Neutron Reflectivity Study of the Structure of pH-Responsive Polymer

pubs.acs.org/Langmuir © 2010 American Chemical Society

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Neutron Reflectivity Study of the Structure of pH-Responsive Polymer Brushes Grown from a Macroinitiator at the Sapphire-Water Interface Mauro Moglianetti,† John R. P. Webster,‡ Steve Edmondson,§, Steven P. Armes,§ and Simon Titmuss*,†

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† Department of Chemistry, Physical & Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, U.K., ‡ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, U.K., and §Department of Chemistry, University of Sheffield, Brook Hill, Sheffield S3 7HF, U.K. Current address: Department of Materials, Loughborough University, Loughborough LE11 3TU, U.K

Received April 19, 2010. Revised Manuscript Received June 15, 2010 Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes have been grown by surface-initiated atom transfer radical polymerization (SI-ATRP) from a polyanionic macroinitiator adsorbed at the sapphire-water interface, and neutron reflectivity has been used to characterize the structures and pH response of the brushes. The polymer brushes are well-described by Gaussian density profiles with an additional thin, dense layer close to the solid-liquid interface for the thicker brushes at pH 7 and 9, which produces a spike in the density profile. The spike in the distribution accounts for less than 5% of the polymer and disappears as the brushes swell at pH 3. The observed swelling behavior has been used in combination with the predictions of scaling theory and previous experimental measurements to determine the grafted density of PDMAEMA chains.

1. Introduction Polymer brushes are layers of polymer chains grafted to an interface at a sufficiently high density such that, in a good solvent, there is an osmotic driving force for the chains to stretch away from the interface.1 This extended conformation makes brush layers useful for the steric stabilization of colloidal dispersions as well as tuning the functional properties of interfaces.2,3 Recently, attention has focused on forming brushes from polymers that respond to physicochemical stimuli, such as pH and temperature, with the aim of producing smart or responsive interfaces that have potential applications as actuators and sensors and in controllable wetting.4 Polymer brushes can be formed either by grafting presynthesized end-functionalized polymers to an interface or by grafting the polymers from the interface by surface-initiated polymerization.5,6 In the grafting-to approach, the brush density is limited by the excluded volume barrier to adsorption that is presented by the previously adsorbed chains.7 Using the grafting-from approach of surface-initiated polymerization removes this barrier, allowing the formation of densely grafted brushes.8 Most surface-initiated polymerizations are performed using a small-molecule initiator that is anchored via thiol or silane chemistry at gold or silica interfaces, respectively.6 Silane and thiol initiators have been used in the preparation of various stimulus-responsive *To whom correspondence should be addressed. E-mail: simon.titmuss@ chem.ox.ac.uk.

(1) Toomey, R.; Tirrell, M. Annu. Rev. Phys. Chem. 2008, 59, 493–517. (2) La Spina, R.; Tomlinson, M. R.; Ruiz-P
erez, L.; Chiche, A.; Langridge, S.; Geoghegan, M. Angew. Chem., Int. Ed. 2007, 46, 6460–6463. (3) Jia, H.; Titmuss, S. Nanomedicine 2009, 4, 951–966. (4) Chen, T.; Ferris, R.; Zhang, J.; Ducker, R.; Zauscher, S. Prog. Polym. Sci. 2010, 94–112. (5) Barbey, R.; Lavanant, L.; Paripovic, D.; Sch€uwer, N.; Sugnaux, C.; Tugul, S.; Lkok, H.-A. Chem. Rev. 2009, 109, 5437–5527. (6) Edmondson, S.; Osbourne, V. L.; Huck, W. T. S. Chem. Soc. Rev. 2004, 33, 14–22. (7) Titmuss, S.; Briscoe, W. H.; Dunlop, I. E.; Sakellariou, G.; Hadjichristidis, N.; Klein, J. J. Chem. Phys. 2004, 121, 11408–11419. (8) Tsujii, Y.; Ohno, K.; Yamamoto, S.; Goto, A.; Fukada, T. Adv. Polym. Sci. 2006, 197, 1–45.

