Quantifying the stress relaxation modulus of polymer thin films via thermal wrinkling

Controlled Preparation of Highly Stretchable, Crack-Free Wrinkled Surfaces with Tunable Wetting and Optical Properties

Constructing wrinkles by utilizing strain-driven surface instability in film-substrate systems is a general method to prepare micronano structures, which have a wide range of applications in smart surfaces and devices such as flexible electronics, reversible wetting, friction, and optics. However, cracks generated during the preparation and use process significantly affect the uniformity of wrinkled surfaces and degrade the functional properties of the film devices. The realization of crack-free wrinkles with high stretchability in hard film systems is still a great challenge. Here, we report on a facile technique for controllable preparation of large-area, highly stretchable, crack-free wrinkled surfaces by ultraviolet ozone (UVO) treatment of Ecoflex. The thickness dependence of the wrinkles and the in situ wrinkling process during mechanical loading are investigated. The wrinkles including striped, labyrinth-like, herringbone, and transitional structures are controllable by changing strain mode (uniaxial or biaxial), loading history (simultaneous or sequential), strain anisotropy, and gradient loading. The wrinkled surfaces obtained using UVO-treated Ecoflex have tunable wetting and optical properties and can maintain excellent mechanical stability under large strains. This study provides a facile method for the preparation of large-area, crack-free wrinkles, which is simple, fast, low-cost, and robust. The resulting wrinkled surfaces remain stable under high stretching, which is beneficial for many practical applications, especially in the cases of large strains.

Hierarchical Surface Instability in Polymer Films

This study demonstrates hierarchical instabilities in thin films. The hierarchical instabilities display three morphological characteristics: (1) windmill-like patterns at the macroscale, (2) Bénard cells and striations at the microscale, and (3) holes at the mesoscale. Such hierarchical instabilities occurred when spin coating was performed on high-volatile solutions under a high relative humidity (RH) but were suppressed when spin coating was performed on low-volatile solutions regardless of the RH. The high-volatile solutions comprise poly(4-vinylpyridine) (P4VP) in methanol or ethanol. The low-volatility solutions comprise P4VP in propanol or butanol. P4VP molecular weights, P4VP concentrations, spin rates, and film thicknesses are not vital factors in forming hierarchical instability in spin-coated P4VP films. Instead, the formation of hierarchical instabilities depends on the RH and solvent types. Namely, the hierarchical instabilities are driven by Bénard-Marangoni convection, water vapor condensation, and disturbance of spin-up and spin-off stages during spin coating of highly volatile solutions under high RH. Mechanisms of hierarchical instabilities are interpreted in detail.

Flat, wrinkled, or ridged: Relaxation of an elastic film on a viscous substrate undergoing continuous compression

We conduct a finite element computational study of the dynamics of a thin elastic film bonded to a much thicker viscous substrate undergoing compression at a fixed rate. The applied compression tends to continuously increase the strain, and hence the elastic energy, of the film. In contrast to the well-studied case of a soft elastic substrate, a viscous substrate cannot store elastic energy; instead it regulates the kinetics of the various mechanisms that dissipate elastic energy of the film. Sufficiently short films remain flat because shear flow in the liquid near the ends allows rapid relaxation of the strain over the entire film length. In longer films, end-relaxation cannot relax film strain in the mid-section, which therefore buckles. Buckles initially appear as packets of approximately-sinusoidal wrinkles. With increasing strain, these packets transform into tall localized ridges separated by nearly flat regions. In all cases, the buckles cause end-relaxation to become dynamically confined to a narrow region near the ends We construct a state map identifying regions of the parameter space of strain vs film length in which the film remains flat, develops wrinkle packets, or develops localized ridges. The evolution of the film energy during continuous compression shows that ridge localization appears due to a competition between two effects: a well-spaced ridges offer a lower energy state than uniform wrinkles, but wrinkles can develop faster because they require the viscous fluid to move over shorter distances.

