Liquid crystal microfibers lead to responsive optoelectronic textiles

Incorporating active materials within fibers holds great promise for tunable, non-woven, optoelectronic textiles that can respond to external stimuli.^1–3 Previous studies have shown that light can be confined by infiltrating the microstructures with liquid crystal (LC) materials.^{4, 5} However, these fibers are mostly silica-based, and the LC material has generally been capillary-filled, limiting the length and flexibility of
                                  the photonic fibers.

We produced and characterized thermally and electro-optically responsive microfibers endowed with LC properties. These include mesophase characteristics and birefringence, as well as molecular-level self-ordering. These LC microfibers are electrospun from a homogeneous solution of 4-pentyl-4'-cyanobiphenyl (5CB) and polylactic acid (PLA) in chloroform/acetone solvent. In the electrospinning process,^{6, 7} the low molecular weight 5CB phase-separates and self-assembles to form a planarly aligned nematic core within a PLA shell. The optical birefringence and dielectric anisotropy of LCs are coupled with the polymer shell's elongated structure to form the LC fibers' light-modulating properties. Most importantly, the orientation of LC domains and, therefore, the optical properties of the 5CB/PLA fibers, can be tuned by applying external fields.

We have shown that the LC content can be increased up to 70wt% while maintaining the core/shell fiber structure.² Figure 1 shows a polarized optical microscope (POM) image of an LC fiber. The interference color shift determined the planar alignment of 5CB in the PLA core. The birefringence of the phase-separated LC phase (Δn ∼ 0:2) in the core could be easily observed by the color changes as the fiber orientation (relative to the incident light polarization) rotated. Differential scanning calorimetry (DSC) analysis of the fibers also confirmed that 5CB is phase-separated and self-assembled in the PLA core at above 28wt% 5CB (see Figure 2).

Figure 1. Determination of the alignment of 5CB in the core of polylactic acid (PLA) by using a first-order retardation plate (RP) (λ=530nm). The fiber is placed at 45^°with respect to polarizer (P) and analyzer (A). Blue represents the additive retardation effect indicated with a plus sign and yellow indicates the relative retardation decrease shown with a minus sign. N is the fiber's long axis.

Figure 2. Dependence of T_NI (nematic-isotropic transition temperature) on the weight % of 5CB (4-pentyl-4'-cyanobiphenyl) in the 5CB/PLA nonwoven fiber mats.

Light modulating properties of LC fibers greatly depend on the fiber structure. For instance, beaded fiber morphology gave rise to randomly oriented large nematic domains (>5μm) with well-known LC defects (see Figure 3). We achieved uniform LC fibers by altering the operational parameters, mainly the collection distance and applied voltage, while keeping the concentration of LC at ∼59wt%. For instance, at higher voltages (>20kV), LC fibers were electrospun more uniformly.

Figure 3. Polarized optical microscope images of electrospun liquid crystal (LC) fibers collected 15.5kV. The inset scale bars indicate 50μm. The top image was taken under crossed polarizers and the bottom image was taken under parallel polarizers.

Additionally, we explored various ways to heighten the optical effects of LC fibers by forming a defect-free polymer shell and aligning the multiple LC fibers as an ordered array. For example, incorporating a metallic grid as a collector further enhanced the morphology while providing control over the optical properties and the LC fibers' orientation. Figure 4 shows the birefringent and an ordered LC fiber array. For these arrays, minimum transmitted light intensity is observed if LC fibers are oriented along one of the polarizers, while maximum transmitted light intensity is obtained when LC fibers are set 45^° between crossed polarizers.

Figure 4. Defect-free and ordered fiber arrays show the light modulating properties of the LC fibers. The inset scale bars indicate 100μm.

Most importantly, the LC domains in the electrospun LC fibers respond to an applied AC-electric field. The switching time upon application of 80V was 4ms, while the relaxation time upon voltage removal was 13ms. When we applied the field, the transmitted light intensity reached a minimum value, indicating that LC molecules aligned along the field and blocked light transmission.³

Encapsulating responsive LC materials into fiber geometry may introduce new photonic and biological applications, such as biosensors, by taking advantage of the self-sustaining and substrate-free form factor of fiber geometry and the biocompatibility of PLA. The soft polymer shell is responsible for maintaining the flexibility and mechanical integrity of the suspended LC fiber array. Tunable optical features of LC microfibers may open up new ways to design and develop birefringent ultrafine photonic fibers.

In the next phase of our research, we will focus on enhancing LC fiber responses to a variety of external stimuli by incorporating different liquid crystal mesophases, ferroelectric particles, and active polymer shells.