Specific Anion Effects on the Internal Structure of a ... - ACS Publications

Article pubs.acs.org/Macromolecules

Specific Anion Effects on the Internal Structure of a Poly(N‑isopropylacrylamide) Brush Timothy J. Murdoch,† Ben A. Humphreys,† Joshua D. Willott,† Kasimir P. Gregory,† Stuart W. Prescott,‡ Andrew Nelson,§ Erica J. Wanless,† and Grant B. Webber*,† †

Priority Research Centre for Advanced Particle Processing and Transport, University of Newcastle, Callaghan, NSW 2308, Australia School of Chemical Engineering, UNSW Australia, UNSW Sydney, NSW 2052, Australia § Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW 2234, Australia ‡

S Supporting Information *

ABSTRACT: The effect of anion identity and temperature on the internal nanostructure of poly(N-isopropylacrylamide) brushes were investigated using neutron reflectometry (NR), atomic force microscopy (AFM), and quartz crystal microbalance with dissipation monitoring (QCM-D). NR and QCM-D measurements showed that addition of strongly kosmotropic acetate anions shifted the lower critical solution temperature (LCST) to lower temperatures relative to pure D2O/H2O, while strongly chaotropic thiocyanate anions shifted the LCST to higher temperatures. Polymer density profiles derived from NR showed direct evidence of vertical phase separation at temperatures around the LCST in all conditions. Results indicate that the density profiles were not simple modulations of structures observed in D2O to higher or lower temperatures, with both anion identity and ionic strength found to influence the qualitative features of the profiles. In particular, the presence of thiocyanate broadened the LCST transition which is attributed to the ability of the thiocyanate anion to electrosterically stabilize the brush above its LCST. Complementary AFM data showed that the acetate ion induced collapsed structures while a broader transition is observed in the presence of thiocyanate.

■

INTRODUCTION

According to mean-field theory, the broad temperature range for the collapse of thermoresponsive polymer brushes arises from a lack of a first-order phase transition during collapse.7 Instead, there is a cooperative conformational transition as the solvent quality decreases. Solvent quality is often represented by the Flory interaction parameter χ(T) .8 However, for thermoresponsive systems χ must be replaced by a concentration-dependent Flory interaction parameter, χeff(ϕ,T), where ϕ is the polymer volume fraction in the brush. By incorporating the empirical χeff(ϕ,T) determined by Afroze et al.9 into numerical self-consistent field (SCF) theory, Halperin and coworkers predicted that a PNIPAM brush not only collapses with increasing temperature but will also undergo vertical-phase separation above a critical grafting density.10,11 That is, at temperatures around the LCST the brush separates into a dense, polymer-rich phase close to the substrate and a dilute tail at the periphery. Vertical phase separation occurs for densely grafted brushes where χ is an increasing function of ϕ as predicted by the n-cluster12,13 and two-state models of polymers.14 However, it is not observed for theories that do not allow χ to vary through the brush.15 A key result of these theories is that the solvent quality at the periphery of the brush

Stimulus-responsive polymer brushes are surface coatings formed from densely end-grafted polymers that change their physicochemical properties depending on their environment.1 Thermoresponsive polymer brushes, such as those formed from poly(N-isopropylacrylamide) (PNIPAM), are particularly promising for applications including chromatography supports2 and cell-culture media3 as they can be switched (i.e., collapse/ expansion of the brush) repeatedly without buildup of additional chemical species. However, an understanding of the effect of additives on the temperature response is important for predicting the behavior in real systems. For example, addition of salt shifts the window of temperature response,4 allowing the design of thermosensitive chromatographic materials that operate at biologically safe ranges.5 PNIPAM has been extensively studied in free solution and undergoes a sharp (1−2 Δ°C) coil−globule phase transition above its lower critical solution temperature (LCST) of ∼32 °C.6 The transition is entropically driven as water associated with hydrophobic hydration of the isopropyl groups and polymer backbone is released.6 When grafted in brush form, the collapse occurs over a much broader temperature range (up to 25 Δ°C).4 Therefore, understanding the relationship between structure and temperature response is important for informed design of future applications. © XXXX American Chemical Society

Received: May 13, 2016 Revised: July 20, 2016

A

DOI: 10.1021/acs.macromol.6b01001 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

kosmotropic character of the anions. Anions may also bind directly to the polymer, raising the LCST through electrostatic stabilization.28,31,40 Binding constants for anions interacting with PNIPAM increase with chaotropic character and hence strong chaotropes raise the LCST at low to intermediate salt concentrations.40 At high salt concentrations chaotropes lower the LCST as binding becomes saturated and the other mechanisms dominate. Until recently, the study of salt effects on the behavior of PNIPAM brushes was limited to one or two kosmotropic salts.41−43 The first detailed study of specific ion effects on PNIPAM brushes was conducted by Naini et al.36 A novel laser temperature-jump technique was used to probe the thermodynamics of the temperature response of a PNIPAM brush in a series of 250 mM sodium halide solutions. Changes in the LCST arose from concentration and ion identity dependent shifts in the switching enthalpy and entropy relative to water. However, the change in switching enthalpy and entropy across the LCST transition was found to be independent of salt identity, suggesting that salts affect the swollen and collapsed brush equally. While this implies that ions are binding directly to the polymer, it does not yield predictions regarding the impact of salt on brush structure at temperatures relative to the LCST. Measurement of a strongly chaotropic salt such as potassium thiocyanate would help to discern whether this conclusion is universal or only applies to conditions which lower the LCST. We have recently demonstrated the effect of the strongly chaotropic potassium thiocyanate and kosmotropic potassium acetate salts at concentrations up to 250 mM on a PNIPAM brush using in-situ ellipsometry, quartz crystal microbalance with dissipation monitoring (QCM-D), and captive bubble contact angle measurements.4 The temperature range of the phase transition was smallest for the contact angle measurements, indicative of vertical phase separation. Consistent with solution studies,28 the presence of kosmotropic acetate lowered the LCST while chaotropic thiocyanate raised the LCST. The effect of salt on the nanostructure of a polymer brush formed from thermoresponsive copolymers of poly[2-(2methoxyethoxy)ethyl methacrylate-co-oligo(ethylene glycol) methacrylate) was studied by Gao et al.44 They concluded that addition of isotonic tris(hydroxymethyl)aminomethanebuffered saline solution resulted in relatively collapsed structures compared to pure D2O at the same temperature with the swollen brush affected to a greater extent. However, their temperature resolution limits insight on the effects of additives across the whole temperature range. To our knowledge, no previous study has systematically probed the influence of a strong chaotrope, potassium thiocyanate, and a strong kosmotrope, potassium acetate, on the internal nanostructure of a thermoresponsive brush across the transition from swollen to collapsed. Herein we present a neutron reflectometry study of the effect of salt identity and concentration on structure in the thermoresponse of a PNIPAM brush. Complementary QCM-D and atomic force microscopy (AFM) normal force measurements support these findings.

