the next generation of LCDs. We have tried to ascertain how much nonlinearity can be introduced into molecules and still have them melt into a stable LC phase. For example, in an effort to find the limiting molecular shapes compatible with liquid crystallinity we "bend" a known linear LC molecule, p-quenquiphenyl (PPPPP), by incorporating 2,5-substituted heterocycles-thiophene (PPTPP) and oxadiazole (PPOPP)-into the center of the five-ring molecular framework.

The figure at right shows the decreasing nematic range (green) with decreasing exocyclic bond angle in the heterocycle for this series of "all-aromatic" nonlinear molecules. Our research program tries to map out, via straight-forward synthesis, the structural requirements of LCs. Our goal is to explore the boundaries of liquid crystallinity in order to engineer new materials for specific end uses, e.g., LCDs and fast electro-optic sensors.

"Non-linear boomerang-shaped liquid crystals derived from 2,5-bis(p-hydroxy-phenyl)-1,3,4-oxadiazole" T. J. Dingemans and E. T. Samulski, Liquid Crystals, 27, 131-136 (2000).

The good solubility of fluoropolymers in CO₂ is critical to the surfactant activity of block co-polymers in this novel solvent and, therefore, key to the widespread adoption of this environmentally- benign solvent. We are using ¹⁹F NMR to see if there is a specific interaction between and fluorocarbons and CO₂ that can account for this unusual solubility. The ¹⁹F chemical shift is sensitive to both intrinsic (molecular structural) and intermolecular contributions.

 An example is shown in the figure at right wherein the CO₂ density-dependence of the relative chemical shifts for the inequivalent fluorine sites in perfluoro-trans-decalin are plotted. We have discovered that above and beyond the diamagnetic contributions to the chemical shift, there appears to be an intermolecular, site-specific change in the chemical shift of solute molecules with varying CO₂ density. By contrasting the behavior of fluorocarbons with hydrocarbons using NMR chemical shifts, relaxation times, diffusion coefficients, etc., under various solution conditions (liquid and supercritical CO₂), we hope to provide a basic understanding of solubility in this unusual solvent.

"Fluorocarbons dissolved in supercritical carbon dioxide. NMR evidence for specific solute-solvent interactions" A. Dardin, J. M. DeSimone, and E. T. Samulski, J. Phys. Chem, 102, 1775-1780 (1998).

As the chain length (molar mass M) exceeds a critical value, M_c, in a polymer melt or concentrated solution, the chains become entangled and the melt viscosity increases very rapidly, as M (see figure at right). Chain dynamics-the large-scale reorientations/reconfigurations executed via diffusive motion and conformational transitions-also abruptly change when M exceeds M_c.

There are related constraints on the global macromolecular dynamics in rubbers, polymer networks wherein chain-ends are covalently tethered together (crosslinked). Such constrained chain dynamics can be detected with NMR even though NMR is primarily sensitive to very rapid, local motions (methyl group rotation and dihedral angle isomerization). This comes about because in polymeric materials the local motions incompletely average nuclear interactions. The "through-space" dipole-dipole interaction between pairs of neighboring protons is such an interaction, and in the schematic diagram at right it is possible to infer that local motion within the chain backbone averages such interactions to some non-zero value projected along the chain's end-to-end vector, R. Hence there remains a residue of the dipolar interactions projected on R and NMR can be exploited to measure its magnitude. In melts and networks the residual interaction is a sensitive function of the librations of R and hence to the molar mass between entanglements (in melts) and covalent crosslinks (in networks).

We have developed a pulse NMR echo sequence, b(2t,t), that give us access to residual dipolar interactions; example data from poly(dimethyl siloxane) networks wherein the fraction of active network chains, fx, is systematically varied, as shown at right. The initial slope of b(2t,t) is a direct measure of the efficacy of constrained chain dynamics for averaging the dipolar interactions in the networks. Our long-range goals are to develop this NMR methodology to a point where we can make contact with computer simulations of chain dynamics and thereby improve our understanding of what determines the ultimate properties of soft macromolecular materials.

"Molecular weight dependence of nuclear spin correlations in PDMS networks" P. T. Callaghan and E. T. Samulski, Macromolecules 33, 3795-3802 (2000).