Langmuir 2007, 23, 2203-2207
2203
Micromechanical Cantilever Technique: A Tool for Investigating the Swelling of Polymer Brushes Gina-Gabriela Bumbu,†,§ Markus Wolkenhauer,† Gunnar Kircher,† Jochen S. Gutmann,*,†,‡ and Ru¨diger Berger*,† Max Planck Institute for Polymer Research, Ackermannweg 10, D 55128 Mainz, Germany, and Institute for Physical Chemistry, Johannes Gutenberg UniVersity, Welderweg 11, D 55099 Mainz, Germany ReceiVed July 21, 2006. In Final Form: NoVember 14, 2006 Polymer brush coatings are well-known for their ability to tailor surface properties in a wide range of applications from colloid stabilization to medicine. In most cases, the brushes are used in solution. Consequently, efforts were expended to experimentally investigate or theoretically predict the swelling behavior of the brushes in solvents of different qualities. Here, we show that the micromechanical cantilever (MC) sensor technique is a tool to perform time-resolved physicochemical investigations of thin layers such as polymer brushes. Complementary to scattering techniques, which measure the thickness, the MC sensor technique provides information about changes in the internal pressure of the brushes during a swelling and deswelling process. We show that the kinetics of both swelling and deswelling are dependent on solvent quality. Comparing the measured data with its thickness evolution, which was calculated based on the Flory-Huggins theory, we found that only the first 10% of the thickness increase of the polymer brush results in a significant pressure increase inside the polymer brush layer.
Introduction Polymer molecules end-tethered to a surface with a high density of attachment points force the chains to stretch away from the solid interface, forming thin films known as polymer brushes.1 Polymer brush layers play an important role to tailor physical and chemical properties of surfaces for applications such as colloidal stabilization,2,3 corrosion inhibition, nonfouling surface technology,4,5 opto-electronic devices,6 tribology,7 chromatography,8 rheology,9 chemical gates,10 and biomedical science.11-13 Stimuli-responsive binary polymer brush layers can be used to create switchable surfaces.14 The properties of polymer brushes depend on their environment, e.g., the quality of a solvent.15 Therefore, both experimental and theoretical swelling and deswelling studies of polymer brushes are mandatory to understand and tailor their properties. * To whom correspondence should be addressed. Telephone: +49-06131379-114. Fax: +49-06131-379-100. E-mail:
[email protected];
[email protected]. † Max Planck Institute for Polymer Research. ‡ Johannes Gutenberg University. § Permanent address: P. Poni Institute of Macromolecular Chemistry, Iasi, Romania. (1) Milner, S. T. Science 1991, 25, 905. (2) Napper, D. H. Polymeric Stabilization of Colloid Dispersions; Academic: New York, 1983. (3) Pincus, P. Macromolecules 1991, 24, 2912. (4) Caster, K. C. In Polymer Brushes; Advincula, R. C., Brittain, W. J., Caster, K. C., Ru¨he, J., Eds.; Wiley-VCH: Weinheim, 2004; pp 329-371. (5) Leckband, D.; Sheth, S.; Halperin, A. J. Biomater. Sci., Polym. Ed. 1999, 10, 1125. (6) Whiting, G. L.; Farhan, T.; Huck, W. T. S. In Polymer Brushes; Advincula, R. C., Brittain, W. J., Caster, K. C., Ru¨he, J., Eds.; Wiley-VCH: Weinheim, 2004; pp 371-381. (7) Klein, J.; Kumacheva, E. Science 1995, 269, 816. (8) van Zanten, J. H. Macromolecules 1994, 27, 6797. (9) Parnas, R. S.; Cohen, Y. Rheol. Acta 1994, 33, 485. (10) Ito, Y.; Ochiai, Y.; Park, Y. S.; Imanishi, Y, J. Am. Chem. Soc. 1997, 119, 1619. (11) Harris, J. M. Poly(ethylene glycol) Chemistry; Plenum Press: New York, 1992. (12) Galaev, I. Y.; Mattiasson, B. Trends Biotechnol. 1999, 17, 335. (13) Aksay, I. A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberger, P. M.; Gruner, S. M. Science 1996, 273, 892. (14) Uhlmann, P.; Ionov, L.; Houbenov, N.; Nitschke, M.; Grundke, K.; Motornov, M.; Minko, S.; Stamm, M. Prog. Org. Coat. 2006, 55, 168. (15) Carignano, M. A.; Szleifer, I. J. Chem. Phys. 1994, 100, 3210.
