Hydration and Viscoelastic Properties of High- and Low-Density

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Hydration and Viscoelastic Properties of High- and Low-Density Polymer Brushes Using a Quartz-Crystal Microbalance Based on Admittance Analysis (QCM-A) Hiroyuki Furusawa,*,†,‡ Tomomi Sekine,‡ and Tomomitsu Ozeki‡ †

Innovative Flex Course for Frontier Organic Material Systems (iFront), Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan ‡ Department of Biomolecular Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8501, Japan S Supporting Information *

ABSTRACT: The hydration and viscoelastic properties of synthetic polymer brushes on a substrate were evaluated using a 27 MHz piezoelectric quartz-crystal microbalance based on admittance analysis (QCM-A). The (−ΔFwater)/(−ΔFair) values, which indicate the hydrodynamic water (bound and vibrated water) ratio per unit polymer mass, and the ΔDwater/ (−ΔFair) values, which indicate the viscoelastic properties (energy dissipation per mass), were obtained for low- and high-density brushes of each polymer. These values indicated that the low-density polymer chains had different properties, which depended on the chemical features of each polymer. In contrast, these values were similar for the high-density brushes of each polymer, indicating that the polymers were dehydrated and more elastic due to limited space for movement of the chains with the solvent. These mechanical properties are expected to relate to functions such as surface friction and biocompatibility.

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INTRODUCTION

polymers including water is required to understand the functions of the material surface, such as biocompatibility. Scanning probe microscopy (SPM), which includes atomic force microscopy (AFM), is a technique that can be used to evaluate the physical properties of polymer surfaces,8 whereas scanning viscoelasticity microscopy (SVM) can be used to map polymer surfaces based on the surface modulus.9 However, quantitative measurement of water amounts in polymers on a substrate is difficult with SPM techniques because the probe cannot distinguish between water and the polymers. As other evaluation methods for water in polymers, Raman spectroscopy are used to observe spectrochemical water and attenuated total reflection infrared (ATR-IR) spectroscopy are carried out to observe the hydrogen-bonding network of water molecules around polymer side chains.10 Neutron reflectometry (NR) can reveal the ratio of water uptake in a polymer.11,12 Although these optical spectrometry and reflectometry techniques and other optical techniques, including surface plasmon resonance and ellipsometry, can provide information on about the volume fraction of water in hydrated polymer films, these techniques cannot easily be used to evaluate mechanical properties, such as viscoelasticity, of polymers including water on a substrate. Thus, a technique that can evaluate both the mechanical

The properties of material surfaces are related to various interface phenomena, such as wettability, friction, adhesiveness, and adsorption. The surface properties of surface-modified materials have attracted attention in the field of materials science because surface modification can impart desirable functions to a material surface. Previously, many studies have reported surface modification by polymer grafting.1 Surface grafting of poly(2-methacryloyloxyethylphosphorylcholine) (PMPC) on a polyethylene film or a silicon wafer was reported to preserve platelet function or dramatically reduce protein adsorption that causes undesirable biological response cascades.2−4 The protein adsorption was affected by both graft density and chain length.4 Moreover, although protein adsorption occurred on a cast film or a low-density brush of poly(2-hydroxyethyl methacrylate) (PHEMA), a high-density brush showed resistance against protein adsorption, independent of the brush thickness.5 Thus, investigation from the viewpoint of not only the polymer chemical structure but also the various physical properties of the polymer brush on the substrate is important to explain the observed interface phenomena. On the other hand, water structures, such as nonfreezing water, freezing bound water, and free water around polymers modified on a substrate, were reported to be related to the functions of a material surface.6,7 This suggests that an evaluation of the physical properties of surface-modified © XXXX American Chemical Society

