Spatially Confining Surface Roughness on Exponentially Growing Polyelectrolyte Multilayer Films

Abstract

approach is direct and effective, how to manipulate the formation of such roughness features remains to be explored. A study on this issue may provide an approach for controlling the surface profiles of these PEM films and facilitate constructing various functional surfaces.The self-roughening behavior of exponentially growing PEM films is essentially a pressure-induced wrinkling behavior, which is caused by the diffusion of polyelectrolytes into the films during the assembling process. [16,17] Since molecular diffusion can be affected dramatically by the presence of steric hindrance, [22,23] the formation of surface wrinkles could be affected by changing the internal construction of these PEM films. In this study, we fabricated a self-wrinkling PEM film capable of being covalently crosslinked under UV light. Such PEM film was LbL assembled from branched polyethylenimine (PEI) and a photoreactive poly(acrylic acid) (PAA) derivative. We found that the wrinkling behavior of this film can be suppressed by applying a pre-crosslinking process, which can produce steric hindrance within the films against the diffusion of oncoming polyelectrolytes. With the aid of photolithography, the surface wrinkling of the film can be "turned off" regioselectively and confined on the areas unexposed to UV irradiation, thus leading to a film with patterned surface roughness. This can provide a spatial control over the properties of film surfaces and thus pave the way for novel applications, such as marine antifouling and tunable wettability.Branched PEI (Figure 1A) and a PAA derivative were alternately assembled onto substrates to generate a photocross-linkable PEM film. The PAA derivative (PAA-N 3 ) was synthesized by modifying PAA with phenyl azido groups via amidation reaction ( Figure 1B) and its grafting ratio was calculated to be 5.3% ( Figure 1C). Figure 1D shows the thickness of (PEI/PAA-N 3 ) n films as a function of the number of layers. The (PEI/PAA-N 3 ) n films grow exponentially in the initial ten layers and linearly in the following layers. Such growth mode of PEM systems generally results from the diffusion of polyelectrolyte chains ''in'' and "out'' of the whole film, as verified by previous studies. [24][25][26] The introduction of phenyl azido groups allows a (PEI/PAA-N 3 ) n film to be covalently cross-linked under UV light. This cross-linking reaction was studied through UV-vis spectroscopy (Figure 2A). The absorption peak at 270 nm corresponds to the π-π* transition of phenyl azido groups. [27][28][29] Upon UV irradiation, it drops off and exhibits a blueshift, which can be ascribed to the dissociation of phenyl azido groups into Introducing roughness features onto materials' surface can serve as an efficient strategy for novel or enhanced surface functionality. Surface roughness can be developed spontaneously on exponentially growing polyelectrolyte multilayer films without any postassembly treatment. This is essentially a pressure-induced wrinkling behavior, which is associated with the diffusion of polyelectro...

Dynamic Porous Pattern through Controlling Noncovalent Interactions in Polyelectrolyte Film for Sequential and Regional Encapsulation

“…After using a photomask with a heart pattern, we successfully created a heart porous pattern in the film. The porous region was off-white because of light scattering, while the black region was a solid and transparent film. , The treatment of the film in 100% RH and dark environment can erase the pattern. We further performed creation and erasure procedures one more time using a circle photomask (Figure c).…”

Laser‐Directed Foaming of Hydroplastic Polyelectrolyte Films toward Tunable Structures and Programmable Routes

“…Detailed procedures have been reported in our previous study. [ 57 ] Briefly, 500 mg PAA, 71.08 mg 4‐azidoaniline hydrochloride, 239.61 mg EDC, and 479.23 mg NHS were dissolved in 50 mL deionized water in sequence. The pH of the mixture was adjusted to 7.0 by dripping a certain amount of 1.0 m NaOH.…”