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brushes, which have been characterized by neutron reflectivity measurements.9-12 In this study, we adopt an alternative approach of electrostatically adsorbing a polyelectrolytic macroinitiator13-15 at the sapphire-water interface, from which we grow PDMAEMA brushes with dry thicknesses in the range of 50-170 A˚. PDMAEMA is a weak cationic polyelectrolyte, and PDMAEMA brushes grown from a silane initiator at the silica/water interface have previously been shown to be pH-responsive,10 as have the related poly(2-diethylamino)ethyl methacrylate) (PDEA) brushes.9 The final architecture of the surface-grown polymer studied here is that of a comb polymer comprising a polyanionic backbone and PDMAEMA side chains. This architecture is closely related to the preformed combs comprising a poly(methyl methacrylate) backbone and PDMAEMA side chains used by Tomlinson et al. to form brushes at a hydrophobized silica interface using Langmuir-Schaeffer deposition.16 Synthetic sapphire serves as a model substrate for technologically relevant metal oxide interfaces. The high neutron scattering length density of the sapphire (5.85
10-6 A˚-2)17 is sufficiently close to that of D2O (6.35
10-6 A˚-2) that the presence of a hydrogenous polymer layer (at a volume fraction greater than ca. 10%) creates an almost symmetrical well in the scattering potential at the (9) Geoghegan, M.; Ruiz-P
erez, L.; Dang, C. C.; Parnell, A. J.; Martin, S. J.; Howse, J. R.; Jones, R. A. L.; Golestanian, R.; Topham, P. D.; Crook, C. J.; Ryan, A. J.; Sivia, D. S.; Webster, J. R. P.; Menelle, A. Soft Matter 2006, 1076–1080. (10) Sanjuan, S.; Perrin, P.; Pantoustier, N.; Tran, Y. Langmuir 2007, 23, 5769– 5778. (11) Zhang, J.; Nylander, T.; Campbell, R. A.; Rennie, A. R.; Zauscher, S.; Linse, P. Soft Matter 2008, 4, 500–509. (12) Yim, H.; Kent, M. S.; Mendez, S.; Lopez, G. P.; Satija, S.; Seo, Y. Macromolecules 2006, 39, 3420–3426. (13) Vo, C.-D.; Schmid, A.; Armes, S. P.; Sakai, K.; Biggs, S. Langmuir 2007, 23, 408–413. (14) Edmondson, S.; Armes, S. P. Polym. Int. 2009, 58, 307–316. (15) Edmondson, S.; Vo, C. D.; Armes, S. P.; Unali, G. F. Macromolecules 2007, 40, 5271–5278. (16) Tomlinson, M. R.; Cousin, F.; Geoghegan, M. Polymer 2009, 50, 4829– 4836. (17) Follows, D. I. DPhil Thesis, University of Oxford, Oxford, U.K., 2005.

Published on Web 06/28/2010

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polymer-decorated interface between the sapphire and the D2O. This symmetrical well in the scattering potential enhances the sensitivity of the reflectivity measurement to the shape of the polymer segment distribution. Adsorption at the cationic sapphire-water interface requires the use of an anionic macroinitiator.18 This macroinitiator was synthesized by esterification of a poly(glycerol monomethacrylate) (PGMA) precursor so as to introduce both the 2-bromoester initiator groups and the anionic benzene sulfonic acid groups.15 Using such a macroinitiator simplifies the functionalization of large-surface-area substrates, such as colloidal dispersions or the relatively large planar substrates employed in neutron reflectivity measurements. (See ref 14 for a review of brush synthesis from macroinitiators.) The ultimate goal is to use the PDMAEMA brushes to study the interaction of PDMAEMA chains with an oppositely charged sodium dodecyl sulfate surfactant at a fixed chemical potential of polymer. This model system is expected to aid our understanding of the driving force for the formation of the multilayer structures that have been observed at the air-water interface.19 With this goal in mind, four polymer brushes (referred to as a-d) have been prepared, with the polymerization time being used to vary the dry thickness of the brushes. Here we focus on the characterization of the brushes and how their swelling behavior varies with the solution pH. These data are combined with the predictions of scaling theory and the results of a previous experimental investigation of PDMAEMA brushes grafted from silane initiators at the silica-water interface10 in order to estimate the interfacial density and degree of polymerization of the PDMAEMA chains grafted from the anionic macroinitiator adsorbed at the sapphire-water interface.

