Switching kinetics of thin thermo-responsive hydrogel films_

Physics Seminar, Tuesday, 17th of April 2012 at 16h15 (coffee at 16h00)
Campus Limpertsberg – Room BS104
Talk by Prof. Dr. Peter Müller-Buschbaum
Technische Universität München, Physik-Department Switching kinetics of thin thermo-responsive hydrogel films
The collapse transition of polymers with lower critical solution temperature (LCST) behavior is of great interest for a number of purposes where a strong change of volume is desired even for small changes of an external stimulus such as temperature. Examples of applications are valves in micro-fluidics, the release of drugs in the body or sensors. A polymer which is a promising candidate in this context is poly(N-ispropylacrylamide), PNIPAM [1]. It exhibits a lower critical solution temperature (LCST) of about 32 °C that is attributed to alterations in the hydrogen-bonding interactions of the amide group. In cross-linked hydrogels, its LCST temperature in water was found to be 33.2°C upon heating. Significant macroscopic volume changes have been obtained and have been characterized by calorimetry, dynamic light scattering and small-angle neutron scattering. Upon the collapse, water is released from the gel.
In thin film geometry, such gels are of interest for applications such as thermosensitive surfaces, artificial pump and muscles, light modulation systems and optical switches. Because the response times of such thermosensitive hydrogels depend strongly on the size of the gel [2], for some bulk applications they are too long. A recent trend in creating responsive, polymeric hydrogels is thus to decrease the size of the responsive units. In thin films, the volume is decreased by the very limited film thickness.
We focus on how the internal structures and the transition kinetics of such thin hydrogel films depend on the polymer
composition and film thickness. The films are freshly made by spin coating [3-6]. The surface and internal structure of the dry films are characterized with grazing incidence small angle neutron scattering (GISANS). The temperature dependent swelling and switching kinetics in water vapour of such sensor films is probed by in-situ neutron reflectometry. In addition, atomic force microscopy (AFM) and x-ray reflectivity are used for characterization. The results of these experimental methods are in good agreement with each other.
[1] H.G.Schild Prog Polym Sci 17, 163 (1992)
[2] T.Tanaka, E.Sato, Y.Hirokawa, S.Hirotsu, J.Peetermans Phys Rev Lett 55, 2455 (1985)
[3] W.Wang, K.Troll, G.Kaune, E.Metwalli, M.Ruderer, K.Skrabania, A.Laschewsky, S.V.Roth, C.M.Papadakis, P.Müller-Buschbaum;
Macromolecules 41, 3209-3218 (2008)
[4] W.Wang, E.Metwalli, J.Perlich, K.Troll, C.M.Papadakis, R.Cubitt, P.Müller-Buschbaum; Macromol. Rapid Commun. 30, 114-119 (2009)
[5] W.Wang, E.Metwalli, J.Perlich, C.M.Papadakis, R.Cubitt, P.Müller-Buschbaum; Macomolecules 42, 9041-9051 (2009)
[6] W.Wang, G.Kaune, J.Perlich, C.M.Papadakis, A.M.Bivigou Koumba, A.Laschewsky, K.Schlage, R.Röhlsberger, S.V.Roth, R.Cubitt,
P.Müller-Buschbaum; Macromolecules 43, 2444-2452 (2010)
Next Physics Seminars
• Tuesday, 8th May 2012 To be confirmed, Belval, 16h15
• Tuesday, 15th May 2012 Prof. Joseph Indeken, Limpertsberg, 16h15 Progress on infinite-order wetting
• Wednesday, 23rd May 2012 Prof. Paul van der Schoot, Limpertsberg, 16h15 To be precised

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