Segmental dynamics in dendrimers with perfluorinated end groups: A study using quasielastic neutron scattering

A series of carbosilane dendrimer generations, GxRF6 (x = 1-3) with perfluorohexyl (-C6F13) groups on the surface and one dendrimer G3 without these end groups, have been studied using X-ray scattering and quasielastic neutron scattering. The GxRF6 form generation dependent superstructures as a result of the microphase separation between the end groups and the carbosilane core. The helical end groups tend to arrange in layers between the carbosilane domains. The dynamic structure factor was measured with two backscattering spectrometers (IN10, IN16). For GxRF6 it was composed of two separable contributions referring to the segmental diffusion in the dendrimer core and the rotational diffusion of the end groups. G3 shows only one component. The segmental diffusion in the G3RF6 is reduced by a factor of 7.4 compared to G3, demonstrating the influence of the end groups on the segmental diffusion. A universal length controls the segmental diffusion of the four dendrimers. It gives rise to a characteristic scattering vector dependence of the quasielastic line width. Increasing generation number slows down the segmental diffusion and extends the lifetime of a local dynamical process. The rotational diffusion of the end groups is not strongly affected by x. It is less strongly dependent on temperature than the segmental diffusion.