Department of Chemical and Biological Engineering, University of Wisconsin-Madison , 1415 Engineering Drive, Madison, Wisconsin 53706, United States.
Langmuir. 2014 Dec 16;30(49):14944-53. doi: 10.1021/la501596b. Epub 2014 Jun 24.
We report a study of the wetting and ordering of thermotropic liquid crystal (LC) droplets that are trapped (or "caged") within micrometer-sized cationic polymeric microcapsules dispersed in aqueous solutions of surfactants. When they were initially dispersed in water, we observed caged, nearly spherical droplets of E7, a nematic LC mixture, to occupy ∼40% of the interior volume of the polymeric capsules [diameter of 6.7 ± 0.3 μm, formed via covalent layer-by-layer assembly of branched polyethylenimine and poly(2-vinyl-4,4-dimethylazlactone)] and to contact the interior surface of the capsule wall at an angle of ∼157 ± 11°. The internal ordering of LC within the droplets corresponded to the so-called bipolar configuration (distorted by contact with the capsule walls). While the effects of dodecyltrimethylammonium bromide (DTAB) and sodium dodecyl sulfate (SDS) on the internal ordering of "free" LC droplets are similar, we observed the two surfactants to trigger strikingly different wetting and configurational transitions when LC droplets were caged within polymeric capsules. Specifically, upon addition of SDS to the aqueous phase, we observed the contact angles (θ) of caged LC on the interior surface of the capsule to decrease, resulting in a progression of complex droplet shapes, including lenses (θ ≈ 130 ± 10°), hemispheres (θ ≈ 89 ± 5°), and concave hemispheres (θ < 85°). The wetting transitions induced by SDS also resulted in changes in the internal ordering of the LC to yield states topologically equivalent to axial and radial configurations. Although topologically equivalent to free droplets, the contributions that surface anchoring, LC elasticity, and topological defects make to the free energy of caged LC droplets differ from those of free droplets. Overall, these results and others reported herein lead us to conclude that caged LC droplets offer a platform for new designs of LC-droplet-based responsive soft matter that cannot be realized in dispersions of free droplets.
我们报告了一项关于热致液晶 (LC) 液滴在被捕获（或“笼困”）于微米级阳离子聚合物微胶囊内时的润湿和有序化的研究，这些微胶囊分散在含有表面活性剂的水溶液中。当它们最初分散在水中时，我们观察到被笼困的近球形 E7 各向异性 LC 液滴占据了聚合物胶囊内部体积的约 40%[直径为 6.7 ± 0.3 μm，通过支化聚乙烯亚胺和聚（2-乙烯基-4,4-二甲基恶唑啉）的共价层层组装形成]，并以约 157 ± 11°的角度与胶囊壁的内表面接触。液滴内 LC 的内部有序性对应于所谓的双极构象（由于与胶囊壁接触而变形）。虽然十二烷基三甲基溴化铵 (DTAB) 和十二烷基硫酸钠 (SDS) 对“自由”LC 液滴的内部有序性的影响相似，但我们观察到两种表面活性剂在 LC 液滴被笼困在聚合物胶囊内时，会引发截然不同的润湿和构象转变。具体来说，当 SDS 被添加到水相时，我们观察到被笼困的 LC 在胶囊内表面的接触角（θ）减小，导致复杂的液滴形状的进展，包括透镜（θ≈130 ± 10°）、半球（θ≈89 ± 5°）和凹半球（θ<85°）。SDS 引起的润湿转变也导致 LC 的内部有序性发生变化，产生拓扑等效于轴向和径向构象的状态。尽管在拓扑上与自由液滴等效，但表面锚定、LC 弹性和拓扑缺陷对笼困 LC 液滴的自由能的贡献与自由液滴不同。总的来说，这些结果和本文报告的其他结果使我们得出结论，笼困 LC 液滴为基于 LC 液滴的响应性软物质的新设计提供了一个平台，而这些设计在自由液滴分散体中是无法实现的。
