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Polymers 2016, 8(4), 149;

2H Solid-State NMR Analysis of the Dynamics and Organization of Water in Hydrated Chitosan

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
Biophysics and Department of Chemistry, University of Michigan, Ann Arbor, MI 48109-1055, USA
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China
Author to whom correspondence should be addressed.
Academic Editor: Jianxun Ding
Received: 25 February 2016 / Revised: 7 April 2016 / Accepted: 12 April 2016 / Published: 19 April 2016
(This article belongs to the Special Issue Functional Polymers for Medical Applications)
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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. View Full-Text
Keywords: water state; hydrated chitosan films; mobility; 2H solid-state NMR; variable-temperature water state; hydrated chitosan films; mobility; 2H solid-state NMR; variable-temperature

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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).

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Wang, F.; Zhang, R.; Chen, T.; Sun, P. 2H Solid-State NMR Analysis of the Dynamics and Organization of Water in Hydrated Chitosan. Polymers 2016, 8, 149.

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