Influence of Substrate Surface Properties on Spin Dewetting, Texture, and Phase Transitions of 5CB Liquid-Crystal Thin Films
Influence of Substrate Surface Properties on Spin Dewetting, Texture, and Phase Transitions of 5CB Liquid-Crystal Thin Films
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DOI：
10.1021/acs.jpcb.9b11569
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发表时间：
2020-02-20
影响因子：
3.3
通讯作者：
Mukherjee, Rabibrata
中科院分区：
文献类型：
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作者：
Dhara, Palash;Mukherjee, Rabibrata
我们报告了 5CB 向列液晶 (LC) 薄膜的质地和稳定性如何随着底层聚甲基丙烯酸甲酯 (PMMA) 基板的紫外线臭氧 (UVO) 暴露而变化。 PMMA基材的UVO曝光不仅增加其表面能，使其更易润湿，而且由于酯的光解作用，导致PMMA表面产生含氧极性官能团。虽然 5CB 薄膜在 UVO 暴露的 PMMA 基材上的稳定性有望增强，以防止热引起的去湿，但薄膜的纹理也会随着 UV 暴露时间 (t(E)) 的变化而变化。我们发现，由于 5CB 分子在基材上的锚定增强，薄膜在 t(E) = 20 分钟内继续呈现向列纹影纹理。此外，暴露于 UVO 的 PMMA 基板增强的润湿性和更强的锚定也抑制了 5CB 薄膜由于旋涂过程中分配的溶液层自发破裂而发生旋转反润湿的趋势，特别是当溶质浓度 (C-n) 非常低时。后一种观察结果使得可以创建更薄的液晶薄膜，否则由于各向异性液晶分子之间的内聚相互作用增强，很难通过旋涂形成液晶薄膜。最后，我们表明，在连续薄膜中，随着逐渐加热和冷却，向列相到各向同性（N -> I）和 I -> N 相变保持完全可逆，无论薄膜的纹理和基材的润湿性如何。
We report how the texture and stability of a nematic liquid-crystal (LC) thin film of 5CB vary as a function of UV-ozone (UVO) exposure of the underlying poly(methyl methacrylate) (PMMA) substrate. UVO exposure of the PMMA substrate not only increases its surface energy, making it more wettable, but also results in the generation of oxygen-containing polar functional groups on the PMMA 'surface due to photolysis of ester. While the stability of the 5CB films is expectedly enhanced on UVO-exposed PMMA substrates against thermally induced dewetting, the texture of the film also changes as a function of the UV exposure time (t(E)). We show that the films continue to exhibit the nematic Schlieren texture for t(E) = 20 min due to enhanced anchoring of the 5CB molecules on the substrates. In addition, enhanced wettability and stronger anchoring by the UVO-exposed PMMA substrates also suppress the tendency of spin dewetting of the 5CB films due to spontaneous rupture of the dispensed solution layer during spin coating, particularly when the solute concentration (C-n) is very low. The latter observation allows possible creation of thinner LC films, which are otherwise difficult to form by spin coating due to enhanced cohesive interactions between the anisotropic LC molecules. Finally, we show that in continuous films, the nematic-to-isotropic (N -> I) and I -> N phase transitions with gradual heating and cooling remain completely reversible, irrespective of the texture of the film and wettability of the substrate.