The Role of Isobutanol as a Precipitant of Cellulose Films Formed from N-Methylmorpholine N-Oxide Solutions: Phase State and Structural and Morphological Features

The process of film formation including the initial treatment of thin layers of cellulose solutions in N-methylmorpholine N-oxide with isobutanol followed by washing with water is investigated in detail, and the structural and morphological features of the obtained films are examined. The phase state of the N-methylmorpholine N-oxide–isobutyl alcohol system is studied by differential scanning calorimetry and optical interferometry, and a diagram describing the crystalline equilibrium and allowing determination of the temperature–concentration “window” of compatibility of components is constructed. The dependence of viscosity of N-methylmorpholine N-oxide solutions in isobutanol on temperature confirms the phase composition of the system. The process of film formation is modeled by analyzing the diffusion zone of the cellulose solution–isobutyl alcohol system. The IR study of the interaction of N-methylmorpholine N-oxide containing 13.3% water with isobutanol shows that the affinity of isobutanol for water is much higher than that for N-methylmorpholine N-oxide. For this reason, when the spinning solution is brought in contact with isobutanol, the redistribution of water between the interacting components occurs and the structure of the heterogeneous gel-like complex cellulose–N-methylmorpholine N-oxide–isobutanol “is frozen,” as proved by the X-ray diffraction study of the films. Complete removal of the solvent and isolation of cellulose from this film proceed only upon subsequent washing with water. The X-ray diffraction and optical interferometry study of the effect of temperature on the interaction of a hot cellulose solution with cold isobutanol suggests that at room temperature the film obtained from solution contains inclusions of the vitrified N-methylmorpholine N-oxide. Under isothermal conditions (at a temperature of 90°С), the rate of interdiffusion grows appreciably and the solution preserves the homogeneous structure. Thus, the precipitation of cellulose from the bicomponent solvent N-methylmorpholine N-oxide–water upon contact first with isobutanol and then with water proceeds via two stages: initially the system undergoes phase separation and a concentrated solution is formed in the isobutanol-N-methylmorpholine N-oxide blend, from which cellulose precipitates upon interaction with water. When the process of primary interaction of the solution with alcohol is conducted under the conditions of compatibility of isobutanol with N-methylmorpholine N-oxide, a more homogeneous morphology of the films can be obtained.