Structural phase transition of alkane molecules in nanotube composites

The structural phase transition and crystallization process of alkane molecules in carbon nanotube (CNT) composites are studied through molecular dynamics simulations. An isotropic-to-nematic transition with molecules aligned with embedded nanotubes is found. Further smectic transition of the alkane molecules is found with small radius armchaired CNT (5, 5) as a nucleation site, where molecules form lamellar layers along the nanotube axis and have two-dimensional structure ordering in planes perpendicular to the tube axis, in analogy to a crystalline polymer. While the translational diffusions are limited in the smectic phase, the molecules have rotation freedom along its long axis with small energy barriers similar to 1-2k(B)T (T=300 K) for decane molecules studied here. The molecule crystallization is found to strongly dependent on CNT chirality, where a lack of two-dimensional ordering of molecules around a similar radius zigzag CNT (10, 0) is due to diverse molecule assembly domains with tilted wrapping angles at the interface, controlled by the interactions with the substrate nanotube lattice. The influence of strength and nature of the interfacial interactions between molecules and nanotubes is also studied. The results in this study would be useful and important for the understanding of structural phases in hybrid polymer materials and for the designs of nanotube based high performance composites.