Poly(alkyl methacrylate) Thin Films Relaxation Probed by Nile Red Rotational Dynamics

ARAOZ, BEATRIZ; CARATTINO, AQUILES; ARAMENDIA, PEDRO F.

Congreso; XI Latin American Meeting on Photochemistry and Photobiology; 2012

Poly(alkyl methacrylate) Thin Films Relaxation Probed by Nile Red Rotational Dynamics Araoz, Beatriz1; Carattino, Aquiles; Aramendía, Pedro F.2 1 INQUIMAE-DQIAQF-FCEN-UBA, Pab. II-Ciudad Universitaria, Buenos Aires, beatriz@qi.fcen.uba.ar 2 INQUIMAE-DQIAQF-FCEN-UBA, Pab. II-Ciudad Universitaria, Buenos Aires pedro@qi.fcen.uba.ar In the present work we study the rotational dynamics of Nile Red (NR) embedded in poly(alkyl methacrylates), methyl-, PMMA; ethyl-,PEMA and butyl-, PBMA, thin polymer films by ensemble and single molecule (SM) polarized fluorescence measurements at 296 K. In the sample preparation process, solvent evaporates and the probe keeps immobilized in the matrix; therefore, no translational diffusion is observed. The detected movements result from oscillations of the probe limited by the volume and rearrangements of the cage. This confinement generates small and high fluctuations, due to thermal motion and large conformational cage changes, respectevily. As a result, molecules have different dynamics and the system is heterogeneously distributed both, temporally and spatially. Polarized fluorescence microscopy is a powerful tool to study molecular orientation of fluorescent probes [1], [2]. It gives information about local friction by measuring temporal fluctuation of SM fluorescence anisotropy. Because excited state lifetime is shorter than the rotational characteristic time, dye reorientational movements are studied in the ground state, between successive excitations. In many cases, fluorescence detection is affected by photobleaching processes; to overcome this problem, we propose an analytical solution of the rotational diffusion equation including photobleaching, under linearly polarized excitation. Experiments were conducted through a wide field microscope. NR environment dynamics in PMMA, PEMA and PBMA thin polymer films of 25 and 200 nm thickness at 296 K was studied. The obtained SM Anisotropy fluctuations were larger and more frequents in PBMA films and in PEMA of 25 nm thickness than in PEMA 200 nm and PMMA films. It can be explained by polymer chain relaxation dependence on film thickness [3]. Ensemble measurements have shown a similar rotational diffusion coefficient in all the samples and faster photobleaching rate in the thinner films. References [1] D. Woll, A. Deres, F. C. D. Schryver, H. Uji-i, and J. Hofkens, Chem. Soc. Rev., vol. 38, no. 2, pp. 313-318, 2009. [2] T. Ha, J. Glass, T. Enderle, D. S. Chemla, and S. Weiss, Phys. Rev. Lett., vol. 80, no. 10, pp. 2093-2096, 1998. [3] R. Priestley, M. Mundra, N. Barnett, L. Broadbelt, and J. Torkelson, Aust. J. Chem, vol. 60, pp. 765-771, 2007.