Deformation-induced mechanical anisotropy of gelatin films

Deformation-induced mechanical and structural anisotropy has been analyzed using a gelatin film model. Specifically, gelatin films cast from water and 2,2,2-trifluoroethanol (TFE) are stretched at various draw ratios, locked-in the extended conformations, crosslinked by glutaraldehyde, and then mechanically analyzed in the longitudinal and transverse directions. Notably, the longitudinal modulus of the gelatin films that are cast from water or TFE linearly increases with draw ratio λ, accompanied by a substantial reduction in transverse modulus. When normalized by the Young's modulus of undrawn films, the transverse moduli as a function of draw ratio follow a power-law relationship E/E0∝λα, where α≈−2.5 for the water-cast films and α≈−1.5 for the TFE-cast films. A Scanning Electron Microscopic analysis of the water- and TFE-cast films shows the formation of wrinkles and microcracks oriented in the drawing direction due to large deformations. Fourier Transform Infrared spectroscopy reveals the dominance of aggregated strands and β-sheets in the secondary structures of the gelatin films and the deformation-induced renaturation of triple-helical structures in the films. A model is thus proposed relate the observed mechanical anisotropy of drawn gelatin films to deformation-induced structural anisotropy.