Determination of Glass Transition Temperatures in Bulk and Micellar Nanoconfined Polymers Using Fluorescent Molecular Rotors as Probes for Changes in Free Volume

Fluorescent molecular rotors embedded in polymer matricescan probethe local changes in the dynamics of the polymer matrix (such as theglass transition of polymers near interfaces/surfaces) that are notaccessible from macroscopic measurements. Yet, there is little consensusas to how the fluorescence data should be analyzed, what propertythe fluorescence intensity actually represents, and what are appropriate/optimalrotors for glass transition measurements. By experimentation witha model fluorescent rotor, farnesyl-(2-carboxy-2-cyanovinyl)-julolidine(FCVJ), and also re-analysis of data available in the literature forother types of molecular rotors, we investigated the correlation betweenthe relaxation processes of photo-excited rotors and free volume changesin matrix polymers. The temperature dependences of the fluorescenceintensities from FCVJ-doped polystyrene (PS), poly(vinyl acetate)(PVAc), and poly(isobutyl methacrylate) (PiBMA) materials across theirglass transition temperatures (T (g)) weresuccessfully modeled using a free volume model which is based on theoriginal theory of Loutfy that relates the fluorescence of a rotorprobe to the free volume of the matrix, and the revised definitionof the total free volume quantity proposed by Lipson and White (theLocally Correlated Lattice model-based free volume) that includesboth the vibrational free volume and the excess free volume. Atomisticmolecular dynamics simulation supports that the timescale of the vibrationalmotion of PS monomers is comparable to the intrinsic timescale ofa rotor's internal rotation known in the literature (on theorder of picoseconds), and thus the rotation of a rotor is controlledby the friction from the hard-core volume (not by the friction fromthe vibrational volume) of the monomers. Measurements on FCVJ-loadedpolystyrene-poly(ethylene glycol) (PS-PEG) micellesin water suggest that the PS core domain of the PS-PEG micellehas a broad distribution of glass transition temperatures; a quantitativeanalysis based on the free volume model enabled to estimate the actualrange of T (g). These results allow us toconclude that fluorescent molecular rotors with a tendency towardrapid non-radiative (rotational) relaxation and bulky rotating subgroups(such as FCVJ) exhibit a strong coupling between the fluorescenceof the rotor and the free volume of the matrix and are thus usefulprobes for T (g) measurements.