UC San Diego

Mechanical properties of elastomeric polymers are highly sensitive to the loading rate, temperature and pressure in their transition zone between glassy state and rubbery state. This study focuses on polyurea, which is a group of elastomeric copolymer which has excellent thermal and mechanical properties. It is environmentally stable, tough, abrasion-resistant and dissipative. Both its storage and loss moduli increase significantly when the strain rate increases in the transition zone. This property gives great potential for polyurea to manage mechanical energy in various loading conditions. When applied as a coating material, it can significantly increase the structure's resistance to the shock or blast loading. Its dissipative nature can also be applied to design composite materials that have excellent properties. Its transition zone spans in a wide frequency range, which can be more than 10 decades in the logarithmic scale.

Even though various types of polyurea are widely applied, the mechanical properties of polyurea are not fully understood in such a wide frequency range. This research studies polyurea from various perspectives and provides knowledge of polyurea and its composites in terms of fabrication, characterization in various frequency ranges, modeling and the interface study of polyurea composites, as listed in the five parts:

1. Understanding polyurea, the effect of molecular structure and the stoichiometric ratio on its dynamic mechanical properties.

2. Novel polyurea-based composites.

3. A computational code to efficiently and effectively obtain constitutive models for polyurea and its composites in a wide frequency range.

4. Modification, characterization and modeling of the interfaces of polyurea composites.

5. A novel testing technique for directly characterizing the viscoelastic properties of polyurea and its composites at mid-level (kHz) frequencies, which fills the frequency gap between dynamic mechanical testing and ultrasonic wave measurement.