Surface morphology and ordering in side-chain liquid crystal diblock and homopolymer ultrathin films

Abstract

Microphase-separated morphologies of side-chain liquid crystal (LC) diblock copolymers have been examined as substrate-supported thin films. The film thicknesses ranged from 0.5 to 10 times the block copolymer period (L). These novel materials are of interest because they exhibit self-oriented electro-optical properties in the bulk state, and may exhibit interesting field dependent nanometer-scale ordering as thin films. A primary challenge in taking advantage of these properties involves the fundamental understanding and control of the nano-domain morphology, which is strongly affected by the substrate, block copolymer properties, and LC alignment within the domains. In these studies, an amorphous side-chain LC diblock polymer system has been synthesized containing a side-chain mesogen with a chiral alkyl end. Experiments were conducted using Atomic Force Microscopy, Transmission Electron Microscopy, and X-ray Specular Reflectometry, to determine the complex surface morphology. Contact angle measurements and angle-resolved X-ray Photoelectron Spectrometry were also conducted to determine LC orientation at different interfaces. From this information, final models of complex morphologies were derived for different block copolymer compositions, including lamellar, cylindrical, and new smectic-bilayered morphologies. To better understand the ordering of the LC mesogen at different interfaces, LC homopolymer films were studied. LC anchoring was planar at the substrate, but homeotropic at the air interface due to exposure of the non-polar mesogen tail. Block copolymer thin films were then examined for compositions ranging from 40% to 85% weight fraction of the LC block.

(cont.) Due to the wetting properties of the LC diblock copolymer, novel LC terraces were observed on the top surfaces corresponding to single smectic layer spacing. For diblock thin films, both blocks coexist at the air surfaces to form a mixed surface. Hybrid morphologies, consisting of both symmetrical and anti-symmetrical wettings, were noted for the lamellar morphology with film thicknesses between 0.5 and 1.5Lo. The surface properties are determined by LC orientation, which depends on the film thickness. Homeotropic anchoring of the smectic LC mesogens at the air interface was observed in both the homopolymer and the block copolymers, resulting in smectic terraces on the top surfaces of the films. In thicker films, when the influence of the substrate was decreased, the lamellar orientation was found to undergo a transition from parallel stacking near the substrate to perpendicular arrangement at the air surface. Effects of annealing and solvent exposure on the resulting thin-film behavior were also addressed.