## 2 Materials and Methods
### THz Time-Domain Spectroscopy Set-Up  
The THz spectra were captured via transmission mode at a temperature of around 300 K, utilizing a Menlo Systems (Planegg, Germany) TERA K15 THz-TDS system. This setup featured photoconductive antennae stimulated by a femtosecond fiber-coupled laser (Menlo Systems T-Light). The femtosecond laser emitted light at a wavelength of 1560 nm, with a repetition frequency of 100 MHz and an optical pulse duration of roughly 90 fs. The THz radiation emitted by the photoconductive antenna underwent collection and focusing through a pair of polymethylpentene (TPX) lenses, each with a 50 mm focal length. The radiation transmitted through the sample pellets was then gathered and directed onto the detector antenna, employing another identical pair of TPX lenses with a 50 mm focal length.  
For all measurements, the delay line scan range covered 200 ps, resulting in a spectral resolution of 5 GHz. The data points were recorded at intervals of 33.3 fs, with a scanning rate of approximately 4 Hz. Each spectrum was derived from an average of 2163 scans, each spanning roughly 0.23 s. This means that the acquisition time for each sample or reference was 500 s. The system's spectral range extended from 0.3 to 4.4 THz.  
The frequency-dependent dynamic range (\(DR\)) measured approximately 85 dB at 0.35 THz and approximately 22 dB at 3.5 THz (refer to Supplementary Materials, Figures S1 and S2).  
The pellets were positioned within the transmission lines, and their two flat parallel surfaces were set perpendicular to the THz beam line axis, all within a sample holder. The sample holder was a plate of 14.5 \(\times\) 14.5 cm\({}^{2}\) and with a thickness of 0.6 cm with 16 circular pellet slots and an opening of 12 mm in diameter, realized in polylactic acid (PLA) by using a 3D printer ANYCUBIC i3 MEGA (Shenzhen, China). The sample holder was affixed to a bidirectional translation stage designed for mapping purposes, enabling movement within the \(xy\) plane perpendicular to the THz beam. This configuration facilitated automated sequential measurements of all samples. Movements along the \(x\) and \(y\) directions were synchronized with THz measurements by using a self-made program in a LabVIEW (NI, Austin, TX, USA) environment. For each pellet, three transmission measurements were performed in different spots separated by 2 mm. The spectral measurements were performed out of the THz focus (approximately 7 mm away from the THz nominal focus) so that the THz pulse could probe a large area, thus, minimizing the pellets' heterogeneity-related effects. The THz beam profile was carefully characterized before measurements by means of knife-edge measurements [40; 41] at the sample position in order to know its size and assure its complete transmission through the pellets. The measured THz spot was approximately 7 mm in diameter.  
Since water vapor, due to environmental humidity, absorbs THz radiation at frequencies corresponding to the molecular rotational transitions [42], the THz set-up was placed into a closed compartment purged with dry nitrogen (N\({}_{2}\)). The relative humidity (RH) inside the compartment was measured by using a data logger HygroLog HL-20D (Rotronic, Switzerland). The spectral measurements were not carried out until the datalogger showed RH = 0 within its accuracy and the water vapor absorption lines were indistinguishable from noise up to 2.5 THz (nearly 120 min of purging before the first spectra acquisition).