Real-Time Imaging of Stress in Single Spherulites and Its Relaxation at the Single-Particle Level in Semicrystalline Polymers

Crystallization-induced microscopic stress and its relaxation play a vital role in understanding crystallization behavior and mechanism. However, the real-time measurements for stress and its relaxation seem to be an unachievable task due to difficulties in simultaneous labeling, spatiotemporal discrimination, and continuous quantification. We designed a micron-sized fluorescent probe, whose fluorescence can respond to stress-induced environmental rigidity and whose three-dimensional (3D) flow can respond to stress relaxation. Using the as-prepared fluorescent probe, we established a versatile strategy to realize the real-time 3D imaging of stress and its relaxation in the crystallization process. The rigidity-responsive fluorescence clearly indicated the stress, while the 3D flow movement could quantify the stress relaxation. It is revealed that stress in spherulites increased dramatically as a result of the suppression of stress relaxation in polymer melts. The developed method provides a novel avenue to simultaneously detect stress and its relaxation in various semicrystalline polymers at the single-particle level. This success would achieve the microscopic ways to guide the development of advanced crystallization-dependent materials.

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.

Direct Comparison of Three Buckling-Based Methods to Measure the Elastic Modulus of Nanobiocomposite Thin Films

To engineer tunable thin-film materials, the accurate measurement of their mechanical properties is crucial. However, characterizing the elastic modulus with current methods is particularly challenging for sub-micrometer thick films and hygroscopic materials because they are highly sensitive to environmental conditions and most methods require free-standing films which are difficult to prepare. In this work, we directly compared three buckling-based methods to determine the elastic moduli of supported thin films: (1) biaxial thermal shrinking, (2) uniaxial thermal shrinking, and (3) the mechanically compressed, strain-induced elastic buckling instability for mechanical measurements (SIEBIMM) method. Nanobiocomposite model films composed of cellulose nanocrystals (CNCs) and polyethyleneimine (PEI) were assembled using layer-by-layer deposition to control composition and thickness. The three buckling-based methods yielded the same trends and comparable values for the elastic moduli of each CNC-PEI film composition (ranging from 15 to 44 GPa, depending on film composition). This suggests that the methods are similarly effective for the quantification of thin-film mechanical properties. Increasing the CNC content in the films statistically increased the modulus; however, increasing the PEI content did not lead to significant changes. For the CNC-PEI system, the standard deviation of elastic moduli determined from SIEBIMM was 2-4 times larger than that for thermal shrinking, likely due to extensive cracking due to the different stress applied to the film when subjected to compression of a relaxed substrate versus the shrinking of a pre-strained substrate. These results show that biaxial thermal shrinking is a reliable method for the determination of the mechanical properties of thin films with a simple implementation and analysis and low sensitivity to small deviations in the input parameter values, such as film thickness or substrate modulus.

Observation of General Entropy-Enthalpy Compensation Effect in the Relaxation of Wrinkled Polymer Nanocomposite Films

Surface-textured polymer nanocomposite (PNC) films are utilized in many device applications, and therefore understanding the relaxation behavior of such films is important. By extending an in situ wrinkle relaxation method, we observed that the thermal stability of wrinkled PNC films, both above and below the glass transition temperature (T_g), is proportional to a film's nanoparticle (polymer grafted and bare) concentration, with a slope that changes sign at a compensation temperature (T_comp) that is determined to be in the vicinity of the film's T_g. This provides unambiguous confirmation of entropy-enthalpy compensation (EEC) as a general feature of PNC films, implying that the stability of PNC films changes from being enhanced to becoming diminished by simply passing through this characteristic temperature, a phenomenon having evident practical ramifications. We suggest EEC will also arise in films where residual stresses are associated with the film fabrication process, which is relevant to nanotech device applications.

High-Definition Optophysical Image Construction Using Mosaics of Pixelated Wrinkles

Despite many efforts in structuring surfaces using mechanical instabilities, the practical application of these structures to advanced devices remains a challenging task due to the limited capability to control the local morphology. A platform that programs the orientation of mechanically anisotropic molecules is demonstrated; thus, the surface wrinkles, promoted by such instabilities, can be patterned in the desired manner. The optics based on a spatial light modulator assembles wrinkle pixels of a notably small dimension over a large area at fast fabrication speed. Furthermore, these pixelated wrinkles can be formed on curved geometries. The pixelated wrinkles can record images, which are naturally invisible, by mapping the gray level to the orientation of wrinkles. They can retrieve those images using the patterned optical phase retardation generated under the crossed polarizers. As a result, it is shown that the pixelated wrinkles enable new applications in optics such as image storage, informative labeling, and anti-counterfeiting.

