Session V17: Hybrid Organic/Inorganic Nanomaterials: Synthesis, Assembly, and Applications II

Photoinduced Reduction of Noble Metal Ions to Metal Nanoparticles on Tubular J-Aggregates

Cyanine dye molecules are well known to serve as sensitizers for photo induced electron transfer processes. Technically, this feature is utilized in photographic films to form elementary silver specks in solid silver halide crystallites. In this contribution it is shown that the photo-induced electron transfer reaction from cyanine dyes to noble metal ions can be utilized to grow metallic nanoparticles at the surface of tubular J-aggregates in solution. The J-aggregates are formed by amphiphilic cyanine dye molecules upon aggregation in aqueous solution. The particles are grown by addition of noble metal salts (Na$_{2}$PdCl$_{4}$ or AgNO$_{3})$ to the cyanine dye aggregate solution and by illumination with visible light. The particles are observed by Cryogenic transmission electron microscopy (cryo-TEM) and are rather uniform in size with a mean diameter of a few nanometres. In case of Pd salt the aggregates are destroyed upon particle formation, while in case of Ag salt the aggregates are unaffected by the formation of silver particles. In parallel to the growth of the particles a dramatic quenching of the aggregate fluorescence is observed. The particles form spontaneously and the number and size of the particles depends on the molar Ag/dye ratio. A systematic study of this photo induced process for AgNO$_{3}$ will be presented.   

The chemical and structural properties of PECVD polymerized ferrocene deposited by the sublimation of the precursor material

The novel deposition of metal containing precursor materials in plasma enhanced chemical vapor deposition through the sublimation of the material in its solid state is investigated. The chemical composition and structural properties of these thin films, examined through XPS, variable angle ellipsometry, FT-IR, AFM and X-ray reflectivity, are reported. Using a custom designed plasma chamber and sublimation system, pp-ferrocene films have shown high chemical and mechanical robustness, are pin-hole free, have extremely smooth surface morphologies, and are highly crosslinked through the bulk. The use of sublimation in PECVD opens up the deposition technique to a wealth of new metal containing monomers. And with PECVD you retain a high amount of control over the deposition parameters and resultant film compositions for these organo-metallic films.   

Selective dispersion of nanofillers in PET/PC blends

The nanocomposite formation of immiscible PET/PC blends with organically-modified montmorillonite layered-silicates was studied as a model system to tailor thermodynamics, so as to achieve (a) selective dispersion in the PET only phase, and (b) promote physical mixing (``compatibilization'') of the PET and PC matrices. Dispersion was controlled by design of appropriate surfactant chemistries used for nanofiller modification. The desired composite structure is obtained even when the organically-modified fillers are premixed (masterbatched) in the ``unfavorable'' polymer. This behavior, i.e. the composite structure being markedly independent of processing conditions, indicates that, for these systems, the thermodynamics of dispersion overwhelmingly determine the resulting structure rather than the processing conditions. The resulting changes in PET crystal morphology afford novel new mechanical properties, that combine substantial increases in modulus with accompanying increases in ductility and toughness.   

The thermal properties and the microstructures of organic-inorganic nano-composite materials

The imaging capability of the atomic force Microscope (AFM) with thermal accessory is utilized to study the nano-scale T$_{g }$and morphological evolution of silica-polyacrylate composites. The polymer matrix is made by irradiation-inducing polymerization of the blend of TEGDA and EOBDA (Ethoxylated (3) Bisphenol A Diacrylate)acrylate. The surface area ratio of the nano-particles and the measured frame is applied to define the melting event on the composite film and make a direct comparison with the macro-scale T$_{g}$ obtained from Differential Scanning Calorimetry (DSC) and Thermo-Mechanical Analyzer (TMA). The decomposition temperature T$_{d}$ of the composites is measured by Thermo-Gravimetic Analyzer (TGA). In analog to the mechanical property transition as the nanoparticle content reaches the critical density, the measured melting temperature of these hybrid materials displays a nonlinear trend as the filler content increases. When the silica particles form a percolation network within the composite, the 2-D melting and 3-D disintegration behave differently. This discrepancy arising either from the fundamental difference of physical nature or the instrumental limits will be discussed.   

Enhanced Oxygen Barrier and Interfacial Adhesion of Polystyrene/Clay Nanocomposites via Plasma Surface Modification

Bentonite layered-silicate was the selected nanofiller to be studied and filled in polystyrene, consequently represented PS/clay nanocomposites. The plasma technique developed in our laboratory was utilized as the clay pretreatment method to provide the radical sites on the clay surface prior to styrene grafting step. Gas barrier property and impact strength, one of the most serious deficiencies of polystyrene, were examined. Oxygen permeability was found to be decreased moderately even small amount of modified clay loaded. As the result of plasma surface modification, it was found that grafting of styrene was occurred mainly on the outer clay layer since FT-IR spectra showed the characteristic peak of polystyrene with the same basel-spacing like pristine clay. The broadening of diffraction peak of PS/clay nanocomposites produced by melt intercalation was observed suggesting the structure of both intercalation and partial exfoliation; however, the impact strength was reasonably improved which can imply to the enhancement of interfacial adhesion between clay particles and polystyrene matrix. In addition, the ratio of styrene and initiator played the significant role on these properties as well.   

