Rheology of lyotropic boron nitride nanotube dispersions

According to National Fire Protection Association (NFPA) statistics, 3,280 civilian deaths,15,700 injuries, and $ 14.3 billion in property damage were caused by fire during 2015 alone.At the same time, traditional high-performing flame retardant materials such as halogen- orformaldehyde-based flame retardants have been banned or limited from commercial use dueto their carcinogenic properties, environmental problems, and bioaccumulative effects. As aresult, there is an increased demand in new coatings that are environmentally friendly,biocompatible and have improved char-forming character upon heating or exposure to aflame.

Boron nitride nanotubes (BNNTs) are a newly emerging nanomaterial with promisingproperties for applications in new flame retardant materials, as well as industry gamechanging materials and processes. Since the industrial scale synthesis of BNNTs took placeonly a few years ago, there has been an increasing need in understanding fundamentals oftheir molecular properties, polydispersity, processing to fulfill their promising nanoscaleproperties to macroscale applications. BNNTs are inherently noncytotoxic, mechanicallyrobust, and have extraordinary chemical and thermal stability. Under air oxidation, BNNTsare stable up to 1000 oC. Structurally, BNNTs have highly crystalline single- or multi-walledstructures with a diameter ranging from 2-8 nm, length of as long as 200 μm, a wide range oflength to diameter (aspect) ratios, high rigidity, the potential for long-range interactions, andhydrophobic surfaces. This structure polydispersity of BNNTs presents special challengesfor its processing and applications. Many proposed applications of BNNTs and 1D tubularstructure in general, including flame retardants, structural materials, electronic andoptoelectronic devices, biosensing and imaging, require controlled homogeneous structuredistribution of building blocks across the nano-, micro- and macro- length scales. Thischallenging material requirement can be accomplished by inventing suitable BNNTdispersions in a liquid medium, which will allow for processing with many existing separationtechniques.

The long term goal of the PI’s research project on BNNTs is to develop advancedflame retardant coatings, as well as elevated temperature and hazardous environmentalcorrosion protection for aerospace applications. The specific aims of the BNNT project areto: 1) identify stable BNNT dispersions through quantifying the triangular affair among solventmolecules (e.g. water, water/alcohol mixtures), dispersant molecules (e.g. DNA, surfactants),and BNNTs in the liquid state; 2) control BNNT structure polydispersity throughthermodynamically driven post-synthesis sorting by length and wall-numbers in aqueoussystems of both single and two phases; and 3) probe effects of polydispersity and shear onthe dispersion microstructure by rheology across multiple regimes and to predict changes ofmicrostructure due to various processing conditions.

The Faculty Research and Development (FRD) Program-assisted work is to focus onrheological characterizations of lyotropic BNNT dispersions to establish structure-processingpropertyrelationships in macroscopic assembly of BNNTs. The results of this researchwill provide a solid foundation for developing a long-term research program at theinterface of nanotechnology, colloid science, and rheology. It will enable fundamentalunderstanding of the hydrodynamic response of nanorod dispersions, as well asfurther advance engineering of molecularly organized materials through bottoms-upassembly of liquid dispersed nanorods.