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

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. [ABSTRACT FROM AUTHOR]