In this lecture, we demonstrate that ultra-thin cellulose microfibril fractions can be derived from spruce wood powder using combined delignification, TEMPO-catalyzed oxidation and sonication processes. Small-angle X-ray scattering of these microfibril fractions in a ‘dilute’ aqueous suspension revealed that their shape was in the form of nanostrip with 4 nm width and only about 0.5 nm thicknesses. These dimensions were further confirmed by TEM and AFM measurements. The 0.5 nm thickness implied that the nanostrip could contain only a single layer of cellulose chains. At a higher concentration, SAXS analysis indicated that these nanostrips aggregated into a layered structure. The X-ray diffraction of samples collected at different preparation stages suggested that microfibrils were delaminated along the (1-10) planes from the Iβ cellulose crystals. The degree of oxidation and solid-state 13C NMR characterizations indicated that in addition to the surface molecules, some inner molecules of microfibrils were also oxidized, facilitating the delamination into cellulose nanostrips. Non-woven nanofibrous scaffolds have unique properties, such as interconnected pores, a very large surface-to-volume ratio, and a high capacity for surface modifications, making them useful for high throughput separation media. We further demonstrate that highly permeable nanofibrous membranes, having a hierarchically tiered structure, can enable higher flux, higher retention and lower energy consumption water purification. The improved performance was based on a new concept of composite membrane design, involving (1) the replacement of conventional flux-limited porous membrane mid-layer with a high-flux non-woven nanofibrous support, containing an asymmetric structure with inter-connected void morphology, and (2) the coating of a very thin nanocomposite barrier layer, containing “directed water channels” formed via the formation of interfaces between the cross-linked nanofibers and the polymer barrier layer matrix, on the nanofibrous mid-layer scaffold. Experiments from the hierarchical design and assembly of this type of unique fibrous membranes have revealed very promising potentials, e.g., several times more flux than the best among all conventional ultrafiltration and nanofiltration media without losing high-rejection and low-fouling criteria.

Highly permeable nanofibrous cellulose membranes for water purification

251st National Meeting (2016)

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