In situ polymerization approach to multiwalled carbon nanotubes-reinforced nylon 1010 composites: Mechanical properties and crystallization behavior

Abstract

A series of polyamide 1010 (PA1010 or nylon 1010) and multiwalled carbon nanotubes (MWNTs) composites were prepared by in situ polymerization of carboxylic acid-functionalized MWNTs (MWNT–COOH) and nylon monomer salts. Mechanical tensile tests and dynamic mechanical analysis (DMA) show that the Young modulus increases as the content of the nanotubes increases. Compared with pure PA1010, the Young's modulus and the storage modulus of MWNTs/PA1010 in situ composites are significantly improved by ca. 87.3% and 197% (at 0 °C), respectively, when the content of MWNTs is 30.0 wt%. The elongation at break of MWNTs/PA1010 composites decreases with increasing proportion of MWNTs. For the composites containing 1.0 wt% MWNTs, the Young modulus increases by ca. 27.4%, while the elongation at break only decreases by ca. 5.4% as compared with pure PA1010 prepared under the same experimental conditions. Compared with mechanical blending of MWNTs with pure PA1010, the in situ-prepared composites exhibit a much higher Young's modulus, indicating that the in situ polycondensation method improves mechanical strength of nanocomposites. Scanning electron microscopy (SEM) imaging showed that MWNTs on the fractured surfaces of the composites are uniformly dispersed and exhibit strong interfacial adhesion with the polymer matrix. Moreover, unique crystallization and melting behaviors for MWNTs/PA1010 in situ composites are observed using a combination of differential scanning calorimetry (DSC) and X-ray diffraction methods. It was shown that only the α-form crystals are observed in our MWNTs/PA1010 in situ composites. This result is quite different from PA1010/montmorillonite and PA6-clay composites, where both of α- and γ-form crystals were found.

Introduction

Accompanying the interest of nanoscience and nanotechnology, is the increased focus in the preparation of nanomaterial/polymer nanocomposites and nanohybrid materials in both academic and industrial fields. It promises an easy way to (1) improve the comprehensive properties of pure nanomaterials, and (2) design, develop and fabricate novel nanomaterials by simple chemical/physical techniques. In this regard, nanocomposites that utilize carbon nanotubes (CNTs) [1] and polymers are of particular interest. CNTs offer advantages, such as, excellent mechanical strength and high electronic and thermal conductivity. Polymeric materials present processing-ability, flexibility and can be chemically combined with CNTs [2]. Such nanocomposites can be generally prepared by melt mixing, solution processing and in situ polymerization in terms of the reaction manner [2]. They can also be prepared through the following four strategies according to the nature of interaction between CNTs and polymer (Fig. 1):

• non-covalent blending or mixing of CNTs with polymers;

• covalent linkage of CNTs with polymers;

• specific adsorption or assembly;

• compounding previously functionalized CNTs with polymers.

Strategy (i) results in poor attraction between CNTs and the mixed polymer. Until now, direct solution [3], [4], [5], melt [6], [7], [8], [9], [10], [11], [12], [13] and polymerization mixing methods have been employed. Direct solution and melt mixing methods are based on mixing CNTs with macromolecules. Due to entanglement, phase-aggregation and steric hindrance between macromolecules, CNT diffusion during the processing is retarded, resulting in relatively poor dispersion in the composites. Therefore, the polymerization mixing method was developed to improve CNT dispersibility. This involves initial CNT dispersal in a solution of monomer (and initiator/catalyst), and subsequent polymerization of the monomer followed by the removal of solvents [14]. Despite efforts, the dispersibility of CNTs in non-covalent systems is limited due to poor wet-ability (high surface tension), strongly associated tube bundles, and weak interaction between the tubes and polymer chains.

