Polymer science

Stress Sensing in Polycaprolactone Films Via an Embedded Photochromic Compound

by Bryan Wong

ACS Applied Materials & Interfaces, 2, 1594 (2010)

Synthesis and characterization of neodymium doped ceria nanocrystalline ceramic structures

by Prof.Dr. İbrahim USLU

Ceramics International
First Online March 2012
İbrahim Uslu, Arda Aytimur, Mustafa Kemal Öztürk, Serhat Koçyiğit

In this study, a new method to synthesize neodymium doped ceria ceramic nanopowders by the electrospinning of the... more In this study, a new method to synthesize neodymium doped ceria ceramic nanopowders by the electrospinning of the hybrid polymers solution of their composite precursor was put forward. Calcined and sintered nanopowders were characterized by FT-IR, XRD, BET, SEM, and AFM techniques. According to the XRD analysis, the obtained powders are single phase and independent of the dopant concentration in the range investigated. The crystallite sizes were calculated using Scherrer equation. Moreover, lattice parameters, dislocation densities and microstrain values were calculated. BET results show that the increase of the neodymium doped content decrease the surface area of the composite powders, confirming the highly ordered micro and mesostructure. SEM and AFM results show that the samples have spherical grains. According to the surface roughness measurements, the increase in the amount of neodymium and the decrease in the amount of cerium decreased the surface roughness.

Dependence of surface free energy on molecular orientation in polymer films

by Urszula Stachewicz

Surface free energy of mechanically drawn polycarbonate films was determined using contact angle measurements and... more Surface free energy of mechanically drawn polycarbonate films was determined using contact angle measurements and shown to increase with orientation. The increase in polymer film surface free energy was attributed to increased polymer chain packing during orientation, supported by film density measurements, which provides enhanced intermolecular interactions. Surface free energy can therefore be increased by, or used to predict, polymer orientation.

Structural and electrical characterization of hybrid metal-polypyrrole nanowires

by Sorin Melinte

Frequency and Temperature Dependence of Dielectric Properties of Au/Polyvinyl Alcohol (Co, Ni-Doped)/n-Si Schottky Diodes

by Prof.Dr. İbrahim USLU

International Journal of Polymeric Materials
Volume 59, Issue 10, 739–756, 2010
T.Tunc, ibrahim Uslu, İ. dökme, Ş. Altundal

The dielectric properties and AC conductivity of Au/polyvinyl alcohol (Co, Ni-doped)/n-Si Schottky diodes (SDs) were... more The dielectric properties and AC conductivity of Au/polyvinyl alcohol (Co, Ni-doped)/n-Si Schottky diodes (SDs) were investigated in the frequency range 1 kHz–1 MHz and in the temperature range 80–400 K. The frequency and temperature dependence of dielectric constant (ϵ′), dielectric loss (ϵ″), loss tangent (tan δ), AC electrical conductivity (σ ac ) and the real and imaginary parts of the electric modulus (M′ and M″) were found to be a strong function of frequency and temperature. The values of ϵ′, ϵ″ and tan δ decrease with increasing frequency, while they increase with increasing temperature, especially above 275 K. The values of σ ac increase with both increasing frequency and temperature. Such temperature-related behavior of σ ac can be attributed to the high mobility of free charges at high temperature. Electric modulus formalism was also analyzed to obtain experimental dielectric data. The values of M′ and M″ increase with increasing frequency, while they decrease with increasing temperature. The interfacial polarization, which more easily occurs at low frequencies and high temperatures, consequently contributes to the improvement of the dielectric properties of SDs.

