Controlling the Orientation of Nanowrinkles and Nanofolds by Patterning Strain in a Thin Skin Layer on a Polymer Substrate

Abstract

We describe herein how to control the orientation of polymer nanowrinkles and nanofolds with large amplitudes. Nanowrinkles were created by chemically treating thermoplastic polystyrene sheets to form a thin skin layer and then heating the substrate to relieve strain. By manipulating the strain globally and locally in the skin layer, we could tune whether wrinkles or folds formed, as well as the distances over which these structures could be produced. This unique materials system provided access to high strain regimes, which enabled mechanisms behind the spontaneous formation of complex structures to be explored.Over every length scale, wrinkles form when a compressive strain is applied to a stiff skin on a soft substrate.[1] The average wrinkle periodicity (l) can be predicted from the skin thickness (h) and the ratio of the Young modulus of the skin (E S ) and substrate (E B ). [1][2][3] For microscale and nanoscale wrinkles, the skin layer is typically a stiffened polymer or deposited metal film, whereas substrate examples include compressible polymers or fluids. [2,[4][5][6][7] Wrinkling has emerged as a way to control the mechanical, electrical, and optical properties of a substrate without changing the bulk properties of the material. [3,4] For example, polymer microwrinkles can increase hydrophobicity while maintaining mechanical flexibility.[8] We previously reported how the wavelengths of nanowrinkles (l < 500 nm) could be tuned down to 30 nm by creating a skin layer on thermoplastic polystyrene (PS) sheets through plasma treatment with fluorinated gases.[9] This chemical patterning method enabled access to wrinkle wavelengths much smaller than those accessible with other systems [1,9] because of the low Youngs modulus ratio (E S / E B < 10) and thin skin layers (h < 20 nm).Besides l, wrinkle amplitude (A) is another important parameter that defines the properties of a substrate. Uniformly distributed 2D strain results in randomly oriented nanowrinkles with a value A that depends on the amount of applied strain (e) until a critical threshold is reached. In this strain regime, A has been limited to less than 10 nm for nanowrinkles. [1,4] Above this threshold, strain is relieved by the formation of a second generation of wrinkles with a larger periodicity [4] or by a wrinkle-to-fold transition, [5,10] whereby strain is concentrated into periodic folds. [7,11] Such structures that form under high strain may enable additional tuning of surface properties; however, the materials characteristics that lead to these structures are not well understood.[3] Patterning of the local strain distribution in the skin layer at the microscale is another strategy to create ordered structures from inherently disordered patterns. [2,[12][13][14][15][16] For example, etched features in a gold skin layer on a poly(dimethylsiloxane) substrate have been used to orient microwrinkles over distances up to 17 times the wrinkle wavelength. [2,12] Although micro-and nanoscale patterns [17] have been used to control the ampli...

Tuning and Erasing Surface Wrinkles by Reversible Visible‐Light‐Induced Photoisomerization

“…Foragiven system, anumber of theoretical and computational models have been proposed that enable us to predict s c . [11,12] However,r eliable manipulation of stress states in the system to control wrinkling patterns remains ac hallenging issue and has received considerable interest in the past few years.A side from controlling the magnitude of the external/internal stimuli, [13] adesired stress/deformation state may be achieved by modifying the topographic features on the underlying substrates [1,14] or stiff films, [15,16] by making use of curvature effects, [17] and/or by adopting template-directed confinement effects. [18] Herein, we report as imple yet powerful method to dynamically tune and/or erase the surface wrinkles on an azocontaining poly(disperse orange 3) (PDO 3 )f ilm bonded to apoly(dimethylsiloxane) (PDMS) substrate with visible light.…”

Concurrent design of quasi-random photonic nanostructures

“…As a proofof-concept demonstration, we rapidly optimized threedimensional structures for light trapping in amorphous silicon and realized >160% enhancement in light absorption over the 800-1,200-nm wavelength range. First, polymer nanowrinkles were generated by strain relief of a reactive ion etching (RIE) CHF 3 -plasma treated polystyrene (PS) substrate (24)(25)(26). The 3D PS wrinkles functioned as a master, where poly(dimethysiloxane) (PDMS) was then cast against the pattern to produce a mold for solvent-assisted nanoscale embossing (SANE) (27).…”

Controlling Wrinkle Propagation in the Bilayer System with Thickness‐Gradient

“…Confinement-induced wrinkle orientation guidance has been demonstrated in a number of studies for control of wrinkle structures. [3,5,[14][15][16] At the boundary of the system, the compressive stress becomes asymmetric; [3,5] the stress along the axis parallel to the boundary of the system converges to a particular value as the axis gets closer to the boundary, while stress in the direction lateral to the boundary converges to zero. [15][16][17] Due to the asymmetry of the stress at the boundary, a 2D wrinkle maintains its direction for Spontaneously formed structures on soft materials induced by mechanical instability have drawn attention for their lithography-free fabrication process and stretchable nature.…”

“…[13] Spontaneous generation of compressive stress through plasma treatment or thermal annealing is another method for inducing wrinkle structures. [3,5,[14][15][16] At the boundary of the system, the compressive stress becomes asymmetric; [3,5] the stress along the axis parallel to the boundary of the system converges to a particular value as the axis gets closer to the boundary, while stress in the direction lateral to the boundary converges to zero. Confinement-induced wrinkle orientation guidance has been demonstrated in a number of studies for control of wrinkle structures.…”