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Journal of Biomaterials Science, Polymer Edition Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tbsp20
Neutron reflectivity study of the swollen structure of polyzwitterion and polyeletrolyte brushes in aqueous solution a
b
Motoyasu Kobayashi , Kazuhiko Ishihara & Atsushi Takahara
acd
a
Japan Science and Technology Agency, ERATO Takahara Soft Interfaces Project, CE80 Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan b
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Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyou-ku, Tokyo 110-8586, Japan c
Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan d
International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan Published online: 02 Sep 2014.
To cite this article: Motoyasu Kobayashi, Kazuhiko Ishihara & Atsushi Takahara (2014) Neutron reflectivity study of the swollen structure of polyzwitterion and polyeletrolyte brushes in aqueous solution, Journal of Biomaterials Science, Polymer Edition, 25:14-15, 1673-1686, DOI: 10.1080/09205063.2014.952992 To link to this article: http://dx.doi.org/10.1080/09205063.2014.952992
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims,
proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content.
Downloaded by [Australian National University] at 01:28 03 January 2015
This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/termsand-conditions
Journal of Biomaterials Science, Polymer Edition, 2014 Vol. 25, Nos. 14–15, 1673–1686, http://dx.doi.org/10.1080/09205063.2014.952992
Neutron reflectivity study of the swollen structure of polyzwitterion and polyeletrolyte brushes in aqueous solution Motoyasu Kobayashia, Kazuhiko Ishiharab and Atsushi Takaharaa,c,d*
Downloaded by [Australian National University] at 01:28 03 January 2015
a
Japan Science and Technology Agency, ERATO Takahara Soft Interfaces Project, CE80 Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan; bDepartment of Materials Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyou-ku, Tokyo 110-8586, Japan; cInstitute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan; dInternational Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan (Received 12 April 2014; accepted 7 August 2014) The swollen brush structures of polycation and zwitterionic polymer brushes, such as poly(2-methacryloyloxyethyltrimethylammonium chloride) (PMTAC), poly(2methacryloyloxyethyl phosphorylcholine) (PMPC), and poly[3-(N-2-methacryloyloxyethyl-N,N-dimethyl)ammonatopropanesulfonate] (PMAPS), in aqueous solutions of various ionic strengths were characterized by neutron reflectivity (NR) measurements. A series of the polyelectrolyte brushes were prepared by surface-initiated controlled radical polymerization on silicon substrates. A high-graft-density PMTAC brush in salt-free water (D2O) adopted a two-region step-like structure consisting of a shrunk region near the Si substrate surface and a diffuse brush region with a relatively stretched chain structure at the solution interface. The diffuse region of PMTAC was reduced with increase in salt (NaCl) concentration. The PMAPS brush in D2O formed a collapsed structure due to the strong molecular interaction between betaine groups, while significant increase in the swollen thickness was observed in salt aqueous solution. In contrast, no change was observed in the depth profile of the swollen PMPC brush in D2O with various salt concentrations. The unique solution behaviors of zwitterionic polymer brushes were described. Keywords: polyzwitterions; polymer brushes; neutron reflectivity; ionic strength; sulfobetaine; phosphobetaine
Introduction Polyelectrolyte brushes have attracted much attention of biomaterial scientists for a number of years because these brush-like structures are found in living organisms and in nature. For instance, proteoglycan aggregates in cartilage consist of hyaluronan grafted with chondroitin sulfate chains forming a bottlebrush structure, which contributes to water retention and extremely low friction.[1,2] In addition, ion-containing polymer brushes are expected to be used in various applications in materials science and nanotechnology. However, precise chain structures and behaviors of polyelectrolyte brushes in solution are complex to understand because of the combination of shortrange-excluded volume interactions and long-range electrostatic interactions. *Corresponding author. Email: [email protected] © 2014 Taylor & Francis
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In general, polyelectrolyte chains, which possess either positive or negative charges in their molecules, in a salt-free solution form relatively extended chain structures as a result of intramolecular repulsive interactions between the charged groups on the polymer chains, whereas they behave like electrically neutral polymers in aqueous salt solutions with high ionic strength because the electrostatic interactions are screened by mobile ions.[3,4] Polyzwitterions, so-called polybetaines, which have oppositely charged groups in the same monomer units, also show characteristic solution behavior depending on the ionic strength of the aqueous solution.