Behavior of Surface-Anchored Poly(acrylic acid) Brushes with Grafting Density Gradients on Solid Substrates:  1. Experiment

Abstract

We describe experiments pertaining to the formation of surface-anchored poly(acrylic acid) (PAA) brushes with a gradual variation of the PAA grafting densities on flat surfaces and provide detailed analysis of their properties. The PAA brush gradients are generated by first covering the substrate with a molecular gradient of the polymerization initiator, followed by the “grafting from” polymerization of tert-butyl acrylate (tBA) from these substrate-bound initiator centers, and finally converting the PtBA into PAA. We use spectroscopic ellipsometry to measure the wet thickness of the grafted PAA chains in aqueous solutions at three different pH values (4, 5.8, and 10) and a series of ionic strengths (IS). Our measurements reveal that at low grafting densities, σ, the wet thickness of the PAA brush (H) remains relatively constant, the polymers are in the mushroom regime. Beyond a certain value of σ, the macromolecules enter the brush regime, where H increases with increasing σ. For a given σ, H exhibits a nonmonotonic behavior as a function of the IS. At large IS, the H is small because the charges along PAA are completely screened by the excess of the external salt. As IS decreases, the PAA enters the so-called salt brush (SB) regime, where H increases. At a certain value of IS, H reaches a maximum and then decreases again. The latter is a typical brush behavior in so-called osmotic brush (OB) regime. We provide detailed discussion of the behavior of the grafted PAA chains in the SB and OB regimes.

Non-monotonic swelling of surface grafted hydrogels induced by pH and/or salt concentration

“…31 Moreover, the thickness of weak polyelectrolyte layers has been predicted to be a non-monotonic function of salt concentration by theoretical considerations. [32][33][34]39 Further, this effect has been observed experimentally for polyacrylic acid grafted polymers, 35 and was quantitatively predicted using the molecular theory. 34 Interestingly, the magnitude of the observed thickness and non-monotonicity are similar to those calculated here for the thin film gel.…”

“…The maximum has been experimentally observed. 35 Similarly, the radius of starbranched weak polyelectrolytes was calculated to have a maximum as a function of the ionic strength by a self-consistentfield model. 40 Recently, a reentrant swelling-to-collapse transition was theoretically predicted for micrometer-sized (or larger) weak polyelectrolyte gels in good solvent, assuming local electroneutrality.…”

“…However, a reentrant swelling of comparable magnitude has been experimentally observed as a function of the salt concentration in grafted weak polyelectrolyte layers of similar thickness. 35 Next, we ask if the height fluctuations modify the response of the material to an external stimulus. This question can be addressed by investigating the isothermal compressibility of the polymer network, κ T , which measures the relative volume change of the polymer network in response to an applied mechanical force.…”

“…33,34 This behavior has been confirmed by experimental findings. 35 Is this non-monotonic behavior also present in grafted hydrogel films of weak polyacid chains? Are there important differences between grafted thin film gels and weak polyelectrolyte grafted layers?…”

Post‐polymerization Modification of Polymer Brushes

“…Several polymerization mechanisms have been used to generate polymer brushes, including free radical polymerization and living polymerizations. These reactions include, but are not limited to, atom transfer radical polymerization (ATRP), nitroxide-mediated polymerization (NMP), and ring-opening metathesis polymerization (ROMP) reactions [6,[14][15][16][17][18][19][20][21][22]. Mediating agents are utilized in living polymerizations to control the build of molecular weight with reaction time [23].…”