is unaffected unless the temperature is considerably in excess of the LCST; i.e., the brush collapses from substrate upward. Several experimental studies have shown indirect evidence of vertical phase separation by comparing techniques sensitive to the entirety of the brush, such as ellipsometry,4 surface plasmon resonance,16 and surface forces apparatus,17,18 to surfacesensitive techniques such as contact angle. These studies show broad transitions in the bulk properties of the brush while contact angle changes over a narrow temperature range above the LCST. Varma et al. found direct evidence of vertical phase separation using optical reflectometry to study a series of PNIPAM brushes of varying grafting density (0.045−0.003 Å−2) and dry brush thickness (290−2000 Å).19 Simple tworegion volume fraction profiles were required for the highest grafting density (0.003 Å−2) above a thickness of 620 Å. A dense interior layer followed by an exponential decay was required by Kooiji et al. to fit spectroscopic ellipsometry data of three PNIPAM brushes of varying grafting density (0.0003− 0.0069 Å−2).20 Both techniques give excellent qualitative proof of vertical phase separation but suffer from a lack of sensitivity for thin brushes. By comparison, neutron reflectometry (NR) measurements provide quantitative determination of the polymer density profile normal to the grafting substrate.21 Yim and co-workers studied a series of PNIPAM brushes in pure D2O covering a wide range of molecular weights and grafting densities (13−220 kg mol −1 , 0.0006−0.0054 Å−2).11,22−24 As predicted by theory, vertical phase separation in the vicinity of the LCST was apparent only for high grafting density and high molecular weight brushes.23 A bilayer fit was also required at 34 °C in the study by Elliott et al.25 Addition of a model hydrophilic drug, Isoniazid, reduced the thickness of the dense interior layer. However, the authors were unable to determine if the additive was causing a brush extension or shifting the LCST as only three temperatures were studied. In addition to molecular weight, concentration, and endgroup effects, additives to PNIPAM solutions have been shown to affect the thermoresponse of the polymer.26,27 For example, both the concentration and identity of salt shift the LCST of free PNIPAM in solution.28 Variations in LCST with salt identity follow the Hofmeister series, with kosmotropic anions lowering the LCST and chaotropic anions raising the LCST of PNIPAM and thermoresponsive polymers in general.28 Traditionally, the classification as a kosmotrope or chaotrope is based on the ability of an ion to “make” or “break” the structure of water.29 However, evidence suggests ions do not influence water beyond the first hydration sheath.30 Regardless, the terms remain useful for describing ions that display similar properties and effects. For example, kosmostropes are strongly hydrated ions, while chaotropes are weakly hydrated and have an affinity for hydrophobic interfaces and molecules.29,31 The nature of specific ion effects on PNIPAM has been studied through simulation32,33 and experiment,4,28,34−39 with specific focus on untethered PNIPAM and PNIPAM-based microgels. While the identity of the cation has a small but measurable effect,33,34 changing the anion has significantly larger impact due to a greater variation in polarizable volume.29 Cremer and co-workers elucidated three mechanisms of anion interaction by correlating the LCST dependence on salt concentration with ion properties.28,37−39 The LCST is lowered by disrupting the hydration of the polymer either through polarization of the first hydration shell surrounding the polymer or by increasing surface tension at the hydrophobe/aqueous interface. The decrease in LCST is directly related to the

■

MATERIALS AND EXPERIMENTAL METHODS

Materials. Native oxide silicon wafers (100 mm diameter, 10 mm thick) for specular neutron reflectometry experiments were purchased from EL-CAT Inc. (USA). Native oxide silicon wafers for AFM experiments were purchased from Silicon Valley Microelectronics B


Article

Macromolecules (USA). Quartz crystal microbalance sensors with a 50 nm silica coating (Q-Sense, QSX 303, 4.95 MHz fundamental frequency) were purchased from ATA Scientific (Australia). Potassium hydroxide (Chem-Supply Pty. Ltd., AR grade) was used during surface preparation steps. Surface functionalization reagents (3-aminopropyl)triethoxysilane (APTES, >99%), triethylamine (Eth3N, 99%), and 2bromoisobutyryl bromide (BIBB, >99%) were purchased from SigmaAldrich and used as received. The tetrahydrofuran (THF, Honeywell Burdick and Jackson, >99%) and triethylamine were dried over 4 Å molecular sieves (ACROS Organics) before use. Polymerization reagents 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA, 97%), copper(II) bromide (99.999%), and L-ascorbic acid (≥99.0%) were purchased from Sigma-Aldrich and used as received. NIsopropylacrylamide (NIPAM, ACROS Organics, 99%) was purified by recrystallization from hexane prior to use. Aqueous solutions of potassium acetate (KCH3COO, Alfa Aesar, >99%) and potassium thiocyanate (KSCN, Alfa Aesar, >98%) were prepared and used. The pH of the acetate solutions was controlled at pH 5.5 ± 0.1, with adjustment made by the addition of acetic acid (Ajax Finechem, 100%). Ethanol (Ajax Finechem, absolute) was distilled before use, methanol (Sigma-Aldrich, anhydrous, 99.8%) was used as received, and Milli-Q water (Merck Millipore, 18.2 MΩ·cm and pH 5.5 ± 0.1 at 23 °C) was used throughout. Specular neutron reflectometry measurements were performed in pure deuterium oxide (D2O) with all salt solutions prepared using prefiltered (0.45 μm disk filters) D2O. PNIPAM Brush Synthesis. Silicon wafers and QCM sensors were cleaned and initiator-functionalized following our established protocol (which was up-scaled for the 100 mm neutron reflectometry silicon wafer).4,45−48 The PNIPAM brushes were synthesized via activators continuously regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP) from the covalently bound bromine initiator sites. Polymerization conditions were identical to our earlier work,4 apart from the solvent mixture. In this study the polarity of solvent was decreased (methanol/water in a 4:1 v/v ratio) to decrease polymerization kinetics in order to synthesize thinner PNIPAM brushes in a well-controlled manner, suitable for neutron reflectometry measurements. Ellipsometry. Ellipsometry experiments were conducted on a Nanofilm EP3 single wavelength (532 nm green laser) imaging ellipsometer controlled by EP3View software. WVASE32 software was used to model the ellipsometric parameters (Δ and Ψ). The experimental methodology of dry and in-situ ellipsometry measurements was identical to our earlier study.4 In-situ ellipsometry measurements were performed on a 186 ± 5 Å dry thickness PNIPAM brush. All brushes are hereafter referred to by their dry thickness as measured by ellipsometry, as all brushes were measured using this technique. X-ray Reflectometry. Dry brushes were characterized by using a Panalytical X’Pert Pro X-ray reflectometer (with Cu Kα radiation λ = 1.541 Å). Specular reflectometry measurements were made as a function of the scattering vector, Q = (4π/λ) sin θ, where λ is the wavelength and θ is the angle of incidence. Data were analyzed using the MOTOFIT package, utilizing the Abeles matrix method.49 Neutron Reflectometry. Neutron reflectometry (NR) measurements were carried out on the Platypus time-of-flight reflectometer at the OPAL 20 MW reactor (Australian Nuclear Science and Technology Organisation, Sydney, Australia). Specular reflectivity measurements were made at angles of incidence of 0.8° and 3.8° (0.6° and 2.8° in air) giving a Q range of 0.009−0.31 Å−1 (0.007−0.24 Å−1 in air). Standard reduction procedures were used with ΔQ/Q = 8.8%.50 Four brushes were measured by NR in air with the most uniform brush chosen for in-situ experiments conducted in a standard solid−liquid flow cell. Complete fluid exchange of salt solutions was confirmed by monitoring solution conductivity at the outlet. The cell was thermostated on top and bottom by a circulating water bath. Readings from a thermocouple attached to the silicon block matched the bath set-point temperatures for the measured temperature range (20−45 °C); data from this thermocouple are used herein. An equilibration time of 30 min was used between each change of solution with 15 min allowed for equilibration between temperature changes.

