Exploring the Tunable Optical and Mechanical Properties of Multicomponent Low-Molecular-Weight Gelators.

A class of amino acid-based low-molecular-weight gelators (LMWGs) was used for single and multicomponent gel studies to investigate their tunable optical properties and their self-assembly process. The optical properties of multicomponent gels were found to be easily tuned by changing the proportion of the components, varying from opaque to highly transparent gels as analyzed using ultraviolet-visible spectroscopy. This phenomenon allows tunability without introducing another variable into the system. Scanning electron microscopy, differential scanning calorimetry, and small-angle X-ray scattering (SAXS) were used to investigate the structures of the gels. It was found that because of the structural similarities of the molecules, the gelators favor coassembly packing over self-sorting. The emergence of transparency was ascribed to changes in the fiber diameters. Moreover, analysis of the SAXS data allowed us to compare the molecular order present in the gel phase with single-crystal X-ray diffraction (SCXRD) data. Our analysis suggests that the packing of molecules seen in the crystalline phase is translated into the gel network. This reveals that the structure of the crystalline phase seen through SCXRD is a useful tool to aid in understanding the molecular packing in the gel phase.

Iridescent Cellulose Nanocrystal Films Modified with Hydroxypropyl Cellulose.

The introduction of polymers into a chiral nematic cellulose nanocrystal (CNC) matrix allows for the tuning of optical and mechanical properties, enabling the development of responsive photonic materials. In this study, we explored the incorporation of hydroxypropyl cellulose (HPC) into a CNC film prepared by slow evaporation. In the composite CNC/HPC thin films, the CNCs adopt a chiral nematic structure, which can selectively reflect certain wavelengths of light to yield a colored film. The color could be tuned across the visible spectrum by changing the concentration or molecular weight of the HPC. Importantly, the composite films were more flexible than pure CNC films with up to a ten-fold increase in elasticity and a decrease in stiffness and tensile strength of up to six times and four times, respectively. Surface modification of the films with methacrylate groups increased the hydrophobicity of the films, and therefore, the water stability of these materials was also improved.

Cellulose Nanocrystal Elastomers with Reversible Visible Color.

Responsive photonic crystals have potential applications in mechanical sensors and soft displays; however, new materials are constantly desired to provide new innovations and improve on existing technologies. To address this, we report stretchable chiral nematic cellulose nanocrystal (CNC) elastomer composites that exhibit reversible visible color upon the application of mechanical stress. When stretched (or compressed) the colorless materials maintain their chiral nematic structure but the helical pitch is reduced into the visible region, resulting in coloration of the CNC-elastomer composite. By increasing the percentage elongation of the material (ca. 50-300 %), the structural color can be tuned from red to blue. The color of the materials was characterized by reflectance optical microscopy and reflectance circular dichroism to confirm the wavelength and polarization of the reflected light. We also probed the mechanism of the structural color using 2D-X-ray diffraction. Finally, by either water-patterning the starting CNC film, or by forming a CNC film with gradient color, through masked evaporation, we were able to prepare encoded stretchable chiral nematic CNC-elastomers.

Post-modification of Cellulose Nanocrystal Aerogels with Thiol-Ene Click Chemistry.

The functionalization of cellulose nanocrystal (CNC) aerogels was achieved through a two-step synthetic procedure. CNC aerogels were prepared under hydrothermal conditions, followed by solvent exchange and critical point drying. The CNC aerogels were functionalized with a methacrylate group and then underwent thiol-ene click chemistry to impart a range of functionalities onto the surface of the CNC aerogel. The use of the functionalized aerogels as oil absorbents was then investigated, with the most hydrophobic CNC aerogel, 1 H,1 H,2 H,2 H-perfluorodecanethiol-functionalized CNC aerogel, exhibiting the highest absorption of xylenes at 2.9 mL g-1.

Scalable Fiber-like Micelles and Block Co-micelles by Polymerization-Induced Crystallization-Driven Self-Assembly.