[5] Polyelectrolyte brushes in aqueous solution also change their chain structure and thickness depending on the salt concentration. Furthermore, the swollen thickness of polyelectrolyte brushes on a flat surface in solution is determined by many factors, such as graft density, excluded volume interactions, osmotic pressure (the concentration of mobile ions inside and outside the brush), electrostatic interactions (Debye length and electrostatic persistence length), restoring force of the stretched chains, and chain elasticity (Kuhn segment length). Recent theoretical studies based on scaling models predict the swollen brush thickness and structures depending on ionic strength [6,7]; however, the experimental characterization at the brush/solution interface is also important to understand the actual brush structure in solution. Recently, we characterized the dimensions of three different types of unbound (free) polyelectrolytes in an aqueous solution depending on salt concentration by light scattering and small-angle X-ray diffractions.[8] Significant changes in the hydrodynamic radius (RH) of poly{(2-methacryloyloxy)ethyltrimethylammonium chloride} (PMTAC, polycation) [9] and poly{3-(N-2-methacryloyloxyethyl-N,N-dimethyl)ammonatopropanesulfonate} (PMAPS, polysulfobetaine) [10] were observed. In contrast, the hydrodynamic radius and intermolecular interactions of poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC, polyphosphobetaine) in aqueous solutions revealed no dependency on the ionic strength,[11] although the reason and detailed mechanism are still unclear. These solution behaviors must be affected by the swollen brush structure in solution with various solvent qualities. PMAPS and PMPC are both zwitterionic polymers. PMPC has attracted much interest because of its excellent biocompatibility and hydrophilicity as a result of phospholipid polar groups in the side chains.[12,13] Excellent wettability,[14] antifouling behavior, low friction,[15–19] and adhesion [20,21] of polyelectrolyte brush surfaces in aqueous environments have also been reported. These unique properties are closely related to the swollen brush chain structures. Therefore, it is meaningful to characterize the ionic strength dependence of the swollen polyelectrolyte brush thickness and behavior. Among the several tools for the analysis of the brush structure in solution, neutron reflectivity (NR) measurement is a useful technique for in situ determination of the static structure of swollen brushes by analysis of the brush–solvent interface via deuterium labeling. High-resolution reflectivity can be obtained by a great contrast of the neutron scattering length density (SLD) of the hydrogenated polyelectrolyte brush and deuterated solvents. In addition, specular NR affords much useful information, which includes not only the total thickness of the swollen brush, but also the depth profile along with the distance from the surface. Recently, grazing incidence small-angle neutron scattering and off-specular scattering have also attracted much attention in order to characterize a lateral structure (in-plane direction) and morphology at the surface and interfaces.[22] These scattering techniques are expected to analyze various aggregate lateral structures formed by functional polymer brushes in dry and wet states.[23,24]
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In this article, we review our recent work on the influence of solvent quality on the swollen structures of polyelectrolyte brushes on flat surfaces by specular NR. This paper particularly describes PMTAC, PMPC, and PMAPS brushes (Figure 1), focusing their dependency of swollen structure on the salt concentration of aqueous solutions.
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Chain dimensions of free polyelectrolytes Before the characterization of swollen brush structure, it would be better to mention that the chain dimension of free polyelectrolyte in aqueous solution and their dependency on salt concentration. We estimated the hydrodynamic radii, RH, of free PMTAC, PMPC, and PMAPS in aqueous solution by dynamic light scattering (DLS). Free polyelectrolytes were synthesized by atom transfer radical polymerization (ATRP) of ionic monomers in the presence of CuBr, 2,2′-bipyridyl, and 1-ethyl-3-methyl imidazolium chloride in 2,2,2-trifluoroethanol at 333 K for several hours in argon atmosphere.[25– 27] The number-average molecular weights (Mn), weight-average molecular weights (Mw), and molecular weight dispersities of free PMTAC are determined by size exclusion chromatography (SEC) using an aqueous acetic acid (0.5 M) containing sodium nitrate (0.2 M) as an eluent at a flow rate of 0.8 mL min−1 at 313 K calibrated with a series of corresponding polymers as standards. An aqueous NaCl solution (0.20 M) as the SEC eluent was used for PMPC and PMAPS. Figure 2 shows the RH values of free PMTAC, PMAPS, and PMPC in aqueous NaCl solutions at Cs = 0–5.0 M measured by DLS at 298 K. DLS experiments on the free polymers in aqueous NaCl solutions at 298 K were carried out at a scattering angle (θ) ranging from 30° to 150°, using a goniometer system (ALV CGS-3-TAC/LSE-5004, Langen, Germany) using an He–Ne laser (λ = 632.8 nm) with a power of 22 mW. The Rayleigh ratio at a scattering angle of θ = 90° was based on pure toluene at a wavelength of 632.8 nm at 298 K. The autocorrelation function was obtained by pseudo-cross-correlation of the signals from two photomultipliers to suppress noise. The samples in aqueous NaCl solutions for DLS measurements were prepared in quartz cells of diameter 10 mm. As shown in Figure 2(a), the RH of PMTAC (Mn = 154,000, Mw/Mn = 1.22) decreases with increase in NaCl concentration. The PMTAC chain in a solution of low ionic strength forms relatively extended chain structures due to the intramolecular repulsive interactions between the charged groups on the polycations, leading to the increase of the chain stiffness and excluded volume strength. On the other hand, the
CH3 CH3 n Cl CO2(CH2)2 N CH3
CH2 C
PMTAC brush CH3 CH3
CH3
CH3
CH2 C
O n CO2(CH2)2O P O(CH2)2N CH3
PMPC brush
Figure 1.
