Structure of pH-Responsive Polymer Brushes Grown at the Gold

Dec 14, 2011 - polymerization (SI-ATRP) from the gold−water interface at different grafting densities ..... As this spike layer is independent of pH...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/Macromolecules

Structure of pH-Responsive Polymer Brushes Grown at the Gold− Water Interface: Dependence on Grafting Density and Temperature Haidong Jia,† Andrew Wildes,‡ and Simon Titmuss*,§ †

Department of Chemistry, Physical & Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, U.K. ‡ Institut Laue-Langevin, BP 156, 38402 Grenoble Cedex 9, France § School of Physics & Astronomy, University of Edinburgh, Edinburgh EH9 3JZ, U.K. S Supporting Information *

ABSTRACT: Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes have been grown by surface-initiated atom transfer radical polymerization (SI-ATRP) from the gold−water interface at different grafting densities, and neutron reflectivity has been used to characterize the response of the brush structures to changes in pH and temperature. Low-density brushes show the greatest response to changes in pH, with the swelling of the highest density brushes essentially independent of pH. The scaling exponent of the swelling ratio with grafting density changes from β ∼ −0.7, which is typical for neutral brushes, at pH 10, to β ∼ −1.4, at pH 3, reflecting the change in the dominant contribution to the osmotic pressure in the brush. At low pH, the osmotic pressure due to the counterions of the charged segments exceeds that of the segmental excluded volume, except in the highest density brushes, resulting in the pH-dependent swelling response of all but the highest density brushes. At pH 10, increasing temperature causes a partial collapse of the brushes with a transition temperature in the range 30−40 °C. The transition is associated with the dehydration of the hydrophilic segments. The magnitude of the deswelling decreases with increasing grafting density, reflecting the decreasing volume fraction of water in the brush.

1. INTRODUCTION Polymer brushes are layers of polymer chains grafted to an interface at a sufficiently high density 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 dispersions2 as well as tuning the functional properties of interfaces.3,4 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, sensors, and in control of wetting properties.5 Control over interfacial wetting of nanoparticles is key in the use of such particles to stabilize Pickering emulsions of immiscible liquids.6,7 Dense polymer brushes can be formed by grafting from both planar or nanoparticle interfaces by surface-initiated polymerization either from a small molecule initiator, anchored via thiol or silane chemistry at gold or silica interfaces, respectively,8 or from polyelectrolytic macroinitiators, adsorbed at charged interfaces.9 Both approaches have been used in the preparation of stimulus-responsive brushes, which have been characterized by neutron reflectivity measurements.10−15 PDMAEMA is a weak polyelectrolyte that exhibits conformational changes in response to both pH and temperature.11,14 We have used SI-ATRP to functionalize gold nanoparticles with layers of responsive PDMAEMA brushes.16 These functional© 2011 American Chemical Society

ized nanoparticles stabilize Pickering emulsions that can be reversibly broken by altering the pH, giving potential applications in controlled release.16 Our overall goal is to relate the microscopic changes in the conformation of the PDMAEMA layer to the macroscopic response of dispersions of gold nanoparticles functionalized with the same responsive polymer layers.16 Here we focus on the characterization of the brushes and how their swelling behavior varies with grafting density, solution pH, and temperature. By using a planar geometry and neutron reflectivity, attention is focused on the conformational changes that occur in the polymer layer, without the complicating effects of a change in structure factor that occur in small-angle scattering measurements of the dispersions. The effect of interfacial curvature on layer structure and the microscopic and macroscopic structural response of the nanoparticle dispersions will be described in a future publication.

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. Received: August 11, 2011 Revised: December 7, 2011 Published: December 14, 2011 305

dx.doi.org/10.1021/ma201839y | Macromolecules 2012, 45, 305−312

Macromolecules

Article

Table 1. Dry Thicknesses, Grafting Densities, and Swollen Thicknesses for the PDMAEMA Brushes sample

G4

G2

G7

G5

initiator/% polymerization time/h hdry/Å σ/Å−2 LpH 3/Å LpH 7/Å LpH 10/Å Tspike/Å ϕspike N f brush