One way to gradually vary the solvent quality is by mixing a solvent and a nonsolvent at different ratios. Birshtein and Lyatskaya modeled the behavior of a polymer brush immersed in a mixture of miscible solvent and nonsolvent. In these studies, the Flory-Huggins interaction parameter between solvent and polymer, χsp, was varied between 0 and 2.16 Results indicated the existence of two regimes during the swelling process of the brushes in mixed solvents: the so-called “decollapse regime”, and the regime of the “swollen brush”. The “decollapse regime” shows up at low concentrations of solvent and is characterized by a small increase in the height of the brush upon increasing the solvent concentration. In this regime, the swelling is realized mainly by the penetration of the solvent molecules in the brush. The “swollen brush” regime appears at higher concentrations of solvent. Here, the increase of the brush thickness is attributed to a replacement of nonsolvent with solvent molecules. Experimentally, the swelling of polymer brushes was mainly studied by X-ray and neutron scattering techniques17-19 pioneered by Auroy and Auvray.17 They investigated the collapsestretching transition of grafted poly(dimethylsiloxane) in various mixtures of dichloromethane/methanol. Knoll et al.18 looked into the swelling behavior of polystyrene brushes with different grafting densities in mixtures of toluene/methanol of various compositions. However, the neutron scattering technique requires perdeuterated solvent mixtures to increase the contrast. This technique is not applicable for mixtures with a high proportion of solvent, since changes in Kiessig fringes could not be resolved. Therefore, the swelling of the brushes cannot be followed up to 100% solvent. Here, we report the investigation of the kinetics of both the swelling and deswelling behavior of poly(methyl methacrylate), PMMA, brushes using a micromechanical cantilever sensor technique. (16) Birshtein, T. M; Lyatskaya, Y. V. Macromolecules 1994, 27, 1256. (17) Auroy, P.; Auvray, L. Macromolecules 1992, 25, 4134. (18) Bunjes, N.; Paul, S.; Habicht, J.; Prucker, O.; Ru¨he, J.; Knoll, W. Colloid Polym. Sci. 2004, 282, 939. (19) Fick, J.; Steitz, R.; Leiner, V.; Tokumitsu, S.; Himmelhaus, M.; Grunze, M. Langmuir 2004, 20, 3948.
10.1021/la062137u CCC: $37.00 © 2007 American Chemical Society Published on Web 01/10/2007
2204 Langmuir, Vol. 23, No. 4, 2007
Bumbu et al. Scheme 1. Schematic Representation of the Functionalization Procedure of a MC Array with PMMA Brushes
Figure 1. Scanning electron micrograph of an Octosensis micromechanical cantilever array chip used as the mechanical transducer.
Materials and Methods Chemicals. Methyl methacrylate (MMA; Acros, 99%) was purified by passing it through an alumina column, distilling it under reduced pressure from CaH2, and storing it under argon at -20 °C. Anisole (Aldrich, 99%) was used after a previous deaerating by bubbling argon for 1 h. CuBr (Aldrich, 98%) was purified by boiling, for a short time, in a 50 vol %/50 vol % Millipore water/acetic acid mixture and subsequent filtrattion. The precipitate was rinsed sequentially with water, ethanol, and diethyl ether and dried under reduced pressure for 24 h. N,N,N′,N′,N′′-pentamethyldiethylenetriamine (PMDETA; Aldrich, 99%) was purified by distillation under reduced pressure. Ethyl 2-bromoisobutyrate (2-EiBBr; Aldrich, 98%) was used without further purification. The supplies were distilled under argon atmosphere, toluene from sodium, and triethylamine from CaH2. All other reagents were used as received. The starter (1), (3-(2-bromoisobutyryl)propyl)dimethyl chlorosilane, was synthesized following the procedure described in ref 20 and purified by distillation under reduced pressure. To prevent its degradation by moisture, the starter (1) was stored under argon atmosphere over silicagel in a desiccator. Ethyl acetate (Fluka, 99.5%, with a refractive index, n, of 1.3720) and isopropanol (Fluka, 99.8%, n ) 1.3770) were filtrated and sonicated before use to eliminate the remaining impurities and air. Micromechanical Cantilever. Octosensis micromechanical cantilever (MC) array chips (Micromotive GmbH, Mainz, Germany) were used as mechanical transducers (Figure 1). Each chip consists of eight rectangular cantilevers having a length of 750 µm, a width of 90 µm, and a thickness of 1.0 µm fabricated at a pitch of 250 µm. The variations of resonance frequencies within one array are less than 1%, indicating similar mechanical properties. Polymer Brush Synthesis. MC arrays were partially functionalized with polymer brushes using the following procedure: a protective gold coating was applied on the entire back side and on half of the top side of the array after a base cleaning of the array by a thermal evaporation procedure at a rate of 0.1 nm‚s-1. The base cleaning ensures the controlled hydration state of the silicon oxide layer of the array in view of functionalization and consists of immersing the arrays in a mixture of NH3, H2O2, and Millipore water at 80-85 °C for 20-25 min. Afterward, the arrays were rinsed with copious amounts of Millipore water and dried under reduced pressure. Subsequently, the starter (1) was immobilized on the unprotected surface of the array by keeping it in a starter solution in dry toluene and triethylamine (c ∼ 25 mmol‚L-1) for 20 h. The starter attached itself only on the uncovered surface of the array (Scheme 1). The resulting silanated arrays were cleaned by extraction in dichloromethane. Poly(methyl methacrylate), PMMA, brushes (20) Ramakrishnan, A.; Dhamodharan, R.; Ru¨he, J. Macromol. Rapid Commun. 2002, 23, 612.