Received: January 7, 2016 Revised: April 7, 2016

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DOI: 10.1021/acs.macromol.6b00035 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules properties and water amounts simultaneously is required for polymer surfaces including water. A piezoelectric quartz-crystal microbalance (QCM), in which a quartz plate acts as a thickness-shear mode resonator, is a sensitive mass-measuring device. The fundamental frequency (the F value) of the QCM decreases linearly with increasing mass (at the nanogram level) on an electrode attached to the quartz plate.13−15 The QCM can work not only in air but also in solution, as long as the quartz plate resonator oscillates. Therefore, the QCM technique has been applied as a biosensor in aqueous solution.16−19 In addition, the energy dissipation of the quartz plate resonator can be obtained as the energy dissipation factor D, which corresponds to the inverse of the frequency quality Q, and the viscoelastic properties of a material on a QCM plate can be related to the D value.20,21 In the case of hydrophilic materials, a frequency change includes the changes in the bound and vibrating water mass together with the loading mass, and the energy dissipation of the QCM oscillation is related to the viscoelastic properties of the loading materials swollen with water.21−23 In our previous study, we connected a 27 MHz QCM plate to a network analyzer to determine the crystal admittance (QCM-admittance method; QCM-A) and obtained the frequency decreases (−ΔFwater) and energy dissipation increases (ΔDwater) during immobilization of hydrophilic biomacromolecules in the water phase together with −ΔFair in the dry air phase (Figure 1).22 Thus, we concluded that QCM-A based on the mechanical resonance of a quartz plate can provide the physical properties, such as hydrodynamic water (bound and vibrated water) mass and viscoelasticity, of a material on the plate in the water phase.23−25 In this study, we applied the QCM-A technique to quantitative evaluation of the hydration and viscoelastic properties of various synthetic polymer brushes on a substrate in aqueous solution (Figure 1). Although ΔF and ΔD have previously been used to describe the thermal phase transition of poly(N-isopropylacrylamide) (PNIPAM) grafted on a QCM plate, quantitative analysis of the hydration and viscoelasticity could not be performed owing to the complex behavior of ΔF in the water phase, which includes polymer mass, bound water mass, and vibrated water mass.26,27 As an improvement strategy, we obtained ΔFair in the dry air phase after immobilization of polymers, together with ΔFwater and ΔDwater in the water phase (Figure 1C). The ΔFair values correspond to the polymer dry mass (Δm) on the QCM plate. The [(−ΔFwater)/(−ΔFair)] values, which indicate the hydrodynamic water (bound and vibrated water) ratio per unit polymer mass, and the [ΔDwater/(−ΔFair)] values, which indicate the energy dissipation per unit mass, were obtained for four synthetic polymers, including positively charged and hydrophilic poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA), neutral PHEMA, thermally dependent PNIPAM, and zwitterionic PMPC. The polymer brushes were grafted on a QCM plate by an amine-coupling method (“grafting-to”)16−18,26 and atom transfer radical polymerization (ATRP) (“grafting-from”)28−31 to prepare low- and highdensity polymer brushes, respectively, which have been reported to show distinctive functions.4,5,32−34 The behavior of the low-density polymers could be characterized by the [(−ΔFwater)/(−ΔFair)] values and the [ΔDwater/(−ΔFair)] values. In contrast, these values were similar for the highdensity polymers.

Figure 1. Schematic illustrations of (A) a 27 MHz QCM system connected to a vector network analyzer for admittance analysis (QCM-A system) and (B) the conductance curves of an oscillator obtained by a frequency sweep of the QCM-A system for determination of the resonance frequency shift (ΔF) and change in energy dissipation (ΔD) using the represented equations.22,24,25 (C) Procedure for the measurement of ΔFwater, ΔDwater, and ΔFair for quartz plates modified with polymer brushes.

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EXPERIMENTAL SECTION

Materials. All of the reagents, except 2-methacryoyloxyethylphosphorylcholine (MPC), were purchased from chemical companies and, unless otherwise noted, used without further purification. MPC was kindly gifted from Prof. Ishihara, Tokyo University. Milli-Q water was used in all experiments. Details of the reagents and materials used in this work are provided in the Supporting Information. A solvent-resistant QCM cell made from polypropylene was specially ordered from Initium, Inc. (Tokyo, Japan). A 27 MHz QCM plate (AT-cut quartz plate with a diameter of 8.7 mm) with Au electrodes (area of 0.049 cm2) on both sides was attached to the cell bottom using a Chemraz O-ring, resulting in a cell volume of 500 μL (Figure 1A). Immobilization of Polymers on QCM by the Grafting-To Method. Details of this procedure are provided in the Supporting Information. Briefly, each polymer was synthesized with an amino group terminus by free-radical polymerization with 2-aminoethanethiol as a chain transfer agent (Figures 2A (upper) and 2C).35 The molar ratios of the reagents and the polymerization results are summarized in Table S1. B

