  We investigate how the microstructure of a colloidal polycrystal influences
its linear viscoelasticity. We use thermosensitive copolymer micelles that
arrange in water in a cubic crystalline lattice, yielding a colloidal
polycrystal. The polycrystal is doped with a small amount of nanoparticles, of
size comparable to that of the micelles, which behave as impurities and thus
partially segregate in the grain boundaries. We show that the shear elastic
modulus only depends on the packing of the micelles and does not vary neither
with the presence of nanoparticles nor with the crystal microstructure. By
contrast, we find that the loss modulus is strongly affected by the presence of
nanoparticles. A comparison between rheology data and small-angle neutron
scattering data suggests that the loss modulus is dictated by the total amount
of nanoparticles in the grain boundaries, which in turn depends on the sample
microstructure.
