Alfred J. Crosby

Polymer Nanocomposites: The “Nano” Effect on Mechanical Properties

Polymer nanocomposites offer significant potential in the development of advanced materials for numerous applications. These novel materials benefit from the synergy between filler particles and polymer chains that are on similar length scales and the large quantity of interfacial area relative to the volume of the material. Although enhanced properties of these materials have been demonstrated by numerous researchers, our fundamental knowledge of the “nano” effect in terms of mechanical properties is not fully developed. In this article, we discuss the important properties of three components in a general polymer nanocomposite: the polymer matrix, the nanoscale filler, and the interfacial region. We highlight theory and experimental observations from several different fields to help guide the future research and development of understanding in this critical field.

Deformation and failure modes of adhesively bonded elastic layers

Adhesively bonded elastic layers with thicknesses that are small relative to their lateral dimensions are used in a wide variety of applications. The mechanical response of the compliant layer when a normal stress is imposed across its thickness is determined by the effects of lateral constraints, which are characterized by the ratio of the lateral dimensions of the layer to its thickness. From this degree of confinement and from the material properties of the compliant layer, we predict three distinct deformation modes: (1) edge crack propagation, (2) internal crack propagation, and (3) cavitation. The conditions conductive for each mode are presented in the form of a deformation map developed from fracture mechanics and bulk instability criteria. We use experimental data from elastic and viscoelastic materials to illustrate the predictions of this deformation map. We also discuss the evolution of the deformation to large strains, where nonlinear effects such as fibrillation and yielding dominate the fai...

Axisymmetric adhesion tests of soft materials

We describe a general, linearized fracture mechanics analysis for studying the adhesive properties of elastic, low modulus materials. Several adhesion tests are described, but all involve an elastic material which is brought into contact with a rigid surface along an axis of radial symmetry. Relationships between the load, displacement, and radius of the circular contact area between the two materials are described. These relationships involve the elastic modulus of the compliant material, the energy release rate (or adhesion energy) and various parameters which characterize the geometry of interest. The ratio of the contact radius to the thickness of the elastic material is shown to be a particularly important parameter. After reviewing some general concepts relevant to the adhesion of soft polymeric materials, we describe the fracture mechanics analysis, and provide examples from our own work on the adhesion of elastomers, thermoreversible gels and pressure sensitive adhesives.

Looking Beyond Fibrillar Features to Scale Gecko‐Like Adhesion

Hand-sized gecko-inspired adhesives with reversible force capacities as high as 2950 N (29.5 N cm(-2) ) are designed without the use of fibrillar features through a simple scaling theory. The scaling theory describes both natural and synthetic gecko-inspired adhesives, over 14 orders of magnitude in adhesive force capacity, from nanoscopic to macroscopic length scales.

Solvent‐Responsive Surface via Wrinkling Instability

Stimuli-responsive materials that undergo large changes in their properties and structures in response to external stimuli such as pH, [ 1 , 2 ] temperature, [ 3 , 4 ] light, [ 5 , 6 ] and solvent environment [ 7 , 8 ] have attracted a great deal of attention due to a wide range of potential applications in switchable surfaces, drug delivery, optical systems, coating, and biosensors. [ 9 ] Several strategies have been adopted for fabricating smart materials with hierarchical structures, such as self-assembly, [ 10 ]

Synthetically Simple, Highly Resilient Hydrogels

Highly resilient synthetic hydrogels were synthesized by using the efficient thiol-norbornene chemistry to cross-link hydrophilic poly(ethylene glycol) (PEG) and hydrophobic polydimethylsiloxane (PDMS) polymer chains. The swelling and mechanical properties of the hydrogels were controlled by the relative amounts of PEG and PDMS. The fracture toughness (G(c)) was increased to 80 J/m(2) as the water content of the hydrogel decreased from 95% to 82%. In addition, the mechanical energy storage efficiency (resilience) was more than 97% at strains up to 300%. This is comparable with one of the most resilient materials known: natural resilin, an elastic protein found in many insects, such as in the tendons of fleas and the wings of dragonflies. The high resilience of these hydrogels can be attributed to the well-defined network structure provided by the versatile chemistry, low cross-link density, and lack of secondary structure in the polymer chains.

