Dynamics and Self-Healing of Layer-by-Layer Hydrogen-Bonded Films of Linear Synthetic Polyphenols

This work explores the dynamics of hydrogen-bonded layer-by-layer (LbL) films of linear synthetic polyphenols (lPPh's) with different backbone and pendant group structures. The polymers feature repeat units with catechol-like or gallol-like polyphenol rings, namely, poly(3,4-dihydroxybenzyl methacrylamide) (P2HMA), poly(3,4-dihydroxybenzyl acrylamide) (P2HAA), poly(3,4,5-trihydroxybenzyl acrylamide) (P3HAA), and poly(3,4,5-trihydroxybenzyl methacrylamide) (P3HMA), and were assembled with linear poly(ethylene oxide) (PEO). The structure of the lPPh's has a major effect on the diffusivity of LbL films, and chain dynamics is asymmetric for lPPh- and PEO-terminated films during film construction. Specifically, diffusivity of polyphenols in the direction perpendicular to the substrate varied from values below ∼10-17 cm2 s-1 to ∼10-14 cm2 s-1 for lPPh's of the P2 and P3 families, as assessed by in situ ellipsometry during film assembly. The uptake of the most diffusive P3HMA exhibits subdiffusive behavior with an anomalous diffusion exponent β ≈ 0.3. Similarly, large differences in film dynamics were revealed by neutron reflectometry, which detected fast penetration of deuterated PEO (dPEO) through the entire film with a diffusion coefficient >10-12 cm2 s-1 through P3HMA/PEO films but accumulation of dPEO only at the film surface for all other lPPh/PEO systems during the first 10 min of exposure to dPEO. The observed trends in film dynamics were consistent with strongly exponential growth of P3HMA/PEO films and largely linear deposition of nondiffusive P2/PEO systems. Finally, the self-healing behavior of lPPh/PEO films in an aqueous environment was quantified by in situ atomic force microscopy experiments, which revealed robust self-healing of P3/PEO films occurring on a time scale of minutes and an absence of film healing for P2/PEO films.