Creep of gelatin-water system was studied covering a wide range of concentration at several different temperatures. The sample studied is unfractionated and photographically inert alkali-processed hide gelatin with isoelectric point of pH4.8. Creep at higher concentrations was measured with automatically extension-recording apparatus or travelling microscope at definite humidities and definite temperatures, and that at lower concentrations with special gel elastometer. The strain values were limited to be less than 1%.
The result of the measurements suggests the existence of three concentration regions showing deformation with different mechanisms in mechanical behavior.
First, glassy state above 85% in concentration at 20°C. It is reasonable to surmise that water molecules are present at hydration centers of the amorphous region of gelatin. Creep compliance curves at various concentrations are superimposed on each other, and compose a smooth curve through usual horizontal shift along time axis.
Second, dispersion region. The concentration range at 20°C is from about 40 to 83%. Young's modules and creep compliance depend remarkably on concentration, and the dependency seems not very simple. The upward shift of creep compliance curves for gelatin at pH 4.8 maintaining its shape was found when concentration decreased, therefore, the composite curve was obtained through vertical shift along creep compliance logJ axis. Horizontal shift as well as vertical shift was necessary to superimpose creep compliance curves at pH6.5, but the former reducing factors were smaller than the latter one. Increasing amount of water saturates all available hydration centers in amorphous region of gelatin, then induces disturbance of crystallites, and then is supposed to decrease the degree of crystallinity remarkably. The large vertical shift above-mentioned is considered to relate with crystallinity changes according to the recent research on synthetic, crystalline polymers. The crystallites in this region may be micelle-like.
Third, the square law region. Elasticity modulus of gelatin gel is proportional to the square of gelatin concentration up to about 30% but from that to 40% the modulus is less than the value predicted from the square law. To superimpose the creep compliance curves, vertical shift along logJ axis was necessary. Here, the gel is essentially rubber-like, and the cross-linkages may be formed between two chain segments, caused by secondary bonds. Since the cross-linkages are evidently dissociated by decreasing gelatin concentration or arising temperature, vertical superposition may become possible.
In conclusion, the viscoelastic properties of gelatin not in glassy state are characterized by vertical shift of the curves denoting them concerning with concentration or temperature, and horizontal reducing factor log b is null or nearly null, otherwise not null but always smaller than vertical reducing factor. The different mechanisms of viscoelastic deformation in three regions in concentration above-mentioned result from the structural difference of gelatin.
