Multi-Core Liquid Crystal-Polymer Composite Fibers Produced by Electrospinning

Abstract

Electrostatic spinning, also known as electrospinning, is a technology which can produce long thin polymer fibers with diameter on the micro- or even nanoscales. In coaxial elecrospinning an additional capillary is introduced into the spinneret, allowing the inclusion of non-spinnable materials in the core of the fiber. As a result, the fiber may obtain new functionality provided by the core material. For instance, a low molar mass liquid crystal cannot be spun into fibers on its own, but it can be injected into a regular polymer fiber via coaxial elecrospinning. Such a liquid crystal-functionalized fiber acquires properties characteristic of liquid crystals, such as birefringence or selective reflection. The liquid crystal core can undergo phase transitions, allowing dynamic tuning of the properties and opening for applications in e.g. organic vapor detection. In this research project, we grant more than one function to a single fiber by introducing two liquid crystals via dual-channel coaxial spinning. In our experiments, we use a glass capillary with theta-shaped cross section as spinneret as this provides a convenient way of introducing two separate capillaries for the two core materials. Stable dual core fibers could be produced provided that experimental factors such as flow rate, spinneret-collector distance, voltage, humidity and the characteristic of the collector are all optimized. We found that the stability of the dual-core structure as well as the shape of fiber strongly depends on whether the collector substrate is hydrophobic or hydrophilic. Samples were characterized by polarizing optical microscopy (POM) and scanning electron microscopy (SEM). The birefringence of the liquid crystal gives a unique color between crossed polarizers and it marks the inner channel structure of fiber. By heating we could determine whether or not the two core materials were well separated or if mixing had occurred. In case of well separated channels each core clears (transitions into an isotropic liquid) at a specific temperature which is different for the two cores. In contrast, if the two liquid crystals have mixed inside the fiber, then both cores become isotropic at one and the same temperature, which is intermediate between the clearing points of the individual materials. The cross section of fibers was monitored in detail through SEM. In conclusion, we produced multichannel fibers functionalized with two different liquid crystals through coaxial elecrospinning. The characteristic of the liquid crystals revealed the inner channel system of fiber as well as its stability. This result shows that it is possible to produce multifunctional fibers via multi-channel coaxial electrospinning, but mixing of the different core materials is a critical problem which can be difficult to avoid.