Stress transfer quantification in gelatin-matrix natural composites with tunable optical properties.

This work reports on the preparation and characterization of natural composite materials prepared from bacterial cellulose (BC) incorporated into a gelatin matrix. Composite morphology was studied using scanning electron microscopy and 2D Raman imaging revealing an inhomogeneous dispersion of BC within the gelatin matrix. The composite materials showed controllable degrees of transparency to visible light and opacity to UV light depending on BC weight fraction. By adding a 10 wt % fraction of BC in gelatin, visible (= 550 nm) and UV (= 350 nm) transmittances were found to decrease by ∼35 and 40%, respectively. Additionally, stress transfer occurring between the gelatin and BC fibrils was quantified using Raman spectroscopy. This is the first report for a gelatin-matrix composite containing cellulose. As a function of strain, two distinct domains, both showing linear relationships, were observed for which an average initial shift rate with respect to strain of -0.63 ± 0.2 cm^-1%^-1 was observed, followed by an average shift rate of -0.25 ± 0.03 cm^-1%^-1. The average initial Raman band shift rate value corresponds to an average effective Young's modulus of 39 ± 13 GPa and 73 ± 25 GPa, respectively, for either a 2D and 3D network of BC fibrils embedded in the gelatin matrix. As a function of stress, a linear relationship was observed with a Raman band shift rate of -27 ± 3 cm^-1GPa^-1. The potential use of these composite materials as a UV blocking food coating is discussed.