TD-NMR analysis of structural evolution in PVDF induced by stress relaxation

Poly(vinylidene fluoride) was relaxed at different temperatures (23, 80 and 120 °C) and strains (3.5, 7 and 10%) during 24 h. The material, as processed and all relaxed conditions, was characterized by tensile tests and time-domain nuclear magnetic resonance (TD-NMR). Tensile tests after stress relaxation showed a huge drop in the elastic modulus, varying from 30% to 45% compared to the as processed material. The TD-NMR technique allowed to correlate the variation of the mechanical property with the evolution of the structure inside the material, namely, decrease of crystalline fraction and increase of constrained amorphous region due to the stress relaxation. However, the free amorphous fraction did not undergo a significant change. The structure evolution described above occurred in nanoscale, even for the smallest strain (3.5%) and the lowest temperature (23 °C) tested here, while no visual change on the specimens was noticed, even for the extreme conditions (10% and 120 °C).