Anisotropic charge transport in ion-conductive photoresponsive polyethylene oxide–based mesomorphic materials

The mechanism of charge motion in conductive and photosensitive mesogenic block copolymers containing polyethylene oxide (PEO) segments is investigated over a wide frequency and temperature range with the broadband dielectric spectroscopy technique. It is found that the ultraviolet (UV) irradiation, the UV intensity, and the anchoring conditions of mesogenic unit in the cells produce changes in conductivity properties and in the molecular arrangement. The anisotropic nature of the conductivity is established.

Figure 1

(a) Chemical structure of rodlike shaped block PEO-based derivatives, (b) UV/Vis spectrum of pure PEOC6 in $\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{l}}_2$ before and after UV light irradiation for 15 min at 365 nm; insert: time dependence of UV irradiation on the intensity of the $\pi -{\pi }^{*}$ transition absorption band $(\lambda =333\mathrm{nm})$, (c) DSC trace of PEOC6 measured on cooling $(\mathrm{rate}5{}^{\circ }\mathrm{C}·\mathrm{mi}{\mathrm{n}}^{-1})$; POM microphotograph $(T=36{}^{\circ }\mathrm{C})$ showing the nematic texture of PEOC6 and PEOMe in homogeneous aligned cell.

Figure 2

Frequency dependences of the complex conductivity and the complex dielectric function for PEOC6:$\mathrm{LiC}{\mathrm{F}}_3\mathrm{S}{\mathrm{O}}_3$ (5:2) at different temperatures; symbols are experimental data points, and solid lines are the best fit curves obtained with the theoretical approach developed by Dyre. The error bars are comparable to the size of the symbols. Attention is paid to the charge transport processes (fitted part of curves) even if electrode polarization (PE) effects occur on the low-frequency side.

Figure 3

(a) Normalized plots of the real part of the complex conductivity versus normalized frequency scaling with respect to temperature for PEOC6:$\mathrm{LiC}{\mathrm{F}}_3\mathrm{S}{\mathrm{O}}_3$ (5:2). (b) BNN plot ${\sigma }_{DC}$ versus ${\omega }_e$; the line is a linear regression to these data. (c) Temperature dependence and inverse temperature dependence of the hopping radial frequency ${\omega }_e$ and the DC conductivity. The continuous solid line represents the fit of experimental data to Eq. (3) [obtained fit values are for hopping radial frequency curve: ${{\omega }_e}_0=2.1(\pm 0.5)\times {10}^{11}{\mathrm{K}}^{0.5}·\mathrm{rad}·{\mathrm{s}}^{-1},E=11100\pm 300\mathrm{J}·\mathrm{mol}$ and $T_0=177\pm 4\mathrm{K}$, and for DC conductivity: ${\sigma }_{DC0}=32\pm 5{\mathrm{K}}^{0.5}·\mathrm{S}·{\mathrm{m}}^{-1},E=10400\pm 300\mathrm{J}·\mathrm{mol}$ and $T_0=177\pm 4\mathrm{K}$].

Figure 4

Temperature dependence and inverse temperature dependence of DC conductivity ${\sigma }_{DC}$ of pure PEO or doped PEO. Symbols are experimental data points and solid lines are the best fit curves to modified Vogel-Fulcher-Tammann equation [Eq. (4)].

Figure 5

Influence of UV irradiation (365 nm) under various intensities for PEOC6:$\mathrm{LiN}{\left(\mathrm{S}{\mathrm{O}}_2\mathrm{C}{\mathrm{F}}_3\right)}_2$ (5:1) into a homogenous cell at $30{}^{\circ }\mathrm{C}$: (a) time dependence of the DC conductivity, insert percentage of loss of conductivity obtained with Eq. (5). Symbols are experimental data points and solid lines are the best fit curves to Eq. (6). (b) Rate constant (1/τ) of the process versus UV intensity.

Figure 6

Influence of anchoring: (a) Temperature dependence and inversed temperature dependence of ionic conductivity for PEOMe:$\mathrm{LiN}{\left(\mathrm{S}{\mathrm{O}}_2\mathrm{C}{\mathrm{F}}_3\right)}_2$ (5:1) for a homogenous or homeotropic anchoring with or without UV irradiation. The solid lines represent the fit of experimental data to the modified VFT equation. The optical micrograph for a homogeneous anchoring before and after UV irradiation, (b) Percentage of DC conductivity loss when UV light is switched on for a reduced temperature $T/T_{NI}$ equal to 0.58.

Figure 7

Schematic of the segmental motion assisted diffusion of $\mathrm{L}{\mathrm{i}}^{+}$ between the PEO moieties of different molecules for a planar (a) and a homeotropic (b) anchoring. The circles and the rectangles, respectively, represent the ether oxygens of PEO chains and the mesogenic blocks of the molecules.