10 1 50 ± 3 0.0015 ± 0.0001 234 142 127 24 0.94 148 0.57

10 2 90 ± 5 0.0014 ± 0.0001 564 338 271 20 0.94 286 0.76

25 1 100 ± 5 0.003 ± 0.0001 340 271 246 16 0.94 148 0.86

100 1 190 ± 5 0.0057 ± 0.0001 360 342 342 5 0.94 148 0.98

Cu(I)Br, Cu(II)Br2, 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA), 11-mercapto-1-undecanethiol (MUT), dichlorobenzene, propan-2-ol, iodine (I2), tetrahydrofuran (THF), and dichloromethane (CH2Cl2) were purchased from Aldrich and used without further purification; mercaptoundecyl bromisobutyrate (MUBB) was purchased from Prochimia and used without further purification. 2.1.2. Preparation of Gold-Coated Quartz Blocks. One-side polished quartz blocks (Crystran) with dimensions 50 × 60 × 11 mm were used as the superphase, through which the neutrons are incident on the interface. Before deposition of gold, the quartz blocks were first washed for 10 min with a 2% Hellmanex solution at 60 °C and then immersed in aqua regia (3:1 mixture of HCl and HNO3) for 2 h. The blocks were then successively rinsed with 18 MΩ cm conductivity water and ethanol, before drying under a nitrogen stream. The blocks were then subjected to 30 min of UV/ozone cleaning. The coating procedure was carried out in the Thin Film Facility at the Department of Physics, University of Oxford, using a Leybold L560 coating plant. The blocks were first coated with a ∼10 nm titanium adhesion layer, followed by a ∼10 nm gold layer. 2.1.3. Self-Assembly of the Initiator Monolayer. The gold-coated quartz blocks were immersed in a 5 mM solution of the initiator (MUBB) in dichlorobenzene for 12 h at room temperature, forming a self-assembled monolayer (SAM) with a thickness of 0.8 nm, as determined by ellipsometry and neutron reflectivity (data shown in Figure S2 and fit parameters in Table S1, in the Supporting Information). The grafting density of the polymer brushes was controlled by varying the molar ratio of the initiator (MUBB) to dilutant (MUT), an approach previously demonstrated by Jones et al.17 SAMs comprising 10%, 25%, and 100% MUBB have been used in this study. 2.1.4. Surface-Inititiated Atom Transfer Radical Polymerization. In a flask, 80 mL of a reaction mixture comprising DMAEMA (335 mmol), CuBr (1.34 mmol), CuBr2 (0.15 mmol), HMTETA (3.2 mmol), and 20 mL of propan-2-ol/water (8:2) was degassed by purging with nitrogen for 30 min. The mixture in the flask was then transferred under nitrogen to a reaction chamber containing the initiator-functionalized gold-coated quartz blocks. To obtain brushes comprising chains of similar molecular weights, the polymerizations on blocks G4, G5, and G7 (see Table 1) were performed under identical conditions for 1 h; for block G2, the polymerization time was doubled. 2.1.5. Fourier Transform Infrared Attenuated Total Reflection (FTIR-ATR). FTIR-ATR spectra were measured using a Bio-Rad FTS 7000 spectrometer, in which the infrared light is reflected back through an objective onto a liquid-nitrogen-cooled narrow-band mercury− cadmium−telluride (MCT) detector. The spectra comprise 256 scans at a resolution of 2 cm−1. 2.1.6. Matrix-Assisted Laser Desorption Ionization Mass Spectrometry (MALDI-MS). To determine the molecular weight of the PDMAEMA chains making up the brushes, the thiol bond was cleaved by oxidation following overnight incubation in a 4 mM solution of iodine in dichloromethane. The iodine/dichloromethane was then removed by reduced pressure evaporation, and the remaining polymer was dissolved in THF and analyzed by MALDI-MS.

MALDI-MS was performed on a Bruker time-of-flight spectrometer. The PDMAEMA cleaved from the interface (∼1 mg) was dissolved in 100 μL of THF, and 2 μL of this was mixed with 10 μL of a matrix solution comprising 0.1 M 2,5-dihydroxybenzoic acid (DHA) in a 1:1 (v/v) water/acetonitrile mixture, to which 2 μL of 0.1 M NaCl was added. Approximately 4 μL of this mixture was placed on the sample plate and dried. The sample was irradiated at λ = 337 nm at 5 Hz with a total accumulation of 200 shots per spectrum at a power slightly above the threshold for ion formation. The molecular weight was determined by numerical integration of the resulting peak area. 2.1.7. Ellipsometry. The dry thickness of the polymer layer was measured using a Beaglehole ellipsometer. The relative change in the phase shift and amplitude of the two perpendicular components of polarized incident light (λ = 632.8 nm) upon reflection from an interfacial layer are used to determine the mean refractive index and average thickness of the film.18 The dry polymer layer was modeled as a single layer sandwiched between gold and air and the refractive index of the PDMAEMA polymer was assumed to be described by Cauchy parameters An = 1.52 and Bn = 0.01. Measurements made at five different points yielded identical dry thicknesses. 2.1.8. Neutron Reflectivity. Specular neutron reflectivity measurements were made in time-of-flight mode using the D17 reflectometer at ILL.19 Reflectivity profiles presented in this work are plots of RQ4, where R is the specular reflectivity and Q = 4π sin θ/λ, with θ the glancing angle of incidence and λ the neutron wavelength, is the momentum transfer. Plotting and fitting the data in Porod form (RQ4) removes the effect of the reflectivity decreasing as R ∼ Q−4 at Q ≫ Qc expected for a sharp interface, highlighting the effects of small changes in reflectivity across the whole Q-range. Glancing angles of incidence of 0.3° and 2.3° were used giving a Q-range of 0.0033−0.22 Å−1. The reflected intensity was normalized to the incident beam spectral distribution and detector efficiency and established on an absolute reflectivity scale with a resolution ΔQ/Q = 5%. The reflectivity at Q > 0.2 Å−1 is dominated by sample-dependent background, which arises primarily from the incoherent scattering from the bulk solution. To remove this contribution, a background measured to the side of the specular ridge was subtracted before evaluating the reflectivity. The thickness and roughness of the gold and titanium layers were first determined by neutron reflectivity measurements from a quartz block (coated with the metal layers during the same deposition as those used to prepare the brush layers) with D2O and cmAu subphases. Typical reflectivity profiles and the metal layer thicknesses are shown in Figure S2 of the Supporting Information. The MOTOFIT package20 was used to fit the reflectivity profiles to model scattering length density profiles. The model comprises a layer of Ti between the quartz superphase and a layer of Au (layer thicknesses and roughnesses given in Table S1) capped by a 8 Å initiator layer (common to all models), followed by two layers with scattering length densities corresponding to mixtures of polymer and solvent to represent the brush region. Quartz was chosen as the superphase (through which the neutrons are incident on the interface) as its scattering length density is close to that of gold, which means that with a subphase that is contrast306

