Wang Fenfen, Zhang Rongchun, Chen Tiehong, Sun Pingchuan
Key Laboratory of Functional Polymer Materials of Ministry of Education and College of Chemistry; Nankai University, Tianjin 300071, China.
State Key Laboratory of Medicinal Chemical Biology, Nankai University, Tianjin 300071, China.
Polymers (Basel). 2016 Apr 19;8(4):149. doi: 10.3390/polym8040149.
Understanding water⁻biopolymer interactions, which strongly affect the function and properties of biopolymer-based tissue engineering and drug delivery materials, remains a challenge. Chitosan, which is an important biopolymer for the construction of artificial tissue grafts and for drug delivery, has attracted extensive attention in recent decades, where neutralization with an alkali solution can substantially enhance the final properties of chitosan films cast from an acidic solution. In this work, to elucidate the effect of water on the properties of chitosan films, we investigated the dynamics and different states of water in non-neutralized (CTS-A) and neutralized (CTS-N) hydrated chitosan by mobility selective variable-temperature (VT) ²H solid-state NMR spectroscopy. Four distinct types of water exist in all of the samples with regards to dynamic behavior. First, non-freezable, rigid and strongly bound water was found in the crystalline domain at low temperatures. The second component consists of weakly bound water, which is highly mobile and exhibits isotropic motion, even below 260 K. Another type of water undergoes well-defined 180° flips around their bisector axis. Moreover, free water is also present in the films. For the CTS-A sample in particular, another special water species were bounded to acetic acid molecules via strong hydrogen bonding. In the case of CTS-N, the onset of motions of the weakly bound water molecules at 260 K was revealed by ²H-NMR spectroscopy. This water is not crystalline, even below 260 K, which is also the major contribution to the flexibility of chitosan chains and thus toughness of materials. By contrast, such motion was not observed in CTS-A. On the basis of the ²H solid-state NMR results, it is concluded that the unique toughness of CTS-N mainly originates from the weakly bound water as well as the interactions between water and the chitosan chains.
理解水与生物聚合物之间的相互作用仍然是一项挑战，这种相互作用会强烈影响基于生物聚合物的组织工程和药物递送材料的功能及性质。壳聚糖是用于构建人工组织移植物和药物递送的重要生物聚合物，近几十年来受到了广泛关注，用碱溶液中和可显著提高由酸性溶液浇铸而成的壳聚糖膜的最终性能。在这项工作中，为了阐明水对壳聚糖膜性能的影响，我们通过迁移率选择性变温（VT）²H 固态核磁共振光谱研究了未中和（CTS-A）和中和（CTS-N）的水合壳聚糖中水的动力学和不同状态。就动力学行为而言，所有样品中存在四种不同类型的水。首先，在低温下的结晶区域中发现了不可冻结、刚性且结合紧密的水。第二种成分是弱结合水，其流动性很高，即使在 260 K 以下也表现出各向同性运动。另一种类型的水围绕其平分线轴进行明确的 180°翻转。此外，膜中还存在自由水。特别是对于 CTS-A 样品，另一种特殊的水物种通过强氢键与醋酸分子结合。在 CTS-N 的情况下，²H 核磁共振光谱揭示了弱结合水分子在 260 K 时开始运动。即使在 260 K 以下，这种水也不是结晶的，这也是壳聚糖链柔韧性以及材料韧性的主要贡献因素。相比之下，在 CTS-A 中未观察到这种运动。基于²H 固态核磁共振结果，得出结论：CTS-N 独特的韧性主要源于弱结合水以及水与壳聚糖链之间的相互作用。