2. Experimental Section 2.1. Materials and Sample Preparation.

2.1.1. Materials.

DMAEMA was purchased from Aldrich and purified by passing it through a basic alumina column (150 mesh) also from Aldrich. Cu(I)Br, Cu(II)Br2, 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA), and 2-propanol were purchased from Aldrich and used as received. A-plane (1120) sapphire wafers, for use in AFM measurements, were purchased from the MTI Corporation. A-plane synthetic sapphire blocks (60
70
15 mm3) for use in neutron reflectivity measurements were obtained from Dr. D. Styrkas. They were lapped and polished by PI-KEM Ltd. and finally polished for 20 min using a 0.1 μm diamond slurry in the Physical & Theoretical Chemistry Laboratory at Oxford. The sapphire samples were cleaned using a mild piranha treatment (incubation for 15-20 min in a mixture of concentrated sulfuric acid, hydrogen peroxide, and ultrapure water in a ratio of 4:1:5 at 80
C) and then subjected to an Ar/H2O plasma (detailed protocol in Supporting Information) for a few minutes to remove any remaining interfacial contamination and to promote the formation of hydroxyl groups at the interface.

2.1.2. Surface-Inititiated Atom Transfer Radical Polymerization. Surface-initiated ATRP was performed from the anionic macroinitiator, illustrated in Figure 1, that was electrostatically adsorbed at the sapphire-water interface. The macroinitiator synthesis has been described previously.15 Briefly, it was (18) Although measurements of the point of zero charge for single-crystal sapphire suggest that it occurs at more acidic pH (pH 5-7)36-38 than that of colloidal alumina (pH 5-9)39 and that it shows some variation with the crystal surface, neutron reflectivity measurements made at the surface studied here clearly demonstrate that the interface is cationic at neutral pH because the anionic surfactant SDS forms 86% of a bilayer.17 (19) Moglianetti, M.; Li, P.; Malet, F. L. G.; Armes, S. P.; Thomas, R. K.; Titmuss, S. Langmuir 2008, 24, 12892–12898.

