Sophia Stepp (Mechanical Undergraduate) and Sophia Do (Bioengineering Graduate) were awarded the President’s Award at the 2018 Student Research Symposium, taking place on March 2nd-March 3rd at the Conrad Prebys Aztec Student Union. They both work in the Experimental Mechanics Laboratory under the supervision of Dr. George Youssef. Sophia Do was invited to represent the lab in the 32nd Annual Student Research Competition being held at California State University, Sacramento. Below is a summary of the research that they conduct.
Synthesis and Characterization of Polyurea Microspheres
Polyurea is an elastomer thermoset with a wide range of implementations in industrial and biomechanical applications. In the past two decades, bulk polyurea was heavily investigated under different environmental, operating and loading conditions. For example, the addition of a relatively thin layer of polyurea to civilian or military armors was found to enhance the impact mitigating properties of these structures. The further exploitation of the superior mechanical and physical properties of polyurea necessitates the exploration of smaller length scales, e.g. micro- and nano-scales. Thus, the objective of this project was to fabricate crosslinked polyurea microspheres, which we hypothesized would inherit some of the properties of dense polyurea such as hygrothermal stability while enhancing other properties such as stiffness. Polyurea microspheres were fabricated using a modified precipitation polymerization. In this process, both the monomer and initiator were added to a solvent, in which they were soluble. The mixture was violently agitated for 90 seconds, which helped disassociate any base chemicals that tend to agglomerate. Once the monomer and initiator reacted to form the polymer, it became insoluble in the solvent and precipitated. The polymerization process continued until the monomer and initiator were completely reacted. The microspheres were then characterized for their physical, thermal, and mechanical properties. The physical properties were examined using a scanning electron microscope (SEM), which showed that the microspheres (see figure) were graded in size, ranging from 0.5μm to 7μm.
The thermal properties of the microspheres were investigated using a thermogravimetric analyzer at a heating rate of 5°C/min in a nitrogen environment, where polyurea microspheres were found to be thermally stable up to 250°C, mirroring the stability of their bulk counterparts. The mechanical properties were characterized using atomic force microscopy, where individual microspheres were loaded with constant force while measuring the tip deflection to determine their stiffness. Preliminary reported microscale elastic modulus of the microsphere was found to be higher than dense polyurea but further confirmations of these measurements continue.
In addition to fabricating and characterizing the individual microspheres, we also plan to fabricate a polymer-polymer composite by reinforcing a polyurea matrix with polyurea microspheres. We hypothesize that this novel polymer-polymer composite will have enhanced strength and stiffness as well as impact mitigation properties of polyurea. To verify our hypothesis, these polyurea-polyurea composites will be characterized and compared to bulk polyurea for their quasi-static and dynamic mechanical properties.