Abstract
Polymer brushes are widely used to alter the properties of interfaces. In particular, poly(ethylene glycol) (PEG) and similar polymers can make surfaces inert toward biomolecular adsorption. Neutral hydrophilic brushes are normally considered to have static properties at a given temperature. As an example, PEG is not responsive to pH or ionic strength. Here we show that, by simply introducing a polymeric acid such as poly(methacrylic acid) (PMAA), the highly hydrated brush barrier can change its properties entirely. This is caused by multivalent hydrogen bonds in an extremely pH-sensitive process. Remarkably, it is sufficient to reduce the pH to 5 for complexation to occur at the interface, which is two units higher than in the corresponding bulk systems. Below this critical pH, PMAA starts to bind to PEG in large amounts (comparable to the PEG amount), causing the brush to gradually compact and dehydrate. The brush also undergoes major rheology changes, from viscoelastic to rigid. Furthermore, the protein repelling ability of PEG is lost after reaching a threshold in the amount of PMAA bound. The changes in brush properties are tunable and become more pronounced when more PMAA is bound. The initial brush state is fully recovered when releasing PMAA by returning to physiological pH. Our findings are relevant for many applications involving functional interfaces, such as capture–release of biomolecules.
Highlights
Polymer brushes have proved important for many applications and for investigating fundamental properties of macromolecules using surface-sensitive techniques
Inspired by previous work on hydrogen bond interactions with polyacidic brushes,[10,12,20] we investigated how a poly(ethylene glycol) (PEG) brush responded when exposed to polymeric acids at different pHs using surface plasmon resonance (SPR) and quartz crystal microbalance with dissipation monitoring (QCMD)
The PEG brushes have the advantage of measurable grafting density (0.25 ± 0.05 nm−2 depending on the exact batch of thiol-PEG and incubation time) since the molecular weight is known
Summary
Polymer brushes have proved important for many applications and for investigating fundamental properties of macromolecules using surface-sensitive techniques. It is established that proteins may bind to polyelectrolytes, but not how this influences brush thickness and rheology Such changes should be investigated using well-defined chemical interactions between additives and the monomers of the grafted coils. In this work we show how intermolecular hydrogen bonding, a phenomenon that has long been studied in the liquid bulk,[19] can be transferred to an interface and used to drastically alter the properties of a simple hydrophilic brush This is achieved by introducing a polymeric acid at low pH where it is protonated to a high extent. All SPR measurements were performed at a set temperature of 25 °C at a flow rate of 20 μL/min for measurements in liquid (except for the PEG grafting solution at 2 μL/min). The substrate was inserted into the SFA cell and probed against a freshly cleaved backsilvered mica surface glued on a glass disk with the same radius of curvature
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