Periodic Stratified Porous Structures in Dynamic Polyelectrolyte Films Through Standing‐Wave Optical Crosslinking for Structural Color

Abstract

Periodic porous structures have been introduced into functional films to meet the requirements of various applications. Though many approaches have been developed to generate desired structures in polymeric films, few of them can effectively and dynamically achieve periodic porous structures. Here, a facile way is proposed to introduce periodic stratified porous structures into polyelectrolyte films. A photo‐crosslinkable polyelectrolyte film of poly(ethylenimine) (PEI) and photoreactive poly(acrylic acid) derivative (PAA‐N3) is prepared by layer‐by‐layer (LbL) self‐assembly. Stratified crosslinking of the PEI/PAA‐N3 film is generated basing on standing‐wave optics. The periodic stratified porous structure is constructed by forming pores in noncrosslinked regions in the film. Thanks to the dynamic mobility of polyelectrolytes, this structural controlment can be repeated several times. The size of pores corresponding to the layer spacing of the film contributes to the structural colors. Furthermore, structural color patterns are fabricated in the film by selective photo‐crosslinking using photomasks. Although the large‐scale structural controlment in thick (micron‐scale and above) films needs to be explored further, this work highlights the periodic structural controlment in polymeric films and thus presents an approach for application potentials in sensor, detection, and ink‐free printing.

Responsive Structural Colors Derived from Geometrical Deformation of Synthetic Nanomaterials

“…Reproduced with permission. [ 111 ] Copyright 2019, John Wiley and Sons. k) Layered stacks prepared using biomass formwork.…”

“…Such structures generally require precise light treatment to obtain an inhomogeneous crosslinking system (see Figure 2j). [ 111 ] Gratings are usually obtained by negative duplicating from templates (see Figure 2e). [ 112 ] The spacing and depth of the template can be precisely controlled by photolithography.…”

“…Ren et al prepared a photocrosslinkable iridescent film by grafting azido groups into a synthetic polyanion system (see Figure a). [ 111 ] Periodic crosslinks formed in the film using standing‐wave optics. Moreover, the periodic stratified macroporous structures only appeared in the noncrosslinked parts.…”

“…Reproduced with permission. [ 111 ] Copyright 2021, John Wiley and Sons. b) On‐demand structural color controlled by the deflection of nano‐/micropillars.…”

Fabrication of “Spongy Skin” on Diversified Materials Based on Surface Swelling Non-Solvent-Induced Phase Separation

“…Several coating methods have been developed to combine porous structures with substrates including the etching of immiscible polymer blends, , water templating and freezing-drying, − and solvent/non-solvent exchange. , Non-solvent-induced phase separation (NIPS) is a common method to prepare porous films that are widely used in membrane separation in industries. , The typical NIPS process involves three steps: solution casting, solvent/non-solvent exchange, and phase separation. Porous structures can be regulated depending on the specific requirements and be applied to various materials including chitosan, polycaprolactone, polylactide, and so forth. − Moreover, a recent study has combined the dip-coating method with the NIPS process for porous coating construction .…”