Rigidity and Other Mechanical Properties

Dependence of Rigidity on Concentration. The early measurements of Leick (1904) showed that the rigidity of a gelatin gel was approximately proportional to the square of the concentration over a concentration range of about 10 to 30%. From later experiments Sheppard and Sweet (1921a) concluded that the rigidity was proportional to c , where n was equal to 2 for some samples and slightly less for others. 

Measurements by the method of transverse vibrations (Ferry, 1948) on gels prepared from a sample of ossein gelatin of = 45,000, at pH 7.0 in the presence of 0.16 M sodium chloride, showed that the rigidity was closely proportional to the square of the concentration at several different temperatures up to a concentration of about 50 g./liter (Fig. 10). Measurements on another sample of ossein gelatin with Af = 39,000, at pH 7 in the absence of salt, showed that the rigidity was almost proportional to the square of the concentration up to 160 g./liter. For 

Dependence of Rigidity on Temperature. The fact that the rigidity of a gelatin gel increases rapidly with decreasing temperature is well known, and is provided for, for example, in the careful temperature control specified for industrial tests of gel strength (Sheely, 1930). 

The values of G /c fall close to the curve throughout the temperature range shown. However, the fact that the melting point does depend on concentration means that the curves for different concentrations must diverge just below the melting point, where the ri dity becomes quite small. This region remains to be investigated in detail. 

Modulus of rigidity divided by square of concentration, plotted against temperature, for gels of a gelatin with Mt, — 45,000. Data of Feny (1948). 

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