Wednesday  |  Conference Center B  |  04:40 PM–05:00 PM

#13333, Spectroscopic Characterization of Thermal Transitions in Polymers

Terahertz-based spectroscopic techniques is a viable pathway for in-situ characterization given their penetrative, noninvasive, and nondestructive attributes. Specifically, polymers are nearly transparent to electromagnetic waves in the terahertz regime; therefore, terahertz time-domain spectroscopy (THz-TDS) operating in the transmission mode is suitable for investigating polymers during thermal loading scenarios. This research aims to elucidate the thermal transition, e.g., glass transition, of an elastomeric polymer (polyurea) using THz-TDS. Polyurea is a thermoset elastomer that has been integrated into a plethora of applications due to its superior physical and mechanical properties. The prime attribute of polyurea is its excellent impact mitigating property, reducing the damage of structures and injury of humans from hypervelocity projectiles. However, the glass transition temperature of polyurea is known to be below room temperature, implying the possibility of ductile to brittle transition during ultrahigh strain loading. Such a transition is associated with changes in molecular vibrations giving rise to a change in the refractive index. This research strives to use the THz-TDS technique to elucidate the structure-property interrelationship of polyurea as a function of temperature spanning from -100°C to room temperature. In the experimental approach the sample temperature is controlled in a cold finger, where it is isothermally soaked at each temperature step for 2 min before collecting the terahertz signal. The sample interacts with the terahertz wave and inherits the molecular information, related change in the refractive index. The collected time-domain data is transformed to the frequency domain using discrete Fourier transform and the optical and dielectric properties are deduced from the spectral data of each sample. A shift in the sensitivity to a change of the optical properties with respect to temperature is reported around the thermal transition of polyurea. In other words, experimental investigation using THz-TDS at cryogenic temperature is able to accurately identify the glass transition temperature and radiation-related changes, since variations in molecular vibrations are associated with conformational changes encoded in the terahertz signal as it propagates through the sample. Molecular dynamic simulations are coupled with the experimental results to accurately elucidate the vibrational modes and source of conformational changes.

Nha Uyen Huynh   San Diego State University
Carlos Gamez   Apple
George Youssef   San Diego State University