Nursel Dilsiz

Surface analysis of unsized and sized carbon fibers

Abstract Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and contact angle analyses were performed on unsized and sized carbon fibers to better understand the mechanism of adhesion in carbon fiber/polymer matrix composites. AFM images and surface roughness analyses showed that the sizing changes the surface topography on a microscopic scale. The total surface energy decreased from 70 mJ/m2 for unsized fiber to 54 mJ/m2 for Ultem® sized and to 36 mJ/m2 for PTPO sized fibers. The percentage of functional groups on the sized fibers decreased slightly compared to the unsized fibers. The surface functional groups and surface energies of fibers are critical properties in predicting fiber/matrix adhesion. Angle dependent XPS, voltage contrast XPS, and perimeter measurements revealed that the thickness of the poly(thioarylene phosphine oxide) (PTPO) sizing on the carbon fiber surface was greater than for the poly(etherimide) (Ultem®) sizing.

Effect of acid–base properties of unsized and sized carbon fibers on fiber/epoxy matrix adhesion ☆

Abstract The surface energies and acid–base character of unsized and sized Zoltek fiber were investigated using dynamic contact angle analysis. The surface’ acid–base property were characterized by calculating the work of acid–base interaction according to Fowkes’ and Good’s theory. To evaluate the effect of sizing on carbon fiber surface properties, Ultem® polyimide and polyurethane (PU) sized fibers were studied in comparison with unsized fibers. It was found that the sizing on the carbon fiber tended to reduce the surface energy and cover acid–base sites. The surface of unsized and sized carbon fibers were analyzed by XPS to investigate changes in surface chemistry. XPS analysis indicated that hydroxyl groups on the fiber surface decreased with sizing (Ultem® and polyurethane). These changes in chemistry are reflected in a decrease in the polar γsp and basic γs− contribution of the surface energy of the fibers. Single fiber fragmentation testing of unsized and Ultem® and polyurethane sized fibers in epoxy matrix showed that there is a direct effect of the surface chemical changes on the fiber/matrix adhesion.

Surface energy and mechanical properties of plasma-modified carbon fibers

The surfaces of carbon fibers were treated by allylcyanide and xylene/air/argon plasma polymers. The surfaces of untreated and treated fibers were examined by scanning electron microscopy. Electron micrographs of the surface of carbon fibers showed that the surface striations and surface roughness were changed increasingly on the fiber surface after plasma treatment. Surface energy determinations of the fibers were carried out from advancing contact angle measurements, together with tensile testing of the single fibers. A qualitative agreement between an increase in the strength of the plasma-coated fibers and an increase in the surface energy of the fibers was observed.

Plasma surface modification of carbon fibers: a review

The properties of the fiber/matrix interface in carbon fiber-reinforced composites play a dominant role in governing the overall performance of the composite materials. Understanding the surface characteristics of carbon fibers is a requirement for optimizing the fiber-matrix interfacial bond and for modifying fiber surfaces properly. Therefore, a variety of techniques for the surface treatment of carbon fibers have been developed to improve fiber-matrix adhesion as well as to enhance the processability and handling of these fibers. Many research groups have studied the effects of plasma treatments, correlating changes in surface chemistry with the interfacial shear strength. This article reviews the recent developments relative to the plasma surface modification of carbon fibers.

Pressure-dependent conduction behavior of various particles for conductive adhesive applications

Abstraet-The efficiency of electrical conduction in particle-filled conductive adhesives largely depends on the interparticle conduction. In order to gain insight into the pressure-dependent conduction behavior with particles of different sizes, shapes, and types, the effects of external pressure on the filler resistance were measured by the four-point probe method using different conductive fillers. The following types of particles were used: Ni powder, Ni flake, Ag powder, Ni filament, magnetite spindles, and Cu particles. Non-filament particle size was in the range 0.7-44 μm. The filaments were 20 μm in diameter and 160 or 260 μm in length. The particle treatment procedures investigated included silver coating using different methods, and the use of acid solutions including H3PO4, HF, and HCl to etch and remove the surface oxide layer. Resistivity levels were measured using a nonconductive hollow cylinder plunger device developed in our laboratories. The results of our work showed that when the pressur...