FABRICATION AND CHARACTERIZATION OF BORON DOPED ZIRCONIUM OXIDE NANOFIBERS

by Prof.Dr. İbrahim USLU

BALKAN PHYSICS LETTERS
BPL, 19, 191026, pp. 232 – 239, (2011)
23 February 2011

ibrahim Uslu, M.K. Öztürk, T. Tunc, T.C. Karabulut, S. Keskin

Recently, considerable attention has been focused on zirconium oxide (ZrO2) which exhibits many desirable structural... more Recently, considerable attention has been focused on zirconium oxide (ZrO2) which exhibits many desirable structural and electronic properties and has been used in many applications. For example it is a very attractive and promising material for low-voltage and short-wavelength electro-optical devices such as light emitting diodes and laser diodes; it also has other applications such as gas sensors and varistors [1-4]. This study is related to the preparation of boron doped ZrO2 material. The first step was the preparation of composite by mixing zirconium acetate with polyvinyl alcohol (PVA) at a suitable temperature [5]. The resulting composite polymer was doped with boron using boric acid as boron source and then electrospun giving nano fibers. Finally, the resulting nano fibers were subjected to a thermal treatment at 250 oC, 500 oC, 800 oC to elucidate their morphological properties. The conductivity of the nano fibers obtained was measured with four probe technique and the boron supported polymer was observed to have a higher conductivity. The Differential Scanning Calorimetry (DSC) results indicate that the glass transition (Tg) and melting temperatures (Tm) showed great change with the addition of boron and boron doped fibers were observed to degrade at higher temperatures. The SEM appearance of the fibers showed that the addition of boron resulted in the formation of cross linked bright surfaced fibers.

The illumination intensity and applied bias voltage on dielectric properties of au/polyvinyl alcohol (Co, Zn‐doped)/n‐Si Schottky barrier diodes

by Prof.Dr. İbrahim USLU

Journal of Applied Polymer Science
Volume 120, Issue 1, pages 322–328, 5 April 2011
H. Uslu, Ş. Altındal, T. Tunç, ibrahim Uslu, T.S. Mehmedov

The Au/polyvinyl alcohol (PVA) (Co, Zn‐doped)/n‐Si Schottky barrier diodes (SBDs) were exposed to various illumination... more The Au/polyvinyl alcohol (PVA) (Co, Zn‐doped)/n‐Si Schottky barrier diodes (SBDs) were exposed to various illumination intensities. Illumination effect on the dielectric properties has been investigated by using capacitance–voltage (C–V) and conductance–voltage (G/ω–V) characteristics at 1 MHz and room temperature. The values of dielectric constant (ε′), dielectric loss (ε″), loss tangent (tanδ), electric modulus (M′ and M″), and AC electrical conductivity (σAC) were found strongly intensity dependent on both the illumination levels and applied bias voltage especially in depletion and accumulation regions. Such bias and illumination dependency of these parameters can be explained on the basis of Maxwell–Wagner interfacial polarization and restructuring and reordering of charges at interface states. In addition, the ε′–V plots also show an intersection feature at ∼ 2.8 V and such behavior of the ε′–V plots appears as an abnormality compared with the conventional behavior of an ideal SBD. The obtained results revealed that illumination intensity enhances the conductivity of Au/PVA(Co, Zn‐doped)/n‐Si SBD.

The Au/polyvinyl alcohol (Co, Zn-doped)/n-type silicon Schottky barrier devices

by Prof.Dr. İbrahim USLU

Synthetic Metals 161 (2011) 474–480

ibilge Dökme, T. Tunc¸ ibrahim Uslu, S. Altındal

Metal/polyvinyl alcohol/n-type silicon Schottky barrier (SB) devices have been fabricated in this study. The... more Metal/polyvinyl alcohol/n-type silicon Schottky barrier (SB) devices have been fabricated in this study. The importance of this study is that PVA (Co, Zn doped) nanofiber film as an interfacial layer was formed by the electrospinning technique on n-type silicon substrate. The forward and reverse bias current–voltage (I–V) characteristics of this device were measured at room temperature. The ΦBo value of about 0.749 eV obtained from I–V characteristics indicates that the contact potential barrier exists at the interface between organic and inorganic semiconductor layer, that is, PVA/n-Si interface. The variation in the capacitance–voltage (C–V) and conductance–voltage (G/ω–V) characteristics of the Au/PVA (Co, Zn doped)/n-Si SB devices have been systematically investigated as a function of frequencies in the frequency range of 2 kHz–2 MHz at room temperature. The effects of density of interface states (Nss) and series resistance (Rs) on I–V, C–V and G/ω–V characteristics were investigated. The high-frequency capacitance (Cm) and conductance (Gm/ω) values measured under reverse bias were corrected to decrease the effects of series resistance. These results show that the locations of interface states between Si/PVA and series resistance have a significant effect on electrical characteristics of the Au/PVA (Co, Zn doped)/n-Si SB devices.

