Polypropylene (PP) is a widely used common type of polymer with excellent chemical and mechanical properties. Thermal aging is a main culprit in degrading its lifetime and effectiveness. In this work, terahertz dielectric frequency response of PP during the process of thermal aging is monitored and analyzed to understand the underlying mechanism, and to qualify terahertz (THz) dielectric spectroscopy as a useful tool for time-stamping and benchmarking thermal aging of polymer materials. Using an improved THz material parameter extraction algorithm for thin films, the real and imaginary parts of the dielectric permittivity of standard PP test coupons were obtained at various stages of the thermal aging process. In agreement with theoretical analysis based on a molecular-level understanding of the thermal aging dynamics, the experimental results indicated that THz wave transmission loss of PP correlated quantitatively with the degree of its thermal aging.

Accurate online measurement of capsule coating's thickness is desirable because coating is one of the key factors to control the release of active pharmaceutical ingredients. Terahertz time-domain spectroscopy offers a novel and potentially useful technique to examine coatings in terms of its thickness and uniformity quickly and nondestructively. However, online measurement usually requires the simultaneous determination of the upper and bottom parts of the cylindrical shell, in either the reflection or transmission mode. There is no method for accurate thickness determination in this two-layer structure of capsule coating simultaneously. Improved matched filter is introduced to obtain each layer's thickness for capsule coating with high resolution and accuracy by terahertz time-domain spectroscopy, without adjusting the focal point. This method can determine the thicknesses of both the upper and under layers of the capsule coating with high accuracy in a single measurement, whereas traditional method based on time delays cannot identify the thickness due to the low signal to noise ratio from the under layer.