Nematic Elastomer Fiber Actuator Jawad Naciri,* ,† Amritha Srinivasan, † Hong Jeon, † Nikolay Nikolov, † Patrick Keller, ‡ and Banahalli R. Ratna † Center for Bio/Molecular Science and Engineering, Naval Research Lab, 4555 Overlook Avenue SW, Code 6950, Washington, D.C. 20375, and Laboratoire Physico-Chimie Curie, CNRS-UMR 168, Institut Curie-Section de Recherche, 11 rue P. et M. Curie, 75231 Paris Cedex 05, France Received July 2, 2003; Revised Manuscript Received September 3, 2003 ABSTRACT: We report the synthesis and physical studies of a liquid crystalline elastomer fiber consisting of two side-chain liquid crystalline acrylates and a nonmesogennic comonomer side group that acts as a reactive site for cross-linking. The terpolymer was synthesized by radical polymerization, and the cross- linking of the network was achieved by using a diisocyanate unit. The fiber formed shows good liquid crystal alignment texture under a cross-polarizer microscope. Thermoelastic response shows strain changes through the nematic-isotropic phase transition of about 30-35%. A retractive force of nearly 300 kPa was measured in the isotropic phase. Static work loop studies show the viscoelastic losses in these materials to be very small. We also present preliminary studies on the effect of doping carbon nanotubes on the induced strain at the nematic-isotropic transition. Introduction There has been considerable effort to develop human- made actuator materials that can mimic muscle per- formance. 1,2 The developmental goal is to generate large mechanical actuation induced by external stimuli such as electric field, temperature, and light. Because of their anisotropic orientational symmetry in combination with rubber elasticity, liquid crystal (LC) elastomers are promising materials for applications in the field of sensors and actuators. The potential for liquid crystal- line to exhibit unusual properties was first suggested by de Gennes. 3 Subsequently, such elastomers have been prepared and their resultant properties investi- gated. 4-19 In general, the elastomers most frequently studied have been those based on side-chain liquid crystalline polymers rather than the main-chain sys- tems considered originally by de Gennes. These elas- tomers exhibit anisotropic shape change under applied fields 5,19,20 as they go through phase transitions and retain network memory 9 which enables them to revers- ibly contract and extend. There are two basic ap- proaches to prepare LC elastomers: the first approach developed by Mitchell and co-workers 21 involves cross- linking an acrylate polymer prealigned in a magnetic field. Such samples are found to show complete recovery from their global orientation on cooling to the nematic phase from the isotropic phase. The second method due to Finkelmann and co-workers 5,20 involves a two-step cross-linking strategy of a siloxane liquid crystal poly- mer. The first stage involves a lightly cross-linking of the polymer while applying a stress field. Subsequently, a second cross-linking reaction is performed which fixes the uniaxial alignment. By this method LC elastomers of large dimensions with permanent alignment and highly anisotropic mechanical properties were produced. An alternative approach to the use of chemical reactions to produce intermolecular cross-linking is photo-cross- linking. 22-24 Although such materials show promises for the generation of elastomers, there may be a number of problems associated with their use. 22 The coupling between the liquid crystalline side group and the polymer backbone is critical for the thermo- strictive behavior of elastomeric materials. Theoreti- cal 25,26 and experimental 27,28 studies have shown that orientational order of the side groups will be ac- companied by some level of orientational order in the polymer backbone. We have chosen to study elastomers with laterally affixed liquid crystal mesogens (Scheme 1), since they have been shown to exhibit large backbone anisotropy. 28,29 In our previous paper 19 we presented detailed studies of mechanical properties of two LC elastomer films. These networked films exhibited musclelike physical properties with strains of 35-40% and blocked stress values of the order of 200 kN/m 2 . The subject of this paper is to expand our previous work to the preparation of ordered fibers. The idea of preparing artificial muscles in the form of fibers is based, in part, on its similarity to the way the natural muscles are organized in bundles of fibers. It is well- known that the LC mesogens are spontaneously ordered during the spinning of the fiber. 30,31 Therefore, in the elastomer with side-on attachment of the liquid crystal mesogen, one expects the orientational order of the mesogen as well as the polymer backbone to be along the fiber axis. Hence, we expect the contraction to occur along the fiber axis similar to what occurs in natural muscle fibers. One can envisage using bundles of these fibers in devices, the number of fibers in each bundle † Naval Research Lab. ‡ Institut Curie-Section de Recherche. * Corresponding author: e-mail Jnaciri@ccs.nrl.navy.mil, Tel 202-404-6056. Scheme 1. Concept of Liquid Crystal Elastomer as Artificial Muscle 8499 Macromolecules 2003, 36, 8499-8505 10.1021/ma034921g CCC: $25.00 © 2003 American Chemical Society Published on Web 10/09/2003