Langmuir 2009, 25, 983-991
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Poly(N-isopropylacrylamide) Thin Films Densely Grafted onto Gold Surface: Preparation, Characterization, and Dynamic AFM Study of Temperature-Induced Chain Conformational Changes Franck Montagne,* Je´rome Polesel-Maris, Raphael Pugin, and Harry Heinzelmann Centre Suisse d’Electronique et de Microtechnique SA, Jaquet-Droz 1, Case Postale CH-2002 Neuchaˆtel, Switzerland ReceiVed September 23, 2008 Thermally responsive poly(N-isopropylacrylamide) (PNIPAM) films are attracting considerable attention since they offer the possibility to achieve reversible control over surface wettability and biocompatibility. In this paper, we first report a new and simple method for the grafting under melt of amine-terminated PNIPAM chains onto gold surfaces modified with a self-assembled monolayer (SAM) of reactive thiols. The formation of homogeneous tethered PNIPAM films, whose thickness can be tuned by adjusting polymer molecular weight or SAM reactivity, is evidenced by using the combination of ellipsometry, X-ray photon spectroscopy, infrared spectroscopy (PM-IRRAS), and atomic force microscopy. The calculation of grafting parameters from experimental measurements indicated the synthesis of densely grafted PNIPAM films and allowed us to predict a “brushlike” regime for the chains in good solvent. In a second part, the temperature-induced responsive properties are studied in situ by conducting dynamic AFM measurements using the amplitude modulation technique. Imaging in water environment first revealed the reversible modification of surface morphology below and above the theoretical lower critical solution temperature (LCST) of PNIPAM. Then, the determination of amplitude and phase approach curves at various temperatures provided direct measurement of the evolution of the damping factor, or similarly the dissipated energy, as a function of the probe indentation into the PNIPAM film. Most interestingly, we clearly showed the subtle and progressive thermally induced chain conformational change occurring at the scale of several nanometers around the expected LCST.
Introduction Stimuli-responsive polymers, also referred to as “smart” polymers, are a very exciting class of macromolecules, since they exhibit marked and rapid physical changes in response to a variation in their environment.1-3 Among them, thermally responsive polymers are of particular interest, because temperature changes can be easily applied and monitored in both aqueous and organic media. Since the pioneering work of Heskins et al.,4 who first evidenced the existence of a sharp and reversible lower critical solution temperature (LCST) for bulk poly(N-isopropylacrylamide) (PNIPAM) at a temperature around physiological conditions (LCST ∼ 32 °C in pure water), PNIPAM and its derivatives have been the topic of intensive studies. It is now well-admitted that, below the LCST, PNIPAM chains are hydrated by the surrounding water molecules and form expanded swollen structures. When the temperature is raised to above the LCST, hydrogen bonds between the amide groups of the PNIPAM backbone and the water molecules are suppressed, inducing the collapse of the chains.5 Thus, unlike most other chemical compounds, PNIPAM becomes less soluble (more hydrophobic) at elevated water temperatures. To date, the fascinating behavior of PNIPAM-based polymers6,7 has been exploited in a wide range of applications, including cell culture,8,9 tissue engineering,10,11 filtration membranes,12,13 medical diagnostic,14,15 controlled delivery,16,17 and * Correspondingauthor.Tel:+41-32-720-5681.E-mail:franck.montagne@ csem.ch (1) Urban, M. W. Stimuli-ResponsiVe Polymeric Films and Coatings; Oxford University Press and American Chemical Society: Washington, DC, 2005. (2) de las Heras Alarcon, C.; Pennadam, S.; Alexander, C. Chem. Soc. ReV. 2005, 34, 276–285. (3) Bag, D. S.; Rao, K. U. B. J. Polym. Mater. 2006, 23, 225–248. (4) Heskins, M.; Guillet, J. E. J. Macromol. Sci. Chem. 1968, 1441–1455. (5) Steitz, R.; Leiner, V.; Tauer, K.; Khrenov, V.; Klitzing, R. Appl. Phys. A: Mater. Sci. Process. 2002, 74, 519–521.
