Different nanocomposites based on isotactic polypropylene (iPP) and mesoporous SBA-15 silica have been prepared by melt extrusion as an attempt of incorporating iPP within the nanometric SBA-15 pores by the action of shear forces during processing. The observed inclusion of iPP chains in the SBA-15 leads to a change in the gas transport properties of the resultant iPP/SBA-15 membranes together with the improvement of their ultimate mechanical performance. Existence of confinement effects is observed, consequently, by the exhibited transport properties and is confirmed through real-time variable-temperature Small Angle X-ray Scattering (SAXS) experiments with synchrotron radiation. The former indicates that the mesostructure of silica is full, i.e., these particles essentially act as fillers. The latest has been proved by the presence of a discontinuity in the intensity of the (100) SBA-15 diffraction, which turn SAXS measurements at variable temperature into a valuable means to corroborate the development of iPP crystallites within SBA-15 channels. Moreover, SAXS profiles provide information about the most probable long spacing of chains located outside the SBA-15 nanospaces. On the other hand, changes in the beta viscoelastic relaxation have been correlated to the influence of SBA-15 in the iPP amorphous regions. All the structural features have been crucial for the understanding the mechanical response shown by these nanocomposites. Mesoporous particles exert a reinforcing role and reduce the deformation capacity in the ultimate materials as their content is increased.