Pristine PPV

As far as applications of the doped polymer are concerned, PPV has the advantage over polyacetylene of an improved environmental stability. Much of the current research is concerned with pristine PPV because of its semiconductor properties, in particular luminescence, instead of with its conducting form. 

The early structural papers deal with pristine PPV (Gagnon et al. [265, 266], Bradley et al. [57, 267]). Higher draw ratios, up to 20, have been attained in the study by Granier et al. [266], which has enabled these 

By means of both x-ray diffraction and infrared dichroism studies, the degree of orientation in similar drawn PPV films has been further characterized by Gagnon el al. [269]. The Hemians molecular orientation function/is determined in an approximate manner from the angular spread of the (llO) reflection. A maximum value of f— 0.96 is attained at draw ratios of 6 and higher, for which also the lateral crystallite size (lower limit) levels off to 20 A. Whereas the value of / corresponds well, the crystallite size is much smaller than that reported by Bradley [267], which is 270 A. 

Additional information comes fixim a high-resolution transmission electron microscopy study of crystal morphology by Masse et al. [272]. From dark-field imaging crystallinity is estimated to be 50% in a film drawn to a ratio of 8-9. The aspect ratio of the crystallites is near 1.0 and their diameter is approximately 50 A. This value is considerably lower than the x-ray based value of 120 A, quoted above. The morphology is characterized as micellar , a network of oriented fibrils. 

More recently, the analysis of pristine PPV has been supplemented by x-ray data obtained by Chen et al. [273] on a four-circle diffractometer, thus avoiding some of the complications inherent in the use of electron diffraction data, such as dynamical effects and the lack of absolute intensities. The samples consist of multilayers of stretched (///o = 10) film stacked to a thickness not exceeding 200 pm a c-axis mosaic spread of less than 7° is determined from equatorial reflections. Since crystallographic results have been found to vary somewhat among the investigations 

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An alternative explanation for the photoinduced absorption in the bulk polymer has been discussed by the Cambridge group [28]. It was shown that the amount of stimulated emission depends critically on the degree of photooxidation of the conjugated polymer. Figure 10-5 compares the stimulated emission of pristine PPV (see Fig. 10-5 a) and its heavily photooxidized counterpart (sec Fig. 10-5b). 

For PPV doped with AsFg anion, a new high BE Cls component appeared at about 286.5 eV, in addition to the two Cls components at 284.6 eV and 285.5 eV for pristine PPV [60], It has been associated with carbon atoms in a highly oxidized state resulting from doping. Thus, the Cls core-level spectrum of doped PPV readily indicates that the positive charge is non-uniformly distributed over the polymeric matrix. Furthermore, the satellite structures of the doped polymer are found to have a substantially enhanced intensity at about 290 eV, which is... 

Figure 3.29. Cls core-level spectra of (a) undoped (b) 5% AsFs-doped and (c) 7% AsF5-doped PPV Components Ca and Cb arc associated with pristine PPV while component Ct is associated with the oxidized stale. (Reprinted with permission from ref. 139, Am Chem. Soc.)...

The PL-enhancing resonance of highly ordered pristine PPV synthesized by Holmes and co-workers [80] is of width 17 G, i.e., almost twice the... 

The PL spectra of the DHO-PPV and the highly-ordered pristine PPV used to obtain the polaron resonance shown in Figure 7.14, and the spectral dependence A/ of the resonance in DHO-PPV are displayed in Figure 7.15. As observed in the P3ATs (Figure 7.10), the spectral dependence of the resonance... 

Figure 9 shows the EL spectra of ITO/polymer/Al LED devices incorporating PPV and its blends with PVP respectively, at a drive voltage of 8 V. ITO was used as a positive electrode, whereas A1 as a negative electrode. The EL spectrum of PPV LED is basically the same as its PL spectrum except that the peak intensity at 520 nm is relatively higher. When PPV was blended with 20 wt% PVP, the EL spectrum became broader and had much higher intensity compared to the pristine PPV. The peak at 520 nm also had a blue shift to 515 nm. However, with further increase of PVP content, the peak intensities decreased. The peak at 550 nm reduced its intensity at much higher rate than the left peak, while the latter further shifted to 502 nm when the content of PVP was increased to 50 wt%. 

Raman spectra of pristine PPV with Aexc 676.4 and 351.1 nm are reported by Lefrant et al. [174]. The same group has reported the Raman spectrum of doped PPV and have derived some structural information [174]. As discussed in section 4 there are some theoretical reasons to believe that the observation of the Raman spectra of doped polyconjugated materials is doubtful. This problem is going to be... 

The PL decay patterns for polymer nanocomposites films with two different Si02 Au nanoparticles are shown in Fig. 15.9. The rate of PL decay in PPV nanocomposites doped with Si02 Au nanoparticles was drastically reduced, as compared with that of the pristine PPV. This suggests that the rate of luminescence-quenching by exciton trap formation is slowed considerably. The protection against photo-oxidation inherited by Si02 Au nanoparticles in PPV could be accomplished with an extremely low concentration, corresponding to... 

Optical absorption (a) and PL (b) spectra of pristine PPV and PPV nanocomposites (doped 0.05vol.% gold-coated silica nanoparticles). Reprinted with permission from Y.T. Lim eta ., Synth. Mat 128, 133 (2002). Copyright 2002 Elsevier. 

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