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Figure 1. Anionic macroinitiator with an overall degree of esterification of 82% (36% hydroxy groups esterified with BIBB and 46% esterified with SBA) as judged by 1H NMR spectroscopy. The macroinitiator structure shown is approximate, and random esterification of the hydroxy groups on the PGMA chains is expected. prepared by a one-pot synthesis whereby the PGMA was sequentially esterified using first 2-bromoisobutyrl bromide (BIBB) followed by excess 2-sulfobenzoic acid cyclic anhydride (SBA) to provide the anionic sulfonic acid groups that promote strong electrostatic adsorption at the cationic sapphire-water interface. The overall degree of esterification was 82%, with 36% of the hydroxy groups esterified with BIBB and 46% esterified with SBA as judged by 1H NMR. The anionic macroinitiator was adsorbed at the sapphirewater interface from a 0.1 wt % solution at pH 4 over a period of 12 h. Adsorption is confirmed by X-ray reflectivity and ellipsometry measurements (not shown), which consistently indicate a 1 ( 0.5 nm macroinitiator layer at the interface, and by the increase in the rms surface roughness observed in tapping-mode AFM (Figure 2), from 0.06 nm prior to adsorption to 0.18 nm after adsorption. These measurements are consistent with the adsorbed mass of 1.03 ( 0.06 mg/m2 observed for this macroinitiator at an aminated silicon oxynitride-water interface.15 Surface-initiated ATRP was conducted under a nitrogen atmosphere in a reaction medium comprising 50% DMAEMA monomer and 50% 9:1 v/v propan-2-ol/water, with Cu(I)Br and Cu(II)Br2 as the activator and deactivator, respectively, and HMTETA as the ligand for the copper catalyst (239:1:0.1:0.39 [DMAEMA]/ [Cu(I)Br]/[Cu(II)Br2]/[HMTETA]).20,21 The polymerization time ranged from 30 min to 1 h, and the resulting brushes were characterized by ellipsometry and X-ray reflectivity measurements. Transmission FT-IR spectra of PDMAEMA brushes grown using this protocol had characteristic bands at 2819 and 2769 cm-1, which can be assigned to C-H stretches of the dimethylamino groups. 2.2. Neutron Reflectivity. Specular neutron reflectivity measurements were made using the SURF reflectometer22 at the ISIS pulsed neutron source at the Rutherford Appleton Laboratory, U.K. Reflectivity profiles presented in this work are plots of the specular reflectivity with respect to the momentum transfer Q = (4πsinθ)/λ, where θ is the glancing angle of incidence and λ is the neutron wavelength. Measurements were made at glancing angles of incidence of 0.1, 0.25, 0.7, and 1.5
covering a Q range of 3.3
10-3 to 0.6 A˚-1, with a resolution of ΔQ/Q = 5%. The reflectivity at Q > 0.3 A˚-1 is dominated by a sample-dependent background that arises primarily from the incoherent scattering from the bulk aqueous solution. This was accounted for by the inclusion of a constant background reflectivity in the model calculations. The MOTOFIT package23 was used to fit the reflectivity profiles to model scattering length density profiles. The model profiles were constructed from three to five layers characterized by a thickness, a scattering length density, and a Gaussian roughness. The first stage of the fitting procedure used a trial-and-error approach to determine the number of layers required to give a reasonable fit between (20) Zhang, X.; Xia; Matyjaszewski, K. Macromolecules 1998, 31, 5167–5169. (21) Xu, C.; Wu, T.; Drain, C. M.; Batteas, J.; Fasolka, M. J.; Beers, K. L. Macromolecules 2006, 39, 3359–3364. (22) Penfold, J.; et al. J. Chem. Soc., Faraday Trans. 1997, 93, 3899–9917. (23) Nelson, A. J. Appl. Crystallogr. 2006, 39, 273–276.

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Figure 2. Tapping-mode AFM images of the dry sapphire interface (a) before and (b) after adsorption of the macroinitiator and (c) after the growth of an 18 nm dry thickness brush grown using the same procedure as for the brushes grown for neutron reflectivity measurements. the measured and calculated reflectivities. The scattering length density profiles, F(z), were then optimized using a genetic algorithm in which the layer thicknesses, scattering length densities, and roughnesses are varied to minimize χ2 between the measured and calculated reflectivities. The monomer volume fraction profiles were then determined as φðzÞ ¼

FðzÞ - FD2 O FDMAEMA - FD2 O

ð1Þ

where FD2O (= 6.35
10-6 A˚-2) is the scattering length density of D2O and FDMAEMA (= 0.8
10-6 A˚-2) is the scattering length density of the polymer.24 From the volume fraction profile, we evaluate the surface excess R of polymer grafted to the interface, γ= ¥ 0 φ(z) dz. If this surface excess differs by more than 20% from the dry brush thickness (determined by X-ray reflectivity and ellipsometry before the neutron reflectivity experiment), then the volume fraction profile is discounted and the fitting procedure repeated. The surface excesses of polymer determined at the two different pH values for each brush were constant to within 15%. The independent measurement of the dry thickness of the brush layer by ellipsometry and X-ray reflectivity provides an important constraint on the surface excess of polymer chains and hence on the volume fraction profile determined by fitting the neutron reflectivity data.

