[5p-C13-1~15] 12.3 Functional Materials and Novel Devices

[5p-C13-8] Liquid-crystalline polymer brushes prepared by surface segregation and self-assembly

〇(D)Koji Mukai¹, Mitsuo Hara¹, Shusaku Nagano², Takahiro Seki¹ (1.Nagoya Univ., 2.Ngaoya Univ. VBL)

Keywords:surface segregation, liquid-crystalline polymer, polymer brush

Polymer brush is an attractive topic to design unique molecular structures and functions in polymer material surface. Approaches based on the synthetic technique via surface-initiated living polymerization have provided precise control of high-density polymer brushes having a high extended chain conformation oriented vertically from a substrate surface. We present herein that a new approach to form the high-density polymer brushes using surface segregation and self-assembly of liquid-crystalline block copolymers. The surface segregation structure of liquid-crystalline (LC) azobenzene block copolymers with polystyrene (PS-b-PAz) exhibits the high-density LC polymer brushes at a surface of binary mixture films composed of base polystyrene (PS) with the addition of the PS-b-PAz. Binary mixture films of PS-b-PAz and PS (PS-b-PAz/PS films) were prepared by spin coating from a chloroform solution and annealed at 130 °C for 24 h. The mixing ratio of the PS-b-PAz and the PS were 5:95 or 10:90 by weight. The surface segregation structure of the resulting PS-b-PAz/PS films was evaluated by water contact angle measurement and X-ray photoelectron spectroscopy (XPS). The results showed that the PAz block with lower surface energy was segregated preferentially at the free surface. Cross-sectional transmission electron microscope (TEM) images of the PS-b-PAz/PS films stained with RuO₄ were captured. In the images, a skin layer assigned to the surface segregated PAz block was observed at the topmost surface of the mixture film. The thickness of the observed PAz layers linearly increases with polymerization degree (N) of the PAz block chain. From the relationship between the PAz layer thickness and the N, the length of the PAz block chains can be estimated to reach approximately 80% of the all-trans-zigzag conformation of PMMA, which indicates that the surface segregated PAz chains adopt highly stretched conformation. The LC structure and orientation were studied by grazing angle incidence small angle X-ray scattering (GI-SAXS) measurement. The X-ray scatterings due to the LC layer were observed only in the in-plane direction at q = 1.83 nm^–1 (d = 3.4 nm). The in-plane scattering indicates that the smectic lamellar of PAz was formed vertically to the surface, and planar orientation for PAz was induced in the film. The orientation structure of the PAz was similar to that of a high-density LC polymer brush prepared by the surface-initiated polymerization method. In this way, the LC polymer brush of PAz block is readily formed by the simple surface segregation process.