The combination of transmission null ellipsometry (TNE) and attenuated total reflection (ATR) methods supported by absorption measurements is shown to be an effective tool to study spontaneous and photoinduced 3D order in azopolymers. We investigated a series of azobenzene containing side-chain polyesters differing by the length of the main-chain spacer (CH2) m (m = 2, 8, 9, 10, 12, 13, 14, 16) and the tail substitutes (NO2, OCH3, and OC4H9) in azochromophore. The 3D order was induced by monochromatic polarized light of several wavelengths strictly distinguished by absorption efficiency of azochromophores. The orientational order under irradiation and after irradiation is studied. A big variety of 3D orientations (biaxial, uniaxial, and isotropic) is realized. The uniaxial and isotropic configurations correspond to the initial state and the saturation state of irradiation, while biaxiality is an attribute of the transient orientations. It is shown that both initial and photoinduced 3D orders are strongly determined by molecular structure of azopolymers. If we exclude the case of ultrathin polymer films (d < 200 nm), the observed regularities can be summarized as follow. The homologues with NO2 tail group and m > 8 demonstrate strong preference for in-plane alignment, which is random in the initial state and uniaxially ordered in the saturated state of irradiation. The change of the tail substitute in the succession NO2 → OCH3 → OC4H9 leads to transition from the in-plane to the out-of-plane alignment of azochromophores (nonirradiated films), increase of the tendency of isotropic ordering (excitation within ππ* absorption band), and transition from the positive (prolate) to the negative (oblate) in-plane uniaxial order (excitation within nπ* absorption band). The observed regularities are explained by competition of photoorientation determined by symmetry of light and intrinsic self-organization determined by the structure of polymer molecules.
The frequency dependences of the imaginary ε″ and real ε′ parts of complex dielectric permittivity inherent to planarly aligned layers of nematic liquid crystals 5CB doped with multiwalled carbon nanotubes (CNT) were investigated in a wide range of frequencies (f = 10-2-10 6 Hz) and CNT concentrations (c = 0-0.25 wt.%). It has been shown that the studied frequency range can be divided in three parts according to behavior of ε′ (f) and ε″ (f) curves. The low-frequency range (10-2 < f < 10 1 Hz) reflects the near-electrode processes in the cell. The ratio ε″/ε′ used to characterize these processes sharply grows if the concentration of CNT exceeds 0.05 wt.%. The moderate frequency range (10 1 < f < 10 5 Hz) corresponds to the alternating current conductivity, σ АС. At the nanotubes concentration less than 0.025 wt.%, σ АС does not depend on the frequency that implies its ionic origin. In its turn, at the c ≥ 0.025 wt.%, σ АС is a power function of the frequency that is typical for electronic hopping mechanism. The transition from the ionic to electronic conductivity can be explained by the percolation theory with the critical concentration of nanotubes 0.031 wt.% and percolation parameter 2.5. The high-frequency range (10 5 < f < 10 6) is mainly attributed to dipole volume polarization. For c < 0.05 wt.% such polarization is well described by the Debye equation. The time of dielectric relaxation in this frequency range increases with nanotubes content. This is explained by effective interaction of nanotubes with 5CB molecules.
In the present study, the influence of diamond nanoparticles (DNPs) on dielectric properties of nematic liquid crystal (LC) E7 from Merck has been considered. It is established that the insertion of DNPs leads to an increase in the dielectric constant ε', as well as to a significant change in the electric conductivity σ of the LC. The growth of ε' with the concentration of DNPs, CDNP, is mainly caused by a contribution of the DNP permittivity to the effective permittivity of the composite. The character of the σ(CDNP) curves depends on the ionic purity of LC E7: for the samples based on pure E7, an increase of the electric conductivity with the concentration of DNPs is detected, whereas the reverse trend is observed for the samples containing impure E7. This behavior is attributed to the competitive adsorption and desorption of ions on/from the surface of DNPs and the ion transfer along the percolation network of theseparticles.
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