Kinetics of Coil-to-Globule Transition of Dansyl-Labeled Poly(N-isopropyl-acrylamide) Chains in Aqueous Solution

The coil-to-globule transition of thermally sensitive linear poly(N-isopropylacrylamide) (PNIPAM) labeled with dansyl group is induced by 1.54 μm laser pulses (width≈10 ns). The dansyl group is used to follow the transition kinetics because its fluorescence intensity is very sensitive to its micro-environment. As the molar ratio of NIPAM monomer to dansyl group increases from 110 to 300, the effect of covalently attached dansyl fluorophores on the transition decreases. In agreement with our previous study in which we used 8-anilino-1-naphthalensulfonic acid ammonium salt free in water as a fluorescent probe, the current study reveals that the transition has two distinct stages with two characteristic times, namely, τ_fast≈0.1 ms, which can be attributed to the nucleation and formation of some "pearls" (lo-cally contracting segments) on the chain, and τ_slow≈0.5 ms, which is related to the merging and coarsening of the "pearls". τ_fast is independent of the PNIPAM chain length over a wide range (M_w=2.8×10⁶-4.2×10⁷ g/mol). On the other hand, τ_slow only slightly increases with the chain length.