Poly valence band

A study of the valence band photoelectron spectrum and the X-ray emission spectrum of poly(ethylene oxide) was carried out by Brena and co-workers [102] in order to understand the effect of conformation on the observed spectra. Up to 12 monomers were used in the calculations for the valence band photoelectron... 

In a series of calculations on ethylene, butadiene and hexatriene, Deleuze and co-workers [105] showed that the ADC(3) method can provide a very accurate picture of the electronic processes associated with ionisation in the valence region. Poly(acetylene) has a large feature above 21 eV, which was previously assigned to shake up. The theoretical work showed conclusively that in fact even the band at around 17eV, which had previously been assigned to a C 2s excitation could not be explained by a single particle picture but was due to satellite excitations. 

Bunz et al. pointed out that it would be of interest to develop materials that combine the stability, electron affinity, and high emissive quantum yield of PPEs with the excellent hole injection capabilities of poly(p-phenylene vinylene)s (PPVs) [48]. In line with this notion,recent synthetic activities have focused on the engineering of the band gap, conduction band, and valence band of PAEs with the objective to render these materials more useful for practical applications that exploit their electrically (semi)conducting nature. Examples of materials that emerged from these efforts are discussed in detail in other portions of this volume (in particular the chapters by Bunz, Klemm, and Yamamoto). They include, among others, poly(heteroarylene ethynylenes) such... 

Thus, the reduced form of poly-3-methyl thiophene is an intrinsic semiconductor and the Fermi level lies between the valence band and the conductivity band. The effect of oxidation is to introduce a surface state in the band gap between n and n orbitals. The Fermi level is decreased when the compound loses electrons, and metallic properties appear when an increasing number of electrons build a new but only half-filled band. These situations are shown in Fig 11.7. 

Table 12 Energy parameters of six bands in the poly(Gly-Ala-Ser) mixed polymer lying in the energy region of the conduction and valence bands of the pure systems (all quantities in eV)...

This problem contains elements of Exercises 6.4 and 6.5 that treat infinite CH and BN chains, respectively. Use the same approach. Instead of two 7t-band pairs there is only one for planar poly- -BHNH- and, as for poly-BN, there will be a band gap. With two 7t electrons per unit cell, the valence band is just filled. Hence, it would be a semiconductor and not subject to a Peierls distortion. 

Substituting a hydrogen in PE by a -CH2 CH radical modifies more drastically the valence band spectrum of poly(l-butene). An interpretation of this spectrum can be extrapolated from the data recorded for the saturated hydrocarbon identified with the monomeric unit, n-butane. The two spectra differ however in some details (relative intensities of the bands, or splittings of the peaks) due to the new bonds created in the polymer by the bonding of the -CH.-CH group to the polyethylene skeleton (10). 

Fluoro-substituted Polymers. The fluoropolymers were between the first to be studied by the XPS technique because the substitution of F atom(s) in the -CH.-CH - unit induced very large modifications in the XPS core level spectra (shifts up to 8eV) that were easy to detect and interpret. The XPS valence band spectra of similar compounds, namely poly(vinyl fluoride) (PVF), poly(vinylidene fluoride) (PVF2), poly(trifluoroethylene) (PVF3), and poly(tetrafluoroethylene) (PTFE) (26, 27, 28) are also expected to reflect the induction of such strong electronic effects at the valence molecular level. 

The compilation of such data constituted a firm basis that was used to study a specific and more complicated system the elucidation of the electronic structure of a copolymer of ethylene (48%) and tetrafluoroethylene (52%) whose synthesis was conducted in order to maximize the alternating sequences. The valence band spectrum of such a compound (Figure 8) was found very similar to the one measured e.g. for poly(vinylidene fluoride). But, by looking to the fine details of the spectrum, by simulating the valence band of a block copolymer (by addition of PE and PTFE spectra), and by comparison with model calculations, it was possible to show that the C-C band width and the distance F2s-top of the C-C band were characteristic of an ethylene-tetrafluoro-ethylene copolymer with dominant alternant structure (28). 

