Supramolecular polymers form tactoids through liquid-liquid phase separation.

Liquid-liquid phase separation (LLPS) of biopolymers has recently been shown to play a central role in the formation of membraneless organelles with a multitude of biological functions1-3. The interplay between LLPS and macromolecular condensation is part of continuing studies4,5. Synthetic supramolecular polymers are the non-covalent equivalent of macromolecules but they are not reported to undergo LLPS yet. Here we show that continuously growing fibrils, obtained from supramolecular polymerizations of synthetic components, are responsible for phase separation into highly anisotropic aqueous liquid droplets (tactoids) by means of an entropy-driven pathway. The crowding environment, regulated by dextran concentration, affects not only the kinetics of supramolecular polymerizations but also the properties of LLPS, including phase-separation kinetics, morphology, internal order, fluidity and mechanical properties of the final tactoids. In addition, substrate-liquid and liquid-liquid interfaces proved capable of accelerating LLPS of supramolecular polymers, allowing the generation of a myriad of three-dimensional-ordered structures, including highly ordered arrays of micrometre-long tactoids at surfaces. The generality and many possibilities of supramolecular polymerizations to control emerging morphologies are demonstrated with several supramolecular polymers, opening up a new field of matter ranging from highly structured aqueous solutions by means of stabilized LLPS to nanoscopic soft matter.

Architecture Controls Phonon Propagation in All-Solid Brush Colloid Metamaterials.

Brillouin light scattering and elastodynamic theory are concurrently used to determine and interpret the hypersonic phonon dispersion relations in brush particle solids as a function of the grafting density with perspectives in optomechanics, heat management, and materials metrology. In the limit of sparse grafting density, the phonon dispersion relations bear similarity to polymer-embedded colloidal assembly structures in which phonon dispersion can be rationalized on the basis of perfect boundary conditions, i.e., isotropic stiffness transitions across the particle interface. In contrast, for dense brush assemblies, more complex dispersion characteristics are observed that imply anisotropic stiffness transition across the particle/polymer interface. This provides direct experimental validation of phonon propagation changes associated with chain conformational transitions in dense particle brush materials. A scaling relation between interface tangential stiffness and crowding of polymer tethers is derived that provides a guideline for chemists to design brush particle materials with tailored phononic dispersion characteristics. The results emphasize the role of interfaces in composite materials systems. Given the fundamental relevance of phonon dispersion to material properties such as thermal transport or mechanical properties, it is also envisioned that the results will spur the development of novel functional hybrid materials.

Network Viscoelasticity from Brillouin Spectroscopy.

Even though the physical nature of shear and longitudinal moduli are different, empirical correlations between them have been reported in several biological systems. This correlation is of fundamental interest and immense practical value in biomedicine due to the importance of the shear modulus and the possibility to map the longitudinal modulus at high-resolution with all-optical spectroscopy. We investigate the origin of such a correlation in hydrogels. We hypothesize that both moduli are influenced in the same direction by underlying physicochemical properties, which leads to the observed material-dependent correlation. Matching theoretical models with experimental data, we quantify the scenarios in which the correlation holds. For polymerized hydrogels, a correlation was found across different hydrogels through a common dependence on the effective polymer volume fraction. For hydrogels swollen to equilibrium, the correlation is valid only within a given hydrogel system, as the moduli are found to have different scalings on the swelling ratio. The observed correlation allows one to extract one modulus from another in relevant scenarios.

Fast Light-Driven Motion of Polydopamine Nanomembranes.

The actuation of micro- and nanostructures controlled by external stimuli remains one of the exciting challenges in nanotechnology due to the wealth of fundamental questions and potential applications in energy harvesting, robotics, sensing, biomedicine, and tunable metamaterials. Photoactuation utilizes the conversion of light into motion through reversible chemical and physical processes and enables remote and spatiotemporal control of the actuation. Here, we report a fast light-to-motion conversion in few-nanometer thick bare polydopamine (PDA) membranes stimulated by visible light. Light-induced heating of PDA leads to desorption of water molecules and contraction of membranes in less than 140 µs. Switching off the light leads to a spontaneous expansion in less than 20 ms due to heat dissipation and water adsorption. Our findings demonstrate that pristine PDA membranes are multiresponsive materials that can be harnessed as robust building blocks for soft, micro-, and nanoscale actuators stimulated by light, temperature, and moisture level.

Unusual High-Frequency Mechanical Properties of Polymer-Grafted Nanoparticle Melts.