dx.doi.org/10.1021/ma201839y | Macromolecules 2012, 45, 305−312

Macromolecules

Article

Figure 1. Porod plots of specular neutron reflectivity measured from PDMAEMA brushes (as in Table 1) at the quartz|Ti|gold−water (cmAu) interface at a temperature of 21 °C; solid lines give best fits to the reflectivities. The plots measured at pH 3 are on the correct absolute scale, whereas those measured at pH 7 and pH 10 are offset by scaling factors of 10 and 100, respectively.

Figure 2. Interfacial volume fraction profiles determined for the PDMAEMA brushes (as in Table 1) at pH 3, pH 7, and pH 10. matched to gold (cmAu), a buildup of hydrogenous material at the gold interface creates a well in the scattering potential between the

gold interface and the bulk cmAu phase, as illustrated by Figure S1 of the Supporting Information. This makes the curvature of the Porod 307

dx.doi.org/10.1021/ma201839y | Macromolecules 2012, 45, 305−312

Macromolecules

Article

plot in the range 0.004 < Q < 0.05 Å−1 sensitive to the shape of the polymer volume fraction profile. The scattering length density profiles of the brush region, ρ(z), were optimized using a genetic algorithm in which the layer thicknesses, scattering length densities and roughnesses are varied to minimize the χ2 between the measured and calculated reflectivities. The monomer volume fraction profiles were then determined as

φ(z) =

ρ(z) − ρsub ρDMAEMA − ρsub

(1)

where ρsub is the scattering length density of the D2O (ρD2O = 6.35 × 10−6 Å−2) or cmAu (ρcmAu = 4.5 × 10−6 Å−2) subphase and ρDMAEMA(= 0.8 × 10−6 Å−2) is the scattering length density of the polymer.21 From the volume fraction profile, we evaluate the surface excess of polymer grafted to the interface, γ = ∫ 0∞ϕ(z) dz, and a measure of the swollen thickness of the layer, L22,23 ∞ 2 ∫ z φ(z) dz 0 L= ∞ ∫0 φ(z) dz (2) Neutron reflectivity was measured with both D2O and cmAu subphases at pH 7. The covalent grafting of the polymer means that the total amount of polymer at the interface must be the same in each of these measurements and should be consistent with the dry thickness measured by ellipsometry, hdry, before the neutron reflectivity experiment. The total amount of polymer obtained from these measurements was used to constrain the fitting of the reflectivities measured, at different pHs and temperatures, with a single cmAu subphase.

Figure 4. Porod plots of specular neutron reflectivity measured from PDMAEMA brush G2 (see Table 1) at the quartz|Ti|gold−water (cmAu) interface at pH 10 with increasing temperature; solid lines give best fits to the reflectivities. The plot measured at 21 °C is on the correct absolute scale, whereas those measured at increasing temperatures are offset by a constant scaling factor of 10.

3. RESULTS AND DISCUSSION 3.1. Polymer Brush Growth and Characterization. Four different brushes, characterized by three different grafting densities and two different degrees of polymerization, have

Figure 5. Interfacial volume fraction profiles determined for the PDMAEMA brush G2 (see Table 1) at pH 10 with increasing temperature.

Figure 3. Plots of swelling ratio for the swellable part of the brush as a function of grafting density at different pHs.

