Acoustic Excitations in a Self-Assembled Block Copolymer Photonic Crystal

High resolution Brillouin light scattering can sensitively detect acoustic phonons in concentrated solutions of a high molecular weight poly(styrene-$b$-isoprene) symmetric copolymer in toluene. This block copolymer lamellar forming system also possesses a photonic stop band in the visible spectrum. Based on the low but finite contrast in mechanical properties between the styrene and isoprene components and taking into account the geometrical characteristics of the layered microstructure, we calculate the acoustic band structure and represent the observed acousticlike and opticlike phonons.