Chemical recognition of antioxidants and UV-light stabilizers at the surface of polypropylene: Atomic force microscopy with chemically modified tips

We show in this paper that it is possible to locally detect additives on the surface of polypropylene with chemically modified atomic force microscopy (AFM) tips. Gold-coated AFM tips modified with methyl and hydroxyl terminated self-assembled alkanethiol monolayers were used to measure adhesion forces on a process-stabilizing agent (Irgafos 168)., an antioxidant (Irganox 1010), and UV-light stabilizers (Tinuvin 770, Dastib 845, Chimassorb 944, and Hostavin N30). Pull-off force measurements carried out on these pure additive films have shown that it is possible to discriminate between antioxidants and UV-light stabilizers. We have evidenced a characteristic fingerprint for each additive, according to the functionality of the tip used and the medium wherein the force measurements are realized (water or nitrogen atmosphere). Similarly we have measured pull-off forces on a melt-pressed polypropylene sample stabilized with Irgafos 168, Irganox 1010, and Tinuvin 770. These adhesion force measurements show that the extreme surface of the polymer is mainly made of a layer of Tinuvin 770. These results have been compared to those obtained from time-of-flight secondary ion mass spectrometry measurements.