O
CH3
CH3 n CO2(CH2)2 N (CH2)3SO3
CH2 C
PMAPS brush
Chemical structures of PMTAC, PMPC, and PMAPS brushes.
CH3
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(a)
PMTAC
RH, nm
(c) 10
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PMPC
5 0.01
0.1
PMAPS
1
10
Cs, M
Figure 2. Dependence of hydrodynamic radii (RH) on NaCl concentration (Cs) for free ((a) triangle plot) PMTAC, ((b) square plot) PMAPS, and ((c) circle plot) PMPC in aqueous NaCl solutions at 298 K.
electrostatic interactions are screened by mobile ions in aqueous solutions of high ionic strength to result in the reduction in the RH of PMTAC. However, PMTAC with Mw lower than 20,000 g mol−1 shows inversion phenomenon, i.e. RH increases with ionic strength. We previously reported that the chain structure of PMTAC in a solution could be explained by the helical wormlike (HW) chain model based on small angle X-ray diffraction measurements.[9] Although the actual conformation of the PMTAC chain in dilute solution differs from that of a regular helix due to thermal fluctuations, RH can be expressed by a hydrodynamic HW cylinder model using the Barrett equation [28] which is a function of the Kuhn length, persistence length, excluded volume parameter, and cross-sectional area of the chain. These indicate that a local conformation of the PMTAC chain in dilute solution forms a relatively anisotropic helical structure at the minimum potential energy. With increasing Cs, in general, the PMTAC chain becomes flexible like a continuous elastic wire to result in the reduction of RH. In the case of polymers with lower Mw, however, local conformational change of a helical wormlike cylinder in response to Cs slightly increases because relatively larger chain dimensions are obtained by square-mean calculation of anisotropic chain structure when total contour length was short. These results indicate that the Cs dependency of RH cannot be simply explained by changes in the excluded volume effect and chain stiffness. The RH of PMAPS (Mn = 247,000, Mw/Mn = 1.12) increased from 7 to 9 nm with increasing Cs. The sulfobetaine polymer, PMAPS, forms rather shrunk chain structures in salt-free water than in an aqueous salt solution because of strong inter- or intramolecular attractive interactions between betaine groups in the polymer chains.[29–33] In contrast, the RH of PMPC (Mn = 234,000, Mw/Mn = 1.12) was almost constant (RH = 11.5 nm), regardless of the salt concentration Cs. Although PMPC is also a zwitterionic polymer like PMAPS, non-dependency of RH on Cs is the quite unique behavior of PMPC.
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NR measurements of polyelectrolyte brushes in aqueous solution NR measurements were carried out using a SOFIA time-of-flight-type reflectometer [34–36] (BL-16, Materials and Life Science Facility, Japan Proton Accelerator Research Complex, Tokai, Japan), providing a 25 Hz pulsed neutron radiation at 210 kW. The wavelength (λ) of the incident neutrons was tuned to around 0.20–0.88 nm using a disk chopper. The reflected neutrons were collected by a two-dimensional position-sensitive scintillation detector, which can collect the neutrons of both specular and off-specular reflection. Polymer brushes were immobilized at the surface of a silicon disk (d = 3 inch, thickness = 8 mm), which was covered with an aluminum trough filled with deuterium oxide (D2O) solution to make a brush–solution interface, as shown in Figure 3. The solution receiver area in the aluminum trough was 45 mm × 45 mm, and the depth was 5 mm. A rubber O-ring was sandwiched between the trough and Si disk to prevent water leaks. An incident neutron beam strikes the Si disk and is reflected at the brush– deuterium solution interface to exit in a downward direction. The neutron momentum transfer vector in NR is qz = (4π/λ) sin θ, where θ is the angle of specular reflection with respect to the sample surface. A 30 mm footprint on the sample surface was maintained using incident slits. The MOTOFIT program [37] was used to fit the reflectivity profiles to the model SLD layers, and the thickness of each layer, the SLD, and the Gaussian roughness were optimized to minimize χ2 between the measured and calculated reflectivity curves. The SLDs of Si, SiO2, and D2O used in this study were 2.07 × 10−4 nm−2, 3.47 × 10−4 nm−2, and 6.38 × 10−4 nm−2, respectively. Surface-initiated ATRP was carried out from alkyl bromide-immobilized silicon substrate (3 inch diameter, 8 mm thickness) in the presence of a sacrificial initiator to produce the polymer brush on the substrate and the corresponding free polymer simultaneously. The Mn of the brush was estimated by the SEC of free polymer. The graft
(a) top view
Si disk (d = 3 inch, t = 8 mm) Solvent port Aluminum trough
(b) side view
D2O
O-ring (silicone rubber)
Aluminum trough (90 mm x 90 mm, t = 10 mm) O-ring (silicone rubber) Polymer brush layer
(
Si disk
Reflected beam
Incident neutron beam foot print 30 mm
Aluminum plate (90 mm x 90 mm, t = 10 mm)
Figure 3. (a) Schematic top view and (b) side view of the solid–liquid interface analysis cell for NR measurement of the polyelectrolyte brush–deuterium solvents interface. Polyelectrolyte brushes were prepared on a silicon disk (75 mm diameter, 10 mm thickness). Irradiated area of neutron beam on brush substrate was 30 × 30 mm2. Incident beam angle θ was 0.2–2.0 °.
