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MIT World Peace University

School of Polymer Engineering

The effect of small concentration of methylene blue dye on photocurrents was studied in the solid-state photoelectrochemical cells fabricated using conducting polypyrrole-coated electrodes sandwiched with solid-polymer electrolyte, namely, polyvinylpyrrolidone with phosphoric acid. A maximum photosensitivity factor (S=ll/ld, where ll is the photocurrent and ld is the dark current) of the order of 5 is observed. The current-voltage (I-V) characteristics in such cells reveal that charge transport is mainly governed by the space charge effect. Comparison of the results presented in this paper with the ones we reported earlier [23] indicates that the matrix in which dye has been incorporated plays an important role in such sensitization processes. A matrix that can efficiently transport the photogenerated charge carriers is observed to be more suitable for such dye-sensitized devices. © 2004 Kluwer Academic Publishers.

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Engineering and Technology

Journal of Materials Science: Materials in Electronics

Effect of solid polymer electrolyte on the sensitization of photocurrents in solid-state electrochemical cells using conducting polypyrrole

The increase in the interaction between man and machine has made display devices indispensable for visual communication. The information which is to be communicated from a machine can be often in the form of color images. Electrochromic display device (ECD) is one of the most powerful candidate for this purpose and has various merits such as multicolor, high contrast, optical memory, and no visual dependence on viewing angle. A large number of electrochromic materials are available from almost all branches of synthetic chemistry. In this review, we have tried to describe the fundamentals of such electrochromic materials and their use in EDDs. The most important examples from major classes of electrochromic materials namely transition metal oxides, Prussian blue, phthalocyanines, viologens, fullerenes, dyes and conducting polymers (including gels) are described. Examples of their use in both prototype and commercial electrochromic devices are given. © 2002 Elsevier Science B.V. All rights reserved.

Materials Chemistry and Physics

Electrochromic materials and devices: Present and future

The effect of small concentration of methylene blue (MB) on photocurrents was studied in solid state photoelectrochemical cells fabricated using conducting polyaniline (PANI) coated electrodes sandwiched with a solid polymer electrolyte, viz. poly(vinyl alcohol) (PVA) with phosphoric acid. These exhibit good photoresponse to visible light (maximum photosensitivity factor of the order of 10). The current-voltage (I-V) characteristics in such cells reveal that the charge transport is ohmic. Also, an electrochemical reaction (motion of Cl- from SPE + MB to PANI) is observed at 1.2-1.3 V, which gives rise to a sudden increase of current at this particular voltage. The presence of illumination enhances this electrochemical reaction. The combined effect, i.e. ohmic charge transport and an electrochemical reaction, is observed in the I-V characteristics. Along with the dye concentration, the photosensitivity is observed to be field/voltage dependent. The sensitization effects are due to lowering of the potential barrier formed at the PANI/SPE + MB interface under illumination.

Journal of Solid State Electrochemistry

Solid state electrochemical reaction in photocells made using conducting polyaniline and sensitized with methylene blue

The thermal stability and degradation of poly(vinyl alcohol) (PVA), PVA + H3PO4 (solid polymer electrolyte, SPE), PVA containing methylene blue (MB) dye and SPE (PVA + H3PO4) + MB were studied by TG/DTA/DTG and DSC techniques. The FTIR and UV-vis study of these materials indicates that MB forms a complex with the polymer by a hydrogen bonding interaction between the -OH group of the polymer and the nitrogen of MB and improves the thermal stability of the polymer. © 2002 Elsevier Science Ltd. All rights reserved.

Polymer Degradation and Stability

Thermal degradation properties of solid polymer electrolyte (poly(vinyl alcohol) + phosphoric acid)/methylene blue composites

The thermal conductivity of a filler and the thermal conductivity of a composite made from that filler influence the heat-transfer process during melt processing. The heat-transfer process from the melt to the mold wall becomes an important factor in developing the skin-core morphology. These aspects were examined in this study. The thermal conductivity of polypropylene-filler composites was estimated with a standard model for various fillers such as calcium carbonate, talc, silica, wollastonite, mica, and carbon fibers. The rate of cooling under given conditions, including the melting temperature, mold wall temperature, mass of the composite, and filler content, was estimated with standard heat-transfer equations. The time to attain the crystallization temperature for polypropylene was evaluated with a regression method with differential temperature steps. The crystallization curves were experimentally determined for the different fillers, and from them, the induction period for the onset of crystallization was estimated. These observations were correlated with the expected trends from the aforementioned formalism. The excellent fit of the curves showed that in all these cases, the thermal conductivity of the filler and composite played a dominant role in controlling the onset of the crystallization process. However, the nucleation effects became important in the later stages after the crystallization temperature was attained.

Journal of Applied Polymer Science

Role of heat transfer and thermal conductivity in the crystallization behavior of polypropylene-containing additives: A phenomenological model

Methylene blue (MB) at small concentration is used as a photosensitizer in the solid state photoelectrochemical cells fabricated using doped and undoped polypyrrole (Ppy) coated electrodes sandwiched with solid polymer electrolyte (SPE) viz. polyvinylalcohol (PVA) doped with phosphoric acid. Different sensitization effects are observed for the same polymer-dye combination. MB sensitizes Ppy in doped state more efficiently as compared to undoped state of the polymer. The actual placement of the energy levels of dye with respect to polymer and ease with which polymer transports photogenerated charge carriers without recombination are observed to be important in such sensitization processes and governs the overall process. © 2003 Elsevier B.V. All rights reserved.

Chemical Physics Letters

Sensitization effect in doped and undoped state of polypyrrole by methylene blue in solid state electrochemical cells

A novel poly 3-(2-benzo thiazolyl) thiophene based conductive polymer have been synthesized and characterized using various experimental techniques. The structure and band gap (difference between Highest Occupied Molecular Orbital and Lowest Unoccupied Molecular Orbital) for the above polymer have been calculated using oligomeric approach employing B3LYP/3-21G* method and semi-empirical AM1 and ZINDO methods. The calculated band gap for monomer, dimmer and trimmer has been plotted versus 1/N (where N is the number units). The value at 1/N=0, is taken as the band gap of the polymer having long chain length. It is evident from the theoretical investigations, the band gap of poly 3-(2-benzothiazolyl) thiophene is 3.77 eV. It is possible to change the band gap by designed alteration in the thiophene framework.

Molecular Crystals and Liquid Crystals

Synthesis, structure and band gap of novel thiophene based conductive polymers

The electronic transport properties of electrochemically deposited conducting polypyrrole (PPy)/Prussian Blue (PB) bilayer films have been studied by investigating the current-voltage (I-V) characteristics in dark and under white light illumination. The I-V characteristics observed are highly non-linear, both in dark and under illumination. Space charge limited conduction (SCLC) appears to be the dominant process governing the charge transport in such composite film structures which is also seen when these films are being used in photoelectrochemical or electrochromic mode. The sensitization effect appears due to relative ease of transport of the photogenerated carriers as compared the carrier transport in dark. © 2002 Elsevier Science B.V. All rights reserved.

Charge transport processes in conducting polypyrrole/Prussian Blue bilayers

The regio-regularity and properties of the poly [3-(2-ethylhexyl)thiophene] were studied, and compared with the poly [3′-(2-ethylhexyl)-2,2′,5′,2″-terthiophene]. The thermal stability of the polymers was improved due to the addition of thiophene rings with (2-ethylhexyl)thiophene. Due to the additional thiophenes at the 2,5 position of ethylhexylthiophene, even though the absorption wavelength was red-shifted, there is no characteristic change in photoemission for the latter polymer. Addition of thiophene rings to the 3-(2-ethylhexyl)thiophene improves the thermal properties without loosing the 3-(2-ethylhexyl)thiophene characteristics.

Macromolecular Materials and Engineering

Studies on 3-(2-ethylhexyl)thiophene polymers

The structure development and crystallization behavior of poly(propylene) (PP) filled with wollastonite mineral was studied with respect to different particle sizes of the filler. The decrease in particle size led to increase in crystallization rates and decrease of crystallization half time, indicating greater nucleation efficiency for smaller sizes of the wollastonite particles. This was also reflected in the higher crystallinity values obtained for small filler particles. The WAXD analysis of PP/wollastonite revealed significant changes in the relative intensity of certain peaks (110 reflection) at small particle size, indicating preferential growth of PP on wollastonite. The exact co-relationship of nucleation rate/crystallization rate and the additive particle size (d) was determined which showed that for any given concentration of the additive, it was proportional to (1/d)3.