Improvement of the photo-stability of polystyrene-block-polybutadiene-block-polystyrene through carbon nanotubes

The photo-stability of Polystyrene-Polybutadiene-Polystyrene (SBS) based nanocomposites containing bare multi-walled carbon nanotubes (CNTs) and carbon nanotubes bearing carboxylic functional groups (CNTs-COOH) in comparison to that of pristine SBS has been studied. The photo-oxidation of pristine SBS occurs through crosslinking reactions and oxidized species formation and both these processes begin at early stage of exposure. The formation of crosslinking, formerly in polybutadiene phase, assessed by spectroscopical (FTIR), mechanical, dynamic mechanical and rheological analysis, leads to occurrence of internal mechanical stresses in the solid state and the SBS samples become prematurely unable to stretch. SBS-based nanocomposites showed a significant improved photo-stability to the respect to pristine matrix, due to the presence of CNTs and even more, of CNTs-COOH, that are able to protect efficiently SBS against UVB exposure. All obtained results suggest that the used nanotubes are able to delay the degradation process because of their acceptor-like electron properties and their ability to shield UV-light. Besides, the CNTs act as radical traps, hindering the crosslinking and slowing down the oxidation process. The presence of carboxylic functional groups onto CNTs outer surface enhances the protection ability of CNTs due to the presence of a larger amount of surface defects, that improves their radical scavenging activity.