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density (σ) of the polymer brush was calculated by the Mn of the brush and dry thickness (L) determined by an ellipsometer using the following equation: (1)
where d is the bulk density of polymer, and NA is Avogadro’s number. We prepared a PMTAC brush (Mn = 114,000 g mol−1, Mw/Mn = 1.22, σ = 0.19 chains nm−2, dry thickness = 32 nm), a PMAPS brush (Mn = 187,000 g mol−1, Mw/Mn = 1.17, σ = 0.11 chains nm−2, dry thickness = 31 nm), and a PMPC brush (Mn = 137,000 g mol−1, Mw/Mn = 1.85, σ = 0.080 chains nm−2, dry thickness = 14 nm) on a Si disk with a 75 mm diameter and a thickness of 10 mm. The aluminum trough filled with deuterium solution was covered with a silicon substrate to make a brush– solution interface. Owing to the large neutron SLD contrast between D2O and hydrogenated brushes, NR profiles can be obtained by the incident neutron beam passing through the Si substrate. Figure 4(a)–(c) shows NR curves of PMTAC brush interfaces in contact with D2O solutions at several NaCl concentrations and the corresponding fits calculated based on the neutron SLD profiles, along with distance from the Si surface. The SLD profile was prepared using a four-layer model consisting of a Si substrate, SiO2 layer, gradient brush layer, and D2O solution layer. The SLDs of PMTAC, PMAPS, and PMPC used in this study were 0.82 × 10−4 nm−2, 0.76 × 10−4 nm−2, and 0.78 × 10−4 nm−2, respectively. In general, the SLD of the swollen brush at a position (z) away from the substrate surface is determined by a volume fraction ϕ(z) of the polymer brush in a solvent. Using SLD of MTAC and D2O, SLD(z) can be expressed as SLDðzÞ ¼ SLDMTAC /ðzÞ þ SLDD2 O ð1 /ðzÞÞ
100
Cs = 1.0 M
10-4
-6
10
10-8
0.2
(d) (e) (f)
Cs = 0.0 M Cs = 0.1 M
0.3
qz , nm-1
1
Volume fraction of MTAC
(a) (b) (c)
10
10-10 0.1
(2)
0.6
-2
Reflectivity
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r ¼ L d NA 1021 =Mn
0.5 0.4
Cs = 1.0 M Cs = 0.1 M Cs = 0.0 M
0.3
1 .0M
0.2
0.1 M 0.1
0.0 M
0 0
50
100
150
200
Distance from surface (z), nm
Figure 4. NR profile at the interface of the PMTAC brush in ((a) circle plot) D2O, ((b) triangle plot) 0.1 M NaCl in D2O, ((c) square plot) 1.0 M NaCl in D2O, and the corresponding fits (solid line), and ((d)–(f)) the volume fraction of MTAC with distance from the substrate surface. Neutron momentum transfer vector qz = (4π/λ) sin θ. NR intensities are offset by arbitrary factors to distinguish the difference in reflectivity.
