SINGLE MOLECULES FLUORESCENCE STUDIES OF POLYMER SEGMENT DYNAMICS IN THIN FILMS
Swift technological progress and instrument development allows one for more than a decade to isolate light waves emitted by single molecules and study their photophysical characteristics. Such single emitters also serve as “nanoreporters” when embedded in materials and can provide information on the structure and dynamics of their surroundings on the molecular scale, for example in polymers. Recently, increasing attention is paid to segmental dynamics in polymer films motivated by the applications of such films in microelectronics, lithography, and nanotechnology. Polymers have been demonstrated to exhibit different chain dynamics behavior when confined into the geometry of ultrathin films, as compared to the bulk. In a collaborative project carried out together with the group of Prof. Niek van Hulst (Optical Techniques, MESA+) we are employing single molecule fluorescence methods to study local, segmental scale dynamics in thin polymer films.
Figure 3. a) Scheme of a single fluorescent molecule embedded in a polymer matrix. b) The fluorescence lifetime is recorded in time. c) A 5 x 5 μm2 fluorescence intensity scan of single emitters embedded in a 200 nm thick polystyrene film. The color scale is related to the molecular orientation of the single chromophores.
By monitoring single molecule fluorescence lifetime fluctuations (Figure 3b) we were able to determine the number of polymer segments taking part in a rearrangement volume (NS). Corresponding results provided us information on local polymer segment mobility, and its variation with temperature and with changes in confinement conditions. For example, for thin films of polystyrene, below a critical film thickness (~60 nm) a decrease of the value of NS was observed with decreasing film thickness (Figure 4). It was found that the effects the surface region has on the segment dynamics propagate deep into the polymer sample over distances larger than the radius of gyration of the polymer used (Rg=10 nm) at temperatures far below the glass transition temperature. The decrease of NS is caused by enhanced segment dynamics at the free surface of the polymer film.
Figure 4. The value of the number of polymer segments in the rearrangement volume around the single molecule emitters as a function of polystyrene film thickness.
Publication N. Tomczak, R.A.L. Vallée, E.M.H.P. van Dijk, L. Kuipers, N.F. van Hulst, G.J. Vancso: J. Am. Chem. Soc. 2004, 126, 4748.