Controlling the Thermomechanical Behavior of Nanoparticle/Polymer Films

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>i.e.</i>, 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

Novel Lipophilic Acetohydroxamic Acid Derivatives Based on Conformationally Constrained Spiro Carbocyclic 2,6-Diketopiperazine Scaffolds with Potent Trypanocidal Activity

We describe novel acetohydroxamic acid derivatives with potent activity against cultured bloodstream-form <i>Trypanosoma brucei</i> 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. <i>S</i>-Enantiomer <b>7d</b> was the most active derivative against <i>T. brucei</i> (IC₅₀ = 6.8 nM) and <i>T. cruzi</i> (IC₅₀ = 0.21 μM)

Probing Stress-Induced Optical Birefringence of Glassy Polymers by Whispering Gallery Modes Light Localization

An optical resonance method for the determination of the strain- and stress-optical coefficients of optically transparent polymers is presented and exemplified for monodisperse and bidisperse molecular weight polystyrene (PS). This method employs whispering gallery modes (WGMs) resonation inside a spheroid polymeric cavity, suspended on an optical fiber taper waist, which, in turn, is used for subjecting the polymeric resonator to controlled strain conditions. The wavelength shifts of equal order transverse electric and transverse magnetic polarization WGMs are measured, as well as their relative birefringence versus applied strain. For monodisperse PS microspheroids (2 and 50 kDa) the stress-optical coefficient is negative, contrary to the results for bulk PS in the glassy state indicating different phenyl group orientation of the PS monomer with respect to the strain direction. In the bidisperse (2 and 50 kDa) spheroid with a symmetric monomer composition, local structural irregularities are probably responsible for the observed coupling between WGMs. The method possesses metrological capabilities for probing the molecular orientation of polymer-based resonators

Structure and Dynamics of Dendronized Polymer Solutions: Gaussian Coil or Macromolecular Rod?

We investigate the conformation of well-defined dendronized polymers (denpols) based on poly­(norborene) (PNB) and poly­(<i>endo</i>-tricycle­[4.2.2.0]­deca-3,9-diene) (PTD) backbones employing static and dynamic light scattering. Their synthesis by ring-opening metathesis polymerization (ROMP) led to fully grafted and high molecular weight denpols with narrow polydispersity. In dilute solutions, the persistence lengths were estimated by static (radius of gyration) and dynamic (translational diffusion) chain conformational properties of the denpols and were compared to their homologue precursor PNB. The conformation of denpols with a third generation side dendron conforms to a semiflexible chain with a persistence length of about 6–8 nm, virtually independent of the contour length. In the semidilute regime, the thermodynamics and cooperative diffusion of denpols resemble the behavior of the precursor solutions as described by the scaling theory of flexible polymers above the crossover concentration. The assumption of extremely high chain rigidity for this class of polymers is clearly not supported, at least for the third generation dendron

Dynamics in Stimuli-Responsive Poly(<i>N</i>‑isopropylacrylamide) Hydrogel Layers As Revealed by Fluorescence Correlation Spectroscopy

We employ fluorescence correlation spectroscopy (FCS) to study the translational mobility of molecular tracers in stimuli-responsive grafted poly­(<i>N</i>-isopropylacrylamide) (PNiPAAm) hydrogels, under variable solvency conditions. Tracer–matrix interactions were tuned by selecting three different molecular tracers. In contrast to a noninteracting tracer (Alexa 647), the mobility of a weakly (Alexa 488) and a strongly interacting (Rhodamine 6G) tracer deviates from a simple single Fickian diffusion. In addition to pure crowding effects, the mobility of both Alexa488 and Rhodamine 6G is influenced by tracer–polymer interactions. We interpret the observed trends in tracer mobility in terms of the interplay between Coulombic repulsions and short-range attractions. Although tracer dynamics and hydrogel swelling ratio are interdependent properties, their relation turns out to be nontrivial and does not allow predictions of tracer dynamics on the basis of polymer structural information. Hence, a universal scaling behavior is not possible, due to tracer–polymer interactions

