Tsunashima Yoshisuke, Kawanishi Hiroyuki, Horii Fumitaka
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan.
Biomacromolecules. 2002 Nov-Dec;3(6):1276-85. doi: 10.1021/bm0200682.
Dynamics of cellulose diacetate (CDA, the total degree of substitution (TDS) = 2.44) in dimethylacetamide (DMAc) in dilute solution was investigated at 2, 10, 20, 30, 40, 49.7, and 61.5 degrees C through dynamic light scattering in the quiescent state. The following three facts were made clear. First, CDA existed in three types of structures in the polar solvent, DMAc; one is a single CDA chain, and the others are dynamic structures, or self-assemblies, which were formed temporarily and locally by the solvent-mediated hydrogen bonding between the intermolecular C-6 position hydroxyls of the anhydroglucose units in the CDA backbone. Second, CDA showed a nature of low-temperature solubility in DMAc, that is, CDA is expected to dissolve molecularly below -12 degrees C but to take a phase separation above 65 degrees C, where two structures such as collapses of a single CDA chain and an aggregate appear. Third, a reorganization in the dynamic structures was detected at the temperature T = 33.8 degrees C. At this temperature, two dynamic structures showed the discontinuity in their correlation lengths, whereas the single CDA realized an uncorrelated chain state in the dynamical sense. In view of the low-temperature solubility of CDA in DMAc, this abnormal behavior around T was explained by dynamical critical-like fluctuations if T were treated as a kind of lower critical solution temperature (LCST) in the CDA/DMAc system. Here, the self-assemblies arise as the dynamical fluctuations under the spinodal decomposition situation and the competition between the hydrogen bonding (HB) and the hydrophobic interaction (HPhI) makes the conformation of CDA chains change drastically. In this scheme, the solvent-mediated HB and HPhI play important roles in the structure reorganization of cellulose derivatives in strong electronegative solvents, though HB and HPhI cooperate with the inherent chain helicality.
通过静态条件下的动态光散射，研究了二醋酸纤维素（CDA，总取代度（TDS）=2.44）在二甲基乙酰胺（DMAc）稀溶液中于2、10、20、30、40、49.7和61.5摄氏度时的动力学。明确了以下三个事实。首先，CDA在极性溶剂DMAc中以三种结构形式存在；一种是单个CDA链，另外两种是动态结构或自组装体，它们是由CDA主链中脱水葡萄糖单元的分子间C-6位羟基之间通过溶剂介导的氢键临时且局部形成的。其次，CDA在DMAc中表现出低温溶解性，即预计CDA在低于-12摄氏度时分子溶解，但在高于65摄氏度时会发生相分离，此时会出现单个CDA链的塌缩和聚集体等两种结构。第三，在温度T = 33.8摄氏度时检测到动态结构的重组。在此温度下，两种动态结构的相关长度出现不连续，而单个CDA链在动力学意义上实现了不相关的链状态。鉴于CDA在DMAc中的低温溶解性，如果将T视为CDA/DMAc体系中的一种下临界溶液温度（LCST），则T附近的这种异常行为可以用类似动力学临界的涨落来解释。在此，自组装体是在亚稳分解情况下作为动力学涨落出现的，氢键（HB）和疏水相互作用（HPhI）之间的竞争使CDA链的构象发生剧烈变化。在该体系中，溶剂介导的HB和HPhI在强负电性溶剂中纤维素衍生物的结构重组中起重要作用，尽管HB和HPhI与固有链螺旋性相互配合。
