Switching time, dynamic scattering

The specific electrical conductivity of pure Azoxy-compounds lies between 10 (12 cm)" and 10 ( 2 cm) h Various dopants are added to the Azoxy-compound to influence the conductivity and orientation. The effect of these additives on the variation with time of the electrical conductivity, of the switching times, and of contrast can be measured. It has been found that after roughly 500 hours of operation for instance the cell conductance when measuring the d.c. current flowing through the cell decreases by one to two powers of ten. As subsequent a.c. measurements revealed, this phenomenon results from the formation of double layers with lower electrical conductivity near the electrodes. These double layers disturb the ion injection from the electrode. As a result of low current densities the dynamic scattering disappears almost entirely. 

From the analysis described above, one can deduce the viscoelastic and surface anchoring parameters of the hquid crystalline material phase separated from the polymer matrix, which in our VIS samples have the values K/rj = (0.3 0.06) 10 °m s and A = 65 35 nm. The observed diffusivity K/t] is almost four times lower than the diffusivity of a pure liquid crystal used to prepare the mixture. This result demonstrates that to properly explain the dynamic properties of the H-PDLC gratings, such as switching-on and -off times, it may not be relevant to take viscoelastic parameters of the liquid crystal substances as used for the mixture, but one needs to measure the properties of a nematogenic material formed in the phase separation process. Since the scattering experiments require no special sample treatment, it is one of the most convenient techniques to perform this task. The observed decrease of K/rj is attributed to the increased viscosity of a liquid crystal medium due to the presence of dissolved parts of the polymer chains [26,40]. The observed value of A results inW = (2.1 1.1) 10 J m , which corresponds to a quite strong anchoring. 

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