21—MOISTURE RELATIONS OF CELLULOSE: VI—THE SPECIFIC VOLUME OF CELLULOSE AND ITS TRANSITION TEMPERATURE

Abstract

The apparent specific volumes of cotton, mercerized cotton, viscose rayon, cellobiose and starch in toluene and in dekalin and those of the three varieties of cellulose in water were measured by a dilatometric method between O°C and 50°C. The dekalin values are greater than the toluene values over the range of temperature covered by the experiments, the increments with the varieties of cellulose being greatest with mercerized cotton and least with cotton. A comparison of the volume contraction in sugar solutions with that in the cellulose–water system with respect to the specific volume of crystalline cellulose (X-ray data) leads to the conclusion that the swelling capacity of cotton fibres at 20°C is about 0·30 g of water per g of cellulose. While the apparent specific volume of different cellulose fibres in non-swelling media increases with increase in the amorphous fraction, the apparent specific volume in water follows the reverse order, but that of cotton appears to be too low owing to formation of cellulose hydrate I, which is estimated to have the formula C₆H₁₀O₅·1/4H₂O, in the crystalline portions of mercerized and regenerated cellulose. The apparent specific volume temperature curves of the three varieties of cellulose in any of the liquids used show a change in direction at 22°—25°C, in agreement with the temperature at which the heat of wetting temperature curve of cotton changes direction (23°C), thus confirming it to be the transition temperature of the variety used. The curve of crystalline cellobiose is linear throughout the range 0°—5O°C, while that of amorphous starch alters its slope (at about 3O°C), indicating that the change in direction of the curve is a property of the amorphous structure. The higher-temperature portions of the curves obtained with non-swelling liquids are, in general, steeper than the lower-temperature portions, indicating that the thermal coefficient of expansion of dry cellulose increases at the transition temperature. The water curves are generally steeper than the toluene and dekalin curves and have decreased slope above the transition temperature. This indicates that the apparent compressibility of adsorbed water decreases, or the swelling action of adsorbed water per unit volume increases, with rise of temperature, the change being less marked above the transition temperature than below it.