Table 2. Scaling Exponents for Brush Swelling Ratio with Degree of Polymerization (α) and Grafting Density (β) pH

prefactor

α

β

3 7 10

0.001 0.03 0.04

−0.19 −0.05 −0.06

−1.39 −0.82 −0.73

been prepared, and their properties are summarized in Table 1. Figure S3 of the Supporting Information shows the FTIR-ATR spectrum of a PDMAEMA (100% MUBB, 1 h polymerization, dry thickness = 19 nm) brush grown from a gold−water

Figure 6. Temperature dependence of the swelling ratio for the PDMAEMA brushes. 308

dx.doi.org/10.1021/ma201839y | Macromolecules 2012, 45, 305−312

Macromolecules

Article

layer is associated with the polymer layer rather than the initiator monolayer. We have previously observed a similar spike layer in our study of PDMAEMA brushes grown from a macroinitiator adsorbed on sapphire,14 and similar density profiles have been observed for PDMAEMA brushes formed by a Langmuir− Schaefer transfer of diblock copolymers, with a hydrophobic PMMA anchoring block, from the air−water interface onto a hydrophobized silicon substrate.25 The spike layer is required to ensure that a good fit is obtained across the complete Qrange of the Porod plots for models consistent with the total amount of polymer at the interface determined by ellipsometry. The absence of such a spike from the density profiles determined for PDMAEMA brushes grown from silica interfaces could be associated with the more limited Q-range employed in their neutron reflectivity measurements,11 as it is the high Q-range that will offer the greatest sensitivity to thin interfacial layers; furthermore, our approach of fitting to a Porod plot treats the high Q-range with a comparable weight to the low Q-range, which is predominantly sensitive to the overall shape of the density profile and total amount of polymer at the interface. As this spike layer is independent of pH, the segments comprising it are clearly not involved in the swelling of the brush. We suggest that an attractive interaction either between the DMAEMA segments and the initiator-coated gold interface or between DMAEMA segments in the lower part of the brush is sufficient to overcome the swelling effect of the osmotic pressure due to the segment excluded volume and the counterions. An increase in either of these contributions to the free energy of the chain will lead to an increase in the swelling force that is exerted on the adsorbed segments comprising the spike. From Table 1 it is clear that as the grafting density, σ, increases the fraction (and total number) of segments that are adsorbed at the initiator−water interface decreases. For a weak polyelectrolyte, there are two osmotic contributions to the free energy of the brush that favor stretching of an individual chain, against the entropic elasticity, driving brush swelling. Within a Flory−Huggins approach, the combinatorial excluded volume contribution to the osmotic pressure scales as Πmix ∼ ϕ2. When the segments are charged, the condition of local electroneutrality means there will also be an osmotic pressure due to counterions confined within the brush, Πcounter ∼ Nσ, which implies that the osmotic stretching force on each chain due to the counterions is independent of the grafting density. This means that the increased swelling force that pulls segments out of the spike layer of adsorbed PDMAEMA must be a consequence of the increased excluded volume contribution to the osmotic pressure at high grafting density. It is the Gaussian part of the brush distribution which changes in response to changes in pH, causing the changes in reflectivity at low-Q, that are associated with changes in the overall shape of the scattering potential well between the quartz|Ti|Au superphase and the cmAu subphase. The response of the outer part of the brush distribution to changes in pH is characterized by the degree of swelling (L*/ γ*), where γ* = hdry − ϕspikeTspike is the amount of polymer in the swollen Gaussian part of the distribution and L* is a measure of the swollen thickness of this part of the distribution, determined by evaluating the moment of the outer part of the brush (i.e., excluding the spike) relative to the base of the

interface. The appearance of C−H stretches at 2700−3000 cm−1, ester carbonyl stretch at 1729 cm−1, and tertiary amine peak at 1150 cm−1 demonstrates the successful growth of PDMAEMA brushes from the gold−water interface. To obtain the molecular weight of the grafted PDMAEMA chains, the chains were first oxidatively cleaved from gold interface using iodine, resulting in the disappearance of these characteristic infrared absorption peaks. MALDI-MS was used to determine the molecular weight. A typical mass spectrum obtained from PDMAEMA cleaved from a gold interface following a 1 h polymerization is shown in Figure S4 of the Supporting Information. Numerical integration of the highest intensity peak (27 463 Da) gives the number-average molecular weight Mn = 26 360 Da; the mass spectrum also shows dimer and trimer peaks at 54 733 and 82 102 Da, respectively. The dry thickness measured by ellipsometry at a given grafting density was found to increase linearly with polymerization time unless the polymerization time exceeded 2 h, meaning that polymerization time can reliably be used to control the degree of polymerization. For each brush, ellipsometry measurements at five different positions yielded the same dry thickness. The grafting density can be controlled by choosing the appropriate ratio of initiator to dilutant thiol: in this study solution compositions of 10%, 25%, and 100% MUBB have been used. The grafting density was evaluated from the measured dry thickness h and grafted chain molecular weight (M) as

