Langmuir 2007, 23, 11083-11088
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Direct Observation of the Phase Transition for a Poly(N-isopropylacryamide) Layer Grafted onto a Solid Surface by AFM and QCM-D Naoyuki Ishida†,‡ and Simon Biggs*,† School of Process, EnVironmental and Materials Engineering, UniVersity of Leeds, Leeds LS2 9JT, United Kingdom, and Research Institute of EnVironmental Management Technology, National Institute of AdVanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba 306-8569, Japan ReceiVed May 18, 2007. In Final Form: July 13, 2007 The temperature-induced structural changes of a thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) layer grafted onto a silica substrate were investigated in aqueous solution using an atomic force microscope (AFM) and a quartz crystal microbalance with dissipation (QCM-D). A PNIPAM layer was grafted onto the silicon wafer surface by free radical polymerization of NIPAM to obtain a high molecular weight polymer layer with low-grafting density overall. By AFM imaging, the transition of the grafted PNIPAM chains from a brush-like to a mushroom-like state was clearly visualized: The surface images of the plate were featureless at temperatures below the LCST commensurate with a brush-like layer, whereas above the LCST, a large number of domain structures with a characteristic size of ∼100 nm were seen on the surface. Both frequency and dissipation data obtained using QCM-D showed a significant change at the LCST. Analysis of these data confirmed that the observed PNIPAM structural transition was caused by a collapse of the brush-like structure as a result of dehydration of the polymer chains.
Introduction Poly(N-isopropylacrylamide) (PNIPAM) is the most thoroughly investigated thermo-responsive polymer in aqueous solution; it has a well-defined lower critical solution temperature (LCST) around 32 °C.1 Below the LCST, PNIPAM chains are strongly hydrated and have an expanded conformation in solution. Above the LCST, they dehydrate and collapse to a globular form. When PNIPAM chains are immobilized onto a solid surface, they are expected to show a similar expanded/collapsed phase transition across the LCST. For low graft density end-grafted polymer chains, the transition from a good solvent to a poor solvent condition is characterized with a change in structure from a brush-like conformation to a collapsed mushroom-like structure. Such a structural transition may have a number of interesting possible applications where a responsive character triggered by temperature is important, including drug delivery systems,2,3 permeation-controlled filters,4,5 attachment-switchable surfaces for biomaerials,6-8 and functional composite surfaces.9 Detailed understanding of the properties of such thermoresponsive grafted polymer layers on solid surfaces in solution is therefore crucial if they are to be used in such applications. As a result, the behavior of PNIPAM chains grafted onto solid surfaces has been intensively studied recently using a range of * To whom correspondence should be addressed. E-mail: S.R.Biggs@ leeds.ac.uk. † University of Leeds. ‡ National Institute of Advanced Industrial Science and Technology. (1) Heskins, M.; Guillet, J. E. J. Macromol. Sci. Chem. 1968, A2, 1441-1455. (2) Hoffman, A. S. J. Controlled Release 1987, 6, 297-305. (3) Kwon, I. C.; Bae, Y. H.; Kim, S. W. Nature 1991, 354, 291-293. (4) Osada, Y.; Honda, H.; Ohta, M. J. Membr. Sci. 1986, 27, 327-338. (5) Park, Y. S.; Ito, Y.; Imanishi, Y. Langmuir 1998, 14, 910-914. (6) Yoshioka, H.; Mikami, M.; Nakai, T.; Mori, Y. Polym. AdV. Technol. 1994, 6, 418-420. (7) Okano, T.; Yamada, N.; Okuhara, M.; Sakai, H.; Sakurai, Y. Biomaterials 1995, 16, 297-303. (8) Stayton P. S.; Shimoboji, T.; Long, C.; Chilkoti, A.; Chen, G.; Harris, J. M.; Hoffman, A. S. Nature 1995, 378, 472-474. (9) Ionov, L.; Stamm, M.; Diez, S. Nano Lett. 2006, 9, 1982-1987.