The effects of temperature, radiation, and illumination on current–voltage characteristics of Au/PVA(Co, Zn-doped)/n-Si Schottky diodes

by Prof.Dr. İbrahim USLU

Journal of Applied Polymer Science
Article first published online: 6 January 2012
DOI: 10.1002/app.36327
İ. Dökme, Ş. Altındal, İbrahim Uslu

Polyvinyl alcohol (PVA)/(Co-Ni) nanofiber film was fabricated on silicon wafer using electrospinning technique. The... more Polyvinyl alcohol (PVA)/(Co-Ni) nanofiber film was fabricated on silicon wafer using electrospinning technique. The topography of the produced PVA/(Co-Ni) nanofiber film was examined by scanning electron microscopy (SEM). The Au/Poly (vinyl alcohol) (Co, Ni-doped)/n-Si Schottky diode (SD) was thermally formed in evaporating system after the spinning process. At first, the current–voltage (I–V) characteristics of Au/PVA (Co, Zn-doped)/n-Si SD was measured at the room temperature (300 K). For the investigating the effect of temperature, illumination and radiation on Au/PVA (Co, Zn-doped)/n-Si SD comparatively, the measurement was performed under the illumination intensity of 200 W, at 380K, and finally the radiation dose of 22 kGy respectively. The diode characteristics such as the zero-bias barrier height (ϕBo), ideality factor (n) and series resistance (Rs) were calculated at room temperature and under the condition of high temperature, illumination, and radiation. It was found that these characteristics were affected by the illumination and radiation as well as the temperature. The density of interface states (Nss) distribution profiles as a function of (Ec - Ess) extracted from the forward I–V measurements were also affected by illumination and radiation even if just a bit.

The effect of PVA (Bi2O3-doped) interfacial layer and series resistance on electrical characteristics of Au/n-Si (110) Schottky barrier diodes (SBDs)

by Prof.Dr. İbrahim USLU

Current Applied Physics
Volume 12, Issue 2, March 2012, Pages 525-530

M. Gökçen, T. Tunç, S. Altındal, ibrahim Uslu

Two types of Schottky Barrier Diodes (SBDs) with and without PVA (Bi2O3-doped) polymeric interfacial layer, were... more Two types of Schottky Barrier Diodes (SBDs) with and without PVA (Bi2O3-doped) polymeric interfacial layer, were fabricated and measured at room temperature in order to investigate the effects of the PVA (Bi2O3-doped) interfacial layer on the main electrical parameters such as the ideality factor (n), zero-bias barrier height (ΦB0), series resistance (Rs) and interface-state density (Nss). Electrical parameters of these two diodes were calculated from the current–voltage (I–V) characteristics and compared with each other. The values of ΦB0, n and Rs for SBDs without polymeric interfacial layer are 0.71 eV, 1.44 and 4775 Ω, respectively. The values of ΦB0, n and Rs for SBDs with PVA (Bi2O3-doped) polymeric interfacial layer are 0.74 eV, 3.49 and 10,030 Ω, respectively. For two SBDs, the energy density distribution profiles of interface states (Nss) were obtained from forward-bias I–V measurements by taking the bias dependence of Rs of these devices into account. The values of Nss obtained for the SBD with PVA (Bi2O3-doped) polymeric interfacial layer are smaller than those of the SBD without polymeric interfacial layer.

