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

Abstract

The focus of this work was on the significant changes in the water dynamics of aqueous poly(N-isopropylacrylamide) (PNIPAM) solution during sol-to-gel transition. Through the use of NMR (particularly two-dimensional 2 H NMR T 1-T 2 relaxation) and rheology, we were able to show that below 34 o C fast exchange occurs among free water and water molecules adsorbed on the surface of PNIPAM molecules. At 34 o C, PNIPAM becomes aggregated; most of the water molecules are trapped in the PNIPAM aggregates, where water molecules with different dynamics are found. Above 34 o C, PNIPAM molecules aggregate further to form a gel network; the free bulk water then becomes dominant at this stage. On the basis of these observations, a model where water molecules interact with PNIPAM in different ways during the transition was proposed. We believe that our experimental approach provides new information and fresh perspectives on the sol-to-gel transition of PNIPAM.

Probing the Structural Dynamics of the Coil–Globule Transition of Thermosensitive Nanocomposite Hydrogels

“…36 Additionally, the water molecules mainly exist as free bulk water, presenting similar or slightly increasing mobility with temperature. 30,32 PN-1% SiO 2 showed a higher T 2L value at 34 °C since this temperature is below its T cp and the mobility of water can increase with temperature before reaching the T cp value at 35 °C and acquire a restricted mobility. 30 However, T 2L values reduced at 37 and 40 °C, indicating that the presence of water molecules with confined mobility probably absorbed in the collapsed state of this nanocomposite.…”

“…This second T 2 value arose from water molecules trapped in polymer globules with spatially restricted mobility, which are still present in higher temperatures for PNVCL systems. 32,46 Kametani et al 32 and Watanabe et al 31 also observed two or even more T 2 values for water molecules with different dynamics due to the presence of heterogeneities of the thermosensitive polymer systems such as poly(N-isopropylacrylamide) and polypoly(asparagine) derivative, respectively.…”

“…NMR spectroscopy has demonstrated to be a useful technique to provide information about the molecular motion of thermosensitive polymers in different solvents, medium conditions, and temperature ranges . Through the employment of proton relaxation methods and the obtainment of spin–lattice relaxation time ( T 1 ) and spin–spin relaxation time ( T 2 ), it is possible to monitor the changes of polymer dynamics and the mobility of water molecules during the sol–gel transition, as well as in distinct heterogeneous structures. , While T 1 is especially related to local segmental motions, T 2 is described by large-scale motions . For example, by investigating the T 2 value of PNVCL microgels with different amounts of cross-linker, Balaceanu et al identified a bimodal distribution of molecular dynamics inside the microgels, indicating that the materials had a heterogeneous core–corona internal structure.…”

Antibacterial polymeric nanocomposites synthesized by in-situ photoreduction of silver ions without additives inside biocompatible hydrogel matrices based on N-isopropylacrylamide and derivatives

“…It can be noticed that the typical signals of hydrogels and corresponding nanocomposite after irradiation are the same. So the carbon main chain is not chemically affected during the photoreduction process of Ag + ions [30,31].…”

Seeking Solvation: Exploring the Role of Protein Hydration in Silk Gelation

“…In this respect, flow-induced fibril formation has recently been observed in various other proteins and peptides [231], with obvious implications for various prion diseases. Indeed, similar effects may also occur with a wider range of water-soluble polymers, such as pNiPAm [232,233].…”