Investigations on polyaniline based systems for technologically important applications and some novel polymers as prospective candidates for fabricating polymer light emitting diodes

Light emitting polymers (LEP) have drawn considerable attention because of their numerous potential applications in the field of optoelectronic devices. Till date, a large number of organic molecules and polymers have been designed and devices fabricated based on these materials. Optoelectronic devices like polymer light emitting diodes (PLED) have attracted wide-spread research attention owing to their superior properties like flexibility, lower operational power, colour tunability and possibility of obtaining large area coatings. PLEDs can be utilized for the fabrication of flat panel displays and as replacements for incandescent lamps. The internal efficiency of the LEDs mainly depends on the electroluminescent efficiency of the emissive polymer such as quantum efficiency, luminance-voltage profile of LED and the balanced injection of electrons and holes. Poly (p-phenylenevinylene) (PPV) and regio-regular polythiophenes are interesting electro-active polymers which exhibit good electrical conductivity, electroluminescent activity and high film-forming properties. A combination of Red, Green and Blue emitting polymers is necessary for the generation of white light which can replace the high energy consuming incandescent lamps. Most of these polymers show very low solubility, stability and poor mechanical properties. Many of these light emitting polymers are based on conjugated extended chains of alternating phenyl and vinyl units. The intra-chain or inter-chain interactions within these polymer chains can change the emitted colour. Therefore an effective way of synthesizing polymers with reduced π-stacking, high solubility, high thermal stability and high light-emitting efficiency is still a challenge for chemists. New copolymers have to be effectively designed so as to solve these issues. Hence, in the present work, the suitability of a few novel copolymers with very high thermal stability, excellent solubility, intense light emission (blue, cyan and green) and high glass transition temperatures have been investigated to be used as emissive layers for polymer light emitting diodes.

Methylene Blue Doped Polymers: efficient Media for Optical Recording

Polymer materials find application in optical storage technology, namely in the development of high information density and fast access type memories. A new polymer blend of methylene blue sensitized polyvinyl alcohol (PVA) and polyacrylic acid (PAA) in methanol is prepared and characterized and its comparison with methylene blue sensitized PVA in methanol and complexed methylene blue sensitized polyvinyl chloride (CMBPVC) is presented. The optical absorption spectra of the thin films of these polymers showed a strong and broad absorption region at 670-650 nm, matching the wavelength of the laser used. A very slow recovery of the dye on irradiation was observed when a 7:3 blend of polyvinyl alcohol/polyacrylic acid at a pHof 3.8 and a sensitizer concentration of 4.67 10 5 g/ml were used. A diffraction efficiency of up to 20% was observed for the MBPVA/alcohol system and an energetic sensitivity of 2000 mJ/cm2 was obtained in the photosensitive films with a spatial frequency of 588 lines/mm.

Methylene Blue Doped Polymers:Efficient Media for Optical Recording

Polymer materials find application in optical storage technology, namely in the development of high information density and fast access type memories. A new polymer blend of methylene blue sensitized polyvinyl alcohol (PVA) and polyacrylic acid (PAA) in methanol is prepared and characterized and its comparison with methylene blue sensitized PVA in methanol and complexed methylene blue sensitized polyvinyl chloride (CMBPVC) is presented. The optical absorption spectra of the thin films of these polymers showed a strong and broad absorption region at 670-650 nm, matching the wavelength of the laser used. A very slow recovery of the dye on irradiation was observed when a 7:3 blend of polyvinyl alcohol/polyacrylic acid at a pHof 3.8 and a sensitizer concentration of 4.67 10 5 g/ml were used. A diffraction efficiency of up to 20% was observed for the MBPVA/alcohol system and an energetic sensitivity of 2000 mJ/cm2 was obtained in the photosensitive films with a spatial frequency of 588 lines/mm.

