## 1 Introduction  
Transmission terahertz (THz) spectroscopy has gained interest in various industries, including the polymer and pharmaceutical industries. This interest can be credited to the development of various applications of terahertz time-domain spectroscopy (THz-TDS) in these sectors [1, 2, 3, 4, 5, 6] (and references therein). With regards to the polymer/plastics industry, THz-TDS has been applied to enhance product quality and to reduce material wastage during the manufacture of plastic components/appliances [3, 7, 8]. Applications in the pharmaceutical industry include the determination of porosity in pharmaceutical compacts [9, 10, 11] and the determination of polymorphic forms [2, 12] in both compacts and powder. These applications and many others require the determination of the refractive index (RI) and loss coefficient of a given material [13, 14], and thus, accurate extraction of these optical properties is of the utmost importance. An important optical property is the effective loss coefficient, which combines material absorbance and scattering. Highly absorbing materials, such as many pharmaceutical drug substances, have strong THz absorption, making transmission measurements difficult and creating the need for dilution by a low-loss material such as a non-polar polymer [15, 16]. The absorbing material must be mixed homogeneously with the polymer and then compacted at pressure to create a solid sample with minimal porosity. There have been various applications for this method of sample preparation such as the dilution of active pharmaceutical ingredients, explosive material and biological samples [17; 18; 19]. Several sample properties can be a potential source of error in THz-TDS analysis, with the most critical attributes being sample porosity, uniformity of sample thickness, surface roughness and structural integrity. Sample porosity must be minimised as it affects the accuracy of the RI obtained from terahertz measurements [1; 9; 10; 14; 20; 21]. Porosity refers to the volume ratio of material to void space in the pellet. The inclusion of air in a compact lowers the measured RI [9]. Variations in pellet thickness and surface planarity can degrade the accuracy of the extracted optical constants. Sample thickness is utilised in the extraction of the RI and thus is a potential source of error [20]. In the case of pellets, however, it is well known that pellets experience material- and pressure-dependent axial relaxation, leading to a change in sample thickness and, in some cases, to porosity [10]. The pellets, therefore, must be allowed to relax for a certain period before THz analysis to minimise this source of error. While sample porosity is minimised by increasing the applied pressure, high compaction pressure can cause cracking, lamination and capping. Lamination and capping refer to the breakage of samples during ejection from the die, where capping is the breakage of the top and lamination is the splitting of the sample into layers [21; 22; 23]. These defects can result in additional interfaces for refraction and reflection that produce variations between samples. Pellets should be created at a pressure that results in a pellet with a tensile strength of at least 2 MPa [10]. This value is generally used in the pharmaceutical industry to ensure that pharmaceutical tablets have the required structural integrity to survive further handling and transport. In terms of THz-TDS, the sample also requires a certain strength to ensure that samples do not change structurally between fabrication and measurement. With all these properties in mind and the inherent difference in polymer batches, it is evident that a procedure is required to investigate the polymer properties before creating analytical samples. Even though sample preparation methods using polymers and hydraulic presses have been used extensively to analyse various materials, comprehensive studies on the polymers employed and the optimal conditions to make these samples are scarce.