The response times of the system were checked by measuring the reflectivity at 0.8° during a temperature change from 20 to 45 °C. The sample temperature stabilized after 6 min, while the reflectivity data indicated the brush reached thermal equilibrium within 7 min, i.e., within 1 min of the temperature change. Therefore, the brush was sufficiently thermally equilibrated for all reported conditions. Neutron Modeling. Neutron reflectometry is sensitive to changes in scattering length density (SLD, ρN) perpendicular to the substrate. Data were analyzed using a custom script implemented in MOTOFIT.49 The model consisted of a native SiO2 layer (whose parameters are known by measurements on the dry film) followed by a small number of layers (≤4) of uniform volume fraction that represents the interior (i.e., closest to the substrate) of the brush. The interface between these slabs is smoothed with a Gaussian roughness term. The exterior tail region of the brush is represented either by a Gaussian, hyperbolic tangent, or parabola with power law tail, as described in eqs 1−3, respectively.51,52

⎛ z2 ⎞ ϕ(z ̅ ) = ϕ0 exp⎜− ̅ 2 ⎟ ⎝ H ⎠ ϕ(z ̅ ) =

ϕ0 ⎛ ⎛ Z − z ̅ ⎞⎞ ⎟⎟ ⎜1 + tanh⎜2 0 ⎝ ⎝ 2 H ⎠⎠

⎡ ⎛z ⎞ ϕ(z ̅ ) = ϕ0⎢1 − ⎜ ̅ ⎟ ⎥ ⎝H⎠ ⎦ ⎣

(1)

(2)

2 ⎤α

(3)

where ϕ(z)̅ is the volume fraction at distance z̅ from the start of the tail region of the brush, H is the characteristic height of the exterior tail region, ϕ0 is the initial volume fraction at the boundary between the interior and exterior tail regions, and Z0 is the height satisfying ϕ(Z0) = ϕ0/2. To calculate the reflectivity, the tail region was discretized into 50 layers of constant volume fraction with a small Gaussian roughness term equal to a third of the layer thickness connecting each layer. 50 layers were sufficient to avoid coarse graining effects on calculated reflectivity as shown in Figure S1 (see Supporting Information). The SLD of each brush layer was then calculated from the volume fraction weighted sum of each component:

ρN = ϕρN,PNIPAM + (1 − ϕ)ρN,Solv

(4)

where ρN,Solv is the SLD of the solvent. The reflectivity was then calculated using the Abeles matrix method.49 Each condition was analyzed with 1, 2, and 3 slabs with each of the three possible tails (eqs 1−3) as well as 1−4 slabs without a tail. The curve with the lowest χ2 error was chosen for presentation. If multiple analyses resulted in similar χ2 values, the curve with lowest number of free parameters was chosen. This resulted in predominantly Gaussian tails in the swollen brush regime. Note that no condition could be successfully fit without at least one interior layer. This is demonstrated for 45, 32.5, and 20 °C in D2O in Figures S2−S4. The interior layers allow the fitted profiles to capture behavior not accounted for by theory. For example, adsorbed layers at the interface are not predicted by most SCF calculations as the grafting interface is treated as a noninteracting, impermeable barrier. The fitting parameters for each condition are summarized in Tables S1−S5. The effective dry thickness (δ) for each profile was calculated by

δ=

∫0

∞

ϕ(z) dz

(5)

This value was used to evaluate the physical veracity of the fits as the polymer is covalently bound to the surface; i.e., δ should be constant throughout all dry and wet measurements. In the swollen state, the diffuse nature of the brush results in a relatively featureless reflectivity profile. Unconstrained fits lead to a family of curves with a wide range of thickness and adsorbed amounts. During fitting, a Lagrange multiplier approach was used to constrain the adsorbed amount in such circumstances (for more details see Supporting Information). The average brush thickness, L1st, was determined by twice the normalized first moment of the volume fraction profile: C


Article

Macromolecules

Figure 1. (a) Measured reflectivity curves (filled circles) for equilibrium temperature measurements on a 191 Å PNIPAM brush immersed in pure D2O. (b) Reflectivity at low Q presented in RQ4. The experiment was conducted from 45 °C downward. The solid lines are the fits to the experimental data. Data have been offset on the reflectivity axes for clarity, with the data for 45 °C plotted using actual recorded data. ∞

L1st = 2

analyzed using wormlike chain (WLC) theory allowing the chain persistence length, contour length, and hence molecular weight to be determined. This procedure is fully outlined in our previous work.55 From the molecular weight determined, the brush grafting density was estimated using the molar mass of the monomer (113.2 g mol−1 for NIPAM), the length of the monomer (0.30 nm, calculated based on the projected C−C bonds lengths), the density of the polymer (1.26 g cm−3 as measured by NR; see Results and Discussion for more detail), and Avogadro’s number. The reduced grafting density was calculated by Σ = πσRg2, where σ is the grafting density (chains per unit area) and Rg is the radius of gyration.55 Reduced grafting density is a measure of brushlike character, with Σ > 5 being the empirical boundary between the mushroom and brush regime.56 The colloid probe measurements were performed over approximately 10 h with the PNIPAM brush immersed in solution throughout. All force curves were collected using a piezo ramp size of 1 μm, a tip velocity of 0.5 μm s−1 during both approach and retraction, no surface dwell, and at a constant maximum indentation load. Force measurements were performed in Milli-Q water and in the presence of aqueous 250 mM potassium acetate and 250 mM potassium thiocyanate electrolyte at five different temperatures: 23, 27.5, 32, 33.5, and 37 °C. The solution pH was maintained at 5.5 ± 0.1. A total of 30 curves were collected at each condition. Average approach and retraction curves were determined using a custom MATLAB script as described in our previous publication and are presented in Figures S5−S7.55 Averaged force curves are not necessarily an accurate representation of the interaction of a colloid probe and a population of chains. Therefore, the average curves were used to select the most representative curve for presentation.

∫0 zϕ(z) dz δdry

(6)

A factor of 2 is used in eq 6 as this corresponds to the thickness of a step-density profile with the same normalized first moment.53 Quartz Crystal Microbalance with Dissipation (QCM-D). QCM-D measurements were performed using a KSV Z500 quartz crystal microbalance with dissipation monitoring (KSV, Finland) on a 209 ± 10 Å dry thickness PNIPAM brush. Experiments were performed as per our earlier study.4 Changes in the measured dissipation response for the third overtone are presented. Zero ΔD (change in dissipation) corresponds to the value the equilibrated brush at high temperature. All data presented were normalized using the temperature response of an unmodified QCM-D sensor. Atomic Force Spectroscopy. Normal force measurements were performed using a Bruker MultiMode 8 atomic force microscope (AFM) with a vertical engage EV scanner in contact mode equipped with a closed fluid cell (Bruker, USA). Description of the cantilevers used in this study, the preparation of the silica colloid probe, and cleaning protocols are provided in the Supporting Information. The single-molecule force spectroscopy (SMFS) and colloid probe measurements were performed on the same PNIPAM brush of dry thickness 183 ± 5 Å. The entire AFM was housed in an incubator allowing accurate temperature control (within 0.5 °C) between 20 and 40 °C. Initially, for both the SMFS and colloid probe studies, the PNIPAM brush was exposed to Milli-Q water at 22 °C. Between measurements at each solution condition 10−15 mL of the new solution was flowed through the fluid cell, after which the brush was allowed to equilibrate for at least 20 min. Our studies revealed that this equilibration time was sufficient, with no drift observed in the measured force curves over time. Cantilever deflection vs displacement data were converted to normal force vs apparent separation curves using standard methods as outlined by Ralston et al.54 All SMFS experiments were performed in Milli-Q water at 22 °C. PNIPAM is highly solvated and stretched in pure water at low temperature.4 Consequently, these conditions provided the best opportunity to adhere and stretch individual chains which is of vital importance for this analysis.55 A total of ∼2000 individual force curves were collected at various locations on the brush surface, covering an area of 0.75 μm by 0.75 μm. The chain stretching behavior was

