Free Volume and Swelling in Thin Films of Poly(N-isopropylacrylamide) End-Capped with n-Butyltrithiocarbonate S. Harms, K. Ra¨tzke,* F. Faupel, W. Egger, L. Ravello, A. Laschewsky, W. Wang, P. Mu ¨ller-Buschbaum Introduction Hydrogels are three-dimensional networks of polymer chains that swell, but do not dissolve, in water. With an external stimulus these hydrogels are switched between a collapsed and an extended chain conformation. One prominent model for a thermosensitive hydrogel is poly(N-isopropylacrylamide) (PNIPAM). [1–15] When heated in water above 33 8C, PNIPAM undergoes a reversible phase transition from a swollen hydrated state to a shrunken dehydrated state. Since PNIPAM expels its liquid contents at a temperature near that of the human body, PNIPAM has been investigated by many researchers for possible applications in controlled drug delivery. Because pure PNIPAM homopolymer miss the possibility of internal cross-links to build up a gel, the addition of hydrophobic chain ends (a triblock copolymer in the extreme case) is a simple way to overcome this problem. However, with increasing glassy, hydrophobic block length the ability to swell decreases. [16,17] Thus short hydrophobically end- capped PNIPAM chains can exhibit the strongest swelling and thereby maintain the gel character. Polystyrene-based triblock PNIPAM copolymers show classical block copoly- mer morphologies, however, the water uptake strongly depends on the respective morphology. [18] Recently, end-capped PNIPAM with a very short hydro- phobic end group, n-butyltrithiocarbonate, at both chain ends (denoted nbc-PNIPAM) was investigated. [3] The swelling of thin films was addressed. Surprisingly, the Communication K. Ra ¨tzke, S. Harms, F. Faupel Institute for Materials Sience – Multicomponent Materials, Christian-Albrechts-University at Kiel, Kaiserstraße 2, 24143 Kiel, Germany Fax: (þ49) 431 880 6229; E-mail: kr@tf.uni-kiel.de W. Egger, L. Ravello University of Armed Forces, Munich, 85577 Neubiberg, Germany A. Laschewsky Inst. Chemie, Potsdam Universita ¨t, Karl-Liebknecht-Str. 24-25, 14476 Potsdam-Golm, Germany W. Wang, P. Mu ¨ller-Buschbaum Physik-Department LS E13, TU Mu ¨nchen, James-Franck-Str.1, 85747 Garching, Germany The free volume in thin films of poly(N-isopropylacrylamid) end-capped with n-butyltrio- carbonate (nbc-PNIPAM) is probed with positron annihilation lifetime spectroscopy (PALS). The PALS measurements are performed as function of energy to obtain depth profiles of the free volume of nbc-PNIPAM films. The range of nbc-PNIPAM films with thicknesses from 40 to 200 nm is focused. With decreasing film thickness the free volume increases in good agreement with an increase in the maximum swelling capability of the nbc-PNIPAM films. Thus in thin hydrogel films the sorption and swelling behavior is gov- erned by free volume. 1364 Macromol. Rapid Commun. 2010, 31, 1364–1367 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/marc.201000067