Stress generation by solvent absorption and wrinkling of a cross-linked coating atop a viscous or elastic base

Growth-Induced Wrinkles and Dotlike Patterns of a Swollen Fluoroalkylated Thin Film by the Reaction of Surface-Attached Polymethylhydrosiloxane

The design of hydrophobic surfaces requires a material which has a low solid surface tension and a simple fabrication process for anchoring and controlling the surface morphology. A generic method for the spontaneous formation of robust instability patterns is proposed through the hydrosilylation of a fluoroalkene bearing dangling chains, R_f = C₆F₁₃(CH₂)₃-, with a soft polymethylhydrosiloxane (PMHS) spin-coated gel polymer (0.8 μm thick) using Karstedt catalyst. These patterns were easily formed by an irreversible swelling reaction due to the attachment of a layer to various substrates. The buckling instability was created by two different approaches for a gel layer bound to a rigid silicon wafer substrate (A) and to a soft nonswelling silicone elastomer foundation (B). The observations of grafted R_f-PMHS films in the swollen state by microscopy revealed two distinct permanent patterns on various substrates: dotlike of wavelength λ = 0.4-0.7 μm (A) or wrinkle of wavelength λ = 4-7 μm (B). The elastic moduli ratios of film/substrate were determined using PeakForce quantitative nanomechanical mapping. The characteristic wavelengths (λ) of the patterns for systems A and B were quantitatively estimated in relation to the thickness of the top layer. A diversity of wrinkle morphologies can be achieved by grafting different side chains on pristine PMHS films. The water contact angle (WCA) hysteresis of fluorinated chain (R_f) was enhanced upon roughening the surfaces, giving highly hydrophobic surface properties for water with static/hysteresis WCAs of 136°/74° in the resulting wrinkle (B) and 119°/41° in the dotlike of lower roughness (A). The hydrophobic properties of grafted films on A with various mixtures of hexyl/fluoroalkyl chains were characterized by static CA: WCA 104-119°, ethylene glycol CA 80-96°, and n-hexadecane CA 17-61°. A very low surface energy of 15 mN/m for R_f-PMHS was found on the smoother dotlike pattern.

Effect of Copolymer on the Wrinkle Structure Formation and Gloss of a Phase-Separated Ternary Free-Radical/Cationic Hybrid System for the Application of Self-Matting Coatings

Hybrid free-radical/cationic systems can generate phase-separated polymers or interpenetrating networks driven by photopolymerization. In this study, phase separation of a ternary mixture composed of a polybutadiene urethane diacrylate (PBUDA), a cycloaliphatic diepoxyde (CE), and hexanediol dimethacrylate (HDDMA) was investigated. Using systematic variations of the initial composition of the mixture, a miscibility phase diagram of the ternary mixture was established. Based on this diagram, a reactive copolymer (poly(butyl acrylate-co-glycidyl methacrylate) (PBGMA)) was introduced in a reference hybrid system to manipulate the crosslinking network, polymer morphology, and properties (e.g., roughness, gloss, strain at break, and glass transition temperature T_g). When cured as a coating, the ternary hybrid system showed a depthwise gradient of epoxy conversion, and thereby developed a mostly cured skin above a viscous sublayer of uncured monomer. This skin can develop compressive stress due to the swelling from the diffusion of unreacted monomers beneath, and if the compressive stress is significantly high, wrinkles appear on the coating's surface. This work highlights how both skin thickness and elastic modulus impact wrinkle frequency and amplitude. It was demonstrated that these wrinkle parameters can be manipulated in the ternary system by the addition of PBGMA. We also demonstrated that by employing UV irradiation and varying the PBGMA content, it is possible to engineer coatings that range from smooth surfaces with high gloss to wrinkled topographies with a very low associated gloss.

Surface Wrinkling on Polymer Films

A series of gold precursor solutions are prepared by dissolving HAuCl₄ and its mixtures with K₂CO₃ of different contents in deionized (DI) water. Neat HAuCl₄ predominately forms AuCl₄^- ions in an aqueous solution. In the presence of K₂CO₃, AuCl₄^- ions hydrolyze to form [AuCl_4-x(OH)_x]^- complex ions. Increasing the content of K₂CO₃ in a gold precursor solution increases the content of [AuCl_4-x(OH)_x]^- complex ions and decreases the content of AuCl₄^- ions. Poly(4-vinyl pyridine) (P4VP) films of two different molecular weights are deposited on SiO_x/Si by spin coating, by which the thicknesses are controlled by polymer weight fractions in butanol. Those P4VP films form periodic wrinkles when immersed in aqueous solutions, followed by drying. The surface wrinkling is induced by swelling pressure that overwhelms the mechanical property of the P4VP film. The periodicity and amplitude of wrinkles grown on the P4VP films strongly correlate with initial thickness, AuCl₄^- ion content, and residual stress.

