Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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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*,† †
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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 tetrabutylammonium 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.
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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 coatings,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 © XXXX American Chemical Society
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 accomplished. 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 silicabased 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.
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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 wellReceived: August 24, 2018 Revised: November 12, 2018
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DOI: 10.1021/acs.macromol.8b01832 Macromolecules XXXX, XXX, XXX−XXX
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plane network featuring Si−O−Si linkages that further stabilize the attachment of the organosilanes to the substrate. The latter may be formed by condensation reaction among neighboring silane molecules once they attach to the surface (or prior to attachment to the substrate). In addition to this arrangement, the silane headgroup may also form linkages that feature double- and single-bonded complexes on the substrate (see Figure S1 in the Supporting Information). TBAF is an ideal degrafting agent as it has the tendency to break Si−O links while forming −OH and Si−F bonds. The degrafting characteristics thus depend on the means of attachment of the initiator to the substrate. Investigating the stability of the brushes on surface (or model silane compounds) provides additional valuable information about the nature of trifunctional silane attachment to the substrate, which is not well understood. When degrafting takes place, up to three bonds may break, and there are possibly multiple steps involved in degrafting the chains, depending on the original structure of the silane self-assembled layer. This is where the series reaction approach provides useful insight into polymer brush degrafting. The tendency of trifunctional silanes to form in-plane linkages (vide infra) causes complications with regard to evaluating properly the liberation of PMMA chains from the substrate. Silanes may form clusters that are held by incomplete Si−O networks. As a result, PMMA chains may be removed from the substrate not only as individual molecules but also as assemblies of chains attached to an imperfect silane cluster. Such a scenario may be more prevalent at high PMMA grafting densities. The true arrangement of silane molecules in grafted layers is not known. We recognize that this limits the applicability of our proposed kinetic model.
established surface-initiated atom transfer radical polymerization (SI-ATRP) protocol.30−33 In a typical degrafting experiment, we monitor the concentration of the remaining PMMA polymers on the surface as a function of degrafting time. We vary systematically both the initial grafting density of PMMA and the concentration of TBAF and establish the rate constants for degrafting PMMA brushes that are held by single-, double-, and triple-bonded initiators to the underlying silica surfaces. We analyze the degrafting of PMMA brushes using TBAF by using a variant of a model developed recently by Miles et al.34 To explore the effect of the grafting density on degrafting PMMA brushes from substrates, we employ an orthogonal gradient approach, which enables us to explore a range of system parameters on a single sample. Specifically, we first form a spatially uniform layer of PMMA brush on the substrate and then immerse the sample vertically into the TBAF solution at a steady rate to form a grafting density gradient of PMMA brushes in one direction. These experiments are conducted with a constant concentration of TBAF (CTBAF = 0.1 M) at 40 °C. The sample is then rotated 90° (vide infra), and the second degrafting takes place in solutions of various TBAF concentrations at 40 °C. In some cases (reported in the Supporting Information), we vary the temperature in the second degrafting step. The TBAF incubation times in the second TBAF dipping are k3 for all F
DOI: 10.1021/acs.macromol.8b01832 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules grafts (i.e., σPMMA,1). In performing the shifts, we keep the data corresponding to the highest value of σPMMA,1 and add a time constant for other σPMMA,1 concentrations to produce the master plot. In Figure 8, we plot the grafting density of PMMA after the second degrafting step (i.e., σPMMA,2) in the original (a) and
consistent with the rate constants reported earlier in the paper. Note that while the rate constants have been determined from the shape of the σPMMA,2 versus degrafting time profiles and the tshift values have been extracted from the superposition of the σPMMA,2 versus degrafting time profiles. The results obtained from both approaches are consistent. PMMA Degrafting at Various TBAF Solution Temperatures. We also conducted PMMA degrafting at three different TBAF temperatures (25, 40, and 50 °C). The data are presented in Figure S6. Generally, the rate of degrafting increases with increasing TBAF concentration and the temperature. We did not conduct any further analysis of the data (i.e., Arrhenius type) to extract, for instance, the activation energy of Si−O bond breaking because of limited data set. We were limited, primarily, by the relatively low boiling point of THF (66 °C). Degrafting in Other Polymer Brush Systems. All discussion thus far concentrated on degrafting of PMMA brushes. Similar behavior can be observed in other hydrophobic brush systems, i.e., polystyrene (data not shown) or poly(tert-butyl methacrylate) (PtBMA). In such THF-soluble systems one observes an exponential-like decay in brush thickness with increasing TBAF incubation time; the rate of degrafting increases with increasing incubation temperature (cf. Figure S7). However, some polymer brush systems, i.e., poly(methacrylic acid) (PMAA), when subjected to TBAF solution, exhibit an initial increase in the dry thickness followed by a decrease similar to an exponential-like decay (cf. Figure S7). Such an initial increase is not observed in PtBMA brushes, from which the PMAA brushes are formed by hydrolysis. This may indicate that TBAF, which has an ammonium group, could associate and form a complex with the PMAA (a weak polyacid). Formation of this complex would increase the apparent thickness of the system due to association and increase in the sample volume. Concurrently, the degrafting also progresses since the thickness decreases eventually (albeit at a slower rate). The initial increase is prominent at a lower temperature of 25 °C since the rate of degrafting at this temperature is relatively slow. With increasing temperature, the rate of degrafting increases, while there is little or no increase in the association between PMAA and TBAF. The initial increase in thickness, due to association, becomes less significant at higher temperatures because degrafting dominates at elevated temperatures. The PMAA system represents an interesting example of the competing effects of complex formation (leading to increasing the thickness) and degrafting (which reduces the thickness). The complex formation and the resulting thickness increase are a weaker function of temperature while the degrafting rate increases significantly with rise in degrafting temperature.
