Fluorescence study of the coil-globule transition of a PEO chain in toluene

The cyclization kinetics of a poly(ethylene oxide) (PEO) chain (M-n = 3280; M-w/M-n = 1.05) labeled at both ends with pyrene was studied in toluene at several temperatures. Above 30 degreesC, the kinetics of pyrene excimer formation is well described by a two-state model. Below this temperature, a new excited species appears, which we identified as being a pyrene dimer. The appearance of the pyrene dimer was associated with a broad coil-globule transition. For lower temperatures, the kinetics of pyrene excimer and dimer formation can only be described by a three-states model. The fluorescence decay curves of the polymer in toluene were analyzed according to these models in order to obtain the rate constants. The activation energies of the cyclization and ring-opening processes were obtained from the Arrhenius plots of the rate constants. The activation energies for the cyclization processes (excimer and excited dimer formation) are close to the viscous flow activation energies. From the excimer dissociation rate constants, we calculated the binding energies of the excimer (36 +/- 4 kJ mol(-1) in the coil and 33 +/- 1 kJ mol(-1) in the globule) and the dinner (22 +/- 2 kJ mol(-1)). The van t'Hoff plots of the corresponding equilibrium constants allowed us to calculate the enthalpy and entropy of cyclization. The entropy values are negative according to a loss of entropy upon chain cyclization. The difference between the enthalpy and the binding energy of the excimer in both the globule and coil sates reflects the variation of polymer conformations between the cyclized and noncyclized chains.