| Summary: | Carboxyl-type boronic acid copolymers (CBACs) were synthesized by a radical polymerization method and used for the preparation of polyvinyl alcohol (PVA)-based composite membranes via a solution mixture method. The as-prepared composite membranes exhibited a water uptake (W<sub>R</sub>) of 122.6–150.0%, an ion exchange capacity (IEC) of 0.0147–0.0518 mmol g<sup>−1</sup>, and excellent mechanical (elongation at break (E<sub>b</sub>) of 103.8–148.4%, tensile strength (TS) of 38.7–58.6 MPa) and thermal stability. The alkali resistances of the as-prepared membranes were tested by immersing the samples into 2 mol L<sup>−1</sup> NaOH solutions at 25 °C for 60 h, and the results were encouraging: the mass loss and swelling degree of the as-prepared membranes were in the ranges of 1.9–5.9% and 222.6–241.9%, respectively. The separation performances of the as-prepared membranes were evaluated by the diffusion dialysis (DD) process with an NaOH/Na<sub>2</sub>WO<sub>4</sub> mixture at room temperature. The results demonstrated that the dialysis coefficients of hydroxide (U<sub>OH</sub>) were in the range of 0.0147–0.0347 m h<sup>−1</sup>, and the separation factors (S) were in the range of 29.5–62.6. The introduced carboxyl groups from CBACs and the –OH groups from PVA were both deemed to play significant roles in the promotion of ion transport: the –COO<sup>−</sup> groups formed negatively charged transport channels for Na<sup>+</sup> by electrostatic attraction, and the –OH groups promoted the transport of OH<sup>−</sup> via hydrogen bonding.
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