We show that the mesoscale (?200 nm) thermomechanical properties of polymer nanocomposites formed from silica nanoparticles (NPs) and poly(2-vinylpyridine) (P2VP) critically depend on their interfacial structure, which can be controlled by the casting solvent. The composite films are solvent cast from either pyridine (PYR) or methylethylketone (MEK), with uniform NP spatial distribution obtained in both cases. In the films cast from MEK, our previous work has shown that a bound layer of P2VP is formed at the NP surfaces, while no such bound layer is formed when PYR is used as the casting solvent. In PYR as-cast films, Brillouin light scattering reveals a single acoustic phonon with its longitudinal sound velocity increasing with NP loading. This implies a homogeneous mixture of the NP and the polymer on the mesoscopic scales for all compositions examined. However, in the MEK as-cast films, two longitudinal and two transverse acoustic phonons are observed at NP loadings above ?20 wt % (or ?11 vol %), reminiscent of two metastable microscopic phases. The dense microphase is attributed to the bridging of NPs by P2VP chains, whereas for the softer medium, we conjecture that there exists an interfacial lower density P2VP layer whose longitudinal sound velocity barely changes with NP loading. These solvent-induced differences in the (elastic) mechanical behavior disappear upon thermal annealing, suggesting that these nanocomposite interfacial structures in the as-cast state (far from equilibrium) locally approach equilibrium (i.e., near equilibrium after annealing). Consistent with these conclusions, the abrupt decrease of the longitudinal sound velocity with temperature occurs at a single glass transition temperature for the annealed nanocomposites irrespective of the casting solvent used, which assumes only a slightly higher (?5 K at 45 wt % or ?29 vol %) value than that in bulk P2VP. The results emphasize the important role of solvent in determining the interfacial structure of nanocomposites, which can be used to tailor their thermomechanical behavior.
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.
The properties of graphene nanoribbons (GNRs) make them good candidates for next-generation electronic materials. Whereas 'top-down' methods, such as the lithographical patterning of graphene and the unzipping of carbon nanotubes, give mixtures of different GNRs, structurally well-defined GNRs can be made using a 'bottom-up' organic synthesis approach through solution-mediated or surface-assisted cyclodehydrogenation reactions. Specifically, non-planar polyphenylene precursors were first 'built up' from small molecules, and then 'graphitized' and 'planarized' to yield GNRs. However, fabrication of processable and longitudinally well-extended GNRs has remained a major challenge. Here we report a bottom-up solution synthesis of long (>200 nm) liquid-phase-processable GNRs with a well-defined structure and a large optical bandgap of 1.88 eV. Self-assembled monolayers of GNRs can be observed by scanning probe microscopy, and non-contact time-resolved terahertz conductivity measurements reveal excellent charge-carrier mobility within individual GNRs. Such structurally well-defined GNRs may prove useful for fundamental studies of graphene nanostructures, as well as the development of GNR-based nanoelectronics.
We employ spontaneous Brillouin light scattering spectroscopy and detailed theoretical calculations to reveal and identify elastic excitations inside the band gap of hypersonic hybrid superlattices. Surface and cavity modes, their strength and anticrossing are unambiguously documented and fully controlled by layer thickness, elasticity, and sequence design. This new soft matter based superlattice platform allows facile engineering of the density of states and opens new pathways to tunable phoxonic crystals.
We employ fluorescence correlation spectroscopy (FCS) and coarse-grained molecular dynamics simulations to study the mobility of tracers in polymer solutions. Excluded volume interactions result in crowding-induced slowdown, depending only on the polymer concentration. With specific tracer-polymer attractions, the tracer is slowed down at much lower concentrations, and a second diffusion component appears that is sensitive to the polymer chain length. The two components can be resolved by FCS, only if the distance traveled by the tracer in the polymer-bound state is greater than the FCS focal spot size. The tracer dynamics can be used as a sensitive probe of the nature and strength of interactions, which-despite their local character-emphasize the role of chain connectivity.
An explanation for the vast difference observed in the trypanocidal activity between the new secondary (N-methylated) hydroxamic acids 5 and 6, and their primary (nonmethylated) congeners 1a and 2, based on their E/Z conformational behaviour in DMSO, is presented.
We have measured the glassy-state structural relaxation of aqueous suspended polystyrene (PS) nanoparticles (the case of soft confinement) and the corresponding silica-capped PS nanoparticles (the case of hard confinement) via differential scanning calorimetry. Suspended and capped PS nanoparticles undergo physical aging under isobaric and isochoric conditions, respectively. With decreasing diameter, suspended and capped PS nanoparticles exhibited reduced and bulk glass transition temperatures (T(g)), respectively. To account for T(g) changes with confinement, all physical aging measurements were performed at a constant value of T(g) - T(a), where T(a) is the aging temperature. With decreasing diameter, aqueous suspended PS nanoparticles exhibited enhanced physical aging rates in comparison to bulk PS. Due to differences in thermodynamic conditions during aging and interfacial effects from nanoconfinement, at all values of T(g) - T(a) investigated, capped PS nanoparticles aged at reduced rates compared to the corresponding aqueous suspended PS nanoparticles. We captured the physical aging behavior of all nanoparticles via the Tool, Narayanaswamy, and Moynihan model of structural relaxation.
