Phase separation and dewetting of polymer dispersed liquid crystal (PDLC) thin films on flat and patterned substrates

Abstract

The morphology, phase separation, instability and dewetting of Polymer dispersed liquid crystal (PDLC) thin film comprising of nematic liquid crystal, 4-n-pentyl-4ʹ-cyanobiphenyl (5CB) and Polystyrene (PS) is reported in this paper. Thin film of PS-5CB PDLC is spin coated on flat and topographically patterned silica substrates from a common solvent (toluene). Films with two different thickness h ≈ 35 nm and h ≈ 85 nm are used in the study. The composition of PDLC is chosen as RB (PS: 5CB) = 1:2 and 1:1. On flat substrate, the thinner films (h ≈ 35 nm) dewet at room temperature (T ≈ 25 °C) over time, forming isolated droplets for both RB. In contrast, the h ≈ 85 nm thick films are stable at room temperature though they exhibit gradual phase evolution of the 5CB domains with time. These films dewet only when they are subject to thermal annealing at T ≈ 70 °C right after spin coating and results in an isotropic collection of nearly equal sized droplets, each of which comprises of an inner polymer core surrounded by a 5CB ring. Finally, we show that the instability patterns as well as the phase separated LC domains can be aligned by casting the films on topographically patterned substrate. While the 35 nm thin film results in spin dewetted array of aligned droplets along the substrates grooves, the 85 nm thick film remains continuous and undergoes progressive phase evolution, resulting in alternating domain of phase separated LC (within the grooves) and PS domains align along the contours of the substrate patterns.