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Using Equation (2), the SLD profiles were converted to volume fraction profiles of MTAC, as shown in Figure 4(d)–(f). The NR curve of the PMTAC brush in salt-free D2O showed a gentle shoulder at qz = 0.3–0.8 nm−1 and a large fringe around at qz = 1.2 nm−1. The NR profile at lower qz (
Journal of Biomaterials Science, Polymer Edition Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tbsp20
Neutron reflectivity study of the swollen structure of polyzwitterion and polyeletrolyte brushes in aqueous solution a
b
Motoyasu Kobayashi , Kazuhiko Ishihara & Atsushi Takahara
acd
a
Japan Science and Technology Agency, ERATO Takahara Soft Interfaces Project, CE80 Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan b
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Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyou-ku, Tokyo 110-8586, Japan c
Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan d
International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan Published online: 02 Sep 2014.
To cite this article: Motoyasu Kobayashi, Kazuhiko Ishihara & Atsushi Takahara (2014) Neutron reflectivity study of the swollen structure of polyzwitterion and polyeletrolyte brushes in aqueous solution, Journal of Biomaterials Science, Polymer Edition, 25:14-15, 1673-1686, DOI: 10.1080/09205063.2014.952992 To link to this article: http://dx.doi.org/10.1080/09205063.2014.952992
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims,
proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content.
Downloaded by [Australian National University] at 01:28 03 January 2015
This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/termsand-conditions
Journal of Biomaterials Science, Polymer Edition, 2014 Vol. 25, Nos. 14–15, 1673–1686, http://dx.doi.org/10.1080/09205063.2014.952992
Neutron reflectivity study of the swollen structure of polyzwitterion and polyeletrolyte brushes in aqueous solution Motoyasu Kobayashia, Kazuhiko Ishiharab and Atsushi Takaharaa,c,d*
Downloaded by [Australian National University] at 01:28 03 January 2015
a
Japan Science and Technology Agency, ERATO Takahara Soft Interfaces Project, CE80 Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan; bDepartment of Materials Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyou-ku, Tokyo 110-8586, Japan; cInstitute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan; dInternational Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan (Received 12 April 2014; accepted 7 August 2014) The swollen brush structures of polycation and zwitterionic polymer brushes, such as poly(2-methacryloyloxyethyltrimethylammonium chloride) (PMTAC), poly(2methacryloyloxyethyl phosphorylcholine) (PMPC), and poly[3-(N-2-methacryloyloxyethyl-N,N-dimethyl)ammonatopropanesulfonate] (PMAPS), in aqueous solutions of various ionic strengths were characterized by neutron reflectivity (NR) measurements. A series of the polyelectrolyte brushes were prepared by surface-initiated controlled radical polymerization on silicon substrates. A high-graft-density PMTAC brush in salt-free water (D2O) adopted a two-region step-like structure consisting of a shrunk region near the Si substrate surface and a diffuse brush region with a relatively stretched chain structure at the solution interface. The diffuse region of PMTAC was reduced with increase in salt (NaCl) concentration. The PMAPS brush in D2O formed a collapsed structure due to the strong molecular interaction between betaine groups, while significant increase in the swollen thickness was observed in salt aqueous solution. In contrast, no change was observed in the depth profile of the swollen PMPC brush in D2O with various salt concentrations. The unique solution behaviors of zwitterionic polymer brushes were described. Keywords: polyzwitterions; polymer brushes; neutron reflectivity; ionic strength; sulfobetaine; phosphobetaine
Introduction Polyelectrolyte brushes have attracted much attention of biomaterial scientists for a number of years because these brush-like structures are found in living organisms and in nature. For instance, proteoglycan aggregates in cartilage consist of hyaluronan grafted with chondroitin sulfate chains forming a bottlebrush structure, which contributes to water retention and extremely low friction.[1,2] In addition, ion-containing polymer brushes are expected to be used in various applications in materials science and nanotechnology. However, precise chain structures and behaviors of polyelectrolyte brushes in solution are complex to understand because of the combination of shortrange-excluded volume interactions and long-range electrostatic interactions. *Corresponding author. Email: [email protected] © 2014 Taylor & Francis
Downloaded by [Australian National University] at 01:28 03 January 2015
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M. Kobayashi et al.