techniques including dynamic light scattering (DLS),10-12 surface plasmon resonance (SPR),13 neutron reflectivity (NR),14-18 quartz crystal microbalance (QCM),19-21 and AFM force measurements.22-24 Although most of these studies have focused on changes in the thickness of the grafted layers, the morphological change of the surface structure across the LCST condition has not been well characterized, and very few papers provide useful information about this thus far. Cunliffe et al.25 observed PNIPAM-grafted surfaces in water both below (10-12 °C) and above (35 °C) the LCST with direct imaging using AFM. However, these authors found no significant change in the morphology of the layer from the images obtained at both temperatures investigated. This is perhaps because of the very short chain length of polymer they used, as is mentioned below. To date, as far as we are aware, no direct visual evidence for a brush-to-mushroom transition across the LCST for an end-grafted PNIPAM layer has been (10) Zhu P. W.; Napper, D. H. J. Chem. Phys. 1997, 106, 6492-6498. (11) Zhu, P. W.; Napper, D. H. Colloids Surf. A 1996, 113, 145-153. (12) Walldal, C.; Wall, S. Colloid Polym. Sci. 2000, 278, 936-945. (13) Balamurugan S.; Mendez, S.; Balamurugan, S. S.; O’Brien II, M. J.; Lo´pez G. P. Langmuir 2003, 19, 2545-2549. (14) Yim, H.; Kent, M. S.; Huber, D. L.; Satija, S.; Majewski, J.; Smith, G. S. Macromolecules 2003, 36, 5244-5251. (15) Yim, H.; Kent, M. S.; Mendez, S.; Balamurugan, S. S.; Balamurugan, S.; Lopez, G. P.; Satija, S. Macromolecules 2004, 37, 1994-1997. (16) Yim, H.; Kent, M. S.; Satija, S.; Mendez, S.; Balamurugan, S. S.; Balamurugan S.; Lopez C. P. J. Polym. Sci. B 2004, 42, 3302-3310. (17) Yim, H.; Kent, M. S.; Satija, S.; Mendez, S.; Balamurugan, S. S.; Balamurugan, S.; Lopez, G. P. Phys. ReV. E 2005, 72, 051801. (18) Yim, H.; Kent, M. S.; Mendez, S.; Lopez, G. P.; Satija, S.; Seo, Y. Macromolecules 2006, 39, 3420-3426. (19) Zhang, G. Macromolecules 2004, 37, 6553-6557. (20) Liu, G.; Zhang, G. J. Phys. Chem. B 2005, 109, 743-747. (21) Liu, G.; Cheng, H.; Yan, L.; Zhang, G. J. Phys. Chem. B 2005, 109, 22603-22607. (22) Kidoaki, S.; Ohya, S.; Nakayama, Y.; Matsuda, T. Langmuir 2001, 17, 2402-2407. (23) Ishida, N.; Kobayashi, K. J. Colloid Interface Sci. 2006, 297, 513-519. (24) Jones D. M.; Smith J. R.; Huck W. T. S.; Alexander, C. AdV. Mater. 2002, 14, 1130-1134. (25) Cunliffe, D.; Alarcon, C. D.; Peters, V.; Smith, J. R.; Alexander, C. Langmuir 2003, 19, 2888-2899.
10.1021/la701461b CCC: $37.00 © 2007 American Chemical Society Published on Web 09/29/2007
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reported. Our primary aim in the work reported here was to observe in situ the morphological behavior of end-grafted PNIPAM chains at the solid-liquid interface across the LCST phase transition using AFM soft contact imaging. When one investigates the phase transition of PNIPAM chains grafted onto a surface, both grafting density and molecular weight of the polymer (i.e., chain length of the grafted polymer) are known to have a critical importance. Balamurugan et al.13 was the first to note that when the grafting density is high the conformational behavior of the grafted PNIPAM chains may show only a gradual transition across a wide temperature range that includes passing through the LCST. This gradual change is considerably different to the sharp structural change at the LCST often reported and/or predicted in the case of high molecular weight (long) polymer chains with a low grafting density.8,10 Yim et al. have also reported in a series of papers using NR that not only the grafting density, but also the molecular weight of the polymer has a significant influence to any observed structural changes.15-18 This finding is in accordance with theoretical predictions.26-28 Zhu et al.29 reported that, in the case of low molecular weight grafted polymers, end-grafted PNIPAM chains do not show a collapse transition even well above the LCST, which is in accordance with the AFM images by Cunliffe et al.25 mentioned above. It was our aim here to directly visualize the phase transition of a PNIPAM grafted layer in situ using soft contact AFM imaging. Hence, a large and sharp structural change is fundamentally desirable. In this study, therefore, we chose to use a surface modified with a high molecular weight end grafted PNIPAM at a low-grafting density overall. On the basis of the earlier reports, discussed above, this system is expected to exhibit a sharp and significant structural change from brush-like to mushroom-like at the LCST. In addition to direct AFM observation of the structural transitions within the grafted polymer layer, we also utilized quartz crystal microbalance (QCM-D) measurements to obtain complementary information about the structural changes. QCM-D is a very sensitive tool for the determination of adsorbed mass on flat substrates. Although QCM was originally designed for use with thin rigid films in vacuum or gaseous atmospheres, it has now become a popular tool for examining adsorption at the solid-liquid interface. More recently, the QCM technique has been extended through the use of the dissipation signal available from the resonance decay.30,31 In cases where the adsorbed mass is invariant, such