ρ hNA σ= m (3) M where ρm is the mass density of the DMEAMA. We found that it was not possible to obtain brushes with a grafting density σ > 0.006 Å−2 even for an incubation solution composition of 100% MUBB and incubation times in excess of 24 h. Although the grafting density increases with the mole fraction of MUBB in the incubation solution, the increase is nonlinear, which raises the possibility that the chain initiation efficiency is dependent on the fractional coverage of initiators in the thiol monolayer. It appears that the initiation efficiency decreases with increasing density of grafted chains. At high chain density, crowding within the polymer layer will affect the local concentration of reactants and hence the relative reaction rates of initiation, propagation, and termination.24 This is likely to exert the greatest influence on reactions between grafted chains, such as termination between grafted radicals and degenarative chain transfer between a grafted chain and a grafted radical. 3.2. pH Response of PDMAEMA Brushes. Figure 1 shows Porod plots of the neutron reflectivity measured from the brushes listed in Table 1 as a function of pH. In all cases the reflectivity at Q > 0.04 Å−1 is dominated by the Kiessig fringes that result from the well-defined titanium, gold, and initiator layers that are sandwiched between the quartz and the PDMAEMA−water interface. Comparison of the high-Q regions for G5/G7 indicate that this fringe structure is modulated by changes in the polymer layer, although it is in the low-Q region, just above the critical edge, that changes in brush structure with pH lead to the most obvious changes in reflectivity. Figure 2 shows the volume fraction profiles that give the best fits to the Porod plots. The volume fraction profiles comprise a thin (layer thickness Tspike), high volume fraction (ϕspike) layer or spike, at the solid−liquid interface, that is independent of pH, followed by a Gaussian-like decay into the liquid phase. From Figure S1, it can be seen that the spike 309

dx.doi.org/10.1021/ma201839y | Macromolecules 2012, 45, 305−312

Macromolecules

Article

ization, which is evident from the values of α in Table 2, was also observed in the SCF study for brushes comprising chains of less than 102 Kuhn segments.28 When designing polymer layers to be responsive to changes in pH, it is clear from Figure 3 that the highest grafting density measured (G5 corresponding to 100% MUBB) is not suitable. At this grafting density, the volume fraction of polymer is sufficiently high that the osmotic pressure due to the segments exceeds that due to the counterions of the charged segments, even at pH 3, resulting in a degree of swelling is essentially independent of pH. It was noted above that the observation of a spike layer at the base of the brush is suggestive of either an attractive interaction between the DMAEMA segments and the initiator-coated gold interface or between DMAEMA segments in the lower part of the brush. Although PDMAEMA has been used to anchor diblock polymers to the interfaces of bare gold nanoparticles,29 in the system studied here, the gold interface is covered by a dense thiol layer, so the electrostatic anchoring mechanism suggested for the PDMAEMA on bare gold nanoparticles is unlikely to be applicable. We suggest two alternative mechanisms for this attractive interaction, depending on the charge on the segments at the base of the brush, closest to the interface. In the case that these segments are either charged or exist as a dipole comprising ionized segment and a bound counterion, a charge−image−charge or dipole−image−dipole interaction at the dielectric discontinuity of the thiol-coated gold−water interface could lead to an attraction between the segment and the interface. In the case that the segments at the base of the brush are uncharged, then the effective solvent quality of these segments will be poorer, resulting in a net attraction for the hydrophobic thiol layer. For sufficiently dense brushes, SCF calculations have predicted that a large fraction of segments nearest the grafting interface of weak polyelectrolyte brushes might be uncharged, as the high local concentration of segments, coupled to a long screening length, shifts the chemical equilbrium of the acid−base ionization toward the uncharged segments.28,30 In addition to driving attraction between the uncharged segments and the hydrophobic thiolcoated interface, this will also favor an attraction between uncharged DMAEMA segments, which will also result in a dense inner layer. Recently, experimental31,32 and theoretical30,33 studies have suggested that weak polyelectrolyte brush systems can be unstable with respect to the formation of lateral inhomogeneities. The driving force for this instability is the existence of a minimum in the chain chemical potential with coverage, caused by the coupling between the chemical equilibrium between charged and uncharged segments, the solvent quality, and the local segment density, which drives the tethered chains to microphase separate into high- and low-density domains.30 The experimental observations were made by AFM on PDMAEMA brushes formed from diblock copolymers transferred from an air−water interface by Langmuir−Blodgett deposition.31,32 The conformational degrees of freedom that are responsible for determining the length scale of the lateral inhomogeneities are absent from the covalently grafted chains that form the brushes that we studied. Furthermore, the brushes studied here are at grafting densities that exceed those in the AFM studies and so outside the lateral instability region. Nevertheless, the authors of the AFM study point out that that in the case of immobile chains the instability that drives the lateral microphase separation in their system will result in a microscopic

swellable brush region at the outer edge of the spike.26 Figure 3 shows a plot of the degree of swelling as a function of grafting density for the three lower molecular weight brushes at the three pHs. From the volume fraction profiles in Figure 2 and the degrees of swelling plotted in Figure 3, it is clear that the low density brushes (G4 and G2) display the greatest pH response and that the response (degree of swelling) depends only weakly on the degree of polymerization. The swelling follows a well-defined power law dependence on the grafting density, σ