In general, polyelectrolyte chains, which possess either positive or negative charges in their molecules, in a salt-free solution form relatively extended chain structures as a result of intramolecular repulsive interactions between the charged groups on the polymer chains, whereas they behave like electrically neutral polymers in aqueous salt solutions with high ionic strength because the electrostatic interactions are screened by mobile ions.[3,4] Polyzwitterions, so-called polybetaines, which have oppositely charged groups in the same monomer units, also show characteristic solution behavior depending on the ionic strength of the aqueous solution.[5] Polyelectrolyte brushes in aqueous solution also change their chain structure and thickness depending on the salt concentration. Furthermore, the swollen thickness of polyelectrolyte brushes on a flat surface in solution is determined by many factors, such as graft density, excluded volume interactions, osmotic pressure (the concentration of mobile ions inside and outside the brush), electrostatic interactions (Debye length and electrostatic persistence length), restoring force of the stretched chains, and chain elasticity (Kuhn segment length). Recent theoretical studies based on scaling models predict the swollen brush thickness and structures depending on ionic strength [6,7]; however, the experimental characterization at the brush/solution interface is also important to understand the actual brush structure in solution. Recently, we characterized the dimensions of three different types of unbound (free) polyelectrolytes in an aqueous solution depending on salt concentration by light scattering and small-angle X-ray diffractions.[8] Significant changes in the hydrodynamic radius (RH) of poly{(2-methacryloyloxy)ethyltrimethylammonium chloride} (PMTAC, polycation) [9] and poly{3-(N-2-methacryloyloxyethyl-N,N-dimethyl)ammonatopropanesulfonate} (PMAPS, polysulfobetaine) [10] were observed. In contrast, the hydrodynamic radius and intermolecular interactions of poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC, polyphosphobetaine) in aqueous solutions revealed no dependency on the ionic strength,[11] although the reason and detailed mechanism are still unclear. These solution behaviors must be affected by the swollen brush structure in solution with various solvent qualities. PMAPS and PMPC are both zwitterionic polymers. PMPC has attracted much interest because of its excellent biocompatibility and hydrophilicity as a result of phospholipid polar groups in the side chains.[12,13] Excellent wettability,[14] antifouling behavior, low friction,[15–19] and adhesion [20,21] of polyelectrolyte brush surfaces in aqueous environments have also been reported. These unique properties are closely related to the swollen brush chain structures. Therefore, it is meaningful to characterize the ionic strength dependence of the swollen polyelectrolyte brush thickness and behavior. Among the several tools for the analysis of the brush structure in solution, neutron reflectivity (NR) measurement is a useful technique for in situ determination of the static structure of swollen brushes by analysis of the brush–solvent interface via deuterium labeling. High-resolution reflectivity can be obtained by a great contrast of the neutron scattering length density (SLD) of the hydrogenated polyelectrolyte brush and deuterated solvents. In addition, specular NR affords much useful information, which includes not only the total thickness of the swollen brush, but also the depth profile along with the distance from the surface. Recently, grazing incidence small-angle neutron scattering and off-specular scattering have also attracted much attention in order to characterize a lateral structure (in-plane direction) and morphology at the surface and interfaces.[22] These scattering techniques are expected to analyze various aggregate lateral structures formed by functional polymer brushes in dry and wet states.[23,24]
Journal of Biomaterials Science, Polymer Edition
1675
In this article, we review our recent work on the influence of solvent quality on the swollen structures of polyelectrolyte brushes on flat surfaces by specular NR. This paper particularly describes PMTAC, PMPC, and PMAPS brushes (Figure 1), focusing their dependency of swollen structure on the salt concentration of aqueous solutions.
Downloaded by [Australian National University] at 01:28 03 January 2015
Chain dimensions of free polyelectrolytes Before the characterization of swollen brush structure, it would be better to mention that the chain dimension of free polyelectrolyte in aqueous solution and their dependency on salt concentration. We estimated the hydrodynamic radii, RH, of free PMTAC, PMPC, and PMAPS in aqueous solution by dynamic light scattering (DLS). Free polyelectrolytes were synthesized by atom transfer radical polymerization (ATRP) of ionic monomers in the presence of CuBr, 2,2′-bipyridyl, and 1-ethyl-3-methyl imidazolium chloride in 2,2,2-trifluoroethanol at 333 K for several hours in argon atmosphere.[25– 27] The number-average molecular weights (Mn), weight-average molecular weights (Mw), and molecular weight dispersities of free PMTAC are determined by size exclusion chromatography (SEC) using an aqueous acetic acid (0.5 M) containing sodium nitrate (0.2 M) as an eluent at a flow rate of 0.8 mL min−1 at 313 K calibrated with a series of corresponding polymers as standards. An aqueous NaCl solution (0.20 M) as the SEC eluent was used for PMPC and PMAPS. Figure 2 shows the RH values of free PMTAC, PMAPS, and PMPC in aqueous NaCl solutions at Cs = 0–5.0 M measured by DLS at 298 K. DLS experiments on the free polymers in aqueous NaCl solutions at 298 K were carried out at a scattering angle (θ) ranging from 30° to 150°, using a goniometer system (ALV CGS-3-TAC/LSE-5004, Langen, Germany) using an He–Ne laser (λ = 632.8 nm) with a power of 22 mW. The Rayleigh ratio at a scattering angle of θ = 90° was based on pure toluene at a wavelength of 632.8 nm at 298 K. The autocorrelation function was obtained by pseudo-cross-correlation of the signals from two photomultipliers to suppress noise. The samples in aqueous NaCl solutions for DLS measurements were prepared in quartz cells of diameter 10 mm. As shown in Figure 2(a), the RH of PMTAC (Mn = 154,000, Mw/Mn = 1.22) decreases with increase in NaCl concentration. The PMTAC chain in a solution of low ionic strength forms relatively extended chain structures due to the intramolecular repulsive interactions between the charged groups on the polycations, leading to the increase of the chain stiffness and excluded volume strength. On the other hand, the
CH3 CH3 n Cl CO2(CH2)2 N CH3
CH2 C
PMTAC brush CH3 CH3
CH3
CH3
CH2 C
O n CO2(CH2)2O P O(CH2)2N CH3
PMPC brush
Figure 1.
