Lue Ang, Zhang Lina
Department of Chemistry, Wuhan University, Wuhan 430072, China.
J Phys Chem B. 2008 Apr 17;112(15):4488-95. doi: 10.1021/jp077685a. Epub 2008 Mar 27.
Cellulose was dissolved rapidly in a 9.5 wt % NaOH/4.5 wt % thiourea aqueous solution pre-cooled to -5 degrees C to prepare its concentrated solution, in which inclusion complexes (ICs) associated with cellulose, NaOH, thiourea, and water clusters were created. Physical gels could form in the cellulose solution at either high temperature or after long storage time, because of aggregation between the ICs. To clarify whether the Winter and Chambon theory could describe the gelation process of this complex system, we have investigated carefully the viscoelastic behavior of the cellulose solution with the advanced rheological expanded system (ARES). In the temperature range from 10 to 25 degrees C, we have successfully measured the loss tangent (tan delta) at the gel point according to the Winter and Chambon theory, showing the independence of tan delta on the frequency for the cellulose solution. The exponents of the scaling laws eta 0 proportional, variant epsilon-gamma and Ge proportional, variant epsilon z for the cellulose solution at 10 degrees C before and beyond the gel point were confirmed to be in agreement with the predicted values based on the percolation theory. The high sensitivity of the cellulose solution on temperature poses a limit for the application of the scaling law for the wide temperature range. The gel formed from the cellulose solution at 30 degrees C at long storage time could undergo a transition to a transparent liquid state after stirring at -5 degrees C. At the same time, the loss modulus (G' ') exceeds the storage modulus (G'), indicating a partially reversible sol-gel transition, as a result of the reconstruction of the hydrogen-bond networks between the solvent and cellulose.
将纤维素迅速溶解于预先冷却至-5℃的9.5 wt%氢氧化钠/4.5 wt%硫脲水溶液中，以制备其浓溶液，其中形成了与纤维素、氢氧化钠、硫脲和水簇相关的包合物（ICs）。由于ICs之间的聚集，物理凝胶可在高温下或长时间储存后在纤维素溶液中形成。为了阐明Winter和Chambon理论是否能描述该复杂体系的凝胶化过程，我们使用先进流变扩展系统（ARES）仔细研究了纤维素溶液的粘弹性行为。在10至25℃的温度范围内，我们根据Winter和Chambon理论成功测量了凝胶点处的损耗角正切（tanδ），结果表明纤维素溶液的tanδ与频率无关。确认了在凝胶点之前和之后10℃下纤维素溶液的标度律η0∝ε-γ和Ge∝εz的指数与基于渗流理论的预测值一致。纤维素溶液对温度的高敏感性限制了标度律在宽温度范围内的应用。在30℃下长时间储存的纤维素溶液形成的凝胶在-5℃搅拌后可转变为透明液态。同时，损耗模量（G''）超过储能模量（G'），这表明由于溶剂与纤维素之间氢键网络的重构，发生了部分可逆的溶胶-凝胶转变。
