The structure and dynamics of the poly(dimethylsiloxane) (PDMS) segregated nanophase in block copolymers with polystyrene (PS) has been analyzed in detail by combining wide- and small-angle X-ray diffraction, infrared absorption, differential scanning calorimetry, and dielectric relaxation spectroscopy. In particular, we have investigated PS-rich PS-PDMS diblocks where the minority PDMS phase is segregated into cylindrical and spherical regions with diameter in the range 10-20 nm and compared the results with those previously, reported on symmetric diblocks with lamellar phases of similar size. It is found that in these highly segregated cylindrical and spherical regions in the copolymers PDMS presents a rather unexpected behavior as probed by X-ray diffraction, infrared absorption, differential scanning calorimetry, and dielectric relaxation spectroscopy. Structural techniques indicate poor packing of the PDMS segments, whereas calorimetric experiments evidence both Strong suppression of PDMS crystallization and significant-reduction of the glass transition temperature range. Connected with that, the dielectric relaxation probing the PDMS segmental dynamics is much more heterogeneous and markedly-faster not only than that observed for PDMS in lamellar nanophases but more strikingly than that of PDMS melt.