Instability of Surface-Grafted Weak Polyacid Brushes on Flat Substrates

Abstract

We study the stability of weak polyacid brush (WPAB) gradients in aqueous media covering a range in grafting density (σ) spanning 0.05−0.5 chains/nm 2 using two analogous surface-anchored bromoisobutyrate-based initiators for atom transfer radical polymerization (ATRP) bearing either an ester or amide linker. Variations in dry thickness of ester-based WPABs as a function of time and pH are consistent with WPAB degrafting via linker hydrolysis catalyzed by mechanical tension in the grafted chains. Sources of tension considered include high σ, as well as swelling and electrostatic repulsion associated with increasing degree of deprotonation (α) of repeat units in the WPAB. Normalized thickness of the WPAB decreases by a maximum amount at intermediate σ between ∼0.05−0.15 chains/nm 2 , implying that contributions to tension by α are counterbalanced by charge regulation in the WPAB at high σ. Amide-based WPABs are more stable up to 264 h incubation, suggesting that commonly used ester-bearing ATRP initiators are more susceptible to hydrolysis over the time scales examined.

Entropic death of nonpatterned and nanopatterned polyelectrolyte brushes

“…The results suggest that as the degree of charging in the brush increases, the stability of the layer is decreased. Similar to our finding, Galvin et al 31 observed an increasing degradation rate depending on the degree of charging. They immersed weak polyacid brushes in buffer solutions with different pHs.…”

“…This explanation is also in agreement with the literature in so far that the presence of nucleophiles enhances the degrafting process of polymer brushes. 31,41,63 It was reported that a tethered macromolecule experiences the largest tension in the surface linker that binds the polymer chain to the rigid surface 45,46 which enhances the degrafting. 30 Since the tension on the polymer brushes is directly related to the grafting density, a series of PMeVP brushes with graft densities ranging from 0.0069 to 0.1693 μmol m −2 were exposed to water at 22 C for different periods of time to determine the effect of grafting density on the stability of strong PEBs.…”

“…Within the last decade, there has been an increasing number of reports about degradation of polymer brushes. [23][24][25][26][27][28][29][30][31][32][33] A first report of Tugulu and Klok described the delamination process of poly[poly(ethylene glycol)methacrylate] brushes upon exposure to a cell culture medium. 23 In another report by Borozenko et al, 27 degrafting was described for poly(acrylic acid) brushes using total internal reflection fluorescence microscopy.…”

“…40 Thus, polymer brushes are inherently metastable systems where the supply of additional energy such as a mechanical activation can lead to chain fracture and degrafting. 31,41 The grafting density plays an important factor in the brush stability. An increasing number of chains per surface area lead to a stronger chain stretching and therefore a larger tensile stress on the chains.…”

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.…”