Self-assembled 1D block copolymer nanoparticles (micelles) are of interest for a range of applications. However, morphologically pure samples are often challenging to access, and precise dimensional control is not possible. Moreover, the development of synthetic protocols that operate on a commercially viable scale has been a major challenge. Herein, we describe the preparation 1D fiber-like micelles with crystalline cores at high concentrations by a one-pot process termed polymerization-induced crystallization-driven self-assembly (PI-CDSA). We also demonstrate the formation of uniform fibers by living PI-CDSA, a process in which block copolymer synthesis, self-assembly, and seeded growth are combined. We have demonstrated that the method is successful for block copolymers that possess the same composition as that of the seed (homoepitaxial growth) and also where the coronal chemistries differ to give segmented 1D fibers known as block co-micelles. We have also shown that heteroepitaxial growth allows the formation of scaled-up block co-micelles where the composition of both the core and corona was varied. These proof-of-concept experiments indicate that PI-CDSA is a promising, scalable route to a variety of polydisperse or uniform 1D nanoparticles based on block copolymers with different crystalline core chemistries and, therefore, functions.

Two-dimensional assemblies from crystallizable homopolymers with charged termini.

The creation of shaped, uniform and colloidally stable two-dimensional (2D) assemblies by bottom-up methods represents a challenge of widespread current interest for a variety of applications. Herein, we describe the utilization of surface charge to stabilize self-assembled planar structures that are formed from crystallizable polymer precursors by a seeded growth approach. Addition of crystallizable homopolymers with charged end-groups to seeds generated by the sonication of block copolymer micelles with crystalline cores yields uniform platelet micelles with controlled dimensions. Significantly, the seeded growth approach is characterized by a morphological memory effect whereby the origin of the seed, which can involve a quasi-hexagonal or rectangular 2D platelet precursor, dictates the observed 2D platelet shape. This new strategy is illustrated using two different polymer systems, and opens the door to the construction of 2D hierarchical structures with broad utility.

Monodisperse Cylindrical Micelles and Block Comicelles of Controlled Length in Aqueous Media.

Cylindrical block copolymer micelles have shown considerable promise in various fields of biomedical research. However, unlike spherical micelles and vesicles, control over their dimensions in biologically relevant solvents has posed a key challenge that potentially limits in depth studies and their optimization for applications. Here, we report the preparation of cylindrical micelles of length in the wide range of 70 nm to 1.10 µm in aqueous media with narrow length distributions (length polydispersities <1.10). In our approach, an amphiphilic linear-brush block copolymer, with high potential for functionalization, was synthesized based on poly(ferrocenyldimethylsilane)-b-poly(allyl glycidyl ether) (PFS-b-PAGE) decorated with triethylene glycol (TEG), abbreviated as PFS-b-(PEO-g-TEG). PFS-b-(PEO-g-TEG) cylindrical micelles of controlled length with low polydispersities were prepared in N,N-dimethylformamide using small seed initiators via living crystallization-driven self-assembly. Successful dispersion of these micelles into aqueous media was achieved by dialysis against deionized water. Furthermore, B-A-B amphiphilic triblock comicelles with PFS-b-poly(2-vinylpyridine) (P2VP) as hydrophobic "B" blocks and hydrophilic PFS-b-(PEO-g-TEG) "A" segments were prepared and their hierarchical self-assembly in aqueous media studied. It was found that superstructures formed are dependent on the length of the hydrophobic blocks. Quaternization of P2VP was shown to cause the disassembly of the superstructures, resulting in the first examples of water-soluble cylindrical multiblock comicelles. We also demonstrate the ability of the triblock comicelles with quaternized terminal segments to complex DNA and, thus, to potentially function as gene vectors.

"Cross" Supermicelles via the Hierarchical Assembly of Amphiphilic Cylindrical Triblock Comicelles.

Self-assembled "cross" architectures are well-known in biological systems (as illustrated by chromosomes, for example); however, comparable synthetic structures are extremely rare. Herein we report an in depth study of the hierarchical assembly of the amphiphilic cylindrical P-H-P triblock comicelles with polar (P) coronal ends and a hydrophobic (H) central periphery in a selective solvent for the terminal segments which allows access to "cross" supermicelles under certain conditions. Well-defined P-H-P triblock comicelles M(PFS-b-PtBA)-b-M(PFS-b-PDMS)-b-M(PFS-b-PtBA) (M = micelle segment, PFS = polyferrocenyldimethylsilane, PtBA = poly(tert-butyl acrylate), and PDMS = polydimethylsiloxane) were created by the living crystallization-driven self-assembly (CDSA) method. By manipulating two factors in the supermicelles, namely the H segment-solvent interfacial energy (through the central H segment length, L1) and coronal steric effects (via the PtBA corona chain length in the P segment, L2 related to the degree of polymerization DP2) the aggregation of the triblock comicelles could be finely tuned. This allowed a phase-diagram to be constructed that can be extended to other triblock comicelles with different coronas on the central or end segment where "cross" supermicelles were exclusively formed under predicted conditions. Laser scanning confocal microscopy (LSCM) analysis of dye-labeled "cross" supermicelles, and block "cross" supermicelles formed by addition of a different unimer to the arm termini, provided complementary characterization to transmission electron microscopy (TEM) and dynamic light scattering (DLS) and confirmed the existence of these "cross" supermicelles as kinetically stable, micron-size colloidally stable structures in solution.

