Single Molecule Studies of Diffusion Dynamics in Polymer Thin Films Near Tg

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Xu, Kewei
Dickson, Robert M.
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For polymers near the glass transition, the dynamics in some regions can be orders of magnitude different compared with the dynamics in other regions only a few nanometers away, so called spatial heterogeneity [1]. In this thesis, single molecule fluorescence microscopy as a powerful tool, was applied to study the spatially heterogeneous dynamics, both orientational and translational, within the polymer matrix near the glass transition temperature. With our total internal reflection fluorescence microscopy (TIRFM) methods, many individual fluorescent dye molecules embedded in the poly (isopropyl acrylate) (PIPA) thin films can be simultaneously excited. Their emission patterns are analyzed using our orientation determination methods [2] to give the true 3D orientational trajectories of the individual molecules. At Tg < T < 1.2 Tg, single molecule tracking was used to study the dye molecules translational diffusion. Results show that, below 1.1 Tg, the probe molecules are in the confined flow region [3]; at T > 1.1 Tg, the diffusion follows normal diffusion model; at T = 1.2 Tg, although the statistical results shows that normal random walk behavior is followed, the individual molecules still show different diffusion behaviors, clear evidence of the spatial heterogeneity that still exists at this temperature. The second part of this thesis is a development of the 3-detector method to determine the 3D orientation of single molecules [4]. This method is based on the work proposed by Fourkas [4] in 2001. Results utilizing this experimental setup are compared with our emission pattern fitting methods. The results show that, with a little bit higher error range (10º in θ, 20º in φ), the 3-detector method can give agreeable orientation fittings, further more, with higher time resolution of < 10 ms. This 3-detector method is useful and can be applied to study the fast orientation dynamics in different systems.
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