David R. E. Snoswell

3D bulk ordering in macroscopic solid opaline films by edge-induced rotational shearing.

A breakthrough in the fi eld of large area photonic structures is reported, based on permanent ordering of solid polymeric fi lms of sub-micrometer spheres by edge rotational-shearing. The resulting high-quality polymer opal thin-fi lms exhibit strikingly intense structural color, as confi rmed by combining a number of spectroscopic approaches. This induced self-assembly on macroscopic length scales represents a step-change away from current surface lithographies, presenting new routes for assembling solid ordered photonic materials. Despite previous reports of shear-ordering in sedimentary colloids in solution, [ 1 , 2 ] no precedents exist for the application of such techniques to these granular solvent-free systems, which allow formation of permanent composite structures in the solid-state. Full 3D ordering of sub-micrometer components into defi ned architectures is a major challenge for bottom-up nanophotonics, nano-electronics, plasmonics and metamaterials. [ 3‐5 ] Even simple structures, such as opaline photonic crystals based on fcc colloidal lattices, have optical properties dominated by defects and cannot be fabricated in any scalable fashion. [ 6‐9 ] Here, we report a signifi cant advance in high-quality polymer opal thin-fi lms exhibiting tunable structural color across visible wavelengths. An edge-induced rotational shearing (EIRS) process produces reproducible highly uniform samples with bulk-ordering of sub-micrometer components, greatly enhancing both the intensity and chromaticity of the observed structural color. The demonstration of reproducible scale-up of these elastomeric synthetic opaline fi lms to industrial length scales makes them very attractive as a route to a wide range of large-area photonics applications, including sensors and coatings [ 10 ] as well as metamaterials when combined with metallic core‐shell particles. The advance reported here is based on a recently developed technique to produce fl exible opals using melting and shearordering under compression of core/shell polymer nanoparticles. [ 11‐13 ] So far, this produces fl exible polymer opals with

Ordering in stretch-tunable polymeric opal fibers.

We demonstrate the production of high-quality polymer opal fibers in an industrially-scalable process. These fibers exhibit structural color, based on the self-assembly of sub-micron core-shell particles, with a spectrum which is stretch-tunable across the visible region. The internal substructure and ordering of fibers, as inferred from variations in spectral bandwidth, is studied using dark-field microscopy. We employ a granular model to examine flow and shear forces during the extrusion process, and the effects on particle ordering. In both theory and experiment, a concentric zone of the fiber near the exposed surface develops particularly strong structural color. Such elastically-tuned structurally colored fibers are of interest for many applications.

Electric-field-tuned color in photonic crystal elastomers

Electrically tuned photonic crystals are produced by applying fields across shear-assembled elastomeric polymer opal thin films. At increasing voltages, the polymer opal films stretch biaxially under Maxwell stress, deforming the nanostructure and producing marked color changes. This quadratic electro-optic tuning of the photonic bandgap is repeatable over many cycles, switches within 100 ms, and bridges the gap between electro-active materials and photonic crystals.

Large-scale ordering of nanoparticles using viscoelastic shear processing

Despite the availability of elaborate varieties of nanoparticles, their assembly into regular superstructures and photonic materials remains challenging. Here we show how flexible films of stacked polymer nanoparticles can be directly assembled in a roll-to-roll process using a bending-induced oscillatory shear technique. For sub-micron spherical nanoparticles, this gives elastomeric photonic crystals termed polymer opals showing extremely strong tunable structural colour. With oscillatory strain amplitudes of 300%, crystallization initiates at the wall and develops quickly across the bulk within only five oscillations. The resulting structure of random hexagonal close-packed layers is improved by shearing bidirectionally, alternating between two in-plane directions. Our theoretical framework indicates how the reduction in shear viscosity with increasing order of each layer accounts for these results, even when diffusion is totally absent. This general principle of shear ordering in viscoelastic media opens the way to manufacturable photonic materials, and forms a generic tool for ordering nanoparticles.

