Temenuzhka Yovcheva

Plovdiv University · Department of Experimental Physics

Polyelectrolyte multilayers (PEMs) deposited on porous composite poly-lactic acid (PDLA)/poly( $\varepsilon $ -caprolactone) (PEC) substrates are investigated. The substrates are charged with a corona discharge. The dependence of the normalized surface potentials on the storage time of the charged substrates is studied. Layer-by-layer (LbL) techni...

The objectives of the present study were to synthesize casein/chitosan nanocomplexes, which are able to immobilize and release Benzydamine in a controlled manner, and to investigate the influence of casein/chitosan ratio on their morphological, physico-chemical and drug carrier characteristics. The complexes were obtained by electrostatic interacti...

In the present paper chitosan and xanthan polyelectrolyte multilayers (PEMs) deposited on composite multicomponent poly(D-lactic acid) (PDLA) / poly(ε-caprolactone) (PEC) substrates were investigated. The substrates' melting phenomena and degree of crystallinity were investigated by differential scanning calorimetry (DSC). Based on the melting beha...

In the present article, chitosan and casein polyelectrolyte multilayers (PEMs) deposited on poly(D-lactic acid) (PDLA) substrates are investigated. PDLA substrates are modified both physically under negative corona discharge and chemically with $N$ -ethyl- $N ^\prime $ -(3-dimethylaminopropyl) carbodiimide hydrochloride (EDAC). The substrates mo...

This research aims to investigate the properties of nano- and micro-sized casein hydrogels crosslinked by sodium tripolyphosphate as drug delivery systems. Benzydamine hydrochloride was chosen as a model hydrophilic drug. The gels were synthesized by varying different parameters: casein concentration, casein/crosslinking ratio, and addition of etha...

Polyelectrolyte multilayers (PEMs) deposited on non lyophilized and lyophilized polylactic acid (PDLA) substrates were investigated. The substrates were charged in a corona discharge. The charging of the substrates was carried out by means of a conventional corona triode system consisting of a corona electrode (needle), a grounded plate electrode a...

In the present paper the effect of pH and ionic strength on the immobilization and release of curcumin from chitosan and casein polyelectrolyte multilayers (PEMs) was investigated. The investigated PEMs were deposited on polylactic acid (PDLA) substrates. The PLA substrates were charged in a corona discharge system, consisting of a corona electrode...

In the present study chitosan and casein polyelectrolyte multilayers (PEMs) deposited on composite polylactic acid (PDLA) / poly(ε-caprolactone) (PEC) substrates were investigated. The substrate’s morphology was investigated using polarized light microscopy and their degree of crystallization was studied by the application of differential scanning...

Polyelectrolyte multilayers (PEMs) are widely used as drug delivery systems, but still remain challenging for their small drug immobilizing capacity. One way to increase the immobilized drug amount may be crosslinking of the PEMs, which stabilize them and increase their porosity. The aim of the present study is fabrication and characterization of c...

The presented paper deals with the formulation of medical pads with potential use as drug delivery systems via buccal mucosa adhesion. The pads consists of a substrate and deposited on it multilayered structure of alternating casein and chitosan. The method of preparation of this multilayer structure is layer-by-layer deposition. The electrolyte na...

The influence of the structure and physico-chemical properties of chitosan/casein multilayer films on their potential use for drug delivery systems was investigated. The multilayer films were prepared using layer-by-layer self-assembly, whereby chitosan and casein were deposited onto poly(lactic acid) substrates pretreated with either a positive or...

The build-up of new functional materials with controlled structures and properties in micro - and nano-dimensional scale is of essential interest because of their use in biomedicine, pharmaceuticals, tissue engineering, and regenerative medicine. From that point of view, the layer-by-layer deposition of polyelectrolytes on a substrate is imposed an...

The potential of several physical methods for investigation of the botanical origin of honey has been discussed. Samples from the three most prevalent types of honey in Bulgaria (acacia, linden and honeydew) have been used. They have been examined by laser refractometry, UV, VIS and FTIR spectroscopy, electric conductivity measurement and different...

The influence of low pressure on the surface potential decay of gamma irradiated polymer films of polypropylene (PP) and poly(ethylene terephtalate) (PET) were studied. Polymer film samples were subjected to integral irradiation doses (Ey = 1.25 MeV, ⁶⁰Co source) of 5 kGy and 25 kGy accumulated in air at a dose rate of 0.26 Mrad/h. After irradiatio...