An Optical Steam Sterilization Sensor Based On a Dual-Responsive Supramolecular Cross-Linked Photonic Polymer

An optical time-temperature steam sensor is presented based on the loss of structural color in a supramolecularly cross-linked cholesteric liquid crystal photonic coating. A gradual decrease in the selective reflection band is observed upon exposure to temperatures above 105 °C related to the cholesteric to isotropic transition temperature. The linear polymers with carboxylic acid side chains provide physical cross-linking through hydrogen bonding that allows a time-temperature-dependent order loss through the dynamic equilibrium between supramolecular dimer and free monomer states. Steam is accelerating the color loss, and autoclave experiments show that the photonic supramolecular polymer is applicable as a steam sterilization sensor for medical applications.

Recent advances in wrinkle-based dry adhesion

Surface wrinkles driven by elastic instabilities have attracted significant interest in the field of materials science and engineering. They are simple and readily fabricated with various patterns of tunable size, morphology and surface topography from a wide range of material systems. Recently, they have been investigated as a new type of dry adhesives. In this review, after a brief introduction of different methods to prepare wrinkle surfaces, we focus on the investigation of dry adhesion mechanisms in different material systems. By exploiting wrinkle dimension, morphology, modulus, curvature, and different contacting surfaces (flat, hemispherical, spherical) and their complementarity, we show adhesion enhancement, reduction and selectivity. By comparing experimental results with theoretical predictions, we aim to provide a guideline to design and engineer wrinkle-based dry adhesives. Several examples of applications of engineered wrinkles are also demonstrated, including pick, release and transfer of nanoparticles and bulk materials, and gecko-like hybrid adhesives. The review is concluded with perspectives on the wrinkling technology for smart dry adhesives.

Folding and birefringence behavior of poly(vinyl alcohol) hydrogel film induced by freezing and thawing

A band-like folding structure with high aspect ratio, parallel or perpendicular to the flat film, and birefringence behavior was observed for an in situ formed thin PVA hydrogel film, the folding structure being formed via freezing–thawing treatment of PVA aqueous solution coated on glass.

Modeling kinetics of diffusion-controlled surface wrinkles

Nonlinear wrinkling of a compressed film on a soft substrate in the presence of inhomogeneous swelling actuation strain caused by solvent diffusion is studied. The simulation relies on a continuum model which integrates phase field microelasticity and Föppl-von Kármán plate theory. We show that the wrinkling morphologies developed in the diffusive domain exceeding a critical compression are confined and become shape and size dependent. A rich variety of wrinkling patterns observed in experiments including hexagonal ordered, dimple, or peanut structures, are numerically recovered, depending on the distribution of diffusion-mediated actuation strain. A cascade feature of the diffusion-coupled wrinkle is demonstrated as well: There are two ranges of solvent concentration within which the sequences of wrinkling pattern are different.

Highly oriented tunable wrinkling in polymer bilayer films confined with a soft mold induced by water vapor

The authors report the formation of highly oriented wrinkling on the surface of the bilayer [polystyrene (PS)/poly(vinyl pyrrolidone) (PVP)] confined by a polydimethylsiloxane (PDMS) mold in a water vapor environment. When PVP is subjected to water vapor, the polymer loses its mechanical rigidity and changes to a viscous state, which leads to a dramatic change in Young's modulus. This change generates the amount of strain in the bilayer to induce the wrinkling. With a shape-controlled mold, they can get the ordered wrinkles perfectly perpendicular or leaned 45 degrees to the channel orientation of the mold because the orientation of the resultant force changes with the process of water diffusion which drives the surface to form the wrinkling. Additionally, they can get much smaller wrinkles than the stripe spacing of PDMS mold about one order. The wrinkle period changes with the power index of about 0.5 for various values of the multiplication product of the film thicknesses of the two layers, namely, lambda approximately (h(PS)h(PVP))(1/2).