Article pubs.acs.org/Langmuir
The Relationship between Charge Density and Polyelectrolyte Brush Profile Using Simultaneous Neutron Reflectivity and In Situ Attenuated Total Internal Reflection FTIR Paul D. Topham,† Andrew Glidle,‡ Daniel T. W. Toolan,§ Michael P. Weir,∥ Maximillian W. A. Skoda,⊥ Robert Barker,# and Jonathan R. Howse*,§ †
Chemical Engineering and Applied Chemistry, Aston University, Birmingham B4 7ET, United Kingdom Division of Biomedical Engineering, School of Engineering, Rankine Building, University of Glasgow, Glasgow G12 8LT, United Kingdom § Department of Chemical and Biological Engineering, University of Sheffield, Sheffield S1 3JD, United Kingdom ∥ Bragg Institute, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia ⊥ STFC, Rutherford Appleton Laboratory, Harwell, Oxford, Didcot, OX11 0QX, United Kingdom # Institut Laue-Langevin, BP 156, 6 rue Jules Horowitz, 38042 Grenoble Cedex 9, France ‡
S Supporting Information *
ABSTRACT: We report on a novel experimental study of a pH-responsive polyelectrolyte brush at the silicon/D2O interface. A poly[2-(diethylamino)ethyl methacrylate] brush was grown on a large silicon crystal which acted as both a substrate for a neutron reflectivity solid/liquid experiment but also as an FTIR-ATR spectroscopy crystal. This arrangement has allowed for both neutron reflectivities and FTIR spectroscopic information to be measured in parallel. The chosen polybase brush shows strong IR bands which can be assigned to the N−D+ stretch, D2O, and a carbonyl group. From such FTIR data, we are able to closely monitor the degree of protonation along the polymer chain as well as revealing information concerning the D2O concentration at the interface. The neutron reflectivity data allows us to determine the physical brush profile normal to the solid/liquid interface along with the corresponding degree of hydration. This combined approach makes it possible to quantify the charge on a polymer brush alongside the morphology adopted by the polymer chains.
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INTRODUCTION AND BACKGROUND Polymer brushes1,2 have received widespread attention as responsive surface coatings, since advances in controlled radical polymerization3 have allowed polymer chains to be easily grafted from (as opposed to grafted to) a wide range of surfaces with good degree of control over aspects such as the grafting density,4 molecular weight, and polydispersity.5 Lateral confinement of the polymer chains leads to a rich range of responsive behavior dependent upon the physical and chemical properties of the brush layer. The presence of a polymer brush influences the nature of the interface, and these interfacial properties © 2013 American Chemical Society
further depend upon the physical or chemical state of the grafted layer, for example, if it is swollen or condensed. Therefore, responsive polymer brushes at surfaces give a route to tunable surface properties such as wettability, affinity toward the adsorption of proteins and macromolecules, and electrochemical characteristics.6,7 Received: February 11, 2013 Revised: April 19, 2013 Published: April 22, 2013 6068
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resonance spectroscopy (SPR),18 and infrared spectroscopy19 and the excellent, and thorough, work by Tran and Sanjuan who used both ellipsometry and NR.20 Responsive polymer brushes at interfaces have also been the subject of a number of excellent reviews.5,21−24 Additionally, real-time swelling of a polyacid brush has been demonstrated using AFM,8 graphically illustrating the extent of swelling and deswelling possible via pH-switching.25 Ellipsometry and NR experiments on weak polyelectrolyte brushes confirmed a number of the predictions of scaling theory,13 while polyampholyte brushes (exhibiting both weakly acidic and basic monomer groups) have shown large ranges in swelling response to pH.20 Recently, poly[2(dimethylamino)ethyl methacrylate] (PDMA) brushes were shown to swell by the application of a voltage between the substrate and an electrode some distance away in the surrounding aqueous environment.7 Surface grafted strong polyelectrolyte brushes have also been shown to collapse dramatically upon the addition of oppositely charged surfactant to the surrounding solution and showed evidence of surface adsorption on the grafted chains.26 In all the work conducted on responsive brushes to date, characterization techniques are used in isolation, often measuring a single parameter through a chosen technique, where that parameter is a function of the pH of the solution. For example, a technique which measures the thickness of the brush film is measuring the height of the polymer chains, which is a function of the degree of dissociation, which in turn is a function of the pH of the solution. Consequently, this approach is only indirectly measuring the degree of dissociation and so limits our understanding of these complex systems. In the novel experimental study described herein, we have directly measured both