Article Cite This: Langmuir 2018, 34, 2448−2454
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Salt Sensitivity of the Thermoresponsive Behavior of PNIPAAm Brushes Meike Koenig,†,¶ Keith Brian Rodenhausen,‡,∇ Sebastian Rauch,† Eva Bittrich,† Klaus-Jochen Eichhorn,† Mathias Schubert,§,∥ Manfred Stamm,†,⊥ and Petra Uhlmann*,†,# †
Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Straße 6, 01069 Dresden, Germany Department of Chemical and Biomolecular Engineering, University of Nebraska−Lincoln, 207 Othmer Hall, Lincoln, Nebraska 68588, United States § Department of Electrical and Computer Engineering and Center for Nanohybrid Functional Materials, University of Nebraska−Lincoln, 209N Scott Engineering Center, Lincoln, Nebraska 68588, United States ∥ Department of Physics, Chemistry, and Biology, IFM, Linköping University, SE-581 83 Linköping, Sweden ⊥ Faculty of Science, Department of Chemistry, Chair of Physical Chemistry of Polymeric Materials, Technische Universität Dresden, Bergstraße 66, 01069 Dresden, Germany # Department of Chemistry, University of Nebraska−Lincoln, Hamilton Hall, 639 North 12th Street, Lincoln, Nebraska 68588, United States ‡
S Supporting Information *
ABSTRACT: We report investigations on the salt sensitivity of the thermoresponsive behavior of PNIPAAm brushes applying the quartz crystal microbalance coupled with spectroscopic ellipsometry technique. This approach enables a detailed study of the optical and mechanical behavior of the polymer coatings. Additional conclusions can be drawn from the difference between both techniques due to a difference in the contrast mechanism of both methods. A linear shift of the phasetransition temperature to lower temperatures with the addition of sodium chloride was found, similar to the behavior of free polymer chains in solution. The thermal hysteresis was found to be decreased by the addition of sodium chloride to the solution, hinting to the interaction of the ions with the amide groups of the polymer, whereby the formation of hydrogen bonds is hindered. The results of this study are of relevance to the application of PNIPAAm brushes in biological fluids and demonstrate the additional potential of the ion sensitivity besides the better known thermosensitivity.
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INTRODUCTION PNIPAAm probably is the most prominent example for polymers exhibiting lower critical solution temperature behavior and has been studied extensively.1−4 One reason for this is that its phase-transition temperature Tc in water lies in the vicinity of the human body temperature, rendering it interesting for biological applications.5−7 Because biological fluids contain high amounts of salt, the effect of additional salt on the phase transition, as it was found for PNIPAAm chains in solution,1,8,9 is of importance, as well. The addition of ions to an aqueous solution was reported to influence the position of the phase transition.1,10 This effect is widely known as the “Hofmeister effect”, describing the ability of anions and cations to influence many crucial properties of a solution.11−14 Initially, it was discussed to stem from a change in the ordering of the water structure, but in recent reports, the Hofmeister effect was argued to partly arise from direct interaction between the ions and the solute.15−18 In the case of © 2018 American Chemical Society
PNIPAAm, Zhang et al. discussed three mechanisms contributing to the shift of Tc: (a) dehydration of the amide groups by kosmotropic ions due to favorable hydration of the small ions, (b) a change in the surface tension affecting the hydration of the hydrophobic parts of the polymer, and (c) direct interaction of the anions with the amide groups.16 While the first two effects lead to salting-out of the polymer (decrease in Tc), the last effect leads to salting-in of the polymer (increase in Tc). The extent to which each of these effects contributes to the overall behavior of an ion varies. Cl− is located in the middle of the Hofmeister series with weak chaotropic properties. Although weak binding to the amide group was measured, overall a salting-out effect is found due to the increase in surface tension.16,19 Received: November 14, 2017 Revised: January 12, 2018 Published: January 22, 2018 2448
DOI: 10.1021/acs.langmuir.7b03919 Langmuir 2018, 34, 2448−2454
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Figure 1. Exemplary data of QCM and SE measurements of the salt- and temperature-dependent swelling of PNIPAAm brushes; (a) ΔF of the seventh overtone, (b) ΔD of the seventh overtone, (c) Ψ at λ = 501 nm, and (d) Δ at λ = 501 nm.