Structure and Morphology of Polymer/Clay Nanocomposites formed by Chaotic Smart Blending

The dispersion and orientation of the nano particles within polymeric matrices determine their properties. They depend on inherent characteristics such as miscibility, entropic barriers, structure of the polymer etc., and on processing conditions under which the polymer and the particles are mixed. Chaotic flow patterns have been used to define the structure and orientation on a micron length scale. In efforts to derive quantitative correlations, we carried out structural studies using X-ray, AFM and TEM on Nylon 6/modified clay nano composites as a function of the strength of the chaotic advection in a smart blending process. On the micron length scale, the clay particles are dispersed into multi layers, where the thickness of these sheets and the orientation and the distribution of the clay particles are controlled by the strength of the chaotic advection. On the molecular level, increasing the strength results in increase of the $\gamma $ crystal form of the Nylon, a higher internal energy state, in which hydrogen bonds are formed between parallel chains. The structure on the micron level is attributed to the chaotic mixing which o the molecular level it is may be attributed to the fractal nature of the process.   

A Nanoparticle Self-Assembled Tactile Sensor with Sensitivity {\&} Resolution of Human Finger

Sensation of touch, primarily the determination of stress distribution over the area of physical contact between the sensor and the object surfaces, is critical for advancement of minimum invasive surgical procedures and for development of humanoid robots. The spatial resolution of current large-area tactile sensor ($>$ 1 cm$^{2})$ lags by over an order of magnitude compared to human finger. Using metal and semiconducting nanoparticles $\sim $100 nm thick, large area thin-film device is self assembled such that the change in current density through the film and the electroluminescent light intensity are proportional to local stress. Both lateral and height resolution of texture are comparable to that of human finger, 40$\mu $m and 2 $\mu $m respectively. The sensitivity of 9 KPa is well within the 10 to 40 KPa range that a human finger applies to sense texture and shape. Stress image can be constructed by directly imaging the electroluminescence on a CCD or by mapping the current density over the surface of the device. The device is based on the principle of electron tunneling. Being solution processed it can be easily assembled on large surfaces and complex shapes..   

Novel Route to Nanoparticle Dispersion Using Supercritical Carbon Dioxide

An experimental study was carried out to determine the effects of supercritical carbon dioxide (scCO$_{2}$) on the dispersion of untreated and modified alumina nanoparticles in polystyrene. For the untreated alumina, the sudden expansion of the carbon dioxide did not alter the size of the agglomerates. This was probably caused by the weak interaction of scCO$_{2}$ with the untreated alumina that keep the agglomerate intact upon depressurization. On the other hand, the large agglomerates of the modified alumina showed signs of catastrophic fragmentation. It is speculated that the smaller agglomerates lacked the surface coating, which may have lead to their intact structure. The fluorinated modifiers used had a high degree of solubility with the scCO$_{2}$ at the processing conditions used, and therefore, the scCO$_{2}$ in the modified alumina composites was able to diffuse easily into the agglomerate compared to the untreated alumina. Large agglomerates can absorb more scCO$_{2}$ than smaller ones, and therefore, have an adequately larger bursting pressure to overcome both the hydrogen bonding between the modified alumina nanoparticles and the weak electrostatic interactions.   

Directing self-assembly of gold nanoparticles in diblock copolymer scaffold

A versatile hierarchical approach for directing self -assembly of gold nanostructures with size 2-3nm in diblock copolymer scaffolds is found. Diblock copolymer polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) is used to form a regular scaffold of highly anisotropic, stripe-like domains, and controlled differential wetting by dichloromethane and thermal annealing guides gold nanoparticles with half hydrophilic ligand to aggregate selectively along the scaffold, producing highly organized metal nanostructures. In as-cast block-copolymer and gold nanoparticles thin films, micelle structure and gold nanoparticles random distribution on scaffold are typically observed. However, samples annealed in dichloromethane exhibit well-defined short-range ordered nanostructure with gold nanoparticles located at the interface of PS and P2VP nanoscale domain. After annealing at 170\r{ }C, the gold nanoparticles at interface migrated into the middle of P2VP phase and exhibited long-range ordered hierarchical structures. Synergistic interactions between the gold nanoparticles and the PS-b-P2VP caused an orientation of the microdomains normal to the film surface.   

