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Electron Beam Irradiation of Poly(Vinyl Methyl Ether) Films: 1. Synthesis and Film Topography Jan Hegewald,† Thomas Schmidt,† Uwe Gohs,‡ Margarita Gu¨nther,§ Rudolf Reichelt,£ Burkhard Stiller,| and Karl-Friedrich Arndt*,† Institut fu¨ r Physikalische Chemie und Elektrochemie, Technische Universita¨ t Dresden, Mommsenstrasse 13, D-01062 Dresden, Germany, Fraunhofer Institut fu¨ r Elektronenstrahlund Plasmatechnik, Winterbergstrasse 28, D-01277 Dresden, Germany, Institut fu¨ r Festko¨ rperelektronik, Technische Universita¨ t Dresden, Mommsenstrasse 13, D-01062 Dresden, Germany, Institut fu¨ r Medizinische Physik und Biophysik, Westfa¨ lische-Wilhelms-Universita¨ t Mu¨ nster, Robert-Koch-Strasse 31, D-48149 Mu¨ nster, Germany, and Institut fu¨ r Physik, Universita¨ t Potsdam, Am Neuen Palais 10, D-14469 Potsdam, Germany Received January 28, 2005. In Final Form: April 12, 2005 Temperature-sensitive hydrogel layers on silicon (Si) substrates were synthesized by electron beam irradiation of spin-coated poly(vinyl methyl ether) (PVME) films. The influences of the used solvent, the polymer concentration, and the spinning velocity on the homogeneity and the thickness of the PVME film were investigated. In the range of concentration cp ) 1-15 wt% PVME in ethanol solution, homogeneous films with a thickness between d ) 50 nm and 1.7 µm were obtained. The films were cross-linked by electron beam irradiation under inert atmosphere and analyzed by sol-gel-analysis. The results were compared with bulkgels formed by electron beam irradiation of PVME in the dry state. The film topography was analyzed by high-resolution field emission scanning electron microscopy and atomic force microscopy. An islandlike structure in the dry, swollen, and shrunken state of the hydrogel films was observed.
* Author to whom correspondence should be addressed. E-mail:
[email protected]. Phone: +49-351-46332013. Fax: +49-351-463-32013. † Institut fu ¨ r Physikalische Chemie und Electrochemie, Technische Universita¨t Dresden. ‡ Fraunhofer Institut fu ¨ r Elektronenstrahl- und Plasmatechnik. § Institut fu ¨ r Festko¨rperelektronik, Technische Universita¨t Dresden. £ Westfa ¨ lische-Wilhelms-Universita¨t Mu¨nster. | Universita ¨ t Potsdam.
homopolymer and copolymer films prepared by a simultaneous polymerization and cross-linking.5,6 Another method of cross-linking PNIPAAm films was investigated by Kuckling et al.7,8 Copolymers of PNIPAAm containing comonomers with a chromophore were cross-linked photochemically, and the swelling properties of the resulting films were investigated. Furthermore, the plasma immobilization of PNIPAAm-graft-PEG copolymers has been investigated.9 PVME shows a lower critical solution temperature (LCST) behavior in aqueous solution with a transition temperature of 34 °C.10,11 The temperature-sensitive behavior is applicable to PVME networks (hydrogels), which then undergo a temperature-induced reversible swelling/deswelling. PVME hydrogels were synthesized by high-energy irradiation of aqueous PVME solutions.12-16 Under such experimental conditions, one makes the use of the indirect effect of radicals formed by the electron beam irradiation-induced radiolysis of water molecules. The H or OH radicals formed attack the PVME chain and transfer the radical center to the chain.17,18 The recombination of the polymer radicals leads to the cross-linking of the polymer. Janik et al. studied the radiation-chemical yield of cross-linking and scission, as well as the gelation
(1) De Rossi, D.; Kaijwara, K.; Osada, Y.; Yamauchi, A. Polymer Gels. Fundamentals and Biomedical Applications; Plenum Press: New York, 1991. (2) Gehrke, S. H. Adv. Polym. Sci. 1993, 110, 80-144. (3) Hoffman, A. S. Adv. Drug Delivery Rev. 2002, 54, 3-12. (4) Tanaka, T.; Fillmore, D. J. J. Chem. Phys. 1979, 70, 1214-1218. (5) Matzelle, T. R.; Ivanov, D. A.; Landwehr, D.; Heinrich, L. A.; Herkt-Bruns, C.; Reichelt, R.; Kruse, N. J. Phys. Chem. B 2002, 106, 2861-2866. (6) Matzelle, T. R.; Geuskens, G.; Kruse, N. Macromolecules 2003, 36, 2926-2931. (7) Kuckling, D.; Harmon, M. E.; Frank, C. W. Macromolecules 2002, 35, 6377-6383. (8) Kuckling, D.; Hoffmann, J.; Plo¨tner, M.; Ferse, D.; Kretschmer, K.; Adler, H. J. P.; Arndt, K. F.; Reichelt, R. Polymer 2003, 44, 44554462. (9) Schmaljohann, D.; Beyerlein, D.; Nitschke, M.; Werner, C. Langmuir 2004, 20, 10107-10114.
