Rheology and microstructure of stable concentrated ionic liquid colloidal suspensions
Date
2017
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Astronauts are constantly threatened by impact from micrometeorite and
orbital debris (MMOD) when conducting extra-vehicular activities (EVA) in low
Earth orbit (LEO). These threats have already become a major challenge to long-term
missions and deep space exploration. Shear thickening fluids (STFs) demonstrate an
abrupt increase in viscosity with applied high shear stress, improving their ability to
dissipate energy and making them good candidates for protective body armor. In my
thesis work, a novel STF formulation in ionic liquid has been developed to improve
the resistance of EVA suit against threats from ballistic, puncture, and hypervelocity
MMOD impacts. Ionic liquids serve as the solvent phase for the STF formulations
because of their low volatility and stability over a broad range of temperatures.
However, dispersing colloidal particles in ionic liquids can be challenging because the
high ionic strength of ionic liquids screens the electrostatic stabilizing forces that are
typically important for stabilizing colloidal dispersions in polar solvents. ☐ Stable nanoparticle dispersions in the ionic liquid [C4mim][BF4] are created
through surface coatings (e.g., fluorinated alkyl chains, alcohol), which induce
solvation layering around the particles. Solvation layers are initiated by hydrogen
bonds between the anion groups [BF4]- and the functionalized particle surface,
providing a stabilizing steric repulsive inter-particle force. Rheology, electron
microscopy, dynamic light scattering (DLS), and small-angle neutron scattering
(SANS) are employed to determine the thickness of the solvation layers and the
microstructure of dispersions for different coating systems. A quantitative model
based on analysis of SANS data is developed to evaluate the inter-particle interactions
and the thickness of the solvation layers. Additionally, the rheological behavior of
dispersions is controlled by tuning the strength of surface hydrogen bonding with
different surface chemistry. The influence of temperature on the thickness of
solvation layers and particle interactions is also investigated through rheology, DLS,
and SANS studies. Destabilization phenomena (from stable dispersion to unstable
gel) are identified due to the change of interfacial structure with increasing
temperature. Furthermore, the influence of impurities (i.e., water) on the
microstructure and thermodynamic properties of ionic liquid are studies using SANS
and small-angle x-ray scattering (SAXS) techniques. A phase diagram for ionic liquid
aqueous solutions (microphase separation, phase inversion, and micelle formation) is
constructed, revealing similarities to traditional oil-water-surfactant systems. This
understanding of ionic liquid phase behavior and formation of solvation layers is
critical for the formulation of colloidal dispersions in ionic liquids with a specific
rheological profile. Ionic liquids based STF-Kevlar® nanocomposites are shown to
provide superior puncture resistance in lab scale quasi-static puncture tests. The
fabricated nanocomposites are proven to provide better protection than traditional
Kevlar® without compromising the flexibility. The results of the present research
demonstrate the feasibility of STF-Kevlar® nanocomposites for astronaut protection
and identify technological challenges that still need to be addressed.