sensing,13,18 for instance. Many of these applications require the covalent immobilization of PNIPAM chains on a solid substrate in order to achieve reversible and permanent control over surface properties.19,20 In this case, the preservation of the bulk temperature-dependent properties, on one side, and the magnitude of the phase transition, on the other side, are two key aspects that must be taken into consideration. This was the topic of numerous theoretical21,22 and experimental23-25 studies in the past years. (6) Xu, F. J.; Zhong, S. P.; Yung, L. Y. L.; Kang, E. T.; Neoh, K. G. Biomacromolecules 2004, 5, 2392–2403. (7) Ebara, M.; Yamato, M.; Hirose, M.; Aoyagi, T.; Kikuchi, A.; Sakai, K.; Okano, T. Biomacromolecules 2003, 4, 344–349. (8) Voit, B.; Schmaljohann, D.; Gramm, S.; Nitschke, M.; Werner, C. Int. J. Mat. Res. 2007, 98, 646–650. (9) Akiyama, Y.; Kikuchi, A.; Yamato, M.; Okano, T. Langmuir 2004, 20, 5506–5511. (10) Wang, L. S.; Chow, P. Y.; Phan, T. T.; Lim, I. J.; Yang, Y. Y. AdV. Funct. Mater. 2006, 16, 1171–1178. (11) Yamato, M.; Konno, C.; Utsumi, M.; Kikuchi, A.; Okano, T. Biomaterials 2002, 23, 561–567. (12) Zhang, L.; Xu, T. W.; Lin, Z. J. Membr. Sci. 2006, 281, 491–499. (13) Morohashi, H.; Nakanoya, T.; Iwata, H.; Yamauchi, T.; Tsubokawa, N. Polym. Journ. 2006, 38, 548–553. (14) Zhou, G.; Veron, L.; Elaissari, A.; Delair, T.; Pichot, C. Polym. Int. 2004, 53, 603–608. (15) Duracher, D.; Elaissari, A.; Mallet, F.; Pichot, C. Langmuir 2000, 16, 9002–9008. (16) Dincer, S.; Tuncel, A.; Piskin, E. Macromol. Chem. Phys. 2002, 203, 1460–1465. (17) Eeckman, F.; Moes, A. J.; Amighi, K. Int. J. Pharm. 2002, 241, 113–125. (18) Abu-Lail, N. I.; Kaholek, M.; LaMattina, B.; Clark, R. L.; Zauscher, S. Sens. Actu. B: Chem. 2006, 114, 371–378. (19) Kizhakkedathu, J. N.; Norris-Jones, R.; Brooks, D. E. Macromolecules 2004, 37, 734–743. (20) Seino, M.; Yokomachi, K.; Hayakawa, T.; Kikuchi, R.; Kakimoto, M.; Horiuchi, S. Polymer 2006, 47, 1946–1952. (21) Baulin, V. A.; Zhulina, E. B.; Halperin, A. J. Chem. Phys. 2003, 119, 10977–10988. (22) Mendez, S.; Curro, J. G.; Mccoy, J. D.; Lopez, G. P. Macromolecules 2005, 38, 174–181. (23) Cunliffe, D.; Alarcon, C. D.; Peters, V.; Smith, J. R.; Alexander, C. Langmuir 2003, 19, 2888–2899.
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Among them, the works describing the use of characterization techniques in liquid media are particularly interesting, since they allow real time observation of conformational changes and surface morphologies. They include neutron reflectivity,26,27 surface plasmon resonance,28,29 quartz crystal microbalance,30,31 IR spectroscopy,32 dynamic light scattering,33 and atomic force microscopy.31,34-38 Main results indicate that the grafting density and molecular weight of PNIPAM chains are the two main parameters influencing the magnitude of the phase transition and the morphology of PNIPAM films above and below the LCST. Recent studies even show that low molecular weight PNIPAM chains immobilized onto silicon at low density do not exhibit the expected coil-to-globule transition and remain expanded in water at temperatures above the LCST.26,39 The method used to produce PNIPAM films also largely influences their final responsive properties. Some studies demonstrated that physical adsorption can modify the chain conformation and then reduce and even suppress the phase transition.21,39 In the ideal case, polymer chains must be tethered to the surface so as to offer minimum perturbation compared to the bulk state. Therefore, besides electron beam9 and plasma25,36 polymerization methods, most of the works related to PNIPAMgrated films describe the use of surface initiated polymerization (SIP) techniques, which allow polymer chains to grow directly from the substrate.34,40-43 In particular, a large majority of studies use atom transfer radical polymerization (ATRP), since this method yields PNIPAM films of adjustable thicknesses (from a few tens of nanometers up to several hundreds of nanometers) and allows the control over chain density.6,24,29,35,37,44-46 However, SIP often requires constringent operating conditions and unfriendly chemicals; moreover, it is difficult to reproducibly (24) Kaholek, M.; Lee, W. K.; Ahn, S. J.; Ma, H. W.; Caster, K. C.; LaMattina, B.; Zauscher, S. Chem. Mater. 