3. Results and Discussion The polymerization results in layers with dry thicknesses determined by X-ray reflectivity measurements (Figure S1) and ellipsometry to be in the range of 50-170 A˚. The tapping-mode AFM image (Figure 2c) displays a significant increase in the rms roughness to 6 A˚ for the 170 A˚ dry thickness layer. The neutron reflectivity profiles measured from the different polymer-coated substrates at different pH values are shown in Figure 3. The volume fraction profiles that correspond to the best fits to the measured reflectivities are shown in Figure 4. From the volume fraction profiles, the surface excess, γ, and a measure of the swollen thickness of the layer, L, have been determined,25,26 R¥ 2 0 zφðzÞ dz L ¼ R¥ ð2Þ 0 φðzÞ dz from which the degree of swelling can be evaluated as L/γ. Table 1 summarizes the values of γ, L, and L/γ obtained for all of the brushes. Estimates of the uncertainties associated with γ and L are obtained from perturbations to the best fit volume fraction profiles that still result in an acceptable fit to the measured reflectivity. In the case of brush c, the total amount of polymer determined from the fit to the neutron reflectivity profile is only 82% of the nominal dry brush thickness determined by X-ray (24) An, S. W.; Thirtle, P. N.; Thomas, R. K.; Baines, F. L.; Billingham, N. C.; Armes, S. P.; Penfold, J. Macromolecules 1999, 32, 2731–2738. (25) Mir, Y.; Auroy, P.; Auvray, L. Phys. Rev. Lett. 1995, 75, 2863. (26) Tran, Y.; Auroy, P.; Lee, L. T. Macromolecules 1999, 32, 8952–8964.

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Figure 3. Specular neutron reflectivity profiles obtained from PDMAEMA brushes a-d (as in Table 1) at the sapphire-D2O interface; solid lines give best fits to the reflectivities. The profile for brush a is on the correct absolute scale, whereas those for brushes b-d are offset by a constant scaling factor of 103.

reflectivity and ellipsometry. We attribute this to the retention of H2O by the hydrophilic PDMAEMA chains during the dry thickness measurement, which is then exchanged for D2O prior to the neutron reflectivity measurement. In all cases, the resulting volume fraction profiles can be approximately represented by Gaussian distributions (φ(z) = φ0 exp(-z2/HG2), shown as dashed lines in Figure 4, for which the parameters are summarized in Table 2. In the case of brushes b-d, at pH 7 there are deviations from the Gaussian distribution that take the form of a spike close to the sapphire interface followed by a small local minimum. These spikes are responsible for the modulation of the reflectivity profiles at Q ≈ 0.2 A˚-1, and the local maximum at 100-200 A˚ is responsible for the oscillatory modulation at lower Q, which is particularly evident for brush b. The spikes account for less than 5% of the total polymer and could be explained by the adsorption of DMAEMA segments at the solid-liquid interface. The presence of these segments at the interface will make it repulsive toward DMAEMA segments in the chains stretching away from the interface, creating the small depletion region. The dimension of the depletion region is expected to scale with chain length and so is not resolved for the shorter chains of brush a. A contributing factor to the adsorption of DMAEMA segments could be that the adsorption Langmuir 2010, 26(15), 12684–12689

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Table 1. Amount of Polymer (γ), Swollen Thickness (L), and Swelling Ratio (L/γ) for the PDMAEMA Brushes sample

polymerization time (mins)

dry thickness (A˚)

γ (A˚)

LpH 7 (A˚)

(L/γ)pH 7

LpH 3 (A˚)

(L/γ)pH 3

a b c

30 45 45

50 ( 5 110 ( 11 170 ( 15

47 ( 5 108 ( 8 140 ( 14

168 ( 20 457 ( 57 477 ( 56

3.6 3.9 3.4

d

60

170 ( 15

172 ( 17

517 ( 73

3.0

261 ( 31 569 ( 78 641 ( 92 LpH 9 (A˚) 538 ( 80

5.6 5.7 4.6 (L/γ)pH 9 3.2

Figure 4. Interfacial volume fraction profiles determined for the PDMAEMA brushes a-d (as in Table 1) at pH 3 (red), pH 7 (green), and pH 9 (blue); dashed lines give the Gaussian fits (black for pH 3). Table 2. Parameters Characterizing Fits of the Volume Fraction Profiles to Gaussian Distributions (O(z) = O0 exp(-z2/HG2)) pH 7

pH 3

sample

φ0

HG (A˚)

a b c d

0.39 0.33 0.37 0.42

130 394 427 473

pH 9

φ0

HG (A˚)