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Figure 2. Schematic illustrations of the polymer brushes prepared by (A) an amine-coupling method (“grafting-to”) and (B) atom transfer radical polymerization (ATRP) (“grafting-from”). (C) Chemical structures of the polymers used in this study. PDMAEMA: poly(2-(dimethylamino)ethyl methacrylate); PHEMA: poly(2-hydroxyethyl methacrylate); PNIPAM: poly(N-isopropylacrylamide); PMPC: poly(2-methacryoyloxyethylphosphorylcholine). Details of these procedures are described in the Supporting Information. Next, the synthesized polymers were immobilized on the Au electrode of a QCM plate using an amine-coupling method, as shown in Figure 2A (lower).16−18 The amino-terminated polymer chain was reacted with the activated esters on the QCM plate for 1−3 h, resulting in various amounts of immobilized polymer, depending on the reaction time. The grafting densities, which were typically 0.02−0.1 chain nm−2, are summarized in Table S1. Immobilization of Polymers on QCM by the Grafting-From Method. As an alternative method, the polymers (except for PNIPAM) were grafted on a QCM plate using ATRP (Figure 2B).5,36−39 The molar ratios of the reagents and the polymerization results for ATRP, as well as the grafting densities, are summarized in Table S2. More details of this procedure are provided in the Supporting Information. The 27 MHz QCM-Admittance (QCM-A) Measurements. Measurements using a 27 MHz piezoelectric quartz crystal with admittance analysis were performed as described previously.22,24,25 The detailed procedure is described in the Supporting Information. Briefly, AFFINIX Q4 (Initium, Inc., Tokyo, Japan) connected with a vector network analyzer (model R3754B, Advantest Co. Ltd., Tokyo, Japan) was used as a QCM-A apparatus, as illustrated in Figure 1A. The admittance curve was converted into a conductance curve (conductance−frequency plot) based on an admittance analysis to obtain both the frequency change (ΔF) and energy dissipation change (ΔD) (Figure 1B). The frequency change in the air phase (ΔFair) and that in aqueous solutions (ΔFwater) can be measured to determine the amount of hydrodynamic water on the polymers (Figure 1C). The energy dissipation change in water (ΔDwater) and the frequency change after

drying (ΔFair) can be used to evaluate the viscoelastic properties in aqueous solutions. Unless otherwise noted, Milli-Q water was used as the measurement medium and the measurements were performed at 20 °C. The ΔFair (Hz) values were converted into Δm (ng cm−2) values based on the Sauerbrey equation.40 We calculated Δm in the air phase using eq 1, as described elsewhere.22−25

Δm [ng cm−2] = − 0.62ΔFair [Hz]

(1)

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RESULTS AND DISCUSSION We have applied our previous approach22 to characterize the hydration and viscoelastic properties of various synthetic polymers. We prepared amino-terminated polymers by freeradical polymerization for immobilization on a QCM plate using amine coupling (“grafting-to”, Figure 2A). For these lowdensity polymer brushes, an increase in −ΔFair (= Δm) indicates an increase in the number of polymer chains, and the (−ΔFwater)/(−ΔFair) and ΔDwater/(−ΔFair) values reflect individual polymer chain properties. We also prepared highdensity polymer brushes by ATRP (“grafting-from”, Figure 2B). In this case, an increase of −ΔFair (= Δm) indicates elongation of the polymers on the QCM plate, and the (−ΔFwater)/ (−ΔFair) and ΔDwater/(−ΔFair) values reflect film properties. PDMAEMA. We chose poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) as a positively charged and hydrophilic C

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Figure 3. (A) −ΔFwater and (B) ΔDwater with −ΔFair values for PDMAEMA brushes grafted on a QCM plate using the (a) “grafting-to” and (b) “grafting-from” method in buffer (5 mM GTA-NaOH, pH 4, 200 mM NaCl) at 20 °C.