The effect of lactose concentration on the activity of the immobilised enzyme β-galactosidase from Aspergillus niger has been evaluated, considering future applications for the production of galactooligosaccahrides with prebiotic potential. The following enzyme was immobilized in xanthan and chitosan polyelectrolyte multilayers (PEMs) deposited by...

In the present work, the refractive characteristics of aqueous solutions of several sweeteners are investigated. These data in combination with ones from other sensors should find application for brief determination of sweeteners content in food and dynamic monitoring of food quality. The refractive indices of pure (distilled) water and aqueous sol...

The aim of the present work is to investigate the effect of pH and ionic strength on the deposition of chitosan/xanthan multilayers on preliminary corona charged substrates from polylactic acid. The multilayerfilms were formed by alternative dipping the substrate into chitosan and xanthan polyelectrolytesolutions. For this purpose 0.1% chitosan sol...

Multilayered polyelectrolyte films are promising structures in the biomedical field. In order to meet the demands for biomedical applications, the structures have to be built from biocompatible and/or biodegradable, nontoxic starting materials, possessing some specific functional properties, depending on the particular application. In the present s...

The effect of BaTiO3 on properties of polylactic acid composites was studied. Measurements of surface potential, electric field strength, effective surface charge density, and isothermal and thermally-stimulated potential relaxation showed that corona-charged films of the PLA – BaTiO3 composites had more stable electret properties compared to PLA f...

Polyelectrolyte multilayers (PEMs) are well-defined nanoarchitectures with many potential applications, usually as biomaterial coatings. They possess excellent characteristics, such as fine tuning of thickness, stiffness, stability, morphology and topography. Hence they may exhibit special biological properties, such as mucoadhesion and local drug...

In this paper, the design and testing of a five-wavelength laser microrefractometer are presented. Five semiconductor lasers are used for the spectral region of 405–1320 nm. The presented device is based on the critical angle method. In this case, the critical angle of total internal reflection is determined with the help of a CCD camera detecting...

In the present paper the influence of both time and storage temperature on the surface potential decay of polypropylene (PP) composite films was investigated. We have investigated the 200 ??m PP composite films with different weight concentrations of the particles (ZnO or CuO) - 0 wt.%, 2 wt.% and 4 wt.%. The samples were charged in a corona discha...

Electret and surface properties of the PLA film treated in a negative or a positive corona discharge at two different temperatures were studied. Vibrating electrode method with compensation was used to measure the electret surface potential value. The changes in the surface free energy were investigated by means of contact angle measurements. The B...

This paper describes a study of the usefulness of some physical methods in the detection of adulteration of extra virgin olive oil with controlled concentration of relatively cheap sunflower oil. We have tested three physical methods measuring refractive indices (RI) and their dispersion curves, fluorescence spectra and color parameters that are re...

The influence of both low pressure and time storage on the surface potential decay of high density polyethylene (HDPE) composite films with different weight concentrations of the zeolite particles - 0 wt.%, 2 wt.% and 6 wt.% were studied. The samples were charged in a corona discharge by means of a corona triode system for ' minute under room condi...

The refractive index of nanoparticles dispersed in water is measured, using the total internal reflection method. The critical angle is determined by the disappearance of diffraction orders from a metal grating. The investigated nanoparticles are titanium dioxide (anatase phase), (35 nm diameter), zinc oxide, (<50 nm diameter), zirconium dioxide, (...

The paper presents the results of holographically investigated photo-diffusion of Ag, Au and Cr ions into nano-sized As2S3 films. Evanescent wave’s holographic recordings with spatial resolution 2 500 min-1 in chalcogenide/metal sandwich like structures with 50 nm thick As2S3 layer, were performed. During the holographic recording 5 kV corona disch...

The present study is focused on the characterization of optical properties of poly(methyl methacrylate) (PMMA) films and the possibilities of modulation and fine tuning of their refractive index by the inclusion of different concentrations of nano-sized titanium dioxide (TiO2) particles (less than 33 nm) and corona poling. The samples are prepared...

Quantification of olive oils is an important issue because of frequent adulteration with cheaper oils such as soybean, sunflower or canola. The standard chemical methods used to determine the chemical content of the oils are usually time-consuming, laboratory intensive and expensive. Therefore, we have tested three physical methods measuring refrac...

In the present paper the influence of the concentration of TiO2 particles of average size 0.5 μm on the electret properties of the PP composite films were investigated. We investigated the PP composite samples with different weight concentrations of the TiO2 particles (0 wt., 2 wt.% and 4 wt.%) and thicknesses 200 μm. The samples were charged by th...

