Nanomembranes are important class of nanomaterials, with significantly advancing applications in a wide range of applications, including molecular separations, energy conversion and storage, sensing, catalysis and biomedical applications such as wound dressing, owing to their nano-scale thickness and high aspect ratios. However, in order to exploit the unique features of self-supporting nanomembranes (transferability onto any arbitrary substrate, high aspect ratio and unique interfacial properties), they need to have sufficient macroscopic stabilities. Conventional mechanical enhancement approaches, such as use of nanoparticle fillers often face material compatibility problems, limiting the range of material selection. In this work, a simple one-step strategy for developing functional free-standing nanomembranes (FS-NMs) from blended types of organic/inorganic composites has been presented. Such blending approach offers atomic scale in-situ interaction of organic and inorganic structures, ensuring nanoscale stability in membranes. Here, a hydroxyl-terminated polyethylene glycol (PEG-OH) was premixed with different metal oxide precursors for sol-gel assisted membrane formation, and the mechanical properties of the resulting FS-NMs were compared. The gas separation behavior of mechanically stable nanomembranes was also discussed. The presented strategy offers an alternative approach to develop functional self-supporting nanomembranes.