Radiopaque materials from natural polymers: Special emphasis on chitosan and natural rubber

The present work is to impart radiopacity in various natural polymers like chitosan, natural rubber and derivatives of chitosan and to characterize it. This thesis collated the radiopaque properties of these radiopaque polymers and various technological applications in the medical field. The applications of radiopaque polymers leads to an exploitation of radiopaque properties like X-ray visibility, optical density, effective atomic number, attenuation coefficient of biopolymers like chitosan, chitosan formate, chitosan acetate, carboxy methyl chitosan and natural rubber. The radiopaqe properties of these materials highly depend upon the size, shape, amount of radiopacifier and crystallinity of the radiopaque material. Radiopaque chitosan microspheres were prepared by cross linking with glutaraldehyde followed by the encapsulation of barium sulpahte. The effect of different emulsion systems on the morphology of chitosan microspheres were studied. The study concentrates radiopaque natural rubber for shielding applications. It reveals that to improve the particle size, morphology and crystalline phase of the zinc oxide particles, a novel method for the preparation of zinc oxide is adopted. A detailed radiopacity study was done in natural rubber containing 100phr precipitated zinc oxide prepared from different zinc salts. One of the significant findings of this investigation is that NR vulcanizates containing precipitated zinc oxide (from zinc acetate) shows higher attenuation coefficient. These interesting findings reveal the applications of these natural radiopaque systems in various fields like surgical tools, medical tubings, catheters, radiation shielding,etc.

Comparison of Microwave and Electrical Properties Selected Conducting Polymers

Microwave properties of conductive polymers is crucial because of their wide areas of applications such as coating in reflector antennas, coating in electronic equipments, firequenry selective .surfaces, EMI materials, satellite communication links, microchip antennas, and medical applications. This work involves a comparative study of dielectric properties of selected conducting polymers such as polyaniline. poly(3,4-eth),lenedio.syt2iophene), polvthiophene, polvpvrrole. and pohparaphenylene diazomethine (PPDA) in microwave and DC,fields. The inicrowave properties such as dielectric constant, dielectric loss. absorption coefficient, heating coefficient, skin depth, and conductivity in the microwave frequency (S hand), and DC fields were compared. PEDOT and polccuiiline were found to exhibit excellent properties in DC field and microwave frequencies, which make thein potential materials in many of the alorenientioned applications

Photoacoustic evaluation of the thermal effusivity in the isotropic phase of certain comb-shaped polymers

The thermal effusivity values in the isotropic phase of certain comb-shaped polymers have been evaluated for the first time using an open photoacoustic cell configuration. The compounds investigated have siloxane and acrylate backbone and they carry mesogenic groups in their side chain. The results indicate that the polymer chain length as well as the side chain length have pronounced influence on the thermal effusivity values in liquid crystalline polymers

Development of Photorefractive Polymers: Evaluation of Photoconducting and Electro-optic Properties

This Thesis discussed molecules suitable for photorefractive effect. Out of the molecules studied, only one system was used to make photorefractive polymers system. Other molecules, especially, the electro-optic polymer, Poly(3-methacrloyl-1-(4'-nitro-4-azo-1'-phenyl)phenylalanine-co- methyl methacrylate) can be subjected to more detailed studies to explore the possibilities of using them for electro-optic applications. Though not included in the thesis, the efficient photoconductor, Poly(6-tert-butyl-3- phenyl-3,4-dihydro-2H-1,3-benzoxazine) sensitized with C60, which was described in Chapter 3 showed a low magnitude photovoltaic effect. This hints at the possibility of using this system for organic solar cells also. The thesis presented the initial observation of photorefractive effect in a polybenzoxazine based polymer system. A detailed analysis of the effect of C60, ECZ and DR1 can be carried out to check for the possibility of a high efficiency photorefractive system.

Development of Photorefractive Polymers: Synthesis and Characterization of Polymers with Donor-Pi-Acceptor Groups

Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology

Studies on Photosensitive polymers for Optical Recording

Department of Polymer Science and Rubber Technology,Cochin University of Science and Technology

Studies on Conducting Polymers and Conductive Elastomer Composites

The primary aim of this work has been to develop conductive silicone and nitrile rubbers, which are extensively used for making conductive pads in telephone sets, calculators and other applications. Another objective of the work has been to synthesise and characterize novel conducting polymers based on glyoxal and paraphenylenediamine- poly(p-phenylenediazomethine. Conducting polymer matrices were developed from polymer blends such as poly(pphenylenediazomethine), polyethylene, PVC and silica and their properties were studied.

