Light and oxygen induce chain scission of conjugated polymers in solution

Abstract

Conjugated polymers have been widely studied as flexible, versatile semiconductors in organic electronics. However, the material stability is one of the problems limiting their applications. Thus, understanding the degradation process of conjugated polymers is crucial. In this work, we monitored the chain scission of the model polymer MEH-PPV in chloroform solutions under different conditions by assessing its molecular weight using gel permeation chromatography and optical spectral measurements. We showed that changes in the UV-VIS spectrum can be seen only when the degradation has already progressed substantially. The fluorescence spectrum was found to be almost totally insensitive to the degradation stage of the polymers. We demonstrate that chain scission in solutions happens even in the dark leading to a 15% decrease of the molecular weight after just one day of storage. If exposed to room light, the chain length decreases by about 10 times over one day of exposure. Using stronger light intensity or enriching the solution with oxygen accelerates the degradation process dramatically. The rate of the reaction follows approximately a square root dependence with light intensity and oxygen concentration. We conclude that some extent of polymer degradation is difficult to avoid in common laboratory practices since to prevent it, one needs to work in an oxygen-free atmosphere in the dark. Preparation of polymer films from partially degraded solutions might lead not only to losing the connection between the molecular weight and the opto-electronic properties but also to unintentional doping of the semiconductor by products of chain scission reactions.