| Abstract |
We studied the coil-globule transition of a poly(dimethylacrylamide) (PDMA) chain (M-n = 2.5 x 10(5); M-w /M-n = 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 coil-globule transition temperature T-cg. We obtained T-cg = 46 +/- 7 degreesC for PDMA05Py (8 x 10(-8) M in methanol) and T-cg = 52 +/- 6 degreesC for PDMA11PY (3 x 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 T-cg = 53 +/- 3 degreesC for PDMA05Py and about T-cg = 55 +/- 5 degreesC 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 x 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 degreesC for PDMA05Py and 55 degreesC 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. |
