Improved Dynamic Strain Hardening in Poly(urethane urea) Elastomers for Transparent Armor Applications

摘要

The U.S. Army Joint Service Chemical/Biological Protective Facemask program requires a lens system that can be folded while providing a high level of optical quality, chemical resistance, ballistic protection, scratch resistance, and environmental durability. In general, materials with good barrier properties lack significant energy absorption/dissipation abilities upon impact, whereas rigid transparent polymeric materials that are extremely tough exhibit poor resistance to chemical hazards and abrasion. The Army Research Laboratory is currently engaged in collaboration with the Institute for Soldier Nanotechnologies (ISN) to help design novel polymeric materials with improved physical and mechanical properties by exploiting novel molecular mechanisms. This paper presents recent experimental findings exploring the role of molecular mechanisms on the dynamic mechanical deformation of a model set of transparent segmented poly(urethane urea), PUU, elastomers. As expected, increasing the hard segment content improved the barrier properties, and also increased the stiffness and flow stress levels. Tailoring of the microstructure was critical in altering their rate-dependent mechanical behavior. It was observed that promoting phase mixing among the hard and soft segment domains of the microphase-separated PUU material greatly enhanced its rate-dependent stiffening and strain hardening behavior. Furthermore, the resulting increase in intermolecular interaction also enhanced the barrier properties. These insights can aid in the design of PUUs for articles that manifest improved protective abilities under impact, while maintaining their flexibility during normal use, which is greatly desired for chemical/biological protective faceshield applications.