In Situ Visualization of Block Copolymer Self-Assembly in Organic Media by Super-Resolution Fluorescence Microscopy.

Analytical methods that enable visualization of nanomaterials derived from solution self-assembly processes in organic solvents are highly desirable. Herein, we demonstrate the use of stimulated emission depletion microscopy (STED) and single molecule localization microscopy (SMLM) to map living crystallization-driven block copolymer (BCP) self-assembly in organic media at the sub-diffraction scale. Four different dyes were successfully used for single-colour super-resolution imaging of the BCP nanostructures allowing micelle length distributions to be determined in situ. Dual-colour SMLM imaging was used to measure and compare the rate of addition of red fluorescent BCP to the termini of green fluorescent seed micelles to generate block comicelles. Although well-established for aqueous systems, the results highlight the potential of super-resolution microscopy techniques for the interrogation of self-assembly processes in organic media.

Synthetic Covalent and Non-Covalent 2D Materials.

The creation of synthetic 2D materials represents an attractive challenge that is ultimately driven by their prospective uses in, for example, electronics, biomedicine, catalysis, sensing, and as membranes for separation and filtration. This Review illustrates some recent advances in this diverse field with a focus on covalent and non-covalent 2D polymers and frameworks, and self-assembled 2D materials derived from nanoparticles, homopolymers, and block copolymers.

Understanding the Self-Assembly of Cellulose Nanocrystals-Toward Chiral Photonic Materials.

Over millions of years, animals and plants have evolved complex molecules and structures that endow them with vibrant colors. Among the sources of natural coloration, structural color is prominent in insects, bird feathers, snake skin, plants, and other organisms, where the color arises from the interaction of light with nanoscale features rather than absorption from a pigment. Cellulose nanocrystals (CNCs) are a biorenewable resource that spontaneously organize into chiral nematic liquid crystals having a hierarchical structure that resembles the Bouligand structure of arthropod shells. The periodic, chiral nematic organization of CNC films leads them to diffract light, making them appear iridescent. Over the past two decades, there have been many advances to develop the photonic properties of CNCs for applications ranging from cosmetics to sensors. Here, the origin of color in CNCs, the control of photonic properties of CNC films, the development of new composite materials of CNCs that can yield flexible photonic structures, and the future challenges in this field are discussed. In particular, recent efforts to make flexible photonic materials using CNCs are highlighted.

Amino Acid-Containing Phase-Selective Organogelators: A Water-Based Delivery System for Oil Spill Treatment.

The simple structural modification of replacing a terminal carboxylic acid with a primary amide group was found to lower the minimum gelation concentration (MGC), by at least an order of magnitude, for a series of N-lauroyl-l-amino acid phase-selective organogelators in decane. The amide-functionalized analogue N-lauroyl-l-alanine-CONH2 was demonstrated to gel a broad range of solvents from diesel to THF at MGCs of 2.5% w/v or less, as well as to produce gels with a higher thermal stability (ca. 30 °C) and enhanced mechanical properties (5 times increase in complex modulus), compared to the carboxylic acid analogue, N-lauroyl-l-alanine-COOH. These improved properties may be due to the additional hydrogen bonding in the primary amide analogue as revealed by SCXRD. Most significantly for this study, the introduction of the primary amide functionality enabled N-lauroyl-l-alanine-CONH2 to form a self-assembled fibrillar network in water. The aqueous network could then actively uptake and rapidly gel decane, diesel, and diluted bitumen ("dilbit") with MGCs of 2.5% w/v or less. This aqueous delivery method is advantageous for oil-remediation applications as no harmful carrier solvents are required and the gel can be easily separated from the water, allowing the oil to be recovered and the gelator recycled.

Manipulation and Deposition of Complex, Functional Block Copolymer Nanostructures Using Optical Tweezers.

