Synthesis and characterization of conjugated polymers with polyhedral oligomeric silsesquioxane pendant groups

Ertan, Salih
In this thesis, a series of monomers covalently bonded to polyhedral oligomeric silsesquioxane (POSS) pendant group was designed and synthesized. POSS group is a hybrid organic-inorganic nanostructure with 1 to 3 nm diameter. POSS which has a cage structure is mainly composed of silicon (Si) (at vertices) and oxygen (O) (in the middle of the edge). POSS nanostructure can be modified through the vertices with organic functional groups. POSS groups provide different properties, such as thermal robustness, mechanical endurance, electrochemical stability etc. to the matrices in which they are added. These groups are attached to the monomers chemically and the possible effects of the POSS units on electrochemical and optical behaviors, thermal stability and solubility of the polymers have been investigated. Alkyl-substituted POSS cage is combined with 3,4-ethylenedioxythiophene (EDOT-POSS), 3,4-propylenedioxythiophene (ProDOT-POSS) and 3,4-ethylenedioxyselenophene (EDOS-POSS). Chemical and electrochemical polymerizations of EDOT-POSS monomer were achieved successfully and the resulting polymers (PEDOT-POSS) were found to be soluble. Both chemically and electrochemically obtained polymers are soluble in common organic solvents like dichloromethane, chloroform, tetrahydrofuran etc. PEDOT-POSS has somewhat higher band gap (1.71 eV with λmax= 618 nm) than its parent PEDOT (1.60 eV with λmax= 627 nm) and as expected PEDOT-POSS exhibits higher optical contrast (74% at 618 nm) and coloration efficiency (582 cm2/C for 100% switching), lower switching time (0.9 s), higher electrochemical stability (93% of its electroactivity retains after 5000 cycles under ambient conditions) when compared to PEDOT. Photocatalytic activity of PEDOT-POSS polymer on dye removal was also investigated. When compared to parent PEDOT polymer, PEDOT-POSS showed better performance on removal of dye in shorter exposure time to UV light. While PEDOT can remove 95% of the dye in an hour, PEDOT-POSS can remove nearly all the dye in the same period of time. In addition, ProDOT-POSS was also polymerized chemically and electrochemically resulting in soluble polymers (PProDOT-POSS). Both chemically and electrochemically obtained polymers were completely soluble in common organic solvents like toluene, dichloromethane, chloroform, tetrahydrofuran, etc. It is possible to dope and dedope the polymers successfully in film and solution forms via both chemical and electrochemical methods. Electroactive polymer film has a band gap of 1.95 eV with a maximum absorption band at 555 nm. Also, soluble polymers have a reddish orange emission centered at 605 nm in toluene solution. Furthermore, PProDOT-POSS polymers have a percent transmittance change of 55% at 555 nm, high transparency at oxidized state, low switching time (~1.0 s) and high coloration efficiency (504 cm2/C for 95% switching). Finally, electroactive EDOS-POSS monomer was successfully polymerized via both chemical and electrochemical methods. The obtained polymer called PEDOS-POSS was solution-processable and soluble in common organic solvents like tetrahydrofuran, toluene, dichloromethane, chloroform, etc. PEDOS-POSS polymer exhibited electrochromic behavior: pure blue when neutralized and highly transparent when oxidized. When compared to the parent PEDOS (1.40 eV with λmax= 673 nm), the polymer film has a somewhat higher band gap (1.50 eV with λmax= 668 nm and 724 nm). Also, PEDOS-POSS exhibited high optical contrast ratio (59%) and coloration efficiency (593 cm2/C for 95% switching) with a low switching time (0.9 s) due to the presence of POSS cage in the polymer backbone. Furthermore, the polymers exhibited fluorescent properties and exhibited a pinkish orange emission centered about at 640 nm.