Digital color in cellulose nanocrystal films.

Cellulose nanocrystals (CNCs) form chiral nematic phases in aqueous suspensions that can be preserved upon evaporation of water. The resulting films show an intense directional coloration determined by their microstructure. Here, microreflection experiments correlated with analysis of the helicoidal nanostructure of the films reveal that the iridescent colors and the ordering of the individual nematic layers are strongly dependent on the polydispersity of the size distribution of the CNCs. We show how this affects the self-assembly process, and hence multidomain color formation in such bioinspired structural films.The research leading to these results has received funding from the BBSRC David Phillips fellowship (BBSRC David Phillips, BB/K014617/1) and Next Generation fellowship (to S.V.) and the Schlumberger Foundation (Faculty for Future fellowship to A.G.D.), the EPSRC Carreer Acceleration Fellowship (EP/ H00338X/2) the Royal Society (E.R.), EPSRC grant EP/ G060649/1 (to U.S. and J.J.B.), and ERC LINASS 320503 (to J.J.B.).This is the final published version. It first appeared at http://pubs.acs.org/doi/abs/10.1021/am501995e

Quantitative imaging of heterogeneous dynamics in drying and aging paints

Drying and aging paint dispersions display a wealth of complex phenomena that make their study fascinating yet challenging. To meet the growing demand for sustainable, high-quality paints, it is essential to unravel the microscopic mechanisms underlying these phenomena. Visualising the governing dynamics is, however, intrinsically difficult because the dynamics are typically heterogeneous and span a wide range of time scales. Moreover, the high turbidity of paints precludes conventional imaging techniques from reaching deep inside the paint. To address these challenges, we apply a scattering technique, Laser Speckle Imaging, as a versatile and quantitative tool to elucidate the internal dynamics, with microscopic resolution and spanning seven decades of time. We present a toolbox of data analysis and image processing methods that allows a tailored investigation of virtually any turbid dispersion, regardless of the geometry and substrate. Using these tools we watch a variety of paints dry and age with unprecedented detail.</p

A mechanistic view of drying suspension droplets

When a dispersion droplet dries, a rich variety of spatial and temporal heterogeneities emerge. Controlling these phenomena is essential for many applications yet requires a thorough understanding of the underlying mechanisms. Although the process of film formation from initially dispersed polymer particles is well documented and is known to involve three main stages - evaporation, particle deformation and coalescence - it is impossible to fully disentangle the effects of particle deformation and coalescence, as these stages are closely linked. We circumvent this problem by studying suspensions of colloidal rubber particles that are incapable of coalescing. Varying the crosslink density allows us to tune the particle deformability in a controlled manner. We develop a theoretical framework of the main regimes and stresses in drying droplets of these suspensions, and validate this framework experimentally. Specifically, we show that changing the particle modulus by less than an order of magnitude can completely alter the stress development and resulting instabilities. Scanning electron microscopy reveals that particle deformability is a key factor in stress mitigation. Our model is the suspension equivalent of the widely used Routh-Russel model for film formation in drying dispersions, with additional focus on lateral nonuniformities such as cracking and wrinkling inherent to the droplet geometry, thus adding a new dimension to the conventional view of particle deformation.</p

Electroplasticization of Liquid Crystal Polymer Networks

Shape-shifting liquid crystal networks (LCNs) can transform their morphology and properties in response to external stimuli. These active and adaptive polymer materials can have impact in a diversity of fields, including haptic displays, energy harvesting, biomedicine, and soft robotics. Electrically driven transformations in LCN coatings are particularly promising for application in electronic devices, in which electrodes are easily integrated and allow for patterning of the functional response. The morphing of these coatings, which are glassy in the absence of an electric field, relies on a complex interplay between polymer viscoelasticity, liquid crystal order, and electric field properties. Morphological transformations require the material to undergo a glass transition that plasticizes the polymer sufficiently to enable volumetric and shape changes. Understanding how an alternating current can plasticize very stiff, densely cross-linked networks remains an unresolved challenge. Here, we use a nanoscale strain detection method to elucidate this electric-field-induced devitrification of LCNs. We find how a high-frequency alternating field gives rise to pronounced nanomechanical changes at a critical frequency, which signals the electrical glass transition. Across this transition, collective motion of the liquid crystal molecules causes the network to yield from within, leading to network weakening and subsequent nonlinear expansion. These results unambiguously prove the existence of electroplasticization. Fine-tuning the induced emergence of plasticity will not only enhance the surface functionality but also enable more efficient conversion of electrical energy into mechanical work.</p