Block copolymer self-assembly has enabled the creation of a range of solution-phase nanostructures with applications from optoelectronics and biomedicine to catalysis. However, to incorporate such materials into devices a method that facilitates their precise manipulation and deposition is desirable. Herein we describe how optical tweezers can be used to trap, manipulate, and pattern individual cylindrical micelles and larger hybrid micellar materials. Through the combination of TIRF imaging and optical trapping we can precisely control the three-dimensional motion of individual cylindrical block copolymer micelles in solution, enabling the creation of customizable arrays. We also demonstrate that dynamic holographic assembly enables the creation of ordered customizable arrays of complex hybrid block copolymer structures. By creating a program which automatically identifies, traps, and then deposits multiple assemblies simultaneously we have been able to dramatically speed up this normally slow process, enabling the fabrication of arrays of hybrid structures containing hundreds of assemblies in minutes rather than hours.

Probing the Growth Kinetics for the Formation of Uniform 1D Block Copolymer Nanoparticles by Living Crystallization-Driven Self-Assembly.

Living crystallization-driven self-assembly (CDSA) is a seeded growth method for crystallizable block copolymers (BCPs) and related amphiphiles in solution and has recently emerged as a highly promising and versatile route to uniform core-shell nanoparticles (micelles) with control of dimensions and architecture. However, the factors that influence the rate of nanoparticle growth have not been systematically studied. Using transmission electron microscopy, small- and wide-angle X-ray scattering, and super-resolution fluorescence microscopy techniques, we have investigated the kinetics of the seeded growth of poly(ferrocenyldimethylsilane)- b-(polydimethylsiloxane) (PFS- b-PDMS), as a model living CDSA system for those employing, for example, crystallizable emissive and biocompatible polymers. By altering various self-assembly parameters including concentration, temperature, solvent, and BCP composition our results have established that the time taken to prepare fiber-like micelles via the living CDSA method can be reduced by decreasing temperature, by employing solvents that are poorer for the crystallizable PFS core-forming block, and by increasing the length of the PFS core-forming block. These results are of general importance for the future optimization of a wide variety of living CDSA systems. Our studies also demonstrate that the growth kinetics for living CDSA do not exhibit the first-order dependence of growth rate on unimer concentration anticipated by analogy with living covalent polymerizations of molecular monomers. This difference may be caused by the combined influence of chain conformational effects of the BCP on addition to the seed termini and chain length dispersity.

Long-range exciton transport in conjugated polymer nanofibers prepared by seeded growth.

Easily processed materials with the ability to transport excitons over length scales of more than 100 nanometers are highly desirable for a range of light-harvesting and optoelectronic devices. We describe the preparation of organic semiconducting nanofibers comprising a crystalline poly(di-n-hexylfluorene) core and a solvated, segmented corona consisting of polyethylene glycol in the center and polythiophene at the ends. These nanofibers exhibit exciton transfer from the core to the lower-energy polythiophene coronas in the end blocks, which occurs in the direction of the interchain π-π stacking with very long diffusion lengths (>200 nanometers) and a large diffusion coefficient (0.5 square centimeters per second). This is made possible by the uniform exciton energetic landscape created by the well-ordered, crystalline nanofiber core.

Higher-order assembly of crystalline cylindrical micelles into membrane-extendable colloidosomes.

Crystallization-driven self-assembly of diblock copolymers into cylindrical micelles of controlled length has emerged as a promising approach to the fabrication of functional nanoscale objects with high shape anisotropy. Here we show the preparation of a series of crystallizable diblock copolymers with appropriate wettability and chemical reactivity, and demonstrate their self-assembly into size-specific cylindrical micelle building blocks for the hierarchical construction of mechanically robust colloidosomes with a range of membrane textures, surface chemistries and optical properties. The colloidosomes can be structurally elaborated post assembly by in situ epitaxial elongation of the membrane building blocks to produce microcapsules covered in a chemically distinct, dense network of hair-like outgrowths. Our approach provides a route to hierarchically ordered colloidosomes that retain the intrinsic growth activity of their constituent building blocks to permit biofunctionalization, and have potential applications in areas such as biomimetic encapsulation, drug delivery, catalysis and biosensing.Functional nanoscale objects can be prepared via crystallization-driven self-assembly of diblock copolymers. Here the authors show the self-assembly of crystalline block copolymers into size-specific cylindrical micelles for the hierarchical construction of mechanically robust colloidosomes with a range of membrane textures.

Scalable and uniform 1D nanoparticles by synchronous polymerization, crystallization and self-assembly.

