Structure-phase morphology - Property relationship of a series of light-emitting alternating copolymers with distyrylbenzenes segments and oligo(ethylene oxide) spacers

A series of light-emitting alternating copolymers: DSB-TEO, DSB-SEO, DSB-PEO1500 and dimethoxy(DM)DSB-SEO, consisting of 1,4-distyrylbenzene (DSB) and different length of ionic conducting oligomer ethylene oxide (OEO) spacers, were synthesized. The thermal stabilities, electrochemical behavior and photoluminescent properties of these polymers were studied systematically. It was found that with increasing OEO chain length, the solubility, film-forming property and relative fluorescence quantum yield were improved accordingly. The introduction of two methoxy groups as side chains into the DSB segment to form DMDSB not only enhanced the film-forming property, but also improved the quantum efficiency of the polymer. Phase contrast microscopy, polarized optical microscopy and atomic force microscopy (AFM) were employed to investigate the phase morphology of the thin films prepared from the pure polymer or polymer/LiCF3SO3 blends. Results revealed that compared with pure polymer thin films, there was significant change in the phase morphology of the thin films prepared from polymer/LiCF3SO3 blends. The reason could be attributed to the contrary roles of dissolved and undissolved lithium salt played in the polymer matrix, which was discussed in detail in the paper. Phase morphology study of DMDSB-SEO revealed that, compared with DSB-SEO, there was much less crystallization of OEO segments and the solubility of lithium salt had increased greatly. Furthermore, AFM was used to investigate the dependence of surface morphology on the spin-coat processing conditions, such as the solvents, substrates and rotation rate. A light-emitting electrochemical cell device of the copolymer was fabricated, and the brightness and maximum external quantum efficiency of the device were investigated.