were grown on the unprotected array surface through a “grafting from” atom transfer polymerization (ATRP) technique. The reaction took place in a Schlenk-like setup, under vacuum in anisole at 35 °C, in the presence of low amounts of sacrificial starter 2-EiBBr (for details, see ref 21). This technique ensures an overall concentration of Br in the system that controls the chain growth of both the immobilized and soluble initiators. Therefore, the ‘‘free polymer” formed by the unbound initiator can be used to monitor the characteristics of the grafted polymer. It is well-known that it is very difficult to determine the exact molecular weight of the polymer grafted on the silicon surface (because of its low amount), but it is expected that the molecular weight of the grafted polymer is similar to that of the polymer formed in solution.22-24 Gel permeation chromatography indicates an average number molecular mass, Mn, of 52 kg‚mol-1 and a polydispersity of 1.39 for the bulk-obtained PMMA. IR Spectroscopy. Attenuated total reflectance (ATR) infrared spectra of PMMA brushes layer-deposited on silicon wafers were recorded with a Nicolet Magna 850 FTIR spectrometer before and after treatment in the KI/I2 solution. The spectral resolution was 4 cm-1. Ellipsometry. The thickness of the polymer brush layer was measured with an imaging ellipsometer (EP3 imaging spectroscopic ellipsometer, Nanofilm, Go¨ttingen, Germany) directly on each of the MCs. The measurements were carried out at an angle of incidence of Ri ) 50°. Using interference filters and a Xenon lamp as the light source, the wavelength was set to λ ) 403 ( 6 nm. The field of view, utilizing a 10× objective, was 0.330 mm × 0.330 mm. On each cantilever, a region of interest with an area of 50 µm × 300 µm was used to average the thickness over this area. To achieve highest accuracy, four zone measurements were carried out. For the calculation of the layer thickness, l, of the polymer brush from the ellipsometric angles, Ψ and ∆, a multilayer model for homogeneous films covering the MCs was applied.25 It has been assumed that the (21) Bumbu, G. G.; Kircher, G.; Wolkenhauer, M.; Berger, R.; Gutmann, J. S. Macromol. Chem. Phys. 2004, 205, 1713. (22) Ejaz, M.; Yamamoto, S.; Ohno, K.; Tsujii, Y.; Fukuda, T. Macromolecules 1998, 31, 5934; (23) Ejaz, M.; Tsujii, Y.; Fukuda, T. Polymer 1998, 42, 6811. (24) Hung, X.; Wirth, M. J. Macromolecules 1999, 32, 1694. (25) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland: Amsterdam, 1979.
Technique for InVestigating Polymer Brush Swelling Scheme 2. Setup of the MC Read-Out Principle Based on the Beam Deflection Technique
multilayer consists of a homogeneous isotropic polymer brush layer in contact with air, and a layer of the starter on a silicon surface with a SiO2-layer on top. The thickness of the silicon oxide layer was determined from the uncovered reference MC and was assumed to be valid for all others as well. From previous measurements, the thickness of the starter film, lstart, was known.21 The following refractive indices have been used in calculations: SiO2, 1.46; starter layer, 1.56; and dry PMMA brush film, 1.49. To check the influence of the different layers, especially the starter and silicon oxide layers, modeling was performed assuming different thicknesses. It was found that the polymer brush film thickness dependency on these layers was smaller than (1 nm. The surface was measured to have ∼1 nm root-mean-square (rms) roughness. Therefore, the accuracy of the reported layer thickness is (1 nm. MC Technique. The deflection change of each MC was monitored with a Scentris platform (Veeco, USA) using the beam deflection principle. This tool allows the sequential recording of the deflection of eight cantilevers at a deflection resolution of
