Kinetic Study of Degrafting Poly(methyl methacrylate) Brushes from Flat Substrates by Tetrabutylammonium Fluoride Rohan Patil, † Jason Miles, † Yeongun Ko, † Preeta Datta, † Balaji M. Rao, † Douglas Kiserow, ‡ and Jan Genzer* ,† † Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905, United States ‡ US Army Research Office, Research Triangle Park, North Carolina 27709-2211, United States * S Supporting Information ABSTRACT: Polymer degrafting is a process in which surface-attached polymer brushes are removed from the substrate by breaking a chemical bond in proximity to the substrate. This paper provides insight into the kinetics of degrafting poly(methyl methacrylate) (PMMA) brushes using tetrabutyl- ammonium fluoride (TBAF) and demonstrates how the process can be modeled using a series of degrafting reactions. The trichlorosilane-based polymerization initiator utilized here to synthesize PMMA grafts by surface-initiated atom transfer radical polymerization anchors to the silica substrate by up to three potential attachment points. During the degrafting sequence this anchoring reduces to two and one chemical bond and finally results in complete liberation of the PMMA macromolecule from the substrate. We investigate the effect of TBAF concentration, the initial grafting density of PMMA grafts on the substrate, and TBAF exposure time on degrafting of PMMA by monitoring the instantaneous areal grafting density of PMMA on the substrate. ■ INTRODUCTION Polymer chains grafted to a surface are called polymer brushes. 1,2 Tethering of macromolecules to the surface limits the number of spatial arrangements the chains may adopt, and it, in turn, imparts some novel attributes, such as reduction of the coefficient of friction. 3 In such systems, the surface areal density (i.e., grafting density) of the grafts, which is a measure of the lateral distance between the attachment points of the macromolecules on the surface, dictates the conformation of the substrate-anchored polymer grafts. When the distance between neighboring chains is sufficiently small, the chains interact through excluded volume interactions, stretch, and form structures termed “brushes”. This is opposed to the “mushroom” regime where the distance between the chains on the surface is comparable to (or larger than) the size of the polymer and where the number of interchain contacts is much smaller than in the “brush” regime. The unique properties of grafted polymer systems are utilized in applications such as creating stimuli-responsive surfaces, 4-6 antibiofouling coat- ings, 7-9 controlled lubrication/adhesion, 10-14 and patterned surfaces. 15-18 Polymer brushes exhibit relatively high stability when they are anchored covalently to the substrate. The stability of the polymer brush system depends strongly on the grafting density, 19 the molecular weight of the polymeric grafts, and the degree of charging. Charged or chargeable polymer brushes with permanent or pH-induced charge, respectively, may degraft spontaneously from the substrate due to large swelling that imposes strong force on the linker, which keeps the polymer attached to the substrate. Indeed, strong swelling inside the brush may induce mechanochemical breakage of the chemical bonds in the linker region and ultimately result in liberating the grafted chains from the surface. 20-28 While these examples illustrate cases of “spontaneous” degrafting, which occurs primarily in polyelectrolyte systems, “on-demand” degrafting of polymers from surfaces may also be accom- plished. This involves chemically assisted cleaving of polymers, regardless of their chemical composition, from the substrate. We have demonstrated previously that tetrabutylammonium fluoride (TBAF) is an effective reagent that breaks Si-O bonds and thus may help to degraft polymer chains from silica- based substrates. 29 Here we endeavor to understand how the rate of degrafting is affected by the initial grafting density of the grafted system on the substrate and TBAF concentration in solution. ■ APPROACH In this work we focus primarily on a simple model system featuring poly(methyl methacrylate) (PMMA) polymer grafts in the brush regime anchored to flat silica surfaces. We grow PMMA directly from flat silicon substrates covered with a thin layer of silica present on top of the silicon support using a well- Received: August 24, 2018 Revised: November 12, 2018 Article pubs.acs.org/Macromolecules Cite This: Macromolecules XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.macromol.8b01832 Macromolecules XXXX, XXX, XXX-XXX Downloaded via UNIV OF RHODE ISLAND on December 9, 2018 at 18:23:49 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.