Thermoreversible gelation of aqueous methylcellulose solutions
Thermoreversible gelation of aqueous methylcellulose solutions
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DOI：
10.1021/ma990242n
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发表时间：
1999-10-19
期刊：
影响因子：
5.5
通讯作者：
Lodge, TP
中科院分区：
文献类型：
--
作者：
Kobayashi, K;Huang, CI;Lodge, TP
用静态和动态光散射(DLS)、小角中子散射(SANS)和流变学研究了甲基纤维素在水溶液中的热可逆凝胶化过程。在20℃时，稀溶液光散射确定聚合物的相对分子质量、第二维里系数、旋转半径和流体动力学半径。半稀溶液在DLS中表现出两种弛豫模式，一种反映合作扩散，另一种归因于PreGel团簇。在这种状态下的流变学测量也表明了弱的超分子缔合。半稀溶液的凝胶化过程分两个阶段进行，温度升高到20摄氏度以上，这与以前的报道一致。第一个阶段是由疏水缔合驱动的链的聚集，并延伸到大约50℃。这个过程伴随着低频动态弹性模数G‘的增加，以及光和中子散射强度的增加。这些溶液的DLS性质和散射强度的角度依赖性不受这种缔合的很大影响。凝胶化的第二阶段在约50℃以上相当突然地发生，并归因于伴随凝胶化的相分离。弹性模数随温度迅速增加，样品变得明显混浊，散射强度在很宽的散射波矢量Q范围内显著增加。相反，稀溶液不会凝聚，但在高温区域提供了明显的聚集证据，这与跨越相界是一致的。凝胶态的SANS结构因子S(Q)可用两项之和很好地描述，对应于两个幂函数区。在前凝胶和凝胶状态下，较低的a遵循S(Q)类似于Q(-1.8)的状态，与介于良好和Theta溶剂条件之间的链一致。当Q较高时，胶凝后的指数从-2.5左右演化到-4左右。后者的指数表明凝胶结构和中间流体之间有一个尖锐的边界，这与凝胶阻止的液-液相分离一致。
The thermoreversible gelation of methylcellulose in aqueous solution has been studied by static and dynamic light scattering (DLS), small-angle neutron scattering (SANS), and rheology. At 20 degrees C, dilute solution light scattering establishes the molecular weight, second virial coefficient, radius of gyration, and hydrodynamic radius of the polymer. Semidilute solutions exhibit two relaxation modes in DLS, one reflecting cooperative diffusion and the other attributable to pregel clusters. Rheological measurements in this regime also suggest a weak supermolecular association. The gelation of semidilute solutions proceeds in two stages with increasing temperature above 20 degrees C, consistent with previous reports. The first stage is attributable to clustering of chains, driven by hydrophobic association, and extends up to approximately 50 degrees C. This process is accompanied by an increase in the low-frequency dynamic elastic modulus, G', and an increase in both light and neutron scattered intensity. The DLS properties of these solutions, and the angular dependence of the scattered intensity, is not greatly affected by this association The second stage of gelation occurs rather abruptly above ca. 50 degrees C and is attributed to phase separation accompanied by gelation. The elastic modulus increases rapidly with temperature, the samples become visibly turbid, and the scattered intensity increases markedly over a wide range of scattering wavevector, q. In contrast, dilute solutions do not gel but give clear evidence of aggregation in the high-temperature regime, consistent with crossing a phase boundary. The SANS structure factor S(q) in the gel state is well described by a sum of two terms, corresponding to two power-law regimes. The lower a regime follows S(q) similar to q(-1.8) in both the pregel and gel states, consistent with chains intermediate between good and Theta solvent conditions. AL higher q the exponent evolves from ca. -2.5 to -4 upon gelation. The latter exponent indicates a sharp boundary between the gel structure and the intervening fluid, consistent with liquid-liquid phase separation that is arrested by gelation.