Internal dynamics in colloidal PNIPAM microgel particles immobilised in mesoscopic crystals

Internal dynamics of colloidal microgels based on N-isopropyl-acrylamide is investigated by means of neutron spin-echo spectroscopy. To avoid interference of the relaxation of interest with the centre of mass diffusion, the microgels were immobilised in a colloidal crystal. Due to the q-range used in these quasi-elastic neutron scattering experiments, it is possible to obtain nearly completely decaying ensemble averaged intermediate scattering functions 〈ISF〉E. These can be fitted by single exponential forms, simplifying largely the data analysis compared with photon correlation spectroscopy. This is due to the fact that for light q2〈ξ2〉→∞ can not be reached, whereas for neutrons this condition is approximately fulfilled. The obtained network diffusion coefficients decrease linearly with increasing cross-linker concentration. This is in contrast to previous results from photon correlation spectroscopy obtained for macroscopic poly(N-isopropyl-acrylamide) gels [Macromolecules, 29 (1996) 8746].