Xue-San Wang

Engineering novel polyelectrolyte complex membranes with improved mechanical properties and separation performance

Polyelectrolyte complexes (PECs) commonly suffer from poor processability owing to their ionic crosslinking nature, a problem which spurs increasing interest in processable PECs. New processing technologies have been exploited to render PECs processable, but usually at the expense of compromising their mechanical properties. Through a conceptually novel strategy of “complexation–sulfation”, here we engineer solution-processable PECs derived from a strong polyacid, in pursuit of high mechanical strength combined with exceptional separation performance. Effects of chemical structures and compositions on mechanical properties of these PEC membranes were studied. It was found that the mechanical properties of these PEC membranes based on strong “ion-pairs” were substantially enhanced, with their tensile strength and elongation at break reaching as high as 108.3 MPa and 5.0%, respectively. In addition, PEC membranes exhibited a high performance in separating water–ethanol mixtures. For example, the flux and water content in the permeate for PEC membranes were 2100 g m−2 h−1 and 99.58 wt%, respectively, in dehydrated 10 wt% water–ethanol mixture at 70 °C.

Insight into Fractal Self-Assembly of Poly(diallyldimethylammonium chloride)/Sodium Carboxymethyl Cellulose Polyelectrolyte Complex Nanoparticles

Poly(diallyldimethylammonium chloride)-sodium carboxymethyl cellulose polyelectrolyte complexes (PDDA-CMCNa PECs) solids were prepared and dispersed in NaOH aqueous solution. Self-assembly of PECs nanoparticles during the solvent evaporation was examined by field emission electron microscopy (FESEM), atomic force microscopy (AFM), and fractal dimension analysis. It was found that tree-shaped fractal patterns formed after the solvent (water) was dried at ambient temperatures, and the fractal pattern is composed of needle-shaped PEC aggregate (PECA) nanoparticles. Time-dependent FESEM observation revealed that the fractal pattern started with the formation of initial nucleon and it is growing, during which the diffusion limited aggregation (DLA) mechanism revealed and made the pattern branched. Physical insight into the DLA mechanism was discussed in detail. Effects of PEC concentrations, PEC compositions, solvent evaporation temperatures, pH of PEC dispersion, and chemical structures of PECs on the formation of self-assembled fractal pattern were studied. Generally, it was found that the morphologies, charge characters of PEC particles, and the solvent evaporation conditions play important roles during the fractal self-assembly process.

Preparation and separation characteristics of polyelectrolyte complex membranes containing sulfated carboxymethyl cellulose for water–ethanol mixtures at low pH

Novel polyelectrolyte complexes based on sulfated carboxymethyl cellulose and chitosan were prepared, containing strongly charged (sulfate (OSO3)) and weakly charged (carboxylate (COO−)) groups (SPECs). SPEC homogeneous membranes (SPECMs) for the pervaporation dehydration of ethanol–water mixtures were investigated. The chemical structures and compositions of SPECMs were characterized by Fourier transform infrared spectroscopy and X-ray photoelectron spectra. The swelling behavior and separation performance of SPECMs in low pH feed were evaluated. It was found that the SPECMs containing OSO3 groups were resistant to acidic feeds and showed very good separation performance even in low pH feed. For instance, the water content in permeate for SPECM-0.35 reached as high as 95.8xa0wt%, which was much higher than the 60xa0wt% for SPECM-0 in the dehydration of water–ethanol mixtures with pH 2. The result was attributed to the interplay between the stable complexation formed by OSO3 groups and the improved ionization degree of COO− groups in SPECMs. Moreover, the SPECMs maintained their operation stability against acidic feeds. The SPECMs could find considerable potential application in the pervaporation dehydration of acidic feeds, especially for enhancing the conversion of ester condensation.

Preparation and properties of PEC nanocomposite membranes with carboxymethyl cellulose and modified silica

Carboxymethyl cellulose (CMC)-modified silica nanocomposites were prepared via in situ incorporation of modified silica during the ionic complexation between CMC and poly(2-methacryloyloxy ethyl trimethylammonium chloride) (PDMC). Ionic bonds were introduced between the poly(2-acrylamido-2-methylproanesulfonic acid) modified silica (SiO2-PAMPS) and the polyelectrolyte complex (PEC) matrix. The PEC nanocomposites (PECNs) and their membranes (PECNMs) were characterized with Fourier transform-infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and tensile testing. PECNM containing 5 wt.% SiO2-PAMPS showed a tensile strength of 68 MPa and elongation at break of 7.1%, which were 1.9 and 2.6 times as high as those of pristine PEC membranes, respectively. Moreover, the pervaporation performance of as-prepared PECNMs was evaluated with dehydration of 10 wt.% aqueous isopropanol mixtures, and the PECNMs exhibited a flux of 2,400 gm(-2)h(-1) with a high separation factor of 4491 at 70°C.

Tunable disintegration of layer-by-layer assembly multilayer films based on hydrolytical-polybetaine at wide-range time.

A cationic hydrolytical-polycarboxybetaine (HPCB), poly(N-ethyl acetate-4-vinylpyridinium bromide) was synthesized by incorporating ester group into the side chain of polycarboxybetaine (PCB). The hydrolytic behaviors of HPCB samples in pH 7.4 phosphate buffer saline (PBS) were investigated by FT-IR and (1)H NMR. The layer-by-layer (LbL) assembly of HPCB/poly (sodium 4-styrenesulfonate) PSS and the disintegration of HPCB/PSS multilayer films were monitored by UV-vis absorption spectroscopy, quartz crystal microbalance (QCM) and atomic force microscopy (AFM). The disintegrated behavior of multilayer films was studied in detail by changing the cationic degree of HPCB and the pH of the immersion solution (PBS) in the disintegration process. The disintegration time of HPCB/PSS multilayer films could be controlled widely from 2 min to 30 days in PBS.