Effect of stress state on wrinkle morphology

Wrinkles in thin films on soft substrates have been shown to self-organize into topological patterns, providing a possible route towards inexpensive generation of surface microstructure. However, the effect of the magnitude of applied stress in relation to the critical buckling stress, or overstress, on the observed patterns has until this point been neglected experimentally. In this paper, we investigate the effect of overstress using poly(dimethylsiloxane) which has been surface-oxidized with a UV-ozone oxidation technique. Using a swelling-based stress application technique, where the applied swelling stress in the thin film is controlled by changing the concentration of vapor-phase swelling agent (ethanol) in a sealed swelling chamber, we are able to impart swelling stresses below, at, and well above the critical stress. We observe a transition from hexagonally packed dimples at low overstress to ridge-based morphologies (herringbone and labyrinth) at high overstress. The observed dimple structures are remarkably widespread, and the hexagonal arrangement of these dimples is confirmed using Fourier analysis. Although analytical results predict that a square arrangement of dimples is preferred to hexagonal for flat wrinkling surfaces, hexagonal arrays are nonetheless unilaterally observed at low overstress. We attribute this observation to an inherent curvature that develops in the swelling film. The overstress is quantified by measuring the radius of curvature of swelling bilayer beams, both confirming the preferential swelling of the surface oxide layer by ethanol and quantifying the swelling extent. Effects of non-equibiaxial stress are investigated by inducing a compressive prestress prior to swelling, and “trapped” non-equilibrium morphologies are discussed briefly.

Adhesive failure analysis of pressure-sensitive adhesives

In this article we use a linear elastic fracture mechanics approach to characterize the adhesive performance of two commercially available pressure-sensitive adhesives (PSAs). An axisymmetric adhesion test involving the contact of a spherical indenter with a thin adhesive layer is used to generate “tack” curves for both adhesives. These curves describe the relationship between the normal loads and displacements during the test. Adhesive failure is understood in terms of crack propagation at the indenter/adhesive interface. We investigate the effects of adhesive layer thickness and crosshead velocity on the tack curves. Using fracture mechanics equations developed for thin layers, we show that the energy release rate is a unique function of the crack velocity for a given adhesive. Based on the tack curves and energy release rates, we discuss the coupling of the bulk and interfacial properties that produce the large adhesion energies typical of pressure-sensitive adhesives.

Self-Wrinkling of UV-Cured Polymer Films

IC A IO N The ability to rapidly fabricate patterned microstructures over large areas is desirable for a wide range of applications, from adhesives [ 1 ] and fl exible electronics [ 2 ] to optics [ 3 ] and biological cell studies. [ 4 ] Conventional patterning methods, i.e. lithography, are not only prohibitively expensive for large area productions but also have inherent technical limits such as the inability to pattern non-planar surfaces. Over the past decade, wrinkling [ 2 , 5–7 ] and creasing [ 8 , 9 ] of thin fi lms has been recognized increasingly as an important route to overcome these limitations. In both phenomena, an in-plane compressive stress is applied to a constrained thin fi lm and exceeds a critical value for the onset of an elastic instability. This critical stress is defi ned by both the fi lm thickness and the mechanical properties of the fi lm, as well as the nature of the fi lm’s constraint. [ 9–11 ] The known relationships between the dimensions of wrinkle/ crease pattern, and materials properties and geometry [ 9–11 ] make these processes ideal for scalable technological use; however, currently demonstrated methods for inducing these patterned microstructures involve multiple steps thereby increasing cost and decreasing convenience. For many applications, it would be ideal to have “self-developing” patterned microstructures, akin to morphogenetic processes often observed in Nature. In this paper, we demonstrate a novel mechanism of selfwrinkling in UV-cured polymer fi lms, which results in a singlestep fabrication of wrinkles with well-controlled wavelength and amplitude, and demonstrate that this mechanism is easily adapted for high-fi delity patterning. In our experiments we used a mixture of 2-phenoxyethyl acrylate, 1,6-hexanediol diacrylate (cross-linker) and Irgacure 184 (photoinitiator) as a UV-curable resin. When a fi lm of UVcurable resin coated on a substrate is exposed to UV in the presence of oxygen ( Figure 1 a), a thin liquid layer of monomers remains uncured on the surface due to the quenching of free radicals by oxygen [ 12 , 13 ] (Figure 1 a). The bottom crosslinked fi lm has a depth wise cross-linking density gradient due to diffusion induced concentration gradient of oxygen in the fi lm, which inhibits polymerization during UV-curing. [ 13 ] Here, we take advantage of this uncured liquid layer to spontaneously swell the underlying substrate-constrained gradient-crosslinked fi lm, leading to in-plane stresses that generate surface wrinkle patterns in a single fabrication step (Figure 1 a). This wrinkling mechanism is quite different from another, previously known wrinkling of UV-cured fi lms, which requires two curing steps. [ 14 ] In that alternate mechanism, due to low penetration

Nanoparticle Stripes, Grids, and Ribbons Produced by Flow Coating

Multicomponent and robust structures of quantum dots, in the form of stripes and grids, are produced by a simple flow coating method giving unprecedented control over macroscopic architectures from nanoscopic components. Crosslinking and lift-off of stripes lead to free-floating structures of nanoparticles that are flexible, robust, and fluorescent.