Całus Sylwia, Rau Daniel, Huber Patrick, Kityk Andriy V
Faculty of Electrical Engineering, Czestochowa University of Technology, 42-200 Czestochowa, Poland.
Phys Rev E Stat Nonlin Soft Matter Phys. 2012 Aug;86(2 Pt 1):021701. doi: 10.1103/PhysRevE.86.021701. Epub 2012 Aug 10.
We explore the nematic ordering of the rodlike liquid crystals 5CB and 6CB, embedded into parallel-aligned nanochannels in mesoporous silicon and silica membranes as a function of mean channel radius (4.7 ≤ R ≤ .3 nm), and, thus, geometrical confinement strength, by optical birefringence measurements in the infrared region. The orientational order inside the nanochannels results in an excess birefringence, which is proportional to the nematic order parameter. It evolves continuously on cooling with a precursor behavior, typical of a paranematic state at high temperatures. These observations are compared with the bulk behavior and analyzed within a phenomenological model. Such an approach indicates that the strength of the nematic ordering fields σ is beyond a critical threshold σ(c) = 1/2 that separates discontinuous from continuous paranematic-to-nematic behavior. In agreement with the predictions of the phenomenological approach, a linear dependency of σ on the inverse channel radius is found and we can infer therefrom the critical channel radii, R(c) separating continuous from discontinuous paranematic-to-isotropic behavior, for 5CB (12.1 nm) and 6CB (14.0 nm). Our analysis suggests that the tangential anchoring at the channel walls is of similar strength in mesoporous silicon and mesoporous silica membranes. A comparison with the bulk phase behavior reveals that the nematic order in nanoconfinement is significantly affected by channel wall roughness, leading to a reduction of the effective nematic ordering.
我们通过红外区域的光学双折射测量，研究了嵌入介孔硅和二氧化硅膜中平行排列纳米通道内的棒状液晶5CB和6CB的向列有序性，该有序性是平均通道半径（4.7≤R≤13nm）的函数，因此也是几何限制强度的函数。纳米通道内的取向有序导致了额外的双折射，其与向列序参量成正比。在冷却过程中，它以前驱体行为连续演化，这是高温下准向列态的典型特征。将这些观察结果与体相行为进行比较，并在一个唯象模型中进行分析。这种方法表明，向列有序场σ的强度超过了一个临界阈值σ(c)=1/2，该阈值将准向列到向列行为的不连续与连续区分开来。与唯象方法的预测一致，发现σ与通道半径的倒数呈线性依赖关系，由此我们可以推断出5CB（12.1nm）和6CB（14.0nm）的临界通道半径R(c)，它将准向列到各向同性行为的连续与不连续区分开来。我们的分析表明，介孔硅和介孔二氧化硅膜中通道壁处的切向锚定强度相似。与体相行为的比较表明，纳米限域中的向列序受到通道壁粗糙度的显著影响，导致有效向列有序性降低。
