Swelling/Deswelling-Induced Reversible Surface Wrinkling on Layer-by-Layer Multilayers

Layer-by-layer (LbL) multilayer film is incorporated in the fabrication of a film/substrate system for the investigation of swelling/deswelling-induced wrinkle evolution for the first time. As one typical example, hydrogen-bonded (PAA/PEG)_<i>n</i> (PAA, poly­(acrylic acid); PEG, poly­(ethylene glycol)) is deposited on a poly­(dimethylsiloxane) (PDMS) substrate via the LbL technique. Heating treatment causes the covalent cross-linking reaction to occur in the H-bonded multilayers with simultaneously spontaneous formation of labyrinth wrinkles. Subsequent water immersion leads to the evolution of a series of the swelling-sensitive wrinkles in the thermally cross-linked (PAA/PEG)_<i>n</i>/PDMS bilayer, ranging from initial labyrinth wrinkles (a) to an intermediate smooth wrinkle-free state (b), hexagonally arranged dimples (c), and the later-segmented labyrinth patterns (d). Upon deswelling by reheating of the swollen bilayer, the reverse wrinkle evolution happens via the process of d → b, or d → b → a, or c → b, or c → b → a, which is dependent on the reheating temperature and the swelling-induced pattern. We investigate the influences of experimental conditions on the swelling kinetics and the resulting wrinkle evolution, which include the thickness of (PAA/PEG)_<i>n</i>, the additionally deposited outermost layer (e.g., Pt and polystyrene), and the swelling solution pH. The involved mechanism has been discussed from the viewpoint of the relation between the wrinkling behavior and the swelling/deswelling-induced stress state. The results indicate that the combined strategy of LbL assembly with the introduction of additional layers endows us with considerable freedom to fabricate multifunctional film/substrate systems and to tune the instability-driven patterns for advanced properties and extended applications

Swelling/Deswelling-Induced Reversible Surface Wrinkling on Layer-by-Layer Multilayers

Layer-by-layer (LbL) multilayer film is incorporated in the fabrication of a film/substrate system for the investigation of swelling/deswelling-induced wrinkle evolution for the first time. As one typical example, hydrogen-bonded (PAA/PEG)_<i>n</i> (PAA, poly­(acrylic acid); PEG, poly­(ethylene glycol)) is deposited on a poly­(dimethylsiloxane) (PDMS) substrate via the LbL technique. Heating treatment causes the covalent cross-linking reaction to occur in the H-bonded multilayers with simultaneously spontaneous formation of labyrinth wrinkles. Subsequent water immersion leads to the evolution of a series of the swelling-sensitive wrinkles in the thermally cross-linked (PAA/PEG)_<i>n</i>/PDMS bilayer, ranging from initial labyrinth wrinkles (a) to an intermediate smooth wrinkle-free state (b), hexagonally arranged dimples (c), and the later-segmented labyrinth patterns (d). Upon deswelling by reheating of the swollen bilayer, the reverse wrinkle evolution happens via the process of d → b, or d → b → a, or c → b, or c → b → a, which is dependent on the reheating temperature and the swelling-induced pattern. We investigate the influences of experimental conditions on the swelling kinetics and the resulting wrinkle evolution, which include the thickness of (PAA/PEG)_<i>n</i>, the additionally deposited outermost layer (e.g., Pt and polystyrene), and the swelling solution pH. The involved mechanism has been discussed from the viewpoint of the relation between the wrinkling behavior and the swelling/deswelling-induced stress state. The results indicate that the combined strategy of LbL assembly with the introduction of additional layers endows us with considerable freedom to fabricate multifunctional film/substrate systems and to tune the instability-driven patterns for advanced properties and extended applications

Large-Area Patterning of Polyaniline Film Based on <i>in Situ</i> Self-Wrinkling and Its Reversible Doping/Dedoping Tunability

Here we report a simple one-pot yet robust approach to fabricate large-scale wrinkle patterns with reversible acid-doping/base-dedoping tunability. A novel swelling-induced self-wrinkling mechanism is responsible for the <i>in situ</i> growth of wrinkled polyaniline (PANI) film on polydimethylsiloxane (PDMS) substrate. The spontaneously formed wrinkles with controlled microstructures such as the wavelength, spatial orientation, and location have been well regulated by PANI film thickness (via polymerization time and monomer concentration) and PDMS substrate modulus as well as the boundary conditions imposed by the substrate. The results indicate that the <i>in situ</i> self-wrinkling is highly desirable for patterning PANI film over large areas with the instability-driven morphologies, even in the case of curved surfaces employed. Interestingly, taking advantage of the swelling/deswelling capability via the unique acid doping/base dedoping of PANI, we have further realized unprecedented reversible modulation between the wrinkled and dewrinkled states. The involved physics underlying the complicated <i>in situ</i> self-wrinkling and the reversible doping/dedoping tunability has been revealed