(10) Horne, R. A.; Almeida, J. P.; Day, A. F.; Yu, N. T. J. Colloid Interface Sci. 1971, 35, 77-84. (11) Scha¨fer-Soenen, M.; Moerkerke, R.; Koningsveld, R.; Berghmans, H.; Dusˇek, K.; Sˇ olc, K. Macromolecules 1997, 30, 410-416. (12) Kabra, B. G.; Akhtar, M. K.; Gehrke, S. H. Polymer 1992, 33, 990-995. (13) Suzuki, M.; Hirasa, O. Adv. Polym. Sci. 1993, 110, 241-261. (14) Arndt, K. F.; Schmidt, T.; Menge, H. Macromol. Symp. 2001, 164, 313-322. (15) Schmidt, T.; Querner, C.; Arndt, K. F. Nucl. Instrum. Methods Phys. Res., Sect. B 2003, 208, 331-335. (16) Janik, I.; Kasprzak, E.; Al-Zier, A.; Rosiak, J. M. Nucl. Instrum. Methods Phys. Res., Sect. B 2003, 208, 374-379. (17) Janik, I.; Ulan˜ski, P.; Rosiak, J. M.; von Sonntag, C. J. Chem. Soc., Perkin Trans. 2 2000, 2034-2040. (18) Janik, I.; Ulan˜ski, P.; Hildenbrand, K.; Rosiak, J. M.; von Sonntag, C. J. Chem. Soc., Perkin Trans. 2 2000, 2041-2048.
1. Introduction Stimuli-sensitive hydrogels undergo an abrupt change in their physical properties (e.g., swelling capacity, mechanical properties, hydrophilicity) when the nature of the surrounding aqueous media exceed a certain critical value (e.g., temperature, pH value, solvent concentrations, ionic strength, etc.).1-3 The rate of the swelling/deswelling transition strongly depends on the hydrogel dimension.4 One opportunity to reduce hydrogel dimension is to synthesize films (typically with 10 nm-200 µm thickness). Recently, several investigation were performed to create temperature-sensitive hydrogel structures on various substrates.5-9 Matzelle et al. studied the mechanical properties of poly(N-isopropyl acrylamide) (PNIPAAm)
10.1021/la0502589 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/18/2005
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dose, Dg, of PVME dependent on the polymer concentration (5-100 wt% polymer in aqueous solution).16 Dg increases with increasing polymer concentration. The cross-linking of polymer chains can also occur in the absence of solvent, as a direct effect of high-energy radiation on the polymer materials. The homolytic breakage of chemical bonds and the intermolecular coupling of the resulting radicals lead to network formation. Crosslinking of polymers initiated by high-energy radiation was studied in detail by Charlesby.19 Less investigations were performed on the cross-linking of dry PVME in the literature. The first radiation cross-linking experiments of bulky PVME were reported by Duffey.20 The work focused on improvements in the mechanical properties of the rubberlike PVME by the addition of inorganic filling materials. Aziz et al. analyzed the protein adsorption on PVME hydrogels films on glass substrates of several micrometer thickness cross-linked by γ-ray irradiation.21 However, thinner PVME films have not yet been investigated. Biocompatible poly(vinyl pyrrolidone) (PVP) hydrogel layers of approximate thickness of 50 nm (dry film thickness) were cross-linked using an industrial electron accelerator onto poly(ethylene therephthalate) (PET)coated Si wafers.22 Furthermore, Krsko et al. studied the