Investigations on some selected conducting polymers and polymer composites for possible optoelectronic applications

Department of Physics, Cochin University of Science and Technology

Photothermal and photoacoustic investigations on certain polymers and semi conducting materials

Material synthesizing and characterization has been one of the major areas of scientific research for the past few decades. Various techniques have been suggested for the preparation and characterization of thin films and bulk samples according to the industrial and scientific applications. Material characterization implies the determination of the electrical, magnetic, optical or thermal properties of the material under study. Though it is possible to study all these properties of a material, we concentrate on the thermal and optical properties of certain polymers. The thermal properties are detennined using photothermal beam deflection technique and the optical properties are obtained from various spectroscopic analyses. In addition, thermal properties of a class of semiconducting compounds, copper delafossites, arc determined by photoacoustic technique.Photothermal technique is one of the most powerful tools for non-destructive characterization of materials. This forms a broad class of technique, which includes laser calorimetry, pyroelectric technique, photoacollstics, photothermal radiometric technique, photothermal beam deflection technique etc. However, the choice of a suitable technique depends upon the nature of sample and its environment, purpose of measurement, nature of light source used etc. The polynler samples under the present investigation are thermally thin and optically transparent at the excitation (pump beam) wavelength. Photothermal beam deflection technique is advantageous in that it can be used for the detennination of thermal diffusivity of samples irrespective of them being thermally thick or thennally thin and optically opaque or optically transparent. Hence of all the abovementioned techniques, photothemlal beam deflection technique is employed for the successful determination of thermal diffusivity of these polymer samples. However, the semi conducting samples studied are themlally thick and optically opaque and therefore, a much simpler photoacoustic technique is used for the thermal characterization.The production of polymer thin film samples has gained considerable attention for the past few years. Different techniques like plasma polymerization, electron bombardment, ultra violet irradiation and thermal evaporation can be used for the preparation of polymer thin films from their respective monomers. Among these, plasma polymerization or glow discharge polymerization has been widely lIsed for polymer thin fi Im preparation. At the earlier stages of the discovery, the plasma polymerization technique was not treated as a standard method for preparation of polymers. This method gained importance only when they were used to make special coatings on metals and began to be recognized as a technique for synthesizing polymers. Thc well-recognized concept of conventional polymerization is based on molecular processcs by which thc size of the molecule increases and rearrangemcnt of atoms within a molecule seldom occurs. However, polymer formation in plasma is recognized as an atomic process in contrast to the above molecular process. These films are pinhole free, highly branched and cross linked, heat resistant, exceptionally dielectric etc. The optical properties like the direct and indirect bandgaps, refractive indices etc of certain plasma polymerized thin films prepared are determined from the UV -VIS-NIR absorption and transmission spectra. The possible linkage in the formation of the polymers is suggested by comparing the FTIR spectra of the monomer and the polymer. The thermal diffusivity has been measured using the photothermal beam deflection technique as stated earlier. This technique measures the refractive index gradient established in the sample surface and in the adjacent coupling medium, by passing another optical beam (probe beam) through this region and hence the name probe beam deflection. The deflection is detected using a position sensitive detector and its output is fed to a lock-in-amplifIer from which the amplitude and phase of the deflection can be directly obtained. The amplitude and phase of the deflection signal is suitably analyzed for determining the thermal diffusivity.Another class of compounds under the present investigation is copper delafossites. These samples in the form of pellets are thermally thick and optically opaque. Thermal diffusivity of such semiconductors is investigated using the photoacoustic technique, which measures the pressure change using an elcctret microphone. The output of the microphone is fed to a lock-in-amplificr to obtain the amplitude and phase from which the thermal properties are obtained. The variation in thermal diffusivity with composition is studied.

Low-voltage driven ZnS:Mn MIS and MISIM thin film electroluminescent devices with Eu2O3 insulator layer

AC thin film electroluminescent devices of MIS and MISIM have been fabricated with a novel dielectric layer of Eu2O3 as an insulator. The threshold voltage for light emission is found to depend strongly on the frequency of excitation source in these devices. These devices are fabricated with an active layer of ZnS:Mn and a novel dielectric layer of Eu2O3 as an insulator. The observed frequency dependence of brightness-voltage characteristics has been explained on the basis of the loss characteristic of the insulator layer. Changes in the threshold voltage and brightness with variation in emitting or insulating film thickness have been investigated in metal-insulator-semiconductor (MIS) structures. It has been found that the decrease in brightness occurring with decreasing ZnS layer thickness can be compensated by an increase in brightness obtained by reducing the insulator thickness. The optimal condition for low threshold voltage and higher stability has been shown to occur when the active layer to insulator thickness ratio lies between one and two.