where, φ (%) is the weight fraction of actual rubber in
crosslinked sheet, α (%) is the weight loss of the rubber
during swelling, ρs (g/cm3) is the density of the solvent
(toluene: ρs = 0.872 g/cm3), ρr (g/cm3) is the density of the
rubber (NR: 0.92 g/cm3).