0.26 0.22 0.28

195 505 571

φ0

HG (A˚)

0.39

494

of the anionic macroinitiator results in a charge reversal of the functionalized sapphire interface (to overall net negative). This will favor the adsorption of protonated DMAEMA segments because of the entropic gain that would result from the release of the hydroxyl counterions. Alternatively, the dense inner region could be attributable to a population of short chains associated with polydispersity in the brush layer: calculations suggest that the short chains tend to be more compressed and the longer chains tend to be more stretched, producing similarly shaped volume fraction profiles compared to those observed in this study.27,28 The effect of polydispersity would be expected to be less significant in brush a, which is grown in the shortest polymerization time. When the solution pH is lowered to 3, the PDMAEMA chains become more protonated and more highly swollen because there is a greater osmotic driving force stretching the chains away from (27) de Vos, W. M.; Leermakers, F. A. M. Polymer 2009, 50, 305–316. (28) Kritikos, G.; Terzis, A. F. Polymer 2007, 48, 638–651.

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the interface. The polymer segment distributions become Gaussian and no longer display the spike and local minimum/maximum. In all cases, there is a reduction in the volume fraction at the interface. The increase in swelling is evident from the increases in L/γ in Table 1 and the increases in HG in Table 2. The volume fraction profiles observed at pH 3 are similar in shape to those observed by Sanjuan et al. at pH 2 for PDMAEMA brushes grown at a grafting density of 0.23 nm-2 from a silica interface.10 At pH 7, these workers also observed a broad Gaussian distribution with a significant increase in the volume fraction at the interface. However, no distinctive spike or local minimum was resolved, which may be due to the more restricted Q range used in the earlier study. Self-consistent field theory predicts a Gaussian distribution for the osmotic regime of weak polyelectrolyte brushes in which the counterions are confined within the brush.29 For weak polybases such as PDMAEMA, the charge on any DMAEMA segment is determined by an acid-base equilibrium, with an increase in the Hþ concentration at low pH favoring charged segments over uncharged segments. (The pKa for PDMAEMA chains in dilute aqueous solution is around 7.0-7.5.30) A recent self-consistent field theory study demonstrated that the fraction of protonated cationic segments varies with the segment density through the brush layer and is lowest in the interior of the brush where the segment density is highest.31 This is a refinement of the quasi-neutral regime predicted by Zhulina and Borisov for brushes at a high grafting density and a low fraction of charged segments.32 In this regime, the osmotic pressure due to the finite volume of the polymer segments is at least equal to that due to the counterions confined within the brush, causing the brush to swell more like a neutral brush rather than like a charged brush. In their study of PDMAEMA brushes grafted from a silane initiator layer on silica, Sanjuan et al. found that the osmotic pressure of the monomers is equal to the osmotic pressure of the counterions at pH 7.10 Consequently, PDMAEMA brushes at pH 7 are expected to swell as neutral brushes. This provides an explanation for the observation that the swelling ratio and segmental distribution of brush d hardly change when the pH is increased from 7 to 9. We note that even at pH 7 and 9 the PDMAEMA brush layers contain a relatively high volume fraction of water. This suggests that, although the polymer chains may be less hydrophilic than at pH 3, water is certainly not a poor solvent. This is consistent with the absence of the complete collapse transition that has been predicted by scaling theory for the poor solvent case.33 For a neutral brush, the swollen thickness is predicted to scale as L  Nσ1=3

ð3Þ

where N is the degree of polymerization and σ is the grafting density. The total amount of polymer is γ = Na3σ, where a3 is the (29) Zhulina, E. B.; Borisov, O. V. J. Chem. Phys. 1997, 107, 5952–5967. (30) B€ut€un, V.; Billingham, N. C.; Armes, S. P. Polymer 2001, 42, 5993–6008. (31) Witte, K. N.; Sangtae, K.; Won, Y.-Y. J. Phys. Chem. B 2009, 113, 11076– 11084. (32) Zhulina, E. B.; Borisov, O. V. Macromolecules 1996, 29, 2618–2626. (33) Ross, R. S.; Pincus, P. Macromolecules 1992, 25, 2177–2183.