■

RESULTS AND DISCUSSION Determination of Brush Properties. The PNIPAM brush used for NR studies had a dry thickness of 181 ± 16 Å, as measured by ellipsometry in air. X-ray reflectivity and NR measurements in air yielded comparable dry brush thickness values of 182 ± 14 and 191 ± 14 Å, respectively. All NR data reported were recorded with this brush which remained immersed throughout the experiment. Values of SLD and thickness for the SiO2 layer were determined by the dry NR D


Article

Macromolecules measurement to be (3.1 ± 0.1) × 10−6 Å−2 and 19 ± 1 Å, respectively. Note that the SLD of the SiO2 was allowed to vary to account for the effects of porosity and the presence of H2O. These values were fixed for the fitting of the solvated brush. The SMFS study conducted on a 183 Å PNIPAM brush gave an estimated grafting density of 7 × 10−4 Å−2 with a corresponding reduced grafting density of ∼40. This is a lower limit as the technique is biased toward longer chains.55 The fitted persistence length of 7.9 ± 5.3 Å was consistent with values reported by Kutnyanszky et al. for single, end-grafted PNIPAM chains.57 It is assumed that the fitted values from the SMFS are representative for all surfaces studied as they were grown using identical synthetic conditions. Table S6 presents key results from the SMFS measurements with Figure S8 showing the resultant molecular weight distribution. There is significant disagreement within the literature on the bulk density of PNIPAM, with values between 1.03 and 1.386 g cm−3 reported.25,58 The large variation and the presence of water in the brush even when measured in air make estimating the amount of polymer, δdry, difficult. Assuming that the amount of polymer must be the same at all conditions, the fitted SLD and thickness of the brush in air and D2O at 45 °C were used to determine ϕ(Z) at each condition as well as ρN,PNIPAM = 0.93 × 10−6 Å−2 (full details are provided in Table S7 and in the accompanying discussion). This corresponds to a bulk density of 1.26 g cm−3, which is within the reported range of values. Thermoresponse of a PNIPAM Brush in Pure Water. Reflectivity curves for equilibrium NR measurements on a PNIPAM brush in pure D2O are shown in Figure 1a. Measurements were performed from 45 °C downward to match our previous ellipsometry and QCM-D studies.4 The choice of the direction of temperature change is expected to have minimal impact on the observed structures, with negligible hysteresis observed for PNIPAM brushes of a similar grafting density and thickness.19 At high temperatures Kiessig fringes indicate a blocklike structure around ∼270 Å thick. The poorer quality of the fit at high Q values for these collapsed structures could be due to macro-scale lateral inhomogeneities in the polymer thickness over the area illuminated by the neutron beam. For collapsed brushes, the dominant brush thickness determines the amplitude and spacing of the initial fringes at low Q. Data at low Q are sensitive to the total amount of polymer and the overall shape of the profile.59 Therefore, the important quantitative features of the profiles have been adequately captured by the fits. In general, the largest variations in reflectivity with decreasing temperature were observed in the low Q region (Q < 0.08 Å−1). To emphasize this, the data in this region have been presented in terms of RQ4 in Figure 1b. This highlights features of the reflectivity profile as it compensates for the Q−4 decay due to Fresnel’s law. As the temperature decreases from 45 to 35 °C, damping of the fringes indicates that the polymer brush has become more diffuse. Between 35 and 32.5 °C this damping increases, and the reflectivity drops off more steeply at Q values just above the critical edge, indicating the formation of a significantly more diffuse interface. The increase in the frequency of the Kiessig fringes at low Q also suggests an expansion of the brush. At lower temperatures the Kiessig fringe minima are further shifted and reduced in intensity indicative of continued swelling of the brush. The reflectivity data were modeled using theoretical volume fraction profiles as described in the previous section; the

corresponding theoretical reflectivity curves for these profiles are shown as solid lines in Figure 1. Volume fraction profiles corresponding to these fits are given in Figure 2. For these

Figure 2. Fitted volume fraction profiles for a PNIPAM brush immersed in D2O as a function of sample temperature. Solid lines highlight examples of the collapsed (45 °C), vertically phase separated (between 35 and 30 °C), and swollen regimes (20 °C). Arrows are provided to guide the eye and correspond to a decrease in temperature. The inset plot at the top right presents the average brush thickness determined from twice the first moment of the volume fraction profile as a function of solution temperature.

profiles, zero perpendicular distance corresponds to the outer edge of the fitted silica (SiO2) layer thickness. Under all conditions, a sharp spike in polymer density is observed at z ≤ 20 Å. This layer is required to fit the low frequency oscillation in the reflectivity (minima between 0.05 and 0.08 Å−1). The layer is significantly thicker than the initiator layer (∼5 Å), which is encompassed in the roughness between the SiO2 and polymer in the dry fit. Thin surface layers have been observed by NR previously and are associated with attractive interactions with the substrate.59,60 This layer accounts for between ∼10 and 15% of the total polymer volume. At temperatures ≥40 °C the brush is collapsed and forms a dense block of polymer with ∼35% solvent. This level of hydration is consistent with previous NR studies.23−25 As the temperature is decreased, the brush expands because it takes up solvent with a corresponding increase in average brush thickness. This can be seen by the increase in first moment thickness given in the inset of Figure 2. No plateau value of brush thickness is seen in the inset of Figure 2 at low temperatures, suggesting that the total transition occurs over a temperature range greater than 20 °C. This broad transition was observed in our previous work4 and in our in-situ ellipsometry studies of a 186 ± 5 Å PNIPAM brush (see Supporting Information Figure S9a). For temperatures below 30 °C, the brush volume fraction profile consists of a single, dilute tail following the initial spike in polymer density at the grafting surface. Here the polymer is hydrophilic and is extended due to the osmotic pressure exerted by the solvent. However, between 30 and 35 °C the profile consists of a dense interior layer near the substrate followed by a dilute tail; i.e., vertical phase separation occurs. The decaying profile of the dilute tail suggests that this region of the brush remains very hydrophilic. Considering the complete transition, the dense region of the brush reduces in thickness and E