O
CH3
CH3 n CO2(CH2)2 N (CH2)3SO3
CH2 C
PMAPS brush
Chemical structures of PMTAC, PMPC, and PMAPS brushes.
CH3
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(a)
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PMPC
5 0.01
0.1
PMAPS
1
10
Cs, M
Figure 2. Dependence of hydrodynamic radii (RH) on NaCl concentration (Cs) for free ((a) triangle plot) PMTAC, ((b) square plot) PMAPS, and ((c) circle plot) PMPC in aqueous NaCl solutions at 298 K.
electrostatic interactions are screened by mobile ions in aqueous solutions of high ionic strength to result in the reduction in the RH of PMTAC. However, PMTAC with Mw lower than 20,000 g mol−1 shows inversion phenomenon, i.e. RH increases with ionic strength. We previously reported that the chain structure of PMTAC in a solution could be explained by the helical wormlike (HW) chain model based on small angle X-ray diffraction measurements.[9] Although the actual conformation of the PMTAC chain in dilute solution differs from that of a regular helix due to thermal fluctuations, RH can be expressed by a hydrodynamic HW cylinder model using the Barrett equation [28] which is a function of the Kuhn length, persistence length, excluded volume parameter, and cross-sectional area of the chain. These indicate that a local conformation of the PMTAC chain in dilute solution forms a relatively anisotropic helical structure at the minimum potential energy. With increasing Cs, in general, the PMTAC chain becomes flexible like a continuous elastic wire to result in the reduction of RH. In the case of polymers with lower Mw, however, local conformational change of a helical wormlike cylinder in response to Cs slightly increases because relatively larger chain dimensions are obtained by square-mean calculation of anisotropic chain structure when total contour length was short. These results indicate that the Cs dependency of RH cannot be simply explained by changes in the excluded volume effect and chain stiffness. The RH of PMAPS (Mn = 247,000, Mw/Mn = 1.12) increased from 7 to 9 nm with increasing Cs. The sulfobetaine polymer, PMAPS, forms rather shrunk chain structures in salt-free water than in an aqueous salt solution because of strong inter- or intramolecular attractive interactions between betaine groups in the polymer chains.[29–33] In contrast, the RH of PMPC (Mn = 234,000, Mw/Mn = 1.12) was almost constant (RH = 11.5 nm), regardless of the salt concentration Cs. Although PMPC is also a zwitterionic polymer like PMAPS, non-dependency of RH on Cs is the quite unique behavior of PMPC.
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NR measurements of polyelectrolyte brushes in aqueous solution NR measurements were carried out using a SOFIA time-of-flight-type reflectometer [34–36] (BL-16, Materials and Life Science Facility, Japan Proton Accelerator Research Complex, Tokai, Japan), providing a 25 Hz pulsed neutron radiation at 210 kW. The wavelength (λ) of the incident neutrons was tuned to around 0.20–0.88 nm using a disk chopper. The reflected neutrons were collected by a two-dimensional position-sensitive scintillation detector, which can collect the neutrons of both specular and off-specular reflection. Polymer brushes were immobilized at the surface of a silicon disk (d = 3 inch, thickness = 8 mm), which was covered with an aluminum trough filled with deuterium oxide (D2O) solution to make a brush–solution interface, as shown in Figure 3. The solution receiver area in the aluminum trough was 45 mm × 45 mm, and the depth was 5 mm. A rubber O-ring was sandwiched between the trough and Si disk to prevent water leaks. An incident neutron beam strikes the Si disk and is reflected at the brush– deuterium solution interface to exit in a downward direction. The neutron momentum transfer vector in NR is qz = (4π/λ) sin θ, where θ is the angle of specular reflection with respect to the sample surface. A 30 mm footprint on the sample surface was maintained using incident slits. The MOTOFIT program [37] was used to fit the reflectivity profiles to the model SLD layers, and the thickness of each layer, the SLD, and the Gaussian roughness were optimized to minimize χ2 between the measured and calculated reflectivity curves. The SLDs of Si, SiO2, and D2O used in this study were 2.07 × 10−4 nm−2, 3.47 × 10−4 nm−2, and 6.38 × 10−4 nm−2, respectively. Surface-initiated ATRP was carried out from alkyl bromide-immobilized silicon substrate (3 inch diameter, 8 mm thickness) in the presence of a sacrificial initiator to produce the polymer brush on the substrate and the corresponding free polymer simultaneously. The Mn of the brush was estimated by the SEC of free polymer. The graft
