Study Of Water Adsorption Mechanism In Cellulose Based On Time-Domain Nuclear Magnetic Resonance

In this paper, salix, cotton, ramie and flax cellulose was selected as materials and the crystal structure of cellulose before and after acid and alkali pretreatment was analyzed using X-ray diffraction. 1H time-domain nuclear magnetic resonance was used to determine the free induction decay (FID) of cellulose soaked by deuteroxide and transverse relaxation time (T2) of cellulose in adsorption process, and the model of crystalline of cellulose was also built, in order to explore the water distribution and water states in cellulose and the relationship between water and cellulose during water adsorption process.The conclusion was summarized as follows:1. The x-ray diffraction spectrum of cellulose showed that the diffraction peak position of cellulose crystal was changed after the treatment of 20% NaOH. The cellulose Ⅰ which has the parallel chain structure transformed cellulose Ⅱ which has the antiparallel chain structure. The intramolecular and intermolecular hydrogen boning also became more complex. The degree of cellulose crystalline was decrease and the crystal size was reduced and the crystal spacing was increase after alkali treatment.2. The crystal form of cellulose did not change after 2.5N HC1 boiling water treatment, but the β-1,4 glycosylic bond in amorphous region was broken and the cellulose chain in amorphous region was hydrolysed. The degree of cellulose crystalline increased, the crystal size was reduced and the crystal spacing was decrease after acid treatment. The structure of cellulose became more compact.3. The hydrogen in hydroxyl groups of cellulose have rigid character, of which the FID signal decay fast. When cellulose was socked in deuteroxide, the hydrogen in hydroxyl groups of cellulose could interaction with deuteroxide by chemical proton exchange, leading to the FID signal decay slowly. According to the structure of cellulose, water accessibility can be calculated by analysis the FID curve of cellulose before and after deuteroxide treatment. Water accessibility of cellulose was decrease along with the increase of the crystalline degree. The model of crystalline of cellulose was also built. Combining the model with the experimental data we can calculate that water can access only one molecule layer of cellulose crystalline region.4. During the adsorption process of cellulose treated by different method in 37℃, 98% relative humidity (RH) environment, the equilibrium moisture content of untreated cotton was 15.2%, and acid treated cotton was 12%, and alkali treated cotton was 25.4%. Water in cellulose existed mainly as bound water. There was also less condensation water in capillary and transient state water.5. Upon the initial addition of water, strong hydrogen bonding occurs between water molecules and free hydroxyl groups, which T2 was short. Then water adsorbed on the unsatisfied hydroxyl groups on the cellulose structure. Some of the hydrogen bond between the hydroxyl groups in cellulose was broken. T2 increased slowly with the increase of water content.Multilayers water began to absorb after the Primary Bound Water was saturated. T2 increased quickly. Amorphous region of cellulose was swelled by water adsorption and then the pore size increased. According to the formula, the maximum content of primary bound water of cotton was 13.2%, acid treated cotton was 10.2% and alkali treated cotton was 19.4%. But not all hydroxyl in cellulose amorphous region can absorb water, so the actual value was less than this calculated value.