Disclination-mediated thermo-optical response in nematic glass sheets

Abstract

Nematic solids respond strongly to changes in ambient heat or light, significantly differently parallel and perpendicular to the director. This phenomenon is well characterized for uniform director fields but not for defect textures. We analyze the elastic ground states of a nematic glass in the membrane approximation as a function of temperature for some disclination defects with an eye toward reversibly inducing three-dimensional shapes from flat sheets of material, at the nanoscale all the way to macroscopic objects, including nondevelopable surfaces. The latter offers a paradigm to actuation via switchable stretch in thin systems. DOI: 10.1103/PhysRevE.81.060701 PACS number͑s͒: 61.30.Jf, 46.32.ϩx, 46.70.De, 46.70.Hg Nematic glasses ͑densely cross-linked networks͒ ͓1͔ are solids with a natural state of elongation along their director and contraction perpendicular that depends on their orientational order. Accordingly they suffer large, reversible length change with heating ͓2͔, illumination ͓3,4͔, solvent uptake, pH change and any other stimulus that causes order change. For glasses these strains can be 2 -3 % and of opposite sign ͑without conserving volume͒ along and perpendicular to the director. They can then exhibit spectacular effects such as large bend resulting from gradients of stimuli ͓5͔ ͑light, solvent͒ or from uniform stimuli ͑such as temperature or weakly absorbing light͒ but with a director gradient through, for instance, the section of a sheet or cantilever ͓2-4͔ ͑usually twist or splay-bend͒. Glasses are distinguished from elastomers by being so heavily cross-linked that the director field only changes as a result of convection due to material shape change from the elastic strain. Nematic glass directors do not rotate relative to the matrix as in nematic elastomers-they are ͓3͔ conventional uniaxial elastic solids, with moduli of about ϫ10 4 higher than those of elastomers. We are most concerned with the thermally or optically inspired length changes which we take to be by a factor along the director and by − perpendicular to the director ͑where fulfills the role of a thermal or optical Poisson ratio, were the deformations to be infinitesimal͒. Elastic energy cost would be associated with imposed changes away from these new natural shapes-we find deformation fields matching these thermo/ optical changes and thus of zero elastic energy in the membrane limit we take. Accordingly we do not need to explore the solid body elasticity specific to uniaxial materials.Director fields can be established in the nematic liquid progenitor phase before cross-linking and are permanently recorded in the solid state achieved after linkage. In fact complex, three-dimensional ͑3D͒ director fields for subtle mechanical response can be achieved in nematic glasses ͓6͔ via holography and surface preparation. From the point of view of device design, the ability to "write" an initial director field into a solid so that it distorts in a predictable ͑and reversible way͒ into a new shape with applied temperature or li...

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