Of several ideas being pursued by NASA for the reduction of radiation dosage to astronauts, the use of ultra-high-molecular-weight polyethylene (UHMWPE)-based composite materials for both radiation shielding and micrometeorite shielding appears to be particularly appealing. UHMWPE has long been understood to provide superior radiation shielding following encounters with energetic nucleons due to its high hydrogen content. Meanwhile, impacts of micrometeorites with UHMWPE tend to vaporize it, rather than causing spallation of the shield material, which then creates additional potentially damaging micrometeorites. Less widely appreciated is the high specific strength of UHMWPE and UHMWPE fibers, which provide structural integrity to the composite. Amongst thermoplastics, UHMWPE has the highest impact strength and is also highly resistant to abrasion. Despite this highly appealing combination of properties, UHMWPE’s key mechanical properties can be improved by forming composites with other nanostructured materials, leading to further performance increases and weight reductions. Such composites will increase the ability of UHMWPE structures to withstand micrometeorite impacts and maintain the structural integrity of a pressurized environment.
The overall goal of this project was to use a nanoparticulate reinforcement for UHMWPE. The UHMWPE to be used in actual spacecraft structures will be reinforced by fibers as well, but the focus of the project was the development and application of chemical approaches to disperse nanoparticles in a UHMWPE matrix.
TDA Research (TDA) proposed to add 1-2 wt.% of hard nanoparticles of boehmite (alumoxane) or fullerenes to UHMWPE as a way to improve the materials properties and micrometeorite shielding ability of UHMWPE. Since these nanoparticles consist of light elements and will be present at very low wt.% loadings, the formation of nanocomposites can be achieved without significantly degrading the outstanding radiation shielding characteristics of UHMWPE. While the theory of nanoparticle reinforcement is still developing, experimental results consistently demonstrate that the addition of nanoparticles, even at such low loadings, dramatically improves key materials properties of other polymers. When larger particles or fibers are used as reinforcements, it is known that debonding at the particle-matrix interface induces shear yielding in the matrix, thereby absorbing considerable energy that would otherwise propagate the crack. As this effect should be correlated to the surface area of the particles, smaller particles can achieve similar dissipations of crack energy with much less volume (weight) present in the composite.
The main challenge of the project was to disperse the nanoparticles into UHMWPE. It is generally very difficult to disperse any particulate material into even lower-molecular-weight grades of PE, and it is generally impossible to obtain a good dispersion of nanoparticles into any polymer by high-shear mixing or any other physical mixing approach. Successful approaches are based on tailoring the surface chemistry of the nanoparticle to match that of the matrix, and forming a solution above the glass transition temperature of the polymer. However, the extremely high viscosity of UHMWPE renders that approach similarly impotent. A gel-processing technique was used to disperse and retain the surface-modified nanoparticles in commercial UHMWPE. While the gel-processing technique is not unique to TDA, the inclusion of nanoparticles by this method appears to be an invention of the project.
A method for dispersing functionalized nanoparticles of fullerenes and alumoxanes into UHMWPE was developed. Increases of up to 26 percent in impact strength and 29 percent in tensile modulus versus non-reinforced UHMWPE were observed.
This work was done by Sheila Thibeault of Langley Research Center. LAR-17564-1