Modification of the refractive-index contrast in polymer opal films

Synthetic opals, based on self-assembled arrays of core–shell (bead/matrix) polymer microspheres, are a promising platform for next-generation photonic structures, coatings and sensors. The refractive index contrast (Δn) between beads and the matrix polymer is essential for the appearance of structural colour in polymer opal films. We report how the index contrast can be modified by engineering the chemical composition of the core–interlayer–shell (CIS) precursor particles. Alternative approaches to emulsion polymerisation, using the fluorinated monomer 2,2,2-trifluoroethyl acrylate and the aromatic monomer benzyl acrylate, yield a much larger range of Δn values than for standard systems made from styrene and ethyl acrylate. Spectroscopic studies reveal striking differences in the transmission properties of thin-films as Δn is varied from 0.045 up to 0.18.

Interplay of index contrast with periodicity in polymer photonic crystals

We report how the strength of resonant Bragg reflection from polymeric photonic crystals (polymer opals) varies linearly with the refractive-index contrast, Δn, in contrast to the quadratic buildup of Fresnel reflections scaling as (Δn)2. This occurs due to the interplay of disorder and periodicity, in agreement with a simple 1-dimensional periodic model. Goniometry experiments show that opal films exhibit “cones” of resonantly scattered light, which extend to ±20° angular deviation from the specular direction. The intensity of the scattering cones varies super-linearly with Δn. Such medium contrast photonic crystals are of significant interest for understanding structural colors exhibited in nature, by structures with inherent disorder.

Anisotropic Resonant Scattering from Polymer Photonic Crystals

Hyperspectral goniometry reveals anisotropic scattering which dominates the visual appearance of self-assembled polymer opals. The technique allows reconstruction of the reciprocal-space of nanostructures, and indicates that chain defects formed during shear-ordering are responsible for the anisotropy in these samples. Enhanced scattering with improving order is shown to arise from increased effective refractive index contrast, while broadband background scatter is suppressed by absorptive dopants.

Electrically conductive polymeric photonic crystals

Electrically conductive polymeric 3D photonic crystals are prepared by the shear ordering of composites consisting of monodisperse core-shell polymer spheres and single-walled carbon nanotubes (SWNTs). Strong iridescent colour indicates that the highly ordered opaline structures are not disrupted by the presence of the conductive nanotube networks. Thermal annealing leads to a significant increase in the overall electrical conductivity of thin-film samples yielding DC conductivities of 10−4 S cm−1, with percolation thresholds of less than 0.4 wt% of SWNT. Such composites with open networks of carbon nanotubes held apart by lattices of hard spheres, give combined conductive properties and structural colour effects, within a tuneable viscoelastic medium, with many potential functional applications.

The rheology and processing of “edge sheared” colloidal polymer opals

This paper is concerned with the rheology and processing of solvent-free core shell “polymer opals” that consist of a soft outer shell grafted to hard colloidal polymer core particles. Strong iridescent colors can be produced by shearing the material in a certain way that causes the initially disordered spheres to rearrange into ordered crystalline structures and produce colors by diffraction and interference of multiple light scattering, similar to gemstone opals. The basic linear viscoelastic rheology of a polymer opal sample was determined as a function of temperature, and the material was found to be highly viscoelastic at all tested temperatures. A Cambridge multipass rheometer was specifically modified in order to make controlled mechanical measurements of initially disordered polymer opal tapes that were sandwiched between protective polyethylene terephthalate sheets. Axial extension, simple shear, and a novel “edge shearing” geometry were all evaluated, and multiple successive experiments of the e...

Real-time measurements of crystallization processes in viscoelastic polymeric photonic crystals.

We present a study of the dynamic shear ordering of viscoelastic photonic crystals, based on core-shell polymeric composite particles. Using an adapted shear-cell arrangement, the crystalline ordering of the material under conditions of oscillatory shear is interrogated in real time, through both video imaging and from the optical transmission spectra of the cell. In order to gain a deeper understanding of the macroscopic influences of shear on the crystallization process in this solvent-free system, the development of bulk ordering is studied as a function of the key parameters including duty cycle and shear-strain magnitude. In particular, optimal ordering is observed from a prerandomized sample at shear strains of around 160%, for 1-Hz oscillations. This ordering reaches completion over time scales of order 10 s. These observations suggest significant local strains are needed to drive nanoparticles through energy barriers, and that local creep is needed to break temporal symmetry in such high-viscosity nanoassemblies. Crystal shear-melting effects are also characterized under conditions of constant shear rate. These quantitative experiments aim to stimulate the development of theoretical models which can deal with the strong local particle interactions in this system.