Surface Aligned Liquid Crystals on Micropatterned Polyimide Relief Structures

The combination of nematic liquid crystals (LCs) and topographically patterned surfaces has enabled the development of a broad range of functional optoelectronic devices, such as displays, LIDAR (light detection and ranging) systems, dynamic lenses, tunable diffractive optical elements and polarisation converters. The interaction between LCs and patterned surfaces dictates the overall surface alignment and thus the bulk optical properties of a device due to the LC’s long-range order. These LC-surface interactions are therefore of great interest and are investigated in this work using planar and homeotropic LC polyimides, patterned with various surface relief structures. The motivation is to demonstrate unique and transferrable surface alignment phenomena, as well as highlight the potential for creating multi-functional LC alignment layers by combining the chemical surface anchoring of polyimides with the presence and physical alignment influence of topography.

Commercially sourced, spontaneously formed and precision engineered microstructures form the three unique starting points for the experimental chapters in this thesis. These structures are transferred in to polyimide alignment layers using a variant of solvent-assisted microcontact moulding (SAMIM); a subcategory of soft lithography. This methodology is discussed in detail to provide LC researchers with an insight in to an accessible and arguably underutilized means of patterning alignment layers, with additional control offered by the inherent surface anchoring properties of conventional LC polyimides.

Micropatterned liquid crystal elastomers (μLCEs) are fabricated using planar polyimide alignment layers imprinted with commercially sourced linear surface relief gratings. Mechanical rubbing overrides the grating alignment effect on these surfaces, allowing for multiple grating orientations on a single monodomain μLCE film. The all-acrylate LCE chosen for this study undergoes anisotropic deswelling upon removal of the non-reactive component (6OCB), enabling controllable miniaturization of surface features, as measured via atomic force microscopy (AFM). The original grating pitch of 1040nm, is reduced to 707nm (-32%) or 1010nm (-3%) when the nematic director is aligned parallel (n ̂_∥) or perpendicular (n ̂_⊥) to the grating grooves respectively. Varying the composition of the precursor mixture for the μLCE, provides additional control over the deswelling, reducing the original pitch to 770nm (-26%) at 45.7 mol% 6OCB and 643nm (-38%) at 64.4 mol% 6OCB in n ̂_∥ samples. The μLCE’s thermal response is quantified using optical diffraction measurements, indicating changes in grating pitch of up to 33% over a 215°C temperature range. Pitch expansion and pitch contraction are exhibited by the n ̂_∥ and n ̂_⊥ samples respectively upon heating, each with linear variations in pitch spanning ≈50% of their operational temperature range, with respective thermal sensitivities of +1.2 ± 0.1 nm/°C and -2.2 ± 0.1 nm/°C. The mechanical response of the μLCEs is explored by applying uniaxial tensile strains (ε) of up to 157%, where optical diffraction measurements are used to demonstrate a maximum pitch elongation of 1110nm. The μLCE topography, and specifically the height of the grating structures, is measured as a function of strain using AFM, which exhibits a non-linear relationship attributed to the auxetic response of this class of LCEs. An initial decrease in height is observed at moderate strains (ε ≤ 40%) followed by an increase at large strains (ε ≥ 80%), exceeding the height at rest by as much as 26% (50 ± 2nm at ε = 0%, 63 ± 3nm at ε = 146%). The first-order diffraction efficiency (η_1) of the μLCEs is also investigated as a function of temperature and strain. Notably, a distinct transition in the linear behaviour of η_1 is observed with increasing strain, where the gradient shifts from 0.67 for
ε < 100%, to 1.79 for ε > 100%. This corresponds closely with the emergence of the auxetic response and associated growth in feature height confirmed by AFM, which increases the grating’s phase modulation and is therefore suspected to be contributing to the higher gradient of η_1 for ε > 100%.