Table 1. Hydrodynamic Water Amounts and Energy Dissipation of Polymer Brushesa low-density polymer PDMAEMA PHEMA

PNIPAM PMPC

pH 4c pH 9c 50% EtOH(aq)d water 20 °C 40 °C

high-density

(−ΔFwater)/ (−ΔFair)

ΔDwater/(−ΔFair)/ 10−8 Hz−1

H2O moleculesb/ residue

(−ΔFwater)/ (−ΔFair)

ΔDwater/(−ΔFair)/ 10−8 Hz−1

H2O moleculesb/ residue

4.9 ± 0.2 1.7 ± 0.1 1.6 ± 0.1

14.8 ± 1.2 2.8 ± 0.4 15.4 ± 1.3

34.1 6.1 4.3

2.0 ± 0.1 1.1 ± 0.1 1.8 ± 0.2

2.3 ± 0.1 1.3 ± 0.1 2.0 ± 0.2

8.7 0.9 5.8

± ± ± ±

1.4 16.4 1.3 11.5

1.1 1.3 1.2 1.6

± ± ± ±

0.7 1.9 1.3 9.8

1.2 3.6 1.2 1.7

± ± ± ±

1.0 0.5 0.1 0.1

3.6 3.3 1.3 2.9

0.3 0.5 0.1 0.3

± ± ± ±

0.6 0.1 0.1 0.1

1.8 1.1 1.1 2.5

0.2 0.1 0.4 0.1

Measured in Milli-Q water at 20 °C, unless otherwise noted. bObtained from [(−ΔFwater)/(−ΔFair) − 1]·(Mw,monomer/Mw,H2O). c5 mM GTANaOH, pH 4 or 9, 200 mM NaCl. dIn this case, ΔFwater and ΔDwater mean ΔFsolvent and ΔDsolvent, respectively. The experimental errors were obtained from linear regression analysis. a

polymer. The −ΔFwater and −ΔFair plot for a pH 4 solution of low-density PDMAEMA is linear with a slope of 4.9 (Figure 3A, line a). Thus, each PDMAEMA chain has a hydrodynamic water mass of 3.9 times its own mass, corresponding to ∼34 H2 O molecules per polymer residue (Table 1). This (−ΔFwater)/(−ΔFair) value is close to those obtained for ssDNA (4.2) and dsDNA (6.5).22 The slope of the linear ΔDwater and −ΔFair plot (14.8 × 10−8 Hz−1, Figure 3B, line a, and Table 1) is also similar to the values obtained for ssDNA (11 × 10−8 Hz−1) and dsDNA (19 × 10−8 Hz−1). Thus, lowdensity PDMAEMA at pH 4 holds water strongly around the positively charged chains, which move independently owing to electrostatic repulsion, similar to DNA. These results are summarized in Figure 4 (aLD). Linear plots are also obtained for the pH 4 solution of highdensity PDMAEMA brushes (Figure 3, lines b). The (−ΔFwater)/(−ΔFair) and ΔDwater/(−ΔFair) values (Figure 4 (aHD) and Table 1) are smaller than those of low-density polymer brushes. Thus, the high-density PDMAEMA chains have smaller hydrodynamic water amounts and are more elastic. This could be explained as an increase of unprotonated (neutral) side chains in the high-density film, even at pH 4, caused by the lower pKa of the secondary amine in the side chain to avoid electrostatic repulsion between chains. To confirm the effect of the neutral side chain, we examined PDMAEMA in a pH 9 solution, where the dimethylaminoethyl

Figure 4. Relationship between hydrodynamic water amount per unit mass [(−ΔFwater)/(−ΔFair)] and energy dissipation per unit mass [ΔDwater/(−ΔFair)] of (a) PDMAEMA at pH 4, (b) PHEMA in 50% ethanol aqueous solution, (c) PNIPAM at 20 °C, and (d) PMPC. LD and HD indicate low- and high-density polymer brushes, respectively.

groups are unprotonated (Figure S1). For low-density polymer brushes at pH 9, the hydrodynamic water amount was smaller D

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Figure 5. (A) −ΔFwater and (B) ΔDwater with −ΔFair values for PHEMA brushes grafted on a QCM plate using the (a) “grafting-to” and (b) “grafting-from” method in 50% ethanol aqueous solution at 20 °C.

