The influence of the rate of dilution of the mesogens along the polymer backbone on the conformation and the structure of two different side-on mesogens polysiloxanes are studied using neutron-scattering techniques. The average backbone conformation in the nematic and smectic C phases is deduced from small-angle neutron-scattering measurements. An inversion in the backbone conformation from prolate in the nematic phase to highly oblate in the smectic phase is evidenced. Distribution profiles of polysiloxane main chains are calculated from neutron diffraction experiments in the smectic phase. A transition in the confinement of the main chain within the layers has been demonstrated: the polymer backbone can be segregated between the layers, but also present in the middle part of the mesogenic layers.