Figure 8. Grafting density of PMMA after the second degrafting step (σPMMA,2) as a function of actual time (a) and “shifted” time (b) for data sets corresponding to various σPMMA,1.
time-shifted (b) format for samples degrafted with CTBAF = 0.3 M in the second degrafting step. As evident from Figure 8b, after shifting, all data fall onto a master plot. The same procedure was conducted for all experiments in which the CTBAF was varied in the second degrafting step. The time shifts for all data are plotted in Figure 9. The complete analysis of PMMA degrafted in the second TBAF degrafting and shifted along the time axis is depicted in Figure S5. Larger time shifts, tshift, indicate longer degrafting times and hence slower degrafting rate. The results in Figure 9 reveal, not surprisingly, that the rate of degrafting increases with increasing TBAF concentration in the second degrafting step. The results also demonstrate that the degrafting rates for 0.1 and 0.3 M TBAF are approximately the same. The findings are
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CONCLUSIONS We have presented a simple method aiming to comprehend the process of degrafting PMMA grafts from silica substrates using an analogy to conventional reaction kinetics. The rate of degrafting of PMMA brushes by TBAF is a strong function of the initial grafting density of the PMMA brush and the incubation time in TBAF solutions. Degrafting of PMMA brushes from silica substrates can be described by invoking a simple model comprising of a series reaction with pseudo-firstorder dependence on the instantaneous brush grafting density. Using PMMA brushes with variable grafting density for degrafting provides an additional insight into the degrafting
Figure 9. Time shift needed to collapse the σPMMA,2 data onto a uniform time scale (cf. Figure 8) plotted as a function of the initial PMMA grafting density (σPMMA,1) before second degrafting for four different values of CTBAF in the second degrafting step. G
DOI: 10.1021/acs.macromol.8b01832 Macromolecules XXXX, XXX, XXX−XXX
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process and potentially reveals useful information about the structure of the silane headgroup present in the initiator. The combinatorial setup employed here provides a convenient platform that minimizes individual stochastic errors associated with using discrete samples and enables studying degrafting of polymer brushes in a systematic and fast screening manner. We have demonstrated that degrafting may be utilized to vary polymer brush density and therefore brush thickness in a controlled fashion. Competing kinetic effects, such as complex formation versus Si−O bond breaking, come into play during degrafting, depending on the initial density of the polymer brush. We presented such an example of complex formation in the poly(methacrylic acid) system. The series reaction model of grafted polymer system, combined with gradient approach for sample preparation, can be further expanded to incorporate competing reactions and the effect of diffusion of TBAF into the polymer layer, which will, in turn, provide new insight into removing grafted polymers from silica surfaces.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01832. Silane attachment configurations, meniscus in TBAF degrafting solutions, single rate constant model analysis, time shifts during PMMA degrafting, effect of temperature on degrafting of PMMA, degrafting of other polymer systems, and estimate of bond tension for PMMA grafts in THF (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Rohan Patil: 0000-0001-8841-4635 Jason Miles: 0000-0002-7916-2816 Yeongun Ko: 0000-0001-5770-6707 Preeta Datta: 0000-0003-1078-1915 Balaji M. Rao: 0000-0001-5695-8953 Jan Genzer: 0000-0002-1633-238X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work was supported by the National Science Foundation (Grant DMR-1404639) and the Army Research Office under their Staff Research Program (Grant W911NF-04-D- 00030016).
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REFERENCES
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DOI: 10.1021/acs.macromol.8b01832 Macromolecules XXXX, XXX, XXX−XXX