Figure 6. (A) −ΔFwater and (B) ΔDwater with −ΔFair values for PNIPAM brushes grafted on a QCM plate using the “grafting-to” method in Milli-Q water at 20 °C. Low-density (0.02−0.2 chain nm−2) and high-density (0.4−0.7 chain nm−2) data are indicated in blue and red, respectively.

(bLD) and (bHD), Table 1), which indicates that PHEMA is aggregated. The (−ΔFwater)/(−ΔFair) values indicated 1.2 and 1.1 for low- and high-density polymer brushes, respectively. Thus, we could use these parameters for quantitative solubility evaluation by establishing the insolubility threshold as (−ΔFwater)/(−ΔFair) ≤ 1.2 empirically for not only this particular polymer but also other polymers generally (see also insoluble PNIPAM at 40 °C, Table 1). The advantage of this method is the direct measurement of solvent amounts independent of solubility, even for insoluble polymers. PNIPAM. PNIPAM is a temperature-responsive polymer that is soluble below 32 °C (lower critical solution temperature; LCST) and insoluble above the LCST owing to conformational changes.11,26,27 Thus, differences in the properties of the soluble and insoluble PNIPAM states were obtained from measurements at 20 and 40 °C, respectively (Figure 6 and Figure S4). Although ATRP of PNIPAM to obtain high-density polymer brushes was unsuccessful, the “grafting-to” method provided polymer brushes with densities of 0.02−0.7 chain nm−2 (Table S1). The boundary between low and high density, where the results should be amenable to scaling, is 0.2−0.4 chain nm−2.41 Therefore, we separated the data into low-density (0.02−0.2 chain nm−2) and high-density (0.4−0.7 chain nm−2) regions (blue and red data in Figure 6 and Figure S4). At 20 °C, low-

and the elastic property increased compared with those at pH 4 (Figure S2 (aLD) and Table 1). Low-density PDMAEMA chains are dehydrated and entangled at pH 9 due to the lack of electrostatic repulsion. For high-density polymer brushes at pH 9, each PDMAEMA chain had a hydrodynamic water mass of only 0.1 times its own mass and a greater elastic property than PDMAEMA in other conditions (Figure S2 (aHD) and Table 1). These results are reasonable for a neutral polymer, indicating that the hydration and viscoelastic properties of polymer brushes on a QCM plate can be evaluated. PHEMA. As PHEMA, with an −OH group in the side chain, is practically insoluble in water (poor solvent, Figure S3), 50% ethanol aqueous solution was used as a good solvent (Figure 5). In 50% ethanol aqueous solution, low-density PHEMA brushes had a smaller dynamic solvent ratio and similar viscoelastic property in comparison with PDMAEMA (Figure 4 (bLD), Figure 5, line a, and Table 1), suggesting that PHEMA chains are flexible and weakly solvated in the good solvent. High-density PHEMA brushes had a similar dynamic solvent ratio and 7.7 times smaller viscoelastic property in comparison with low-density PHEMA (Figure 4 (bHD), Figure 5, line b, and Table 1), indicating that a rigid film is formed with little solvation. In water, regardless of the polymer density, the PHEMA brushes were rigid and hardly solvated (Figure S2 E

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Figure 7. (A) −ΔFwater and (B) ΔDwater with −ΔFair values for PMPC brushes grafted on a QCM plate using the (a) “grafting-to” and (b) “graftingfrom” method in Milli-Q water at 20 °C.

