Mechanical properties and transition temperatures of cross-linked oriented gelatin: 1. Static and dynamic mechanical properties of cross-linked gelatin

This study is an extension of previous work on cellulosics [(1994) Colloid Polym Sci 272:284, 393] that showed that unusually good mechanical properties can be obtained by drying a swollen network of semirigid chains in a state of strain. This novel approach is applied in this investigation to gelatin, because of its attractive environmental characteristics but poor mechanical properties in the unmodified form. Since drawing of non-crosslinked gelatin is not practical, crosslinking by formaldehyde was used, followed by swelling, drawing and drying at fixed length. Mechanical tests were performed in static and dynamic modes. In this way improvements of Young's modulus E, and stress at break σ_b, were determined as a function of gelatin concentration during drying. An increase in E and σ_b up to 2-3 times, and in the dynamic modulus E′ up to 6 times, was obtained when the draw ratio λ reached 4-5, after which E, E′, and σ_b were found to decrease. Such behavior is explained by the highest orientation being achieved at λ = 4-5, as proved by x-ray analysis. At λ = 10-20 the orientation is lost due to relaxation of chain segments, which is preceded by partial destroying of the network structure (chemical and physical), possibly via chain scission, but probably mostly by the pulling out of chains from crystallites. In any case, the mechanical properties become poor again. The improvements reported above were referred to the undrawn but crosslinked gelatin. Compared to the starting isotropic non-crosslinked material, the improvement is slightly higher. The observation that the improvements are less than those obtained for the cellulosics is explained by the coexistence of interpenetrating chemical and physical networks, which is typical of gelatin. This structural feature drastically reduces the orientability of the chains and the improvements that can be expected in the mechanical properties.