2004, 16, 3688–3696. (25) Schmaljohann, D.; Beyerlein, D.; Nitschke, M.; Werner, C. Langmuir 2004, 20, 10107–10114. (26) Yim, H.; Kent, M. S.; Huber, D. L.; Satija, S.; Majewski, J.; Smith, G. S. Macromolecules 2003, 36, 5244–5251. (27) Yim, H.; Kent, M. S.; Satija, S.; Mendez, S.; Balamurugan, S. S.; Balamurugan, S.; Lopez, C. P. J. Polym. Sci. Part B: Polym. Phys. 2004, 42, 3302–3310. (28) Balamurugan, S.; Mendez, S.; Balamurugan, S. S.; O’Brien, M. J.; Lopez, G. P. Langmuir 2003, 19, 2545–2549. (29) Mitsuishi, M.; Koishikawa, Y.; Tanaka, H.; Sato, E.; Mikayama, T.; Matsui, J.; Miyashita, T. Langmuir 2007, 23, 7472–7474. (30) Annaka, M.; Yahiro, C.; Nagase, K.; Kikuchi, A.; Okano, T. Polymer 2007, 48, 5713–5720. (31) Ishida, N.; Biggs, S. Langmuir 2007, 23, 11083–11088. (32) Katsumoto, Y.; Tanaka, T.; Sato, H.; Ozaki, Y. J. Phys. Chem. Part A 2002, 106, 3429–3435. (33) Ye, J.; Xu, J.; Hu, J. M.; Wang, X. F.; Zhang, G. Z.; Liu, S. Y.; Wu, C. Macromolecules 2008, 41, 4416–4422. (34) Kidoaki, S.; Ohya, S.; Nakayama, Y.; Matsuda, T. Langmuir 2001, 17, 2402–2407. (35) Jones, D. M.; Smith, J. R.; Huck, W. T. S.; Alexander, C. AdV. Mater. 2002, 14, 1130–1134. (36) Cheng, X. H.; Canavan, H. E.; Stein, M. J.; Hull, J. R.; Kweskin, S. J.; Wagner, M. S.; Somorjai, G. A.; Castner, D. G.; Ratner, B. D. Langmuir 2005, 21, 7833–7841. (37) Plunkett, K. N.; Zhu, X.; Moore, J. S.; Leckband, D. E. Langmuir 2006, 22, 4259–4266. (38) Fitzgerald, P. A.; Dupin, D.; Armes, S. P.; Wanless, E. J. Soft Matter 2007, 3, 580–586. (39) Zhu, X.; Yan, C.; Winnik, F. M.; Leckband, D. Langmuir 2007, 23, 162–169. (40) Binder, W. H.; Gloger, D.; Weinstabl, H.; Allmaier, G.; Pittenauer, E. Macromolecules 2007, 40, 3097–3107. (41) Benetti, E. M.; Zapotoczny, S.; Vancso, J. AdV. Mater. 2007, 19, 268. (42) Wang, Y. P.; Yuan, K.; Li, Q. L.; Wang, L. P.; Gu, S. J.; Pei, X. W. Mater. Lett. 2005, 59, 1736–1740. (43) Ista, L. K.; Mendez, S.; Perez-Luna, V. H.; Lopez, G. P. Langmuir 2001, 17, 2552–2555. (44) Sun, T. L.; Song, W. L.; Jiang, L. Chem. Commun. 2005, 1723–1725. (45) Yim, H.; Kent, M. S.; Mendez, S.; Lopez, G. P.; Satija, S.; Seo, Y. Macromolecules 2006, 39, 3420–3426. (46) Lokuge, I.; Wang, X.; Bohn, P. W. Langmuir 2007, 23, 305–311.
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apply the technique to large solid substrates. These are severe limitations when industrial scale production is envisioned. Alternatively, simpler methods, referred to as “graft to” or “click chemistry”, consist of using preformed polymers containing reactive groups able to establish covalent binding with a functional surface. Such reactions are usually conducted in solution;23,47 however, due to excluded volume interactions that restrict the access of polymer chain to the surface, they often lead to low grafting densities. To overcome this issue, coupling of endfunctional polymers under melt (i.e., at a temperature above the glass transition) had been first proposed by Mansky et al.48 and then successfully applied to the grafting of poly(styrene),49 poly(2vinylpyridine),50 and poly(ethylene glycol)51 onto reactive silicon substrates. Using this process, ultrathin polymer films exhibiting high grafting densities and bushlike structures were produced. To our knowledge, there have been only a few attempts to prepare PNIPAM films according to the melt process. Yim et al.26 reported the preparation of PNIPAM films onto silicon substrates using molten carboxylic acid-terminated polymer, but they mainly attributed the grafting to noncovalent interactions with the hydroxylated surface. Hirata et al.52 produced cross-linked PNIPAM-based hydrogels onto thiol-modified gold and showed significant changes in swelling ratios below and above the LCST, demonstrating the preservation of responsive properties. In the present paper, we report a new and simple method for the formation of dense PNIPAM brushes onto gold based on the melt reaction of amine-terminated PNIPAM chains with a selfassembled monolayer (SAM) of reactive thiol. Then, we use the dynamic AFM technique in amplitude modulation mode to demonstrate the preservation of the bulk responsive behavior. In particular, we evidence the temperature-dependent length variation of the repulsive swollen brush below the LCST and the gradual chain conformational changes at the macromolecular level during the phase transition from brush to mushroom regime.
