Synthesis and characterization of photorefractive polymer based on chemically hybridized CdS-PVK nanocomposite with a new azo chromophore

We report an organic/inorganic polymer composite based on the chemically hybridized photoconductor CdS-PVK nanocomposite doped with a new second-order optically nonlinear chromophore 1-n-butoxy-2-methyl-(4-p-nitrophenylazo)benzene (BMNPAB) and plasticizer 9-ethyl-carbazole (ECZ) to manifest a photorefractive (PR) effect. A detailed description of the synthesis and characterization of BMNPAB is presented. The poled film including PVK-10-CdS nanocomposite and BMNPAB exhibits a high second harmonic generation (SHG) coefficient of 31 pm/V The photoconductivity of PVK-CdS nanocomposite also was studied here. Two-beam coupling experiment clearly indicated an asymmetric optical energy exchange between two beams on the polymer composite at zero electrical field, and the two-beam coupling gain of 50.0 cm(-1) and diffraction efficiencv of 4.2% were obtained at 647.1 nm wavelength. (C) 2007 Elsevier Ltd. All rights reserved.

Local gratings due to angular hole burning in a photorefractive polymer

We investigate the processes involved in writing real-time holographic gratings in a photorefractive polymer (PRP) that incorporates an azo-dye. In such systems there may be gratings due to mechanisms associated with trans–cis isomerization (angular hole burning (AHB) and/or angular redistribution), which appear in addition to those arising from the photorefractive (PR) effect. The work presented here helps to understand the interactions which may occur between these different gratings. The formation of local gratings due to mechanisms associated with photoisomerization is studied, in a new PRP based on the photoconductor poly(N-vinylcarbazole):2, 4, 7-trinitro-9-fluorenone, plasticized with N-ethylcarbazole. The polymer includes the azo-dye 4-nitro-4'-pentyloxy-azobenzene and we observe both PR and photoisomerization gratings. The gratings are shown to be both polarization-sensitive and reversible. The presence of the photoisomerization gratings (which diffract almost as strongly as the PR gratings) significantly affects the field-dependent diffractive behaviour of the composite. A measurement of the lifetime of the cis state is made (τcis = 38 s) using photoinduced dichroism. This is close to the decay time constant of the local gratings (τdecay = 42 s), and it is suggested that the local grating mechanism is AHB of the azo-dye. This is the first time (to the knowledge of the authors) that a local grating due to AHB has been demonstrated in a PRP.

Atom transfer radical polymerisation - towards the synthesis of a fully-functional photorefractive polymer

Atom transfer radical polymerisation (ATRP) of styrene in xylene solution initiated with 1-phenylethyl bromide and mediated by CuBr/N-propyl-2- pyridinemethanimine catalyst complex was studied. The polymerisation was ill-controlled, yielding polymers with broad molecular weight distributions and values of number average molecular weight considerably higher than the theoretical values calculated from 100% initiator efficiency. The degree of control afforded over the polymerisation was enhanced by use of a more soluble catalyst complex, CuBr/N-octyl-2-pyridinemethanimine. Furthermore, the use of a more polar solvent, diglyme, generated a homogeneous catalyst complex that facilitated the production of polymers having narrow molecular weight distributions (1.10 < PDi < 1.20). The kinetics of the atom transfer radical polymerisation of methyl methacrylate at 90°C in diglyme solution initiated with ethyl-2-bromoisobutyrate and mediated by CuBr/N-octyl-2-pyridinemethanimine was studied and the orders of the reaction were established. The effect on the rate of polymerisation of the ratio of CuBr:N-octyl-2-pyridinemethanimine was also determined. The temperature dependencies of the rate of polymerisation of methyl methacrylate in diglyme solution and xylene solution were studied, and were found to be non-linear and dependent upon the polarity of the solvent. The use of highly polar aprotic solvents, such as N,N-dimethylformamide and dimethylsulphoxide, was found to be detrimental to the degree of control afforded over the polymerisation of methyl methacrylate. This was circumvented by use of a 5-fold excess, over that conventionally used, of catalyst complex. The atom transfer radical polymerisation of (4-nitrophenyl)-[3-[N-[2- (methacryloyloxy)ethyl]carbazolyl]]diazene in dimethyl sulphoxide solution was studied. Although homopolymerisation yielded only oligomers, copolymerisation of this monomer with methyl methacrylate was found to be readily achievable. Keywords: ATRP, Styrene; Methyl methacrylate; Polar solvents; Fully-functional photorefractive polymer. 2

Studies on Some Aspects of Light Beam Propagation Through Certain Nonlinear Media

This thesis deals with the study of light beam propagation through different nonlinear media. Analytical and numerical methods are used to show the formation of solitonS in these media. Basic experiments have also been performed to show the formation of a self-written waveguide in a photopolymer. The variational method is used for the analytical analysis throughout the thesis. Numerical method based on the finite-difference forms of the original partial differential equation is used for the numerical analysis.In Chapter 2, we have studied two kinds of solitons, the (2 + 1) D spatial solitons and the (3 + l)D spatio-temporal solitons in a cubic-quintic medium in the presence of multiphoton ionization.In Chapter 3, we have studied the evolution of light beam through a different kind of nonlinear media, the photorcfractive polymer. We study modulational instability and beam propagation through a photorefractive polymer in the presence of absorption losses. The one dimensional beam propagation through the nonlinear medium is studied using variational and numerical methods. Stable soliton propagation is observed both analytically and numerically.Chapter 4 deals with the study of modulational instability in a photorefractive crystal in the presence of wave mixing effects. Modulational instability in a photorefractive medium is studied in the presence of two wave mixing. We then propose and derive a model for forward four wave mixing in the photorefractive medium and investigate the modulational instability induced by four wave mixing effects. By using the standard linear stability analysis the instability gain is obtained.Chapter 5 deals with the study of self-written waveguides. Besides the usual analytical analysis, basic experiments were done showing the formation of self-written waveguide in a photopolymer system. The formation of a directional coupler in a photopolymer system is studied theoretically in Chapter 6. We propose and study, using the variational approximation as well as numerical simulation, the evolution of a probe beam through a directional coupler formed in a photopolymer system.

Photorefractive performance of a novel multifunctional inorganic-organic hybridized nanocomposite sensitized by CdS nanoparticles

A novel multifunctional inorganic-organic photorefractive (PR) poly(N-vinyl)-3-[p-nitrophenylazolcarbazolyl-CdS nanocomposites with different molar ratios of US to poly(N-vinyl)-3-[p-nitrophenylazo]carbazolyl (PVNPAK) were synthesized via a postazo-coupling reaction and chemically hybridized approach, respectively. The nanocomposites are highly soluble and could be obtained as film-forming materials with appreciably high molecular weights and low glass transition temperature (T,) due to the flexible spacers. The PVNPAK matrix possesses a highest-occupied molecular orbital value of about -5.36 eV determined from cyclic voltammetry. Second harmonic generation (SHG) could be observed in PVNPAK film without any poling procedure and 4.7 pm/V of effective second-order nonlinear optical susceptibility is obtained. The US particles as photosensitizers had a nanoscale size in PVNPAK adopting transmission electron microscopy. The improvement of interface quality between US and polymer matrix is responsible for efficient photoinduced charge generation efficiency in the nanocomposites. An asymmetric optical energy exchange between two beams on the polymer composites PVNPAK-CdS/ECZ has been found even without an external field in two-beam coupling (TBC) experiment, and the TBC gain and diffraction efficiency of 14.26 cm(-1) and 3.4% for PVNPAK-5-CdS/ECZ, 16.43 cm(-1) and 4.4% for PVNPAK-15-CdS/ECZ were measured at a 647.1 nm wavelength, respectively.

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