Fabrication and characterization of cellulose nanofiber based thin-film nanofibrous composite membranes

Abstract

Nanoscale cellulose fibers with nominal diameters in the range of 5–10nm were fabricated from wood pulp by using the TEMPO/NaBr/NaClO system (TEMPO-mediated oxidation) followed by a mechanical treatment. The morphology of the resulting cellulose nanofibers was found to be dependent on the cellulose concentration, pH value and ionic strength of the oxidized cellulose aqueous suspension, where nanofibers with ~5nm diameters could be obtained when the cellulose suspension was lower than 0.20wt%. Variations in the degree of crystallinity and thermal stability between the initial wood pulp and resulting cellulose nanofibers were investigated by means of X-ray diffraction and thermo-gravimetric techniques. A novel ultrafiltration (UF) thin-film nanofibrous composite (TFNC) membrane was prepared by using cellulose nanofibers as the top barrier layer, polyacrylonitrile (PAN) electrospun scaffold as the mid-layer and polyethylene terephthalate (PET) non-woven as supporting substrate. The maximum pore size of the cellulose nanofibrous (CN) based TFNC membrane was ~55nm as estimated by the molecular weight cut-off (MWCO) method. Microsphere with 0.10±0.01μm diameter was used as a feed solution to determine the UF efficiency. The permeate flux of the CN-TFNC membrane was found to be about 5-times higher than that of commercial UF membranes (e.g., PAN10) produced with the same polymer components without the cellulose nanofiber barrier layer, while maintaining an even higher rejection ratio (>99.9%) of the microsphere during a 48-h filtration period. For ultrafiltration of oil/water emulsions, the permeate flux of the CN-TFNC membrane was about 8-fold higher than that of the commercial PAN10 membrane. In addition, the CN-TFNC membranes showed excellent chemical resistance, high anti-biodegradation, high hypochlorite resistance and a wide applicable pH range.