The paper presents the results of investigation of the surface refractive index and the contact angle analysis of layers of poly(methyl methacrylate) (PMMA). The following PMMA trademarks Vedril (Italy), Diakon (USA) and Plexigum (Germany) with different molecular weights were used. Film of PMMA were prepared by drop casting techniques using 10 wt....

In this paper the silver mobility in thin chalcogenide films is investigated. The diffraction gratings in 96 nm thick As2S3 films are holographically recorded using evanescent wave, created by total internal reflection and normally incident plane one. The grating step Λ is 447 nm and corresponding spatial frequency is 2237 mm-1. 5 kV corona dischar...

Electric impedance spectroscopy, as a fast and non-invasive method, was used to monitor apples' properties during aging. This method provides information about the physical properties of apples which are closely related to the chemical ones. Two different analytical techniques for assessment of the changes of apples' properties during aging time we...

Holographic gratings with diffraction efficiency (DE) more than 8% and a spatial resolution 2237 mm-1 are recorded in very thin As2S3 films with thickness 100 nm. Silver photo-diffusion is observed during the holographic recording process while applying a corona discharge. We used the method of holographic grating relaxation spectroscopy (Forced Ra...

Polypropylene (PP), poly(lactic acid) (PLA) and poly(ethylene terephthalate) (PET) samples, charged in a positive or in a negative corona were studied. The charging of the samples in a corona discharge was carried out by means of a conventional corona triode system. After charging the surface potentials were measured using the method of the vibrati...

Electret stability of poly(L‐lactide) (PLA) films, gamma‐irradiated up to 100 kGy has been investigated by measuring the surface potential during the storage period. PLA samples—40‐μm thick films—were prepared by the casting method and then irradiated in a 60Co radiation facility at a dose rate of 0.25 kGy/h. The structural changes during the irrad...

The nano-scale holographic gratings ware recorded in 29 nm and 56 nm thick As2S3 films. The chalcogenide layers were deposited on a transparent chromium electrode with thickness 10 nm, produced on a glass substrate. Both chromium and chalcogenide films were deposited in one vacuum cycle by e-beam and thermal evaporation, respectively. The diode 532...

In this work, the effect of the surface modification of polypropylene films via corona treatment was investigated. Polypropylene films were treated with negative and positive corona discharge, at atmospheric pressure, for 5 minutes, at two different temperatures - 25 °C and 90 °C. The changes in the surface free energy were investigated by means of...

The refractive index (RI) of polymer nanocomposite of poly(vinyl alcohol) films with TiO 2 nanoparticle inclusions with low concentration up to 1.2 wt. % was investigated. Accurate refractometric measurements, by a specially designed laser microrefractometer, were performed at wavelengths 532 and 632.8 nm. The influence of TiO 2 concentration on th...

The investigated holographic gratings in 56 nm and 100 nm thick As2S3 films were recorded using Stetson-Nassenstein optical arrangement simultaneously with their charging in a corona field. The thin chalcogenide layers were deposited on transparent chromium electrode with thickness of 10 nm on glass substrate. Both chromium and chalcogenide films w...

The influence of the gas media in which electrets have been obtained on the electrets behaviour of samples stored at low pressures was investigated. Non-metalized samples of poly(ethylene terephtalate) (PET) films were studied. Initially the PET films were cleaned with alcohol in an ultrasonic bath for 4 minutes, washed in distilled water, and drie...

The theoretical background as well as the experimental results of evanescent wave holographic recording with grating periods of 3.2, 0.88 and 0.37 μm (spatial frequencies of 310 mm−1, 1140 mm−1 and 2700 mm−1) in nano-sized As2S3 films with thicknesses of 29 nm, 15 nm and 10 nm are presented. The grating recording has been performed using the interf...

The book describes the new advances in the science and technology of hydrocolloids which are used in food and related systems. The focus is on the technofunctionality and the biofunctionality of hydrocolloids, giving an appropriate emphasis to the manipulative skills of the food scientist and recognising the special part hydrocolloids can play in s...

(Full text in Bulgarian) In the past decade polymer nanocomposite materials have attracted great interest from many researchers. The organic/inorganic nanocomposites combine the unique properties of the two constituents and therefore passes new properties different from the original components. The present study is focused on the characterization o...