Production and characterization of poly(vinyl alcohol)/poly(vinylpyrrolidone) iodine/poly(ethylene glycol) electrospun fibers with (hydroxypropyl)methyl cellulose and aloe vera as promising material for wound dressing

by Prof.Dr. İbrahim USLU

Journal of Applied Polymer Science
Volume 124, Issue:4 pp: 3520-3524, 2012

DOI: 10.1002/app.35525

ibrahim Uslu, A. Aytimur

Biocompatible poly(vinyl alcohol)/poly (vinylpyrrolidone) iodine/poly(ethylene glycol) fibers containing... more Biocompatible poly(vinyl alcohol)/poly (vinylpyrrolidone) iodine/poly(ethylene glycol) fibers containing (hydroxypropyl)methyl cellulose (HPMC) and aloe vera were successfully prepared by electrospinning their aqueous solution. Aloe vera which is known to be effective in the treatment of various wounds was added to the polymer solution. HPMC was added to the system as the water retention agent. The hybrid fiber mats were subjected to detailed analysis using a differential scanning calorimeter, a scanning electron microscope (SEM), and a Fourier transform infrared spectrometer. Images obtained from the SEM showed that the polymer fibers were linear, homogenous, and contained no beading. The fiber diameters ranged between 100 and 900 nm. It was seen that the electrospun mats obtained could potentially be used as a material for dressing wounds. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Nanofibers by Electrospinning of Nylon 6 Doped with Boron

by Prof.Dr. İbrahim USLU

Hacettepe Journal of Biology and Chemistry volume 37 Issue 1 pp 55-60 2009
ibrahim Uslu, A. Altaş, M.L. Aksu, F. Gökmeşe

In this study, Nylon 6 was successfully cross-linked with boron using formic acid as solution. Electrospinning process... more In this study, Nylon 6 was successfully cross-linked with boron using formic acid as solution. Electrospinning process was performed by putting hybrid nylon 6/boric acid solutions at different applied voltage in a range from 20 to 25 kV using a high voltage power supply. Nylon has been widely used in industry because of its good mechanical properties. The use of boron improves the strength, flame retardant characteristics and flexibility of the nylon 6 fibers.
The structures of the nanofibers synthesized are characterized by the Fourier transformed infrared spectroscopy (FTIR), thermo gravimetric and differential thermal analysis (TG/DTA), differential scanning calorimetry (DSC), atomic force microscopy (AFM), and scanning electron microscopy (SEM).
DSC and TGA analysis revealed that stability of electrospun nylon 6 fiber is higher than non spun sample and increase of boric acid content increases the stability of the polymer. It is observed from the SEM images that the diameters of the fibers decreased with boron content increased. FTIR analysis revealed that all characteristic N-H structures are observed and consistent with literature.

Layer-by-Layer Assembly of Polyelectrolyte Chains and Nanoparticles on Nanoporous Substrates: Molecular Dynamics Simulations

by Jan-Michael Carrillo

Adsorption kinetics of ultrathin polymer films in the melt probed by dielectric spectroscopy and second harmonic generation

by Simone Napolitano

Langmuir 2011, 27 13533-13538

Effects of Nanoclay and Polyurethanes on Inhibition of Mild Steel Corrosion

by Abhinay Mishra

published in Journal of Nanoscience and Nanotechnology, 2011

Dielectric relaxation in polyimide nanofoamed films with low dielectric constant

by Shanming Ke

Published in "Appl. Phys. Lett.", 2008

Friction between Brush Layers of Charged and Neutral Bottle-Brush Macromolecules. Molecular Dynamics Simulations

by Jan-Michael Carrillo

Co-authored with Daniel Russano and Andrey Dobrynin

Measuring the micro-polarity and hydrogen-bond donor/acceptor ability of thermoresponsive N-isopropylacrylamide/N-tert-butylacrylamide copolymer films using solvatochromic indicators.

by Alan Ryder

B. Szczupak, A.G. Ryder, D.M. Togashi, Y.A. Rotchev, A. Gorelov, and T.J. Glynn, Applied Spectroscopy, 63(4), 442-449, (2009).

