2H Solid-State NMR Analysis of the Dynamics and Organization of Water in Hydrated Chitosan
作者：Fenfen Wang, Rongchun Zhang, Tiehong Chen, Pingchuan Sun*
關(guān)鍵字：water state, hydrated chitosan films, mobility, 2H solid-state NMR, variable-temperature
論文來源：期刊
具體來源：Polymers 2016, 8, 149-160
發(fā)表時(shí)間：2016年
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.