Crystallization and melting behavior of multi-walled carbon nanotube-reinforced nylon-6 composites

The crystallization and melting behavior of neat nylon-6 (PA6) and multi-walled carbon nanotubes (MWNTs)/PA6 composites prepared by simple melt-compounding was comparatively studied. Differential scanning calorimetry (DSC) results show two crystallization exotherms (TCC, 1 and TCC, 2) for PA6/MWNTs composites instead of a single exotherm (TCC, 1) for the neat matrix. The formation of the higher-temperature exotherm TCC, 2 is closely related to the addition of MWNTs. X-ray diffraction (XRD) results indicate that only the α-phase crystalline structure is formed upon incorporating MWNTs into PA6 matrix, independently of the cooling rate and annealing conditions. These observations are significantly different from those for PA6 matrix, where the increase in cooling rate or decrease in annealing temperature results in the crystal transformation from α-phase to γ-phase. The crystallization behavior of PA6/MWNTs composites is also significantly different from those reported in PA6/nanoclay systems, probably due to the difference in nanofiller geometry between one-dimensional MWNTs and two-dimensional nanoclay platelets. The nucleation sites provided by carbon nanotubes seem to be favorable to the formation of thermodynamically stable α-phase crystals of PA6. The dominant α-phase crystals in PA6/MWNTs composites may play an important role in the remarkable enhancement of mechanical properties