Thermal and morphological characterization of nanocomposites prepared by in-situ polymerization of high-density polyethylene on carbon nanotubes

The morphology, nucleation, and crystallization of polyethylene/carbon nanotubes nanocomposites were studied. The nanocomposites were prepared by in-situ polymerization of ethylene on carbon nanotubes (CNT) whose surface had been previously treated with a metallocene catalytic system. The effects of composition (5-22% CNT) and structure of the nanotube (single, double, or multiwall, i.e., SWNT, DWNT, and MWNT) were evaluated, and an excellent nucleating effect on polyethylene matrix was found regardless of the CNT type in comparison to neat high-density polyethylene (HDPE) prepared under identical conditions. The CNT were found to be more efficient in nucleating the HDPE than its own crystal fragments, a result obtained by self-nucleation studies. Differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) results showed that under both isothermal and dynamic crystallization conditions the crystals produced within the nanocomposite HDPE matrix were more stable than those produced in neat HDPE or in physical blends prepared by melt mixing of HDPE and untreated CNT. The remarkable stability of the crystals was reflected in melting points up to 5 degrees C higher than neat HDPE and concomitant thicker lamellae. The changes induced on HDPE by CNT are due to the way the nanocomposites were prepared; since the macromolecular chains grow from the surface of the nanotube where the metallocene catalyst has been deposited, this produces a remarkable nucleating effect and bottle brush morphology around the CNT. Isothermal crystallization kinetics results showed that the in-situ nanocomposites crystallize much faster at equivalent supercoolings than neat HDPE because of the nucleating effect of CNT. Wide-angle X-ray scattering studies demonstrated that the crystalline structure of the HDPE matrix within the in-situ-polymerized HDPE/CNT nanocomposites was identical to that of neat HDPE and did not change during isothermal crystallization, keeping its orthorhombic unit cell.