The influence of corona charging on holographic recording in acrylamide-based photopolymer nanocomposite containing MFI zeolite nanoparticles has been studied. The holographic recording was carried out in two different geometries —transmission grating recording and total internal reflection grating recording. During the recording process, the layer...

The experimental results showed that the zinc oxide filler of PMMA polymer films improved charge storage stability independent of the polarity. The temperature dependences of the electret potential showed the appearance of additional charge trapping levels of different depths in the electret PMMA/ZnO composite films. The effect of low pressures on...

Acrylamide-based photopolymerizable nanocomposites containing three types of nanosized crystals with controlled microporosity, Silicalite-1 (MFI-structure), AlPO-18 (AEI-structure) and Beta (BEA-structure) are studied. The influence of the porous nanoparticles on the average refractive index, optical scattering and holographic recording properties...

In the present paper the influence of both relative humidity and low pressure on the surface potential decay of charged 20 μm polypropylene films was investigated. Sample charging was performed in a three-electrode corona discharge system. After charging the initial surface potential was measured using the method of the vibrating electrode with com...

The dielectric function of polymer nanocomposites is examined in a small filling factor approximation (SFFA). The linear concentration dependence on SFFA is experimentally confirmed by refractometric investigations of PVA thin films doped with titania and diamond nano-sized particles.

We report the results of electric field influence on holographic recording in very thin chalcogenide glass films. The total internal reflection prism recording technique (Stetson's scheme) is applied for holographic recording. The main advantage of this scheme is the possibility of holographic recording in micro- and nanometer thick photosensitive...

The holographic recording properties of charged zeolite containing photopolymerisable nanocomposites were studied. Photopolymer samples doped with pure silica MFI‐type zeolite nanoparticles were charged using corona discharge. The holographic properties of the nanocomposites were characterised in real time using the Stetson geometry of recording....

Polymer nanocomposite materials have attracted increasing attention because of their unique optical properties. Of practical interest is the controllable increase in the refractive index (RI) values of the polymer nanocomposite materials by using nanoparticle inclusions with high refractive index values. This paper presents a study of the optical c...

Polypropylene samples, 20 pm thick, charged in a negative corona were studied. Each sample was placed on a metal pad. The samples were charged to different initial surface potentials, by means of a conventional corona triode system. The negative corona voltage was 5 kV and the grid voltages of the same polarity were 350 V. 500 V. 650 V. 800 V and 9...

The aim of the present research is to investigate the formation and stability of pectin/chitosan multilayer films on preliminary corona charged polylactide substrate with different polarity. For this purpose two different multilayer structures with alternative ordering of polycation and polyanion layers were prepared. Layer‐by‐layer (LbL) depositi...

The photo-induced changes in As-S-Se ternary glasses offer a possibility of using amorphous chalcogenide layers for high-density information storage, high-resolution display devices and fabrication of diffractive optical elements. In this paper we report results of the photo-induced changes in the optical properties of optically uniform thin As40S6...

Polypropylene electrets 20 μm thick charged in a negative corona were studied. The samples were charged to different initial surface potentials and were stored under various pressures for 30 minutes. Then, the surface potential was again measured. The dependence of the electret surface potential on the pressure, under which the electrets were kept...

The aim of the present work is the development of technology for formation of corona charged electret films from polylactic acid and investigation of their structural, optical and electret properties. Polylactide films with different degree of crystalinity were prepared by casting of poly-L-lactide and poly-DL lactide blended solutions. Then glass...

The thermostimulated discharge current method (TSDC) is widely used for the study of charge storage mechanisms in electrets. A new discharged technique, called differential, which consists in discharging a charged sample through an otherwise identical but uncharged one, has been proposed by J.-P. Reboul and A. Toureille. In the present paper a new...

This chapter is a brief review of the investigations devoted to polymer film changes initiated by corona treatment and their possible applications. Polymer films, among which arepolypropylene, polyethylene terephthalate and polytetrafluoroethylene, are most widely used as materials for the production of stable charged films. One of the simplest and...

Poly-L-lysine/carboxymethylcellulose multilayers were formed by a self-assembly deposition technique on negative corona charged polypropylene (PP) films by consecutive dipping of these PP films in acidic solutions of poly-L-lysine (PLL) and carboxymethylcellulose (CMC). The binding of the biopolymers was irreversible over the time of the deposition...

In the present work the optical and holographic characteristics of acrylamide-based photopolymer layers are studied. For the first time the refractive index change of a liquid acrylamide photopolymer due to exposure at 532 nm is obtained using a critical angle laser micro-refractometer. The 30 mm thick solid photopolymer films are prepared by casti...