Controlled Free Edge Effects in Surface Wrinkling via Combination of External Straining and Selective O₂ Plasma Exposure

Herein the edge effect from the traction-free boundary condition is utilized to direct the spontaneous surface wrinkling. This boundary condition is attained by a simple combination of mechanical straining and selective exposure of polydimethylsiloxane (PDMS) substrate to O₂ plasma (OP) through a copper grid. When the strained PDMS sheet is subjected to selective OP treatment, a patterned heterogeneous surface composed of the OP-exposed “hard” oxidized SiO_<i>x</i> region (denoted as <i>D</i>₁) and the OP-unexposed “soft” region (denoted as <i>D</i>₂) is produced. The subsequent full release of the prestrain (ε_pre) leads to the selective wrinkling in <i>D</i>₁, rather than in <i>D</i>₂. It is seen that even in <i>D</i>₁, no wrinkling occurs in the vicinity of the <i>D</i>₁ edge that is perpendicular to the wavevector. Furthermore, the average wrinkle wavelength in <i>D</i>₁ (λ_<i>D</i>1) is smaller than that of the exposed copper grid-free blank area (λ_blank). This wavelength decrement between λ_<i>D</i>1 and λ_blank, which can be used to roughly estimate the edge-effect extent, increases with the applied mesh number of copper grids and exposure duration, while decreases with the increase of ε_pre. Meanwhile, there exists a decrease in the amplitude of the patterned wrinkles, when compared with that of the blank region. Additionally, hierarchical wrinkling is induced when the strain-free PDMS substrate is selectively exposed to OP, followed by uniaxial stretching and the subsequent blanket exposure. Consequently, oriented wrinkles perpendicular to the stretching direction are generated in <i>D</i>₂. With respect to <i>D</i>₁, no wrinkling happens or orthogonal wrinkles occur in this region depending on the applied mesh number, exposure duration, and ε_pre. In the above wrinkling process, the combinative edge effects in two perpendicular directions that are involved sequentially have been discussed

Patterning Poly(dimethylsiloxane) Microspheres via Combination of Oxygen Plasma Exposure and Solvent Treatment

Here a simple low-cost yet robust route has been developed to prepare poly­(dimethylsiloxane) (PDMS) microspheres with various surface wrinkle patterns. First, the aqueous-phase-synthesized PDMS microspheres are exposed to oxygen plasma (OP), yielding the oxidized SiO_<i>x</i> layer and the corresponding stiff shell/compliant core system. The subsequent solvent swelling and solvent evaporation induce the spontaneous formation of a series of curvature and overstress-sensitive spherical wrinkles such as dimples, short rodlike depressions, and herringbone and labyrinth patterns. The effects of the experimental parameters, including the radius and Young’s modulus of the microspheres, the OP exposure duration, and the nature of the solvents, on these tunable spherical wrinkles have been systematically studied. The experimental results reveal that a power-law dependence of the wrinkling wavelength on the microsphere radius exists. Furthermore, the induced wrinkling patterns are inherently characteristic of a memory effect and good reversibility. Meanwhile, the corresponding phase diagram of the wrinkle morphologies on the spherical surfaces vs the normalized radius of curvature and the excess swelling degree has been demonstrated. It is envisioned that the introduced strategy in principle could be applied to other curved surfaces for expeditious generation of well-defined wrinkle morphologies, which not only enables the fabrication of solids with multifunctional surface properties, but also provides important implications for the morphogenesis in soft materials and tissues

Synergism of Dewetting and Self-Wrinkling To Create Two-Dimensional Ordered Arrays of Functional Microspheres

Here we report a simple, novel, yet robust nonlithographic method for the controlled fabrication of two-dimensional (2-D) ordered arrays of polyethylene glycol (PEG) microspheres. It is based on the synergistic combination of two bottom-up processes enabling periodic structure formation for the first time: dewetting and the mechanical wrinkle formation. The deterministic dewetting results from the hydrophilic polymer PEG on an incompatible polystyrene (PS) film bound to a polydimethylsiloxane (PDMS) substrate, which is directed both by a wrinkled template and by the template-directed in-situ self-wrinkling PS/PDMS substrate. Two strategies have been introduced to achieve synergism to enhance the 2-D ordering, i.e., employing 2-D in-situ self-wrinkling substrates and boundary conditions. As a result, we achieve highly ordered 2-D arrays of PEG microspheres with desired self-organized microstructures, such as the array location (e.g., selectively on the crest/in the valley of the wrinkles), diameter, spacing of the microspheres, and array direction. Additionally, the coordination of PEG with HAuCl₄ is utilized to fabricate 2-D ordered arrays of functional PEG–HAuCl₄ composite microspheres, which are further converted into different Au nanoparticle arrays. This simple versatile combined strategy could be extended to fabricate highly ordered 2-D arrays of other functional materials and achieve desirable properties and functionalities