The in-plane and out-of-plane elastic properties of thin films of "quasi-one-component" particle-brush-based nanocomposites are compared to those of "classical" binary particle-polymer nanocomposite systems with near identical overall composition using Brillouin light scattering. Whereas phonon propagation is found to be independent of the propagation direction for the binary particle/polymer blend systems, a pronounced splitting of the phonon propagation velocity along the in-plane and out-of-plane film direction is observed for particle-brush systems. The anisotropic elastic properties of quasi-one-component particle-brush systems are interpreted as a consequence of substrate-induced order formation into layer-type structures and the associated breaking of the symmetry of the film. The results highlight new opportunities to engineer quasi-one-component nanocomposites with advanced control of structural and physical property characteristics based on the assembly of particle-brush materials.
We describe novel acetohydroxamic acid derivatives with potent activity against cultured bloodstream-form Trypanosoma brucei and selectivity indices of >1000. These analogues were derived from conformationally constrained, lipophilic, spiro carbocyclic 2,6-diketopiperazine (2,6-DKP) scaffolds by attaching acetohydroxamic acid moieties to the imidic nitrogen. Optimal activity was achieved by placing benzyl groups adjacent to the basic nitrogen of the 2,6-DKP core. S-Enantiomer 7d was the most active derivative against T. brucei (IC(50) = 6.8 nM) and T. cruzi (IC(50) = 0.21 ?M).
We performed fluorescence correlation spectroscopy measurements to assess the long-time self-diffusion of a variety of spherical tracer particles in periodic porous nanostructures. Inverse opal structures with variable cavity sizes and openings in the nanometer domain were employed as the model system. We obtained both the exponent of the scaling relation between mean-square displacement and time and the slow-down factors due to the periodic confinement for a number of particle sizes and confining characteristics. In addition, we carried out Brownian dynamics simulations to model the experimental conditions. Good agreement between experimental and simulation results has been obtained regarding the slow-down factor. Fickian diffusion is predicted and seen in almost all experimental systems, while apparent non-Fickian exponents that show up for two strongly confined systems are attributed to polydispersity of the cavity openings. The utility of confining periodic porous nanostructures holds promise toward understanding of constrained diffusion with a wide range of applications ranging from water purification and drug delivery to tissue engineering.
We present a novel light scattering setup that enables probing of dynamics near solid surfaces. An evanescent wave generated by a surface plasmon resonance in a metal layer is the incident light field in the dynamic light scattering experiment. The combination of surface plasmon resonance spectroscopy and dynamic light scattering leads to a spatiotemporal resolution extending a few hundred nanometers from the surface and from microseconds to seconds. The comparison with evanescent wave dynamic light scattering identifies the advantages of the presented technique, e.g., surface monitoring, use of metal surfaces, and biorelevant systems. For both evanescent wave geometries, we define the scattering wave vector necessary for the analysis of the experimental relaxation functions.
Spiro[piperidine-2,2-adamantane] 4 is one of the most potent synthetic anti-influenza A aminoadamantanes or other cage structure amines tested so far. Based on previous results Tataridis et al. (2007) [5h] which demonstrate the boost of in vitro potency by the presence of an additional amino group, we examined whether the incorporation of a second amino group into this heterocycle would increase the anti-influenza A virus activity. The new synthetic molecules 5-7 are capable of forming two hydrogen bonds within the receptor. We identified the diamino derivatives 5 and 6, which are active against influenza A H3N2 virus although less potent than amantadine and its equipotent spiropiperidine 4.
Anodic aluminum oxide (AAO) containing arrays of aligned cylindrical nanopores infiltrated with polymers is a well-defined model system for the study of hypersound propagation in polymer nanocomposites. Hypersonic phononic properties of AAO/polymer nanocomposites such as phonon localization and anisotropic sound propagation can be tailored by adjusting elastic contrast and density contrast between the components. Changes in density and elastic properties of the component located in the nanopores induced by phase transitions allow reversible modification of the phononic band structure and mode switching. As example in case, the crystallization and melting of poly(vinylidene difluoride) inside AAO was investigated.
Experiments addressing supramolecular dynamics at interfaces are of paramount importance for the understanding of the dynamic behaviour of polymers, particles, or cells at interfaces, transport phenomena to and from surfaces, thin films or membranes. However, there are only few reports in the literature due to the paucity of experimental methods that offer the required spatial and time resolution. Evanescent wave dynamic light scattering originally developed to meet these needs has limited sensitivity and is restricted to glass substrates. Here we report the first experimental realization of a dynamic light scattering experiment close to an interface using surface plasmon polaritons as light source offering a strong increase in the signal to noise ratio and allowing for the use of metallic interfaces. As a proof of concept, we consider the diffusion of particles with radii down to 10nm in dilute dispersions close to a gold surface.
