Photo-Stability Improvement of Polystyrene-block-Polybutadiene-block-Polystyrene through Carbon Nanotubes

The photo-oxidation stability of Polystyrene-Polybutadiene-Polystyrene (SBS) based nanocomposites containing bare multi-walled carbon nanotubes (CNTs) and carbon nanotubes bearing hydroxyl functional groups (CNTs-OH) in comparison to that of pristine SBS has been widely investigated. 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-OH, 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. CNTs and even more CNTs-OH, are thus able to protect SBS against UVB exposure and this protection is particularly efficient at early stage of exposure, as established by FTIR analysis and trend of main mechanical properties as a function of photo-oxidation time. The efficient protection of SBS provided by CNTs could be understand considering different features: CNTs are able to shied UV-light and to act as radical scavenger. If the CNTs form aggregates into the matrix, their UV-light shielding effectiveness could be reduced, or indeed rescinded. Moreover, the CNTs aggregates lead to a local increase of the temperature, facilitating the radicals formation at the interface between the nanofillers and the host polymeric matrix. The interface region is a critical area for the oxidative degradation of the nanocomposite materials because the rapid formation of radicals and the availability of environmental oxygen, that advance significantly the degradation phenomena. Furthermore, in the interface region between the nanofillers and matrix, the radical scavenger activity of CNTs can play a determinant role for a deceleration of the degradation phenomena, protecting efficiently the polymers against thermo- and photo-degradation. The CNTs radical scavenging activity is related to the formation of acceptor-like localized states due to the presence of structural defects onto CNTs sidewalls. Additionally, the CNTs upon UV light are able to physisorbe the oxygen molecules which, being strongly bonded on the walls of carbon nanotubes, are not available for the oxidative phenomena. Furthermore, the presence of hydroxyl 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. The CNTs-OH can act as chain-breaking electron donor, similarly to the sterically hindered phenols, becoming responsible for radical termination during SBS degradation