Polymer pellet fabrication for accurate THz-TDS measurements

Murphy, Keir N., Mira Naftaly, Alison Nordon, and Daniel Markl. "Polymer pellet fabrication for accurate THz-TDS measurements." Applied Sciences 12, no. 7 (2022): 3475.

for full paper see https://www.mdpi.com/2076-3417/12/7/3475/htm

Abstract

We investigate fabrication of compacts using polytetrafluoroethylene (PTFE) and polyethylene (PE), and the effect of compaction conditions on their terahertz transmission properties. The conditions used to fabricate compressed powder samples for terahertz time-domain spectroscopy (THz-TDS) can impact the accuracy of the measurements and hence the interpretation of results. This study investigated the effect of compaction conditions on the accuracy of the THz-TDS analysis. Two polymers that are commonly used as matrix materials in terahertz spectroscopy studies were explored using a compaction simulator and a hydraulic press for sample preparation. THz-TDS was used to determine the refractive index and loss coefficient to compare the powder compacts (pellets) to the values of solid material. Sample porosity, axial relaxation and tensile strength were measured to assess the material’s suitability for terahertz spectroscopy. It was found that PTFE is the preferable material for creating THz-TDS samples due to its low porosity and high tensile strength. PE was found to show significant porosity at all compaction pressures, making it an unsuitable material for the accurate determination of optical parameters from THz-TDS spectroscopy measurements. The larger particle sizes of PE resulted in compacts that exhibited significantly lower tensile strength than those made from PTFE making handling and storage difficult.

Keywords: terahertz time domain spectroscopy; sample preparation; polymers; porosity

"2.2.3. THz-TDS Measurements

THz-TDS measurements were carried out on a commercial system (TeraPulse Lx, Teraview), with a frequency (ν$$) resolution of 0.04 THz. All measurements were performed in a nitrogen-purged chamber. Sample thickness (L) was measured using a micrometer (±0.005mm$$) prior to the TDS measurement and was used for the calculation of the frequency-dependent refractive index (n(ν))$$ and loss coefficient (α(ν))$$."