L* ∼ (fN )α σβ γ*

(4)

where f is the fraction of the N monomers of the polymer that are in the swollen Gaussian part of the distribution. There is a clear change in the scaling exponent β from β = −0.82 at pH 7 to β = −1.4 at pH 3. These values are comparable to the β = −0.7 obtained for uncharged PDMAEMA in methanol and β = −0.95 obtained for PTMAEMA, the quaternized strong polyelectrolyte analogue, in water.11 From the small differences in the degrees of swelling observed for the two brushes with different degrees of polymerization (G2 and G4), the scaling parameter α has been evaluated at the three pHs and is tabulated in Table 2. The classic scaling approach to weak polyelectrolyte brushes predicts27

L* ∼ N 0 σ−4/3cs−1/3 γ*

(5)

where cs is the concentration of added salt (i.e., ions that cannot participate in the acid−base equilibrium). Using a continuum numerical SCF approach, Witte et al. explicitly demonstrated that such a scaling law is only obtained when the compressibility constraint is removed, i.e., in the limit that the effective segment volume tends to zero.28 For a finite segmental excluded volume they obtain a range of scaling exponents for brush height with grafting density (L ∼ σl) from l = 0.14 to l = 0.3, depending on the solvent quality, segmental excluded volume, pH, and ionic strength. As γ* ∼ Nσ, the corresponding values of β lie in the range −0.86 to −0.7. The values of β obtained experimentally here, shown in Table 2, range from close to the classic scaling result of eq 5 at pH 3 to the results of the numerical SCF study at pH 7 and 10. In particular, the value of βpH 3 = −1.4 implies that the brush height at pH 3 scales only weakly with grafting density (l = −0.4). The small negative scaling exponent indicates that the brush is truly in the osmotic regime in which the osmotic pressure due to the counterions dominates over that due to the segmental excluded volume. This explains the low prefactor observed at pH 3: the osmotic pressure due to the counterions is sufficiently high that the average volume occupied by each segment in the swollen brush is greater than the excluded volume of the segment, which is equivalent to relaxing the incompressibility constraint described by Witte et al.28 The nonzero scaling exponent with grafting density is a result of the increase in the local concentration of monomers, with increasing σ, which shifts the acid−base equilibrium toward the reactants (i.e., unprotonated monomers), decreasing the average degree of charging, lowering the osmotic pressure of the confined counterions, leading to a less swollen brush compared to a lower grafted density brush. The weak scaling dependence of the degree of swelling on degree of polymer310

dx.doi.org/10.1021/ma201839y | Macromolecules 2012, 45, 305−312

Macromolecules

Article

temperature-induced collapse transition highlights the twophase nature of these brushes in this regime and the volume fraction dependence of the Flory χ parameter.37,39,40 The dependence of solvent quality on volume fraction results in incomplete brush collapse, occurring first at the base of the brush where the monomer density is highest. We suggest that it is the swelling of the low-density outer region of the brush that provides an osmotic force pulling the chain out of the brush. Only for the brush formed from high molecular weight chains is this force sufficiently large for chains to desorb from the gold interface. The origin of the volume fraction and temperature dependence of the χ parameter that is manifested as a lower critical solution temperature has been explained in terms of a hydrophilic/hydrophobic two-state model,39 in which hydrogen-bonding interactions and the translational entropy of water play a key role. As the temperature increases, dehydration of the DMAEMA monomers is favored, provided that the volume fraction of monomers is sufficiently high for water−monomer hydrogen bonds to be replaced by monomer−monomer hydrogen bonds. The high average volume fraction of polymer in the densest brush means that the monomers are already less hydrated than in the lower density brushes, so the decrease in hydration that occurs on increasing the temperature above the transition results in a smaller decrease in the excluded volume and hence a smaller deswelling than in the lower density brushes.

