Study of the semidilute solutions of poly(N,N-dimethylacrylamide) by fluorescence and its implications to the kinetics of coil-to-globule transitions.

The large scale motions of poly(N,N-dimethylacrylamide) chains randomly labeled with pyrene (Py-PDMA) were monitored by steady-state and time-resolved fluorescence in semidilute solutions of naked PDMA in acetone and DMF for polymer concentrations ranging from 0 to 550 g/L. Although increasing the polymer concentration of the solution led to a decrease of the mobility of the chromophore attached onto the PDMA backbone, this reduction was rather modest when compared to the large increase of the macroscopic viscosity. This result indicated that locally, the monomer constituting the chains experienced freedom of movement despite the high solution viscosity. The restricted mobility of the chromophore was characterized by the number of monomers occupying the volume probed by the excited chromophore during its lifetime, referred to as a fluorescence "blob". The number of monomers constituting a fluorescence blob, N(F)(-)(blob), and the volume of a fluorescence blob, V(F)(-)(blob), were found to decrease as the polymer concentration of the solution increased, reflecting the decreased mobility experienced by the chromophore. In DMF, the radius of an F-blob was found to scale as N(nu)(F)(-)blob, where nu equaled 0.66 +/- 0.03, very close to the expected value of the Flory exponent of 0.6 for a polymer in a good solvent. The combined knowledge of how N(F)(-)(blob) varies with the fluorescence lifetime of the chromophore and the coil density of the polymer was used to propose a new means of studying coil-to-globule transitions with potential implications for predicting the rate of protein folding.