of these important factors (acid−base dissociation and polymer brush profile) and so are able to determine, and correlate, the interplay between both parameters simultaneously, offering new insight into the behavior of these systems. In this experiment, we have exploited both the infrared and neutron transmission of silicon. Silicon is a common substrate for neutron reflectivity experiments and is also frequently used in attenuated total (internal) reflection (ATR) spectroscopy when making measurements in contact with a liquid such as water (which is opaque to IR). Therefore, a cell in which a polymer brush is grown on the surface of the silicon and placed against a liquid allows for investigation of the brush profile via standard neutron reflectivity, while also allowing for the changes in molecular bonding and configuration to be investigated via FTIR-ATR spectroscopy. Attenuated Total (Internal) Reflection Fourier Transfer Infrared Spectroscopy (ATR-FTIR). The ATR method of collecting infrared spectra has a particular advantage when studying materials immersed in highly (infrared) absorbing media such as water. To achieve total internal reflection, infrared light reflects off a substrate surface at an angle above the critical angle, leading to the generation of an evanescent (IR) field on the opposite side of the substrate interface, which decays exponentially into the bulk, with a penetration depth defined by eq 1. In our particular setup the evanescent field probes both the polymer and aqueous solution phase due to the depth of the evanescent wave, as shown in Figure 2. For thin layers of non, or weakly, absorbing (IR) polymers on the surface of the silicon prism, the intensity of the evanescent wave [I(z)] decays exponentially with increasing distance from the interface (z) according to the eq 1.27,28
In weak polyelectrolyte brushes, where the responsive unit is either a weak acid or base, the fraction of charges along the polymer chain is related to the pH through an equilibrium between association and dissociation of the relevant chemical groups on the monomers.8 More specifically, for a weak polyacid, dissociation contributes a positively charged counterion to the solution. Under acidic conditions, where the solution contains a high [H+], dissociation is unfavorable and the brush charge fraction is low. Alternatively, at high pH, the brush becomes charged and is swollen by the osmotic pressure arising from the strong confinement of counterions within the brush. Weak polybase brushes exhibit the contrary relationship between swelling and pH, becoming swollen in acidic conditions as shown in Figure 1.9 To date, there have been a number of structural studies on the swelling of polyelectrolyte brushes, principally using the techniques of ellipsometry,10,11 neutron reflectivity (NR),12−16 quartz crystal microbalance (QCM),17 surface plasmon
Figure 1. Schematic showing the molecular and physical changes occurring for a weak polybase brush as a function of pH and how the signals measured change in response to these changes. 6069
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three regions of the IR spectrum which can be used, and these are shown in Table 1. Table 1. Summary of the Bands Visible and the Effect of Protonation of the Polymer Chain band N −D (polymer) CO (polymer) D−O−D (solvent) +
where
d=
effect of protonation
1955
intensity ↑
1730
intensity ↓, band shifts to lower frequency and broadens intensity ↑
2500
Neutron Reflectivity. Specular neutron reflectivity (NR) measures the reflection intensity as a function of q, where q (=4π/λ sin θ) is the out-of-plane momentum transfer; that is, R(q) samples the differences in neutron scattering length density (Nb), or neutron “refractive index”, perpendicular to the sample surface, while averaging over in-plane structures of the whole sample.29 For a layered sample, such as polymer brushes attached to a solid substrate, this technique generally enables the determination of three parameters [thickness, Nb (which changes as function of solvation), and interfacial roughness] for each layer in the stack. In the simplest case, fitting a typical model Nb profile comprising a series of stacked slabs, each with their respective thicknesses, Nb and interfacial roughness to a neutron reflectivity curve. This allows us to quantify the polymer brush thickness, its physical density, and the roughness between the brush and solvent. This in turn makes it possible to observe brush swelling (or collapse) as a function of pH in consecutive measurements. In this study, we have developed a setup (shown in Figure 2) which allows for simultaneous measurement of both neutron reflectivity and ATR-FTIR from the same polymer brush sample, for the first time. Adjusting the pH of the solution allows us to investigate the effect of pH on the protonation/ deprotonation of a surface-bound polyelectrolyte brush (by FTIR), and the subsequent effect that this charge has upon the physical brush profile (by NR). This is represented schematically in Figure 1.