Tunable Hypersonic Bandgap Formation in Anisotropic Crystals of Dumbbell Nanoparticles

Phononic materials exhibit mechanical properties that alter the propagation of acoustic waves and are widely useful for metamaterials. To fabricate acoustic materials with phononic bandgaps, colloidal nanoparticles and their assemblies allow access to various crystallinities in the submicrometer scale. We fabricated anisotropic crystals with dumbbell-shaped nanoparticles via field-directed self-assembly. Brillouin light spectroscopy detected the formation of direction-dependent hypersonic phononic bandgaps that scale with the lattice parameters. In addition, the local resonances of the constituent nanoparticles enable metamaterial behavior by opening hybridization gaps in disordered structures. Unexpectedly, this bandgap frequency is robust to changes in the dumbbell aspect ratio. Overall, this study provides a structure–property relationship for designing anisotropic phononic materials with targeted phononic bandgaps

Elastic Modulus and Thermal Conductivity of Thiolene/TiO₂ Nanocomposites

Metal oxide based polymer nanocomposites find diverse applications as functional materials, and in particular thiol-ene/TiO₂ nanocomposites are promising candidates for dental restorative materials. The important mechanical and thermal properties of the nanocomposites, however, are still not well understood. In this study, the elastic modulus and thermal conductivity of thiol-ene/TiO₂ nanocomposite thin films with varying weight fractions of TiO₂ nanoparticles are investigated by using Brillouin light scattering spectroscopy and 3ω measurements, respectively. As the TiO₂ weight fraction increases from 0 to 90%, the effective elastic longitudinal modulus of the films increases from 6.2 to 37.5 GPa, and the effective thermal conductivity from 0.04 to 0.76 W/m K. The former increase could be attributed to the covalent cross-linking of the nanocomposite constituents. The latter one could be ascribed to the addition of high thermal conductivity TiO₂ nanoparticles and the formation of possible conductive channels at high TiO₂ weight fractions. The linear dependence of the thermal conductivity on the sound velocity, reported for amorphous polymers, is not observed in the present nanocomposite system

Incorporation of Nanoparticles into Polymersomes: Size and Concentration Effects

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 SiO₂ nanoparticles by poly(dimethylsiloxane)-<i>block</i>-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

Plasmonic Core–Satellite Assemblies as Highly Sensitive Refractive Index Sensors

Highly sensitive and spectrally tunable plasmonic nanostructures are of great demand for applications such as SERS and parallel biosensing. However, there is a lack of such nanostructures for the midvisible spectral regions as most available chemically stable nanostructures offer high sensitivity in the red to far red spectrum. In this work, we report the assembly of highly sensitive nanoparticle structures using a hydroxylamine mediated core–satellite assembly of 20 nm gold nanoparticle satellites onto 60 nm spherical gold cores. The average number of satellites allows tuning the plasmon resonance wavelength from 543 to 575 nm. The core–satellite nanostructures are stable in pH ranges from 5 to 9 and show about 2-fold higher plasmonic sensitivity than similar sized gold nanospheres

Role of Polymer Graft Architecture on the Acoustic Eigenmode Formation in Densely Polymer-Tethered Colloidal Particles

The concurrent evaluation of the vibration eigenfrequencies in densely polymer-tethered particle systems (“particle brushes”) by Brillouin light scattering and elastodynamic theory reveals a distinctive change of acoustic eigenmode formation associated with polymer graft modification of colloidal particles. The eigenfrequencies of particle brushes reveal a characteristic red-shift compared to uniform core-shell particles that can only be rationalized by assuming imperfect boundary conditions and anisotropic elastic properties of the graft layer. The distinct characteristics of vibration modes in particle brush materials provide direct evidence for the implications of chain confinement on the nanomechanical properties of tethered chains. The results highlight a rich and hitherto unexplored parameter-space for controlling properties and interactions in particle–brush based systems that could spur the development of hybrid materials with novel functionalities