effect the Tc according to their order in the Hofmeister series. The effect of kosmotropic and chaotropic anions was compared by the group of Wanless using neutron reflectivity, AFM, and QCM-D32 or QCM-D, ellipsometry, and contact angle measurements.33 Also here, a behavior in accordance with the Hofmeister series was found, with kosmotropic acetate ions shifting the Tc to lower temperatures and the chaotropic thiocyanate ions shifting the Tc to higher temperatures. A linear dependence on the concentration of the ions was detected, similar to the behavior of free PNIPAAm chains in solution. However, neither of these reports uses viscoelastic modeling to further elucidate the detected changes in QCM-D data. Moreover, the discrepancies found by Humphreys et al. comparing the results of QCM-D and ellipsometry measurements could not be fully explained by the authors.33 We used a combinatorial setup of QCM-D and spectroscopic ellipsometry (SE) to investigate the phase transition, and its shift with NaCl content, of grafting-to PNIPAAm brushes. Using viscoelastic and optical modeling, this setup allows a comprehensive study of the thickness, composition, optical, and mechanical properties of the polymer layer.35−37 Here, the combined setup ensures the direct comparability of the two techniques for further evaluation.
Polymer brushes are a special type of polymeric coating, which have received increasing interest in recent years.20−23 These systems consist of polymer chains, tethered by one end to a planar or curved substrate, in close proximity to each other such that the chains are forced to stretch away from the surface in a “brush conformation”.24 Responsive polymer brushes are capable of reacting to external stimuli, generally by the reversible swelling−deswelling behavior. PNIPAAm brushes have been reported to exhibit a broader transition than in solution; this has been discussed to stem from the increased interaction between the closely tethered polymer chains.25,26 Few reports have dealt with the influence of additional salt on the thermoresponsive behavior of grafted PNIPAAm chains.27−34 Jhon et al. investigated the influence of NaCl on grafting-from PNIPAAm brushes with an unknown grafting density by quartz crystal microbalance with dissipation monitoring (QCM-D). They found a nonlinear dependence of the shift of Tc to lower temperatures, with a lower rate of decline at increased concentrations of NaCl. This is unlike the behavior found for PNIPAAm in solution, where the addition of NaCl lowers the transition temperature in a linear manner.27 Ishida et al. investigated the response of PNIPAAm brushes with a very low grafting density (∼0.008 chains nm−2) to the addition of Na2SO4 using in situ atomic force microscopy (AFM) and QCM-D. They found a very unusual peaklike change in frequency and dissipation with increasing salt content, which they attributed to the drastic structural changes occurring in the polymer layer.28 Naini et al. measured the influence of different sodium halides on the switching kinetics and thermodynamics of PNIPAAM brushes using a laser temperature-jump technique.30 The halides were found to
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RESULTS AND DISCUSSION The effect of the concentration of NaCl was investigated by measuring the temperature-dependent swelling at four different salt concentrations (0, 0.1, 0.5, and 1 M) for a PNIPAAm brush with a dry thickness of 12 ± 0.5 nm and a grafting density of 0.16 nm−1. The measurements were recorded in a quasi-static state, allowing the sample to equilibrate for 30 min at each 2449
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phase transition to lower temperatures. The plot of Tc as calculated from the heating curves is shown in the Supporting Information (Figure S2). As found for PNIPAAm in solution, the transition is linearly shifted upon the addition of NaCl. At c(NaCl) = 1 M, the transition is shifted by ∼13 degrees down to 20 °C. This change is slightly larger than the change reported for PNIPAAm in solution, where the Tc was found to shift from 31 °C to about 22 °C at c(NaCl) = 1 M.8 The shape of the transition curve was found to remain unchanged upon the addition of NaCl, and no broadening or sharpening was observed. This indicates that the addition of NaCl does not alter the mechanism of deswelling of the chains. PNIPAAm is known from the literature for the exhibition of a hysteresis between the heating and the cooling cycle because of the formation of inter- and intramolecular hydrogen bonds in the collapsed state that have to be broken for reswelling.38−40 For grafted polymer chains, the hysteresis was found to depend on the grafting density.4 Comparing the results of the two techniques, a distinction can be seen in the hysteresis behavior: while QCM data show almost no difference in between the heating and cooling cycle, the transition temperature derived from the SE data varies by about one degree. This can be attributed to the different sensitivities of SE and QCM-D to the limit of the outer boundary of the brush against the bulk ambient.41,42 There is a very low optical contrast between the ambient bulk and the top of the swollen brush, where dilute polymer chains dangle out; SE is more biased to the denser region of the brush layer near the substrate. QCM-D is more
temperature step, before the temperature was raised or cooled further. Because a bare QCM-D sensor also shows temperaturedependent frequency and dissipation shifts, the response of a bare sensor was subtracted from the data to only evaluate the effect of the polymer coating (see Figure S1 in the Supporting Information). The effect of the salt concentration on the response of the bare sensor was found to be negligible. Figure 1 shows the exemplary data from QCM-D and SE measurements of one heating and cooling cycle; similar curves can be plotted for each of the overtones and wavelengths measured. The transition temperature was determined from those curves as the maximum of the first derivative, by averaging the slopes of two adjacent points (Table 1). Table 1. Transition Temperatures Tc Determined from Raw Data of QCM and SE Measurements of the Salt- and Temperature-Dependent Swelling of PNIPAAm Brushes Tc,heating (Tc,cooling) [°C] c(NaCl) [mol/L] 0 0.1 0.5 1