Plasma polymerization of selected organic compounds

Plasma polymerization reactions and plasma polymers of xylene/air, xylene/argon, xylene/air/argon, toluene/air/argon, dioxane and allyl cyanide were compared with respect to discharge power (between 50 and 500 W), where all other plasma operational parameters were kept constant. In plasma polymerization, the glow characteristics are known to play an important role depending on the factor WF, where W is the discharge power and F is the monomer flow rate. In plasma polymerization reactions, it is known that decomposition of the monomer and/or polymer produced competes with polymer formation. Above a certain power level, polymer decomposition may become the predominant reaction, and hence polymer deposition rates decrease with increasing discharge power. The plasma polymer films were characterized by Fourier transform infra-red spectroscopy, elemental analysis and thermogravimetric analysis. Infra-red spectra of the plasma polymers obtained indicated that when the discharge power was increased, the absorption intensities of all the functional groups decreased. The deposition rates and chemical structures of the plasma polymers prepared appeared to depend strongly on the structure and characteristics of the monomer used and on the conditions of discharge and other operational parameters.

Study of sol–gel processing for fabrication of low density alumina microspheres

Abstract A sol–gel process for producing microspheres of low density alumina has been developed. A Dispal alumina sol and aluminum monohydrate were used as starting materials. Microspheres with spherical shape and controllable size have been produced by using a drop generation technique. The technique consists of producing a continuous stream of uniform droplets of alumina sol and then converting the drops into a rigid form by exposing them to an ammonium hydroxide gelation medium. By using this technique, alumina spheres with surface area 351 m2 g−1 and pore size 10.7 A° have been prepared.

Molecular design of photoswitchable surfaces with controllable wettability

In this work, we developed a photocontrollable substrate which was prepared using an azobenzene-containing self-assembled monolayer (SAM) on the silicon surface via the chemisorption of 3-glycidoxypropyltrimethoxysilane (GPTS) and 4-(4′-aminophenylazo) benzoic acid (APABA). The prepared surfaces were chemically characterized by X-ray photoelectron spectroscopy (XPS). The reversible photoswitching performance of APABA molecules were investigated by UV spectroscopy in dimethylsulfoxide (DMSO) solution. To understand and control this reversible photoswitchable mechanism and wettability properties, contact angle measurements were performed by using a variety of liquids after UV and visible light irradiation. These contact angle results are used to approximate the components of the APABA-modified surface energy under UV and visible light using the Lifshitz–van der Waals/acid–base approach. The total surface energy (γs) after visible light irradiation (for trans formation) was calculated to be 37.28 mJ m−2, whereas the value after UV light exposure (for cis formation) was also calculated to be 36.95 mJ m−2. All the results demonstrate the great potential to control molecular events within and on the surfaces of molecular constructs using light.

Macroporous Poly(Acrylamide) Hydrogels: Swelling and Shrinking Behaviors

Macroporous poly(acrylamide) hydrogels have been synthesized by using poly(ethylene glycol) (PEG) with three different molecular weights as the pore‐forming agent. Scanning electron microscope graphs reveal that the macroporous network structure of the hydrogels can be adjusted by applying different molecular weights of PEG during the polymerization reaction. The swelling ratios of the PEG‐modified hydrogels were much higher than those for the same type of hydrogel prepared via conventional method. However, the swelling/deswelling ratios of the PEG‐modified hydrogels were affected slightly by the change in the amount of the PEG. Scanning electron microscopy experiments, together with swelling ratio studies, reveal that the PEG‐modified hydrogels are characterized by an open structure with more pores and higher swelling ratio, but lower mechanical strength, compared the conventional hydrogel. PAAm has potential applications in controlled release of macromolecular active agents.

Photocontrollable DNA hybridization on reversibly photoresponsive surfaces

Herein we have developed a new DNA sensor which displays the possibility of photocontrollable DNA hybridization by changing the orientation of azobenzene layers on the silicon wafer surface. Basically, the trans-form layers present a coordinating surface, the “on” state that can be switched “off” in the cis-form. Water contact angles of the prepared substrates have been measured after UV and visible light irradiation to understand the switchable properties of each form of the surface. The photocontrollable DNA chip was prepared using Cy3 labeled amino terminated single stranded probe-DNA (Cy3–ssDNA–NH2) molecules which were immobilized onto a COOH-terminated photoswitchable surface. The optimum hybridization conditions were performed with Cy5 labeled complementary-DNA (Cy5–ssDNA) using fluorescence microscopy. Furthermore, the photocontrolling of DNA hybridization onto prepared surfaces was verified by confocal microscopy before and after light irradiation. The percentage hybridization ratios from confocal microscopy for on and off positions of the DNA sensor were calculated to be 61% and 5%, respectively. These results show that the prepared surfaces can be reversibly photoswitched between two states efficiently. As a result we believe that the results demonstrate the great potential to control DNA hybridization within and on the surfaces of molecular constructs using light.