Monitoring in Real-Time the Degrafting of Covalently Attached Fluorescent Polymer Brushes Grafted to Silica Substrates—Effects of pH and Salt

Abstract

Polymer-bearing surfaces are of particular interest because of their thermal and solvent response, 1,2 their prospective use as protein and cell adhesive platforms, 3À5 or as self-biolubricating substrates, among others. 6 Surface-tethered polymer chains provide the means to tailor the surface properties by undergoing externally stimulated conformational changes. This is particularly true for polyelectrolyte polymers such as poly(acrylic acid) (PAA) whose degree of ionization is highly influenced by pH, ionic strength, and the presence of multivalent species. 7,8 Various polymer conformations including pancake and brushes are possible with end-tethered polyelectrolytes by varying surface density and the media. 9 The control of conformational properties have allowed for advances in areas such as surface friction modulation, switchable wettability, autophobicity, 10À12 antifouling 13À16 and lubricity, 6,17À21 to name but a few.Surface-bound polymers of well-defined discrete degrees of polymerization and high surface homogeneity are desired for ensuring control of surface properties and reversible surface response with external stimuli such as temperature, pH and ionic strength. These polymers can be obtained by controlled polymerization with an ATRP initiator chemically linked to the substrate. For silica substrates, the substrateÀinitiator linkage (Si substrate ÀOÀSi initiator bond) is generally robust enough for both sustaining the reaction conditions required for ATRP and subsequent polymer brush formation and characterization. We recently showed that PAA brushes built on mica by anchoring polystyrene-poly(acrylic acid)(PS-b-PAA) diblock copolymers in a polystyrene monolayer covalently attached to OH-activated mica surfaces resist to cleavage at pH 5.5 with added salt for several days. 22 However, there is still uncertainty regarding the robustness of the substrateÀpolymer bond and whether it is resistant to the extreme pH and ionic strengths that are required for conformational analyses of polyelectrolyte brushes. It is unknown whether these extreme conditions hydrolyze the SiÀOÀSi bond and cleave the polymer from the substrate leading to undesired decrease in polymer grafting density, similar to what was reported for poly(methacrylic acid) brushes and PAA brushes grafted from silica-based substrates. 23À25 Knowing the conditions under which undesired polymer cleavage occurs is pivotal for accurate surface-property studies. They are also important for controlling reversible polymer conformational changes, while preventing polymer degrafting. Although we previously provided indirect evidence for polymer cleavage from mica at high pH and salt concentrations, 23 determining the exact conditions under which PAA cleavage occurred was not possible. Also, unequivocal evidence for polymer degrafting from silica substrates has not yet been demonstrated.ABSTRACT: Poly(acrylic acid) (PAA) covalently immobilized on glass substrates was made fluorescent by grafting a BODIPY derivative (PMOH) via an ester linkage. Although...

Instability of Surface-Grafted Weak Polyacid Brushes on Flat Substrates

“…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. At low σ, when the distance between neighboring chains (d) is much greater than the gyration radius of the grafted chains (R g ), the grafted chains will adopt a Gaussian conformation.…”

“…For the surface-grafted initiator commonly employed with silicon substrates, [11-(2-bromo-2-methyl)-propionyloxy] undecyltrichlorosilane (eBMPUS), shown in Figure 1b, two possible functional groups are liable to breakage due to hydrolysis: (1) the siloxane networks at the initiator/ substrate interface and/or (2) the ester group contained within the initiator. 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.…”

Chain End‐Functionalized Polymer Brushes with Switchable Fluorescence Response

“…End anchoring of polymer chains at a high density to a surface to form polymer brushes is a powerful way to prepare functional coatings . Fluorescent dye‐labeled polymer brushes have been prepared and utilized in ion sensing and organic light emitting diode (OLED) development or in studying the degrafting kinetics of brushes . In this context, responsive polymer brushes, which undergo conformational change upon certain, for example, pH, temperature, ionic strength, and solvent have been modified with fluorescent probes and widely used in the design of sensors .…”

“…To date, most fluorescently labeled polymer brushes have been prepared by functionalizing side chains with dye molecules as a post‐polymerization step, by copolymerization with a fluorescent monomer or by self‐assembly of charged fluorescent molecules on charged polymer brushes by electrostatic interactions . One disadvantage of these techniques is that physicochemical properties of the system such as swelling, crystallinity, or wettability are strongly altered by the introduction of a dye, which limits the applicability.…”

Synthesis and stability of BODIPY-based fluorescent polymer brushes at different pHs

“…The substrate‐organic molecule linkage (SiC) is strong enough for both supporting the reaction conditions necessary for living radical polymerization methods and subsequent polymer brush formation and characterization. Recently, Giasson and coworkers reported that poly(acrylic acid) brushes built on mica by tethering polystyrene‐poly(acrylic acid) diblock copolymers in a polystyrene layer covalently linked to OH‐activated mica surfaces resist to cleavage at pH 5.5 with added NaCl for several days. They also investigated effects pH and salt on the detaching resist to cleavage of covalently attached fluorescent poly(acrylic acid) brushes grafted to silicon substrate.…”