Giant-block twist grain boundary smectic phases

doi:10.1073/pnas.0500664102

Giant-block twist grain boundary smectic phases

^†Department of Physics and Liquid Crystal Materials Research Center, University of Colorado, Boulder, CO 80309-0390; ^††Department of Chemistry and Biochemistry and Liquid Crystal Materials Research Center, University of Colorado, Boulder, CO 80309-0215; ^∥Department of Chemical Engineering, University of California, Santa Barbara, CA 93106; ^‡Centre de Recherche Paul Pascal, Centre National de la Recherche Scientifique, Avenue Albert Schweitzer, 33600 Pessac, France; ^¶Raman Research Institute, Bangalore 560080, India; and ^§Groupement de Droit Comparé, Unités Mixtes de Recherche 5581, Université Montpellier II, F 34095 Montpellier Cedex 05, France

Edited by Tom C. Lubensky, University of Pennsylvania, Philadelphia, PA (received for review March 9, 2005)

Abstract

Study of a diverse set of chiral smectic materials, each of which has twist grain boundary (TGB) phases over a broad temperature range and exhibits grid patterns in the Grandjean textures of the TGB helix, shows that these features arise from a common structure: “giant” smectic blocks of planar layers of thickness l_b > 200 nm terminated by GBs that are sharp, mediating large angular jumps in layer orientation between blocks (60° < Δ < 90°), and lubricating the thermal contraction of the smectic layers within the blocks. This phenomenology is well described by basic theoretical models applicable in the limit that the ratio of molecular tilt penetration length-to-layer coherence length is large, and featuring GBs in which smectic ordering is weak, approaching thin, melted (nematic-like) walls. In this limit the energy cost of change of the block size is small, leading to a wide variation of block dimension, depending on preparation conditions. The models also account for the temperature dependence of the TGB helix pitch.

Footnotes

↵ ‡‡ To whom correspondence should be addressed. E-mail: noel.clark{at}colorado.edu.
Author contributions: J.F., L.H., R.-F.S., J.E.M., L.N., M.B., N.V.M., O.M.-M., C.B., J.Z., J.A.R., D.M.W., and N.A.C. designed research; J.F., L.H., R.-F.S., J.E.M., L.N., M.B., N.V.M., O.M.-M., C.B., J.Z., J.A.R., D.M.W., and N.A.C. performed research; N.V.M., J.A.R. and D.M.W. contributed new reagents/analytic tools; J.F., L.H., R.-F.S., J.E.M., L.N., M.B., N.V.M., O.M.-M., C.B., J.Z., J.A.R., D.M.W., and N.A.C. analyzed data; and J.F. and N.A.C. wrote the paper;
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations: LC, liquid crystal; SmA, smectic A; SmC, smectic C; GB, grain boundary; TGB, twist GB; MGB, melted GB; GBTGB, giant-block TGB; GBTGBA, GBTGB smectic A; GBTGBC, GBTGB smectic C; FFEM, freeze-fracture electron microscopy; XRD, x-ray diffraction; CN, chiral nematic.
↵ §§ Clark, N. A. & Shao, R. F., University of Pennsylvania TGB Symposium, April 22-24, 1998, Philadelphia.