Simple Fabrication of Mesoporous Silica with Remarkable High Temperature Stability at Neutral pH and Ambient Conditions from TEOS

Traditional silica synthesis processes can yield well ordered materials, but the synthesis conditions also lead to incomplete condensation of the silica network, which results in significant structural contraction upon calcination and limited thermal, hydrothermal and mechanical stability. Here we report the synthesis that, surprisingly, yields nearly complete condensation of the silica network (virtually all Q4 linkages) using cysteamine as the catalyst and polyoxyethylene surfactants as the structure directing agents in buffered solution at neutral pH and ambient temperature. Recently, small molecule bifunctional amines, including cysteamine, were evaluated by Morse and co-workers and found to produce silica from TEOS(JACS 2005, 127, 35). Our work combines the cysteamine catalyst system with structure-directing block copolymer surfactants at neutral pH and ambient temperature to produce mesoporous silica. The addition of tetraethyl orthosilicate (TEOS) to a solution of containing cysteamine, citrate buffer (pH 7.2) and 5wt Brij amphiphilic block copolymer (polyethylene oxide-polyethylene) yields mesoporous silica. The resulting mesoporous silica powder was analyzed using XRD, TGA, FTIR, TEM, and NMR. The materials were found to exhibit stability under extreme temperature calcinations (up to 800 C) in the presence of water. SAXS shows that 1.0 shrinkage upon calcination up to 800C. 29Si NMR analysis indicates a fully condensed silica network, Q4 linkages, in accordance with this observation.   

Hydrophilic Silica-Polypeptide Composite Particles

Composite, pH-responsive particles have been synthesized by covalently attaching a simple polypeptide to a silica core. The synthesis begins with the production of organophilic poly(benzylglutamate)-coated silica particles.\footnote{Fong,B.; Russo, P.S. Organophilic Colloidal Particles with a Synthetic Polypeptide Coating. \textit{Langmuir} \textbf{1999}, $15$, 4421-4426.} The particles are rendered hydrophilic by cleaving the benzyl side group by treatment with hydrogen bromide in benzene. The resulting poly(glutamic acid)-coated silica spheres exhibit a change in hydrodynamic radius in response to pH stimulus. The size transition is due to a change in the polypeptide conformation, as deduced from circular dichroism measurements.   

Polymer-Graphite Nanocomposites: Comparison to Clay- and Carbon Nanotube-Based Hybrids

Although polymer-layered silicate and polymer-carbon nanotube nanocomposites have been widely studied in the last decade, hybrids containing nanoscale entities of graphite have been studied far less. Its structural analogy to layered silicates and chemical analogy to carbon nanotubes make graphite an attractive nanofiller in both scientific study and technological application. A common challenge of efficient dispersion of the nanofiller in the polymer matrix associated with conventional fabrication methods is overcome by processing using the solid-state shear pulverization technique. The level of dispersion and presence of graphite nanosheets are confirmed by X-ray diffraction and electron microscopy, while enhanced mechanical, thermal, and electrical properties of the resulting materials are characterized using tensile testing, dynamic mechanical testing, differential scanning calorimetry, thermogravimetric analysis and impedance spectroscopy.   

Organic-Inorganic Photovoltaic Composite Materials Based on Polymer-Functionalized Semiconductor Nanorods

Polymer-based photovoltaics, using composite materials consisting of inorganic particles embedded within the polymer, are considered to be promising candidates for the enhancement of power conversion efficiency, due to the directionality of electron transport enabled by the nanorods. We have demonstrated that oriented nanorods phase separate into close-packed micron-scale arrays within a matrix of photoactive polymer matrix with the assistance of electric field. However, for the material to be useful in photovoltaic applications, the separation distance between elements must be about 10 nm or less due to the exciton diffusion length. Here, we describe the attachment of photoactive polymers to CdSe nanorods. The photoluminescence spectra of these photoactive polymer-functionalized CdSe nanorods exhibited photoluminescence quenching of both nanorods and polymer, indicating charge transfer between donor and acceptor, a prerequisite for a successful photovoltaic material.   

Controlled Clustering of Oxide Nanoparticles using Block Copolymers for Coating and Biomedical Applications

During the past years, we have investigated the complexation between nanocolloids and oppositely charged polymers. The nanocolloids examined were ionic surfactant micelles and inorganic oxide nanoparticles. For the polymers, we used homopolyelectrolytes or block copolymers with linear and comb architectures. The attractive interactions between oppositely charged species are strong and in general the simple mixing of disperse solutions yield to a precipitation, or to a phase separation. We have developed means to control the electrostatically-driven attractions and to preserve the stability of the mixed solutions. With these approaches, we designed novel core-shell nanostructures, as those obtained recently with polymers and cerium and iron oxide nanoparticles (Berret et al., J. Am. Chem. Soc. 2006, 128, 1755). In this presentation, we show that electrostatic complexation can be used to tailor new functionalized nanoparticles and provide examples related to surface coating and biomedical applications.   

Novel Nanocomposite Materials Synthesis and Their Application

This paper focuses on our effort to develop novel nanocomposite materials. A new method, in situ nanoparticle synthesis, has been used to make a variety of polymer-metal oxide nanocomposites. This method provides a simple and versatile tool to overcome the difficulty of controlling particle dispersion during nanocomposite synthesis. The resulting synthesized nanocomposites have well-controlled morphology and enhanced optical properties. The structure-property relation of these nanocomposites will also be discussed.