Project 1.3: Design & Testing of Polymers for Improved Soldier Protection

PIs: Nelson, Swager, Rutledge

 

There is a crucial need to develop lightweight materials with superior ballistic and blast mitigation capabilities. Toward this end, Project 1.3 initiates an integrated effort including novel synthesis, modeling, and characterization methods. Polymer networks, elastomers, and semicrystalline polymers will be both synthesized at MIT and obtained from ARL collaborators, and direct real-time observations of their dynamical responses to shock and microparticle hypervelocity impact will be measured. The results will be used to guide molecular and multiscale modeling at both MIT and ARL. The project will yield improved understanding of the dynamic nonlinear mechanical behavior of existing materials and will guide the synthesis of new materials with improved performance.

Recently developed experimental measurements of dynamic shock loading and microparticle hypervelocity impact will be conducted on materials provided by ARL scientists - greatly expanding an ongoing collaboration - and on new materials synthesized at MIT. Dynamic mechanical responses will be observed to allow direct comparison between particle impact responses on the microscale measured at MIT and projectile impact responses measured on the macroscale at ARL. Spectroscopic observations of materials under shock loading will allow assessment of the extent of hydrogen bonding and crystallinity on a dynamic basis, permitting comparison to static vibrational spectroscopy measurements conducted at ARL.

New materials will be developed that are designed to test the role of hydrogen bonding within and between different polymers; rigid polymers as well as flexible ones that can be hydrogen bond accepting and/or hydrogen bond donating materials. Complementary interpenetrating and phase separated networks between these materials and polyurethanes will be studied to determine structure-property relationships relevant to ballistic protection. A variety of polymer architectures will be produced, including linear, hyperbranched, and brush polymer structures.

Molecular simulations will be used to reveal the fundamental mechanisms that underlie the high rate mechanical response of these materials with particular attention to morphologically complex, heterogeneous polymeric materials with nanometer-sized domains, under conditions of shock deformation. This work will build upon pre-existing cooperation between ARL and MIT around the simulation and analysis of sub-sonic deformations in semicrystalline polyethylene. Computational results will be compared to both the real-time visualization of the high-rate nonlinear mechanical response and molecular vibrational spectroscopy under shock loading and particle impact for model refinement. Key attributes for energy absorption and dissipation will be identified to guide materials synthesis. The industrially important case of semicrystalline polyethylene, such as high-performance Spectra® fibers, as well as more novel materials such as such as phase-segregated polyurethanes, polyurea or the hydrogen-bonding networks synthesized within the project, will be examined.

The outcome will be an improved understanding of the high rate mechanical response mechanisms of currently available material compositions and the development of new compositions with improved performance.

 

Coefficient of restitution as a function of impact speed for TripPEES-g-pEtOx and previously-tested poly(urethane urea) elastomers [1].
(a) Synthesis of TripPEES-g-poly(ethyl oxazoline).
(b) Image sequence recorded showing silica particle impact on a TripPEES-g-poly(ethyl oxazoline) – PU blend sample. at 330 m/s. The time stamps, shown at the top of the frames, indicate the delay in acquisition time relative to the first frame.
(c) Results from constant uniaxial stress Hugoniostat (NPzzHug) simulations at four different uniaxial stress (Pzz) values, plotted along with data from Agrawal et al. (left) Shock velocity vs. particle velocity, and (right) pressure vs. specific volume.