simultaneous cross-linking and patterning of poly(ethylene oxide) (PEO). Using a field emission scanning electron microscope, it was observed that this is a method leads to film thickness limitations.23 However, an advantage of this method is the simple synthesis of hydrogel structures with small lateral sizes. The aim of this work is the synthesis of temperaturesensitive hydrogel films based on PVME by electron beam irradiation of the spin-coated polymer on a Si substrate. The investigations will be focused on the influence of the experimental conditions (polymer concentration during casting, cp, applied radiation dose, D) on the film thickness, d. The gel content, g, the swelling capacity, Q, and the film topography are investigated. The temperaturesensitive swelling behavior of the formed PVME hydrogel films will be the object of further investigations.24
Hegewald et al. Nuclear Physics Nowosibirsk, Russia) with an energy of E ) 1.5 MeV and a beam power of 20 kW. At constant beam current (I ) 4 mA), the absorbed dose depends on the exposure time.25 The dry bulkgels were weighed to obtain mnon-extracted and then subjected to acetone extraction in a Soxhlett apparatus for 5 days. The gel content, g, was calculated using eq 1 after drying the gels in a vacuum and weighing the mextracted.
g)
(19) Charlesby, A. Atomic Radiation and Polymers; Pergamon Press: Oxford, 1960. (20) Duffey, D. Ind. Eng. Chem. 1958, 50, 1267-1272. (21) Aziz, C. A.; Sefton, M. V.; Anderson, J. M.; Ziats, N. P. J. Biomed. Mater. Res. 1996, 32, 193-202. (22) Meinhold, D.; Schweiss, R.; Zschoche, S.; Janke, A.; Baier, A.; Simon, F.; Dorschner, H.; Werner, C. Langmuir 2004, 20, 396-401. (23) Krsko, P.; Sukhishvili, S.; Mansfield, M.; Clancy, R.; Libera, M. Langmuir 2003, 19, 5618-5625. (24) Hegewald, J.; Schmidt, T.; Eichhorn, K.; Grundke, K.; Kretschmer, K.; Kuckling, D.; Arndt, K. F. Electron beam irradiation of poly(vinyl methyl ether) films. 2. Temperature-dependent swelling behavior. Langmuir, submitted for publication.
(1)
The cross-linking behavior of irradiated polymers is typically analyzed by using the Charlesby-Pinner26 equation (eq 2).
p0 1 + q0 q0MnD
(2)
λ 2 - λ Dv + Dg + 2 2 Dv + D
(3)
s + xs ) s + xs )
where p0 ) the fracture density per unit dose, q0 ) the density of cross-linked units per unit dose, λ ) 2 p0/q0 ) the scission/ cross-linking radiation yield ratio, Dv ) a virtual dose, and Dg ) the dose of gelation. However, eq 2 is only valid for polymers with a molecular weight distribution of Mw/Mn ) 2. PVME has a broader distribution due to its cationic polymerization (Mw/Mn ) 2.5). Therefore, a modified version of this equation was used (Charlesby-Rosiak27) that is valid for all distributions (eq 3). The gel-sol analysis was performed by using the software GelSol95 obtained from the homepage of the group of J. M. Rosiak/ Ło´dz´ (www.mitr.p.lodz.pl/biomat). The equilibrium mass swelling degree, Qm, of the extracted PVME bulkgels (mdry ≈ 0.3 g) is dependent on the temperature and was measured by weighing the samples tempered for 24 h to obtain mswollen (eq 4).
