Macromolecules, 31 (9), 2972 -2976, 1998. ma971873p S0024-9297(97)01873-1
Web Release Date: April 14, 1998

Copyright © 1998 American Chemical Society

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

Xiaohui Wang, Xingping Qiu,and Chi Wu*

Department of Chemistry, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, and The Open Laboratory of Bond Selective Chemistry, Department of Chemical Physics, University of Science and Technology of China, HeFei, China

Received December 29, 1997

Revised Manuscript Received March 3, 1998

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.