Photo-Cross-Linkable PNIPAAm Copolymers. 5. Mechanical Properties of Hydrogel Layers Marianne E. Harmon, †,‡ Dirk Kuckling,* ,†,§ and Curtis W. Frank* ,† Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025, Institut fu ¨ r Makromolekulare Chemie und Textilchemie, Technische Universita ¨ t Dresden, D-01062 Dresden, Germany Received June 5, 2003. In Final Form: September 17, 2003 Atomic force microscopy (AFM) was used to study the mechanical properties of photo-cross-linked, temperature-responsive hydrogel layers in water. Photo-cross-linkable linear polymers based on N- isopropylacrylamide and 2-(dimethylmaleimido)-N-ethyl-acrylamide were spin-coated to produce uniform thin films of cross-linked responsive hydrogels. These films were imaged using AFM, and force-distance curves were used to measure the temperature-dependent elastic modulus. The volume phase transition of the hydrogel layers is constrained by the presence of a fixed substrate, and the length scale of these effects is related to the modulus. These materials were also studied with surface plasmon resonance and optical waveguide spectroscopy to determine the polymer volume fraction as a function of the temperature. The modulus varies as a function of the polymer volume fraction and is in good agreement with previous measurements on bulk hydrogels. The effect of the cross-linking density and degree of ionization on the modulus was investigated, and a comparison of these results to rubber elasticity theory for a swollen network was used to study the hydrogel morphology. The concentration of elastically active network chains is lower than expected but increases as the network collapses at temperatures above the volume phase transition temperature. Introduction Atomic force microscopy (AFM) has been widely used for imaging and mechanical measurements of soft materials. 1-3 The materials range from polymer films above or below their glass transition temperatures 3,4 to highly swollen polymer networks. 5,6 Force-distance curves can be generated, and the resulting indentation of the cantilever under a given load is used to calculate the surface elastic modulus. 7 The resulting elastic modulus tends to be high, compared to bulk values, and substrate effects can be significant for thin polymer films. However, the results from different models are in good agreement when the films are sufficiently thick and the indentation is small relative to the total film thickness. 1,4 Substrate effects can be minimized by using reference experiments that measure the modulus as a function of both the film thickness and the indentation, and the Hertz model has been used to obtain elastic modulus values as low as 2 kPa. 7 AFM imaging of hydrogel networks has revealed domains on the submicrometer length scale that are related to inhomogeneous gel network structures. 8-10 AFM has also been used to observe the swelling behavior of micropatterned pH-responsive polymers, 11 and studies of responsive hydrogels range from single-chain experi- ments 12 to tribology 13 and modulus 6,14 measurements. Responsive polymers and hydrogels have been studied extensively for a wide range of applications, 15 and a number of techniques have been used to make films, membranes, and responsive surfaces from these materials. 16-18 One of the most intensively studied systems is poly(N-isopropylacrylamide) (PNIPAAm) gel, which exhibits a temperature-induced volume phase transition in water upon heating above 32 °C. 15,19 The change in the transition temperature with applied stress often compli- cates the mechanical measurements of responsive hy- drogels near the transition temperature, 20 but the me- chanical properties of bulk hydrogels can be measured by indentation, 21 simple tension and compression, 22 small- * To whom correspondence should be addressed. E-mail: dirk.kuckling@chemie.tu-dresden.de (D.K.); curt.frank@stanford.edu (C.W.F.). † Stanford University. ‡ Current address: General Electric Global Research Center, 1 Research Circle, Niskayuna, NY, 12309. § Technische Universita ¨ t Dresden. (1) Domke J.; Radmacher, M. Langmuir 1998, 14, 3320-3325. (2) Jena, B. P., Ho ¨ rber, J. K. H., Eds. Atomic Force Microscopy in Cell Biology; Academic Press: San Diego, 2002. (3) Tsukruk, V. V.; Gorbunov, V. V.; Huang, Z.; Chizhik, S. A. Polym. Int. 2000, 49, 441-444. (4) Chizhik, S. A.; Huang, Z.; Gorbunov, V. V.; Myshkin, N. K.; Tsukruk, V. V. Langmuir 1998, 14, 2606-2609. (5) Radmacher, M.; Fritz, M.; Hansma, P. K. Biophys. J. 1995, 69, 264-270. (6) 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. (7) Van Landingham, M. R.; Villarrubia, J. S.; Guthrie, W. F.; Meyers, G. F. Macromol. Symp. 2001, 167, 15-43. (8) Suzuki, A.; Yamazaki, M.; Kobiki, Y. 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