Probing conformational transitions of polymer chains by microrheology

We report the first quantitative demonstration of the use of particle tracking microrheology (PTμR) in probing the pH-induced coil–globule transition (CGT) of poly-2-vinylpyridine (P2VP). The calculated radii R of the P2VP coils and globules are found to vary with molecular weight M respectively according to: R∝M0.592 ± 0.006 and R∝M0.339 ± 0.010, in excellent agreement with the classical Flory–de Gennes theory. The sphere-like small-angle X-ray scattering spectrum and the sharp rise in pyrene emission suggest that the observed CGT is likely triggered by drastic changes in the chainʹs hydrophobicity, sensitive to the degree of protonation near the coil–globule transition point. A more in-depth theoretical analysis further reveals that the phenomenon strongly depends on interplays between excluded-volume, electrostatic, and hydrophobic interactions at the subunit level of a P2VP chain. This new application of PTμR could have potential uses in exploring the physics of a variety of polymer systems at the nano/molecular scales.