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Table 3. Grafting Densities (σ), Degrees of Polymerization (N), and Corresponding Chain Molecular Weights (Mw) sample

polymerization time (min)

γ (A˚)

σ (nm-2)

N

Mw (kg/mol)

a b c d

30 45 45 60

47 108 140 172

0.13 ( 0.02 0.12 ( 0.02 0.14 ( 0.02 0.18 ( 0.03

155 443 430 434

24 ( 5 70 ( 16 68 ( 15 68 ( 15

monomer volume, so the swelling ratio scales as L  σ - 2=3 γ

ð4Þ

Sanjuan et al. determined the swelling ratio for PDMAEMA brushes grafted from a silane initiator layer on silica for a range of grafting densities.10 They find a swelling ratio of L/γ = 2.5 for a brush at a grafting density of 0.23 nm-2 at pH 7. Because the osmotic pressure due to the segment excluded volume equals that due to the counterions in this brush layer, it will also swell as a neutral brush and so should scale as eq 4 with the same constant of proportionality (monomer volume) as for the PDMAEMA brushes studied here. We use eq 4 to estimate the grafting densities, σ, for the brushes grown from the macroinitiator, which we list in Table 3. The density of the grafted chains is approximately constant at 0.14 ( 0.03 nm-2, which corresponds to an average chain spacing of 2.6 nm. The density of grafted chains is significantly lower than the density of 2-bromoester groups (1.03 ( 0.06 nm-2) determined by Edmondson et al.15 for this macroinitiator adsorbed at a silicon oxynitride-water interface rendered cationic by an aminefunctionalized silane monolayer. This suggests that initiation is less than 100% efficient, which is also the case for small-molecule initiators.8,34,35 An initiation efficiency of 10% has been estimated for methyl methacrylate brushes grown from a thiol initiator on gold.35 The slightly lower chain density observed for brush b suggests that the underlying coverage of the macroinitiator must be slightly lower than for the other brushes. The driving force for macroinitiator adsorption is the release of counterions (OH-) from the sapphire interface, which bears a positive charge associated with the protonated hydroxyl groups. We suggest that there could be some variability in the plasma treatment of the sapphire substrate that was used to increase the surface density of hydroxyl groups. If this is correct, then it would reduce the adsorbed amount of macroinitiator, which in turn would result in a lower density of grafted PDMAEMA chains in brush b. From the surface excess, γ, and the estimate of the grafting density, σ, it is possible to estimate the degree of polymerization of the chains as γ ð5Þ N ¼ συm where υm (= 225 A˚3) is the volume of a DMAEMA monomer.24 The resulting degrees of polymerization and corresponding molecular weights are summarized in Table 3. The overall trend is for longer polymerization times to produce higher degrees of polymerization, as expected. Nevertheless, it is clear that the PDMAEMA brushes grown for this study do not follow the (34) Jones, D. M.; Brown, A. A.; Huck, W. T. S. Langmuir 2002, 18, 1265–1269. (35) Kim, J.-B.; Bruening, M. L.; Baker, G. L. J. Am. Chem. Soc. 2000, 122, 7616–7617. (36) Kershner, R. J.; Bullard, J. W.; Cima, M. J. Langmuir 2004, 20, 4101–4108. (37) Franks, G. V.; Gan, Y. J. Am. Ceram. Soc. 2007, 90, 3373–3388. (38) Zhang, L.; Tian, C.; Waychunas, G. A.; Shen, Y. R. J. Am. Chem. Soc. 2008, 130, 7686–7694. (39) Kosmulski, M. J. Colloid Interface Sci. 2009, 337, 439–448.