Experimental Section Material. Highly polished silicon wafers (〈100〉, N-doped, thickness 525 µm) were provided by Silicon Materials Inc. (Carnegie, PA). Amine-terminated poly(N-isopropylacrylamide) (PNIPAM) of different number average molecular weights (A-NIP2K ) 2000 g/mol, PI ) 1.6; A-NIP15K ) 15 000 g/mol, PI ) 1.7; and A-NIP41K ) 41 000 g/mol, PI ) 1.5) and nonfunctional PNIPAM (NF-NIP39K ) 39 000 g/mol, PI ) 1.5) were purchased from Polymer Source and used as received. 11-Mercaptoundecanoic acid (99%), trifluoroacetic acid (99%), trietylamine (>99%), and all the organic solvents of the highest available purity were purchased from Sigma-Aldrich. Ultrapure deionized water was purified using Milli-Q system from Millipore. 11-Mercaptoundecanoic Acid Self-Assembled Monolayer (SAM-COOH) onto Gold. Silicon wafers were diced into 10 × 10 mm samples, degreased in acetone and 2-propanol in an ultrasonic bath, and then dried under a stream of nitrogen. The substrates were cleaned in a piranha solution (H2O2:H2SO4, 1:3 v/v %) at 120 °C for 30 min and rinsed in boiling deionized water for 20 min. (Caution: Piranha solution is a hazardous oxidizing agent and must be handled with extreme care.) Thin layers of chromium (200 Å) and gold (1200 Å) were then successively deposited onto dry substrates by physical vapor deposition. SAM-COOH was prepared at room temperature by overnight immersion of gold-coated silicon substrates (47) Walters, K. B.; Hirt, D. E. Macromolecules 2007, 40, 4829–4838. (48) Mansky, P.; Liu, Y.; Huang, E.; Russell, T. P.; Hawker, C. Science 1997, 275, 1458–1460. (49) Luzinov, I.; Julthongpiput, D.; Malz, H.; Pionteck, J.; Tsukruk, V. V. Macromolecules 2000, 33, 1043–1048. (50) Minko, S.; Patil, S.; Datsyuk, V.; Simon, F.; Eichhorn, K. J.; Motornov, M.; Usov, D.; Tokarev, I.; Stamm, M. Langmuir 2002, 18, 289–296. (51) Zdyrko, B.; Klep, V.; Luzinov, I. Langmuir 2003, 19, 10179–10187. (52) Hirata, I.; Okazaki, M.; Iwata, H. Polymer 2004, 45, 5569–5578.
