Polymer Chemistry

Stress Sensing in Polycaprolactone Films Via an Embedded Photochromic Compound

by Bryan Wong

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

Electronic Properties of Vinylene-Linked Heterocyclic Conducting Polymers: Predictive Design and Rational Guidance from DFT Calculations

by Bryan Wong

Journal of Physical Chemistry C, 115, 18333 (2011)

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.

Janus Hybramers: Self-Adapting Amphiphilic Hyperbranched Polymers

by ashok samuel

Ashok Zachariah Samuel and S. Ramakrishnan

Janus structures have attracted a great deal of interest because of their fascinating properties and potential for... more Janus structures have attracted a great deal of interest because of their fascinating properties and potential for applications. In this study, we demonstrate that hyperbranched polymers, bearing randomly placed docosyl (C22 alkyl segment) and PEG segments on their periphery, can readily reconfigure so as to segregate the alkyl and PEG segments, thereby generating Janus-type structures that we have termed Janus hybramers. DSC studies clearly reveal an endothermic transition that corresponds to the melting of the docosyl domains, while Langmuir isotherms demonstrate that these polymers form stable monolayers that appear to undergo a slight densification beyond a critical surface pressure; this suggested possible crystallization of the docosyl segments at the air–water interface. AFM studies of the transferred monolayers reveal various interesting aggregate morphologies at different surface pressures suggestive of island formation at the air–water interface; at the same time they also provided an estimate of the monolayer thickness. These Janus HBPs also form vesicles as evident from TEM and AFM studies; the AFM height of the deposited vesicles, as expected, was roughly 4 times that of the monolayer. SAXS studies revealed the formation of lamellar structures; the interlamellar spacing was largest when the relative mole fractions of docosyl and PEG segments were similar, but the spacing decreased when the mole fraction of either of these peripheral segments is substantially smaller; this suggested the possible presence of interdigitation within the domains of the minor component.

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

Preparation and Characterization of PVA/Zinc Acetate/Boron Composite Fibers

by Prof.Dr. İbrahim USLU

e-Polymers 2007, no. 145

Ibrahim Uslu, Burcu Başer, Ahmet Yaylı, Mehmet Levent Aksu

The objective of this work was to synthesize boron incorporated PVA/Zinc acetate nano precursor composite polymer... more The objective of this work was to synthesize boron incorporated PVA/Zinc acetate nano precursor composite polymer fibers using electrospinning technique. Produced fibers, are useful reagents in organic synthesis of zinc oxide nano structures which have gained wide interest because of their potential applications in the fabrication of microelectronic devices. Boron incorporated PVA/Zinc acetate not only improve the electrical properties of ZnO films but also has an influence on the crystal structure of ZnO films. The fibers were measured and characterized by conductiometer (with four-point probe), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The formation of boron incorporated PVA/Zinc acetate hybrid polymer was confirmed by FT-IR spectra. SEM micrographs clearly reveal that there are random-growth oriented fibers on aluminum foil with diameters ranging from 0.5 to 1 μm. According to the TGA analysis decomposition of zinc acetate from the PVA chain was observed around 273 oC, 368 oC and 447 oC. Boron incorporated PVA/Zinc acetate nano composite polymer fibers have been successfully synthesized for the first time using electrospinning technique and characterized in detail as precursor material for ZnO. Electrospinning process has been a widely used, simple and versatile method and in future may be used for large-scale preparation of nanofibers of ZnO.

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.

Preparation of PVA/Chitosan Doped with Boron Composite Fibers and Their Characterization

by Prof.Dr. İbrahim USLU

Hacettepe Journal of Biology and Chemistry volume 36 Issue 2 pp: 117-122, 2008.

ibrahim Uslu, H. Celikkan, O. Atakol, M.L. Aksu

In this study, chitosan and poly(vinyl alcohol) were cross-linked with boron in varying concentrations and chitosan... more In this study, chitosan and poly(vinyl alcohol) were cross-linked with boron in varying concentrations and chitosan and poly(vinyl alcohol) blend fibers were obtained with electrospinning process.
Viscosity and electrical conductivity results revealed that the increase in chitosan and boron content increased the viscosity of the solution and but decreased the electrical conductivity of the fibers. The peaks obtained with FT-IR analyses were in good agreement with literature. SEM micrographs also showed that wavy and curly fibers with relatively small diameters were obtained as the chitosan and boron concentrations were increased. Thermogravimetric data revealed that the formation of a more stable polymer structure as a result of the cross linking with boron

