Self-wrinkling polyelectrolyte multilayers: construction, smoothing and the underlying mechanism

Abstract

Introducing wrinkling or rough features into substrates is of great practical significance to construct various functional surfaces. Due to the sensitivity of assembled units towards environmental stimuli, the internals of layer-by-layer films can be readily adjusted to generate various micro- and nanostructures. We previously described a self-roughening polyelectrolyte multilayer (PEM) to facilitate the introduction of surface microstructures. In the present work, the growth process of PEI/PAA multilayer films was investigated and the mean size of the surface microstructures was found to increase linearly with the film thickness. The spontaneous formation of these surface features can be attributed to swelling-induced film deformation during the assembling process, which is similar to the surface wrinkling of hydrogels undergoing a volume phase transition. When exposed to saturated humidity, the internal stress as well as the surface microstructures can be diminished spontaneously, leading to a flat surface over the substrates. Given the effect of the underlying film thickness on the characteristic wavelength of the surface wrinkles, multiscale surface microstructures can be readily realized by means of spatially presetting the distribution of the film thickness.

Controlling Structural Transformation of Polyelectrolyte Films for Spatially Encapsulating Functional Species

“…As soon as the number of bilayers reaches a certain threshold, the dipping time (15 min in this case) becomes inadequate to saturate the entire film with incoming polyelectrolyte . Then the amount of polyelectrolytes involved in the film buildup is decoupled from film thickness and maintains at a constant level, leading to the subsequent linear growth …”

“…After the loading process, the microporous structures of the (PEI/PAA‐N 3 ) 8 film can be eliminated at saturated humidity, leading to the encapsulation of Nile red (Figure B). This is mainly due to the incorporation of water molecules into PEI/PAA‐N 3 film, which enhances the mobility of polyelectrolyte chains by plasticizing effect . The structural transformation of the microporous (PEI/PAA‐N 3 ) 8 film can also lead to the change of its optical properties.…”

“…Without any postassembly treatment, many worm‐like microstructures were created spontaneously on the surfaces of PEI/PAA‐N 3 films. For ease of further applications and characterizations, the PEI/PAA‐N 3 films were exposed to a 100% relative humidity environment for 24 h, thus eliminating these surface microstructures . In a small sealed container, equilibrium water vapor over deionized water (25 °C) was employed to build a 100% RH environment …”

“…In the past decades, the dynamic features of polyelectrolyte films have aroused considerable interests in a variety of fields, including separation, electronics, energy, and optics . We recently demonstrated that the structural transformation of microporous polyelectrolyte films can be enabled at saturated humidity, under the condition that the interchain interactions can be highly weakened and the mobility of polyelectrolyte chains can be activated . In this study, we describe a facile and effective strategy to spatially control the encapsulation of various species by locally regulating the structural transformation of polyelectrolyte films.…”

“…[4,[13][14][15] We recently demonstrated that the structural transformation of microporous polyelectrolyte films can be enabled at saturated humidity, under the condition that the interchain interactions can be highly weakened and the mobility of polyelectrolyte chains can be activated. [16][17][18] In this study, we describe a facile and effective strategy to spatially control the encapsulation of various species by locally regulating the structural transformation of polyelectrolyte films. On the basis of our research, cationic poly(ethylenimine) (PEI) was layer-by-layer (LbL) assembled with a poly(acrylic acid) (PAA) derivative (PAA-N 3 ) to yield a photoreactive polymeric film.…”