Gelation, Phase Separation, and Fibril Formation in Aqueous Hydroxypropylmethylcellulose Solutions
Gelation, Phase Separation, and Fibril Formation in Aqueous Hydroxypropylmethylcellulose Solutions
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DOI：
10.1021/acs.biomac.7b01611
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发表时间：
2018-03-01
影响因子：
6.2
通讯作者：
Sammler, Robert L.
中科院分区：
文献类型：
--
作者：
Lodge, Timothy P.;Maxwell, Amanda L.;Sammler, Robert L.
通过流变学、比浊法、低温透射电子显微镜 (cryoTEM)、光散射、小角中子散射 (SANS) 和小角 X 射线散射 (SAXS) 等表征工具的强大组合，研究了羟丙基甲基纤维素 (HPMC) 样品在水溶液中的热响应行为。与现有文献一致，浓度范围为 0.3 至 3 wt% 的溶液在加热到接近 57°C 时，动态粘弹性模量 G' 和 G" 急剧下降。模量的下降伴随着浊度的突然增加。所有证据都与液-液相分离相对应，导致在聚合物耗尽的基质中产生富含聚合物的液滴。进一步加热后，模量增加，G' 超过G”，对应于凝胶化。稀溶液中的 CryoTEM 显示 HPMC 在模量增加的相同温度范围内形成原纤维。 SANS 和 SAXS 证实了在一定浓度范围内原纤维的出现，并且它们的平均直径约为 100 微米。 18纳米；因此，凝胶化可归因于样品跨越原纤维网络的形成。这些结果与密切相关且经过充分研究的甲基纤维素 (MC) 进行了详细比较。 HPMC 原纤维通常比 MC 更短、更柔韧，并且含有更多的水，并且在高温下生成的凝胶具有低得多的模量。除了原纤维结构的差异外，HPMC 和 MC 之间的主要区别在于，前者在形成原纤维和相关凝胶化之前经历液-液相分离，而后者首先形成原纤维。鉴于最近发现 MC 和 HPMC 在加热时自组装成原纤维的倾向，将这些结果及其解释与先前的文献进行了比较。
The thermoresponsive behavior of a hydroxy-propylmethylcellulose (HPMC) sample in aqueous solutions has been studied by a powerful combination of characterization tools, including rheology, turbidimetry, cryogenic transmission electron microscopy (cryoTEM), light scattering, small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAXS). Consistent with prior literature, solutions with concentrations ranging from 0.3 to 3 wt % exhibit a sharp drop in the dynamic viscoelastic moduli G' and G" upon heating near 57 degrees C. The drop in moduli is accompanied by an abrupt increase in turbidity. All the evidence is consistent with this corresponding to liquid-liquid phase separation, leading to polymer-rich droplets in a polymer-depleted matrix. Upon further heating, the moduli increase, and G' exceeds G", corresponding to gelation. CryoTEM in dilute solutions reveals that HPMC forms fibrils at the same temperature range where the moduli increase. SANS and SAXS confirm the appearance of fibrils over a range of concentration, and that their average diameter is ca. 18 nm; thus gelation is attributable to formation of a sample spanning network of fibrils. These results are compared in detail with the closely related and well-studied methylcellulose (MC). The HPMC fibrils are generally shorter, more flexible, and contain more water than with MC, and the resulting gel at high temperatures has a much lower modulus. In addition to the differences in fibril structure, the key distinction between HPMC and MC is that the former undergoes liquid-liquid phase separation prior to forming fibrils and associated gelation, whereas the latter forms fibrils first. These results and their interpretation are compared with the prior literature, in light of the relatively recent discovery of the propensity of MC and HPMC to self-assemble into fibrils on heating.