Relaxation dynamics in single polymer microcapsules probed with laser-generated GHz acoustic waves

Using an ultrafast optical technique we probe coherent phonon propagation inside a single microcapsule composed of a nanometric polymer shell made of poly(lactide-co-glycolide) (PLGA) encapsulating a liquid perfluorooctyl bromide (PFOB) core. Longitudinal storage and loss moduli are measured simultaneously in the transparent shell and core at frequencies similar to 18 and similar to 4 GHz, respectively, using time-resolved Brillouin spectroscopy. A time-frequency analysis allows determination of the thicknesses of several capsule shells ranging from 620 down to 80 nm. Comparison with lower frequency data shows a weak power-law frequency dependence of phonon attenuation in the PLGA shell, the signature of thermally activated processes in glasses.