Highly Sensitive Wearable Pressure Sensors Based on Three-Scale Nested Wrinkling Microstructures of Polypyrrole Films

Pressure sensors have a variety of applications including wearable devices and electronic skins. To satisfy the practical applications, pressure sensors with a high sensitivity, a low detection limit, and a low-cost preparation are extremely needed. Herein, we fabricate highly sensitive pressure sensors based on hierarchically patterned polypyrrole (PPy) films, which are composed of three-scale nested surface wrinkling microstructures through a simple process. Namely, double-scale nested wrinkles are generated via in situ self-wrinkling during oxidative polymerization growth of PPy film on an elastic poly­(dimethylsiloxane) substrate in the mixed acidic solution. Subsequent heating/cooling processing induces the third surface wrinkling and thus the controlled formation of three-scale nested wrinkling microstructures. The multiscale nested microstructures combined with stimulus-responsive characteristic and self-adaptive ability of wrinkling morphologies in PPy films offer the as-fabricated piezoresistive pressure sensors with a high sensitivity (19.32 kPa^–1), a low detection limit (1 Pa), an ultrafast response (20 ms), and excellent durability and stability (more than 1000 circles), these comprehensive sensing properties being higher than the reported results in literature. Moreover, the pressure sensors have been successfully applied in the wearable electronic fields (e.g., pulse detection and voice recognition) and microcircuit controlling, as demonstrated here

Highly Sensitive Wearable Pressure Sensors Based on Three-Scale Nested Wrinkling Microstructures of Polypyrrole Films

Pressure sensors have a variety of applications including wearable devices and electronic skins. To satisfy the practical applications, pressure sensors with a high sensitivity, a low detection limit, and a low-cost preparation are extremely needed. Herein, we fabricate highly sensitive pressure sensors based on hierarchically patterned polypyrrole (PPy) films, which are composed of three-scale nested surface wrinkling microstructures through a simple process. Namely, double-scale nested wrinkles are generated via in situ self-wrinkling during oxidative polymerization growth of PPy film on an elastic poly­(dimethylsiloxane) substrate in the mixed acidic solution. Subsequent heating/cooling processing induces the third surface wrinkling and thus the controlled formation of three-scale nested wrinkling microstructures. The multiscale nested microstructures combined with stimulus-responsive characteristic and self-adaptive ability of wrinkling morphologies in PPy films offer the as-fabricated piezoresistive pressure sensors with a high sensitivity (19.32 kPa^–1), a low detection limit (1 Pa), an ultrafast response (20 ms), and excellent durability and stability (more than 1000 circles), these comprehensive sensing properties being higher than the reported results in literature. Moreover, the pressure sensors have been successfully applied in the wearable electronic fields (e.g., pulse detection and voice recognition) and microcircuit controlling, as demonstrated here

Light-Modulated Surface Micropatterns with Multifunctional Surface Properties on Photodegradable Polymer Films

Photodegradable polymers constitute an emerging class of materials that are expected to possess advances in the areas of micro/nano- and biotechnology. Herein, we report a green and effective strategy to fabricate light-responsive surface micropatterns by taking advantage of photodegradation chemistry. Thanks to the molecular chain breakage during the photolysis process, the stress field of photodegradable polymer-based wrinkling systems undergoes continuous disturbance, leading to the release/reorganization of the internal stress. Revealed by systematic experiments, the light-induced stress release mechanism enables the dynamic adaption of not only thermal-induced labyrinth wrinkles, but uniaxially oriented wrinkle microstructures induced by mechanical straining. This method paves the way for their diverse applications, for example, in optical information display and storage, and the smart fabrication of multifunctional surfaces as demonstrated here

Simple and Versatile Strategy to Prevent Surface Wrinkling by Visible Light Irradiation

A stiff film bonded to a compliant substrate is susceptible to surface wrinkling when it is subjected to in-plane compression. Prevention of surface wrinkling is essential in many cases to maintain the integrity and functionality of this kind of system. Here we report a simple versatile technique to restrain surface wrinkling of an amorphous poly­(<i>p</i>-aminoazobenzene) (PAAB) film by visible light irradiation. The key idea is to use the combined effects of photosoftening of the PAAB film and the stress release induced by the reversible photoisomerization. The main finding given by experiments and dimensional analysis is that the elastic modulus <i>E</i>_f of the film is well modulated by the ratio of light intensity and the release rate, i.e., <i>I</i>/<i>V</i>. Furthermore, the explicit solution describing the correlation of <i>I</i>/<i>V</i> with <i>E</i>_f is derived for the first time. The difference between the calculated critical wrinkling strain ε_c,t based on <i>E</i>_f and the experimentally measured value ε_c enables us to quantitatively evaluate the release amount of the compressive stress in the film. These key solutions provide a simple strategy to prevent the undesired surface wrinkling. Additionally, they allow us to propose a wrinkling-based technique to investigate photoinduced changes in the mechanical properties of azo-containing materials