Deformation of a Polydomain, Smectic Liquid Crystalline Elastomer C. Ortiz, † M. Wagner, ‡ N. Bhargava, C. K. Ober,* and E. J. Kramer § Cornell University, Department of Materials Science and Engineering and The Materials Science Center, Bard Hall, Ithaca, New York 14853 Received September 25, 1997; Revised Manuscript Received August 12, 1998 ABSTRACT: A main-chain, polydomain, smectic liquid crystalline elastomer (LCE) was prepared by reacting the LC epoxy monomer, diglycidyl ether of 4,4′-dihydroxy-R-methylstilbene, with the aliphatic diacid, sebacic acid. When deformed in uniaxial tension, a “polydomain-to-monodomain” transition took place leading to bulk, macroscopic orientation. With this process was associated a plateau in the nominal stress-versus-strain curve and a dramatic change in optical properties from opaque to translucent. Polarized optical microscopy showed that the transition took place by an elongation of the LC domains and a rotation of the local director orientations along the stress axis. The strain and orientation of the deformed samples were retained upon unloading, even after annealing above Tg for extended periods. Upon heating, the oriented LCEs disordered at the same temperature as the undeformed polydomains and “remembered” their original polydomain microstructure and sample dimensions when subsequently cooled from the isotropic state. Introduction Liquid Crystalline Elastomers (LCEs) are loosely cross-linked networks that have rigid-rod, LC molecules incorporated directly into the polymer backbone (i.e., “main-chain” LCEs) or attached to the polymer back- bone via a flexible spacer group (i.e., “side-chain” LCEs). These materials typically have low glass transition temperatures (T g < 35 °C) and low moduli (E ≈ 0.5 MPa), deform at nearly constant volume, and exhibit liquid crystalline phase transitions due to the high mobility of the network strands. The networks used in this study were ordered locally into a smectic phase, on a scale less than 1 µm; i.e., the molecules exhibited orientational order along a unit vector called the direc- tor, n ˆ , in addition to positional order in two-dimensional planes or sheets (Figure 1a). On a larger scale (>1 µm), the LCEs exhibited both a continuous reorientation of n ˆ as well as abrupt discontinuities in n ˆ , line defects called disclinations. This unique isotropic “polydomain” microstructure (Figure 1b) results in a finely scaled Schlieren texture, 1 when viewed under the polarizing optical microscope. These brushes emanate from dis- clinations and are visible where n ˆ is oriented along the polarizer or analyzer axes. The characteristic length scale of the texture or LC “domain size” can be ap- proximated by the mean distance between disclinations, and since this parameter is typically about the wave- length of light, the material appears opaque in bulk form. The unique properties of LCEs 2,3 originate from the coupling between an applied mechanical stress and the LC director(s). One of the most remarkable character- istics is the ability to undergo a polydomain-to-mono- domain transition; i.e., stress-induced macroscopic ori- entation leading to the formation of a “liquid single- crystal elastomer.” The average degree of orientation of the LC domains with respect to the tensile direction can be represented by the orientation parameter, S, which is defined according to eq 1: where  is the angle between the individual domain directors, n ˆ , and the tensile direction and <> denotes an average of cos 2  over all the LC domains. S ) 1 for a perfectly uniaxially oriented sample; S ) 0 for a completely random, isotropic sample; and S )-0.5 for a planar orientation. The polydomain-to-monodomain transition is a well- known, universal phenomenon and has been reported for side-chain LCEs based on siloxanes, 4,5 side-chain polyacrylate and polymethacrylate networks, a main- chain polymalonate, a combination of these to form side- chain/main-chain networks, 6-18 a main-chain, epoxide- based network, 19 and main-chain, semirigid, epoxide- based networks. 20,21 Most experimental work to date has focused on side-chain, nematic LCEs. In 1989, Scha ¨ tzle et al. 10 conducted uniaxial tensile experiments on a side-chain, nematic, methacrylate-based elastomer just below the nematic-to-isotropic (“clearing”) transition temperature. They found a three-region nominal stress- versus-nominal strain curve and a unique relationship between orientation parameter, S, and nominal stress (Figure 2). † Current address: Department of Polymer Chemistry, Univer- sity of Groningen, Nijenborgh 4, 9747 AG Groningen, The Neth- erlands. ‡ Current address: Fachbereich Chemie und Pharmazie, Uni- versita ¨ t Mainz, 55099 Mainz. § Current address: Materials Department, University of Santa Barbara, Santa Barbara, California 93106-5050. Figure 1. A liquid crystalline elastomer with (a) smectic-type local order and a (b) polydomain microstructure (as viewed under the polarizing optical microscope). S ) 3〈cos 2 〉 -1 2 (1) 8531 Macromolecules 1998, 31, 8531-8539 10.1021/ma971423x CCC: $15.00 © 1998 American Chemical Society Published on Web 11/10/1998