Article

Macromolecules decreases in polymer concentration (solid arrow) while the dilute region extends away from the substrate (dashed arrow) as temperature is decreased. The narrow temperature range over which vertical phase separation is present is consistent with our previous contact angle study where the surface of the brush increased in hydrophobicity between ∼31 and 34 °C.4 The steep drop-off in reflectivity around the critical edge at 32.5 °C relates to the presence of two diffuse interfaces; the combination of hydrogenated polymer and deuterated solvent for this layer results in low contrast with the underlying silica. More extended structures at low temperature actually have a higher reflectivity at low Q as additional entrained deuterated solvent leads to a greater contrast with the silica. In a previous neutron reflectivity study, Yim and co-workers suggested that the presence of a vertical phase separation corresponded to a maximum in the first moment of the density profile as a function of temperature. However, this is inconsistent with other NR data25 and SCF theory.11,15 The discrepancy may result from the significantly higher reduced grafting density (173 and 136) of Yim et al.23 compared to Elliott et al.25 (∼48) and the current work (∼40, Table S6). Furthermore, it is unclear to what degree chain dispersity and specific interactions with the substrate influence the degree of vertical phase separation. Varma et al. argue that the lower LCST exhibited by longer chains in solution will reduce the sharpness of the vertical phase separation as shorter chains will remain solvated while longer chains collapse near the “average” LCST.19 This is supported by comments of Baulin et al., who suggest that chain dispersity and variations in grafting density would smooth the discontinuity as the initial volume fraction at the grafting substrate and the height at which the discontinuity occurs will vary. While NR measurements reflect the weighted average of these areas, Baulin et al. did not directly test this in their SCF calculations.61 In fact, dispersity may encourage the presence of vertical phase separation since long chains are preferentially stretched, resulting in compression of shorter chains.62 Complementary to the NR profiles, normalized force versus separation curves for the approach of a silica colloid to the brush as a function of temperature in pure H2O are given in Figure 3a. At 23 °C the brush is highly swollen, with repulsive interactions on approach out to ∼600 Å. As the temperature is increased, the range of repulsive interaction during approach decreases. This is shown clearly in the inset and is consistent with the transition from a swollen brush to a collapsed brush consisting of a single dense, incompressible layer.17 The exponential shape of the force curve at intermediate compression is characteristic of the compression of a polymer brush in a good solvent.63 The origin of the attractive interaction is likely polymer bridging between the brush and the silica colloid probe.64,65 The distance over which bridging occurs is significantly larger than the extent of the volume fraction profile at a given temperature as measured by NR shown in Figure 2, even though the dry thickness of the brushes used for NR and AFM studies are within 10%. The prospect of long-range bridging suggests there is a small population of longer chains extending further into solution than the majority of the chains. Goodman et al. used this argument to explain the presence of a bridging interaction in a small fraction of curves at higher grafting densities (up to 0.0017 Å−2).64 Bridging was also observed by Malham et al. and Ishida et al. for PNIPAM brushes at temperatures greater than 37 °C.17,66 The presence of these

Figure 3. Force versus apparent separation curves for the approach of a silica colloid toward a 183 Å PNIPAM brush in (a) pure H2O, (b) 250 mM potassium acetate, and (c) 250 mM potassium thiocyanate. The inset plots show the repulsive region of the data with log−linear axes.

chains would reconcile conflicting grafting density requirements for vertical phase separation (higher grafting densities) and bridging interactions (lower grafting density). Bridging is also dependent on the chemical identity of the monomer, with attraction observed at lower grafting densities for PNIPAM than for example, poly(N,N-dimethylacrylamide).64 However, in Figure 3a and at temperatures ≥33.5 °C we propose that the range and magnitude of the attraction decrease as a result of the reduced number of contacts between the collapsed brush and the colloid probe. At all temperatures transition from attractive bridging to osmotic repulsion results in a monotonic increase in force at low separations. However, at 37 °C there is an increase in the compliance of the brush between ∼100 and 150 Å. This feature was seen reproducibly and is present in the average curve (see Figure S5). Anion Effects on the Thermoresponse of a PNIPAM Brush. The thermoresponse of PNIPAM brushes in the presence of salt is shifted to higher or lower temperatures dependent on the location of the anion in the Hofmeister series.4,28,36 However, it was previously unclear whether these shifts are accompanied by changes in the overall structure of the brush. The effect of the addition of kosmotropic potassium F


Article

Macromolecules

Figure 4. Fitted volume fraction profiles for a PNIPAM brush immersed in potassium acetate (a, c) and thiocyanate (b, d) as a function of salt concentration and temperature. Solid lines highlight examples of the collapsed, vertically phase separated, and swollen regimes.

electrolyte on brush behavior, we would expect identical profiles in, for example, D2O at 32.5 °C and 100 mM potassium acetate at 30 °C or in 250 mM thiocyanate at 35 °C and in 250 mM acetate at 27.5 °C. Clearly, the nanostructure of the brush is affected by both salt concentration and identity. The SCF calculations of Halperin et al. show large structural variations over the narrow temperature range (100 mM thiocyanate > D2O > 100 mM acetate >250 mM acetate. Regardless of salt concentration or identity the brush thickness reaches a plateau at high temperatures, corresponding to a collapsed structure consisting of ∼65% polymer. This is contrary to previously reported ellipsometry data where the thickness at the high temperature plateau region (>40 °C) increased monotonically with potassium thiocyanate concentration.4 Note, ellipsometry and NR are both sensitive to the average thickness of brush.67 Therefore, this difference may arise from the significantly higher dry thickness of the previously studied brush (566 ± 14 Å).4 The discontinuity temperature at which the brush starts to swell from the collapsed state follows the expected Hofmeister

acetate and chaotropic potassium thiocyanate on the structure of a PNIPAM brush may be understood through the volume fraction profiles presented in Figure 4 with the corresponding reflectivity curves and fits shown in Figures S10−S13. Consistent with the pure D2O data in Figure 2, at high temperatures the brush consists of a single, dense layer with the degree of solvation decreasing with increasing temperature. At low temperatures the brush consists of a single swollen phase, with polymer density decaying away from the grafting surface for all solvents measured at 20 °C. Addition of potassium acetate results in relatively collapsed structures (compared to pure D2O) for the same temperatures, while potassium thiocyanate has the opposite effect. For example, a single, dense phase is present in temperatures as low as 30 °C in 250 mM potassium acetate, while it is only present at 40 °C and higher in 250 mM potassium thiocyanate. This matches our previous ellipsometry studies.4 Vertical phase separation is again present under all conditions. It is most prominent in the 27.5 °C 250 mM potassium acetate data, which exhibits a structure similar to the 32.5 °C D2O data shown in Figure 2. The transition between collapsed and swollen profiles is more gradual with respect to temperature in the presence of thiocyanate. This is evident in 35 °C profile in 250 mM thiocyanate where there is a smooth decay after an initial block of ∼130 Å. It should be noted that the profiles are not simply shifted to higher or lower temperatures in accordance with the shift in LCST in the presence of specific anions. If this was the sole impact of added G


Article

Macromolecules

Figure 5. Averaged overall brush thickness determined by twice the first moment of volume fraction profiles as a function of solution temperature for the brush in D2O and in the presence of 100 and 250 mM potassium acetate and thiocyanate.

trend. In this region, the slope of thiocyanate data is significantly shallower, corresponding with a more gradual transition from the collapsed to swollen state seen in the brush structures in Figure 4. This likely results from the capacity of thiocyanate to bind directly with the monomer resulting in electrosteric stabilization of the collapsed brush.4,28,31 An alternate representation of these data with temperatures offset relative to the discontinuity temperature is presented in Figure S14 and clearly shows the influence of thiocyanate on brush behavior in the early stages of swelling; i.e., at temperatures just below where swelling begins. In all data except the 250 mM thiocyanate solution, there is a rapid change in first moment around the discontinuity point followed by a decrease in slope at lower temperatures where the brush is acting as a single, swollen phase. This matches the predictions from SCF theory.11 The rate of increase in thickness with decreasing temperature for the swollen phase are similar, suggesting salt plays a less significant role on the structure of the swollen brush. While the average brush thickness gives a useful comparison for the swelling behavior of the brush, it does not yield a useful transition temperature for comparison with the QCM-D data. For this we have defined the LCST as the midpoint temperature for the transition from a single dense phase to a swollen phase. To determine this temperature, the volume fraction profiles were parametrized by calculating the fraction of total polymer contained within the dense interior phase of the brush independent of solvent content. Details of the parametrization are given in Figure S15 and accompanying discussion. The fraction of total polymer in the inner dense region determined in this way is shown in Figure 6a as a function of temperature. The LCST was taken as the point of inflection of a sigmoidal fit to the data as shown in Figure 6a. For comparison, the shifts in LCST relative to pure water calculated from the half height of sigmoidal fits to QCM dissipation data are also presented in Figure 6b. Example QCM-D data in H2O are given in Figure S9b. Absolute values of the LCST in the absence of added electrolyte are 31.6 ± 0.1 and 33 ± 0.25 °C for NR and QCM-D, respectively. This discrepancy likely arises from different sensitivities and definitions for the LCST in both techniques as opposed to slight variations between the samples. As inferred from the volume fraction profiles in Figure 4, the NR data show a decrease in LCST with increasing concentration of the kosmotropic potassium acetate while increasing concentrations