Imaging the Molecular Motions of Autonomous Repair in a Self-Healing Polymer

Self-healing polymers can significantly extend the service life of materials and structures by autonomously repairing damage. Intrinsic healing holds great promise as a design strategy to mitigate the risks of damage by delaying or preventing catastrophic failure. However, experimentally resolving the microscopic mechanisms of intrinsic repair has proven highly challenging. This work demonstrates how optical micromechanical mapping enables the quantitative imaging of these molecular-scale dynamics with high spatiotemporal resolution. This approach allows disentangling delocalized viscoplastic relaxation and localized cohesion-restoring rebonding processes that occur simultaneously upon damage to a self-healing polymer. Moreover, frequency- and temperature-dependent imaging provides a way to pinpoint the repair modes in the relaxation spectrum of the quiescent material. These results give rise to a complete picture of autonomous repair that will guide the rational design of improved self-healing materials

Systematic variation of membrane casting parameters to control the structure of thermo-responsive isoporous membranes

Fouling is a critical issue in membrane process operation as it greatly compromises the efficiency of the treatment processes. A promising approach to overcome this problem is the production of easy-to-clean membranes by incorporating stimuli-responsive pores. In this study, we fabricated thermo-responsive polystyrene-b-poly(N-isopropyl acrylamide) (PS-b-PNIPAM) block copolymer membranes using the self-assembly and non-solvent induced phase separation (SNIPS) method and systematically varied several membrane casting parameters, i.e. evaporation time, polymer concentration, solvent type and water content, to obtain nano- and isoporous membranes. Membranes with a disordered surface were obtained for PS selective solvents, whereas isoporous membranes were obtained when the block copolymers were dissolved in PNIPAM selective solvent mixtures. Using 1,4-dioxane/ tetrahydrofuran mixtures resulted in isoporous membranes for a large parameter space, indicating the robustness of structure formation in the PS-b-PNIPAM system. Permeability tests at various temperatures demonstrated fully reversible thermo-responsive behavior of the membranes

In-situ and quantitative imaging of evaporation-induced stratification in binary suspensions

The drying of a multi-component dispersion, such as water-based paint, ink and sunscreen to form a solid film, is a widespread process. Binary colloidal suspensions have proven capable of spontaneous layer formation through size segregation during drying. To design bespoke stratification patterns, a deeper understanding of how these emerge is crucial. Here, we visualize and quantify the spatiotemporally evolving concentration profiles in situ and with high resolution using confocal fluorescence microscopy of custom-designed binary dispersions in a well-defined geometry. Our results conclusively establish two distinct stratification routes, which give rise to three layered structures. A first thin layer develops directly underneath the evaporation front in which large particles are kinetically trapped. At later times, asymmetrical particle interactions lead to the formation of two subsequent layers enriched in small and large particles, respectively. The spatial extent and magnitude of demixing strongly depend on the initial volume fraction. We explain and reproduce the experimental concentration profiles using a theoretical model based on dynamic arrest and higher-order thermodynamic and hydrodynamic interactions. These insights unravel the key mechanisms underlying colloidal auto-stratification in multi-component suspensions, and allow preprogramming of stratification patterns in single-deposition formulations for future applications

Systematic variation of membrane casting parameters to control the structure of thermo-responsive isoporous membranes

Fouling is a critical issue in membrane process operation as it greatly compromises the efficiency of the treatment processes. A promising approach to overcome this problem is the production of easy-to-clean membranes by incorporating stimuli-responsive pores. In this study, we fabricated thermo-responsive polystyrene-b-poly(N-isopropyl acrylamide) (PS-b-PNIPAM) block copolymer membranes using the self-assembly and non-solvent induced phase separation (SNIPS) method and systematically varied several membrane casting parameters, i.e. evaporation time, polymer concentration, solvent type and water content, to obtain nano- and isoporous membranes. Membranes with a disordered surface were obtained for PS selective solvents, whereas isoporous membranes were obtained when the block copolymers were dissolved in PNIPAM selective solvent mixtures. Using 1,4-dioxane/ tetrahydrofuran mixtures resulted in isoporous membranes for a large parameter space, indicating the robustness of structure formation in the PS-b-PNIPAM system. Permeability tests at various temperatures demonstrated fully reversible thermo-responsive behavior of the membranes

Systematic variation of membrane casting parameters to control the structure of thermo-responsive isoporous membranes

Fouling is a critical issue in membrane process operation as it greatly compromises the efficiency of the treatment processes. A promising approach to overcome this problem is the production of easy-to-clean membranes by incorporating stimuli-responsive pores. In this study, we fabricated thermo-responsive polystyrene-b-poly(N-isopropyl acrylamide) (PS-b-PNIPAM) block copolymer membranes using the self-assembly and non-solvent induced phase separation (SNIPS) method and systematically varied several membrane casting parameters, i.e. evaporation time, polymer concentration, solvent type and water content, to obtain nano- and isoporous membranes. Membranes with a disordered surface were obtained for PS selective solvents, whereas isoporous membranes were obtained when the block copolymers were dissolved in PNIPAM selective solvent mixtures. Using 1,4-dioxane/ tetrahydrofuran mixtures resulted in isoporous membranes for a large parameter space, indicating the robustness of structure formation in the PS-b-PNIPAM system. Permeability tests at various temperatures demonstrated fully reversible thermo-responsive behavior of the membranes