  We investigate the local viscosity of a polymer glass around its glass
transition temperature using environment-sensitive fluorescent molecular rotors
embedded in the polymer matrix. The rotors' fluorescence depends on the local
viscosity, and measuring the fluorescence intensity and lifetime of the probe
therefore allows to measure the local free volume in the polymer glass when
going through the glass transition. This also allows us to study the local
viscosity and free volume when the polymer film is put under an external
stress. We find that the film does not flow homogeneously, but undergoes shear
banding that is visible as a spatially varying free volume and viscosity.
