Solution processing: fabrication and characterization of polymeric nanocomposite films and polystyrene nanoparticles

An approach to bottom-up design of new materials was developed starting from homogenous solutions. Solution processing techniques were used to fabricate advanced solid state materials, and processing parameters were identified and characterized. Three studies related to this work are reported herein: i) Polymeric electrospun nanofibers were metallized with transition metals for potential use as catalysts in organic reactions or sensing elements. Two different polymer-metal systems, which were palladium/poly(acrylonitrile-co-acrylicacid) and silver/poly(acrylonitrile-coglycidylmethacrylate), were employed. Polyacrylonitrile based copolymers were chosen as carrier material in view of their facile spinnability and established utility as precursor materials of carbon fibers. Nanofibers, in both cases, were obtained by electrospinning of homogeneous solutions in dimethylformamide. Metals were deposited on electrospun films starting from the metal salts by following two different procedures. In one route, palladium-II-chloride and the polymer were dissolved in dimethylformamide and subjected to electrospinning. Salt molecules were homogeneously distributed into nanofibers. Palladium cations were reduced after the electrospun film was immersed into an aqueous solution of hydrazine. The parameters affecting and tuning the particle size were determined. In particular, the amount of acrylic acid on the polymer backbone and palladium salt concentration in solution described two key factors. Palladium particles, called clusters, were afforded as polycrystalline structure consisting of smaller crystal units. Catalytic activity of palladium produced on electrospun film was investigated in a hydrogenation reaction of unsaturated alcohols. It was found that electrospun-supported palladium particles displayed 4.5 times higher catalytic activity than alumina-supported palladium. In the second route, silver was coated on poly(acrylonitrile-co-glycidylmethacrylate) nanofibers by use of electroless plating techniques. Reagent-accessible oxirane groups supported on the nanofibers were modified with a reducing agent, hydrazine. Surface-modified electrospun nanofibers were allowed to react with an ammonic solution of silver nitrate. A redox reaction took place during which time metallic silver was nucleated along the fiber surface, affording silver nanoparticles of 40 nm diameter. These particles featured typical separation distances of 5-50 nm. ii) Thermoelasticity of silica reinforced poly(dimethylsiloxane) networks was examined. Poly(dimethylsiloxane) networks exhibit rubber-like elasticity; that is, they recover their original state following deformation. Elasticity is an entropy driven phenomena for polymers. Uniaxial stretching of a network elongates the chains, resulting in a decreased conformational entropy due to the restricted number of low energy conformations that the extended chains can adopt. When the stress is removed, the chains recoil into the relaxed state with higher entropy. Elastomeric force, f, applied in uniaxial extension has an entropic and an energetic component affecting the network chains at the molecular level. The entropic component, fs, is used in changing the configurations of the chains into less disordered state. The rest of the force, fe, is used in changing the conformations of the chains. The ratio of fe/f can be determined by thermoelasticity experiments which are based on stress-strain measurements at constant volume. An ideal network was prepared from hydroxylended poly(dimethysiloxane) chains. They were dissolved in toluene. Fumed silica was introduced into the polymer solution prior to end-linking. A tetrafunctional crosslinker, tetraethoxysilane, was added into the homogenous solution and end-linked in the presence of Tin(II) 2-ethylhexanoate as a catalyst. The thickness of the nanocomposite film is on the order of 2 mm. The filler content was varied in the range 0-5 wt%. Tapping mode Atomic Force Microscopy was performed to characterize the silica particles, which become larger as the silica concentration increases. The temperature coefficient and the energetic part of the force in uniaxial extension are found to increase with increasing silica content. The elastic modulus of the reinforced networks was determined by mechanical experiments and swelling measurements. The modulus increases linearly with increasing silica concentration. iii) Amorphous polystyrene molecules/clusters were isolated and investigated. Erman and Flory showed that long polystyrene molecules undergo large dimensional changes in cyclohexane at 35°C. This event is known as coil-globule transition. Here the dimensional changes at dry state after the transition takes place were imaged and measured. Dilute solution of cyclohexane was cast on mica by the drop deposition technique. Solvent evaporation left behind a discontinuous film consisting of separated polystyrene islands. Atomic Force Microscopy was employed to determine the morphology and dimensions (volume, height and diameter) of the polystyrene particles. The experiment was performed at four different temperatures. It is found that the dimensions are strongly temperature dependent and exhibit a Gaussian-like distribution. Polystyrene chains tend to form clusters as the temperature increases. Two scenarios were discussed for whether the particles contain single or several chains

