Molecular dynamics and crystallization precursors in polylactide and poly(lactide)/CNT biocomposites in the insulating state

The early stages of the cold crystallization process and the formation of a rigid amorphous phase as seen by the dielectric response of polylactide, PLA, and composites polylactide/carbon nanotubes, PLA/CNT, are studied here by broadband dielectric spectroscopy for CNT concentrations below percolation. The presence of precursors during the nucleation and crystallization process is demonstrated by the existence of a time shift between the decline in the number of mobile segments and the growth of a 3D ordered phase as seen by variable temperature wide angle X-ray scattering measurements. Also, the loss of the mobile amorphous phase is not justified by the slow lamellar growth in identical conditions. The variation of the molecular dynamics, either for short range reorientations or cooperative motions, is followed in both amorphous and semicrystalline states. The changes observed in the composites PLA/CNT show a moderate heterogeneous nucleating effect of the nanofiller and a sensitivity of the three subglass transitions to the chain ordering. The relaxation parameters of the segmental relaxation are not very sensitive to the presence either of lamellae or of the nanofiller.