Solvent-Free Phase Separation of Polystyrene-block-poly(2-hydroxyethyl methacrylate) Forming Freestanding Photonic Films

A solvent-free approach to the formation of freestanding photonic material from amphiphilic polystyrene-block-poly(2-hydroxyethyl methacrylate) (PS-b-PHEMA) is reported, where the application of shear force and pressure induces phase separation. This work demonstrates access to high molecular weight (HMW; >100 kg mol-1) PS-b-PHEMA with PHEMA contents up to 62 vol% using sequential anionic polymerization. By exploring hot pressing, the dependency of microstructure formation on temperature, pressure, and time is demonstrated using transmission electron microscopy and small-angle X-ray scattering measurements. Within 30 min, phase-separated block copolymer (BCP) films are obtained. Although no highly ordered equilibrium structures are formed, photonic properties are observed for PS-b-PHEMA films with molecular weights higher than 140 kg mol-1 and PHEMA contents between 20 and 51 vol%. The photonic properties are investigated by ultraviolet–visible (UV-vis) and fluorescence spectroscopy as well as confocal fluorescence microscopy. The BCP films exhibit tailored transmittance that is dependent on molecular weight and microstructure, making them suitable for UV and blue light filter applications. Also, structure-dependent reflection and fluorescence are demonstrated. Finally, the application in the field of sensors is addressed by demonstrating a reversible color change of BCP films with a co-continuous microstructure, achieved through polar solvent infiltration and evaporation.