Enhancing the photo-oxidation resistance of PPV-type polymers: effect of a radical scavenger addition
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Enhancing the photo-oxidation resistance of PPV-type polymers: effect of a radical scavenger addition G. R. Ferreira1, B. Nowacki3, E. R. deAzevedo2, L. C. Akcelrud3 and R. F. Bianchi1 1 Universidade Federal de Ouro Preto, Departamento de Física, Laboratório de Polímeros e Propriedades Eletrônicas de Materiais 2 Universidade de São Paulo, Instituto de Física de São Carlos 3 Laboratório de Polímeros Paulo Scarpa, Universidade Federal do Paraná key words: PPV-type polymers, photodegradation, radical scavenger ABSTRACT A hindered phenol type radical scavenger (Irganox 1010®) was used to improve the resistance to photo-oxidation in PPV-type polymers. The degradation was performed under blue light (460 nm), and the effect of the added compound was investigated following the intensity variation of absorbance and photoluminescence spectra. The results showed that the addition of the compound brought about a significant enhancement in photodegration resistance without interfering in the electronic properties of the polymers. These results suggest that the free radical scavenger attenuates the decrease of the polymer conjugation length induced by the radiation. FTIR measurements confirm this evidence. INTRODUCTION Since the discovery of electroluminescence properties of conjugated polymers, a considerable number of researches have been done to investigate the semiconducting properties of this class of materials and their related applications in optoelectronic devices [1,2]. However, the structural degradation caused by radiation exposure is an obstacle for commercial applications of conjugated polymers in such a way that the control and investigation of optical and structural properties these materials under environmental conditions are important to improve their optoelectronic device stability and performance [3-15]. Due to the growing interest in the luminescent polymers stability, there are many studies oriented to their chemical degradation, mainly focusing on the role of oxygen, water and electrode material reactions with the active polymer layer. Although the free radical character of the degradation reactions has been demonstrated [5,16], a detailed study of the mechanism involved is still lacking and deserves further exploration, due its relevance. On the other hand, apart from clarifying the degradation pathways, these studies open the possibility of designing and developing dosimeters where the effects of nonionizing radiation on the optical properties of organic materials are more important than improving the luminance and lifetime of the light-emitting diodes made from them [17-21]. In this sense, understanding the PPVs photoxidation and stability are important to control the lifetime of these devices and thus to improve the performance of the radiation sensors [17-21]. In the present work, we investigated the photo-oxidation process and its effects in the optical properties and chemical structure of poly[2-methoxy-5-(2-ethyl-hexyloxy)-pphenylene-vinylene] (MEH-PPV), poly[2,5-bis(-dimethyl-octyloxy)1,4-phenylenev
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