Electrospinning and wet-spinning of elastic fibers

In this thesis, two fiber spinning processes were designed and studied; conventional wet spinning and electrospinning which is relatively novel technique. Several process parameters were identified and characterized for both techniques. Diameter, surface roughness, surface elasticity and surface morphology of fibers were characterized using optical, atomic force, and scanning electron microscopes. The electrospinning process produces nanoscale fibers by applying electrical force to a fiber forming polymer solution. A charged liquid jet was ejected from polymer solution to the grounded conductive sheet. After the solvent evaporation, a nonwoven mat with a porous structure composed of unusually thin fibers was left on the sheet. The effect of the solution and instrumental characteristics on fiber morphology including viscosity, conductivity and applied electrical field strength were investigated. Polyurethane and polyacrylonitrile based polymers were electrospun succesfully within a viscosity controlled interval. The diameter of ultrathin fibers was found to depend mainly on viscosity with a power-law relationship. High viscous polyurethane based polymers exhibited curly, wavy and straight structures whereas fibers obtained from low viscous solutions demonstrated beads on strings morphology. Additionally, the nanofibers were not uniform in diameter. Macroscale fibers were produced by using the wet spinning technique. The polyurethane based polymer solution was extruded into a water coagulation bath through a nozzle. After coagulation, the single elastic filament was dried with air blow and wound up. The effect of two process variables, the rate of drawing and the rate of extrusion, on the fiber diameter were investigated. The rate of drawing was inversely proportional whereas the rate of extrusion was directly proportional to fiber diameter. AFM characterization has shown that the surface of fibers was heterogeneous in nature including disordered, fibrilliar, and flat structures. The morphology exhibited on fiber surface did not depend on the two process variables. Fiber diameters in the range of 7 nm to 150 um were succesfully spun from polyurethane based polymer with electrospinning and wet spinning, respectively. In the electrospinning process, nanoscale diameter fibers were obtained, and these fibers provided high surface area to volume ratios. Furthermore, it was found that nanofibers obtained from polyurethane solutions have rougher surface than the wet-spun fibers. In order to compare surface elasticity of two fibers, AFM sensitivity of reference materials (Glass slide, teflon film and parafilm) were examined. Polyurethane based polymer solution was processed with electrospinning, wet spinning and film casting. Parafilm was found to be the softest material and glass slide was the hardest. Elasticities of the materials processed, were found to be between that of glass and parafilm. Electrospun fibers were harder than the film of the same polymer. The stiffness difference between film and electrospun fibers can be explained by the orientation of electrospun fibers due to the electrical force

Ultrafine conducting fibers: metallization of poly(acrylonitrile-co-glycidyl methacrylate) nanofibers

Electrospun poly(glycidylmethacrylate) (PGMA) and poly(acrylonitrile-co-glycidyl methacrylate) (P(AN-GMA)) nanofibers were coated with monodisperse silver nanoparticles by using an electroless plating technique at ambient conditions. Oxirane groups on the surface of nanofibers were replaced with reducing agent, hydrazine. Surface modified nanofibers were allowed to react with ammonia solution of AgNO3. A redox reaction takes place and metallic silver nucleate on fibers surface. Parameters affecting the particle size were determined

A brief overview on geothermal scaling

Hot spring waters are rich in terms of minerals. Since there are dramatic changes in thermodynamic parameters in geothermal power plants, such as a decrease in temperature and pressure, severe precipitation occurs throughout the system components in an uncontrolled manner. There are three main chemistries in deposits: carbonates (mainly calcium carbonates), silicates (metal silicates), and sulphides (antimony sulphide-stibnite). Energy harvesting is remarkably reduced out of the insulating nature of the deposit. Various actions need to be taken to mitigate this undesirable issue of scaling in geothermal systems. Geothermal systems are in fact quite complex, and the composition of brine and, accordingly, the chemistry of the deposit are not identical. Therefore, each system should be studied individually, and a tailor-made remedy should be developed. In this overview, the types of deposits in terms of chemistry and the actions (pH modification or antiscalant dosing) that should be taken to reduce scaling are mentioned, and potential chemistries of antiscalants are given

Dimensions of polystyrene particles deposited on mica from dilute cyclohexane solution at different temperatures

