Visualization of Mechanochemically-Assisted Degrafting of Surface-Tethered Poly(Acrylic Acid) Brushes

Abstract

We report visualization of mechanochemically assisted degrafting of surface-tethered poly­(acrylic acid) (PAA) brushes in a basic aqueous buffer at nanometer to micrometer length scale by monitoring changes in local etching of silicon substrates. PAA brushes were prepared by surface-initiated atom transfer radical polymerization and incubated in 0.1 M ethanolamine buffer (pH 9.0) with 0.5 M NaCl. Morphological changes of the underlying substrates were monitored by scanning electron microscopy and atomic force microscopy. The appearance of regular-shaped pits indicated etching of the substrate, and both their number and size grew with increasing incubation time. We compared the etching behaviors for PAA, poly­(methyl methacrylate) (PMMA), and poly­(poly­(ethylene glycol) methacrylate) (PPEGMA) brushes grafted on silicon substrates. After incubation for 7 days, the substrate of PMMA brush remained intact. In PAA brush systems, we detected the formation of a few large pits whose size grew in time. Many pits showed up on the substrate of PPEGMA brush but with substantially smaller size compared to PAA. Our findings suggest that hydrophobicity and stability of the grafted polymers play an important role in the morphological changes of the underlying silicon substrates under given incubation conditions.

Polyelectrolytes as Building Blocks for Next-Generation Membranes with Advanced Functionalities

“… 103 , 104 This can potentially lead to degrafting of the PEBs, as has been observed for PAA brushes attached to Si wafers kept in 0.1 M ethanolamine buffer (pH 9.0) with 0.5 M NaCl. 105 Similar degrafting has been observed for carboxylated poly(oligo(ethylene glycol) methacrylate-random-2-hydroxyethyl methacrylate) (poly(OEGMA-r-HEMA)) brushes grafted from metal surfaces and kept in phosphate buffered saline solutions 106 and even for PSPMA brushes attached to Si wafers, exposed to humid air. 107 To prevent such degrafting of PEBs, several strategies for strong anchoring of the brushes have been developed (see Figure 2 ).…”

Spontaneous Degrafting of Weak and Strong Polycationic Brushes in Aqueous Buffer Solutions

“…We can make a few conclusions from the experimental results discussed previously and from earlier publications. 40,44,45,[65][66][67] From the data in Figure 1, degrafting involves base-catalyzed hydrolysis that takes place in the initiator centers. The BMPUS initiators have two kinds of bonds that can be hydrolyzed: 1) ester/amide bond close to the initiating center, and 2) siloxane bond.…”

“…The fluorine ions in TBAF selectively cleave the Si–O bonds, leading to degrafting. − We classified this mode as “on-demand degrafting”. In a series of studies, we reported that polyanionic brushes detached from substrates upon increasing pH of the solutions that induced strong swelling due to electrostatic charging and concurrent hydrolysis of ester bonds in the initiator. − This is considered as “spontaneous degrafting”. Here, we explore the effects of charge density, MW, and grafting density on degrafting of weakly and strongly charged polycationic grafts and discuss the contributions due to the structure of the polymer chain as well as the initiator molecule. ,,,− …”

Kinetic Study of Degrafting Poly(methyl methacrylate) Brushes from Flat Substrates by Tetrabutylammonium Fluoride

“…Charged or chargeable polymer brushes with permanent or pH-induced charge, respectively, may degraft spontaneously from the substrate due to large swelling that imposes strong force on the linker, which keeps the polymer attached to the substrate. Indeed, strong swelling inside the brush may induce mechanochemical breakage of the chemical bonds in the linker region and ultimately result in liberating the grafted chains from the surface. − While these examples illustrate cases of “spontaneous” degrafting, which occurs primarily in polyelectrolyte systems, “on-demand” degrafting of polymers from surfaces may also be accomplished. This involves chemically assisted cleaving of polymers, regardless of their chemical composition, from the substrate.…”