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Real-Time Monitoring of Polymer Swelling on the Nanometer Scale by Atomic Force Microscopy J. I. Paredes,* S. Villar-Rodil, K. Tamargo-Martı´nez, A. Martı´nez-Alonso, and J. M. D. Tasco´n Instituto Nacional del Carbo´ n, CSIC, Apartado 73, 33080 OViedo, Spain ReceiVed September 6, 2005. In Final Form: March 10, 2006 The swelling of a polymer surface has been monitored in real time on the nanometer scale by atomic force microscopy (AFM). After modification by oxygen plasma treatment, poly(p-phenylene terephthalamide) (PPTA) displays a characteristic nanostructured surface morphology consisting of high-lying features alternating with topographically depressed areas. Selective swelling of the least cross-linked, depressed areas after the adsorption of ambient water or water from saturated humid atmospheres was observed by tapping mode AFM operated in the attractive interaction regime. The swollen areas could be distinguished from the nonswollen ones by local variations in the sample indentation made by the AFM tip when imaging in the tapping mode repulsive interaction regime. Monitoring the swelling of the plasma-treated polymer surface provided a means to reveal the nanometer-scale heterogeneity that this type of treatment creates on the polymer surface, which is something that would not be possible otherwise. Measurement of AFM tip-sample adhesion forces evidenced rapid water adsorption onto the oxygen plasma-treated surface, supporting the idea of water-induced swelling. This high hydrophilicity was interpreted as arising from the incorporation of polar oxygen functionalities, as demonstrated by X-ray photoelectron spectroscopy (XPS).
1. Introduction It is well known that polymers can swell upon exposure to certain vapors or solvents,1 in particular, water, which has found many applications in biomedicine and biotechnology including the controlled delivery of drugs,2 the engineering of tissues,3 the development of artificial organs,4 and the control of proteinligand recognition.5 Despite the relevance of the swelling phenomenon, few reports can be found in the literature on the direct visualization of swollen polymers. Several techniques have been used to provide insight into this aspect at different resolutions such as conventional photography,6 magnetic resonance imaging,7 imaging ellipsometry,8 and nanometer-scale atomic force microscopy (AFM).9-11 AFM has been particularly useful in elucidating fundamental aspects of the swelling of polymers in water and organic solvents, such as the influence of both solvent affinity and cross-link density on the swelling behavior. However, to the best of our knowledge and with only one exception,12 studies on the direct, step-by-step visualization of the swelling * Corresponding author. E-mail:
[email protected]. Tel: (+34) 985 11 90 90. Fax: (+34) 985 29 76 62. (1) Cowie, J. M. G. Polymers: Chemistry and Physics of Modern Materials, 2nd ed.; Blackie Academic & Professional: London, 1991. (2) Kurisawa, M.; Terano, M.; Yui, N. J. Controlled Release 1998, 54, 191200. (3) Nishida, K.; Yamato, M.; Hayashida, Y.; Watanabe, K.; Yamamoto, K.; Adachi, E.; Nagai, S.; Kikuchi, A; Maeda, N.; Watanabe, H.; Okano, T.; Tano, Y. N. Engl. J. Med. 2004, 351, 1187-1196. (4) Osada, Y.; Okuzaki, H.; Hori, H. Nature 1992, 355, 242-244. (5) Stayton, P. S.; Shimoboji, T.; Long, C.; Chilkoti, A.; Chen, G. H.; Harris, J. M.; Hoffman, A. S. Nature 1995, 378, 472-474. (6) Cao, Q. R.; Choi, H. G.; Kim, D. C.; Lee, B. J. Int. J. Pharm. 2004, 274, 107-117. (7) Quijada-Garrido, I.; Prior-Cabanillas, A.; Garrido, L.; Barrales-Rienda, J. M. Macromolecules 2005, 38, 7434-7442. (8) Schmaljohann, D.; Nitschke, M.; Schulze, R.; Eing, A.; Werner, C.; Eichhorn, K.-J. Langmuir 2005, 21, 2317-2322. (9) James, P. J.; McMaster, T. J.; Newton, J. M.; Miles, M. J. Polymer 2000, 41, 4223-4231. (10) Bonaccurso, E.; Graf, K. Langmuir 2004, 20, 11183-11190. (11) Affoune, A. M.; Yamada, A.; Umeda, M. Langmuir 2004, 20, 69656968. (12) Gallyamov, M. O.; Tartsch, B.; Knokhlov, A. R.; Sheiko, S. S.; Bo¨rner, H. G.; Matyjaszewski, K.; Mo¨ller, M. Chem.sEur. J. 2004, 10, 4599-4605.
