Nanoscience Laboratory, Department of Physics, University of Trento, Via Sommarive 14, 38123, Povo (TN), Italy.
Department of Engineering and Architecture, University of Trieste, Piazzale Europa 1, 34127, Trieste, Italy.
Sci Rep. 2017 Sep 11;7(1):11129. doi: 10.1038/s41598-017-11649-4.
Sol-gel transition of carboxylated cellulose nanocrystals has been investigated using rheology, SAXS, NMR and optical spectroscopies to unveil the distinctive roles of ultrasound treatments and addition of various cations. Besides cellulose fiber fragmentation, sonication treatment induces fast gelling of the solution. The gelation is independent of the addition of cations, while the final rheological properties are highly influenced by the type, concentration and sequence of the operations since the cations must be added prior to sonication to produce stiff gels. The gel elastic modulus was found to increase proportionally to the ionic charge rather than the cationic size. In cases where ions were added after sonication, SAXS analysis of the Na hydrogel and Ca hydrogel indicated the presence of structurally ordered domains in which water is confined, and 1H-NMR investigation showed the dynamics of water exchange within the hydrogels. Conversely, separated phases containing essentially free water were characteristic of the hydrogels obtained by sonication after Ca addition, confirming that this ion induces irreversible fiber aggregation. The rheological properties of the hydrogels depend on the duration of the ultrasound treatments, enabling the design of programmed materials with tailored energy dissipation response.
羧基化纤维素纳米晶体的溶胶-凝胶转变已通过流变学、小角 X 射线散射、NMR 和光谱学研究进行了研究，以揭示超声处理和添加各种阳离子的独特作用。除了纤维素纤维的碎片化之外，超声处理还会促使溶液快速胶凝。凝胶化与添加阳离子无关，而最终的流变性能受到阳离子的类型、浓度和操作顺序的强烈影响，因为阳离子必须在超声处理之前添加才能产生刚性凝胶。凝胶弹性模量与离子电荷成正比，而与阳离子尺寸无关。在添加阳离子后进行超声处理的情况下，对 Na 水凝胶和 Ca 水凝胶的 SAXS 分析表明，在其中存在限制水的结构有序域，而 1H-NMR 研究表明水在水凝胶内的交换动力学。相反，含有基本自由水的分离相是通过在 Ca 之后进行超声处理获得的水凝胶的特征，这证实了该离子会诱导不可逆的纤维聚集。水凝胶的流变性能取决于超声处理的持续时间，从而能够设计具有定制能量耗散响应的程控材料。
