Plasmas offer a unique environment to enhance and change the properties of polymeric materials. These enhancements can be well characterized by various methods based on radiation measurement schemes. Using a novel approach based on a dual-RF plasma generation scheme, polypropylene samples were processed in argon-rich environments for varying treatment parameters such as power and duration of exposure. By using terahertz (THz) time-domain spectroscopy (TDS) the change in crystallinity of thin polypropylene sheets was observed through a correlated change in refractive index of the material in the low-frequency THz range (0.1-0.6 THz). The THz measurements were further supported by the increase in the strength of the first order X-ray diffraction (XRD) peaks from the semi-crystalline polypropylene unit cell structure typically associated with the alpha-phase. The corresponding change in refractive index in the low-frequency THz range with increase in crystallinity as observed in the XRD measurements is explained through the increase in structural order of the polymer sample. These measurements show that tools such as THz-TDS can be useful in understanding the relative change in structure of polymer materials when processed using plasma-based techniques.