Molecular-Scale Analysis of the Polymerization of Poly (3,4- Ethylenedioxythiophene) (PEDOT)

Abstract

The conjugated polymer Poly (3,4- ethylenedioxythiophene) (PEDOT) and its derivates have attracted a great amount of interest as organic conducting materials due to their high chemical stability, low oxidation potential, biocompatibility and high conductivity. There has been interest in the use of PEDOT as an interfacing material between hard electronic devices and soft ionically conducting human tissue. Due to its compliant mechanical properties and ability to conduct both ionically and electronically, PEDOT becomes a promising candidate material for this application.

Despite the promise of these materials there are challenges in developing this application. PEDOT is insoluble in all known solvents. This puts limitations on our ability to assign chemical signatures for these molecules. It makes it difficult to measure the molecular weight of these polymers since traditional techniques require solubility for the measurement. Vibrational spectroscopy is a chemical characterization technique that does not require solubility to give us chemical information. In the second and third chapters of our thesis we design a method for using Raman spectroscopy and inelastic neutron scattering for predicting the molecular weight and degree of polymerization. We use vibrational spectra calculations using Density function theory to extrapolate our experimental results and predict the molecular weight of molecules from their vibrational spectra. This also allowed us to assign peaks to specific vibrational modes and thus we were able to catalogue modes and their characteristics for PEDOT and its intermediates. We were also able to comment on the transitions we see in the chemical signation as the the polymerization proceeds.

We found that the peaks at 1424 cm^-1 and 3110 cm^-1 in the Raman spectra which are indicative of the thiophene C=C and thiophene C-H were the most useful for measuring the changes during polymerization. The C=C peak increases in intensity as the polymerization progresses due to the increase in the conjugation of PEDOT, while the C-H stretching peak decreases in intensity due to the abstraction of the proton from the thiophene C-H. Combing experimental results with DFT calculations we were able to form a model based on DFT calculations to relate the molecular weight of PEDOT with the relative intensities of these two modes.

We performed inelastic neutron scattering studies on PEDOT and its intermediates and for the first time catalogued these modes. We found that the resolution and intensity of C-H bending modes was excellent. Using DFT calculations we were able to predict the changes seen in these modes as a function of chain length.

We also studied properties of newly synthesized maleimide functionalized PEDOTs (PEDOT maleimide) and their monomers as a possible neural interfacing material. We measured the thermal and crystallographic properties of the homochiral and racemic versions of the monomers. Homochiral EDOT-maleimide form large crystals with a trigonal unit cell symmetry while the racemic crystals have a monoclinic unit cell with a threefold helical stack. We analyse these differences and their effect on thermal properties of the material.

We also explored the possibility of using PEDOT-maleimide for biofunctionalization and biosensing by successfully demonstrating the attachment of Immunoglobulin-G antibodies on PEDOT films using Quartz crystal microscopy. The new functionalized molecules together with the older established PEDOT form an important part of the solution to neural interfacing. The development of characterization methods is a important process allowing us to unlock the potential of these molecules.