as for grafted polymer films, QCM-D measurements may provide an opportunity to explore conformational changes in the adsorbed material since the signal recorded reflects the quality of the coupling between the surface, the adsorbed polymer, and the background solution; changes in these coupling factors are expected if the conformation changes. In the case of adsorbed stimulus-responsive polymer micelles, it has previously been shown that the QCM-D data can indicate conformational changes on the surface. However, in this earlier work, the adsorbed mass was also seen to vary as the system was (26) Zhulina, E. B.; Borisov, O. V.; Pryamitsyn, V. A.; Birshtein, T. M. Macromolecules 1991, 24, 140-149. (27) Baulin, V. A.; Zhulina, E. B.; Halperin, A. J. Chem. Phys. 2003, 119, 10977-10988. (28) Baulin, V. A.; Halperin, A. Macromol. Theory Simul. 2003, 12, 549559. (29) Zhu, X.; Yan, C.; Winnik, F. M.; Leckband, D. Langmuir 2007, 23, 162-169. (30) Rodahl, M.; Ho¨o¨k, F.; Krozer, A.; Brzezinski, P.; Kasemo B. ReV. Sci. Instrum. 1995, 66, 3924-3930. (31) Rodahl, M.; Kasemo, B. ReV. Sci. Instrum. 1996, 67, 3238-3241.
Ishida and Biggs
exposed to different background solutions, and so interpretation of the data purely in terms of conformational changes is difficult.32,33 There have been several reports to date concerning measurements with QCM sensor crystals grafted by polymer layers.20,34-36 Of particular interest here is the work of Liu and Zhang who grafted PNIPAM onto a sensor at a relatively high grafting density and showed that the frequency and dissipation data changed gradually and monotonically across a temperature range of 2038 °C.20 However, these authors estimated the corresponding structural changes for the polymer chains only from these QCM data. In the present study, we compare directly data from QCM-D measurements to in situ AFM images for equivalent grafted surfaces to give a greater insight of the structural transitions across the LCST for end-grafted PNIPAM layers. Experimental Section Materials. Silicon wafers (Nilaco, Japan) were used as test surfaces and they were cut to ∼1.5 × 1.5 cm pieces. Polished, AT-cut quartz crystal QCM sensors (1.4 cm in diameter) with a coating of a thin silicon oxide layer were obtained from Q-Sense AB and were used for the QCM-D measurements reported here. N-Isopropylacrylamide (NIPAM, Aldrich) was recrystallized from hexane. Potassium persulfate and N,N,N′,N′-tetramethylethylenediamine (TMEDA, Aldrich) were of synthesis grade and were used without further purification. Dimethylvinylchlorosilane, toluene, chloroform, and sodium sulfate of reagent grade (Aldrich) were used as received. All water used in experiments was purified using a Milli-Q system (Millipore). Grafting of PNIPAM on Surfaces. An end-grafted PNIPAM layer was prepared on both the silicon wafer surfaces and the QCM-D sensors by free radical graft polymerization of NIPAM. Prior to grafting, the silicon substrates were cleaned in a 7:3 mixture of concentrated sulfuric acid and hydrogen peroxide (piranha solution) at 70-80 °C for 30 min, to remove contamination and ensure the formation of an oxide layer on the surface. Caution: Piranha solution is a hazardous oxidizing agent and can react Violently with organics. After rinsing with pure water, the substrates were sonicated in 5 mM sodium hydroxide solution for 5 min and rinsed again with pure water. The QCM-D sensors were cleaned by UV irradiation for at least 15 min, followed by ultrasonication in 5 mM sodium hydroxide and then a thorough rinsing with water. The polymer-grafted surfaces were prepared by free radical graft polymerization using the following protocol: A cleaned silicon wafer or QCM-D sensor was first immersed in a 1 mM dimethylvinylchlorosilane solution in toluene for 1 h to produce a firmly immobilized layer of active vinyl moieties on the surface. After this, the surface was washed with toluene and chloroform to remove any unreacted reagent before it was dried in a stream of nitrogen gas. The surface was then immersed in a 15 wt % aqueous NIPAM solution in a Teflon reaction vessel. After thoroughly removing oxygen from the solution by bubbling with nitrogen gas for 30 min, 0.2 mL of TMEDA and 0.01 g of potassium persulfate were added to initiate the polymerization. The vessel was then sealed and kept at 4 °C in a refrigerator during the reaction for 12 h. After the reaction, the surface was washed with copious amounts of water to remove any unbound polymers and other unreacted reagents. Free polymer generated in the bulk solution during the polymerization is collected by precipitation into cold methanol to estimate the molecular weight of the grafted PNIPAM. Gel permeation (32) Sakai, K.; Smith, E. G.; Webber, G. B.; Wanless, E. J.; Butun, V.; Armes, S. P.; Biggs, S. J. Colloid Interface Sci. 2006, 303, 372-379. (33) Sakai, K.; Smith, E. G.; Webber, G. B.; Baker, M; Wanless, E. J.; Butun, V.; Armes, S. P.; Biggs, S. Langmuir 2006, 22, 8435-8442. (34) Muller, M. T.; Yan, X. P.; Lee, S. W.; Perry, S. S.; Spencer, N. D. Macromolecules 2005, 38, 5706-5713. (35) Kurosawa, S.; Aizawa, H.; Talib, Z. A.; Atthoff, B.; Hilborn, J. Biosens. Bioelectron. 2004, 20, 1165-1176. (36) Liu, G. M.; Yan L. F.; Chen, X.; Zhang, G. Z. Polymer 2006, 47, 31573163.
