Stress relaxation studies of chemically crosslinked gelatin films

Stress relaxation, carried out in 85 : 15 methyl alcohol–water mixture at 60°C., of gelatin films crosslinked with oxystarch or difluorodinitrobenzene may be represented by the sum of three exponential rates of stress decay. Fresh formaldehyde-crosslinked films relax rapidly and only require two such terms. The second term is shown to be due to the breaking of crosslinks. All excepting 10% of the broken crosslinks do not re-form in unstressed positions as the formaldehyde is easily able to diffuse out of the layer. This 10% is suggested as being due to crosslinks involving a histidine residue. A large percentage of the broken oxystarch crosslinks do re-form because the oxystarch is not easily able to diffuse out of the layer.