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perfectly linear dependence expected for living polymerization. The deviation observed at short reaction times could perhaps be explained by inefficient activation at the beginning of the polymerization, as has been suggested by Jones et al.34 For longer reaction times, the polymerization may have lost its living character within 1 h. Although the reaction conditions used here clearly do not provide the same control that can be achieved for the polymerization of other methacrylic monomers from this macroinitiator,15 they nevertheless afford sufficient control to achieve our main aim of growing PDMAEMA brushes at an approximately constant grafting density and dry thicknesses that increase in steps of approximately 5 nm. The observed absence of any change in the swelling behavior between pH 7 and 9 contrasts with the continuous variation in swelling observed for PDMAEMA brushes anchored by means of a hydrophobic methyl methacrylate block at a hydrophobized silicon interface.24 In that case, the grafting density and layer thickness were significantly lower than in the brushes studied here, meaning that the segment excluded volume osmotic pressure will not dominate the counterion osmotic pressure. Between pH 7 and 9, Sanjuan et al. observed only small changes in the outer regions of the PDMAEMA brush grown on silica, where the volume fraction is low.10 Recently, Tomlinson et al. achieved a grafting density of ∼0.1 nm-2 for PDMAEMA side chains of slightly lower molecular weight (∼50K) by Langmuir-Schaeffer transfer of comb polymers comprising a poly(methyl methacrylate) backbone and PDMAEMA side chains onto hydrophobized silicon. This value is comparable to that achieved using the macroinitiatorfunctionalized interface used in the present study.16 The volume fraction profiles observed by Tomlinson et al. at pH 8 for prepared PDMAEMA brushes are similar in shape to those observed in this study at pH 7, although the volume fractions that they observe are slightly higher and fall off more rapidly.16 They observe a dramatic swelling of the layer when the pH is lowered to 4, which is comparable to the swelling behavior observed at pH 3 in this study. However, the middle of their distribution has a local maximum that is attributed to the effect of greater charge density toward the free chain ends, as in their related study of PDEA brushes grafted from silicon.9 At pH 3, a sufficiently large fraction of the segments become protonated that the osmotic pressure of the associated hydroxyl counterions now exceeds that due to the segment volume. This tends to pull segments away from the interface, removing the spike layer and leading to the more swollen distributions and higher swelling ratios that can be seen in Figure 4 and Table 1.

4. Conclusions Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes have been grown by surface-initiated atom transfer radical polymerization (SI-ATRP) using an anionic macroinitiator adsorbed at the sapphire-water interface. Neutron reflectivity profiles measured from the polymer-functionalized sapphireD2O interfaces can be fit to volume fraction profiles that are close to Gaussian distributions. The swelling ratio of the brushes increases from ∼3.5 ( 0.5 at pH 7 to ∼5.1 ( 0.5 at pH 3 but does not change for the PDMAEMA brush studied at pH 9. At pH 7 and 9, excluded volume interactions between the polymer segments dominate, whereas at pH 3 the weak polyelectrolyte is protonated. This results in an additional counterion contribution to the osmotic pressure, which swells the brushes. The additional swelling removes the dense spike layer that is observed near the sapphire interface in all of the brushes except the one with the Langmuir 2010, 26(15), 12684–12689

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lowest degree of polymerization. By assuming that at pH 7 the brushes swell according to scaling predictions for neutral brushes, the results of a previous investigation of PDMAEMA brushes grafted from silica interfaces10 can be used to estimate the interfacial chain density and corresponding chain lengths of the chains grafted from the macroinitiator at the sapphire interface. Acknowledgment. M.M. acknowledges MRTN-CT-2004512331 (SOCON) for the award of an Early Stage Researcher

Langmuir 2010, 26(15), 12684–12689

Article

Fellowship. S.T. thanks the Royal Society for a University Research Fellowship. We thank ISIS (Rutherford Appleton Laboratory, STFC) for the award of beam time. Supporting Information Available: X-ray reflectivity profiles measured from the dry collapsed brushes at the sapphire interface and the corresponding fits from which the dry thicknesses are determined. This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la101550w

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