Switchable tack in side-chain liquid crystalline polymers

Abstract

With the aim of developing a material with switchable tack properties, a side-chain liquid crystalline polymer containing a poly(oxyethylene) backbone and n-heptylsulfonylmethyl side chains was synthesized. The inner structure, surface properties, and tack behavior of this polymer film were investigated. An X-ray reflectivity study of a film of the polymer coated onto a silicon wafer showed that, upon annealing above the glass transition temperature, the film has a lamellar structure in which the ordered layers are parallel to the substrate surface. The layer thickness is roughly twice the length of the fully extended side chain, indicating a double-layered structure of the polymer with side chains normal to the polymer backbone having an almost all-trans conformation. The contact angle decreases abruptly at the isotropic transition temperature, which results from the disappearance of the smectic order. The coiling of the polymer backbone and disordering of side chains occur above the isotropic transition temperature. NEXAFS spectroscopy and AFM results confirmed that at temperatures below the isotropic transition temperature the side chains in the top layer are oriented such that they are almost perpendicular to the surface, which results in the packing of the CH3 groups at the surface and hence a very low surface energy. A remarkable change in tack properties is observed at the smectic-to-isotropic transition temperature. The dramatic change in tack properties, which occurs within a narrow temperature range, is due to the change in the surface energy and viscoelastic behavior of the film. This reversible and dramatic change in the tack force over a narrow temperature range could potentially be exploited in applications that require materials with switchable tackiness.