Qm )
mgel + mwater mswollen ) mgel mdry
(4)
Qm of the PVME hydrogels at T ) 25 °C was used for calculating the cross-linking density, νC, and number-average of molecular weight between cross-links, MC, respectively, by the FloryRehner equation (eq 5).28
2. Experimental Section 2.1. Materials. For all experiments, a commercially available aqueous solution of PVME (50 wt%), Lutonal M40 (BASF), was used. The molecular weight was determined by static light scattering in 2-butanone (Merck) to Mw ) 57 000 g/mol. The stock solution was evaporated to dryness in a vacuum oven at 40 °C for several days. Ethanol, chloroform, toluene, and acetone obtained from Merck were used without further purification. Si wafers (1 in., Silchem GmbH, Freiberg, Germany) exhibiting a native oxide layer of thickness d ) 1.6 nm (determined by spectroscopic ellipsometry) were used as substrate. 2.2. Synthesis of PVME Bulkgels. To familiarize ourselves with the cross-linking of dry PVME, we initially synthesized PVME bulkgels by irradiation of dried PVME in Petri dishes (max thickness of the sample 3.5 mm). The electron beam irradiation of the PVME was carried out under inert atmosphere (argon) on an electron accelerator ELV-2 (Budker Institute of
mgel mextracted ) mgel + msol mnon-extracted
νC )
Fp ln(1 - φp) + φp + χφ2p )MC VA(Aηφ1/3 p - Bφp)
(5)
where Fp is the polymer density, φp the volume fraction of the polymer in the swollen network, η the memory term with η ) (volume fraction of polymer at cross-linking)2/3)1, A the microstructure factor, R the gas constant, and T the temperature. The affine phantom network model was used, where A ) 1 and B ) 2/f (functionality of the cross-links f ) 4). The Huggins interaction parameter, χ, was calculated to be 0.495 from the second virial coefficient, A2 (determined by static light scattering of PVME in water at T ) 25 °C).14 The calculated absolute values of MC are to be seen as relative due to the incorrectness of χ (calculated from A2 of a dilute polymer solution). 2.3. Synthesis of PVME Films by Electron Beam Irradiation. PVME solutions were prepared by dissolving the dried polymer in the corresponding solvent (for most studies ethanol was used). Before the casting process, the solutions were filtered through a 0.45 µm Nylon filter. The typical spin-coating procedure was carried out with a spinning velocity of ω ) 3000 rpm using Spin Coater P6700 (Speedline Technologies, Franklin, MA). The prepared films were dried in a vacuum for several hours in order (25) Dorschner, H.; Jeschke, W.; Lunkwitz, K. Nucl. Instrum. Methods Phys. Res., Sect. B 2000, 161-163, 1154-1158. (26) Charlesby, A.; Pinner, S. H. Proc. R. Soc. A 1959, 249, 367-389. (27) Olejnniczak, J.; Rosiak, J. M.; Charlesby, A. Rad. Phys. Chem. 1991, 38, 113-118. (28) Flory, P. J.; Rehner, J. J. Chem. Phys. 1943, 11, 512-516.
Electron Beam Irradiation of PVME Films
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Figure 1. Experimental scheme of the electron beam irradiation of the thin PVME films. Experiments were performed in inert atmosphere (N2). The radiation dose, D, was determined as the exposure time of the film during the electron beam irradiation. to remove the remaining solvent and to avoid the uptake of atmospheric water. The cross-linking of the thin PVME film was performed by using electron beam irradiation under inert atmosphere. The samples were placed into PE bags, which were then purged with N2 (remaining O2 content: 50-100 ppm). The electron accelerator (ANDREA 1, FEP Dresden, Germany) operates with an energy of E ) 90-120 keV. However, PVME films were irradiated with electrons of a constant energy of E ) 120 keV. The applied radiation doses were adjusted to D ) 150-450 kGy at different dose rates by changing the beam current (I ) 2-20 mA) and the speed of the conveyor belt (v ) 1-18 m/min). The radiation dose, D, absorbed by the PVME films was determined using alanine film dosimeter29 of 10 µm thickness and an overall uncertainty of