Liquid crystal polymers with motile surfaces

In analogy with developments in soft robotics it is anticipated that soft robotic functions at surfaces of objects may have a large impact on human life with respect to comfort, health, medical care and energy. In this review, we demonstrate the possibilities and versatilities of liquid crystal networks and elastomers being explored for soft robotics, with an emphasis on motile surface properties, such as topographical dynamics. Typically the surfaces reversibly transfer from a flat state to a pre-designed corrugated state under various stimuli. But also reversible conversion between different corrugated states is feasible. Generally, the driving triggers are heat, light, electricity or contact with pH changing media. Also, the macroscopic effects of those dynamic topographies, such as altering the friction, wettability and/or performing work are illustrated. The review concludes with the existing challenges as well as outlook opportunities.

Cumulative energy analysis of thermally-induced surface wrinkling of heterogeneously multilayered thin films

Wrinkling is a well-known example of instability-driven surface deformation that occurs when the accumulated compressive stress exceeds the critical value in multilayered systems. A number of studies have investigated the instability conditions and the corresponding mechanisms of wrinkling deformation. Force balance analysis of bilayer systems, in which the thickness of the capping layer is importantly considered, has offered a useful approach for the quantitative understanding of wrinkling. However, it is inappropriate for multilayer wrinkling (layer number > 3) consisting of heterogeneous materials (e.g. polymer/metal or inorganic), in which the thickness variation in the substrate is also crucial. Therefore, to accommodate the additive characteristics of multilayered systems, we thermally treated tri- or quad-layer samples of polymer/metal multilayers to generate surface wrinkles and used a cumulative energy balance analysis to consider the individual contribution of each constituent layer. Unlike the composite layer model, wherein the thickness effect of the capping layer is highly overestimated for heterogenously stacked multilayers, our approach precisely reflects the bending energy contribution of the given multilayer system, with results that match well with experimental values. Furthermore, we demonstrate the feasibility of this approach as a metrological tool for simple and straightforward estimation of the thermomechanical properties of polymers, whereby a delicate change in the Young's modulus of a thin polymeric layer near its glass transition temperature can be successfully monitored.

Competitive concurrence of surface wrinkling and dewetting of liquid crystalline polymer films on non-wettable substrates

Polymeric thin films coated on non-wettable substrates undergo film-instabilities, which are usually manifested as surface deformation in the form of dewetting or wrinkling. The former takes place in fluidic films, whereas the latter occurs in solid films. Therefore, there have rarely been reports of systems involving simultaneous deformations of dewetting and wrinkling. In this study, we propose polymeric thin films of liquid crystalline (LC) mesogens prepared on a non-wettable Si substrate and apply a treatment of plasma irradiation to form a thin polymerized layer at the surface. The resulting compressive stress generated in the surface region drives the formation of wrinkles, while at the same time, dipolar attraction between LC molecules induces competitive cohesive dewetting. Intriguing surface structures were obtained whereby dewetting-like hole arrays are nested inside the randomly propagated wrinkles. The structural features are readily controlled by the degree of surface cross-linking, hydrophilicity of the substrates, and the LC film thickness. In particular, dewetting of LC mesogens is observed to be restricted to occur at the trough regions of wrinkles, exhibiting the typical behavior of geometrically confined dewetting. Finally, wrinkling-dewetting mixed structures are separated from the substrate in the form of free standing films to demonstrate the potential applicability as membranes.

Controlling self-patterning of acrylate films by photopolymerization

Acrylate formulations can spontaneously generate surface patterns by UV-curing in open-air.

