Investigation of thermochromism in a series of side-chain, liquid-crystalline, azobenzene-containing polymers

The thermochromic properties of a series of liquid crystalline polymethacrylates, containing azobenzene side-chains with variable spacer lengths, were investigated. Annealing the amorphous polymer thin films above the glass transition temperature results in a rearrangement of the azobenzene moieties, causing a hypsochromic shift in the electronic absorption spectra. A detailed investigation of the spectral shift was performed by in situ UV-vis spectroscopy and indicated the formation of H-type aggregates by the side-chain chromophores above the glass transition temperature. The rate at which the hypsochromic shift occurs is faster for polymers with shorter spacers since their high glass transition temperature results in a higher thermal energy during the thermochromic effect. Experimentally determined activation energies show that the aggregation occurs primarily due to side-chain relaxation (beta-relaxation) and main-chain relaxation (alpha-relaxation). Further annealing above the isotropization temperature resulted in the onset of deaggregation and in most cases showed that the chromophores were freed from the ordered state.