Nanofibrous Microfiltration Membrane Based on Cellulose Nanowhiskers

A multilayered nanofibrous microfiltration (MF) membrane system with high flux, low pressure drop, and high retention capability against both bacteria and bacteriophages (a virus model) was developed by impregnating ultrafine cellulose nanowhiskers (diameter about 5 nm) into an electrospun polyacrylonitrile (PAN) nanofibrous scaffold (fiber diameter about 150 nm) supported by a poly­(ethylene terephthalate) (PET) nonwoven substrate (fiber diameter about 20 μm). The cellulose nanowhiskers were anchored on the PAN nanofiber surface, forming a cross-linked nanostructured mesh with very high surface-to-volume ratio and a negatively charged surface. The mean pore size and pore size distribution of this MF system could be adjusted by the loading of cellulose nanowhiskers, where the resulting membrane not only possessed good mechanical properties but also high surface charge density confirmed by the conductivity titration and zeta potential measurements. The results indicated that a test cellulose nanowhisker-based MF membrane exhibited 16 times higher adsorption capacity against a positively charged dye over a commercial nitrocellulose-based MF membrane. This experimental membrane also showed full retention capability against bacteria, for example, <i>E. coli</i> and <i>B. diminuta</i> (log reduction value (LRV) larger than 6) and decent retention against bacteriophage MS2 (LRV larger than 2)

Exploring the Nature of Cellulose Microfibrils

Ultrathin cellulose microfibril fractions were extracted 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 (concentration 0.077 wt %) 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 (0.15 wt %), 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 (11̅0) planes from the I_β 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

Efficient Removal of UO₂²⁺ from Water Using Carboxycellulose Nanofibers Prepared by the Nitro-Oxidation Method

Carboxycellulose nanofibers (NOCNF) were extracted from untreated jute fibers using a simple nitro-oxidation method, employing nitric acid and sodium nitrite. The resulting NOCNF possessed high surface charge (−70 mV) and large carboxylate content (1.15 mmol/g), allowing them to be used as an effective medium to remove UO₂²⁺ ions from water. The UO₂²⁺ (or U­(VI)) removal mechanism was found to include two stages: the initial stage of ionic adsorption on the NOCNF surface following by the later stage of uranyl hydroxide mineralization, as evidenced by the Fourier transform infrared, scanning electron microscopy with energy dispersive spectroscopy capabilities, transmission electron miscroscopy, and wide-angle X-ray diffraction results. Using the Langmuir isotherm model, the extracted NOCNF exhibited a very high maximum adsorption capacity (1470 mg/g), about several times higher than the most efficient adsorbent reported (poly­(acrylic acid) hydrogel). It was also found that the remediation of UO₂²⁺ ions by NOCNF was pH dependent and possessed the maximum adsorption at pH = 7. The removal efficiency of NOCNF was between 80 and 87% when the UO₂²⁺ concentration was below 1000 ppm, while it decreased to 60% when the UO₂²⁺ concentration was around 1250 ppm

A Simple Approach to Prepare Carboxycellulose Nanofibers from Untreated Biomass

A simple approach was developed to prepare carboxycellulose nanofibers directly from untreated biomass using nitric acid or nitric acid-sodium nitrite mixtures. Experiments indicated that this approach greatly reduced the need for multichemicals, and offered significant benefits in lowering the consumption of water and electric energy, when compared with conventional multiple-step processes at bench scale (e.g., TEMPO oxidation). Additionally, the effluent produced by this approach could be efficaciously neutralized using base to produce nitrogen-rich salts as fertilizers. TEM measurements of resulting nanofibers from different biomasses, possessed dimensions in the range of 190–370 and 4–5 nm, having PDI = 0.29–0.38. These nanofibers exhibited lower crystallinity than untreated jute fibers as determined by TEM diffraction, WAXD and ¹³C CPMAS NMR (e.g., WAXD crystallinity index was ∼35% for nanofibers vs 62% for jute). Nanofibers with low crystallinity were found to be effective for removal of heavy metal ions for drinking water purification