ΔF 33.8 31.8 26.8 20.9
(33.8) (31.8) (26.8) (19.9)
ΔD 33.8 31.8 25.9 19.9
(32.8) (31.8) (25.9) (19.9)
Ψ 33.8 31.8 25.9 19.9
(31.8) (30.9) (24.9) (19.5)
Δ 33.8 31.8 25.9 19.9
(32.8) (30.9) (25.9) (19.9)
The addition of salt to the solution has a very prominent effect: increasing the concentration of NaCl gradually shifts the
Figure 2. Difference between the heating and cooling cycle in exemplary data of QCM and SE measurements of the salt- and temperature-dependent swelling of PNIPAAm brushes; (a) ΔF of the seventh overtone, (b) ΔD of the seventh overtone, (c) Ψ at λ = 501 nm, and (d) Δ at λ = 501 nm. 2450
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Figure 3. Effect of salt and temperature on the amount of aqueous solution in PNIPAAm brushes; amount of aqueous solution detected by SE (a) and QCM-D (b), (c) difference in the amount of aqueous solution detected by QCM-D and SE.
aqueous solution acoustically coupled and adsorbed to the polymer brush in this state is unknown, and the derived mass change only contains the amount of aqueous solution added to the polymer layer upon further swelling. SE data was modeled using a multilayer box model, applying a two-component effective medium approach (EMA) to fit for the optical properties of the brush layer. This rather simple box model has turned out to work very well for thin polymer films as investigated here because more complex models led to correlations in the modeled parameters.45 Within the box layer, a homogenous distribution of the polymer segment density within the box is assumed. From this model, the thickness dbrush and the volume fraction of the aqueous solution in the film f H2O can be derived. The mass density of aqueous solution within the swollen polymer film was then calculated using ΓH2O,SE = f H2O·ρH2O·dbrush. A density of 1 g cm−3 for the aqueous solution was assumed. Figure 3 displays the best-fit model data of the saltdependent measurements for the amount of aqueous solution ΓSE and ΓQCM. The critical temperature can be clearly detected as a strong decrease in the amount of aqueous solution in the polymer layer. The shift seen in the raw data upon the addition of NaCl is mirrored in the amount of aqueous solution, as well. As in the raw data, the actual shape of the temperaturedependent swelling is not changed by the presence of the ions. The difference in the amount of aqueous solution between the two techniques ΔΓQCM−SE can be attributed to a difference in
sensitive to the dilute regime of the brush, where rehydration of the PNIPAAm chains and breaking of intermolecular hydrogen bonds occur more easily. To further investigate the hysteresis behavior, the difference between the heating and cooling cycle at each temperature is plotted in Figure 2. The biggest difference between heating and cooling is found at the Tc, as expected. Of greater interest is the observation that in almost all cases the hysteresis decreases with increasing salt content. A similar effect has been reported so far only for the strong chaotropic anion SCN− by Shechter et al.43 They proposed that the presence of bound ions diminishes the segment−segment interaction, leading to less inter- and intrachain hydrogen bonds, which would have to be broken for reswelling. Even though in our case the direct interaction of NaCl with the amide group is much weaker,19 the effect is obviously large enough to be detected by QCM-D and SE. For kosmotropic anions, a decreasing effect on the hysteresis was found as well, but here a relative stabilization of the collapsed coiled stage was hypothesized.44 Because in our case the shape of the transition curve does not change upon the addition of NaCl, this effect can be excluded. A Voigt−Voinova approach was used to evaluate changes in areal mass and viscoelasticity of the QCM-D data. Negative dissipation shift values are not treatable by the Voigt−Voinova model. To avoid these, ΔD was offset such that the minimum ΔD value obtained at the highest respective temperature was set as the zero reference point. At this reference point, the brush is already solvated to some extent, but the amount of 2451
DOI: 10.1021/acs.langmuir.7b03919 Langmuir 2018, 34, 2448−2454
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CONCLUSIONS Summarizing, in this report, the effect of salt concentration on the thermoresponsive behavior of PNIPAAm brushes was investigated. A large linear shift of Tc to lower temperatures was observed upon the addition of NaCl. This shift was in a range similar to that found for PNIPAAm in solution. The comparison of the two applied techniques indicated that SE detects a larger thermal hysteresis than QCM-D, probably because SE has an increased sensitivity for the dense region of the brush, where the effect of interchain hydrogen bonds would be more prominent. The measured hysteresis effect decreased with increasing NaCl content, which indicates the interaction of ions with the amide groups and disruption of hydrogen bonding. The insights reported here provide further knowledge for the development and application of thermoresponsive coatings utilizing PNIPAAm brushes and demonstrate the interesting potential for applications of the additional ion sensitivity. The results also show that salt effects have to be considered when PNIPAAm brushes are supposed to be used for the temperature-sensitive control of biointeractions.
