Kinetics of laser-heating-induced phase transition of poly(N-isopropylacrylamide) chains in dilute and semidilute solutions.

The kinetics of the phase transition of poly(N-isopropylacrylamide) (PNIPAM) chains from the dilute regime to the semidilute regime was studied by using a homemade fluorescence spectrometer equipped with an ultrafast pulsed infrared laser (width ∼10 ns and λ = 1.54 μm). We used 8-anilino-1-naphthalensulfonic acid ammonium salt (ANS) free in water as a fluorescent probe to monitor the conformation changes of the PNIPAM chains. Our results have revealed that in the dilute regime there exists two characteristic transition times that are less than 1 ms, namely, τ(pearls) (∼0.02 ms), which can be attributed to the nucleation and initial growth of some "pearls" (locally contracting segments) on the chain, and τ(coarsening) (∼0.2 ms), which is related to the merging and coarsening of the pearls. At semidilute regime, an extra (third) process (τ ∼ 1.5 ms) appeared, and the contribution from the third process increased with PNIPAM concentration, which can presumably be related to the process of disentanglement of the polymer. A new method to detect the inhomogeneities of the polymer chain segments has been proposed.

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