Details of the forces between nanoparticles determine the ways in which the nanoparticles can self-assemble into larger structures. The use of directed interactions has led to new concepts in self-assembly such as asymmetric dendrons, Janus particles, patchy colloids and colloidal molecules. Recent models that include attractive regions or patches on the surface of the nanoparticles predict a wealth of intricate modes of assembly. Interactions between such particles are also important in a range of phenomena including protein aggregation and crystallization, re-entrant phase transitions, assembly of nanoemulsions and the organization of nanoparticles into nanowires. Here, we report the synthesis of 6-nm nanoparticles with dynamic hydrophobic patches and show that they can form reversible self-assembled structures in aqueous solution that become topologically more connected upon dilution. The organization is based on guest-host supramolecular chemistry with the nanoparticles composed of a hydrophobic dendrimer host molecule and water-soluble hydrophilic guest molecules. The work demonstrates that subtle changes in hierarchal composition and/or concentration can dramatically change mesoscopic ordering.
We employed fluorescence correlation spectroscopy (FCS) to study the diffusion of molecular and macromolecular tracers in polystyrene solutions over a broad range of concentrations (c) and molecular weights (M(w,m)) of the matrix polymer. Molecular tracer diffusion scales only with the matrix concentration and superimposes on a single, nonpolymer specific, curve. On the contrary, the diffusion of macromolecular tracers in solutions of matrix polymers with M(w,m) sufficiently larger than the tracer molecular weight scales with c/c(p)*, where c(p)* is the tracer overlap concentration. We further demonstrate that FCS can address local and global dynamics simultaneously.
Because of the rapidly growing field of nanoparticles in therapeutic applications, understanding and controlling the interaction between nanoparticles and membranes is of great importance. While a membrane is exposed to nanoparticles its behavior is mediated by both their biological and physical properties. Constant interplay of these biological and physicochemical factors makes selective studies of nanoparticles uptake demanding. Artificial model membranes can serve as a platform to investigate physical parameters of the process in the absence of any biofunctional molecules and/or supplementary energy. Here we report on photon- and fluorescence-correlation spectroscopic studies of the uptake of nanosized SiO(2) nanoparticles by poly(dimethylsiloxane)-block-poly(2-methyloxazoline) vesicles allowing species selectivity. Analogous to the cell membrane, polymeric membrane incorporates particles using membrane fission and particles wrapping as suggested by cryo-TEM imaging. It is revealed that the incorporation process can be controlled to a significant extent by changing nanoparticles size and concentration. Conditions for nanoparticle uptake and controlled filling of polymersomes are presented.
We report on the versatile effect of weak red laser light impinging on diblock copolymer [poly(isoprene-b-styrene)] dispersions in two selective solvents for each block. In the strongly scattering but transparent micellar solutions in hexane (a good solvent for polyisoprene), higher refractive index copolymer-rich fibers were formed. In the turbid dispersions of the same copolymer in ethyl acetate (a good solvent for polystyrene), the effect of self-induced transparency was observed. A two-step patterning mechanism caused the generation of a transparent microchannel, increasing light transmission. The analogy between the current effect and that observed in homopolymer polyisoprene solutions in different solvents is discussed toward an understanding of the unanticipated light-soft-matter interaction.
A modular one-component supramolecular transient network in water, based on poly(ethylene glycol) and end-capped with four-fold hydrogen bonding units, is reported. Due to its nonlinear structural formation, this system allows active proteins to be added to the hydrogel during formation. Once implanted in vivo it releases the protein by erosion of both the protein and polymer via dissolution.
We report on the full control of phononic band diagrams for periodic stacks of alternating layers of poly(methyl methacrylate) and porous silica combining Brillouin light scattering spectroscopy and theoretical calculations. These structures exhibit large and robust on-axis band gaps determined by the longitudinal sound velocities, densities, and spacing ratio. A facile tuning of the gap width is realized at oblique incidence utilizing the vector nature of the elastic wave propagation. Off-axis propagation involves sagittal waves in the individual layers, allowing access to shear moduli at nanoscale. The full theoretical description discerns the most important features of the hypersonic one-dimensional crystals forward to a detailed understanding, a precondition to engineer dispersion relations in such structures.
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.
Related JoVE Video
Journal of Visualized Experiments
What is Visualize?
JoVE Visualize is a tool created to match the last 5 years of PubMed publications to methods in JoVE's video library.
How does it work?
We use abstracts found on PubMed and match them to JoVE videos to create a list of 10 to 30 related methods videos.
Video X seems to be unrelated to Abstract Y...
In developing our video relationships, we compare around 5 million PubMed articles to our library of over 4,500 methods videos. In some cases the language used in the PubMed abstracts makes matching that content to a JoVE video difficult. In other cases, there happens not to be any content in our video library that is relevant to the topic of a given abstract. In these cases, our algorithms are trying their best to display videos with relevant content, which can sometimes result in matched videos with only a slight relation.