Figure 2. Exploded schematic showing the paths of the IR beam (red) and neutron beam (blue), and setup of the solid−liquid cell (top). The polymer brush was tethered to the lower planar surface shown in this schematic. Schematic representation of the path of the IR beam and orientation of the decaying evanescent waves generated (bottom). In our particular setup, there were four reflections at the solid/liquid interface.
I(z) = I0 exp−z / d
wavenumber (cm‑1)
λ 2
4π (n1 sin 2 θ − n2 2) (1)
where I0 is the intensity of the incoming beam at the interface, and n1 and n2 are the refractive indices of the silicon and the aqueous phase, respectively. In general for this substrate/ solution combination, d is typically less than 10% of the incident wavelength. The penetration depth is independent of the incident light polarization direction and decreases as the reflection angle increases. When the incident angle equals the critical value, d tends to infinity and the wave fronts of refracted light are normal to the surface. Adjusting the IR beam incidence angle allow the penetration depth of the evanescent field to be controlled, and for an angle of incidence of 45° at a Si/H2O interface, it is approximately 200−400 nm in the mid-IR region. This small penetration depth means that for a 10−100 nm thick polymer film, the magnitude of absorbance due to polymer chromophores will be 2.5−5% of that due to the solvent phase (before taking into account differences in extinction coefficients). Thus, by exploiting a low noise detector, a stable optical configuration, and long integration times, it is possible to measure the IR bands of solvated polymer films and the corresponding shifts that occur in these bands as a result of protonation and deprotonation (due to solution pH). It should also be noted that the degree of hydration within the polymer film will also be revealed as an increase in the signal from the solvent (D2O), as the solvent moves closer to the interface, and hence closer to a region of higher IR intensity. In effect, more solvent is moving into the path of the beam. In our particular brush, there are
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EXPERIMENTAL SECTION
Materials. The substrates used were single crystal silicon in the form of blocks (80 mm × 50 mm × 6 mm) (Crystran UK) polished to the (100) crystal plane on the largest faces, with large 45° faces polished on the longer edges to allow for the FTIR beam to be brought into and out of the silicon (Figure 2). The ATRP initiator, (11-(2-bromo-2-methyl) propionyloxy)undecyl trichlorosilane, was synthesized according to the method used by Matyjaszewski et al.30 2-(Diethylamino)ethyl methacrylate (DEA, Aldrich, 99%) was passed through basic alumina to remove the phenothiazine inhibitor and was stored at 4 °C prior to use. Distilled water was used in all cases, and copper(I) bromide (Aldrich, ≥ 98.0%), copper(II) bromide (Acros Organics, 99+ %), methanol (Fisher, Laboratory Reagent grade), acetone (Fisher, Laboratory Reagent grade), and 2,2′-dipyridyl (Aldrich, 99%) were used as received. Brush Synthesis, Step 1: Initiator Surface Preparation. The silicon blocks were first cleaned by immersing them overnight with the large faces vertical in freshly prepared piranha solution (200 mL of 30% H2O2 and 200 mL of concentrated H2SO4) in a 500 mL PTFE beaker. Upon removal of the block from solution, the now hydrophilic surface was repeatedly rinsed with pure water and blown dry under a stream of nitrogen. The block was then placed with the smaller of the larger two faces resting upon a lens tissue, situated upon a large PTFE 6070
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min−1 and the pD recorded throughout the duration of the NR data collection. Typically, the drift in pH was
