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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - INTERACT (Intelligent Non-woven Textiles and Elastomeric Responsive materials by Advancing liquid Crystal Technology)

Teaser

Summary

A grand challenge in todayâ€™s materials research is the realization of flexible materials that are also intelligent and functional. They will be the enablers of true breakthroughs in the hot trends of soft robotics and wearable technology. The standard approach to the latter is to decorate rubber sheets with electronic components, yielding two serious flaws: rubber is uncomfortable as it does not breath and solid state electronics will eventually fail as a garment is flexed and stretched when worn. While the softness of rubber is ideal it must be used in the form of textile fibers to provide breathability, and for long-term failure resistance we need intelligent components that are soft. A solution to this conundrum was recently presented by the PI with the concept of liquid crystal (LC) electrospinning. The extreme responsiveness of LCs is transferred to a non-woven textile by incorporating the LC in the fiber core, yielding a smart flexible mat with sensory function. Moreover, it consumes no power, providing a further advantage over electronics-based approaches. In a second research line the INTERACT team uses microfluidics to make LC rubber microshells, functioning as autonomous actuators which may serve as innovative components for soft robotics, and photonic crystal shells. This interdisciplinary project presents an ambitious agenda to advance these new concepts to the realization of soft, stretchable intelligent materials of revolutionary character. Five specific objectives are in focus: 1) develop understanding of the dynamic response of LCs in these unconventional configurations; 2) establish interaction dynamics during polymerization of an LC precursor; 3) elucidate LC response to gas exposure; 4) establish correlation between actuation response and internal order of curved LCE rubbers; and 5) assess usefulness of LC-functionalized fibers and polymerized LC shells, tubes and Janus particles in wearable sensors, soft robotic actuators and high-security identification tags.

Work performed

The project has three main tracks, on liquid crystal-functionalized fibers responding to gas exposure and tensile strain; microfluidics-produced liquid crystal elastomer actuators, and polymer-stabilized photonic liquid crystal shells for secure authentication, respectively, where each track had one pure fundamental research component and one component that explores the opportunities for applications. The fundamental research has been done from the very beginning and has been continued throughout the project, whereas the application-oriented ones have been pursued to different degrees, depending on the results of the most fundamental components. The main achievements in the INTERACT project to date can be summarized as follows.

1. Electrospinning of liquid crystal-functionalized fibers.
a. First systematic study of gas sensing with liquid crystal-functionalized fibers was published (Liq. Cryst. Gold Open Access) and the results were presented at multiple international conferences and symposia. The study also revealed a previously unknown type of response, with very high speed and excellent sensitivity, recently investigated quantitatively and in depth.
b. An unexpectedly complex phase diagram between a commonly used liquid crystal and a standard polymer solvent was discovered and mapped out in detail, revealing phase separation by spinodal decomposition or nucleation and growth. Moreover, we found that the presence of even a very small amount of water has great influence on the phase diagram, raising the temperatures to these phase separation phenomena from about 0Â°C without water to about 50Â°C with water. As water thus shifts the phenomena into the range of room temperature, they have significant impact on the fiber spinning process. The study was published in Soft Matter, with feature on the cover page (open access after embargo period).
c. Fibers with dual liquid crystal core channels, well separated from each other such that their individual properties could be detected in one and the same fiber, were successfully spun. The study was published in Journal of Materials Chemistry C (open access since embargo expired).
d. Fibers with LCE core and varying sheaths were spun, revealing interesting morphology variations as a result of the polymer glass transition in combination with LCE actuation. The study was published (open access) in the journal Materials.
e. We developed a new microfluidics-based approach to wet spin rubber sheath fibers with liquid crystal core, giving responsive fibers that are very stretchable in nature. The study was published in J. Mater. Chem. C.
f. We have successfully electrospun fibers with a mixture of polyvinylalcohol (PVA) and polyacrylic acid (PAA) that is crosslinked after spinning, making the fibers resistive to water immersion. Upon incorporating a liquid crystal core into these fibers, we discovered that a certain degree of miscibility between core and sheath is highly beneficial for stable spinning conditions and an extended core continuity of the final fibers (publication in preparation).
g. We developed a new chemistry for making cholesteric liquid crystal elastomers that selectively reflect light, i.e. they show irridescent colors, due to a helical modulation of their internal structure, with period in the visible light wavelength range. Significantly, the reflection color changes continuously across the visible spectrum, from red to blue, allowing a direct strain measurement by analysis of the color. The study on flat films is being published in Advanced Functional Materials (accepted) and we are now developing the chemistry to be compatible with spinning into fibers.

2. Microfluidics-produced liquid crystal elastomer actuators.
a. Together with external collaborators three variations of a new LCE chemistry for two-stage polymerization was developed, requiring solely commercially available components, and the microfluidic shell production technique was adapted for each variety.
b.

Final results

Several unexpected discoveries of significant fundamental interest were made as a result of the research in each of the three main tracks. As a result of the research in coaxial electrospinning with liquid crystal coreâ€“polymeric sheath composite fibers, we discovered (1) an unexpectedly complex phase behavior between the most commonly studied LC compounds (5CB) and one of the most standard solvents, ethanol, as well as the very strong effect of water on this phase diagram. Most conspicuously, the phase diagram features a broad miscibility gap with isotropicâ€“isotropic phase coexistence, by spinodal decomposition or nucleation and growth, with further complications added as the transition to nematic order happens upon cooling. The work was published in the leading journal Soft Matter, with feature on the front cover.

Our exploration of liquid crystal elastomer synthesis in shell geometry led us to discover the first example of negative order liquid crystalline order, with actuators responding in an inverse way to stimuli compared to conventional positive order parameter LCEs. We also discovered an elegantly simple approach to make large-scale cholesteric liquid crystal elastomer with strong selective reflection color that changes in response to tensile or contractile strain, thus acting as a non-electronic strain sensor with direct visual read-out.

Our resarch in polymerized cholesteric liquid crystal shells for secure authentication purposes has receive exceptional interest also from outside the scientific community, including industrial players interested in using our materials for anti-counterfeiting, traceability, and also for an infrastructure guiding autonomous vehicle/robot navigation and augmented reality. Our publications on the topic are classified among the 4% top impact articles of all articles tracked by the service Almetric.