The preparation of well-defined nanoparticles based on soft matter, using solution-processing techniques on a commercially viable scale, is a major challenge of widespread importance. Self-assembly of block copolymers in solvents that selectively solvate one of the segments provides a promising route to core-corona nanoparticles (micelles) with a wide range of potential uses. Nevertheless, significant limitations to this approach also exist. For example, the solution processing of block copolymers generally follows a separate synthesis step and is normally performed at high dilution. Moreover, non-spherical micelles-which are promising for many applications-are generally difficult to access, samples are polydisperse and precise dimensional control is not possible. Here we demonstrate the formation of platelet and cylindrical micelles at concentrations up to 25% solids via a one-pot approach-starting from monomers-that combines polymerization-induced and crystallization-driven self-assembly. We also show that performing the procedure in the presence of small seed micelles allows the scalable formation of low dispersity samples of cylindrical micelles of controlled length up to three micrometres.

Dimensional Control and Morphological Transformations of Supramolecular Polymeric Nanofibers Based on Cofacially-Stacked Planar Amphiphilic Platinum(II) Complexes.

Square-planar platinum(II) complexes often stack cofacially to yield supramolecular fiber-like structures with interesting photophysical properties. However, control over fiber dimensions and the resulting colloidal stability is limited. We report the self-assembly of amphiphilic Pt(II) complexes with solubilizing ancillary ligands based on polyethylene glycol [PEGn, where n = 16, 12, 7]. The complex with the longest solubilizing PEG ligand, Pt-PEG16, self-assembled to form polydisperse one-dimensional (1D) nanofibers (diameters <5 nm). Sonication led to short seeds which, on addition of further molecularly dissolved Pt-PEG16 complex, underwent elongation in a "living supramolecular polymerization" process to yield relatively uniform fibers of length up to ca. 400 nm. The fiber lengths were dependent on the Pt-PEG16 complex to seed mass ratio in a manner analogous to a living covalent polymerization of molecular monomers. Moreover, the fiber lengths were unchanged in solution after 1 week and were therefore "static" with respect to interfiber exchange processes on this time scale. In contrast, similarly formed near-uniform fibers of Pt-PEG12 exhibited dynamic behavior that led to broadening of the length distribution within 48 h. After aging for 4 weeks in solution, Pt-PEG12 fibers partially evolved into 2D platelets. Furthermore, self-assembly of Pt-PEG7 yielded only transient fibers which rapidly evolved into 2D platelets. On addition of further fiber-forming Pt complex (Pt-PEG16), the platelets formed assemblies via the growth of fibers selectively from their short edges. Our studies demonstrate that when interfiber dynamic exchange is suppressed, dimensional control and hierarchical structure formation are possible for supramolecular polymers through the use of kinetically controlled seeded growth methods.

Uniform patchy and hollow rectangular platelet micelles from crystallizable polymer blends.

The preparation of colloidally stable, self-assembled materials with tailorable solid or hollow two-dimensional (2D) structures represents a major challenge. We describe the formation of uniform, monodisperse rectangular platelet micelles of controlled size by means of seeded-growth methods that involve the addition of blends of crystalline-coil block copolymers and the corresponding crystalline homopolymer to cylindrical micelle seeds. Sequential addition of different blends yields solid platelet block comicelles with concentric rectangular patches with distinct coronal chemistries. These complex nano-objects can be subject to spatially selective processing that allows their disassembly to form perforated platelets, such as well-defined hollow rectangular rings. The solid and hollow 2D micelles provide a tunable platform for further functionalization and potential for a variety of applications.

Non-covalent synthesis of supermicelles with complex architectures using spatially confined hydrogen-bonding interactions.

Nature uses orthogonal interactions over different length scales to construct structures with hierarchical levels of order and provides an important source of inspiration for the creation of synthetic functional materials. Here, we report the programmed assembly of monodisperse cylindrical block comicelle building blocks with crystalline cores to create supermicelles using spatially confined hydrogen-bonding interactions. We also demonstrate that it is possible to further program the self-assembly of these synthetic building blocks into structures of increased complexity by combining hydrogen-bonding interactions with segment solvophobicity. The overall approach offers an efficient, non-covalent synthesis method for the solution-phase fabrication of a range of complex and potentially functional supermicelle architectures in which the crystallization, hydrogen-bonding and solvophobic interactions are combined in an orthogonal manner.