Structural evolution in poly(acrylic-co-acrylamide) pH-responsive hydrogels by low-field NMR

Polyacrylate hydrogels are widely used in various aspects in daily life and industrial application. Therefore, the research on its structure is also a very important topic. A poly(acrylic-co-acrylamide) hydrogel synthesized by solution polymerization demonstrated two-step pH-responsive water absorption, which experienced two dramatical increments for pH 3-5 and 10-12. The structure was identified by Fourier transformed infrared spectrum (FT-IR), thermal analysis (TA) and pyrolysis gas-chromatography-mass spectrum. The relationship between structural evolution and pH-responsive absorption of the copolymer, which was investigated by using a low field nuclear magnetic resonance (LF-NMR), indicated that the absorption water in the copolymer could be categorized into three types: bound, semi-bound, and free water. With increasing pH, a change in the contents of these absorption water types with the structural evolution of the three-dimensional network was induced by mesh expansion and emergence of internal forces. The results provided insights into the structural design and control of various pH-responsive hydrogels.