Figure 6. (a) Fraction of total polymer in dense interior region of the brush as a function of temperature for the brush in D2O and in the presence of 100 and 250 mM potassium acetate and thiocyanate. The sigmoidal fits to the data (solid and dashed lines) yield the LCST values used for part b. (b) Shift in LCST as a function of salt concentration and identity relative to water for QCM-D (dashed lines) and NR (solid lines) measurements. For reference the LCST values in the absence of added salt are 31.6 and 33.0 °C measured by NR and QCM-D, respectively.

of potassium thiocyanate increase the LCST. These trends are supported by the QCM-D data and are consistent with previously published results.4 The close agreement of the two techniques suggests that synthesis conditions were highly reproducible. Normalized force versus separation curves in 250 mM potassium acetate and thiocyanate are presented in Figures 3b and 3c, respectively. As with the pure water data (Figure 3a), the range of the compression decreases with increasing temperature for both salts. Changes in compression are clearest in the log−linear scale of the inset plots. The range of compression for acetate is lower than water for the same temperature and is consistent with the relatively collapsed structures at constant temperature seen in the NR data. Furthermore, the range of compression is significantly less in 250 mM acetate at 23 °C than for conditions with a similar average thickness (27.5 °C in pure water and between 27.5 and 30 °C in 250 mM thiocyanate). This suggests that the reduction in compression range is not solely due to a shift in the LCST; the presence of the kosmotropic acetate ion increases the resistance to compression of the brush. Note: this conclusion is only possible through the combination of NR and AFM measurements. Further study of the equilibrium brush profile under mechanical confinement using a combination NR and surface force type apparatus would help elucidate the molecular origins of the increased resistance to compression.21 H


Article

Macromolecules For temperatures greater than 23 °C the brush is significantly more compressible in thiocyanate than in water. The reduction in the compressible region with increasing temperature is also significantly less than in water. This parallels the temperature response of the average brush thickness where 250 mM thiocyanate has shallower response of thickness with temperature such that D2O is thicker at sufficiently low temperatures (Figure 6). Note that the large reduction in range of compression at higher temperatures in water suggests that there is a negligible contribution of double layer forces to the long-range compression at higher temperatures. Thiocyanate shows a gradual decrease in the distance of the bridging interaction with increasing temperature as seen in water (Figure 3a). This is also the case for acetate between 23 and 27.5 °C. However, at higher temperatures ≥32.5 °C there are two minima present in the approach curve. Here, the attractive bridging interactions are significantly stronger in acetate than in water or thiocyanate, though it must be noted that the extent of brush compression is much less in acetate. The strength of PNIPAM bridging interactions are expected to increase with polymer concentration and temperature, due to the increase in segment−segment interactions.68 Therefore, the promotion of segment−segment interactions by the kosmotropic acetate anion cannot be discounted.28

■

AUTHOR INFORMATION

Corresponding Author

*(G.B.W.) E-mail [email protected]; phone +61 2 4033 9067. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

This work was supported by a Bragg Institute Program Grant (PP4274, P4275) and the Australian Research Council Discovery program (DP110100041). The authors thank AINSE Ltd. for providing financial assistance (Award PGRA to T.J.M. and B.A.H.) to enable work on PP4274. The Faculty of Science at the University of Newcastle is thanked for providing a summer scholarship to K.P.G., and UNSW Engineering is thanked for a Faculty Research Grant supporting S.W.P.

■

CONCLUSIONS Neutron reflectivity has been used to determine the internal nanostructure of a PNIPAM brush as a function of temperature, salt identity, and salt concentration. In pure D2O, the brush increased in thickness with decreasing temperature. This included separation of the brush into a dense interior phase and a dilute tail near the periphery at 27.5 °C. Relative to D2O, the addition of kosmotropic acetate anions collapsed the brush while chaotropic thiocyanate anions swelled the brush such that for a given temperature the thickness of the brush increased acetate < D2O < thiocyanate. The magnitude of the saltinduced changes increased with concentration. Structures were not simple modulations of the observed structures in D2O to higher or lower temperatures, showing that the addition of salt affected the internal brush nanostructure. In particular, addition of potassium thiocyanate results in a more gradual collapse over a wider temperature range. This was attributed to the ability of thiocyanate anions to bind directly to the polymer and provide electrosteric stabilization of the collapsing brush. The shift in LCST of the brush determined by NR matched the shift determined by QCM-D. Along with determination of the grafting density, AFM measurements provided valuable complementary evidence for changes in the internal structure of the brush. As with the NR measurements, addition of potassium thiocyanate resulted in more gradual reduction in range of compression with increasing temperature relative to water. Interestingly, addition of acetate resulted in a brush with a higher resistance to compression. These results highlight the potential use of the salts at either end of the Hofmeister series to modulate the behavior of thermoresponsive materials.

■

Details of neutron modeling, SMFS results, tabulated data from models and figures of the average force curves, ellipsometry, QCM-D, and fitted reflectivity curves (PDF)

(1) Minko, S. Responsive Polymer Brushes. J. Macromol. Sci., Polym. Rev. 2006, 46, 397−420. (2) Lamprou, A.; Storti, G.; Soos, M.; Morbidelli, M. Application of Polymeric Macroporous Supports for Temperature-Responsive Chromatography of Pharmaceuticals. J. Chromatogr. A 2015, 1407, 90−99. (3) Takahashi, H.; Nakayama, M.; Yamato, M.; Okano, T. Controlled Chain Length and Graft Density of Thermoresponsive Polymer Brushes for Optimizing Cell Sheet Harvest. Biomacromolecules 2010, 11, 1991−1999. (4) Humphreys, B. A.; Willott, J. D.; Murdoch, T. J.; Webber, G. B.; Wanless, E. J. Specific Ion Modulated Thermoresponse of Poly(Nisopropylacrylamide) Brushes. Phys. Chem. Chem. Phys. 2016, 18, 6037−6046. (5) Liu, Z.; Su, R.; Liang, X.; Liang, Y.; Deng, Y.; Li, Y.; Dai, R. A Thermally Switchable Chromatographic Material for Selective Capture and Rapid Release of Proteins and Nucleotides. RSC Adv. 2014, 4, 15830−15834. (6) Schild, H. Poly(N-isopropylacrylamide): Experiment, Theory and Application. Prog. Polym. Sci. 1992, 17, 163−249. (7) Zhulina, E.; Borisov, O.; Pryamitsyn, V.; Birshtein, T. CoilGlobule Type Transitions in Polymers. 1. Collapse of Layers of Grafted Polymer Chains. Macromolecules 1991, 24, 140−149. (8) de Vos, W. M.; Leermakers, F. A.; Lindhoud, S.; Prescott, S. W. Modeling the Structure and Antifouling Properties of a Polymer Brush of Grafted Comb-Polymers. Macromolecules 2011, 44, 2334−2342. (9) Afroze, F.; Nies, E.; Berghmans, H. Phase Transitions in the System Poly(N-isopropylacrylamide)/Water and Swelling Behaviour of the Corresponding Networks. J. Mol. Struct. 2000, 554, 55−68. (10) Baulin, V. A.; Halperin, A. Signatures of a ConcentrationDependent Flory χ Parameter: Swelling and Collapse of Coils and Brushes. Macromol. Theory Simul. 2003, 12, 549−559. (11) Halperin, A.; Kroger, M. Collapse of Thermoresponsive Brushes and the Tuning of Protein Adsorption. Macromolecules 2011, 44, 6986−7005. (12) Halperin, A. Compression Induced Phase Transitions in PEO brushes: The n-Cluster Model. Eur. Phys. J. B 1998, 3, 359−364. (13) Wagner, M.; Brochard-Wyart, F.; Hervet, H.; de Gennes, P.-G. Collapse of Polymer Brushes Induced by n-Clusters. Colloid Polym. Sci. 1993, 271, 621−628.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01001. I