process on the nanometer scale have never been reported. This is mainly due to the fact that in most cases such a process takes place too rapidly to be monitored at very high resolution by AFM techniques. On the other hand, plasma treatment is regarded as a suitable, environmentally friendly approach to the modification of surface properties (e.g., wettability, adhesion, and biocompatibility) of polymers and has also found many applications in biomedicine, for the development of composite materials, in the packaging and textile industries, and so forth.13,14 As a result of the changes in cross-link density and/or surface energy after plasma treatment, the polymer surface can undergo swelling when exposed to water and organic solvents or to high-humidity atmospheres,10,14-17 which can have important consequences in terms of applications. Again in this case, direct visualization on the nanometer scale of the swelling process of plasma-treated polymers has never been reported, even though such a study offers the possibility of unveiling details of their surface heterogeneity that would not be addressable otherwise.10 We are interested in the plasma treatment of polyaramid fibers in connection with their application as reinforcing agents in composite materials.18 In this context, we have found that certain types of plasma treatment of poly(p-phenylene terephthalamide) (PPTA) bring about modified polymer surfaces that swell on the nanometer scale upon adsorption of ambient water or water from saturated humid atmospheres. Swelling of plasma-treated PPTA takes place on a time scale of hours, thus offering the opportunity to follow the process in detail by AFM. Therefore, in the work reported here AFM was employed for the first time to monitor (13) Chan, C. M.; Ko, T. M.; Hiraoka, H. Surf. Sci. Rep. 1996, 24, 3-54. (14) Fo¨rch, R.; Zhang, Z.; Knoll, W. Plasma Process. Polym. 2005, 2, 351372. (15) Yasuda, T.; Okuno, T.; Tsuji, K.; Yasuda, H. Langmuir 1996, 12, 13911394. (16) Dupont-Gillain, Ch. C.; Adriaensen, Y.; Derclaye, S.; Rouxhet, P. G. Langmuir 2000, 16, 8194-8200. (17) Selli, E.; Mazzone, G.; Oliva, C.; Martı´n, F.; Riccardi, C.; Barni, R.; Marcandalli, B.; Massafra, M. R. J. Mater. Chem. 2001, 11, 1985-1991. (18) Montes-Mora´n, M. A.; Paredes, J. I.; Martı´nez-Alonso, A.; Tasco´n, J. M. D. Macromolecules 2002, 35, 5085-5096.
10.1021/la052428n CCC: $33.50 © 2006 American Chemical Society Published on Web 04/15/2006
AFM Monitoring of Nanometer-Scale Polymer Swelling
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the swelling of a plasma-treated polymer (PPTA) in real time on the nanometer scale. As shown below, useful information concerning the surface heterogeneity and behavior of plasmatreated PPTA that is relevant to the mentioned application was obtained from these studies. 2. Experimental Section The starting PPTA material used in the present study was a finishfree version of Kevlar 29, provided in fiber form by DuPont de Nemours. Bundles of as-received PPTA fibers were exposed to an oxygen plasma generated by microwave radiation (2.45 GHz) in a Technics Plasma 200-G treatment chamber. The treatments were accomplished at a power of 70 W with an oxygen pressure of 1 mbar and an exposure time of 4 min. For comparison purposes, as-received fibers were also exposed to nitrogen and helium plasmas under the same conditions as those used for the oxygen plasma treatments. Immediately after exposure to the plasma, the fibers were mounted onto metallic sample holders by means of double-sided carbon adhesive tape and transferred to a Nanoscope IIIa Multimode apparatus (Veeco Instruments), where the AFM investigations were carried out under ambient conditions (RH ∼45%, ∼22-23 °C). Both the tapping and contact modes of operation were used to examine the samples. The former mode was used for imaging, whereas the contact mode was mainly employed for adhesion force measurements and, to a lesser extent, for imaging. For the tapping mode studies, rectangular silicon cantilevers with spring constants of about 40 N m-1 and resonance frequencies around 250 kHz were used. To keep the tip-sample interaction as gentle as possible and thus avoid disturbing soft, delicate structures during imaging, the tapping mode measurements were mostly performed in the attractive interaction regime. As discussed elsewhere,19-22 tapping mode AFM can be operated under two different tip-sample interaction regimes: attractive and repulsive, the former being preferable for accurate topographic imaging of extremely fragile specimens because sample deformation by the tip is avoided.23,24 Thus, in the present work, the attractive regime was used to follow the topographical evolution of the polymer samples without disturbing them, whereas the repulsive regime was employed only to obtain additional information on the polymer properties (e.g., to map local variations in sample indentation by the AFM tip). Following procedures reported previously,19,21,23 the operating regime (attractive or repulsive) was unambiguously established through proper setting of the free (A0) and setpoint (Asp) amplitudes of cantilever oscillation with the aid of amplitude and phase versus distance curves. The typical parameters employed were 15-30 nm for A0 and 0.8-0.9 for rsp (rsp ) Asp/A0) when operating in the attractive regime and 125-300 nm (A0) and 0.5-0.8 (rsp) when working in the repulsive regime. For the contact mode, triangular silicon nitride cantilevers with a nominal spring constant of 0.06 N m-1 were used. Tip-sample adhesion forces were determined by measuring the pull-off forces from force-displacement curves recorded on random locations of the polymer surface.25 The given adhesion values are the average of several tens of individual measurements. Topographic and lateral force imaging in contact mode was also carried out on the plasma-treated PPTA surfaces to compare with the results obtained in tapping mode. Further characterization of the samples was accomplished by X-ray photoelectron spectroscopy (XPS). A CLAM2/4S (Fisons) spectrometer, using an Al KR X-ray source and operating at a pressure
