Thermomorphological study of the terahertz lattice modes in polyvinylidene fluoride and high-density polyethylene

Semicrystalline polymers possess a complex morphology comprising both crystalline and amorphous domains. We investigate two representatives from this group, polyvinylidene fluoride and high-density polyethylene, with terahertz time-domain spectroscopy. Intriguingly, the thermal gradient of lattice modes arising from the crystalline phase changes at the glass transition. Thus, terahertz spectroscopy can identify the glass transition temperature Tg even in highly crystalline polymers, where conventional methods such as differential scanning calorimetry fail. Furthermore, these findings provide experimental evidence of the strong interactions between the amorphous and the crystalline domains, as the glass transition is an exclusive feature of the mobile amorphous fraction.