CAS Key Laboratory of Engineering Plastics, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190 P. R. China.
J Phys Chem B. 2010 May 13;114(18):6006-13. doi: 10.1021/jp1013863.
The phase transition and rheological behaviors of concentrated solutions of microcrystalline cellulose (MCC) in an ionic liquid of 1-allyl-3-methylimidazolium chloride (AMIMCl) have been investigated. Polarized optical microscopy (POM) measurements indicate that the two critical cellulose concentrations for the appearance of biphase and fully anisotropic phase for MCC/AMIMCl solutions are 9 and 16 wt%, respectively. POM and differential scanning calorimetry (DSC) measurements coherently indicate that the clearing temperature, T(c) increases with increasing cellulose concentration. Oscillatory shear measurements show that the crossover frequency first moves to lower values and then moves back to higher values with increasing cellulose concentration, which indicates that most cellulose chains are aligned or oriented to reduce chain entanglements when the cellulose concentration is above 14 wt%. From the steady shear measurements, it is surprising to find that the viscosity versus shear rate curves exhibit four flow regions including two plateaus and two shear-thinning regions when the cellulose concentration exceeds 9 wt%. The influences of cellulose concentration and temperature on the first normal stress differences (N(1)) are analyzed according to the Larson theory. The peak of N(1) always appears at the intermediate part of the first shear-thinning region, and the following minimum of N(1) appears at the onset of the second shear-thinning region. The viscosity versus shear rate curves only exhibit two flow regions when temperature is above the respective T(c); meanwhile, negative N(1) values disappear and N(1) increases monotonically. The above results suggest that melting of the liquid crystal domains at high temperature results in the disappearance of the second plateau for the viscosity versus shear rate curves.
已研究了微晶纤维素 (MCC) 在 1-烯丙基-3-甲基咪唑氯化物 (AMIMCl) 离子液体中的浓溶液的相转变和流变行为。偏光显微镜 (POM) 测量表明，MCC/AMIMCl 溶液出现双相和完全各向异性相的两个关键纤维素浓度分别为 9wt%和 16wt%。POM 和差示扫描量热法 (DSC) 测量一致表明，清亮点温度 T(c) 随纤维素浓度的增加而升高。振荡剪切测量表明，随着纤维素浓度的增加，交叉频率先移至较低值，然后再移至较高值，这表明当纤维素浓度高于 14wt%时，大多数纤维素链被排列或定向以减少链缠结。从稳态剪切测量中令人惊讶的是，当纤维素浓度超过 9wt%时，粘度与剪切速率曲线表现出四个流动区域，包括两个平台和两个剪切变稀区域。根据拉森理论分析了纤维素浓度和温度对第一法向应力差 (N(1)) 的影响。N(1)的峰值总是出现在第一剪切变稀区域的中间部分，随后 N(1)的最小值出现在第二剪切变稀区域的开始部分。当温度高于各自的 T(c)时，粘度与剪切速率曲线仅表现出两个流动区域；同时，负 N(1)值消失，N(1)单调增加。上述结果表明，高温下液晶域的熔融导致粘度与剪切速率曲线的第二个平台消失。
