Temperature-dependent photonic bandgap in a self-assembled hydrogen-bonded liquid-crytalline diblock copolymer

We take advantage of self-assembly in a hierarchically structured, hybrid material to develop photonic bandgaps in the visible which may be systematically tuned by application of thermal or electric fields. Hydrogen bonding between a host polymer and a guest small molecule is used to augment the molecular weight of an appropriately selected coil-coil diblock copolymer to bring the microdomain structure onto the length scale needed for significant interaction with visible light. Further, the use of liquid-crystal-mesophase-forming moieties as the guest hydrogen-bonding units adds functionality to the system as the optical properties of the liquid-crystalline domains can be modulated by external stimuli. We use hydrogen bonding to sequester varying amounts of imidazole terminated mesogens within the acid domains of a poly[styrene-block-poly-(methacrylic acid)] (PS-b-MAA) diblock copolymer. The resulting PS-b-MAA/LC side-group liquid crystalline diblock copolymer possesses a photonic bandgap in the green with the exact location and structure of the gap dependent on the composition of the system. Here, we discuss the structure and optical properties of these materials as a function of their composition and the response of the optical properties to temperature. Varying the order parameter of the LC domains by heating into the isotropic state changes the peak reflectivity by 40 nm. resulting in a color change from green to orange.