Photo-responsive nanoporous liquid crystal polymer films for selective dye adsorption

Abstract

Photo-responsive nanoporous polymer films (AZOF-R(NC6)) have been developed by a template method based on a hydrogen-bonding supramolecular liquid crystal (LC) and a light-sensitive azobenzene LC crosslinker in this work. Anionic nanopores were obtained after the removal of template NC6 using KOH solution. The AZOF-R(NC6) demonstrates charge-selective dye adsorption and the maximum adsorption capacity for Rh6G is 504.6 mg g^-1. The AZOF-R(NC6) film without UV light treatment shows a 32% higher adsorption capacity for Rh6G than the AZOF-R(NC6) film treated with UV light within the initial 10 min. In addition, UV light can trigger the release of the adsorbed dye from the polymer film due to the pore size change arising from the trans-cis isomerization. Besides, the used polymer film can be effectively regenerated using a HCl solution. Such functional polymer films with highly ordered nanopores and photo-responsive properties hold great promise in selective adsorption and mass separations.

Fig. 1. (a) Chemical structures of the monomer mixture containing the hydrogen-bonding 6OBA·NC6 complex and the azobenzene crosslinker A6MA. (b) Schematic illustration of the fabrication process of the nanoporous polymer films.

Fig. 2. Characterization of the structure and the mesomorphism of the monomer mixture 6OBA·NC6·A6MA. (a) FT-IR spectrum, (b) DSC spectra, (c and d) POM pictures at (c) 90 and (d) 115 °C, (e) XRD spectra and (f) schematic diagram of the 6OBA·NC6·A6MA at 90 and 115 °C.

Fig. 3. Characterization of the structure and thermal stability of the polymer films. The POM pictures of the (a) planar aligned pristine film and (b) its 45° rotation view. (c) FT-IR spectra and (d) TGA results of the pristine and NC6-removal films. SEM images of the surface of the pristine (e), the NC6-removal (f) films. The arrow represents the direction of alignment.

Fig. 4. (a) UV-vis spectra and the corresponding samples (the inset) of Rh6G solutions before and after adsorption by the AZO-P and AZO-R(NC6) films for 24 h. (b) Charge-selective MB adsorption over MO. UV-vis spectra and vials (the inset) of the dye solutions before and after adsorption. (c) SEM image of the surface of the dye adsorption film.

Fig. 5. (a) Fitting of the Pseudo-first-order kinetics and (b) the Pseudo-second-order kinetics of Rh6G adsorption onto the AZO-R(NC6) film.

Fig. 6. (a) Adsorption kinetics of Rh6G onto the AZO-R(NC6) film and the UV-treated AZO-R(NC6) film. (b) Schematic diagram of the isomerization of the azobenzene groups and the corresponding structures of the AZO-R(NC6).

Fig. 7. Photo regulated Rh6G release of the AZOF-R(NC6) film.

Fig. 8. (a) UV-vis spectra and the corresponding solution samples (the inset) before and after regeneration. (b) Reusability of the AZOF-R(NC6) film for Rh6G adsorption.