Controlled Formation of Surface Patterns in Metal Films Deposited on Elasticity-Gradient PDMS Substrates

Controlled surface patterns are useful in a wide range of applications including flexible electronics, elastomeric optics, fluidic channels, surface engineering, measurement technique, biological templates, stamps, and sensors. In this work, we report on the controlled formation of surface patterns in metal films deposited on elasticity-gradient polydimethylsiloxane (PDMS) substrates. Because of the temperature gradient during the curing process, the PDMS substrate in each sample successively changes from a purely liquid state at one side to a purely elastic state at the opposite side. It is found that surface folds appear in the liquid or viscous PDMS region while wrinkles form in the elastic region. In the transition region from the liquid to elastic PDMS, a nested pattern (i.e., the coexisting of folds and wrinkles) can be observed. The folding wave is triggered by the intrinsic stress during the film deposition and its wavelength is independent of the film thickness. The wrinkling wave is induced by the thermal compression after deposition and its wavelength is proportional to the film thickness. The report in this work could promote better understanding of the effect of substrate elasticity on the surface patterns and fabrication of such patterns (folds and wrinkles) by tuning the substrate property.

Self-organized wrinkling patterns of a liquid crystalline polymer in surface wetting confinement

Self-organized wrinkling patterns of a liquid crystalline polymer, dictated by the chemico-physically anisotropic nature of surface wettability, are demonstrated in confined geometries. The symmetry of the geometrical constraints of the confinement primarily governs the periodic wrinkling patterns of such a polymer in the wetting region. In a circular geometry, the number of the radial domains with multi-fold symmetries is linearly proportional to the radius of the confinement. The physical origin of the wrinkling process comes from the periodic bend-splay distortions through the relaxation of the curvature elasticity.

Tunable formation of ordered wrinkles in metal films with controlled thickness gradients deposited on soft elastic substrates

Controlled wrinkled surface is useful for a wide range of applications, including flexible electronics, smart adhesion, wettability, stamping, sensoring, coating, and measuring. In this work, thickness-gradient-guided spontaneous formation of ordered wrinkling patterns in metal films deposited on soft elastic substrates is revealed by atomic force microscopy, theoretic analysis, and simulation. It is observed that in the thicker film region, broad cracks form, and the film surface remains flat. In the thinner film region, the cracks attenuate along the direction of the thickness decrease, and various wrinkle patterns including branched stripes, herringbones, and labyrinths can coexist. The interplay between the residual compression and the thickness gradient leading to the formation of such wrinkling patterns is discussed based on a nonlinear wrinkling model. The simulated wrinkling patterns as well as the variation trends of the wrinkle wavelength and amplitude along the gradient direction are in good agreement with the experimental observations. The report in this work could promote better understanding and fabrication of such ordered wrinkling patterns by tunable thickness gradient.

Tunable wrinkle and crease surface morphologies from photoinitiated polymerization of furfuryl alcohol

Addition of a small fraction of hydrophobic photoacid generator (PAG) to furfuryl alcohol provides a facile route to generate wrinkle topology by acid-catalyzed polymerization that is induced by ultraviolet (UV) light. Here, we describe how the primary characteristic parameters, wavelength and amplitude, of these periodic wrinkles can be tuned through control of the thickness of this furfuryl alcohol-PAG solution prior to UV exposure and the environmental humidity. As the initial coating thickness is increased, the wavelength remains unchanged at fixed temperature and PAG concentration, but the amplitude of the wrinkles increases exponentially with increased coating thickness. A wrinkle to crease transition is observed in some cases as the thickness of the solution coating is increased; this behavior is dependent on the PAG selection. Conversely, variation in relative humidity does not significantly impact the amplitude of the wrinkles, but there is a step change in the wavelength of the wrinkles near approximately 45% relative humidity with a factor of 3 decrease in the wavelength at high humidity. Through this knowledge, we have been able to fabrication wrinkles with an aspect ratio greater than 0.7 in a single step by UV exposure. These simple processing parameters to independently control wavelength and amplitude provide a facile route to systematically examine the role of aspect ratio of wrinkles on physical properties.

Surface wrinkling induced by photofluidization of low molecular azo glasses

Photoinduced surface wrinkling is demonstrated for a low molecular azo compound confined between a substrate and a thin inorganic elastic layer. The wrinkling process is investigated by time-resolved light scattering as well as by two-dimensional autocorrelation analysis of microscopic images. It is shown that the temporal evolution of the wrinkling pattern is directly controlled by the amount of photons absorbed by the sample and that there is no significant dependence of the equilibrium wavelength on irradiation intensity. Finally, the comparison of thermal and photoinduced wrinkling revealed that photoinduced wrinkles are characterized by a narrower mode distribution and less coarsening.