Chlorine-containing Polymers. Polymers containing one chlorine atom in various environments (other sustituents) were studied by XPS poly(vinyl chloride) PVC, poly(chlorotrifluoro-ethylene) PCTFE, an (ethylene-chlorotrifluoroethylene) copolymer, and poly(epichlorohydrine) PEPI, were chosen because besides carbon atoms they contain chlorine in presence of hydrogen, fluorine, and oxygen atoms. The valence band spectra of these compounds (see Figure 9) show that features can be easily and unambiguously assigned to a contribution from the chlorine molecular orbitals. 

Figure 9. XPS valence band spectra of (A) polyvinyl chloride, (B) polyfepi chloro-hydrine), (C) poly(chlorotrifiuoroethylene), (D) cofethylene-tetrafluoroethylene...

The linear polyethers are model compounds to show the effect of the insertion of one oxygen atom at different positions into the polyethylene -CH2-CH -repetitive unit. What will be the influence on the polymer XPS valence band spectrum, compared to the one of polyethylene (Figure 2) Our study was conducted on three materials poly(methylene oxide) or PMO, poly(ethylene oxide) or PEO, and poly(tetramethylene oxide) or PTMO, that are... 

Figure 10. XPS valence band spectra of the linear polyethers (I) polyfmethylene oxide), (2) polyfethylene oxide), (4) poly(tetramethylene oxide) (A).

Figure 11. XPS valence band spectra of poly( 1-butene) and poly(butadiene l,4)cis...

Figure 12. XPS valence band and core level spectra of poly (propylene oxide) and polyfvinyl methyl ether) (4)...

Stereoisomerism. The first trials to use the XPS valence band spectra to distinguish between two stereoisomers were unsuccessful. Cis- and trans- poly(isoprene) - with short branched chain and small substitution effects-, as well as cis- and trans-poly(l,4dichloro-2,3epoxybutene) - with longer branched chain and more intense substituent effects- did not show in our first measurements significant differences in their valence band spectra that could be attributed to the searched effect. Before... 

On the other hand, a -7T band mixing does not take place because of the orbital symmetry, tt states are localized at phenyl side chains and thus are not dispersed. The a- n band mixing was confirmed by UPS measurement (Figure 13) (8, 9). Feature A, which corresponds to the top of the valence band of poly(methylphenylsilane), is lifted by about 1 eV compared with the top of the valence band of poly(dimethylsilane) (A ). On the other hand, feature B, which is derived from electrons localized at phenyl sites, remains at the benzene HOMO level B. ... 

Figure 12. Calculated energy band structure of poly(methylphenylsihm). Abbreviations and symbols are defined as follows CB, conduction band LU, lowest unoccupied molecular orbital VB, valence band HO, highest occupied molecular orbital F, k = 0 point and X, Brillouin zone edge. Bu, A, Bg, b2g, 62u, and eig denote orbital symmetries.

An LCAO (linear combination of atomic orbitals) local-density functional approach was used to calculate the band structures of a series of polymer chain conformations unsubstituted polysilane in the all-trans conformation and in a 411 helical conformation, and all-trans poly(dimethylsilane). Calculated absorption spectra predict a highly anisotropic absorption for the all-trans conformation of polysilane, with the threshold absorption peak arising strictly from polarizations parallel to the chain axis. The absorption spectrum for the helical conformation is much more isotropic. Results for the dimethyl-substituted polysilane chain suggest that the states immediately surrounding the Fermi level retain their silicon-backbone a character upon alkyl-group substitution, although the band gap decreases by I eV because of contributions from alkyl substituent states both below the valence band and above the conduction band to the frontier states. 

Both of the potassium and polybromide intercalation compounds are good conductors of electricity. In the potassium intcrcalant, the electrons in the conduction band can carry the current directly, as in a metal. In the compounds of graphite with poly bromide, holes in the valence band conduct by the mechanism discussed previously for semiconductors (Chapter 7). 

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