Rheology and dynamics of a solvent segregation driven gel (SeedGel).

Bicontinuous structures promise applications in a broad range of research fields, such as energy storage, membrane science, and biomaterials. Kinetically arrested spinodal decomposition is found responsible for stabilizing such structures in different types of materials. A recently developed solvent segregation driven gel (SeedGel) is demonstrated to realize bicontinuous channels thermoreversibly with tunable domain sizes by trapping nanoparticles in a particle domain. As the mechanical properties of SeedGel are very important for its future applications, a model system is characterized by temperature-dependent rheology. The storage modulus shows excellent thermo-reproducibility and interesting temperature dependence with the maximum storage modulus observed at an intermediate temperature range (around 28 °C). SANS measurements are conducted at different temperatures to identify the macroscopic solvent phase separation during the gelation transition, and solvent exchange between solvent and particle domains that is responsible for this behavior. The long-time dynamics of the gel is further studied by X-ray Photon Correlation Spectroscopy (XPCS). The results indicate that particles in the particle domain are in a glassy state and their long-time dynamics are strongly correlated with the temperature dependence of the storage modulus.

Finely tunable dynamical coloration using bicontinuous micrometer-domains.

Nanostructures similar to those found in the vividly blue wings of Morpho butterflies and colorful photonic crystals enable structural color through constructive interference of light waves. Different from commonly studied structure-colored materials using periodic structures to manipulate optical properties, we report a previously unrecognized approach to precisely control the structural color and light transmission via a novel photonic colloidal gel without long-range order. Nanoparticles in this gel form micrometer-sized bicontinuous domains driven by the microphase separation of binary solvents. This approach enables dynamic coloration with a precise wavelength selectivity over a broad range of wavelengths extended well beyond the visible light that is not achievable with traditional methods. The dynamic wavelength selectivity is thermally tunable, reversible, and the material fabrication is easily scalable.

Tunable thermo-reversible bicontinuous nanoparticle gel driven by the binary solvent segregation.

Bicontinuous porous structures through colloidal assembly realized by non-equilibrium process is crucial to various applications, including water treatment, catalysis and energy storage. However, as non-equilibrium structures are process-dependent, it is very challenging to simultaneously achieve reversibility, reproducibility, scalability, and tunability over material structures and properties. Here, a novel solvent segregation driven gel (SeedGel) is proposed and demonstrated to arrest bicontinuous structures with excellent thermal structural reversibility and reproducibility, tunable domain size, adjustable gel transition temperature, and amazing optical properties. It is achieved by trapping nanoparticles into one of the solvent domains upon the phase separation of the binary solvent. Due to the universality of the solvent driven particle phase separation, SeedGel is thus potentially a generic method for a wide range of colloidal systems.

Controlling Bicontinuous Structures through a Solvent Segregation-Driven Gel.

The past decade has seen increased research interest in studying bicontinuous structures formed via colloidal self-assembly due to their many useful applications. A new type of colloidal gel, solvent segregation-driven gel (SeedGel), has been recently demonstrated as an effective approach to arrest bicontinuous structures with unique and intriguing properties, such as thermoreversibility, structural reproducibility, and sensitive temperature response. Here, using a model system with silica particles in the 2,6-lutidine/water binary solvent, we investigate the factors controlling the domain size of a SeedGel system by varying the particle concentration, solvent ratio, and quenching protocol. A phase diagram is identified to produce SeedGels for this model system. Our results indicate that by adjusting the sample composition, it is possible to realize bicontinuous domains with well-controlled repeating distances (periodicities). In addition, the effect of quenching rate on the domain size is systematically investigated, showing that it is a very sensitive parameter to control domain sizes. By further heating SeedGel up into the spinodal region, the structure evolution under high temperatures is also investigated and discussed. These results provide important insights into how to control bicontinuous structures in SeedGel systems.

Self-assembly of donor-acceptor conjugated polymers induced by miscible 'poor' solvents.

The solution-phase self-assembly of donor-acceptor conjugated polymer (DACP) poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl))thieno[3,2b]thiophene] (DPPDTT), is demonstrated and investigated from binary solvent mixtures. It is found that the polarity of a miscible 'poor' solvent (e.g. methanol, dimethyl sulfoxide), which is added to a stable polymer solution in chloroform (i.e. 'good' solvent), strongly affects the resulting nanostructure. Nanoribbons are formed by the addition of certain polar (e.g. methanol) 'poor' solvents to the mixture, while amorphous aggregates are formed upon addition of non-polar 'poor' solvent, such as n-hexane. Atomic force microscopy (AFM), scanning transmission electron microscopy (sTEM) and small angle neutron scattering (SANS) are used to characterize the shape and size of the nanostructures. Experiments show complex self-assembly in solution occurs for DACPs when compared to conjugated homopolymers. SANS results also provide quantitative analysis of DACP conformations in solution before self-assembly occurs. The addition of different polar 'poor' solvents could also alter the size of the assembled nanostructures, as well as the fraction of polymers that self-assemble. The surface orientation and the crystal structure of the nanostructures is also probed by grazing-incidence wide-angle X-ray scattering (GIWAXS). Organic field effect transistors (OFETs) are used to characterize charge transport properties for nanoribbons where enhancement of the average hole mobility is observed.

Colloidal systems with a short-range attraction and long-range repulsion: Phase diagrams, structures, and dynamics.

