where WSW is the weight of the solvent-swollen film, WAD is the weight of the oven-dried film, ρp (=0.93 g/cm3) is the density of the polymer, and ρs (=0.87 g/cm3) is the density of the solvent. The cross-link density (ρc, mmol/cm3) was then calculated by using the Flory−Rehner equation: