Poled, doped polymers

Thermal Effects on Dopant Orientation in Poled, Doped Polymers... 

Recently photorefractivity in photoconductive polymers has been demonstrated (92—94). The second-order nonlinearity is obtained by poling the polymer doped with a nonlinear chromophore. Such a polymer may or may not be a good photoconductor. Usually sensitizers have to be added to enhance the charge-generation efficiency. The sensitizer function of fuUerene in a photorefractive polymer has been demonstrated (93). 

The susceptibility tensors measure the macroscopic compliance of the electrons. Since the second order polarization is a second rank tensor, SHG is zero in a centrosymmetric or randomly oriented system. To make the material capable of SHG, the NLO dopants must be oriented noncentrosymmetricaly in the polymer matrix (2-3). When modeling the poled, doped films using a free gas approximation, the poled second order susceptibilities are given by (2.19)... 

Other chromophores were also investigated this way, example includes cyclobutadiene based doped polymer, of about 8 pm V 1 d33 nonlinearity with about 10 wt % loading, for green 532 nm generation and poled pNAn-PVA for blue 443 nm generation, in this case below the 480 nm cut-off wavelength of the material [36,37]. 

Detailed information about the components of the second-order susceptibility y2)(-2w to, w) can be obtained from second harmonic measurements on well-defined samples such as single crystals or oriented thin films, the latter obtained by procedures such as the asymmetric Langmuir-Blodgett deposition technique or electric-field poling of NLO chromophore-doped polymers.31 In the case of single-crystal samples, the second harmonic is... 

An earlier study of the molecularly doped polymer liquid crystalline system shown in Figure 6.9 revealed more complex behavior (27, 32). In this study, the amount of induced nonlinearity was path dependent, with the largest observed value occurring when poling was done near Tg ( 25 C). The host polymer exhibits a nematic-to-isotropic transition at = 100 C and a nematic texture between Tg and. Poling at 50 C produces a decrease in approximately 1 order of magnitude. The l/T dependence... 

The main issues that need to be considered in the approach with mo-lecularly doped polymer are concentration of dopant molecules that must be maximized, electric fields required for poling, retention of field-induced alignment, and compatibility of poling with device concepts. 

Blanchard PM, Mitchell GR. 1993a. A comparison of photoinduced poling and thermal poling of azo dye doped polymer films for second order nonlinear optical applications. Appl Phys Lett 63(15) 2038 2040. 

Fig. 6 Orientation of chromophoric dipoles under high electric field using corona poling in doped polymer films...

In molecularly doped polymers and polymers with nlo-active pendant groups the main polymer chain has a saturated chemical structure, which does not contribute significantly to the overall nlo properties. As noted earlier, it is necessary to induce polar order in such systems to obtain finite values of This is usually achieved by the application of an electric field while the polymer is heated above its glass transition temperature (Tg). This produces a partial ordering of the polar additive or pendent group. This order is then frozen in by cooling the polymer below Tg. For such poled films there are a limited number of finite, unequal components of the tensor The symmetry of such films is 00 mm, i.e. an infinite-fold rotation symmetry about the poling direction. The non-zero coefficients of the tensor are... 

Figure 26.7 shows the chemical structures of an NLO chromophore (APAN) and an epoxy-based polymer (BPAZO) where NLO moieties are attached to the backbone [81]. Both the dye and the polymer are functionalized with thermally cross-linkable acryioyl groups. As the dye-doped polymer is subjected to heat as part of the simultaneous poling/curing process, the inter- and intramolecular cross-linking reactions occur simultaneously (Fig. 26.8). The 7g of the cross-linked polymer-dye network is lower than that of the undoped polymer network because of the plasticizing effect of the dissolved dye. However, the temporal stability at 100°C of the polymer-dye network is better than that of the undoped polymer network (Fig. 26.9) as a direct result of the increased cross-linking density in the cross-linked guest-host system. Therefore, the addition of the thermally cross-linkable NLO dye not only increases the...

Fig. 26.9 Temporal stability of the effective second harmonic coefficients of two samples monitored at 100°C. BPAZO/APAN represents a poled/cured polymer film that was prepared from an APAN-doped BPAZO polymer solution. For comparison, a poled/cured BPAZO sample was investigated concurrently.

Technically important electrochemical reactions of pyrrole and thiophene involve oxidation in non-nucleophilic solvents when the radical-cation intermediates react with the neutral molecule causing polymer growth [169, 191], Under controlled conditions polymer films can be grown on the anode surface from acetonitrile. Tliese films exhibit redox properties and in the oxidised, or cation doped state, are electrically conducting. They can form the positive pole of a rechargeable battery system. Pyrroles with N-substituents are also polymerizable to form coherent films [192], Films have been constructed to support electroactive transition metal centres adjacent to the electrode surface fomiing a modified electrode,... 

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