NMR Study of Poly(N-isopropylacrylamide) Gels Near Phase Transition


NMR studies have been made on the nonionic and ionic poly(N-isopropylacrylamide) gels near their phase transition. The gels undergo continuous second-order volume phase transition or discontinuous first-order transitions in water in response to temperature changes. The results are compared with the NMR data on the nonionic and ionic poly(N-isopropylacrylamide) polymers, which undergo a phase separation in water. From the analysis of the proton NMR line shapes and relaxation times, we have characterized the environments and motions of the side groups and the backbone polymer chains in terms of molecular dynamic parameters and structures. Some of the results are as follows: (1) The phase transition was characterized by the emergence of a new broad peak in the collapsed state and the disappearance of high-resolution spectra observed in the swollen state. The relaxation behavior of the broad peak characterizes the collapsed state as a solidlike aggregate. (2) The ionic gels consist of two portions, stiff and rather mobile portions. The nonionic gel has a homogeneous structure. (3) There is no appreciable change in the motions of the methylene and methyne protons in the backbone chains throughout the discontinuous phase transition. They remain mobile. (4) The methyl proton in the N-isopropyl group has a large mobility in the swollen state, however, the mobility decreases substantially upon collapse of the gel. It continues to decrease with rising temperature in the collapsed state, and eventually the immobilized portion becomes a significant part of the broad peak.



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© 1991 American Chemical Society (ACS), All rights reserved.

Publication Date

01 Jan 1991