Formation of polymer/carbon nanotubes nano-hybrid shish–kebab via non-isothermal crystallization

Abstract

Multi-walled carbon nanotubes (MWNTs) periodically decorated with polyethylene (PE) lamellar crystals had been prepared using the non-isothermal crystallization method. The morphology and structure of polyethylene attached to MWNTs were investigated by means of transmission electron microscopy (TEM). A nano-hybrid shish–kebab (NHSK) structure was observed wherein the average diameter of PE lamellar crystals varies from 30 to 150 nm with average periodicity of 35–80 nm. The TEM images of samples obtained at 125 °C showed that MWNTs were first wrapped by a homogeneous coating of PE with few subglobules, then PE chains epitaxially grew from the subglobule and formed lamellar crystals perpendicular to the carbon nanotube axis. It is suggested that the homogeneous coating plays a key role in the formation of NHSK structures. And the formation process was discussed based on the intermediate state images of samples obtained at 95 °C. While NHSK structures cannot be formed by using polypropylene (PP). This may attribute to the zigzagged conformation of PP chains on the surface of MWNTs, which hinders the formation of homogeneous coating of PP on it.

Introduction

The discovery of carbon nanotubes and carbon nanotube based materials has inspired scientists for a range of potential applications due to their extraordinary mechanical, electrical, and optical properties [1], [2], [3]. The polymer/carbon nanotube composites have attracted wide attention. However, fabrication of homogeneous nanocomposites with carbon nanotubes remains a great technical challenge. Due to the CNTs' intrinsic poor solubility, they often self-assembled in bundles, which limited their application. Surface functionalization is an effective and necessary approach to improve its compatibility with solvent or polymer matrix. Covalent functionalization of carbon nanotubes has been envisaged as a very important method for nanotubes processing and application [4], [5], [6]. Although a variety of chemical routes have been investigated to achieve nanotube solubility, the conjugation of the CNTs sidewall is disrupted, and electrical and mechanical properties of the covalent functionalized CNTs decrease dramatically compared to the pristine ones due to the breaking of the sp² conformation of the carbon atom. On the other hand, the non-covalent method involves wrapping of the nanotubes by surfactants, polymerizable monomer and polymers [7], [8], [9], [10], [11], [12]. And the integrity of CNTs' sidewall structure is preserved, since the wrapping molecules is fixed onto CNTs by Van der Waals force or by forming π-stacking.

Numerous studies show that carbon materials, in various forms, are able to induce polymer crystallization. Tracz et al. [13] observed highly ordered contact layers of polyethylene crystallized from the melt on highly oriented pyrolytic graphite (HOPG) by atomic force microscopy. Czerw et al. [14] used the carbon nanotubes to pattern a high molecular weight polymer. The resulting regularity of poly(propionylethylenimine-co-ethylenimine) (PPEI-EI) on CNTs suggested a “nanotube-driven” crystallization process. Based on the former studies, Li et al. [15], [16], [17] reported the controlled polymer solution crystallization method to achieve polymer decorated CNTs. PE and Nylon 66 were found to be able to periodically grow along the CNT axis and form nano-hybrid shish–kebab (NHSK) structures. The formation mechanism of the NHSK was attributed to “size-dependent soft epitaxy”. Zhang et al. [18] also reported the formation of NHSK using supercritical CO₂ as antisolvent to induce polymer epitaxial growth on CNTs. These studies offered a new route to functionalize any kinds of pristine CNTs with different diameter or chirality. And the NHSK can be used as templets to prepare nanomaterials with special properties, because of the unique structures of the NHSK.

Meanwhile there is plenty of research work being done in the field of molecular dynamics simulations of the interaction between polymer chains and carbon nanotubes [19], [20], [21], [22], [23], [24], [25]. Yang et al. [19] studied the isothermal crystallization process of PE chain on SWNT (10, 10), which indicated that PE chain was first adsorbed onto the SWNT surface, and then orientated to an ordered lamellae. Wei [20] found that CNT induced high orientation of the PE chains along the nanotube axis, which was correlated with the high density in the first adsorption layer around CNT (10, 0). Liu et al. [21] also reported that PE tended to form an extended conformation to wrap carbon nanotubes, while PP adopted a zigzagged conformation on the nanotubes surface instead of helical conformation.

In current work, we explored the feasibility of synthesizing PE/MWNTs NHSK structures by non-isothermal crystallization method. Compared with previously reported crystalline polymer periodically wrapping to functionalize CNTs via an isothermal solution crystallization technique, non-isothermal crystallization method would be attractive since the manipulation process is easier and similar to the actual industrial process. The formation mechanism of this method was investigated based on the intermediate state obtained at different stages and the MD simulation results reported. PP was also employed in our studies due to the different repeat unit arrangement and conformation between PP and PE.