Abstract

The thermotropic liquid crystalline behavior of poly[2′,5′-bis(hexadecyloxy)-1,4-phenylene-1,3,4-oxadiazol-2,5-diyl] C16pod was investigated using differential scanning calorimetry, small- and wide-angle X-ray scattering techniques. An order–order transition was found at about 120 °C, which was attributed to the transition from a smectic H phase at a low temperature to a smectic A phase at a high temperature. The repeat unit of the smectic H phase is d = 3.70 nm which is composed of a polymer-rich layer (d₁ = 1.70 nm) and an alkyl chain rich layer (d₂ = 2.00 nm). For the smectic A phase we calculated d = 3.85 nm, d₁ = 1.75 nm and d₂ = 2.10 nm. The lamellae of the smectic A phase are assumed to show undulations with a periodicity of 6.3 nm. By contrast, the smectic H phase has no periodic undulations. The C16pod changes its color from yellow–green at 25 °C to blue at 130 °C. UV-vis and fluorescence spectroscopy were used for a quantitative determination of the thermochromicity of C16pod. It was found that the optical properties change continuously within a temperature range of 25 to 140 °C. Even at the order–order transition the UV-vis and fluorescence intensities change continuously. It was therefore concluded that the effective conjugation length of the C16pod reduces continuously with increasing temperature. The order–order transition is caused predominantly by the melting of the side-chains.