Poster Presentation, 3rd International Symposium Advanced Micro- and Mesoporous Materials, 6-9 September 2009, Albena, Bulgaria.

Poly(galacturonic) acid/chitosan multilayers were formed on corona pretreated substrates from biodegradable poly-lactide by LBL deposition. The binding of the biopolymers was irreversible over the time of the deposition. The environmental response and stability of the multilayers to changes in pH, ionic strength, pressure and temperature were inves...

Pectin/chitosan multilayers were formed by self-assembly deposition technique on two types of corona charged poly-lactide substrates, differing in degree of crystallinity. The characterization of the poly-lactide substrates was done by a DSC method. The multilayer films were examined by spectroscopic methods (FTIR, NIR and UV-VIS). The intensity of...

The optical characteristics of photopolymer films doped with nanoparticles are studied. The investigated systems consist of a soft polymer matrix containing porous zeolite nanoparticles with concentrations varied in the range from 0wt.% to 7wt.%. 40μm thick layers are obtained by casting the photopolymer nanocomposite solution on glass substrates....

Acrylamide-based photopolymerisable nanocomposites containing three different types of nanozeolites - Si-MFI, AlPO and BEA, were studied. The influence of the porous nanoparticles on the average refractive index, optical scattering, holographic recording properties and shrinkage were characterised.

The optical properties of acrylamide-based photopolymer doped with pure silica MFI-type zeolites are studied by refractometric and spectrophotometric means. Dynamic light scattering and transmission electron microscopy are used for zeolite characterization, and laser refractometry and white light interferometric profilometry are used for surface ch...

The work presents design and testing of a four-wavelength laser micro-refractometer for determination of refractive indices of liquid and solid thin films. The main goal is to achieve widening of the measured spectral region - from the violet end of the visible spectrum to the near infrared. Semiconductor lasers are used as light sources at wavelen...

In this paper, the charge storage performance of electrospun poly(ethylene terephthalate) (PET) mats with high specific surface area was evaluated in comparison to that of PET film electrets. Corona discharge method was used to charge the electrospun PET mats and PET films. The surface potential decay measurements revealed that the corona charged P...

The samples under investigation are 20 µm thick isotactic polypropylene films corona treated at different corona polarities and grid voltages in a three-electrode system. In order to control the results of the applied treatment a combination of two refractive index (RI) measuring techniques (laser refractometry and spectrophotometry) is used to st...

In the present paper we propose an optical method used to determine refractive index distribution on the surface of polymer film electrets, and by this distribution one can judge about the surface modification. We have studied rectangular samples of isotactic polypropylene film, 20 μm thick, charged by the corona discharge method in two corona devi...

In the present paper poly (ethylene terephthalate) (PET) electrets, 40 μm thick, produced in a corona discharge, were studied. The electrets were charged to different initial surface potentials and were placed into a vacuum chamber under various pressures down to 0.1 mbar for 30 minutes. The surface potential was measured after charging the samples...

Polypropylene (PP) and polytetrafluorethylene (PTFE) electrets 20μm thick obtained in positive and negative corona discharges were studied. Each sample was placed on a metal pad. After charging the electrets to different initial surface potentials (350, 500, 650, 800 and 950)V they were put into a vacuum chamber under various pressures (0.1, 1, 10,...

In the present study, we use X-ray photoelectron spectroscopy (XPS) analysis to clarify how different polarities of corona initiate various changes in the surfaces of polypropylene (PP) electrets subject to corona discharge. The samples were charged in three-electrode corona discharge system using positive and negative corona polarities at both −20...

Optical and electret characteristics of thin 20 μmn polypropylene (PP) corona electrets have been studied in the present work. Refractive index (RI) in a surface layer with submicrometer thickness was determined by the method of the disappearing diffraction pattern. Electret surface potential was measured by the method of the vibrating electrode wi...

The influence of CuBr vapor (Cu-Br) and CO2 laser radiation on polypropylene corona electrets is investigated in the present paper. The surface potential was measured before (V0) and after (Virr) irradiation using the method of the vibrating electrode with compensation. The sample surfaces were characterized by XPS.

The paper studies the influence of low-energy laser radiation on the electret performance of polypropylene (PP) corona electrets. A 20 μm PP foil was charged in a positive and negative corona at 20 °C and 80 °C. The corona voltage was ±5kV, while that of the grid was ±1kV. The surface potential was measured using the method of the vibrating electro...