Electrically switchable polymer membranes with photo-aligned nematic structures for photonic applications

Highlights

• New composite material containing liquid-crystalline microstructures is proposed.

• Liquid crystals are photo-aligned within the porous membranes for the first time.

• Photoalignment technique is successfully modified for porous polymer membranes.

• Different nematic configurations are realized within the porous film.

• Photopatterned nematics tune an electro-optical response of the composite film.

Abstract

Composite materials containing liquid crystalline microstructures, providing fast stimuli responsive variations of birefringence at low consumption power, have great potential applications in photonics. Here, a composite material based on polymer membranes with photo-aligned nematic microstructures is proposed for the first time. For this purpose, porous polyethylene terephthalate (PET) films were covered with an azo-dye and subjected to a modified photoalignment technique to control the azimuthal orientation and anchoring strength of nematic liquid crystals (LCs) at the pore walls. Nematic internal ordering was identified using polarized optical microscopic observations assisted by theoretical simulations. The proposed approach provides spatial distribution of nematic microstructures characterized by specific internal ordering and fingerprint in near infrared spectra at an electro-optical response. These composites can be used as flexible photonic LC elements such as spatial light modulators, optical filters and lenses.

Introduction

Liquid crystals (LCs) are optically anisotropic fluids comprised of elongated molecules exhibiting local orientational order. Their ability to fill various nano and microcavities in host media allows the formation of stimuli responsive LC composite materials having great potential for various applications [[1], [2], [3]]. For instance, application of electric fields to LC-infiltrated composites induces optical response at lower driving voltages and low consumption power, which is useful for photonic applications [4]. Among various LC composite materials (polymer dispersed liquid crystals (PDLCs) [5,6], LC-in-polymer capsules [7], LC shells [8,9], LC emulsions [[10], [11], [12]], LC/polymer fiber mats [13], etc.), PET-NLC films can be easily obtained by directly filling the channels of polymer membranes after appropriate surfactant treatment, making them flexible, relatively durable, and transparent. The electro-optical properties of such composite materials strongly depend on internal nematic LC (NLC) ordering, well represented by the nematic director field within the channels. At the same time, nematic orientational ordering reveals interplay among confining geometry, surface anchoring conditions, and elasticity determined by the Frank elastic constants $K_{11}$, $K_{22}$, $K_{33}$ and $K_{24}$ [[14], [15], [16], [17]].

In the past decades, several theoretical [[18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]] and experimental [21,[29], [30], [31]] investigations have been performed to study the influence of strong curvature on the physical properties of liquid crystals in cylindrical cavities, which are of special fundamental interest. In particular, nuclear magnetic resonance studies of NLCs confined to polycarbonate (Nuclepore) [17,32,33] or aluminum (Anopore) [[33], [34], [35]] membranes showed that tangential or homeotropic anchoring at pore walls after appropriate surfactant treatment [36] could stabilize various director configurations [2,37]. Typical director configurations (Fig. 1a–e) are planar-bipolar (PB), axial (AX), planar-polar (PP), escaped radial (ER) and escaped radial with point defects (ERPD).

Investigations of porous silicon [38,39] and polymer [[40], [41], [42]] films with submicrometer-sized pores filled with NLCs showed the existence of a pronounced electro-optical response assigned to the changes of the initial orientational structure and the effective refractive index of LC under the action of an electric field. Recently, composite PET-NLC materials have been proposed to control the propagation of electromagnetic radiation of visible and THz frequency ranges [43,44]. The stacked LC composites are favorable for THz applications, since THz radiation is characterized by submillimeter wavelengths, requiring thick LC layers, which are slow and difficult to control [45].

In the present paper, we studied for the first time internal ordering and electro-optical properties of PET-NLC membranes with multiple LC structures, varying the anchoring strength and easy axis direction via special photoalignment techniques. Previously, the photoalignment method was successfully used to align LCs in typical sandwich-like cells [46,47] and transparent hollow fibers [48,49]. The method relies on irradiation of flat or curved surfaces coated with a thin layer of an azo-dye by polarized UV or blue light [49]. The irradiation induces reorientation of the dye's molecules and imposes a definite easy axis direction of LC, depending on the direction of the polarization plane. For the porous membranes, we modified the photoalignment method to control the easy axis direction and anchoring strength directly at the pore walls obtaining multiple photo-aligned LC structures within the single polymer sample, which was impossible using traditional surfactant treatment. The proposed composite LC membranes might promote flexible photonic elements such as spatial light modulators, retarders, switches, lenses, etc.