To address this issue, strategy (ii) was adopted and based on covalent bonding between tubes and polymers. The so-called ‘grafting to’ (attaching macromolecules with terminal functional groups to CNTs) and ‘grafting from’ (in situ polymerization of monomers in the presence of CNTs or CNT-based macroinitiators) approaches were employed to make the CNT-polymer adducts or hybrids. Hence, various linear (PS [15], [16], PMMA [17], polyimide [18], poly(vinyl alcohol) (PVA) [19], poly(m-aminobenzene sulfonic acid) (PABS) [20], poly(sodium 4-stryrenesulfonate) [21], poly(N-isopropyl acrylamide) [22], poly(4-vinylpyridine) [23] and poly(N-vinylcarbazole) [24]) and highly branched polymers (poly(3-ethyl-3-hydroxymethyloxetane) [25], poly(amidoamine) [26] and dendrons [27]) have been successfully grafted onto CNT surfaces by the ‘grafting to’ or ‘grafting from’ approach. After the polymer is grafted to the CNTs, the resulting nanohybrids show good solubility and dispersibility in solvents, especially for adducts coated with a high density of polymer. In these cases, the CNTs are shortened to several micrometers or hundreds of nanometers, and are individually separated from each other. Consequently, individual core-shell nanocables composed of a carbon nanotube core and polymer shell can be attained [17], [25]. There is little doubt that covalent functionlization of CNTs presents an important route for the design, synthesis and application of CNT-based nanomaterials and nanodevices. However, large-scale availability of these hybrid materials remains a challenge.

Strategy (iii) introduces new methods for the functionalization of CNTs as well as the preparation of CNT/polymer composites, based on specific interactions like π–π stacking [28], [29], [30], [31], π-charge [32], [33], [34] and charge–charge attractions [21], [35], [36], [37], [38], [39] or supramolecular assembly [21], [39], [40] of polymers on the surface of CNTs. Obviously, this strategy was based on the ‘specific’ interaction and is only effective for compounds which can show ‘specific’ action with CNTs.

Strategy (iv) presents an alternate route for the preparation of CNTs/polymer composites by utilizing functionalized CNTs, and satisfies both requirements for good dispersibility and large-scale synthesis. The nature of this strategy is defined by the polymer chains, which are covalently linked to the CNT surface (or anchored polymer chains), while other polymer chains are free in the composites. The anchored polymer ensures good dispersibility, and the free polymer fraction is controllable, affording an easily tunable CNT:polymer weight ratio.

In this strategy, three methods can be utilized. Firstly, there is condensation compounding, in which oxidized or other functionalized CNTs are reacted with macromolecules by amidation [41], [42], [43] and other reactions [7], [30], [44]. As mentioned before, such a reaction and dispersion is limited because of the macromolecular effect. Nevertheless, it exhibited better performance as compared to strategy (i), as demonstrated by several groups. Secondly, there is mix compounding, from which polymer-grafted CNTs are mechanically mixed with pure polymer. Using this method, Sun et al. [18] prepared CNT-polyimide/polyimide composites with excellent CNT dispersion, due to dissolution of the polyimide-functionalized CNTs with a pure polyimide solution, and followed by evaporation of the solvent. Thirdly, there is polymerization compounding, in which polymer chains can be either tethered to functionalized CNTs or be free in the solution during the polymerization of the monomer. Due to the higher mobility of the monomer compared to that of macromolecules, the dispersion of CNTs in the ‘polymerization compounding’ nanocomposites are expected to be better than that in ‘condensation compounding’ nanocomposites, therefore better mechanical and other properties are expected. However, there are few reports using this method, which will be addressed and discussed in this paper.

The primary aim of this work is to prepare CNTs/polymer composites by the polymerization compounding (or in situ polymerization) method, followed by investigation of their mechanical and thermal properties. Considering polyamide 1010 (PA1010 or nylon 1010) as an important engineering plastic due to high intensity, elasticity, toughness and abrasive resistance (but poor module [45]), we focus on CNTs/PA1010 composites herein. Multiwalled carbon nanotubes (MWNTs) were initially functionalized using a solution of sulphuric acid and nitric acid in order to introduce carboxylic acid groups on the tube surfaces. The MWNTs/PA1010 composites were then prepared by the polymerization compounding method (i.e. polymerization of PA1010's monomer salts in the presence of the oxidized MWNTs). In order to evaluate the mechanical performance of the composites, MWNTs/PA1010 composites, made by the ‘blending’ or ‘condensation compounding’ methods, were obtained for comparison.