Department of Physics, Zhejiang Sci-Tech University, Hangzhou 310018, China.
Soft Matter. 2018 Apr 25;14(16):3003-3011. doi: 10.1039/c7sm02525d.
The main-chain liquid crystal (LC) copolyethers in which the nematic-nematic phase transition was first experimentally observed were revisited and re-characterised. Grazing incidence X-ray scattering revealed that the low-T nematic (Ntb) phase could be highly aligned by shearing, more so than in previously studied bent LC dimers. This was evidenced by a four-point wide-angle X-ray scattering pattern, which originates from convolution of two tilt distributions. Through intensity simulation the orientational order parameter associated with each of the distributions, as well as the conical angle of the Ntb phase, was calculated. Information regarding the polymer chain conformation was obtained using polarised infrared spectroscopy. The findings suggest the average conformation of the chains is a helix, and that the bend angle between mesogenic units is inversely related to temperature. All experimental evidence, including a jump in birefringence at the Ntb-nematic (N) phase transition, shows that copolyether samples mirror the behaviour of bent LC dimers over the transition. This confirms that the low-T nematic phase in copolyethers is indeed the same as that in LC dimers, now known to be the Ntb. The unusual broadening of transition peaks in complex heat capacity, obtained by modulated DSC experiments, is discussed.
首次观察到向列-向列相转变的主链液晶 (LC) 共聚醚被重新研究和重新表征。掠入射 X 射线散射表明，低 T 向列 (Ntb) 相可以通过剪切高度取向，比以前研究的弯曲 LC 二聚体更明显。这可以通过四点广角 X 射线散射图案证明，该图案源自两个倾斜分布的卷积。通过强度模拟，计算了与每个分布相关的取向有序参数以及 Ntb 相的锥形角。使用偏振红外光谱获得了有关聚合物链构象的信息。研究结果表明，链的平均构象为螺旋形，介晶单元之间的弯曲角度与温度成反比。所有实验证据，包括在 Ntb-向列 (N) 相转变处双折射的跳跃，都表明共聚醚样品在转变过程中反映了弯曲 LC 二聚体的行为。这证实了共聚醚中的低 T 向列相确实与 LC 二聚体中的相相同，现在已知为 Ntb。通过调制 DSC 实验获得的复杂热容转变峰的异常展宽进行了讨论。
