1974 Volume 2 Issue 2 Pages 66-69
The dynamic viscoelastic properties of several liquid crystal states of cholesteryl myristate were studied. Cholesteryl myristate was selected as a material for study because it appears both in the form of cholesteric liquid crystals and smectic liquid crystals, depending on the temperature.
At small strain amplitudes, the dynamic viscoelastic properties were measured by means of a coaxial cylinder rheometer in the angular frequency range 0.23-2.96 sec-1, in the temperature range 70-80°C. Hysteresis loops for the cholesteric state change with frequency and show a non-linear pattern at the higher frequencies. Both the storage modulus G′ and the loss modulus G″ for the cholesteric state change with frequency. In contrast, the absolute modulus |G| for the smectic state does not change with frequency and the liquid crystal behaves as an elastic solid. At large strain amplitudes, the dynamic viscoelastic properties were measured by means of a cone-plate rheometer in the angular frequency range 0.135-5.39 sec-1, in the temperature range 90-50°C. Hysteresis loops for this material at a fixed frequency change with temperature. The hysteresis loops for the cholesteric state are elliptical, which is typical linear viscoelastic response, but for the smectic state, they are very much distorted. It has been pointed out that the dynamic behavior of liquid crystals is very strongly affected by their mechanical histories. And even cholesteric liquid crystals show liquid-like flow behavior after many cycles of sinusoidal strain. Although such rheotropic phenomena are very complicated ones, preliminary studies indicate that they are more evident in the cholesteric state than in the smectic state. Data were compared after 10 cycles of sinusoidal strain at a frequency of 0.27 sec-1. The dissipation energy ED for the cholesteric state increases with frequency in a somewhat different way from that for the smectic state. For the smectic state ED0, the extrapolated value of ED at zero frequency, decreases with increasing temperature.