Skip to main content

Intelligent Non-woven Textiles and Elastomeric Responsive materials by Advancing liquid Crystal Technology

Periodic Reporting for period 4 - INTERACT (Intelligent Non-woven Textiles and Elastomeric Responsive materials by Advancing liquid Crystal Technology)

Reporting period: 2019-10-01 to 2020-03-31

Versatile functional materials that can convey information, respond to stimuli, and adapt in various ways to changes in the environment will be enablers in hot trends like soft robotics, wearable technology, autonomous navigation and augmented reality. Wearable technology materials should withstand flexing and be breathable, like the textiles in ordinary clothing. To this end, INTERACT explored a new class of responsive textile fibers realized by electrospinning liquid crystals (LCs) inside a polymer sheath, targeting wearable detectors of hazardous gases and sensors of tensile or compressive strain. We also used microfluidics to make LC-based microshells and tubes with exciting properties. If made of LC elastomers (LCEs), they function as autonomous actuators (artificial muscles), useful for soft robotics. A different LC enables photonic crystal retroreflectors, with diverse application potential. The key conclusions of INTERACT are, organized along the three main themes, (A) LC-functionalized fibers, (B) curved LCE actuators, and (C) photonic crystal shell retroreflectors:

A. LCs can be incorporated inside polymer sheaths, from highly stretchable rubbers to strongly crosslinked tough composites, to make responsive fibers suitable for wearable sensors. To succeed, it is critical to (1) know the phase diagram of mixtures of the LC core and the solvent used for the sheath, (2) minimize the core-sheath interfacial tension, and (3) use a dry spinning atmosphere, as water condensing onto the Taylor cone can lead to catastrophic failure. Thicker LC core fibers can be realized by means of a newly developed microfluidic wet spinning approach.

B. Microfluidic production enables a variety of unconventional LCE actuators, with positive as well as negative order parameter ground states, and with diverse actuation modes. We can tune the LC ordering field as well as the number of internal compartments, and tube-shaped LCE acuators are in sight; we hope to reach this milestone before the end of 2020.

C. Spherical shells of cholesteric LC can be made with excellent radial helix alignment, preserved after complete or partial polymerization. These new optical elements are highly selective retroreflectors and they generate unique Physical Unclonable Functions (PUFs). Undesired scattering can be minimized by embedding in a transparent refractive index-matching matrix. The resulting composites have vast application opportunities, from anti-counterfeiting and supply chain tracing to support for autonomous vehicle navigation and augmented reality.
A1. First systematic study of gas sensing with LC-functionalized fibers (Liq. Cryst., SPIE proceedings).
A2. Fibers with dual LC cores, with different properties, were successfully spun (J. Mater. Chem. C), as were fibers with LCE core (Materials).
A3. We discovered a complex phase diagram with phase separation by spinodal decomposition or nucleation and growth between a commonly used LC and ethanol, strongly impacted by small fractions of water (Soft Matter, feature on cover page).
A4. We showed that this phase separation can take place in the Taylor cone during electrospinning, leading to catastrophic spinning failure (ACS Appl. Mater. Interf.). We also showed that the stability of the Taylor cone, and thus of the fiber spinning process, is highly sensitive to atmospheric humidity.
A5. We developed a new microfluidic wet spinning approach to make rubber sheath fibers with LC core. The responsive fibers can be stretched several 100% (J. Mater. Chem. C).
A6. We crosslinked the sheath polymers after electrospinning to make fibers resistive to water immersion. A continuous LC core can be achieved by ensuring partial miscibility of the core and sheath fluids (publication in preparation).
A7. We developed a new chemistry for making cholesteric LCEs that show strain-dependent color, demonstrating quantitative correlation in a soft strain sensor (Adv. Funct. Mater.).

B1. Three new LCE chemistries were developed and adapted for microfluidic shell production.
B2. We realized the first ever negative order parameter LC, in the form of LCE shells (Sci. Adv.). Fragments cut from the shells act as autonomous microswimmers.
B3. Using LCE oligomers, we made positive order parameter shells that buckle upon actuation (Adv. Funct. Mater.).
B4. Photopolymerization in shells of nematic and smectic type was systematically studied, revealing dramatic lifetime prolongation and new opportunities for programming specific polymer network structures (Adv. Mater.).
B5. We significantly improved the understanding of the impact of stabilizer molecules (surfactants and polymers) on the alignment and stability of LC and LCE shells (Soft Matter, cover page; Liq. Cryst.; Langmuir; Phys. Rev, Res.).

C1. We developed protocols for reproducibly fabricating cholesteric shells with excellent photonic crystal properties, even after polymerization.
C2. We demonstrated photonic cross communication between shells with different photonic bandgaps (Sci. Rep.), and analyzed 3-shell communication (Liq. Cryst., invited), revealing a much richer information content than previously realized, with great value for applications in secure authentication.
C3. We discovered and elucidated a new type of cholesteric shell reflection pattern, resulting from internal Bragg reflection (Adv. Opt. Mater., feature on cover).
C4. We realized air-stable composites, with good optical properties, functioning as secure authentication tags and/or as selective retroreflectors (Adv. Mater., invited).
C5. We developed a method for polymerizing LC shells that allows refractive index matching in- and outside and removing all stabilizers, minimizing scattering (patent pending, publication in preparation).
A1. We discovered a complex phase diagram in LC–solvent mixtures, with a broad miscibility gap triggering phase separation by spinodal decomposition or nucleation and growth, involving isotropic and anisotropic liquid phases (Soft Matter, front cover).
A2. We demonstrated that the common notion that core and sheath liquids in coaxial electrospinning should be immiscible is incorrect: immiscible liquids have such high interfacial tension that the coaxial flow breaks up due to a Rayleigh instability. We demonstrated very good results with partially miscible liquids.
A3. We put the focus on the critical importance of water condensation into the Taylor cone during electrospinning, an aspect largely ignored until now. Counterintuitively, the water condensation speeds up solvent evaporation and it can generate uncontrolled phase separation, leading to catastrophic spinning failure.
A4. We developed a new microfluidic wet spinning approach and demonstrated its usefulness by realizing coaxial fibers with rubber sheath surrounding an LC core.

B1. We discovered the first example of negative liquid crystalline order, with LCE actuators responding in an inverted way compared to conventional LCEs.
B2. We discovered an elegantly simple approach to make large-scale cholesteric LCEs with selective reflection color that changes in response to tensile or compressive strain.

C1. Our research in cholesteric shells received exceptional interest also outside the scientific community, including industrial players interested in the potential of our materials for anti-counterfeiting, traceability, and for an infrastructure guiding autonomous vehicle/robot navigation and augmented reality.
Internal reflection in cholesteric shells