Comparison of the Coil-to-Globule and the Globule-to-Coil Transitions of a Single Poly(<i>N</i>-isopropylacrylamide) Homopolymer Chain in Water

TLDR

The coil‑to‑crumpled coil transition is described by the Birshtein and Pryamitsyn theory. We demonstrated that a single high‑molar‑mass PNIPAM chain in dilute water reversibly collapses from a coil to a globule and back, with the globule retaining ~66 % water, exhibiting hysteresis due to intrachain hydrogen bonding, and revealing intermediate crumpled‑coil and molten‑globule states, while the coil‑to‑globule transition is irreversible.

Abstract

Using a newly prepared nearly monodisperse (Mw/Mn < 1.05) high molar mass (Mw = 1.3 × 107 g/mol) poly(N-isopropylacrylamide) (PNIPAM) sample, we successfully, for the first time, made the conformation of individual PNIPAM chains change from a coil to a fully collapsed thermodynamically stable single chain globule and then back to a coil in an extremely dilute aqueous solution (∼6.7 × 10-7 g/mL). The average chain density in the globule state is ∼0.34 g/mL, close to 0.40 g/cm3 predicted on the basis of a space-filling model, indicating that the globule still contains ∼66% water even in its fully collapsed state. At a given temperature around the lower critical solution temperature, the chains are smaller in the globule-to-coil transition than in the coil-to-globule transition, revealing that the coil-to-globule transition is an irreversible process. The hysteresis can be attributed to the formation of intrachain structures, presumably the intrachain hydrogen bonding, in the globule state. We confirmed the existence of the crumpled coil and the molten globule states between the random coil and the collapsed globule states. The coil-to-crumpled coil transition can be reasonably described by the Birshtein and Pryamitsyn theory.