From Spherical Mesopores to Worm‐Shaped Mesopores: Morphology Transition in Titania–Polystyrene‐ b ‐poly(ethylene oxide) Composite Films with Increasing Sol–Gel Reaction Time

Abstract

A morphology transition from spherical mesopores to worm-shaped mesopores within titania block copolymer composite thin films has been observed by varying the sol–gel reaction time from 40 min to 48 h in the four-component templating system of polystyrene-b-poly(ethylene oxide) (PS-b-PEO), 1,4-dioxane, concentrated HCl, and titanium tetraisopropoxide (TTIP) with a PS-b-PEO mass concentration of 0.25 wt.-%. The impact of the sol–gel reaction time on the local structure, long-range lateral structure, and vertical structure of the as-prepared, calcined, and UV-degraded thin films as well as the structural changes in solution have been systematically investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM), grazing-incidence small-angle X-ray scattering (GISAXS), X-ray reflectivity (XRR), and dynamic light scattering (DLS). With sol–gel reaction times of up to 5 h, hexagonally organized spherical micelles are present within the as-prepared composite films, in which the core of the spherical micelles is composed of the polystyrene (PS) block, and the corona is composed of the poly(ethylene oxide)–titania (PEO–titania) hybrid. Upon calcination or UV exposure, ordered mesoporous structures are formed owing to the removal of the PS block. With the sol–gel reaction time extended to 25 and 48 h, worm-shaped micelles appear, and their quantity increases with increasing sol–gel reaction time. Worm-shaped mesopores are formed by calcination or UV degradation. The GISAXS results prove that the local structural changes are representative over a macroscopic scale. The XRR results suggest that with the sol–gel reaction time extended to 48 h there is an additional thin layer beneath the mesoporous titania layer owing to the presence of a large amount of worm-shaped micelles. The results of the DLS studies imply that the morphology transition from spherical micelles to worm-shaped micelles is caused by a fusion process of the spherical micelles in solution.