title: Better Actuation Through Chemistry: Using Surface Coatings to Create Uniform Director Fields in Nematic Liquid Crystal Elastomers.
abstract: Controlling the molecular alignment of liquid crystal monomers (LCMs) within nano- and microstructures is essential in manipulating the actuation behavior of nematic liquid crystal elastomers (NLCEs). Here, we study how to induce uniformly vertical alignment of nematic LCMs within a micropillar array to maximize the macroscopic shape change using surface chemistry. Landau-de Gennes numerical modeling suggests that it is difficult to perfectly align LCMs vertically in every pore within a poly(dimethylsiloxane) (PDMS) mold with porous channels during soft lithography. In an untreated PDMS mold that provides homeotropic anchoring of LCMs, a radially escaped configuration of LCMs is observed. Vertically aligned LCMs, a preferred configuration for actuation, are only observed when using a PDMS mold with planar anchoring. Guided by the numerical modeling, we coat the PDMS mold with a thin layer of poly(2-hydroxyethyl methacrylate) (PHEMA), leading to planar anchoring of LCM. Confirmed by polarized optical microscopy, we observe monodomains of vertically aligned LCMs within the mold, in agreement with modeling. After curing and peeling off the mold, the resulting NLCE micropillars showed a relatively large and reversible radial strain (∼30%) when heated above the nematic to isotropic transition temperature.
authors.affiliation: Department of Materials Science and Engineering, University of Pennsylvania , 3231 Walnut Street, Philadelphia, Pennsylvania 19104, United States.
authors.forename: Yu
authors.initials: Y
authors.lastname: Xia
authors.affiliation: Department of Materials Science and Engineering, University of Pennsylvania , 3231 Walnut Street, Philadelphia, Pennsylvania 19104, United States.|Engineering Directorate, Lawrence Livermore National Laboratory , 7000 East Avenue, Livermore, California 94550, United States.
authors.forename: Elaine
authors.initials: E
authors.lastname: Lee
authors.affiliation: Department of Materials Science and Engineering, University of Pennsylvania , 3231 Walnut Street, Philadelphia, Pennsylvania 19104, United States.
authors.forename: Hao
authors.initials: H
authors.lastname: Hu
authors.affiliation: Department of Materials Science and Engineering, University of Pennsylvania , 3231 Walnut Street, Philadelphia, Pennsylvania 19104, United States.|Department of Physics and Astronomy, University of Pennsylvania , 209 South 33rd Street, Philadelphia, Pennsylvania 19104, United States.
authors.forename: Mohamed Amine
authors.initials: MA
authors.lastname: Gharbi
authors.affiliation: Paulson School of Engineering and Applied Sciences, Harvard University , 29 Oxford Street, Cambridge, Massachusetts 02138, United States.
authors.forename: Daniel A
authors.initials: DA
authors.lastname: Beller
authors.affiliation: Institute of Organic Chemistry, Johannes Gutenberg-Universität Mainz , Duesbergweg 10-14, 55128 Mainz, Germany.
authors.forename: Eva-Kristina
authors.initials: EK
authors.lastname: Fleischmann
authors.affiliation: Department of Physics and Astronomy, University of Pennsylvania , 209 South 33rd Street, Philadelphia, Pennsylvania 19104, United States.
authors.forename: Randall D
authors.initials: RD
authors.lastname: Kamien
authors.affiliation: Institute of Organic Chemistry, Johannes Gutenberg-Universität Mainz , Duesbergweg 10-14, 55128 Mainz, Germany.
authors.forename: Rudolf
authors.initials: R
authors.lastname: Zentel
authors.affiliation: Department of Materials Science and Engineering, University of Pennsylvania , 3231 Walnut Street, Philadelphia, Pennsylvania 19104, United States.
authors.forename: Shu
authors.initials: S
authors.lastname: Yang
keywords: actuator; liquid crystal elastomer; monodomain; soft lithography; surface chemistry