Off-axis photoelasticity by anisotropic molecular deformation of uniaxially aligned cellulose nanofiber films

We report the angle dependence of the photoelasticity for unidirectionally aligned films of bacterial cellulose nanofibers (CNFs) by applying the off-axis stress at 0 degrees-90 degrees with respect to the CNF orientation. The photoelastic coefficient was positive when the stress direction was close to the CNF axial direction, and it was negative when the stress was applied close to the lateral direction. The positive to negative photoelasticity was also observed in the off-axis photoelasticity of the cellophane film and in the density functional theory (DFT) calculations for the stretching between two atoms in the cellobiose model. On the other hand, unlike the cellophane film, the CNF film showed a positively and negatively asymmetric photoelasticity of 5 to 10 TPa 1, reflecting the anisotropy of the crystal modulus tensor. We found that the presence or absence of cellulose crystals controls the anisotropy of photoelasticity by constraining the molecular deformation.