Thin polymer films are important in many areas of biomaterials research, biomedical devices, and biological... more Thin polymer films are important in many areas of biomaterials research, biomedical devices, and biological sensors.  The accurate, in-situ, measurement of multiple physicochemical properties of thin polymer films is critical in understanding biocompatibility, polymer function, and performance.  In this work we demonstrate a facile, spectroscopic methodology for accurately measuring the micro-polarity and hydrogen-bond donor/acceptor ability for a series of relatively hydrophilic thermoresponsive copolymers.  The micro-polarity of the N-isopropylacrylamide (NIPAM) and N-tert-butylacrylamide (NtBA) co-polymers was evaluated by means of the ET(30), alpha, beta, and pi* empirical solvatochromic polarity parameters.  The data shows that increasing the NtBA fraction in the dry copolymer film reduces polarity and hydrogen-bonding ability.  Within the Kamlet-Taft polarity framework, the NIPAM/NtBA copolymer films are strong hydrogen-bond acceptors, strongly dipolar/polarisable, and rather moderate hydrogen-bond donors.  This characterization provides a more comprehensive physicochemical description of polymers, which aids the interpretation of film performance.  Comparison of the measured ET(30) values with literature data for other water-soluble polymers show that dry NIPAM/NtBA copolymers are slightly more polar than poly(ethylene oxide), less polar than polyvinylalcohol, and approximately the same polarity as poly(N-vinyl-2-pyrrolidone).  These findings indicate that this spectroscopic method is a facile, rapid, and non-destructive methodology for measuring polymer properties in-situ, suitable for most biomaterials research laboratories.

Hybrid Titanium/Biodegradable Polymer Implants with an Hierarchical Pore Structure as a Means to Control Selective Cell Movement

by Nihal Engin Vrana

Published in Plos One

In order to improve implant success rate, it is important to enhance their responsiveness to the prevailing conditions... more In order to improve implant success rate, it is important to enhance their responsiveness to the prevailing conditions following implantation. Uncontrolled movement of inflammatory cells and fibroblasts is one of these in vivo problems and the porosity properties of the implant have a strong effect on these. Here, we describe a hybrid system composed of a macroporous titanium structure filled with a microporous biodegradable polymer. This polymer matrix has a distinct porosity gradient to accommodate different cell types (fibroblasts and epithelial cells). The main clinical application of this system will be the prevention of restenosis due to excessive fibroblast migration and proliferation in the case of tracheal implants.
Methodology/Principal Findings

A microbead-based titanium template was filled with a porous Poly (L-lactic acid) (PLLA) body by freeze-extraction method. A distinct porosity difference was obtained between the inner and outer surfaces of the implant as characterized by image analysis and Mercury porosimetry (9.8±2.2 µm vs. 36.7±11.4 µm, p≤0.05). On top, a thin PLLA film was added to optimize the growth of epithelial cells, which was confirmed by using human respiratory epithelial cells. To check the control of fibroblast movement, PKH26 labeled fibroblasts were seeded onto Titanium and Titanium/PLLA implants. The cell movement was quantified by confocal microscopy: in one week cells moved deeper in Ti samples compared to Ti/PLLA.
Conclusions

In vitro experiments showed that this new implant is effective for guiding different kind of cells it will contact upon implantation. Overall, this system would enable spatial and temporal control over cell migration by a gradient ranging from macroporosity to nanoporosity within a tracheal implant. Moreover, mechanical properties will be dependent mainly on the titanium frame. This will make it possible to create a polymeric environment which is suitable for cells without the need to meet mechanical requirements with the polymeric structure.

Molecular Dynamics Simulations of Nanomolding Process

by Jan-Michael Carrillo