L. A. Pesin

In situ observation of the modification of carbon hybridization in poly(vinylidene fluoride) during XPS/XAES measurements

Abstract The effect of fluorine elimination from the surface of poly(vinylidene fluoride) with the formation of CF-groups during XPS measurements has been confirmed. The XPS analysis shows that the fluorine content values found individually from F1s- and F2s-lines differ markedly due to the difference in kinetic energies of corresponding photoelectrons but decrease monotonically in the same way during the measurements (ca. 970 min) down to almost a half of their original values. The fluorine elimination results in the –CH2–CF2–→–CHCF– transformation thus modifying the hybrid state of carbon atoms. The sensitivity of relative intensity of Auger CKVV spectra to the hybridization mode of the carbon valence electrons made it possible to observe the effect in situ. During the exposure time the intensity of a normalized CKVV peak declines by ca. 10%, which is in reasonable accordance with previous semi-quantitative calculations. The possibility of detecting and monitoring the valence transformations of carbon by measuring the XAES/XPS intensity ratio has been demonstrated.

Kinetic of poly(vinylidene fluoride) defluorination under X-ray irradiation

Modification of the photoelectron and C KVV spectra during the long-term surface degradation of partially crystalline PVDF under simultaneous soft X-ray and electron irradiation are reported. Deep radiative carbonization brings about the formation of carbynoid structures (chain-like carbon) in the surface; as a result the shape of the electron emission spectra of carbon in the carbonized sample essentially differs from that of graphite and PVDF. Analysis of carbon core-level electron spectra via decomposition onto spectral components shows presence of partially (CH, CF) and fully (=C=, -C≡) carbonized units of polymeric chain thus pointing on two-step mechanism of polyme ric chain transformation from the initial to a carbynoid. Results of the mathematical modeling of the first step of the chain transformation show it to be a second order process.

Electron emission features of the derivatives of radiation carbonization of poly(vinylidene fluoride)

It has been studied how photoelectron and CKVV spectra of partially crystalline poly(vinylidene fluoride) (PVDF) are modified during a long-term degradation of its surface under soft X-rays (AlKα), which is accompanied by a flow of secondary electrons having different energies, and upon exposure to a unfocused beam of 600 eV Ar+ ions. In both cases, the surface layer of the sample is enriched with carbon owing to defluorination. The shape of the electron emission spectra of the carbonized layer depends on an external effect; that is, whether soft X-ray photons or ions are used for defluorination. In the case of bombardment with Ar+, there is clear evidence for the dominance of the sp2 bonds between carbon atoms, as can be seen from the specific shape of the C KVV band and the C1s spectrum. The most surprising result of this study is that both photons and ions produce the same depth gradient of residual fluorine at an equal fluorine concentration in the carbonized surface layer. The reason for this is not clear and needs further investigation.

Depth distribution of the fluorine concentration during radiative carbonization of PVDF

The XPS integral intensity of the F1s line and its satellite is measured during the long-term radiative carbonization of PVDF (polyvinylidene fluoride). A model is proposed that describes the effect of the fluorine depth distribution on the shape and intensity of the F1s spectra. A comparison of the experimental data with the model calculations provides estimates for the concentration inhomogeneity during the radiative carbonization of PVDF, for the photoelectron escape depth, and for the probability of a single energy loss by a photoelectron in its motion towards the surface. A technique determining the fluorine concentration is presented. It is based on the occurrence of chemical shifts of the C1s line towards larger bond energies for the carbon atoms chemically bonded to one or two fluorine atoms.

Inhomogeneous depth distribution of fluorine atoms in PVDF upon radiative carbonization

Under the action of ionizing radiation on a PVDF film, fluorine and hydrogen atoms bound to its linear carbon chain with single chemical bonds detach. Free atoms and HF molecules diffuse toward the film surface and escape from it. As a result of irradiation of the sample surface, a fluorine concentration depth profile arises. The fluorine distribution in the PVDF films subjected to long-term X-ray exposure was studied using X-ray photoelectron spectroscopy and Rutherford backscattering spectroscopy. Both methods yield close values of the fluorine concentration at a depth of ∼10 nm.

On the possibility of synthesizing one-dimensional carbon during radiative carbonization of PVDF

The modification of x-ray photoelectron spectra (XPS) and C KVV spectra of a partially crystalline polyvinylidene fluoride (PVDF) film under the long action of soft x-rays and secondary electrons followed by argon ion bombardment of its surface is investigated. Deep radiative carbonization leads to the formation of carbynoid structures (chain carbon) on the PVDF surface. Hence, the carbon XPS of the carbonized sample differs from those obtained for graphite and PVDF. Ion bombardment shows the instability of the carbonized sample surface, giving rise to formation of sp2 hybrid bonds of carbon atoms. The obtained results are indirect experimental evidence that, before ion bombardment, sp-type bonds are dominant on the carbonized PVDF surface.

Special aspects of the temperature dependence of EPR absorption of chemically carbonized polyvinylidene fluoride derivatives

The temperature dependences of electron paramagnetic resonance (EPR) absorption of two samples of chemically carbonized derivatives of polyvinylidene fluoride (PVDF) synthesized under different conditions have been measured in the range of 100–300 K. It has been found that the temperature dependence of the integrated intensity of the EPR signal of both samples is nonmonotonic and does not obey the classical Curie dependence characteristic of free radicals. An analytical expression that is consistent with experimental data and suggests the presence of an activation component of paramagnetism in the test samples has been obtained. The presence of a term independent of temperature in this equation also indicates the paramagnetic contribution of free electrons. The magnitude of the activation energy of the singlet–triplet transitions has been evaluated: δ = 0.067 eV. The HYSCORE spectra of chemically carbonized PVDF derivatives have been obtained for the first time.

Manifestation of auger processes in C1s-satellite spectra of single-walled carbon nanotubes

Using the equipment of the Russian-German beamline of the synchrotron radiation at the BESSY II electron storage ring, satellite spectra accompanying the C1s core lines in the cases of single-walled carbon nanotubes and highly ordered pyrolytic graphite have been measured with a high energy resolution. The Auger spectra corresponding to shaking of the valence system of carbon by the core vacancy have been found and investigated. The Auger spectra of the studied single-walled carbon nanotubes and highly ordered pyrolytic graphite are caused by annihilation of the excited π* electron with a hole in the π subband. It has been established that the electron states in the conduction band have 3π* (gT, K, M) symmetry; i.e., they correspond to flat 3π* subband, which is localized by 12–13 eV above the Fermi level. It has been revealed that the general regularities of the distribution of electron states in the valence system insignificantly change during its shake-up by the excited core.