Kinetics of simultaneous decrystallization and crystallization induced in high‐modulus rayon by bromine water

Abstract

AbstractStudies of the effect of aqueous Br2 solution on the fine structure of cellulose have shown that the accessibility of the cellulose is a complex function of the duration of treatment. Following immersion of rayon‐cellulose in the Br2 solution, its accessibility is observed to increase until a peak is reached, after which the accessibility decreases with time. For higher temperatures and greater concentrations of Br2, a sharper peak and faster rates of increase and decrease of accessibility are obtained. In order to explain this behavior, a mathematical model of the kinetics is postulated involving two opposing first‐order processes occurring simultaneously. One process describes the decrystallization of crystalline regions within the cellulose by opening intermediate linkages due to Van‐der‐Waals forces involved. The second process relates to the crystallization of the less‐ordered regions. The rate constants for these processes are determined by fitting experimental curves to the postulated analytical expression with the aid of a digital computer program. Experiments carried out on high‐modulus rayon showed that for this type of cellulose, the effect described above was very pronounced. From the calculated rate constants, the activation energies of the crystallization and decrystallization reactions were determined. The plots of the rate constants versus 1/T showed that a break occurs at 25°C, corresponding to a second‐order transition point of cellulose. The energies of activation of the crystallization process are in the range of 7–9 and 10–13 kcal/mol in the temperature ranges 10–25 and 25–40°C, respectively. The activation energies of the decrystallization process are 13 and 18–24 kcal/mol in these temperature ranges.