Studies of nanocomposites of carbon nanotubes and a negative dielectric anisotropy liquid crystal.

It has been widely recognized that the combination of carbon nanotube (CNT) and liquid crystals (LCs) not only provides a useful way to align CNTs, but also dramatically enhances the order in the LC phases, which is especially useful in liquid crystal display (LCD) technology. As the measure of this phase behavior, the complex specific heat is presented over a wide temperature range for a negative dielectric anisotropy alkoxyphenylbenzoate liquid crystal (9OO4) and CNT composites as a function of CNT concentration. The calorimetric scans were performed under near-equilibrium conditions between 25 and 95 °C, first cooling and then followed by heating for CNT weight percent ranging from ϕ(w) = 0 to 0.2. All 9OO4/CNT mesophases have transition temperatures ~1 K higher and a crystallization temperature 4 K higher than that of the pure 9OO4. The crystal phase superheats until a strongly first-order specific heat feature is observed, 0.5 K higher than in the pure 9OO4. The transition enthalpy for the nanocomposite mesophases is 10% lower than that observed in the pure 9OO4. The strongly first-order crystallization and melting transition enthalpies are essentially constant over this range of ϕ(w). Complementary electroclinic measurement on a 0.05 wt. % sample, cooling towards the smectic-C phase from the smectic-A, indicates that the SmA-SmC transition remains mean-field-like in the presence of the CNTs. Given the homogeneous and random distribution of CNTs in these nanocomposites, we interpret these results as arising from the LC-CNT surface interaction pinning the orientational order uniformly along the CNT, without pinning the position of the 9OO4 molecule, leading to a net ordering effect for all phases. These effects of incorporating CNTs into LCs are likely due to "anisotropic orientational" coupling between CNT and LC, the change in the elastic properties of composites and thermal anisotropic properties of the CNTs.

人们普遍认识到，碳纳米管（CNT）和液晶（LCs）的结合不仅为碳纳米管的取向提供了一种有用的方法，而且极大地增强了液晶相的有序性，这在液晶显示（LCD）技术中尤为有用。作为这种相行为的度量，针对负介电各向异性的烷氧基苯基苯甲酸酯液晶（9OO4）和碳纳米管复合材料，在较宽的温度范围内给出了复比热容随碳纳米管浓度的变化关系。量热扫描在25到95°C之间的近平衡条件下进行，先冷却，然后加热，碳纳米管的重量百分比从φ(w)=0到0.2。所有9OO4/碳纳米管中间相的转变温度比纯9OO4高约1K，结晶温度比纯9OO4高4K。晶相过热，直到观察到强烈的一级比热容特征，比纯9OO4高0.5K。纳米复合材料中间相的转变焓比纯9OO4中观察到的低10%。在φ(w)这个范围内，强烈的一级结晶和熔化转变焓基本恒定。对0.05 wt.%的样品进行的补充电倾测量（从近晶A相冷却到近晶C相）表明，在存在碳纳米管的情况下，近晶A - 近晶C转变仍然类似于平均场转变。鉴于这些纳米复合材料中碳纳米管均匀且随机的分布，我们将这些结果解释为液晶 - 碳纳米管表面相互作用使取向有序性沿着碳纳米管均匀固定，而不固定9OO4分子的位置，从而对所有相产生净有序化效应。将碳纳米管掺入液晶中的这些效应可能是由于碳纳米管和液晶之间的“各向异性取向”耦合、复合材料弹性性质的改变以及碳纳米管的热各向异性性质。