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CONCLUSION To evaluate the hydration and the viscoelastic properties of synthetic polymer brushes on a substrate, we measured the resonance frequency decreases (−ΔFwater) and energy dissipation increases (ΔDwater) of the polymers (PDMAEMA, PHEMA, PNIPAM, and PMPC) in the water phase, together with −ΔFair in the dry air phase. The low- and high-density brushes of each polymer were characterized based on the (−ΔFwater)/(−ΔFair) values (hydrodynamic water (bound and vibrated water) ratio per unit mass) and ΔDwater/(−ΔFair) values (energy dissipation per unit mass). With low-density brushes, these values were different for each polymer and depended on the features of the side chain. The PDMAEMA chain had strong hydration and flexibility, the PHEMA chain in the good solvent had greater flexibility than that in the poor solvent, and the PNIPAM chain in the insoluble state was less hydrated with more elasticity than that in soluble state. In contrast, the high-density brushes of each polymer showed similar (−ΔFwater)/(−ΔFair) and ΔDwater/(−ΔFair) values, indicating less hydration and more elasticity due to the close proximity of the polymer chains and the limited movement of the chains with solvent. However, the polymer density had no effect on the behavior of the PMPC chains, and thus lowdensity PMPC chains should be elastic with elastic water, similar to the high-density polymer brush. We were able to evaluate mechanical properties of various polymer brushes on a substrate using a QCM-A device with a thickness-shear mode resonator. We expect that the mechanical properties of solvated polymer films are related to functions such as surface friction and biocompatibility, and determining the factors affecting these relationships will be the next challenge.

density PNIPAM chains had a hydrodynamic water mass of 2.6 times its own mass ((Figure 6, blue, Figure 4 (cLD), and Table 1). Moreover, the elastic property was smaller than that of lowdensity PDMAEMA, which is likely due to inter- or intrapolymer chain interactions, such as hydrogen bonding between amide groups. However, high-density PNIPAM chains at 20 °C (Figure 6, red) showed distinctly different behavior from low-density PNIPAM with lower (−ΔFwater)/(−ΔFair) and ΔDwater/(−ΔFair) values (Figure 4 (cHD) and Table 1). These results suggest negligible solvation of high-density PNIPAM brushes, which form a rigid film on the substrate owing to hydrogen bonding of the polymer side chains. The results obtained for low- and high-density PNIPAM brushes at 40 °C were very similar (Table 1). The (−ΔFwater)/ (−ΔFair) and ΔDwater/(−ΔFair) values were quite small and corresponded to those of insoluble materials (Figure S2 ((cLD) and (cHD)), such as a water-insoluble spin-coated polystyrene film ((−ΔFwater)/(−ΔFair) = 1.0 and ΔDwater/(−ΔFair) = 0.16 × 10−8 Hz−1).22 Thus, low- and high-density PNIPAM brushes at 40 °C are dehydrated and rigid, resulting in insolubility. In addition, we measured the temperature dependence (20−45 °C) of PNIPAM brushes on a QCM plate. The hydrodynamic water amount and elasticity of low-density PNIPAM decreased at 32 °C, which is the LCST (Figure S5A). In contrast, the properties of high-density PNIPAM showed no changes (Figure S5B), indicating that they were already dehydrated and in an elastic state below the LCST. PMPC. PMPC has a zwitterionic phosphatidylcholine group in the side chain, which imparts biocompatibility to surface coatings, with a reduction of nonspecific absorption or stabilization of proteins and biomolecules due to large amounts of free water around the chains.42−44 The hydrodynamic water amount and viscoelasticity measured for low- and high-density PMPC brushes were similar (Figure 7, Figure 4 (dLD) and (dHD), and Table 1). These results suggest that the hydrodynamic water amount of each PMPC chain is independent of the density and the water−polymer structures are elastic, even at low densities. High-density polymer brushes do not allow nonspecific absorption of proteins due to the size-exclusion effect.5 Rigid PMPC brushes that include rigid water around the polymer chain may exert a similar effect to reduce nonspecific absorption, even at low densities.

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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.macromol.6b00035. Details of Experimental Section; results under alternative conditions (PDMAEMA in pH 9, PHEMA in Milli-Q water, and PNIPAM at 40 °C), and temperature dependence of PNIPAM (PDF) F

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.F.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Mr. Yasuoka, Dr. Yoshikawa, Prof. Tsujii, and Prof. Fukuda of Kyoto University for useful advice on the preparation of high-density polymer brushes by ATRP. The authors are also thankful to Prof. Okahata of Tokyo Institute of Technology for helpful discussions regarding the results of this study. This work was financially supported by JSPS Grants-in-Aid for Scientific Research Grant 19750177.

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DOI: 10.1021/acs.macromol.6b00035 Macromolecules XXXX, XXX, XXX−XXX