Confined in a Fiber: Realizing Flexible Gas Sensors by Electrospinning Liquid Crystals

Liquid crystalline phases (LCs) readily exhibit optical responsivity to small fluctuations in their immediate environment. By encapsulating LC phase forming compounds within polymer fibers through the electrospinning process (a fiber spinning method known for being a fast way of forming chemically diverse non-woven mats), it is possible to create functionalized LC-polymer fiber mats that are responsive as well. As these fiber mats can be handled macroscopically, a usercan observe the responses of the mats macroscopically without the need for bulky electronics.
This thesis presents several non-woven fiber mats that were coaxially electrospun to contain LC within their individual polymer fibers cores for use as novel volatile organic compound (VOC) sensors. The mats are flexible, lightweight, and shown to both macroscopically and microscopically respond to toluene gas. Such gas responsive mats may be incorporated into garments for visually alerting the wearer when they are exposed to harmful levels of VOCs for example. Additionally, the interaction and re-prioritization of several electrospinning variables (from the chemistry based to the processing based) for forming the LC-mats are also discussed. The balance of these variables determines whether a wide range of phenomena occur during fiber formation. For instance, unexpected phase separation between the polymer sheath solution and the LC core can mean the difference between forming fully dried fibrous mats and wet/meshed films. A chapter is devoted to discussing the impact that solvent miscibility with an LC can have on fiber production, including also the effect that water can have when condensed into the electrospinning coaxial jet.
The fiber shapes that the polymer fiber sheaths adopt (beaded versus non-beaded), as well as the continuity of the LC core, will influence the visual app earance of the mats. These optical properties, in turn, influence the mats’ responsivity to gases and whether the responses can be macroscopically observed with or without additional polarizers. During two types of gas sensing experiments --mats exposed to gas when contained in a cell, and mats exposed to gas diffused in ambient air without containment, we see that not all fibers within a mat respond at the same time. Moreover, different segments of the fibers within the same non-woven mat also show slightly different rates of response due to variations in fiber thickness, LC content, and whether the fiber cores had variations in LC filling (i.e. LC director twists, and gaps).