reorganization of the chain conformation in both the lateral and vertical directions.32 The specular neutron reflectivity measurements we present provide a laterally averaged density profile. The dense spike layer could be a manifestation of a vertical reorganization into a dense inner layer and a dilute outer layer, consistent with the prediction of SCF calculations, which predict a low fraction of charged segments in the dense inner region even at low pH.28 3.3. Temperature Response of PDMAEMA Brushes. PDMAEMA is also responsive to changes in temperature, and the thermoresponsive properties are dependent on pH, with the cloud point decreasing from 76 °C at pH 7 to around 38 °C at pH 10, for solutions of a linear PDMAEMA of comparable molecular weight to the chains forming the brushes in this study.34 Figure 4 shows the variation in the neutron reflectivity from brush G2 at pH 10 as the temperature is changed (temperature-dependent neutron reflectivities for the other brushes are given in Figure S5 of the Supporting Information). The corresponding volume fraction profiles that give the best fits to the measured reflectivities are shown in Figure 5 for G2 and in Figure S6 in the Supporting Information for the other brushes. There is a clear change in the curvature of the Porod plots that occurs at Q ∼ 0.009 Å−1 at a temperature between 30 and 35 °C. (Figure S7 in the Supporting Information highlights this region of the experimental reflectivity profiles.) This can be attributed to a pronounced increase in the volume fraction of polymer just beyond the interfacial spike that is accompanied by a more rapid decay into the solution phase, resulting in a less swollen layer. For the other brushes, fewer measurements were possible due to time constraints, so it is only possible to identify that the transition occurs between 30 and 40 °C. The volume fraction profiles have some similarities with those determined by Yim et al. in their neutron reflectivity study of PNIPAM brushes.13,35,36 They attribute the bilayer structure to a vertical phase separation into an inner dense phase and an outer dilute phase, as has been predicted for brushes formed from chains that are at a temperature above the LCST.37 The response of the brushes to temperature is summarized in Figure 6, which plots L*/γ* as a function of temperature; the amount of polymer in the spike layer adjacent to initiatorcoated gold interface does not change with temperature, so L*/ γ* is again evaluated for the swollen part of the distribution, excluding the spike. As is evident from Table 1 at 21 °C, the spike layer is most pronounced for the low grafting density brushes (G2 and G4). For the higher density brushes, in which the average volume fraction is higher, there is a smoother transition between the inner spike layer and the swollen Gaussian part of the distribution. As the temperature increases beyond the partial collapse transition, and the average volume fraction increases in the less swollen low-density brushes, the overall shape of the distributions become more similar to those of the higher-density brushes. For G2 a second transition occurs between 45 and 50 °C, although this is associated with a loss of 16% of the polymer from the interface, which means that unlike the other brushes, the temperature response would not be fully reversible on cooling. The loss of chains from the interface is a consequence of the unstable nature of the gold−thiol bond at elevated temperatures.38 That degrafting is only observed for the highest molecular weight chains is easily understood for the case of a good solvent, as there is an osmotic driving force pulling the chain out of the high density brush that will increase with chain length. That it is observed for chains in a brush that is above the

4. CONCLUSIONS SI-ATRP has been used to graft PDMAEMA brushes from thiol initiators at different grafting densities at the gold−water interface. The swelling ratio of these layers display different scalings with grafting density at pH 3 compared with pH 7 and 10, which can be attributed to a switch in the dominant contribution to the osmotic pressure within the brush from the counterions of the charged segments at pH 3 to the segment excluded volume at pH 7 and 10. The highest density brush (corresponding to 0.006 chains/Å2) displays the weakest response to changes in pH. At pH 10, the brushes undergo a partial collapse when a transition temperature in the range 30− 40 °C is exceeded. Above this transition temperature a twophase brush develops, with a dense inner region and a dilute outer region. The observation that the dense inner region of this two-phase brush region does not reach the very high density of the thin spike layer, which is always present independent of pH or temperature, suggests that in the spike layer there is an attractive interaction for the hydrophobic thiolcoated interface in addition to the volume-fraction-dependent solvent quality driven interaction between DMAEMA segments that is present in the dense region that forms following temperature-induced partial collapse. The temperature response is also greatest for the lowest density brushes, which contain the highest volume fraction of water and so are the most sensitive to dehydration of the hydrophilic segments. These quantitative observations of the response of PDMAEMA brush conformation to changes in pH and temperature have helped in the formulation of design rules for the functionalization of gold nanoparticles to act as temperature and pH controllable emulsifiers.16 311

dx.doi.org/10.1021/ma201839y | Macromolecules 2012, 45, 305−312

Macromolecules

■

Article

(24) Edmondson, S. Ph.D. Thesis, University of Cambridge, 2006. (25) Tomlinson, M. R.; Cousin, F.; Geoghegan, M. Polymer 2009, 50, 4829−4836. (26) L* = [2∫ T∞spike (z − Tspike)ϕ(z) dz]/[∫ T∞spike ϕ(z) dz]. (27) Zhulina, E. B.; Birshtein, T. M.; Borisov, O. V. Macromolecules 1995, 28, 1491−1499. (28) Witte, K. N.; Sangtae, K.; Won, Y.-Y. J. Phys. Chem. B 2009, 113, 11076−11084. (29) Yuan, J.-J.; Schmid, A.; Armes, S. P.; Lewis, A. L. Langmuir 2006, 22, 11022−11027. (30) Gong, P.; Genzer, J.; Szleifer, I. Phys. Rev. Lett. 2007, 98, 018302. (31) Witte, K. N.; Hur, J.; Sun, W.; Kim, S.; Won, Y.-Y. Macromolecules 2008, 41, 8960−8963. (32) Hur, J.; Witte, K. N.; Sun, W.; Won, Y.-Y. Langmuir 2009, 26, 2021−2034. (33) Tagliazucchi, M.; de la Cruz, M. O.; Szleifer, I. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 5300−5305. (34) Plamper, F. A.; Ruppel, M.; Schmalz, A.; Borisov, O.; Ballauff, M.; Müller, A. H. E. Macromolecules 2007, 40, 8361−8366. (35) Yim, H.; Kent, M. S.; Mendez, S.; Balamurugan, S. S.; Balamurugan, S.; Lopez, G. P.; Satija, S. Macromolecules 2004, 37, 1994−1997. (36) Yim, H.; Kent, M. S.; Datija, S.; Mendez, S.; Balamurugan, S.; Lopez, G. P. Phys. Rev. E 2005, 72, 051801. (37) Baulin, V. A.; Zhulina, E. B.; Halperin, A. J. Chem. Phys. 2003, 119, 10977−10988. (38) Kim, J.-B.; Bruening, M. L.; Baker, G. L. J. Am. Chem. Soc. 2000, 122, 7616−7617. (39) Baulin, V. A.; Halperin, A. Macromolecules 2002, 35, 6432− 6438. (40) Mendez, S.; Curro, J. G.; McCoy, J. D.; Lopez, G. P. Macromolecules 2005, 38, 174−181.

