Order and dynamics of poly(styrene-b-1,4-isoprene), P(S-b-I), diblock copolymers in nanometer thin layers with different isoprene volume fractions (f(PI)) and identical molecular weight of the styrene blocks are studied by a combination of grazing-incidence small-angle X-ray scattering (GISAXS), atomic force microscopy (AFM), and broadband dielectric spectroscopy (BDS). GISAXS and AFM reveal randomly oriented lamellar structures in the films and a parallel orientation at the top surface, respectively. Using BDS, three well separated relaxation processes are detected: (i and ii) the dynamic glass transitions (segmental mode) in the styrene and isoprene blocks, respectively, and (iii) the normal mode relaxation representing fluctuations of the isoprene chain as a whole or parts of it. While the first two do not show any thickness dependence in their spectral positions, the latter becomes faster with decreasing sample thickness. This reflects the difference in the length-scale on which the molecular fluctuations take place.