Brillouin light spectroscopy is used to measure the elastic moduli of spherical polymer-grafted nanoparticle (GNP) melts as a function of chain length at fixed grafting density (0.47 chains/nm^{2}) and nanoparticle radius (8 nm). While the moduli follow a rule of mixtures (Wood's law) for long chains, they display enhanced elasticity and anomalous dissipation for graft chains <100 kDa. GNP melts with long polymers at high σ have a dry zone near the GNP core, surrounded by a region where the grafts can interpenetrate with chain fragments from adjacent GNPs. We propose that the departures from Wood's law for short chains are due to the effectively larger silica volume fraction in the region where sound propagates-this is caused by the short, interpenetrated chain fragments being pushed out of the way. We thus conclude that transport mechanisms (of gas, ions, sound, thermal phonons) in GNP melts are radically different if interpenetrated chain segments can be "pushed out of the way" or not. This provides a facile new means for manipulating the properties of these materials.

Quantization of Acoustic Modes in Dumbbell Nanoparticles.

The vibrational eigenmodes of dumbbell-shaped polystyrene nanoparticles are recorded by Brillouin light spectroscopy (BLS), and the full experimental spectra are calculated theoretically. Different from spheres with a degeneracy of (2l+1), with l being the angular momentum quantum number, the eigenmodes of dumbbells are either singly or doubly degenerate owing to their axial symmetry. The BLS spectrum reveals a new, low-frequency peak, which is attributed to the out-of-phase vibration of the two lobes of the dumbbell. The quantization of acoustic modes in these molecule-shaped dumbbell particles evolves from the primary colloidal spheres as the separation between the two lobes increases.

Optically formed rubbery waveguide interconnects.

Light induced self-written waveguides (LISWs) with unique elongation characteristics and low optical loss are formed in a monodispersed polyisoprene solution using a low-power laser photopolymerization process, while their light transmission characteristics are exemplified in the flexible interconnection of two single-mode optical fibers operating in the visible/near infrared wavelengths. The LISWs formed exhibit rubbery properties, allowing extensibilities upon cases from 400% to 800%, while still retaining significant optical transmission. The rubber elasticity enables sustaining LISWs at stressed lengths longer than 500 µm propagation losses from 1.0 to 2.9 dB/mm.

Surface contacts strongly influence the elasticity and thermal conductivity of silica nanoparticle fibers.

Granular materials are often encountered in science and engineering disciplines, in which controlling the particle contacts is one of the critical issues for the design, engineering, and utilization of their desired properties. The achievable rapid fabrication of nanoparticles with tunable physical and chemical properties facilitates tailoring the macroscopic properties of particle assemblies through contacts at the nanoscale. Models have been developed to predict the mechanical properties of macroscopic granular materials; however, their predicted power in the case of nanoparticle assemblies is still uncertain. Here, we investigate the influence of nanocontacts on the elasticity and thermal conductivity of a granular fiber comprised of close-packed silica nanoparticles. A complete elastic moduli characterization was realized by non-contact and non-destructive Brillouin light spectroscopy, which also allowed resolving the stiffness of the constituent particles in situ. In the framework of effective medium models, the strong enhancement of the elastic moduli is attributed to the formation of adhesive nanocontacts with physical and/or chemical bondings. The nanoparticle contacts are also responsible for the increase in the fiber thermal conductivity that emphasizes the role of interface thermal resistance, which tends to be ignored in most porosity models. This insight into the fundamental understanding of structure-property relationships advances knowledge on the manipulation of granular systems at the nanoscale.

Multiband Hypersound Filtering in Two-Dimensional Colloidal Crystals: Adhesion, Resonances, and Periodicity.

The hypersonic phonon propagation in large-area two-dimensional colloidal crystals is probed by spontaneous micro Brillouin light scattering. The dispersion relation of thermally populated Lamb waves reveals multiband filtering due to three distinct types of acoustic band gaps. We find Bragg gaps accompanied by two types of hybridization gaps in both sub- and superwavelength regimes resulting from contact-based resonances and nanoparticle eigenmodes, respectively. The operating GHz frequencies can be tuned by particle size and depend on the adhesion at the contact interfaces. The experimental dispersion relations are well represented by a finite element method model enabling identification of observed modes. The presented approach also allows for contactless study of the contact stiffness of submicrometer particles, which reveals size effect deviating from macroscopic predictions.

Precision Anisotropic Brush Polymers by Sequence Controlled Chemistry.

