Influence of Substrate Surface Properties on Spin Dewetting, Texture, and Phase Transitions of 5CB Liquid-Crystal Thin Films

Abstract

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, although the disclination point density (|m|) gradually reduces with an increase in t E . However, the texture changes completely to a spherulite or fanlike texture in 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.

Young and Young–Laplace equations for a static ridge of nematic liquid crystal, and transitions between equilibrium states

“…In this situation, the isotropic interfacial tension for the isotropic–nematic interface γnormalIN would be much smaller than the gas–nematic interfacial tension γnormalGN and may become comparable to the anchoring strength CnormalGN. For instance, γnormalIN was measured for the nematic MBBA as γnormalIN=10−5 normalN normalm−1 [64], which is three orders of magnitude smaller than a typical isotropic interfacial tension for a gas–nematic interface γnormalGN [57]. Such a situation could be realized experimentally by using controlled heating and cooling of regions of a substrate coated in a nematic film [34,65].…”

“…|CnormalGN| and |CnormalNS|) [7]. For example, the isotropic interfacial tension of an interface between air and the nematic 4-cyano-4’-pentylbiphenyl (5CB) has been measured as γnormalGN=4.0×10−2 normalN normalm−1, and the isotropic interfacial tension of an interface between the substrate poly(methyl methacrylate) (PMMA) and 5CB has been measured as γnormalNS=4.051×10−2 normalN normalm−1 [57].…”

Young and Young--Laplace equations for a static ridge of nematic liquid crystal, and transitions between equilibrium states

“…In this situation, the isotropic interfacial tension for the isotropic-nematic interface γ IN would be much smaller than the gas-nematic interfacial tension γ GN and may become comparable with the anchoring strength C GN . For instance, γ IN was measured for the nematic MBBA as γ IN = 10 −5 Nm −1 [62], which is three orders of magnitude smaller than typical isotropic interfacial tension for a gas-nematic interface γ GN [54]. Such a situation could be realised experimentally by using controlled heating and cooling of regions of a substrate coated in a nematic film [31,63].…”

“…|C GN | and |C NS |) [7]. For example, the isotropic interfacial tension of an interface between air and the nematic 4-Cyano-4'-pentylbiphenyl (5CB) has been measured as γ GN = 4.0 × 10 −2 N m −1 , and the isotropic interfacial tension of an interface between the substrate poly(methyl methacrylate) (PMMA) and 5CB has been measured as γ NS = 4.051 × 10 −2 N m −1 [54].…”

Feature Size Modulation in Dewetting of Nanoparticle-Containing Ultrathin Polymer Films

“…Meso patterns obtained by dewetting have been used in various applications such as enhancing photoluminescence, water harvesting, − fabrication of water stable UV detectors, and so on . On the other hand, the inherent randomness of the dewetted features can be circumvented by dewetting the film on either topographically or chemically patterned substrates to obtain large area ordered structures with programmable wetting, optical, and transport properties. − Recently, Verma and Sharma fabricated an array of microlenses by submerged dewetting, where the dewetted droplets have a very high contact angle due to reduced surface tension and act as microlenses. , Apart from homopolymer thin films, dewetting has also been observed in various other types of films such as liquid crystals, − sol–gel derived inorganic thin films, and so on.…”