Structure and macroscopic tackiness of ultrathin pressure sensitive adhesive films

Ultrathin layers of the statistical copolymer P(nBA-stat-MA) with a majority of n-butyl acrylate (nBA) and a minority of methyl acrylate (MA) are characterized with respect to the film morphology and the mechanical response in a probe tack test. The probed copolymer can be regarded as a model system of a pressure sensitive adhesive (PSA). The films are prepared by spin-coating which enables an easy thickness control via the polymer concentration of the solution. The film thickness is determined with x-ray reflectivity (XRR) and white light interferometry (WLI). Grazing incidence small angle x-ray scattering (GISAXS) provides detailed and statistically significant information about the film morphology. Two types of lateral structures are identified and no strong correlation of these structures with the PSA film thickness is observed. In contrast, prominent parameters of the probe tack test, such as the stress maximum and the tack energy, exhibit an exponential dependence on the film thickness.

Hierarchical nanoflake surface driven by spontaneous wrinkling of polyelectrolyte/metal complexed films

A mechanical or physical change observed in nanocomposite thin films has recently offered new opportunities to generate intriguing nanostructures. In this study, we present a novel means of creating a hierarchically developed nanoflake structure by exploiting surface wrinkles that occur during the incorporation process of metallic nanoparticles into layer-by-layer assembled polyelectrolyte multilayer (PEM) thin films. The PEM film composed with linear polyethylenimine (LPEI) and poly(acrylic acid) (PAA) allows for facilitated cationic exchange reaction within the film even after the electrostatic complexation and chemical cross-linking reaction. The subsequent reduction process induces an in situ complexation of metallic nanoparticles with a PEM matrix, causing an accumulation of lateral compressive stress for surface wrinkling. The wrinkling characteristics of the complexed films can be theoretically interpreted by employing the gradationally swollen film model, whereby a gradual change in the elastic property along the axial direction of the film can be appropriately reflected. In addition, wrinkled surfaces are further processed to form vertically aligned and hierarchically ordered nanoflakes after selective removal of the PEM matrix with plasma ashing. Consequently, superhydrophobic surface properties (water contact angle = 170°, sliding angle <1°) can be attained from the hierarchical nanoflake structure. The method presented here is advantageous in that large-scale preparation can be readily implemented by a stepwise dipping process without resorting to specific patterning or a serially applied complex structuring process, which can provide a promising platform technique for various surface engineering applications.

Self-organized anisotropic wrinkling of molecularly aligned liquid crystalline polymer

Anisotropic wrinkling which utilizes the anisotropic nature of liquid crystalline polymer (LCP) is demonstrated as a means of physical self-assembly to produce periodic microstructures. Through the plasma treatment on the molecularly aligned LCP film surface, one-dimensionally ordered wrinkle pattern was spontaneously formed on glass substrates without employing external thin-film deposition or prestrain control of the system. Experimental results indicate that the directionality of the wrinkle pattern can be tailored by the structural ordering of LCP molecules in the bilayer system of a hard skin layer on a soft substrate. Studies on process variables, such as the plasma treatment time and the film thickness, were conducted to figure out the effect on the wrinkling morphology. Due to its spatial periodicity over a large area and undemanding requirement of the process, this approach can be a candidate for the microfabrication in various applications.

Swelling-induced delamination causes folding of surface-tethered polymer gels

When a polymer film that is weakly attached to a rigid substrate is exposed to solvent, swelling-induced compressive stress nucleates buckle delamination of the film from the substrate. Surprisingly, the buckles do not have a sinusoidal profile, instead, the film near the delamination buckles slides toward the buckles causing growth of sharp folds of high aspect ratio. These folds do not result from a wrinkle-to-fold transition; instead, the film goes directly from a flat state to a folded state. The folds persist even after the solvent evaporates. We propose that patterned delamination and folding may be exploited to realize high-aspect ratio topological features on surfaces through control of a set of boundary constraints arising from the interrelation of film-surface adhesion, film thickness and degree of swellabilty.