Systems Requiring Extrinsic Fluorescent Labels

As nonconjugated and nonaromatic polymers are nonfluorescent, studies of their phase behavior using fluorescence spectroscopy require the use of extrinsic fluorescent labels [38-40]. These can be either selectively dissolved into the polymer phases [41] or, most commonly, attached covalently to the polymer chains [42-47]. A criticism that is often made of using extrinsic fluorescent probes is the possible local perturbation induced by the probe itself on the nanoenvironment to be probed. In order to minimize such perturbation, the size and shape of the probe should be chosen so as to cause the minimum possible perturbation on the probed region. 

The use of fluorescent probes to obtain information about materials distribution in space is most effective when the labeling of both components (in blends) or of different parts of a single component (as is the case for block copolymers) is such that one is labeled with an energy donor fluorophore (D) and the other with an energy acceptor (A) fluorophore. Time-resolved fluorescence studies of the donor can then be used as a tool to assess the relative spatial distribution of the donor-and acceptor-carrying parts. Considering the FRET process that occurs between D and A, and the distance at which this process is effective, an analysis of the dynamics of the D excited states (D ) will provide information on the interface between the two components of a blend, or between the blocks in a copolymer. When addressing the time-dependence of the donor, Eq. (25.1) can be modified to Eq. (25.18), when in the presence of the acceptor, 

Atvars ef al. [41] performed steady-state fluorescence and time-correlated singlephoton counting measurements on samples of poly(vinyl alcohol) (PVA), poly (vinyl acetate) (PVAc) and their blends, labeled noncovalently with fluorescein and/or anthracene probes. The authors determined the interface thickness as being approximately 2.8 run, and also concluded that the surface-to-volume ratio of PVAc domains was increased in blends with a larger PVAc content. 

Py-PS in a (45/5)/50 (PS/PS )/PVME blend as a function of temperature. Reproduced with permission from Ref [42] 2012, John Wiley Sons, Ine 

Steady-state fluorescence spectroscopy has been used by Torkelson et al. [43,44] to determine the components glass transition temperature (Tg) in various blends, including blends of PS with poly(tert-butyl acrylate) (PtBA), PMMA, and poly(w-butyl methacrylate) (PnBMA) [43], as well as in miscible blends of pyrene-labeled PMMA (MPy-labeled PMMA) with poly(ethylene oxide) (PEO) or poly(vinyl chloride) (PVC) over a broad composition range [44]. In the particular case of this latter study [44], the blend Tg-values were measured upon heating by increasing the 

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