Thermochromism of a liquid crystalline dialkoxy substituted poly(1,4-phenylene-1,3,4-oxadiazol-2,5-diyl)

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 dif ferential scanning calorimetry, small- and wide-angle X-ray scattering tech niques. An order-order transition was found at about 120 degreesC, which wa s 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 (d(1) = 1. 70 nm) and an alkyl chain rich layer (d(2) = 2.00 nm). For the smectic A ph ase we calculated d = 3.85 nm, d(1) = 1.75 nm and d(2) = 2.10 nm. The lamel lae of the smectic A phase are assumed to show undulations with a periodici ty of 6.3 nm. By contrast, the smectic H phase has no periodic undulations. The C16pod changes its color from yellow-green at 25 degreesC to blue at 1 30 degreesC. UV-vis and fluorescence spectroscopy were used for a quantitat ive determination of the thermochromicity of C16. It was found that the opt ical properties change continuously within a temperature range of 25 to 140 degreesC. Even at the order-order transition the UV-vis and fluorescence i ntensities change continuously. It was therefore concluded that the effecti ve conjugation length of the C16 reduces continuously with increasing tempe rature. The order-order transition is caused predominantly by the melting o f the side-chains.