Article

Macromolecules (14) Baulin, V.; Halperin, A. Concentration Dependence of the Flory χ Parameter within Two-State Models. Macromolecules 2002, 35, 6432−6438. (15) Mendez, S.; Curro, J. G.; McCoy, J. D.; Lopez, G. P. Computational Modeling of the Temperature-Induced Structural Changes of Tethered Poly(N-isopropylacrylamide) with Self-Consistent Field Theory. Macromolecules 2005, 38, 174−181. (16) Balamurugan, S.; Mendez, S.; Balamurugan, S. S.; O’Brie, M. J.; López, G. P. Thermal Response of Poly(N-isopropylacrylamide) Brushes Probed by Surface Plasmon Resonance. Langmuir 2003, 19, 2545−2549. (17) Malham, I. B.; Bureau, L. Density Effects on Collapse, Compression, and Adhesion of Thermoresponsive Polymer Brushes. Langmuir 2009, 26, 4762−4768. (18) Plunkett, K. N.; Zhu, X.; Moore, J. S.; Leckband, D. E. PNIPAM Chain Collapse Depends on the Molecular Weight and Grafting Density. Langmuir 2006, 22, 4259−4266. (19) Varma, S.; Bureau, L.; Débarre, D. The Conformation of Thermoresponsive Polymer Brushes Probed by Optical Reflectivity. Langmuir 2016, 32, 3152−3163. (20) Kooij, E. S.; Sui, X.; Hempenius, M. A.; Zandvliet, H. J.; Vancso, G. J. Probing the Thermal Collapse of Poly(N-isopropylacrylamide) Grafts by Quantitative in situ Ellipsometry. J. Phys. Chem. B 2012, 116, 9261−9268. (21) Abbott, S. B.; de Vos, W. M.; Mears, L. L.; Cattoz, B.; Skoda, M. W.; Barker, R.; Richardson, R. M.; Prescott, S. W. Is Osmotic Pressure Relevant in the Mechanical Confinement of a Polymer Brush? Macromolecules 2015, 48, 2224−2234. (22) Yim, H.; Kent, M.; Mendez, S.; Lopez, G.; Satija, S.; Seo, Y. Effects of Grafting Density and Molecular Weight on the Temperature-Dependent Conformational Change of Poly(N-isopropylacrylamide) Grafted Chains in Water. Macromolecules 2006, 39, 3420−3426. (23) Yim, H.; Kent, M.; Satija, S.; Mendez, S.; Balamurugan, S.; Balamurugan, S.; Lopez, G. Evidence for Vertical Phase Separation in Densely Grafted, High-Molecular-Weight Poly(N-isopropylacrylamide) Brushes in Water. Phys. Rev. E 2005, 72, 051801. (24) Yim, H.; Kent, M.; Mendez, S.; Balamurugan, S.; Balamurugan, S.; Lopez, G.; Satija, S. Temperature-Dependent Conformational Change of PNIPAM Grafted Chains at High Surface Density in Water. Macromolecules 2004, 37, 1994−1997. (25) Elliott, L. C.; Jing, B.; Akgun, B.; Zhu, Y.; Bohn, P. W.; Fullerton-Shirey, S. K. Loading and Distribution of a Model Small Molecule Drug in Poly(N-isopropylacrylamide) Brushes: A Neutron Reflectometry and AFM Study. Langmuir 2013, 29, 3259−3268. (26) Xia, Y.; Burke, N. A.; Stöver, H. D. End Group Effect on the Thermal Response of Narrow-Disperse Poly(N-isopropylacrylamide) Prepared by Atom Transfer Radical Polymerization. Macromolecules 2006, 39, 2275−2283. (27) Patel, T.; Ghosh, G.; Yusa, S. i.; Bahadur, P. Solution Behavior of Poly(N-Isopropylacrylamide) in Water: Effect of Additives. J. Dispersion Sci. Technol. 2011, 32, 1111−1118. (28) Zhang, Y.; Cremer, P. S. Chemistry of Hofmeister Anions and Osmolytes. Annu. Rev. Phys. Chem. 2010, 61, 63−83. (29) Lo Nostro, P.; Ninham, B. W. Hofmeister Phenomena: An Update on Ion Specificity in Biology. Chem. Rev. 2012, 112, 2286− 2322. (30) Omta, A. W.; Kropman, M. F.; Woutersen, S.; Bakker, H. J. Negligible Effect of Ions on the Hydrogen-Bond Structure in Liquid Water. Science 2003, 301, 347−349. (31) Rembert, K. B.; Okur, H. I.; Hilty, C.; Cremer, P. S. An NH Moiety Is Not Required for Anion Binding to Amides in Aqueous Solution. Langmuir 2015, 31, 3459−3464. (32) Algaer, E. A.; van der Vegt, N. F. Hofmeister Ion Interactions with Model Amide Compounds. J. Phys. Chem. B 2011, 115, 13781− 13787. (33) Du, H.; Wickramasinghe, R.; Qian, X. Effects of Salt on the Lower Critical Solution Temperature of Poly(N-isopropylacrylamide). J. Phys. Chem. B 2010, 114, 16594−16604.

