Here, σ 0 is the membrane's prestress. The biaxial modulus Y was extracted as the slope of the linear functions fitted to the data with ascending pressure values (Figure ). Comparing the stress-strain data in Figure for ascending and descending pressure reveals only minor hysteresis indicating little deviation from purely elastic behavior. In agreement with previous studies on other types of nanocomposite membranes, we attribute this deviation to retarded elastic deformation and pure viscous creep. Figure (Supporting Information, Section S3) presents the stress-strain characteristics of membrane M1 and confirms that the two membranes M1 and M2 had very similar elastic properties. The mechanical data of both membranes are presented in Table . For both membranes the biaxial modulus Y was ∼33 GPa. Using a Poisson ratio of 0.33, this value translates into a Young's modulus of