Using atomic force microscopy, the height, diameter, and volume of polystyrene particles deposited on mica from dilute cyclohexane solution at different temperatures are determined. Dimensions exhibit a strong temperature dependence. The heights of the deposited particles are only a few atomic diameters, the major dimension being parallel to the mica surface. The number of single polystyrene molecules in a deposited particle cannot be determined directly by atomic force microscopy. However, the maximum number of molecules that may be present in a particle may be estimated, Below 35 degreesC, the particles possibly consist of single collapsed molecules. This number increases with temperature and becomes as large as about 30 molecules per particle at 80 degreesC. The volume occupied by a single chain in solvent is calculated as a function of temperature and compared with observed volumes of dry particles on mica. A linear relationship is observed between single chain volumes in solvent and corresponding particle volumes on mica. On the average, the volume of a single chain in solution is 10 times the volume of a dry particle at the corresponding temperature. Fluctuations of chain volumes that are dominant in solution are also present in the volumes of particles deposited and dried on mica

Effect of chain topology on plasmonic properties of pressure sensor films based on poly(acrylamide) and Au nanoparticles

Au nanoparticles have been recognized as a colorimetric sensing element in polymeric systems because clustering shifts the red color of individual particles into saturated blue due to distinct plasmonic variation. The mechanism of pressure sensing is based on the disintegration of the particle clusters into the individual particles in polymers upon application of pressure. Polymers are usually composed of linear chains that provide a viscoelastic medium for their diffusion. Changing topology of polymer chains from linear to crosslinked under fixed pressure makes a clear change in spectral features of the particles probably due to the hindrance of particle diffusion by the crosslinking points. Therefore, the working range of the sensor films can be increased to higher-pressure values. In this work, polyacrylamide/Au nanoparticle films were prepared by various concentrations of formaldehyde as a crosslinking agent from 0.5 to 5.0 wt %. The initial absorption signal gradually shifts from 690 to 545 nm for linear chains upon application of pressure while shifting goes down to 571 nm for crosslinked ones. The colorimetric change is also examined under humid environments. Contrary to the crosslinking process, humid environment facilitates the diffusion of particles since the chains swell with water molecules that provide a convenient medium for particle diffusion.TUBITAK (117Z331

Non-iridescent structural colors from uniform-sized SiO2 colloids

Structural colors have recently attracted interest from diverse fields of research due to their ease of fabrication and eco-friendliness. These types of colors are, in principle, achieved by periodically arranged submicron-diameter colloidal particles. The interaction of light with a structure containing long-range ordered colloidal particles leads to coloration; this usually varies depending on the angle of observation (iridescence). However, the majority of the applications demand constant color that is independent of the viewing angle (non-iridescence). In this work, silica colloids were obtained using the Stöber method at different sizes from 150 to 300 nm in an alcoholic dispersion. The casting of the dispersion on a substrate leaves behind a photonic crystal showing a colorful iridescent film. However, centrifugation and redispersion of the SiO2 particles into fresh solvent may cause the formation of small, aggregated silica domains in the new dispersion. The casting of this dispersion allows for the development of photonic glass, presumably due to the accumulation of aggregates showing stable colloidal film independent of viewing angle. Moreover, depending on the size of the silica colloids, non-iridescent photonic glasses with various colors (violet, blue, green, and orange) are obtained.Turkish Academy of Science

Monitoring excimer formation of perylene dye molecules within PMMA-based Nanofiber via FLIM Method

Conference on Nanophotonics VI -- APR 03-07, 2016 -- Brussels, BELGIUMWOS:000378220200013Confocal fluorescence lifetime imaging microscopy method is used to obtain individual fluorescence intensity and lifetime values of aromatic Perylene dye molecules encapsulated into PMMA based nanofibers. Fluorescence spectrum of aromatic hydrocarbon dye molecules, like perylene, depends on the concentration of dye molecules and these dye molecules display an excimeric emission band besides monomeric emission bands. Due to the dimension of a nanofiber is comparable to the monomer emission wavelength, the presence of nanofibers does not become effective on the decay rates of a single perylene molecule and its lifetime remains unchanged. When the concentration of perylene increases, molecular motion of the perylene molecule is restricted within nanofibers so that excimer emission arises from the partially overlapped conformation. As compared to free excimer emission of perylene, time-resolved experiments show that the fluorescence lifetime of excimer emission of perylene, which is encapsulated into NFs, gets shortened dramatically. Such a decrease in the lifetime is measured to be almost 50 percent, which indicates that the excimer emission of perylene molecules is more sensitive to change in the surrounding environment due to its longer wavelength. Fluorescence lifetime measurements are typically used to confirm the presence of excimers and to construct an excimer formation map of these dye molecules.SPIE, B PHOT Brussels Photon Team, Res Fdn Flanders, Visit Brussel