ASSOCIATED CONTENT S Supporting Information * Table S1 and Figures S1−S7. This material is available free of charge via the Internet at http://pubs.acs.org.

■ ■

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ACKNOWLEDGMENTS H.J. acknowledges the EPSRC & Royal-Dutch Shell for the award of a Dorothy Hodgkin Postgraduate Award (EP/ P501954/1), and S.T. thanks the Royal Society for a University Research Fellowship, the Scottish Universities Physics Alliance (SUPA), and the National Physical Laboratory’s Strategic Research Programme. We thank the Institut Laue Langevin (France) for the award of beam-time, ISIS (Rutherford Appleton Laboratory, STFC) for consumables support, and Mr. Rick Makin (Department of Physics, Thin Film Facility, University of Oxford) for depositing the gold/titanium layers.

■

REFERENCES

(1) Toomey, R.; Tirrell, M. Annu. Rev. Phys. Chem. 2008, 59, 493− 517. (2) Jia, H.; Grillo, I.; Titmuss, S. Langmuir 2010, 26, 7482−7488. (3) La Spina, R.; Tomlinson, M. R.; Ruiz-Pérez, L.; Chiche, A.; Langridge, S.; Geoghegan, M. Angew. Chem., Int. Ed. 2007, 46, 6460− 6463. (4) Jia, H.; Titmuss, S. Nanomedicine 2009, 4, 951−966. (5) Chen, T.; Ferris, R.; Zhang, J.; Ducker, R.; Zauscher, S. Prog. Polym. Sci. 2010, 94−112. (6) Haase, M. F.; Grigoriev, D.; Moehwald, H.; Tiersch, B.; Shchukin, D. G. Langmuir 2011, 27, 74−82. (7) Saigal, T.; Dong, H.; Matyjaszewski, K.; Tilton, R. D. Langmuir 2010, 26, 15200−15209. (8) Edmondson, S.; Osbourne, V. L.; Huck, W. T. S. Chem. Soc. Rev. 2004, 33, 14−22. (9) Edmondson, S.; Armes, S. P. Polym. Int. 2009, 58, 307−316. (10) Geoghegan, M.; Ruiz-Pérez, 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, 2, 1070−1080. (11) Sanjuan, S.; Perrin, P.; Pantoustier, N.; Tran, Y. Langmuir 2007, 23, 5769−5778. (12) Zhang, J.; Nylander, T.; Campbell, R. A.; Rennie, A. R.; Zauscher, S.; Linse, P. Soft Matter 2008, 4, 500−509. (13) Yim, H.; Kent, M. S.; Mendez, S.; Lopez, G. P.; Satija, S.; Seo, Y. Macromolecules 2006, 39, 3420−3426. (14) Moglianetti, M.; Webster, J. R. P.; Edmondson, S.; Armes, S. P.; Titmuss, S. Langmuir 2010, 26, 12684−12689. (15) Moglianetti, M.; Webster, J. R. P.; Edmondson, S.; Armes, S. P.; Titmuss, S. Langmuir 2011, 27, 4489−4496. (16) Jia, H. Ph.D. Thesis, University of Oxford, 2010. (17) Jones, D. M.; Brown, A. A.; Huck, W. T. S. Langmuir 2002, 18, 1265−1269. (18) Azzam, R, M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland: Amsterdam, 1989. (19) Cubbitt, R.; Fragneto, G. Appl. Phys. A: Mater. Sci. Process. 2002, 74, S329−S331. (20) Nelson, A. J. Appl. Crystallogr. 2006, 39, 273−276. (21) 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. (22) Mir, Y.; Auroy, P.; Auvray, L. Phys. Rev. Lett. 1995, 75, 2863. (23) Tran, Y.; Auroy, P.; Lee, L. T. Macromolecules 1999, 32, 8952− 8964. 312

dx.doi.org/10.1021/ma201839y | Macromolecules 2012, 45, 305−312