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in 2 mM ethanolic solution of 11-mercaptoundecanoic acid.53 Samples were finally sonicated for 1 min in ethanol and dried under a nitrogen stream. Preparation of PNIPAM-Grafted Surfaces. The grafting of amine-terminated PNIPAM was conducted under melt54 onto both carboxylic (COOH) and anhydride (ANH) reactive SAMs (Scheme 1). In this latter case, transformation into activated anhydride groups was achieved by treatment of SAM-COOH in a solution of dimethylformamide containing 0.1 M of trifluoroacetic acid and 0.2 M of triethylamine for 20 min followed by several washing steps in dichloromethane.55 Since anhydride groups were highly subject to hydrolysis, polymer grafting was performed just after the activation step. In a standard grafting procedure, a PNIPAM film was first spin-coated from 1% w/v ethanolic solution (filtrated through 0.2 µm pore Teflon filter) onto the reactive SAM at 3000 rpm for 30 s. The typical ellipsometric thickness of the deposited film was ∼40 nm. The substrate was then placed in an oven for annealing at 150 °C (i.e., above the glass transition temperature of PNIPAM, Tg ∼ 100 °C) for 5 h to enable amine end groups to react with the functional SAM. PNIPAM-grafted surface was finally thoroughly rinsed with ethanol, DMF, and deionized water at 4 °C. Control grafting experiments with NF-NIP39K were carried out according to the same procedure. Scheme 1. Synthetic Pathway for the Grafting of Tethered PNIPAM Film onto Gold
Surface Characterization Techniques. Static contact angle measurements were conducted using a drop shape analysis system (Kruss, DSA10) coupled with a Peltier element. Ellipsometry measurements were conducted in ambient conditions using an UVISEL ellipsometer between 400 and 800 nm at a constant incident angle of 70°. The average dry thickness of reactive SAMs was determined by fitting the data with a one-layer model, taking gold as the substrate and n ) 1.43 as the refractive index for the thiol layer. The average dry thicknesses of PNIPAM-grafted films were measured using a SAM/PNIPAM two-layer model, taking n ) 1.47 for PNIPAM.37 Grafting parameters, including surface coverage, S (mg/m2); grafting density, σ (chain/nm2); and average distance between grafting sites, D (nm), were calculated from ellipsometric thicknesses, h (nm), using eqs 1-349
S ) Fh
(1) -21
σ)
SNA × 10 Mn
(2)
( πσ4 )
(3)
D)
1/2
where F (1.27 g/cm3) is the density of dry PNIPAM film,31 NA is Avogadro’s number, and Mn (g/mol) is the number-average molecular weight of PNIPAM chains. The expected PNIPAM chain conforma(53) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321–335. (54) Jones, R. A. L.; Lehnert, R. J.; Schonherr, H.; Vancso, J. Polymer 1999, 40, 525–530. (55) Yan, L.; Huck, W. T. S.; Zhao, X. M.; Whitesides, G. M. Langmuir 1999, 15, 1208–1214.
tion in good solvent was deduced from the comparison of D with the corresponding Flory radius (RF) of the unperturbed chains calculated from eq 456
RF ) bN3 ⁄ 5
(4)
where N is the number average degree of polymerization and b is the effective segment length (assumed to be 0.3 nm for NIPAM monomer).39 We specify here that given the large polydispersity index of the PNIPAM samples (PI ∼ 1.5), the calculated values of RF were probably slightly underestimated. Finally, the theoretical length of the swollen PNIPAM chains, L (nm), was calculated using eq 5, extracted from the Alexander-de Gennes model,57,58 which is valid for tethered polymer chains dissolved in a good solvent:
L)
M b b M0 D
2⁄3
()
(5)
where M and M0 are the average polymer and monomer molecular weight, respectively. Atomic force microscopy (AFM) measurements were conducted on dry PNIPAM films using standard scanning probe equipment (Veeco, AFM Dimension 3100, controller Nanoscope IIIA). The topography images were zero-order flattened using a standard algorithm of Nanoscope III software. X-ray photon spectroscopy (XPS) analysis were performed using a VG Theta Probe spectrometer (Thermo Electron Corporation, West Sussex, UK) equipped with a concentric hemispherical analyzer and a twodimensional channel plate detector with 112 energy and 96 angle channels. Spectra were acquired at a base pressure of 5 × 10-9 mbar using a monochromatic Al KR source with a spot size of 300 µm. The instrument was run in the standard lens mode with electrons emitted at 53° to the surface normal and an acceptance angle of (30°. The analyzer was used in the constant analyzer energy mode. Pass energy used for survey scans was 200 and 50 eV for detail scans. The signals were integrated following Shirley background subtraction. Data were analyzed using the CasaXPS program. The detection accuracy under the chosen condition is approximately (5% for Au, C, N, and O and (50% for S due to the very low amounts being close to the detection limit. Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was conducted using a Bruker PMA 50 connected to the external beam port of a Bruker Tensor 27 FT-IR spectrometer. The substrate was mounted on an insert inside the PMA 50 compartment. After reflection at an incident angle of 85°, the beam was focused on a liquid nitrogen-cooled photovoltaic MCT detector in the PMA 50 cabinet. A photoelastic modulator (Hinds, PEM 90) was used to modulate the polarization of the light at a frequency of 50 kHz. Demodulation was performed with a lock-in amplifier (Stanford Research Systems, SR830 DSP). All spectra were recorded using a sample scan time of 15 min at 4 cm-1 spectral resolution. The final PM-IRRAS reflectance spectra were calculated using the bare gold as a reference. The dynamic AFM study was performed in amplitude modulation mode (AMAFM) using a Nanowizard II AFM from JPK (Berlin, Germany) equipped with a BioCell to work in liquid environment with a temperature control of (0.1 °C. The probe was a CSC38 silicon cantilever (lever c) from MikroMash with a resonance frequency in air of 18 kHz. The tip curvature radius
