23—MOISTURE RELATIONS OF CELLULOSE: VII. THE HEAT OF WETTING OF DRY VISCOSE RAYON AND ITS TRANSITION TEMPERATURE

Abstract

The heat of wetting in water of dry stabilized viscose rayon has been measured al a series of temperaturcs between 1°C and 50°C. Comparison with the results obtained previously with cotton cellulose shows that:

1. The heat of wetting lemperaiure curves of the two varieties of cellulose are similar, each being represented by two straight lines meeting at about 23°C, the line on the lower-temperature side being steeper. The two curves thus indicate the same transition temperature for the rayon and cotton samples used.

2. The evolution of heat takes practically the same time for apparent completion with both rayon and cotton and is markedly slower the lower the temperature, indicating that the kinetics of the swelling process is likely to be similar with regenerated and native cellulose.

3. The reproducibility of heat measurements was poorer with rayon than with cotton, especially at the higher temperatures, indicating the variability of rayon.

4. The ratio of the heats of wetting of dry rayon and dry cotton at various temperatures is practically constant, being 2·11, which may only approximately equal the ratio of amorphous fractions in the two varieties of cellulose.

The effect of temperature on the factors governing the heat of wetting is discussed in the light of the apparent thermal expansion of cellulose in a non-swelling liquid and in water which gives a nearly bulk swelling effect. This indicates that the binding energy of cellulose increases more rapidly with rise of temperature above the transition temperature than below it, an effect which itself influences the variation with temperature of other factors such as adsorptivity of cellulose and swelling action of adsorbed water. Sorptivity decreases and swelling action increases with rise of temperature, both effects tending to decrease the heat of wetting, more rapidly below the transition temperature than above it. The increase in binding energy with rise of temperature tends to increase the heat of wetting. The result of such changes in the thermal coefficients of these factors at the transition temperature is a more rapid decrease of the heat of wetting below than above the transition temperature.