Abstract:

Intermolecular interactions are the key to control the final structure and properties of polymers; however, molecular-level detection of complex interactions remains a challenge. In this study, a series of poly(acrylic acid)/poly(ethylene oxide) (PAA/PEO) solid films were prepared from aqueous solutions at different pHs. Multinuclear solid-state NMR (SSNMR) experiments, including one- and two-dimensional (1D and 2D) ¹H CRAMPS (Combined Rotation And Multiple Pulse NMR Spectroscopy) based on the continuous phase modulation technique, high-resolution ¹³C CPMAS (Cross-Polarization and Magic-Angle Spining), and ²³Na MQMAS (Multiple-Quantum MAS) experiments, were used to this in situ investigation of the structure and dynamics of these polymer blends. The ¹H CRAMPS experiments revealed different types of protons in the blends from the mutually hydrogen-bonded COOH groups, from the free COOH groups, the COOH groups bounded with water that undergo fast chemical exchange mutually, and the COOH groups interacting with PEO and from main chain groups. With increasing pH, most of these peaks decreased except for the main chain protons owing to the decrease in the hydrogen bonding interaction among PAA and PEO as well as water. These CRAMPS NMR techniques were also used to elucidate the molecular mobility of the different groups. Furthermore, 2D ¹H-¹H spin-exchange NMR experiments provided more detailed information about the interpolymer and water–polymer interactions. ¹H spin-diffusion experiments indicated the presence of phase separation in these blends, and the determined domain size of the mobile phase was approximately 17 nm. Two types of ²³Na sites were revealed by MQMAS experiment; in particular, the Na⁺ ionic location and interaction between individual polymers was revealed by ¹H detected ²³Na-¹H CP experiments, which showed that ²³Na is in the proximity of PAA instead of PEO. These SSNMR experimental results provide detailed information about the influence of hydrogen bonding interactions on the microcosmic structure and dynamics of PAA/PEO blends at the molecular level. The influence of different pH levels on the hydrogen bonding interactions, miscibility between PAA and PEO, microstructure, water–polymer interactions, and molecule mobility of individual compositions was clarified. Based on the above-mentioned NMR studies, we proposed a novel structural model of these PAA/PEO blends. This model successfully revealed the influence of pH on the microstructure and dynamics of PAA/PEO blends at the molecular level for the first time. Our results indicate that solid-state NMR is a powerful tool that can be used to study the complex interactions of multiphase polymer materials. Our research is of great significant to both the development of new methods to probe the weak interactions in polymers and the development of new polymer materials based on hydrogen bonding interactions.

Key words: PAA, PEO, Polymer blends, Hydrogen bonding interaction, Molecular motion, Solid-state NMR