Wrinkled surface textures are spontaneously formed on polydimethylsiloxane (PDMS) slabs via plasma oxidation under tensile strain. Planar and homeotropic wrinkle-imprinted polyimide (WIP) surfaces are then fabricated using these wrinkle textures, with a wrinkle pitch of 900 ± 20nm and height of
125 ± 10nm. Aperiodic valleys in the PDMS slab formed by surface cracking, are inverted into peaks or ‘wall’ features on the WIP surfaces during imprinting. Although only artefacts of the wrinkling and imprinting processes, these wall features with a mean spacing of 13 ± 1μm and a mean height of 230 ± 5nm, are found to have a remarkable impact on LC surface alignment. A planar WIP surface in a hybrid aligned nematic (HAN) device results in uniform director alignment parallel to the wrinkles, due to the azimuthal anchoring strength of ≈4 × 10-6 Jm-2 associated with the wrinkles’ grating-like structure. The alignment properties of a wrinkled surface with homeotropic anchoring are explored for the first time, contained in a vertically aligned nematic (VAN) device geometry. Conoscopic examination of the VAN device shows a ≈7° degenerate tilt of the optic axis at 20°C (T⁄T_NI = 0.86), corresponding to a near-vertical surface pretilt (θ_P) on the wrinkles of 77±2°, thought to be caused by director splay over the peaks and troughs of the wrinkles. Heating the VAN device to T⁄T_NI ≈0.95 results in a stark change in optical appearance from near-extinct to transmissive, with a retardation (Γ) of 250 ± 50nm. This is attributed to the director undergoing a surface anchoring transition from near-vertical to significantly tilted (θ_P=35±9°) on the wrinkled surface, with the director tilting perpendicular to the wrinkle grooves as confirmed by full-wave plate observations. In this tilted state, the aforementioned wall features are shown to produce elongated alignment domains via physical confinement, within which the azimuthal director orientation exhibits an intriguing spatial modulation with periodicities between 10 and 15μm.

Precision engineered linear gratings are fabricated with electron beam lithography and imprinted in to homeotropic polyimides using composite-PDMS replicas as stamps. Scanning electron microscopy identifies the structural limits of the stamps, in terms of grating linewidth (l=150 – 350nm) and mark-to-space ratio (1:1 – 1:2.5), highlighting lateral collapse at l= 150nm and structural stability for all mark-to-space ratios at l≥ 250nm. Faithful transfer of 500 to 1200nm pitch gratings in to polyimide is confirmed via optical diffraction patterns, with pitch measurements agreeing within ≤2% of the design dimensions. Once assembled in to a VAN device geometry, room temperature polarised microscopy shows that the homeotropic gratings induce a transmissive optical appearance (Γ= 150 - 250nm), in an otherwise extinct VAN device, where Γ varies as a function of grating dimensions. This is attributed to surface anchoring transitions on the grating surface, driven by topographic distortions of the director field inducing a HAN profile with director tilt perpendicular to the grating grooves, as confirmed by full-wave plate analysis. Conoscopic examination of the induced HAN profiles, shows that lower mark-to-space ratio and shorter pitch gratings result in greater optic axis (OA) tilt (ϕ) and thus lower induced surface pretilt (θ_P) on the gratings. Angular calibration of the microscope back focal plane allows ϕ to be directly measured from the shifted melatope positions in the conoscopy figures, where ϕ= 8.1 - 14.7° for grating pitches of 1200 to 500nm, corresponding to θ_P≈ 74 - 61° at 20°C. Conoscopy also enables the identification of opposing tilt directions on either side of tilt domain walls, demonstrated over length scales of ≤100μm.

In summary, the work presented in this thesis demonstrates that a wealth of LC device functionality can be accessed by tailoring microscale surface topographies and their interactions with LCs. The work on μLCEs demonstrates controllable miniaturization of surface features, thermo- and mechano-responsive diffractive properties and the first evidence of auxetic microscale surface relief structures, with collective applications spanning microfabrication, spectroscopy, diffractive optimization and surface wetting. The work on wrinkled LC alignment layers demonstrates topography-driven modulated director profiles, highlighting an alternative and unique approach to patterning the azimuthal director orientation, with potential applications in LC-based flat optics. Finally, the focused study of frustrated surface anchoring on homeotropic grating structures shows how varying grating dimensions allows for precise control over surface pretilt, which is a notoriously difficult property to fine-tune, with broad applications in all LC devices.