Chain Length and Grafting Density Dependent Enhancement in the Hydrolysis of Ester-Linked Polymer Brushes

Abstract

Poly(N,N-dimethylacrylamide) (PDMA) brushes with different grafting density and chain length were grown from an ester group-containing initiator using surface-initiated polymerization. Hydrolysis of the PDMA chains from the surface was monitored by measuring thickness of the polymer layer by ellipsometry and extension length by atomic force microscopy. It was found that the initial rate of cleavage of one end-tethered PDMA chains was dependent on the grafting density and chain length; the hydrolysis rate was faster for high grafting density brushes and brushes with higher molecular weights. Additionally, the rate of cleavage of polymer chains during a given experiment changed by up to 1 order of magnitude as the reaction progressed, with a distinct transition to a lower rate as the grafting density decreased. Also, polymer chains undergo selective cleavage, with longer chains in a polydisperse brush being preferentially cleaved at one stage of the hydrolysis reaction. We suggest that the enhanced initial hydrolysis rates seen at high grafting densities and high chain lengths are due to mechanical activation of the ester bond connecting the polymer chains to the surface in association with high lateral pressure within the brush. These results have implications for the preparation of polymers brushes, their stability under harsh conditions, and the analysis of polymer brushes from partial hydrolysates.

Instability of Surface-Grafted Weak Polyacid Brushes on Flat Substrates

“…17,19 This tension is directly proportional to the MW of the grafted chains, and longer chains of a nonionic polymer in the presence of a strong base have been shown to cleave preferentially from a solid substrate. 18 In the case of polyelectrolytes, pH-induced swelling can produce sufficient tension to activate the grafting chemistry for hydrolysis. 12,15,17,19 The σ of a polymer brush can also generate tension.…”

“…This extension will induce tension along the chains and can lead to degrafting of the tethered chains. 18 Since both brushes were polymerized at pH 9, brush swelling due to electrostatic repulsion and highly solubilized chains generates tension along the chain backbone. This tension is modulated by variations in the MW and σ levels of the grafted chains.…”

“…polymer systems, which have exhibited decreased degrafting rates with decreasing σ. 18 One question that remains unanswered by the experiments in this article is whether the tension generated by strongly swelling the grafted chains with increasing pH originates with electrostatic repulsion or increased solubility or both. The presence of 10 mM of sodium ions in the incubation solution leaves the possibility for charge screening to occur in the brush, which would diminish the impact of electrostatic repulsion and possibly lower tension in the polymer brush.…”

“…10,11 Examples include polyelectrolytes in water 12−16 and branched macromolecules, 13,17 as well as polyacrylamide in methanol with a strong base. 18 A proposed mechanism 12 for the instability of polyelectrolyte brushes in water, illustrated in Figure 1a, suggests that strong swelling of the brush due to electrostatic charging in the brush generates increased tension along the grafted chain backbone. This tension is ultimately focused at the bottom-most section of the polymer brush close to the substrate.…”

“…While some researchers have identified siloxane linkage cleavage in replacement reactions in silane monolayers 20 and as the point of scission in brush systems, 13,15 there is a possibility that the ester in the initiator can also be activated for and undergo hydrolysis. 12,18,21,22 Regardless of the bond that undergoes hydrolysis and leads to degrafting of the polymer from the surface, one testable hypothesis that arises from this mechanism is that a weak polyacid (e.g., poly(methacrylic acid), PMAA) will exhibit increasing levels of instability with increasing pH. Since increasing pH will result in a higher degree of dissociation (α) within the polyacid brush, and thus increased solubility as well as electrostatic repulsion, the tension along the polymer backbone will also increase.…”