The effect of ultrasonic waves in conducting polymer solution. Ultrasonics Sonochemistry, v. 13, p. 433-437, 2005.

by Alexandre Luna

de Azevedo WM, de Oliveira Luna AJ, Silva EF, Silva RO. The effect of ultrasonic waves in conducting polymer solution. Ultrason Sonochem. 2006 Jul;13(5):433-7. Epub 2005 Dec 15.
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Departamento de Química Fundamental, Laboratório de Química do Estado Sólido, CCEN-UFPE 50670-901, Recife, Pernambuco, Brazil. wma@ufpe.br

The effects of ultrasonic wave on the conducting polymer polyaniline dissolved in DMSO were observed. The UV-visible,... more The effects of ultrasonic wave on the conducting polymer polyaniline dissolved in DMSO were observed. The UV-visible, infrared and NMR analysis show that the polymer undergo a redox and doping transition when the ultrasound wave interacts with the polymer dissolved into the solvent. The proposed mechanism to explain these effects is based on the solvent's hygroscopicity properties. The interaction with the ultrasonic wave, homolitically dissociates the water molecule producing radical species, and these species interact with the dissolved conducting polymer changing its oxidation and doped state. The resulting effects of that interaction are the modification of oxidation state of the conducting polymer and the decrease of the amount of water molecule in the solvent. From those results, we have proposed one straightforward method to eliminate water contamination in the solvent DMSO using ultrasonic waves.

Biodegradable dextran based microgels: a study on network associated water diffusion and enzymatic degradation

by Mark Telling

Sustained drug delivery represents a major challenge in nanomedicine. Solutions to the many requirements posed by this... more Sustained drug delivery represents a major challenge in nanomedicine. Solutions to the many requirements posed by this field are not easy to address using a unique delivery vehicle. In recent years, our goal has been to implement such requirements in a single device by manipulating the structural and functional features of “soft” biocompatible drug delivery platforms. In this paper we describe a set of biocompatible drug delivery materials, with controlled structure and dimension, which are both biodegradable in a time frame of interest and designed as drug vectors for therapeutic approaches. These microdevices were obtained using ultrasound assisted “water-in-water” emulsification. The resulting material was a spherical shaped microgel with controlled pore size and water content. The dynamic behaviour of water in these matrixes showed a remarkable supercooling effect, an effect which was more pronounced for those microgels with smaller mesh sizes. The biodegradability of the microgel was monitored by observing the enzymatic breakdown of the material both as a whole, i.e. by observing a large number of microgel particles, and by focussing on single particles. A complex degradation pattern was observed, with the particles first increasing their size followed by a complete structural demolition. The time required to fully degrade a microgel can be tuned by varying the relative enzyme content and/or the degree of crosslinking of the network.

Nanostructure to Microstructure Self-Assembly of Aliphatic Polyurethanes: The Effect on Mechanical Properties

by Abhinay Mishra

published in Journal of Physical Chemistry- B, 2010

Effect of dark and salinity treatment in the yield and quality of agar from Gracilaria cornea (Rhodophyceae)

by Daniel Robledo

Co-authored with Yolanda Freile-Pelegrín, M Pedersén, E Bruno and J Rönnqvist

The alkali treatment used previous to agar extractions from the Gracilaria genus reduces, among other reactions, the... more The alkali treatment used previous to agar extractions from the Gracilaria genus reduces, among other reactions, the sulphate content and improves the gel strength; however, at an industrial level it requires expensive effluent processing to reduce its polluting charge. The red alga Gracilaria cornea was cultivated under dark and salinity treatments to replace this alkali treatment. The different treatments tested were: (a) darkness and 33‰ salinity for 8 days, [dark treatment]; (b) darkness and 50‰ salinity or 4 days, followed by darkness and 25‰ salinity for 4 days, [4+4]; and (c) darkness and 50‰ salinity for 8 days, followed by darkness and 25‰ salinity for 4 days, [8+4]. The treatment [4+4] increased the agar yield in 26% (from 36.6 to 46.1%). All treatments reduced the sulphate content of the agar in approximately 24% when compared with agar obtained without any treatment. For G. cornea the [4+4] treatment might become in the future an additional treatment to a mild alkali treatment using less alkaline reagents for agar extraction.