Stretch-oriented PPV

Fig. 2.2. Polarized transmittance (dashed line) and near-normal-incidence reflectance (continuous line) spectra of highly stretch-oriented PPV...

FIGURE 49.14. Polar plot (orientational anisotropy) of the square root of the DFWM signal Intensity for the 10 1 stretch-oriented PPV film. Scattered are the experimental data points continuous line Is the theoretical fit [201]. 

Figures Polarized transmittance (dashed lines) and reflectance (full lines) spectra of highly stretch-oriented PPV free standing films. Elongation ratio 3. Spectra are plotted in nanometers (Left panel) to better display the interference patterns in the near infrared or electron volts (right panel) to show the main electronic transition in the UV-Vis.

The electronic spectrum of PPV has been studied in detail by Obrzut et al. [170]. The infrared spectra of stretch oriented PPV have been recorded and widely studied by Bradley et al. [171] who have also studied the infrared spectra of doped [172] and pohotoexcited materials [173]. 

The energetic aspects of the lattice modes, including the phenyl ring librations, have been probed by an inelastic neutron scattering study on stretch-oriented, highly crystalline PPV films [277]. This topic is outside the scope of this review. 

Disubstituted PPVs have also been explored by Martens et al. [284, 285], using x-ray and electron methods and iinstrctchcd films as well as oriented filnis. Both are obtained from spin-coated precursor films. Stretch alignment is carried out in vacuum during the initial phase of the conversion procedure above 80°C, after which conversion completes above 200°C. In the mote recent of these papers, the results for ethoxy substituents are also presented stretch-orienting has not been possible for this polymer [285],... 

Figure 6.26 shows the chemical structure of PPV. Block letters from A to F show the eight carbon sites in one PPV monomer unit. The principal axes of proton hyperfine coupling of an unpaired tt-electron, defined in Figure 6.18, arc also shown for two proton sites of inequivalent bond orientations. ESR spectra of stretch-oriented undoped PPV films have shown anisotropic lineshapes with an average g value of 2.003 and a spin concentration of about 1 spin/10 PPV monomer units. The observed anisotropic behaviour of the. g value and the linewidth, both larger for the stretch direction, are qualitatively similar to those found in stretch-oriented polyacetylene films mentioned in Section 3.2 and... 

Figure 6.29. Orientation dependenee of the ESR spectra at 4 K with and without illumination of visible light in a 10 times stretched iindoped PPV film. The external field is parallel to the stretch direction for the upper curve and pei pcndicular to it for the lower curve [131] (after ref [105])...

Oriented free-standing trans-(CH)j films (about 2 /xm thick) have been prepared by the Durham/Graz-method, i.e. by stretch-orienting a precursor polymer during thermal conversion (120 C) to (CH). In a similar way, PPV has been fully converted from a water soluble precursor polymer by heating at... 

Similar results also has been obtained by Swiatkiewicz and co-workers [205] in stretch-oriented films of poly-(2,5-dimethoxy-p-phenylenevinylene) thin films. Also, oriented films of PPV have been fabricated with the LB technique [201]. The value (at A = 1.064/i,m)ofanLBfilmofpoly-... 

In a process similar to the "Durham polyacetylene", PPV can be stretch-oriented to relatively high molecular alignment. ... 

The method has also been applied to PAni [53]. The plasticizer is a solvent of the polymer N-methylpyrrolidone (NMP), and the film to be stretched is not, or is very poorly, crystalline and may contain up to 20 wt% NMP. Tg decreases as the NMP content increases, down to 110°C heating and stretching swollen EB films above that Tg value produces X > 6, increased crystallinity (Fig. 11), and sizable orientation and anisotropic properties [27,99,100]. Such films, doped with HC1, show a conductivity anisotropy = 25 [99-101]. Contrary to what is observed with PPV, the mechanical properties of stretched PAni remain modest the Young s modulus is below 0.5 GPa [53]. 

The unstructured polymer spectrum on Fig. 14 corresponds to an unoriented film. It has now been shown, for instance on MEH-PPV [108], that orientation and stretching can result in much more structured absorption spectra. Moreover, the corresponding emission is highly structured, even more than the hexamer emission on Fig. 12a and the wavelength of... 

Moreover, the extended polymer chains should be oriented along the fiber axis due to the strong stretching of the liquid jet during electrospinning better n electron delocalization. In the absorption spectra of MEH-PPV/PFO blend nanofibers a 20-30 nm red shift of the PFO absorption band to 400-410 nm was observed, suggesting that the PFO chains are also extended and oriented along the fiber axis. ... 

At temperatures above 800 °C, PPV can be converted into graphite [79,80], The electrical conductivity of PPV that is pyrolyzed at 3000 °C is strongly influenced by stretching. By doping with sulfur trioxide a conductivity of 10 S cm is reached, which is comparable to that of a highly oriented pyrolytic graphite or natural graphite. Another study [81] in which PPV and other related polymers are pyrolyzed up to 1700 °C reports an electric conductivity of... 

In stretched PPV films, a preferential distribution of crystallites respective to the film surface has been observed. It has been found that two sets of planes, 200 and 110, are preferentially oriented parallel to the film surface. A convolution of a distribution of crystallites with the planes [110] ( 25%), [200] ( 50%), and [iTO] ( 25%) parallel to the film surface resulted in calculated peak profiles similar to the experimental data [130]. 

When doped with AsFs, PPV oligomers were reported to have conductivities of about 10 S m [48]. Films prepared by the precursor method can be stretched to give a structure with a high degree of uniform molecular orientation. After doping, the conductivity in the direction parallel to the orientation has been reported to be as high as 2.5 X 10 S [49],... 

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