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))$$ and loss coefficient (α(ν))$$."

Detecting Crystallinity Using Terahertz Spectroscopy in 3D Printed Amorphous Solid Dispersions

Santitewagun, Supawan, Rishi Thakkar, J. Axel Zeitler, and Mohammed Maniruzzaman. "Detecting Crystallinity Using Terahertz Spectroscopy in 3D Printed Amorphous Solid Dispersions." Molecular Pharmaceutics (2022). 

for full paper see https://pubs.acs.org/doi/full/10.1021/acs.molpharmaceut.2c00163#

Abstract

This study demonstrates the applicability of terahertz time-domain spectroscopy (THz-TDS) in evaluating the solid-state of the drug in selective laser sintering-based 3D printed dosage forms. Selective laser sintering is a powder bed-based 3D printing platform, which has recently demonstrated applicability in manufacturing amorphous solid dispersions (ASDs) through a layer-by-layer fusion process. When formulating ASDs, it is critical to confirm the final solid state of the drug as residual crystallinity can alter the performance of the formulation. Moreover, SLS 3D printing does not involve the mixing of the components during the process, which can lead to partially amorphous systems causing reproducibility and storage stability problems along with possibilities of unwanted polymorphism. In this study, a previously investigated SLS 3D printed ASD was characterized using THz-TDS and compared with traditionally used solid-state characterization techniques, including differential scanning calorimetry (DSC) and powder X-ray diffractometry (pXRD). THz-TDS provided deeper insights into the solid state of the dosage forms and their properties. Moreover, THz-TDS was able to detect residual crystallinity in granules prepared using twin-screw granulation for the 3D printing process, which was undetectable by the DSC and XRD. THz-TDS can prove to be a useful tool in gaining deeper insights into the solid-state properties and further aid in predicting the stability of amorphous solid dispersions.

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