NMR studies of water dynamics during sol-to-gel transition of poly (N-isopropylacrylamide) in concentrated aqueous solution

Authors: KAMETANI, SHUNSUKE - Mitsui Chemicals, Inc; SEKINE, SOKEI - Mitsui Chemicals, Inc; OHKUBO, TAKAHIRO - Chiba University; HIRANO, TOMOHIRO - University Of Tokushima; UTE, KOICHI - University Of Tokushima; Cheng, Huai; ASAKURA, TETSUO - Tokyo University Of Agriculture & Technology

Submitted to: Polymer
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/25/2016
Publication Date: 1/27/2017
Citation: Kametani, S., Sekine, S., Ohkubo, T., Hirano, T., Ute, K., Cheng, H.N., Asakura, T. 2017. NMR studies of water dynamics during sol-to-gel transition of poly (N-isopropylacrylamide) in concentrated aqueous solution. Polymer. 109:287-296.

Interpretive Summary: Stimuli-response polymers are among the most popular research topics today. These are the materials that can change their physical state, form, or attribute in response to a small environmental change (such as temperature, humidity, pH, or electromagnetic field). A well-known stimuli-responsive polymer is poly(N-isopropylacrylamide) (PNIPAM), which, in an aqueous solution, changes from a liquid to a gel at 34°C, near the temperature of the human body. This property enables PNIPAM to be used in many applications, e.g., controlled drug delivery, medical diagnostics, tissue engineering, sensors, and gel actuators. Yet, the exact role of water in gelation is not fully known. In this work, PNIPAM was dissolved in deuterated water, and 2H NMR was used to obtain T1-T2 NMR relaxation maps, which permitted a detailed study of the liquid-to-gel transition from the viewpoint of the water. A working model of the PNIPAM-water interactions was devised on the basis of available data. The new information should provide greater flexibility in formulating new or improved products involving PNIPAM. This NMR technique should also be useful in studying other water-based colloids and gels.

Technical Abstract: The thermo-sensitive polymer, poly(N-isopropylacrylamide) (PNIPAM) undergoes a coil-to-globule transition in an aqueous solution as the temperature is raised through the lower critical solution temperature. Thus far, little is known about the dynamical states of the water molecules that contribute to the transition. In this paper, we applied NMR methods, especially 2H T1-T2 two-dimensional relaxation spectroscopy coupled with inverse Laplace transform, to study the dynamics of water molecules during the transition. Below 34°C, fast exchange among free water and water molecules adsorbed on the surface of PNIPAM molecules was observed in the 2H T1-T2 NMR map. At 34°C, aggregation of PNIPAM molecules occurred suddenly and most of the water molecules became trapped in the PNIPAM aggregates, where discontinuous states of water molecules with different dynamics were found. Above 34°C, PNIPAM molecules aggregated further and formed a gel network; the free bulk water became dominant at this stage. These observations were almost the same for three PNIPAM samples with different tacticities and molecular weights, although small differences in the discontinuous dynamical states of water could be discerned among these samples. On the basis of NMR and rheology data, we proposed a model where water molecules interact with PNIPAM in different ways during the transition. Our experimental approach, which included NMR studies of water dynamics, provided new information and fresh perspectives on the coil-to-globule transition of PNIPAM.