ARAMENDIA Pedro Francisco
congresos y reuniones científicas
Poly(alkyl methacrylate) Thin Films Relaxation Probed by Nile Red Rotational Dynamics
Congreso; XI Encuentro Latinoamericano de Fotoquímica y Fotobiología; 2012
Poly(alkyl methacrylate) Thin Films Relaxation Probed byNile Red Rotational Dynamics1 2Araoz, Beatriz ; Carattino, Aquiles; Aramendía, Pedro F.1 INQUIMAE-DQIAQF-FCEN-UBA, Pab. II-Ciudad Universitaria, Buenos Aires,beatriz@qi.fcen.uba.ar2INQUIMAE-DQIAQF-FCEN-UBA, Pab. II-Ciudad Universitaria, Buenos Airespedro@qi.fcen.uba.arIn the present work we study the rotational dynamics of Nile Red (NR) embedded inpoly(alkyl methacrylates), methyl-, PMMA; ethyl-,PEMA and butyl-, PBMA, thin polymer filmsby ensemble and single molecule (SM) polarized fluorescence measurements at 296 K.Polarized fluorescence microscopy is a powerful tool to study molecular orientation offluorescent probes [1], [2]. It gives information about local friction by measuring temporalfluctuation of SM fluorescence anisotropy. Because excited state lifetime is shorter than therotational characteristic time, dye reorientational movements are studied in the ground state,between successive excitations. In many cases, fluorescence detection is affected byphotobleaching processes; to overcome this problem, we propose an analytical solution ofthe rotational diffusion equation including photobleaching, under linearly polarized excitation,which we apply to the ensemble measurements to derive rotational diffusion coefficients andphotobleaching rates.Experiments were conducted in a wide field microscope on probes containing NR inPMMA, PEMA and PBMA thin polymer films of 25 and 200 nm thickness at 296 K. In thesample preparation process, solvent evaporates and the probe keeps immobilized in thematrix; therefore, no translational diffusion is observed. The detected movements result fromthe confinement that generates small and high fluctuations, due to thermal motion and largeconformational cage changes. Anisotropy time traces of individual molecules show differentdynamics: as expected for a polymer environment, the system is heterogeneous both,temporally and spatially. SM anisotropy fluctuations were larger and more frequent in PBMAfilms and in PEMA of 25 nm thickness than in PEMA 200 nm and PMMA films. It can beexplained by polymer chain relaxation dependence on film thickness [3]. Ensemblemeasurements show a similar rotational diffusion coefficient in all the samples and fasterphotobleaching rate in the thinner films.References[1][2][3]D. Woll, A. Deres, F. C. D. Schryver, H. Uji-i, J. Hofkens, Chem. Soc. Rev., vol. 38,no. 2, 313-318, 2009.T. Ha, J. Glass, T. Enderle, D. S. Chemla, S. Weiss, Phys. Rev. Lett., vol. 80, no. 10,2093-2096, 1998.R. Priestley, M. Mundra, N. Barnett, L. Broadbelt, J. Torkelson, Aust. J. Chem, vol.60, 765-771, 2007.