Partially carboxymethylated feather keratins. 2. Thermal and mechanical properties of films

Free cysteine thiol groups of keratin extracted from chicken feathers were partially carboxymethylated with iodoacetic acid (25-76% cysteine modificat ion). Stable dispersions were used for the preparation of films by solution casting. Glycerol. was used as a plasticizer (0.05-0.47 g/g of keratin), a nd films were stored at a constant relative humidity (20, 30, 50, 70, or 90 %). The degree of crystallinity in the films was higher when more cysteine residues were carboxymethylated. The films displayed an optimum in mechanic al properties at similar to 50% cysteine carboxymethylation. The tensile st rength at this optimum was 25 MPa, the E modulus, 350 MPa, and the elongati on at break, 50%. Probably, this optimum was the result of both a decreasin g amount of disulfide bonds and an increasing degree of crystallinity for h igher degrees of cysteine-modification. The influences of a higher amount o f glycerol and of different storage conditions on the mechanical properties of films from keratin with a defined degree of cysteine modification were also investigated.