A mixed amphiphilic system composed of the anionic surfactant Aerosol OT (AOT), in water forming a lamellar phase, to which is added a neutral noninteracting polymer, poly(N,N-dimethylacrylamide), is studied experimentally by SAXS, 2 H NMR, and microscopy, in a range of surfactant and polymer compositions. Addition of the polymer produces a decrease in the lamellar spacing, the decrease by the polymer being almost twice that produced by an equal volume of AOT. Microscopy reveals heterogeneity, but no macroscopic phase separation occurs. 2 H NMR detects that on increasing the polymer concentration some water is in an isotropic environment. It is inferred that the presence of the polymer induces a microscopic phase separation into a polymer-rich isotropic phase and a surfactant-rich lamellar phase, and this is tested theoretically by calculating the osmotic pressures in these two phases. In the lamellar phase, the effect of electrostatic, undulation, van der Waals, and hydration forces on the AOT bilayer is considered; in the isotropic phase, the osmotic contribution of the polymer is considered. These two pressures correlate well, supporting theoretically the hypothesis of the two phases in equilibrium.

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Fluorescence spectra and fluorescence decays of dimethylphenyldisiloxane (DS) and of poly-(methylphenylsiloxane) (PMPS), in methylcyclohexane dilute solution, were measured in a range of temperatures (-132 to +50 °C). This range is shown to cover both the low-and high-temperature limits and the crossover region. The steady-state results for DS show a typical Arrhenius behavior, compatible with a simple Birks kinetics. DS monomer fluorescence decays are biexponential, showing evidence for excimer dissociation, compatible with an isoemissive point observed in the range 5-27 °C. An excimer rise time could be observed for DS at the lowest temperatures, but the sum of the preexponential factors is larger than zero, proving that a certain fraction of the excimers come from ground-state dimers. In order to account for the presence of such preformed dimers, the kinetic scheme has been modified and the proper equations for this modified scheme have been derived. The steady-state results for PMPS indicate that about 50% of the chromophores form part of the ground-state preformed dimer conformations. Fluorescence decays of PMPS are too complex to yield kinetic parameters with physical meaning, but the steady-state results allow the extraction of the rate constant for excimer formation and its activation energy. A complete set of the Birks scheme rate constants was obtained as a function of temperature for DS, both from steady-state and transient experiments, with a good coincidence of the results obtained by the two methods. Apparent and corrected values of the rate constant for excimer formation, taking into account the contribution of ground-state preformed dimers, were also obtained.

The dynamics of linear polymethylphenylsiloxane chains in dilute methylcyclohexane solution was probed with picosecond time-resolved fluorescence. Experiments were performed, for one monodisperse sample with an average number of skeletal bonds equal to 25, at temperatures covering a wide range (193-293 K). Triple exponential decays were observed at the monomer and excimer emission wavelengths. The three relaxation times were interpreted and full analyzed on the basis of a kinetic scheme, which involves three kinetically coupled species in the excited state: the excimer (E) and two different types of monomers (M nh and M h ). The transition of these monomers to excimer occurs at different rates, M nh by a fast transition (k a ), and M h by a slower transition (k u ). Molecular dynamics simulations for the approach of two chromophores to the excimer configuration suggest that there are two time regimes that can be ascribed to these transitions. The fast one to unrestricted motions controlled just by local bond rotations at the level of a single dyad, and the slower one to retarded motions in which the local bond rotations of the dyad occur only after a delay time caused by the coupling of the dyad to the attached chain. The corresponding to theoretical reciprocal relaxation times are in qualitative agreement with the experimental relative values of k a and k u . These results reveal that the dynamics of dyads is influenced by the rest of the backbone, something that can be responsible for the generally complex excimer formation kinetics in polymers. The rates and activation energies of these two transition modes of the chain were measured: Many of the Si-O-Si double (synchronized) rotations leading to the approach of two neighbor phenyl rings to the close distance excimer configuration occur fast, as in a single diad, with k a (20 °C) ) 1.4 × 10 10 s -1 and E a ) 2.2 kcal mol -1 , but a few suffer a lag (like frozen in the nonexcimer configuration), due to retardation imposed by the polymer, giving the slower rate k u (20 °C) ) 1.2 × 10 9 s -1 and E u ) 5.6 kcal mol -1 . The fractions of "frozen" monomers, β ) 0.04, of ground-state dimers, R ) 0.05, and the rate of energy transfer between "frozen" neighbor phenyl rings, k t ) 5.6 × 10 8 s -1 , were also measured. Steady state fluorescence results are accurately reproduced by using the proposed kinetic scheme and the parameters evaluated from time-resolved results.