Novel ionizing radiation induced reactions of poly(3-hexylthiophene)

Abstract

Conjugated polymers are integral components in many optoelectronic devices, and many studies have focused on the connection between polymer morphology and device performance. There are many methods used to probe this correlation, including the use of synchrotron sources for x-ray scattering and reflectivity experiments. Though these types of experiments are useful in studying these materials, as x-ray sources have gotten stronger, alterations to the polymer chain molecular structure can occur from exposure to ionizing radiation. Most studies have examined thin films of conjugated polymers and the effects of ionizing radiation, with few examining solutions of conjugated polymers. This study monitors the time evolution of changes in poly(3-hexylthiophene) (P3HT) molecular structure when dissolved in deuterated styrene and exposed to synchrotron x-rays. In particular, the kinetics of structural evolution after exposure to X-rays is monitored, where we report the unexpected results that the ionizing radiation induced a reaction cascade, that continues long after removal of the x-ray beam. The scattering data, in conjunction with size exclusion chromatography and Fourier transform infrared spectroscopy, show a growth in the polymer structure that continues long after removal of the x-ray beam. Fitting the scattering data to the Gaussian Lorentz gel model and normalization to the sample concentration and exposure time offers insight into the relative dependence of the observed changes to the beam exposure time. These analyses lead to the conclusion that the structural changes are induced by the incorporation of styrene on the polymer chains, which can then further polymerize to produce grafts of deuterated polystyrene. The results in this paper therefore provide insight into the stability of conjugated polymers in the presence of reactive diluents and ionizing radiation, that reactions induced by exposure to ionizing radiation may cascade long after exposure, and that exposure time to the x-ray beam controls the reaction rate. This also opens novel reaction pathways to forming graft copolymers.