Colloidal systems with both a short-range attraction and long-range repulsion (SALR) have rich phases compared with the traditional hard sphere systems or sticky hard sphere systems. The competition between the short-range attraction and long-range repulsion results in the frustrated phase separation, which leads to the formation of intermediate range order (IRO) structures and introduces new phases to both equilibrium and nonequilibrium phase diagrams, such as clustered fluid, cluster percolated fluid, Wigner glass, and cluster glass. One hallmark feature of many SALR systems is the appearance of the IRO peak in the interparticle structure factor, which is associated with different types of IRO structures. The relationship between the IRO peak and the clustered fluid state has been careful investigated. Not surprisingly, the morphology of clusters in solutions can be affected and controlled by the SALR potential. And the effect of the SALR potential on the dynamic properties is also reviewed here. Even though much progress has been made in understanding SALR systems, many future works are still needed to have quantitative comparisons between experiments and simulations/theories and understand the differences from different experimental systems. Owing to the large parameter space available for SALR systems, many exciting features of SALR systems are not fully explored yet. Because proteins in low-salinity solutions have SALR interactions, the understanding of SALR systems can greatly help understand protein behavior in concentrated solutions or crowded conditions.

A small-angle scattering environment for in situ ultrasound studies.

Ultrasonic devices are common tools in laboratory and industrial settings to produce cavitation events for cleaning, emulsification, cell lysis and other materials applications. Effects of sonication at the macroscopic scale can be visible while effects at the molecular and nano-scales are not easily probed and, therefore, not fully understood. We present a new small angle scattering sample environment designed specifically to study structural changes occurring in various types of dispersions at the nano-scale due to ultrasonic acoustic waves. The sample environment features two face-to-face high-intensity focused ultrasound transducers coaxially aligned and normal to the neutron/X-ray beam propagation direction. A third broadband transducer is fixed beneath the scattering volume to acoustically monitor for cavitation events. By correlating acoustic data to scattering data, measured structural changes can be correlated to changes in parameters such as frequency, acoustic pressure, or cavitation pressure threshold. Several example applications of colloidal systems effectively influenced by ultrasound fields are also presented to demonstrate the capabilities of the device and to motivate future work on in situ scattering analysis of ultrasound materials processing methods.

Sonocrystallization of conjugated polymers with ultrasound fields.

Ultrasound acoustic waves are demonstrated to assemble poly-3-hexylthiophene (P3HT) chains into nanofibers after they are fully dissolved in what are commonly considered to be 'good' solvents. In the absence of ultrasound, the polymer remains fully dissolved and does not self-assemble for weeks. UV-vis spectroscopy, ultra-small angle X-ray scattering (USAXS) and small angle neutron scattering (SANS) are used to characterize the induced assembly process and to quantify the fraction of polymer that forms nanofibers. It is determined that the solvent type, insonation time, and aging periods are all important factors affecting the structure and final concentration of fibers. The effect of changing polymer regio-regularity, alkyl chain length, and side chain to thiophene ratio are also explored. High intensity focused ultrasound (HIFU) fields of variable intensity are utilized to reveal the physical mechanisms leading to nanofiber formation, which is strongly correlated to cavitation events in the solvent. This in situ HIFU cell, which is designed for simultaneous scattering analysis, is also used to probe for structural changes occurring over multiple length scales using USAXS and SANS. The proposed acoustic assembly mechanism suggests that, even when dispersed in 'good' solvents such as bromobenzene, dichlorobenzene and chloroform, P3HT chains are still not in a thermodynamically stable state. Instead, they are stabilized by local energy barriers that slow down and effectively prevent crystallization. Ultrasound fields are found to provide enough mechanical energy to overcome these barriers, triggering the formation of small crystalline nuclei that subsequently seed the growth of larger nanofibers.

Electric field directed formation of aligned conjugated polymer fibers.

Alternating current (AC) electric fields effectively align poly(3-alkylthiophene) (P3AT) fibers during a one-dimensional crystallization process. Structural, mechanical and electrical properties have been probed using microscopy, small angle neutron scattering (SANS), atomic force microscopy (AFM), X-ray diffraction (XRD), rheology, UV-Vis spectroscopy, and dielectric spectroscopy. Optimum frequency and amplitudes were identified for specific P3AT concentrations and for variable solvent quality. Optical microscopy along with SANS and XRD demonstrate alignment persists over both micrometer and nanometer length scales. Small amplitude oscillatory shear rheology also showed that the shear modulus increased for aligned fibers. The structural changes were correlated to improvements in electric conductivity as probed by dielectric spectroscopy. XRD experiments performed with variable sample orientations also showed that the P3HT π-π stacking direction was aligned parallel to the direction of the fiber axis. Electric field alignment was also possible for other alkyl-thiophene polymers and for polymers with simple backbone repeat units.

Structure Characterization and Properties of Metal-Surfactant Complexes Dispersed in Organic Solvents.

This work describes the synthesis and characterization of metal-surfactant complexes. Dioctyl sulfosuccinate and dodecylbenzenesulfonate are associated with multivalent aluminum, iron, and vanadium ions using an ion exchange reaction. The metal complexes are dispersible in various organic solvents. In solvents with low polarity, the complexes form "inverse" macromolecular structures with multiple metal ions. In contrast, in alcohols, the complex size is reduced, showing a more disperse conformation. The metal and surfactant ions are still strongly bonded to each other in all the solvents probed. Small-angle X-ray and neutron scattering (SAXS and SANS) are used to characterize the structures. Simultaneous fitting of neutron and X-ray scattering spectra is performed in order to obtain an accurate description of the system. Scattering results are also validated by performing molecular dynamics (MD) simulations. The conductive and electrochemical properties of the complexes in solution are also evaluated. The dispersion of metal-organic complexes significantly increases electric conductivity, and some metal ions in the core of the complexes are shown to be electrochemically active in apolar solvents.