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

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

Conformation Selected Direct Formation of Form I in Isotactic Poly(butene-1)

Though the transition from metastable to stable crystal (from form II to form I) of isotactic poly­(butene-1) (iPB-1) after melt crystallization looks to be unavoidable, form I direct formation in bulk iPB-1 via a bypass of form II is still a charming approach to the crystalline structure control in iPB-1. However, the physics behind this transition still remains elusive and there are many arguments. In this work, DSC and WAXS were used to investigate the crystallization behaviors of iPB-1 in detail and direct formation of form I in bulk iPB-1 was found able to occur within a temperature range from the glass transition temperature <i>T</i>_g to a critical temperature <i>T</i>_cr = 35 °C, far below the crystallization temperature <i>T</i>_c of form II. The temperature-dependent 3/1 helix conformation formation within this temperature range was proposed to interpret the further direct formation of form I. Formation of iPB-1 polymorphic structures seem to be controlled by temperature-dependent helix conformation formation. The results may trigger further discussions or shed light on the understanding of the physics behind polymer crystallization and polymorph selection processes as well as the development of crystal structure controlling techniques and applications of iPB-1

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

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

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

Bioinspired Fabrication of Free-Standing Conducting Films with Hierarchical Surface Wrinkling Patterns

Mechanical instability has been shown to play an important role in the formation of wrinkle structures in biofilms, which not only can adopt instability modes as templates to regulate their 3D architectures but also can tune internal stresses to achieve stable patterns. Inspired by nature, we report a mechanical–chemical coupling method to fabricate free-standing conducting films with instability-driven hierarchical micro/nanostructured patterns. When polypyrrole (PPy) film is grown on an elastic substrate <i>via</i> chemical oxidation polymerization, differential growth along with <i>in situ</i> self-reinforcing effect induces stable wrinkle patterns with different scales of wavelengths. The self-reinforcing effect modifies the internal stresses, hence PPy films with intact wrinkles can be removed from substrates and further transferred onto target substrates for functional device fabrication. To understand the buckling mechanics, we construct a model which reveals the formation of hierarchical wrinkle patterns