Localization and length-scale doubling in disordered films on soft substrates

Macroscopic Freestanding Nanosheets with Exceptionally High Modulus

Macroscopic single-wall carbon nanotube (SWCNT) films of nanoscale thickness have significant potential for an array of applications that demand thin, transparent, conductive coatings. Using macroscopic micrometer thick polystyrene sheets as a reference, we characterize the elastic response of freestanding multifunctional SWCNT nanosheets possessing both exceptionally high Young's modulus and good durability. Thin SWCNT films (20-200 nm thick) asymmetrically "doped" with dilute concentrations of superparamagnetic colloids were suspended in ethanol as freestanding nanosheets. Through repeated and controlled deformation in an external magnetic field, we measure the temporal relaxation of nanosheet curvature back to equilibrium. From the relaxation time and its dependence on nanosheet thickness and length, we extract the SWCNT nanosheet modulus through a simple viscoelastic model. Our results are consistent with nearly ideal SWCNT rigidity percolation with moduli approaching 200 GPa and limited plasticity for sufficiently thick sheets, which we attribute to the screening of van der Waals interactions by the surrounding solvent and the macroscopic nature of the deformation.

Enhancing the Elasticity of Ultrathin Single-Wall Carbon Nanotube Films with Colloidal Nanocrystals

Thin bilayers of contrasting nanomaterials are ubiquitous in solution-processed electronic devices and have potential relevance to a number of applications in flexible electronics. Motivated by recent mesoscopic simulations demonstrating synergistic mechanical interactions between thin films of single-wall carbon nanotubes (SWCNTs) and spherical nanocrystal (NC) inclusions, we use a thin-film wrinkling approach to query the compressive mechanics of hybrid nanotube/nanocrystal coatings adhered to soft polymer substrates. Our results show an almost 2-fold enhancement in the Young modulus of a sufficiently thin SWCNT film associated with the presence of a thin interpenetrating overlayer of semiconductor NCs. Mesoscopic distinct-element method simulations further support the experimental findings by showing that the additional noncovalent interfaces introduced by nanocrystals enhance the modulus of the SWCNT network and hinder network wrinkling.

Rigidity of lamellar nanosheets

Lamellar nanosheets of contrasting materials are ubiquitous in functional coatings and electronic devices. They also represent a unique paradigm for polymer nanocomposites. Here, we use fluid-assembled lamellar nanosheets - alternating layers of polymer and single-wall carbon nanotubes (SWCNTs) - to gain insight into the flexural mechanics of such hybrid films. Specifically, we measure the modulus and yield strain as a function of both layer thickness and the total number of layers. Overall, we find that the multi-layered films exhibit the greatest synergistic effects near a layer thickness of 20 nm or less, which we relate to the characteristic width of the SWCNT-polymer interface. For all layer thicknesses, we find that the nanosheets have realized the bulk limit by six layers. Our results have potentially profound implications for controlling the rigidity and durability of polymer nanocomposites, thin hybrid films and flexible heterojunctions.

Localization in an idealized heterogeneous elastic sheet

Localized deformation is ubiquitous in many natural and engineering materials as they approach failure, and a significant effort has been made to understand localization processes with simple continuum models. Real materials are much more commonly heterogeneous but it is unclear exactly how heterogeneity affects outcomes. In this work we study the response of an idealized heterogenous elastic sheet on a soft foundation as it is uniaxially compressed. The patterned surface layers are created by selective ultraviolet/ozone treatment of the top surface of a polydimethylsiloxane (PDMS) sample using a TEM grid as a mask. By controlling the exposure time of UV/O₃, samples ranging from continuous thin films to sets of isolated small plates were created. We find that patterned regions noticeably localize while bulk regions appear as uniform wrinkles, and that local and global strains depend on the pattern pitch, exposure levels and the treatment protocol. Remarkably, various responses can be modeled using well-understood theory that ignores pattern details aside from the small distance between the adjacent boundaries and the local value of strain.

