Design and study of lyotropic liquid crystal-butyl rubber nanocomposites for chemical agent vapor barrier applications

The lack of water vapor permeability associated with traditional chemical warfare agent (CWA) protective garment materials causes heat stress and fatigue problems for the wearer. This thesis research aims to develop and study a new type of breathable, nanoporous CWA protective material based on cross-linkable lyotropic liquid crystal (LLC) monomers and commercial butyl rubber (BR). The resulting LLC-BR nanocomposite materials would selectively allow water vapor to pass through, while retaining the majority of the protective properties of cross-linked BR. This thesis describes work over the last five years in the design, development, and study of LLC-BR nanocomposite membranes with the inverted hexagonal (HII) phase (which contains 1-D cylindrical water nanochannels), and the type I bicontinuous cubic (QI) phase (which contains a 3-D interconnected water nanopore network), respectively. First, a HII-phase LLC-BR copolymer material based on a taper-shaped LLC monomer 1 was successfully developed. Vapor permeability studies showed that supported, solvent-cast, HII-phase 1 -BR membranes provide excellent water vapor permeability compared to pure, cross-linked BR membranes made on the same support material. Simultaneously, the 1-BR membranes are still able to reject CEES (2-chloroethyl ethyl sulfide, a stimulant of sulfur mustard) vapor to a higher degree. However, a major drawback of this first-generation LLC-BR composite is that the H II-phase structure requires macroscopic alignment and continuity to achieve maximum water vapor transport. The alignment of the cylindrical channels in HII-phase LLC materials has always been a challenging problem and has not been resolved. In order to overcome this materials engineering problem, a second-generation LLC-BR composite material was developed with a QI LLC phase architecture. This material was based on a phosphonium gemini LLC monomer (monomer 2) that forms a QI phase containing a three-dimensional interpenetrating water manifold network that eliminates the need for pore alignment. A QI-phase LLC-BR material with either the Ia3d or Pn3m structure was successfully obtained by blending and copolymerizing specific amounts of 2, water, and BR at 65--75°C. Hot-pressing was required to successfully fabricate supported QI-phase 2-BR membranes. The resulting Q I-phase 2-BR membranes showed two orders of magnitude higher water vapor permeability, and more than 500 times better water vs. CEES molar selectivity compared to pure cross-linked BR, as well as the first-generation HII -phase 1-BR membranes. Control experiments verified that the improvement in water vapor permeability relies on the interconnected Q I-phase nanopore morphology. After obtaining promising results with CEES, an effectively non-water-soluble, hydrophobic CWA simulant, the permeability of a very water-soluble nerve agent stimulant, dimethyl methyl phosphonate (DMMP), was studied. The absolute DMMP permeability of the QI-phase 2-BR membrane was found to be comparable to that of pure cross-linked BR membranes and lower than that of commercial BR protective gloves, resulting in a three-order of magnitude higher water vs. DMMP molar selectivity. Work by other members of our research group showed that the water layer gap spacing in the BR-free QI-phase polymer of 2 is ca. 0.75 nm. The rejection results obtained with water-soluble DMMP suggest that the water vs. DMMP vapor selection mechanism in the QI-phase 2-BR membranes is based on molecular size-discrimination. CWA protective materials that work by such a mechanism are unprecedented.