AFM STUDIES OF SELF-HEALING POLYMER FILMS BASED ON THIOL-DISULFIDE EXCHANGE REACTIONS

have attracted attention due to their built-in ability to repair physical damage, effectively extending the material lifetime. Many self-healing systems that have been developed focus on molecular level interactions, in which reversible interactions are of particular interest due to the ability to heal damage repeatedly at the same position. Some systems are still limited in practice due to the requirement for external stimuli; Herein, we present a new approach to self-healing polymeric materials based on reversible covalent cross-linking utilizing thiol/disulfide dynamic exchange reactions. Polymers with a branched architecture 5-7 were selected for this study due to the expected low intrinsic viscosity. The polymer gel structure was realized by synthesizing star polymers containing thiol (SH) groups at the arm peripheries, via atom transfer radical polymerization (ATRP). 8-11 The synthesized polymers were solution processed into films in which the peripheral SH groups were oxidatively cross-linked to form S-S bonds. The subsequent evaluation of the self-healing process was conducted by micro- machining damage into the film surface, facilitated by atomic force microscopy (AFM), and followed by time-lapse AFM imaging of the repair process. Experimental Materials. All monomers and the cross-linker, including n-butyl acrylate (BA, 99%) and ethylene glycol diacrylate (EGDA, 90%) were purchased from Aldrich and purified by passing through a column with basic alumina to remove antioxidants and inhibitors. CuBr (98%, Acros) was purified with acetic acid. All other reagents: ethyl 2-bromopropionate (EBrP), N,N,N',N'',N''-pentamethyldiethylenetriamine (PMDETA), CuBr2, tributylphosphine (Bu3P), iron trichloride (FeCl3), iodine, 1,2,7,8- diepoxyoctane, 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU), and solvents were purchased from Aldrich at the highest purity and used as received. SS cross- linking agent bis(2-methacryloyloxyethyl) disulfide (DSDMA) 12-13 was