(34) Fu, H.; Hong, X.; Wan, A.; Batteas, J. D.; Bergbreiter, D. E. Parallel Effects of Cations on PNIPAM Graft Wettability and PNIPAM Solubility. ACS Appl. Mater. Interfaces 2010, 2, 452−458. (35) Pastoor, K. J.; Rice, C. V. Anion Effects on the Phase Transition of N-Isopropylacrylamide. J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 1374−1382. (36) Naini, C. A.; Thomas, M.; Franzka, S.; Frost, S.; Ulbricht, M.; Hartmann, N. Hofmeister Effect of Sodium Halides on the Switching Energetics of Thermoresponsive Polymer Brushes. Macromol. Rapid Commun. 2013, 34, 417−422. (37) Liu, L.; Shi, Y.; Liu, C.; Wang, T.; Liu, G.; Zhang, G. Insight into the Amplification by Methylated Urea of the Anion Specificity of Macromolecules. Soft Matter 2014, 10, 2856−2862. (38) Zhang, Y.; Furyk, S.; Bergbreiter, D. E.; Cremer, P. S. Specific Ion Effects on the Water Solubility of Macromolecules: PNIPAM and the Hofmeister Series. J. Am. Chem. Soc. 2005, 127, 14505−14510. (39) Zhang, Y.; Furyk, S.; Sagle, L. B.; Cho, Y.; Bergbreiter, D. E.; Cremer, P. S. Effects of Hofmeister Anions on the LCST of PNIPAM as a Function of Molecular Weight. J. Phys. Chem. C 2007, 111, 8916− 8924. (40) Chen, X.; Yang, T.; Kataoka, S.; Cremer, P. S. Specific Ion Effects on Interfacial Water Structure Near Macromolecules. J. Am. Chem. Soc. 2007, 129, 12272−12279. (41) Ishida, N.; Biggs, S. Salt-Induced Strutural Behavior for Poly(Nisopropylacrylamide) Grafted onto Solid Surface Observed Directly by AFM and QCM-D. Macromolecules 2007, 40, 9045−9052. (42) Jhon, Y. K.; Bhat, R. R.; Jeong, C.; Rojas, O. J.; Szleifer, I.; Genzer, J. Salt-Induced Depression of Lower Critical Solution Temperature in a Surface-Grafted Neutral Thermoresponsive Polymer. Macromol. Rapid Commun. 2006, 27, 697−701. (43) Kizhakkedathu, J. N.; Norris-Jones, R.; Brooks, D. E. Synthesis of Well-Defined Environmentally Responsive Polymer Brushes by Aqeuous ATRP. Macromolecules 2004, 37, 734−743. (44) Gao, X.; Kucerka, N.; Nieh, M.-P.; Katsaras, J.; Zhu, S.; Brash, J. L.; Sheardown, H. Chain Conformation of a New Class of PEG-Based Thermoresponsive Polymer Brushes Grafted on Silicon as Determined by Neutron Reflectometry. Langmuir 2009, 25, 10271−10278. (45) Willott, J. D.; Murdoch, T. J.; Humphreys, B. A.; Edmondson, S.; Webber, G. B.; Wanless, E. J. Critical Salt Effects in the Swelling Behavior of a Weak Polybasic Brush. Langmuir 2014, 30, 1827−1836. (46) Willott, J. D.; Humphreys, B.; Murdoch, T. J.; Edmondson, S.; Webber, G. B.; Wanless, E. Hydrophobic Effects within the Dynamic pH Response of Polybasic Tertiary Amine Methacrylate Brushes. Phys. Chem. Chem. Phys. 2015, 17, 3880−3890. (47) Willott, J.; Murdoch, T.; Humphreys, B.; Edmondson, S.; Wanless, E. J.; Webber, G. B. Anion-specific Effects on the Behavior of pH-Sensitive Polybasic Brushes. Langmuir 2015, 31, 3707−3717. (48) Cheesman, B. T.; Smith, E. G.; Murdoch, T. J.; Guibert, C.; Webber, G. B.; Edmondson, S.; Wanless, E. J. Polyelectrolyte Brush pH-Response at the Silica−Aqueous Solution Interface: A Kinetic and Equilibrium Investigation. Phys. Chem. Chem. Phys. 2013, 15, 14502− 14510. (49) Nelson, A. Co-Refinement of Multiple-Contrast Neutron/X-Ray Reflectivity Data using MOTOFIT. J. Appl. Crystallogr. 2006, 39, 273− 276. (50) James, M.; Nelson, A.; Holt, S. A.; Saerbeck, T.; Hamilton, W. A.; Klose, F. The Multipurpose Time-of-Flight Neutron Reflectometer “Platypus” at Australia’s OPAL Reactor. Nucl. Instrum. Methods Phys. Res., Sect. A 2011, 632, 112−123. (51) Karim, A.; Satija, S. K.; Douglas, J. F.; Ankner, J. F.; Fetters, L. J. Neutron Reflectivity Study of the Density Profile of a Model EndGrafted Polymer Brush: Influence of Solvent Quality. Phys. Rev. Lett. 1994, 73, 3407−3410. (52) Sudre, G.; Hourdet, D.; Creton, C.; Cousin, F.; Tran, Y. pHResponsive Swelling of Poly(acrylic acid) Brushes Synthesized by the Grafting Onto Route. Macromol. Chem. Phys. 2013, 214, 2882−2890. (53) Habicht, J.; Schmidt, M.; Rühe, J.; Johannsmann, D. Swelling of Thick Polymer Brushes Investigated with Ellipsometry. Langmuir 1999, 15, 2460−2465. J


Article

Macromolecules (54) Ralston, J.; Larson, I.; Rutland, M. W.; Feiler, A. A.; Kleijn, M. Atomic Force Microscopy and Direct Surface Force Measurements (IUPAC Technical Report). Pure Appl. Chem. 2005, 77, 2149−2170. (55) Willott, J. D.; Murdoch, T. J.; Webber, G. B.; Wanless, E. J. Nature of the Specific Anion Response of a Hydrophobic Weak Polyelectrolyte Brush Revealed by AFM Force Measurements. Macromolecules 2016, 49, 2327−2338. (56) Brittain, W. J.; Minko, S. A Structural Definition of Polymer Brushes. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 3505−3512. (57) Kutnyanszky, E.; Embrechts, A.; Hempenius, M. A.; Vancso, G. J. Is there a Molecular Signature of the LCST of Single PNIPAM Chains as Measured by AFM Force Spectroscopy? Chem. Phys. Lett. 2012, 535, 126−130. (58) Bae, Y. H.; Okano, T.; Kim, S. W. Temperature Dependence of Swelling of Crosslinked Poly(N, N′-alkyl substituted acrylamides) in Water. J. Polym. Sci., Part B: Polym. Phys. 1990, 28, 923−936. (59) Jia, H.; Wildes, A.; Titmuss, S. Structure of pH-Responsive Polymer Brushes Grown at the Gold−Water Interface: Dependence on Grafting Density and Temperature. Macromolecules 2011, 45, 305− 312. (60) Moglianetti, M.; Webster, J. R.; Edmondson, S.; Armes, S. P.; Titmuss, S. Neutron Reflectivity Study of the Structure of pHResponsive Polymer Brushes Grown from a Macroinitiator at the Sapphire−Water Interface. Langmuir 2010, 26, 12684−12689. (61) Baulin, V. A.; Zhulina, E. B.; Halperin, A. Self-Consistent Field Theory of Brushes of Neutral Water-Soluble Polymers. J. Chem. Phys. 2003, 119, 10977−10988. (62) de Vos, W. M.; Leermakers, F. A. Modeling the Structure of a Polydisperse Polymer Brush. Polymer 2009, 50, 305−316. (63) Block, S.; Helm, C. A. Conformation of Poly(styrene sulfonate) Layers Physisorbed from High Salt Solution Studied by Force Measurements on Two Different Length Scales. J. Phys. Chem. B 2008, 112, 9318−9327. (64) Goodman, D.; Kizhakkedathu, J. N.; Brooks, D. E. Attractive Bridging Interactions in Dense Polymer Brushes in Good Solvent Measured by Atomic Force Microscopy. Langmuir 2004, 20, 2333− 2340. (65) Prescott, S. W.; Fellows, C. M.; Considine, R. F.; Drummond, C. J.; Gilbert, R. G. The Interactions of Amphiphilic Latexes with Surfaces: The Effect of Surface Modifications and Ionic Strength. Polymer 2002, 43, 3191−3198. (66) Ishida, N.; Kobayashi, M. Interaction Forces Measured Between Poly(N-isopropylacrylamide) Grafted Surface and Hydrophobic Particle. J. Colloid Interface Sci. 2006, 297, 513−519. (67) Zhuang, P.; Dirani, A.; Glinel, K.; Jonas, A. M. Temperature Dependence of the Surface and Volume Hydrophilicity of Hydrophilic Polymer Brushes. Langmuir 2016, 32, 3433−3444. (68) Gong, X.; Wu, C.; Ngai, T. Surface Interaction Forces Mediated by poly(N-isopropylacrylamide) (PNIPAM) Polymers: Effects of Concentration and Temperature. Colloid Polym. Sci. 2010, 288, 1167−1172.

K


Specific Anion Effects on the Internal Structure of a ... - ACS Publications

Recommend Documents