Preparation And Characterization Of Cellulose Hollow Fiber Membranes Made By A Solvent Method

Novel solvent N-Methylmorpholine-N-Oxide (NMMO) was applied inLyocell process of textile industry to physically dissolve cellulose, and this cangreatly avoid chemical degradations of cellulose so the natural characteristics ofcellulose can be satisfactorily protected. Based on the process this thesis carriedout the preparation and characterization of cellulose hollow fiber membranes(CHFM) made by a solvent method.For the first time we prepared CHFM for gas separation with NMMO assolvent and by wet spinning method. CHFM prepared showed homogeneouslydense structure after being naturally dried. The dry CHFM showed hardly anygas permeability. While for the wet CHFM, water can enter the amorphousregion of cellulose and play a role of "permeation paths"for gases following"dissolution-diffusion"mechanism in water. Because of its higher solubility inwater, CO₂ showed excellent permeability in wet CHFM with high idealseparation factors over N2, CH4 and even H₂.Also the influence of drying method on the morphology and properties ofasymmetric CHFM was systematically studied, and a new drying mechanism forCHFM was proposed. Because there were strong hydrogen bonds betweenliquid molecules and during their evaporation great force would be generated topull cellulose molecules together, thus naturally drying and ethanol exchangedrying would greatly destroy the original asymmetric structure of wet CHFM andthe dry CHFM showed very dense structure and poor gas permeability.Compared with them, 2-butannone exchange and ethanol-hexane exchange drying,especially the latter could satisfactorily protect the original asymmetricmorphology of wet CHFM and the corresponding dry CHFM showed obvious gaspermeability. Spinning parameters showed great influence on the morphology and gaspermeation performance of CHFM. With the addition of low boiling pointalcohol to bore liquid, liquid-liquid phase separation was accelerated and morefinger-like pores could be formed. While because of the evaporation of alcohol,"air bubble fibers"would be prepared and this greatly destroyed the spinningstability. Compared with alcohol, the addition of solvent NMMO to bore liquidwould inhibit the liquid-liquid phase separation and the finger-like pores woulddecrease with the increase of NMMO concentration in bore liquid. With theincrease of dry spinning length, the finger-like pore region would increase and thebigger pores could be seen. Also the temperature of coagulation bath showedgreat influence on the morphology of CHFM prepared. With the increase ofcoagulation bath temperature, the outer side part of CHFM would become looserand sponge-like region could be seen. Of course the gas permeationperformance of all these dry CHFM would change greatly according to theirmorphology. At last the gas humidification performance of CHFM was systematicallyinvestigated. The results showed CHFM owned excellent water permeation rate,very low gas permeability and obvious gas humidification ability, and it wasproven that CHFM had the potential to be applied in the gas humidificationsystem for proton exchange membrane fuel cell (PEMFC).