Stretching-induced wrinkling in plastic-rubber composites

We examine the mechanics of three-layer composite films composed of an elastomeric layer sandwiched between two thin surface layers of plastic. Upon stretching and releasing such composite films, they develop a highly wrinkled surface texture. The mechanism for this texturing is that during stretching, the plastic layers yield and stretch irreversibly whereas the elastomer stretches reversibly. Thus upon releasing, the plastic layers buckle due to compressive stress imposed by the elastomer. Experiments are conducted using SEPS elastomer and 50 micron thick LLDPE plastic films. Stretching and releasing the composites to 2-5 times their original length induces buckles with wavelength on the order of 200 microns, and the wavelength decreases as the stretching increases. FEM simulations reveal that plastic deformation is involved at all stages during this process: (1) during stretching, the plastic layer yields in tension; (2) during recovery, the plastic layer first yields in-plane in compression and then buckles; (3) post-buckling, plastic hinges are formed at high-curvature regions. Homogeneous wrinkles are predicted only within a finite window of material properties: if the yield stress is too low, the plastic layers yield in-plane, without wrinkling, whereas if the yield stress is too high, non-homogeneous wrinkles are predicted. This approach to realizing highly wrinkled textures offers several advantages, most importantly the fact that high aspect ratio wrinkles (amplitude to wavelength ratios exceeding 0.4) can be realized.

Pattern transitions in a compressible floating elastic sheet

Compressible thin layers floating on a liquid surface develop wrinkled and folded patterns under lateral pressure.

Wrinkling of structured thin films via contrasted materials

Regular surface patterns induced by the wrinkling of thin films have received intense attention in both science and engineering. We investigate the wrinkling of structured thin films that consist of two types of materials arranged in periodic patterns. A mechanical model is proposed to understand the physics of the wrinkling, and a set of scaling laws for the wrinkle wavelength are obtained. Periodic wrinkles are generated in the local regions of structured films via in-plane contrasted elastic modulus between heterogeneous materials. The wrinkle morphology and location can be tailored by designing structured thin films in a controllable way. Our findings provide the basis for understanding the wrinkling of structured thin films and for the manufacture of regular surface patterns via wrinkling.

Wrinkles and folds in a fluid-supported sheet of finite size

A laterally confined thin elastic sheet lying on a liquid substrate displays regular undulations, called wrinkles, characterized by a spatially extended energy distribution and a well-defined wavelength λ. As the confinement increases, the deformation energy is progressively localized into a single narrow fold. An exact solution for the deformation of an infinite sheet was previously found, indicating that wrinkles in an infinite sheet are unstable against localization for arbitrarily small confinement. We present an extension of the theory to sheets of finite length L, accounting for the experimentally observed wrinkle-to-fold transition. We derive an exact solution for the periodic deformation in the wrinkled state, and an approximate solution for the localized, folded state. We find that a second-order transition between these two states occurs at a critical confinement Δ(F)=λ(2)/L.

Wrinkling and folding of nanotube-polymer bilayers

The influence of a polymer capping layer on the deformation of purified single-wall carbon nanotube (SWCNT) networks is analyzed through the wrinkling of compressed SWCNT-polymer bilayers on polydimethylsiloxane. The films exhibit both wrinkling and folding under compression and we extract the elastoplastic response using conventional two-plate buckling schemes. The formation of a diffuse interpenetrating nanotube-polymer interface has a dramatic effect on the nanotube layer modulus for both metallic and semiconducting species. In contrast to the usual percolation exhibited by the pure SWCNT films, the capped films show a crossover from "composite" behavior (the modulus of the SWCNT film is enhanced by the polymer) to "plasticized" behavior (the modulus of the SWCNT film is reduced by the polymer) as the SWCNT film thickness increases. For almost all thicknesses, however, the polymer enhances the yield strain of the nanotube network. Conductivity measurements on identical films suggest that the polymer has a modest effect on charge transport, which we interpret as a strain-induced polymer penetration of interfacial nanotube contacts. We use scaling, Flory-Huggins theory, and independently determined nanotube-nanotube and nanotube-polymer Hamaker constants to model the response.

Purifying colloidal nanoparticles through ultracentrifugation with implications for interfaces and materials

Liquid-phase processing and colloidal self-assembly will be critical to the successful implementation of nanotechnology in the next generation of materials and devices. A key hurdle to realizing this will be the development of efficient methods to purify nanomaterials composed of a variety of shapes, including nanocrystals, nanotubes, and nanoplates. Although density-gradient ultracentrifugation (DGU) has long been appreciated as a valuable tool for separating biological macromolecules and components, the method has recently emerged as an effective way to purify colloidal nanoparticles by size and optical and electronic properties. In this feature article, we review our recent contributions to this growing field, with an emphasis on some of the implications that our results have for interfaces and materials. Through transient or isopycnic DGU performed in both aqueous and organic environments, we demonstrate some explicit examples of how the mechanical, electronic, and optical properties of thin films assembled from two specific colloidal nanomaterials--single-walled carbon nanotubes and silicon nanocrystals--can be modified in response to fractionation.