contrast, thereby differing in the definition of the outer boundary of the thin film, comprising vibrationally coupled ambient molecules and dangling polymer chains.41,42 Thus, the magnitude of the discrepancy between the two techniques can provide insight into the sharpness of the film−ambient interface. Low values of ΔΓQCM−SE imply that the contrast between the edge of the brush and the bulk ambient is more well-defined, whereas higher values imply a decrease in optical contrast. As described in a previous report,37 two different swelling regimes can be clearly defined: starting from low temperatures ΔΓQCM−SE stays almost constant or slightly increases, indicating that although the amount of aqueous solution decreases, the outer boundary of the polymer layer does not change. With a further rise of temperature, the amount of aqueous solution detected by both techniques quickly converges. This can be explained by the transition from a parabolic to a stepwise profile of the chain segment density as the polymer chains in the brush layer collapse, resulting in a more defined interface between the polymer layer and ambient, which can analogously be detected by both techniques. Interestingly, ΔΓQCM−SE of the swollen state decreases slightly with increasing NaCl concentration. This could be caused by a more defined interface between the brush and the ambient because of the interaction with the ions. Best match model data for the layer viscosity, shown in Figure 4, exhibit a shift to higher values at high salt
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EXPERIMENTAL SECTION
Brush Preparation. Polymer brushes were grafted on silica-coated quartz crystals with a ca. 50 nm thick silicon oxide layer (QSX 303, QSense, Frölunda, Sweden). Samples were cleaned with ethanol abs. (VWR, Germany) and dried with N2 gas. Then, the samples were activated by oxygen plasma for 1 min at 100 W (440-G Plasma System, Technics Plasma GmbH, Germany). A macromolecular anchoring layer was spin-coated onto the surface from a 0.02 wt % solution of poly(glycidyl methacrylate) (PGMA, Mn = 17 500 g mol−1, Mw/Mn = 1.12, Polymer Source, Inc., Canada) in chloroform (Sigma-Aldrich, Germany) and annealed in vacuum for 20 min at 100 °C to chemically bind the polymer to the activated SiO2 surface. Carboxy-terminated PNIPAAm (Mn = 48 900 g mol−1, Mw/Mn = 1.3) was synthesized by controlled radical polymerization (atom transfer radical polymerization) as described previously46 and grafted onto the surface from a 0.8 wt % solution in chloroform, by annealing in vacuum at 150 °C, overnight. Noncovalently bound polymer was extracted in ethanol. The process was followed by ellipsometry in the dry state after each modification step. The grafting density of the resulting polymer brush was calculated from the dry thickness,24 using literature values for the bulk density of PNIPAAm.47 Combinatorial QCM-D/SE. The combinatorial QCM-D/SE setup consists of an E1 QCM-D and an ellipsometry compatible module (QSense, Frölunda, Sweden), mounted onto the sample stage of an alpha-SE spectroscopic ellipsometer with a fixed angle of incidence (AOI) of 65° and a spectral range of 370−900 nm. Accidentally, measurements were performed with an instrument setting of 70° AOI. Subsequent data analysis was corrected by appropriate numerical models for the liquid cell internal AOI of 68.8° by considering small, calibrated refractive polarization variations across the entrance and exit windows. The ellipsometry compatible module is an air-tight liquid flow chamber, into which the sample is inserted for in situ analysis; the module has windows for the probing light beam of an ellipsometry measurement. The flow of the liquid medium was facilitated by means of chemical-resistant polytetrafluoroethylene tubing (VICI AG International, Switzerland), which is connected to a peristaltic flow pump (Ismatec IPC high precision multichannel dispenser, IDEX Health & Science GmbH, Wertheim-Mondfeld, Germany). Millipore water purified with a PURELAB Plus Ultrapure instrument was used, with and without the addition of NaCl. The ellipsometry compatible module temperature was controlled by the QCM-D software, and the system temperature was maintained, stepwise, at 30 min intervals. Temperature-dependent experiments were done at stagnant flow conditions. Ellipsometry Modeling. Modeling of the optical properties (refractive index) and the thickness (d) of the polymer films from
Figure 4. Effect of salt and temperature on the viscosity of PNIPAAm brushes.
