Isothermal crystallization of polyethylene monitored by in situ NMR and analyzed within the “Avrami” model framework

The shape of the solid-state proton NMR Free Induction Decay (FID) of a molten polyethylene (PE) sample has been examined as a function of time during isothermal crystallization in situ. The crystallization rate as a function of crystallization temperature is derived and the results discussed within a thermodynamic framework. A slight increase in molecular correlation times (in the microsecond range) with crystallization time was revealed for both the crystalline and intermediate phases. In particular, the molecular mobility within the intermediate phase is found to be approximately four times faster than in the crystalline phase. During isothermal crystallization the more mobile molecular phase is claimed to be composed of two “sub-phases”, an amorphous phase and a molten phase. The latter “sub-phase”, which represents 100% of the polymer at the start of the crystallization (completely molten sample), transforms completely to crystalline, amorphous and intermediate phases during the crystallization process. The actual temperature region of super-cooling, which can be probed by the present NMR technique, covers approximately 10 K and is discussed in the text.