The programming of nanomaterials at molecular length-scales to control architecture and function represents a pinnacle in soft materials synthesis. Although elusive in synthetic materials, Nature has evolutionarily refined macromolecular synthesis with perfect atomic resolution across three-dimensional space that serves specific functions. We show that biomolecules, specifically proteins, provide an intrinsic macromolecular backbone for the construction of anisotropic brush polymers with monodisperse lengths via grafting-from strategy. Using human serum albumin as a model, its sequence was exploited to chemically transform a single cysteine, such that the expression of said functionality is asymmetrically placed along the backbone of the eventual brush polymer. This positional monofunctionalization strategy was connected with biotin-streptavidin interactions to demonstrate the capabilities for site-specific self-assembly to create higher ordered architectures. Supported by systematic experimental and computational studies, we envisioned that this macromolecular platform provides unique avenues and perspectives in macromolecular design for both nanoscience and biomedicine.

Determination of the Complete Elasticity of Nephila pilipes Spider Silk.

Spider silks are remarkable materials designed by nature to have extraordinary elasticity. Their elasticity, however, remains poorly understood, as typical stress-strain experiments only allow access to the axial Young's modulus. In this work, micro-Brillouin light spectroscopy (micro-BLS), a noncontact, nondestructive technique, is utilized to probe the direction-dependent phonon propagation in the Nephila pilipes spider silk and hence solve its full elasticity. To the best of our knowledge, this is the first demonstration on the determination of the anisotropic Young's moduli, shear moduli, and Poisson's ratios of a single spider fiber. The axial and lateral Young's moduli are found to be 20.9 ± 0.8 and 9.2 ± 0.3 GPa, respectively, and the anisotropy of the Young's moduli further increases upon stretching. In contrast, the shear moduli and Poisson's ratios exhibit very weak anisotropy and are robust to stretching.

Disentangling the Role of Chain Conformation on the Mechanics of Polymer Tethered Particle Materials.

The linear elastic properties of isotropic materials of polymer tethered nanoparticles (NPs) are evaluated using noncontact Brillouin light spectroscopy. While the mechanical properties of dense brush materials follow predicted trends with NP composition, a surprising increase in elastic moduli is observed in the case of sparsely grafted particle systems at approximately equal NP filling ratio. Complementary molecular dynamics simulations reveal that the stiffening is caused by the coil-like conformations of the grafted chains, which lead to stronger polymer-polymer interactions compared to densely grafted NPs with short chains. Our results point to novel opportunities to enhance the physical properties of composite materials by the strategic design of the "molecular architecture" of constituents to benefit from synergistic effects relating to the organization of the polymer component.

Tunable Thermoelastic Anisotropy in Hybrid Bragg Stacks with Extreme Polymer Confinement.

Controlling thermomechanical anisotropy is important for emerging heat management applications such as thermal interface and electronic packaging materials. Whereas many studies report on thermal transport in anisotropic nanocomposite materials, a fundamental understanding of the interplay between mechanical and thermal properties is missing, due to the lack of measurements of direction-dependent mechanical properties. In this work, exceptionally coherent and transparent hybrid Bragg stacks made of strictly alternating mica-type nanosheets (synthetic hectorite) and polymer layers (polyvinylpyrrolidone) were fabricated at large scale. Distinct from ordinary nanocomposites, these stacks display long-range periodicity, which is tunable down to angstrom precision. A large thermal transport anisotropy (up to 38) is consequently observed, with the high in-plane thermal conductivity (up to 5.7 W m-1 K-1 ) exhibiting an effective medium behavior. The unique hybrid material combined with advanced characterization techniques allows correlating the full elastic tensors to the direction-dependent thermal conductivities. We, therefore, provide a first analysis on how the direction-dependent Young's and shear moduli influence the flow of heat.

The Ultrafast Assembly of a Dipeptide Supramolecular Organogel and its Phase Transition from Gel to Crystal.

A gel-to-crystal phase transition of a dipeptide supramolecular assembly mediates active water transportation in oils. The addition of water into ultrafast-assembling dipeptide organogels can induce a lamellar-to-hexagonal structural transformation of dipeptide molecular arrangement. Consequently, a phase transition from gel to crystal occurs and in turn water is transported in the dipeptide crystal via well-defined channels. On a macroscopic scale, water transport in the bulk system exhibits an anisotropic characteristic, which can be tuned by the presence of ions in the Hofmeister series. These favorable features enable the automatic separation of dispersed nanoparticles from dissolved electrolytes in aqueous solution. These findings demonstrate the potential of this assembled system for active filtration without external pressure.

Nonlinear control of high-frequency phonons in spider silk.

