The coil−globule transition of a polystyrene chain (M w = 113 000, M w/M n = 1.03) randomly labeled with a pyrene derivative was studied in cyclohexane under dilute conditions by fluorescence. The coil−globule transition temperature was found to be around 25 °C from the presence of a break point in the pyrene excimer-to-monomer fluorescence intensity ratio plot with temperature. Analysis of the pyrene monomer fluorescence intensity decays with the fluorescence blob model modified to consider the presence of ground-state pyrene pairs allowed for the determination of the chain radii at several temperatures between 12 and 50 °C and confirmed the occurrence of a coil−globule transition. Fluorescence was found to be a reliable technique for studying the coil−globule transitions in dilute solutions of low-molecular-weight polymers where stable globules exist without the interference of polymer aggregates in a large temperature interval.

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We studied the coil-globule transition of a poly(dimethylacrylamide) (PDMA) chain (Mn ) 2.5 × 10 5 ; Mw /Mn ) 2.2) in methanol at several temperatures by steady-state and time-resolved fluorescence and by dynamic light scattering. The polymer was randomly labeled with 0.5 mol % (PDMA05Py) and 1.1 mol % (PDMA11Py) of a pyrene derivative. Temperature breakpoints in both the excimer to monomer and dimer to monomer fluorescence intensity ratios were identified with the coilglobule transition temperature T cg. We obtained Tcg ) 46 ( 7 °C for PDMA05Py (8 × 10 -8 M in methanol) and Tcg ) 52 ( 6 °C for PDMA11Py (3 × 10 -8 M in methanol). Time-resolved fluorescence measurements of these solutions were analyzed using an approach based on Tashiya's model for excimer formation in micelles. Using this model we were able to calculate the polymer radii for different temperatures and identify the coil-globule transition occurring around Tcg ) 53 ( 3 °C for PDMA05Py and about Tcg ) 55 ( 5 °C for PDMA11Py. Finally, the hydrodynamic radii of the polymers in methanol were measured by dynamic light scattering. In these measurements we had to use more concentrated solutions (ca. 3 × 10 -5 M in methanol) in order to detect the transition of the coiled chains. Although we also detect extensive formation of multichain aggregates at this concentration, an abrupt variation of the isolated chain radius was observed at about 50 °C for PDMA05Py and 55 °C for PDMA11Py. Both the transition temperatures and the polymer radii are in good agreement with the results obtained from the fluorescence data and, to our knowledge, provide the first evidence that the coil-globule transition detected by fluorescence techniques coincides with the transition observed in light scattering measurements.

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 °C, 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 dimer ( 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.