固体NMR研究PAA/PEO共混物中氢键相互作用与结构演化

分子间相互作用是决定材料结构和性能的关键因素之一,而如何在分子水上实现对复杂相互作用分子的检测仍然是一个挑战性课题。本工作首先在不同p H值条下以聚丙烯酸/聚环氧乙烷(PAA/PEO)的混合水溶液制备了系列的固体薄膜,然后采用多种基于连续相调制多脉冲技术的一维和二维~1H多脉冲去耦(CRAMPS)固体NMR新技术,并结合高分辨~(13)C交叉极化魔角旋转(CPMAS)、~(23)Na多量子(MQ)等多核固体NMR实验,对PAA/PEO聚合物共混物的微观结构和动力学进行了原位和系统的研究。通过不同类型的~1H高分辨CRAMPS实验检测到共混物中包含多种不同类型质子:通过氢键相互作用形成二聚体的COOH基团、自由COOH基团、与水结合的COOH基团和主链基团。随着p H值的升高,除主链质子外,大部分其它区域的信号都明显降低,这是由于PAA与PEO以及水的氢键作用减弱所致。这些CRAMPS NMR技术也被用来阐明不同p H值制备的样品中不同基团的分子运动性。此外,二维~1H-~1H自旋交换NMR实验提供了关于聚合物PAA与PEO大分子链间、以及水与聚合物的相互作用。~1H自旋扩散实验表明,在这些共混物中明显存在相微观相分离的结构,并且测定的分散相区尺寸约为17 nm。~(23)Na MQMAS实验揭示了在共混物中存在两种类型~(23)Na位,一种是自由的钠离子,另一种是与大分子相互作用的Na离子。特别是通过~1H-检测的~(23)Na-~1H CPMAS实验揭示了Na~+离子的位置远离PEO而与PAA临近。上述这些SSNMR实验结果在分子水平上提供了氢键相互作用对PAA/PEO共混物微观结构和动力学影响的详细信息,可以获得不同p H值对PAA与PEO的氢键作用、相容性、微观结构、水-聚合物相互作用和不同组分分子运动性的影响。在上述核磁共振研究的基础上,我们提出了一种新的PAA/PEO共混物的结构模型,该模型首次成功地揭示了不同的p H值对PAA/PEO共混物中微观结构和动力学的影响。本工作清楚地表明,固态核磁共振是在分子水平上研究具有复杂相互作用的多相聚合物材料的有力工具。本文的研究工作对于探索检测聚合物弱相互作用的新方法和发展基于氢键相互作用的聚合物新材料的开发具有重要意义。

Solid-State NMR Studies on Hydrogen Bonding Interactions and Structural Evolution in PAA/PEO Blends

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.