(a) top view
Si disk (d = 3 inch, t = 8 mm) Solvent port Aluminum trough
(b) side view
D2O
O-ring (silicone rubber)
Aluminum trough (90 mm x 90 mm, t = 10 mm) O-ring (silicone rubber) Polymer brush layer
(
Si disk
Reflected beam
Incident neutron beam foot print 30 mm
Aluminum plate (90 mm x 90 mm, t = 10 mm)
Figure 3. (a) Schematic top view and (b) side view of the solid–liquid interface analysis cell for NR measurement of the polyelectrolyte brush–deuterium solvents interface. Polyelectrolyte brushes were prepared on a silicon disk (75 mm diameter, 10 mm thickness). Irradiated area of neutron beam on brush substrate was 30 × 30 mm2. Incident beam angle θ was 0.2–2.0 °.
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density (σ) of the polymer brush was calculated by the Mn of the brush and dry thickness (L) determined by an ellipsometer using the following equation: (1)
where d is the bulk density of polymer, and NA is Avogadro’s number. We prepared a PMTAC brush (Mn = 114,000 g mol−1, Mw/Mn = 1.22, σ = 0.19 chains nm−2, dry thickness = 32 nm), a PMAPS brush (Mn = 187,000 g mol−1, Mw/Mn = 1.17, σ = 0.11 chains nm−2, dry thickness = 31 nm), and a PMPC brush (Mn = 137,000 g mol−1, Mw/Mn = 1.85, σ = 0.080 chains nm−2, dry thickness = 14 nm) on a Si disk with a 75 mm diameter and a thickness of 10 mm. The aluminum trough filled with deuterium solution was covered with a silicon substrate to make a brush– solution interface. Owing to the large neutron SLD contrast between D2O and hydrogenated brushes, NR profiles can be obtained by the incident neutron beam passing through the Si substrate. Figure 4(a)–(c) shows NR curves of PMTAC brush interfaces in contact with D2O solutions at several NaCl concentrations and the corresponding fits calculated based on the neutron SLD profiles, along with distance from the Si surface. The SLD profile was prepared using a four-layer model consisting of a Si substrate, SiO2 layer, gradient brush layer, and D2O solution layer. The SLDs of PMTAC, PMAPS, and PMPC used in this study were 0.82 × 10−4 nm−2, 0.76 × 10−4 nm−2, and 0.78 × 10−4 nm−2, respectively. In general, the SLD of the swollen brush at a position (z) away from the substrate surface is determined by a volume fraction ϕ(z) of the polymer brush in a solvent. Using SLD of MTAC and D2O, SLD(z) can be expressed as SLDðzÞ ¼ SLDMTAC /ðzÞ þ SLDD2 O ð1 /ðzÞÞ
100
Cs = 1.0 M
10-4
-6
10
10-8
0.2
(d) (e) (f)
Cs = 0.0 M Cs = 0.1 M
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qz , nm-1
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Volume fraction of MTAC
(a) (b) (c)
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10-10 0.1
(2)
0.6
-2
Reflectivity
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r ¼ L d NA 1021 =Mn
0.5 0.4
Cs = 1.0 M Cs = 0.1 M Cs = 0.0 M
0.3
1 .0M
0.2
0.1 M 0.1
0.0 M
0 0
50
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Figure 4. NR profile at the interface of the PMTAC brush in ((a) circle plot) D2O, ((b) triangle plot) 0.1 M NaCl in D2O, ((c) square plot) 1.0 M NaCl in D2O, and the corresponding fits (solid line), and ((d)–(f)) the volume fraction of MTAC with distance from the substrate surface. Neutron momentum transfer vector qz = (4π/λ) sin θ. NR intensities are offset by arbitrary factors to distinguish the difference in reflectivity.
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Using Equation (2), the SLD profiles were converted to volume fraction profiles of MTAC, as shown in Figure 4(d)–(f). The NR curve of the PMTAC brush in salt-free D2O showed a gentle shoulder at qz = 0.3–0.8 nm−1 and a large fringe around at qz = 1.2 nm−1. The NR profile at lower qz (