Ionic Cross-Linked Poly(acrylonitrile-<i>co</i>-acrylic acid)/Polyacrylonitrile Thin Film Nanofibrous Composite Membrane with High Ultrafiltration Performance

A new method for fabrication of thin film nanofibrous composite (TFNC) ultrafiltration (UF) membrane consisting of an ultrathin poly­(acrylonitrile-<i>co</i>-acrylic acid) (PAN-AA) barrier layer based on a polyacrylonitrile (PAN) nanofibrous support layer was proposed in this study. First, a thin PAN-AA nanofibrous layer was electrospun and deposited on a thicker PAN nanofibrous substrate. Then, the as-prepared PAN-AA nanofibers were swollen in the alkaline buffer solution and merged imperceptibly as an integrated nonporous hydrogel layer on the PAN substrate. The PAN-AA hydrogel layer was cross-linked with different bivalent metal cations (Ca²⁺, Mg²⁺) to form an ultrathin barrier layer, of which the thickness and porosity were optimized by controlling the depositing time of PAN-AA nanofibers and pH value of buffer solution. Proteins with different molecular weights were used to evaluate the ultrafiltration performance of the resultant composite membranes. Due to its hydrophilic and negative charged barrier layer, the PAN-AA-Mg and PAN-AA-Ca TFNC UF composite membranes exhibited excellent permeate flux (221.2 and 219.2 L/m² h) and rejection efficiency (97.8% and 95.6%) for bovine serum albumin (BSA) aqueous solution (1 g/L) at 0.3 MPa. The PAN-AA TFNC UF membranes could be used to retain solutes, of which the radius was larger than 4.6 nm

Inducing Order from Disordered Copolymers: On Demand Generation of Triblock Morphologies Including Networks

Disordered block copolymers are generally impractical in nanopatterning applications due to their inability to self-assemble into well-defined nanostructures. However, inducing order in low molecular weight disordered systems permits the design of periodic structures with smaller characteristic sizes. Here, we have induced nanoscale phase separation from disordered triblock copolymer melts to form well-ordered lamellae, hexagonally packed cylinders, and a triply periodic gyroid network structure, using a copolymer/homopolymer blending approach, which incorporates constituent homopolymers into selective block domains. This versatile blending approach allows one to precisely target multiple nanostructures from a single disordered material and can be applied to a wide variety of triblock copolymer systems for nanotemplating and nanoscale separation applications requiring nanoscale feature sizes and/or high areal feature densities

Dual-Biomimetic Superhydrophobic Electrospun Polystyrene Nanofibrous Membranes for Membrane Distillation

A new type of dual-biomimetic hierarchically rough polystyrene (PS) superhydrophobic micro/nano-fibrous membrane was fabricated via a one-step electrospinning technique at various polymer concentrations from 15 to 30 wt %. The obtained micro/nano-fibers exhibited a nanopapillose, nanoporous, and microgrooved surface morphology that originated from mimicking the micro/nanoscale hierarchical structures of lotus leaf and silver ragwort leaf, respectively. Superhydrophobicity and high porosity of such resultant electrospun nanofibrous membranes make them attractive candidates for membrane distillation (MD) application with low energy water recovery. In this study, two kinds of optimized PS nanofibrous membranes with different thicknesses were applied for desalination via direct contact MD. The membranes maintained a high and stable permeate water vapor flux (104.8 ± 4.9 kg/m²·h, 20 g/L NaCl salt feed for a thinner PS nanofibrous membrane with thickness of 60 μm; 51 ± 4.5 kg/m²·h, 35 g/L NaCl salt feed for the thicker sample with thickness of 120 μm; Δ<i>T</i> = 50 °C) for a test period of 10 h without remarkable membrane pores wetting detected. These results were better than those of typical commercial polyvinylidene fluoride (PVDF) MD membranes or related PVDF nanofibrous membranes reported in literature, suggesting excellent competency of PS nanofibrous membranes for MD applications

Time-Resolved Synchrotron X-ray Scattering Study on Propylene–1-Butylene Random Copolymer Subjected to Uniaxial Stretching at High Temperatures

Synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) were used to characterize the structure evolution of propylene–1-butylene (P–B) random copolymer subjected to uniaxial tensile deformation at 100 °C. Polymorphism and preferred orientation of the crystal phases were examined quantitatively by 2D WAXD. The results indicated that three ensembles of crystalline modifications with distinctive orientation modes coexisted during stretching. The orthorhombic γ-form adopted a tilted cross-β configuration, in which the <i>c</i>-axis had a tilt angle with respect to the fiber axis. The monoclinic α-form in the mother lamellae had a <i>c</i>-axis orientation with polymer chains parallel to the fiber axis. In the α-phase daughter lamellae, the unit cell assumed an <i>a-</i>axis orientation, where the <i>c</i>-axis had an 80° angle with respect to the fiber axis. Stretching transformed the γ-phase into the energetically more stable α-phase. In the late stage, the system was dominated by the α-phase with parallel chain packing. Complemented by qualitative SAXS analysis, simultaneous inter- and intralamellar chain slips were observed during the early stage of stretching. After yielding, a fibrillation process followed. The formation of fibril bundles together with a cross-linked network after yielding might account for the stress-hardening behavior in the late stage of stretching

Shear-Induced Precursor Relaxation-Dependent Growth Dynamics and Lamellar Orientation of β‑Crystals in β‑Nucleated Isotactic Polypropylene

Although a shear flow field and β-nucleating agents (β-NAs) can separately induce the formation of β-crystals in isotactic polypropylene (iPP) in an efficient manner, we previously encountered difficulty in obtaining abundant β-crystals when these two factors were applied due to the competitive growth of α- and β-crystals. In the current study, to induce the formation of a high fraction of β-crystals, a strategy that introduces a relaxation process after applying a shear flow field but before cooling to crystallize β-nucleated iPP was proposed. Depending on the relaxation state of the shear-induced oriented precursors, abundant β-crystals with a refined orientation morphology were indeed formed. The key to producing these crystals lay in the partially dissolved shear-induced oriented precursors as a result of the relaxation process’s ability to generate β-crystals by inducing the formation of needlelike β-NAs. Therefore, the content of β-crystals gradually increased with relaxation time, whereas the overall crystallization kinetics progressively decreased. Moreover, more time was required for the content of the β-phase to increase to the (maximum) value observed in quiescent crystallization than for the effect of flow on crystallization kinetics to be completely eliminated. The <i>c</i>-axis of the oriented β-lamellae was observed to be perpendicular, rather than parallel, to the fiber axis of the needlelike β-NAs, as first evidenced by the unique small-angle X-ray scattering patterns obtained. The significance of the relaxation process was manifested in regulating the content and morphology of oriented β-crystals in sheared, β-nucleated iPP and thus in the structure and property manipulation of iPP

Suppressing of γ-Crystal Formation in Metallocene-Based Isotactic Polypropylene during Isothermal Crystallization under Shear Flow

The effect of shear flow on isothermal crystallization behavior of γ-crystals in metallocene-based isotactic polypropylene melt was investigated by in situ synchrotron wide-angle X-ray diffraction (WAXD). In the sample under weak shear (at strain of 300% for 30 s duration), simultaneous evolution of α- and γ-crystals occurred, and the final fraction of γ-crystals (<i>f</i>_γ) was 0.66, which was identical to the undeformed sample (PP-Static). In this scenario, α-crystals probably served as effective seeds for nucleation of γ-crystals. In the samples under strong shear (at strain of 500% for 30 s duration or long-time continuous shear at strains of 100% and 500%), the sequential emergence of α- and γ-crystals was observed. In this case, molten polymer chains were probably constrained by the surrounding crystals after intense short-time shear and/or maintained their extended chain conformation after long-time shear. These oriented chains had little chance to form the γ-crystals directly, behaving very differently from the relaxed chains. Under strong shear fields, the emergence of γ-crystals was delayed or inhibited, whereas the <i>f</i>_γ value was also decreased rapidly. A simple model for the possible pathway of γ-crystal formation in the strong shear environment was proposed