concentration. Here, the polymer chains are surrounded by a more viscous ambient, which leads to a higher dissipation of vibrational energy of the swollen brush layer. For pure water and c(NaCl) = 0.1 M, a local minimum is detected before the most collapsed stage with a substantial hysteresis between heating and cooling. The reason for this is unclear because this effect is not mirrored in the raw data. Further studies are needed to reveal the origin of this feature. To investigate the influence of the grafting density on the salt-dependent swelling behavior, PNIPAAm brushes with a grafting density of 0.09 nm−1 were measured in a similar manner (see Figures S4 and S5 in the Supporting Information). As described in a previous publication,37 a lower grafting density results in a lower swelling ratio and a slight shift of Tc to lower temperatures. The thermal hysteresis was found to be diminished as well. However, the effect of additional sodium chloride was found to be similar to that for the polymer brush with a higher grafting density. 2452
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the ellipsometry-measured primary data Δ (relative phase shift) and tan(ψ) (relative amplitude ratio) was performed using a multilayer box model, consisting of a silica-coated quartz sensor, PGMA as an anchoring layer, and PNIPAAM as a brush layer. Measurements of the blank silica-coated quartz sensors were parameterized by basis spline (B-spline) functions and considered “virtual substrates” for further modeling.48 The modification with polymer layers in the dry state was modeled with fixed refractive indices described by Cauchy dispersions (PGMA: A = 1.525 and B = 0 μm2 and PNIPAAm: A = 1.442 and B = 0.0056 μm2, determined by measurements of a thick polymer layer) in an optical box model. For in situ measurements, a two-parameter Bruggemann-EMA model was used to determine the thickness of the polymer brush dbrush, the combined refractive index neff, and the volume fractions of the polymers f polymer and the ambient solution f H2O, using fixed values of refractive index as for the dry measurements. For the optical properties of the ambient, the refractive index, n(λ), was determined with a digital multiple wavelength refractometer (DSR-lambda, Schmidt + Haensch GmbH & Co.) at eight different wavelengths from 435.8 to 706.5 nm. QCM-D Modeling. Shifts in frequency and dissipation of the odd overtones (j = 3, 5, 7, 9, and 11) were modeled using a Voigt−Voinova approach49 for one homogeneous viscoelastic layer with a fixed density of 1 g cm−3. Measurements on PNIPAAm brushes were referenced to the measurement with the smallest dissipation value. The temperature change in frequency and dissipation of a bare QCM-D sensor was subtracted from the QCM-D data, to eliminate the temperature sensitivity of the bare quartz substrate. For modeling, the software QTools (Q-Sense, Frölunda, Sweden) was used.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b03919. Exemplary data of the temperature response of a bare QCM-D sensor; plot of the shift of the calculated Tc; modeled thickness data; swelling behavior at a lower grafting density (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Petra Uhlmann: 0000-0001-9298-4083 Present Addresses ¶
Karlsruhe Institute of Technology, Institute of Functional Interfaces, Hermann-von-Helmholtz-Platz 1, 76344 EggensteinLeopoldshafen, Germany. ∇ Biolin Scientific, Inc., Paramus, New Jersey 07652, USA. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support was granted by the German Science Foundation (DFG) within the DFG-NSF cooperation project (DFG Proj. Nr. STA 324/49-1 and EI 317/6-1) in the frame of the “Materials World Network” and in the frame of the priority program SPP 1369 “Polymer-Solid Contacts: Interfaces and Interphases” (DFG Proj. Nr. STA 324/37-1). This work was supported in part by the National Science Foundation (NSF) through the Center for Nanohybrid Functional Materials (EPS1004094). 2453
DOI: 10.1021/acs.langmuir.7b03919 Langmuir 2018, 34, 2448−2454
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DOI: 10.1021/acs.langmuir.7b03919 Langmuir 2018, 34, 2448−2454
