Small Molecule Probe Diffusion in Thin Polymer Films near the Glass Transition: A Novel Approach Using Fluorescence Nonradiative Energy Transfer. Deppe, Denise D.; Dhinojwala, Ali; Torkelson, John M. Department of Materials Science, Northwestern University, Evanston, IL, USA. Macromolecules (1996), 29(11), 3898-908. CODEN: MAMOBX ISSN: 0024-9297. Journal written in English. CAN 124:318402 AN 1996:259843 CAPLUS

A novel exptl. approach involving fluorescence nonradiative energy transfer (NRET) is employed to study the Fickian diffusion of small mols. in rubbery polymer films near the glass transition. A theor. formalism has been developed which directly relates the small mol. translational diffusion coeff., .SCRIPTD., to changes in the energy transfer efficiency, E. Values of .SCRIPTD. as low as 5 ´ 10-16 cm2/s have been measured. In this approach, two thin polymer films are sandwiched together, one labeled with either NRET donor or acceptor chromophores and the second doped with the complementary chromophore. Upon annealing for a time t, dopant chromophore diffusion occurs in which E is proportional to (.SCRIPTD.t)1/2/w, where w is the donor film thickness. Values of .SCRIPTD. for pyrene, N-(2-hydroxyethyl)-N-ethyl-4-(tricyanovinyl)aniline (TC1), bis(phenylethynylanthracene) (BPEA), and decacyclene in poly(iso-Bu methacrylate) (PiBMA) and for BPEA in poly(Et methacrylate) (PEMA) have been measured over temps. ranging from ca. Tg to Tg + 20 °C. Among these chromophores, significant differences in both the magnitude and temp. dependence of .SCRIPTD. were obsd. and are attributed to differences in mol. size, shape, and flexibility. Two anomalous effects are obsd. from a comparison of translational diffusion and rotational reorientation dynamics of TC1 in PiBMA near Tg. The first is an apparent enhancement in translational diffusion relative to rotational reorientation dynamics, with the av. translational displacement of a chromophore during an av. rotational relaxation time, átrotñ, being a couple orders of magnitude larger than the length of the mol. This behavior may be explained by significant local-scale heterogeneity in the polymer, i.e., the broad distribution of polymer a-relaxation times. The second regards the different temp. dependencies of átrotñ and .SCRIPTD. near Tg. This may be explained qual. by a strong temp.

dependence of the breadth of the distribution of a-relaxation times, an effect known to be present in the TC1-PiBMA system employed in this comparison as well as a variety of other polymer systems near Tg.