Spider dragline silk possesses superior mechanical properties compared with synthetic polymers with similar chemical structure due to its hierarchical structure comprised of partially crystalline oriented nanofibrils. To date, silk's dynamic mechanical properties have been largely unexplored. Here we report an indirect hypersonic phononic bandgap and an anomalous dispersion of the acoustic-like branch from inelastic (Brillouin) light scattering experiments under varying applied elastic strains. We show the mechanical nonlinearity of the silk structure generates a unique region of negative group velocity, that together with the global (mechanical) anisotropy provides novel symmetry conditions for gap formation. The phononic bandgap and dispersion show strong nonlinear strain-dependent behaviour. Exploiting material nonlinearity along with tailored structural anisotropy could be a new design paradigm to access new types of dynamic behaviour.

Covalently Interlocked Cyclohexa-m-phenylenes and Their Assembly: En Route to Supramolecular 3D Carbon Nanostructures.

In our search to cluster as many phenylene units as possible in a given space, we have proceeded to the three-dimensional world of benzene-based molecules by employing covalently interlocked cyclohexa-m-phenylenes, as present in the unique paddlewheel-shaped polyphenylene 10. A precursor was conceived, in which freely rotating m-chlorophenylene units provide sufficient solubility along with the necessary proximity for the final ring closure to give 10. Monitoring the assembly of solubilized tert-butyl derivatives of 10 into supramolecular carbon nanostructures by dynamic light scattering (DLS) and Brillouin light scattering (BLS) revealed the dimensions of the initially formed aggregates as well as the amorphous character of the solid state.

Synthesis and conformation of 3,6-connected cyclohexadiene chains.

3,6-Connected cyclohexadienes as precursors for polyphenylenes are synthesized and characterized by mass spectrometry and NMR spectroscopy. Pure fractions of trimers, hexamers, and nonamers are collected after separation of the product mixture by recycling GPC. The anticipated formation of rigid linear structures, due to the trans-configuration of the monomeric units, is supported by density functional theory and experimentally confirmed by dynamic light scattering from dilute solution at low scattering angles. The obtained translational diffusion coefficients are represented by rigid rod-like or prolate ellipsoid-like molecular shapes. The measurements of diffusion coefficients reveal a length-dependent ratio of 1:2:3 between the three oligomers, which directly correlates to the expected length extension from trimer to nonamer.

Surface asymmetry of coated spherical nanoparticles.

We validate the nonspherical grafting arrangement of isotropically coated spherical nanoparticles as very recently proposed. We utilize localized surface plasmon resonance enhanced dynamic polarized and depolarized light scattering from Au nanoparticles, the spherical symmetry of which was revealed by single-particle dark-field spectroscopy. The same Au nanospheres are grafted with ligands of different chemistry and length. The wavelength dependent depolarization ratio and the two transport coefficients of these nanoparticles, obtained from the dynamic light scattering experiment, can only be reconciled with the TEM data, the single UV/vis extinction spectrum, and the dark-field spectroscopy experiments if their coating is described as asymmetric. Spatially anisotropic graft distribution on spherical nanoparticles impacts their assembly and understanding its origin will help control the structure and properties of polymer nanocomposites.

Anisotropic hypersonic phonon propagation in films of aligned ellipsoids.

A material with anisotropic elastic mechanical properties and a direction-dependent hypersonic band gap is fabricated using ac electric field-directed convective self-assembly of colloidal ellipsoids. The frequency of the gap, which is detected in the direction perpendicular to particle alignment and entirely absent parallel to alignment, and the effective sound velocities can be tuned by the particle aspect ratio. We hypothesize that the band gap originates from the primary eigenmode peak, the m-splitted (s,1,2) mode, of the particle resonating with the effective medium. These results reveal the potential for powerful control of the hypersonic phononic band diagram by combining anisotropic particles and self-assembly.

Ultrastable nanostructured polymer glasses.

Owing to the kinetic nature of the glass transition, the ability to significantly alter the properties of amorphous solids by the typical routes to the vitreous state is restricted. For instance, an order of magnitude change in the cooling rate merely modifies the value of the glass transition temperature (T(g)) by a few degrees. Here we show that matrix-assisted pulsed laser evaporation (MAPLE) can be used to form ultrastable and nanostructured glassy polymer films which, relative to the standard poly(methyl methacrylate) glass formed on cooling at standard rates, are 40% less dense, have a 40 K higher T(g), and exhibit a two orders of magnitude enhancement in kinetic stability at high temperatures. The unique set of properties of MAPLE-deposited glasses may make them attractive in technologies where weight and stability are central design issues.