Phase Transition for a PNIPAM Layer
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Figure 1. AFM images (2 × 2 µm2) of a PNIPAM layer on silicon wafer in water. Images were obtained at (a) 26.5, (b) 30.1, (c) 32.0, (d) 33.0, (e) 36.5, and (f) 40.1 °C. chromatography (GPC, Waters) with analysis of this polymer was conducted using DMF with 0.01 M LiBr as mobile phase and polystyrene for calibration standards. For the layer grafted on silica, a weight average molecular weight of free polymer was 655 kDa and the polydispersity was 2.4. The dry thickness of the grafted layer measured with an ellipsometer (Gaertner Scientific) was 7.6 nm. The graft density calculated from thickness and molecular weight data assuming the dry density value of PNIPAM45 to be 1.269 g/cm3 was 0.0088 chains/nm2. The mass of grafted layer on the QCM sensor was measured directly in air by comparing the resonance frequency of the sensor before and after the grafting reaction: the mass was determined by assuming that the polymer film in air is a thin rigidly attached layer. The molecular weight of free polymer was 715 kDa. The grafting density of the polymer was then calculated using the molecular weight data to be 0.0074 chains/ nm2. AFM Imaging. In situ imaging of the grafted polymer films on silica in aqueous solution was performed with a Nanoscope IV atomic force microscope (Veeco Instruments). Images were collected using the soft-contact method.37 All images presented are height images and have been zero-order flattened using a standard algorithm within the Nanoscope software to remove artificial height offsets between consecutive scan lines of the raw images. Triangular cantilevers with an integral silicon nitride tip (NanoProbe, Olympus) were used for all AFM experiments and were cleaned using UV irradiation (approximately 9 mW‚cm-2 at 254 nm) prior to use. All of the measurements were conducted in Milli-Q water. The water was passed through 0.2 µm filter (GHP Acrodisc, Pall Gelman Science) mounted on a syringe as they were injected into the AFM fluid cell. The water temperature was varied using a temperature-controlling (37) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409-4413. (38) Ducker, W. A.; Senden, T. J.; Pashley R.M. Langmuir 1992, 8, 18311836. (39) Wang, X.; Qiu, X.; Wu, C. Macromolecules 1998, 31, 2972-2976. (40) Katsumoto, Y.; Tanaka, T.; Sato, H.; Ozaki, Y. J. Phys. Chem. A 2002, 106, 3429-3435. (41) Maeda, Y.; Higuchi, T.; Ikeda, I. Langmuir 2000, 16, 7503-7509. (42) Zhang, Y.; Furyk, S.; Bergbreiter, D. E.; Cremer, P. S. J. Am. Chem. Soc. 2005, 127, 14505-14510. (43) Wang, X.; Qiu, X.; Wu, C. Macromolecules 1998, 31, 2972-2976. (44) Ho¨o¨k, F.; Kasemo, B.; Nylander, T.; Fant, C.; Sott, K.; Elwing, H. Anal. Chem. 2001, 73, 5796-5804. (45) Lele, A. K.; Hirve, M. M.; Badiger, M. V.; Mashelkar, R. A. Macromolecules 1997, 30, 157-159.
system (Veeco Instruments). The system consists of a heat stage mounted on top of the scanner and a controller to adjust the temperature of the heat stage by controlling the electric current to the stage. The water temperature was monitored with a small thermocouple inserted into the liquid cell. The deviation of temperature was estimated to be
