A comparative study of melt spun polyamide-12 fibres reinforced with carbon nanotubes and nanofibres

Abstract

A range of multi-wall carbon nanotubes and carbon nanofibres were mixed with a polyamide-12 matrix using a twin-screw microextruder, and the resulting blends spun to produce a series of reinforced polymer fibres. The aim was to compare the dispersion and resulting mechanical properties achieved for nanotubes produced by the electric arc and a variety of chemical vapour deposition techniques. A high quality of dispersion was achieved for all the catalytically-grown materials and the greatest improvements in stiffness were observed using aligned, substrate-grown, carbon nanotubes. The use of entangled multi-wall carbon nanotubes led to the most pronounced increase in yield stress, most likely as result of increased constraint of the polymer matrix due to their relatively high surface area. The degrees of polymer and nanofiller alignment and the morphology of the polymer matrix were assessed using X-ray diffraction and differential scanning calorimetry. The carbon nanotubes were found to act as nucleation sites under slow cooling conditions, the effect scaling with effective surface area. Nevertheless, no significant variations in polymer morphology as a function of nanoscale filler type and loading fraction were observed under the melt spinning conditions applied. A simple rule-of-mixture evaluation of the nanocomposite stiffness revealed a higher effective modulus for the multi-wall carbon nanotubes compared to the carbon nanofibres, as a result of improved graphitic crystallinity. In addition, this approach allowed a general comparison of the effective nanotube modulus with those of nanoclays as well as common short glass and carbon fibre fillers in melt-blended polyamide composites. The experimental results further highlight the fact that the intrinsic crystalline quality, as well as the straightness of the embedded nanotubes, are significant factors influencing the reinforcement capability.

Introduction

Natural materials such as bone, tooth and nacre are very effective examples of nanocomposites consisting of proteins and minerals. The concept of exploiting the unusual properties of nanoscale reinforcement has found widespread interest in the polymer field. In fact, some of these nanocomposites have been under investigation for many years and have long found their commercial applications. Carbon-black-filled rubber for automotive applications is an excellent example. Within the last decade, carbon nanotubes have stimulated considerable scientific interest due to their exciting physical and mechanical properties, which result from their unique microstructure. Recent experimental investigations have demonstrated the potential of carbon nanotubes as small volume fraction reinforcements in polymer matrix systems; for comprehensive reviews see Refs. [1], [2].

The production of carbon nanotubes in large quantities, high purities, and with a uniform size still poses significant challenges, but an increasing number of companies are focussing on scaled-up synthesis routes. Although highly crystalline materials can be most readily grown at high temperatures using electric arcs or laser ablation, large-scale synthesis efforts are mostly based on chemical vapour deposition (CVD) techniques that use moderate temperatures to decompose hydrocarbons over transition metals. The mechanical properties of individual multi-wall carbon nanotubes have been shown experimentally to depend strongly on the intrinsic crystalline quality of the material; the presence of stacking defects and pronounced structural disorder can reduce the elastic modulus by orders of magnitude [3]. Furthermore, CVD processes can lead to significant entanglement of the nanotubes. On the other hand, high-temperature growth processes such as the electric arc-discharge produce low nanotube volumes and purities. Although it is clearly important to select the appropriate nanotube material for a given application, very few systematic comparisons have been made.

The most promising current approaches towards increasing the orientation of nanoscale reinforcements within a matrix include optimisation of the extrusion die [4] and stretching the composite melt to form films [5], [6], [7] and fibres [8], [9], [10], [11], [12], [13], [14]. One complication is that the microstructure of semicrystalline thermoplastic polymer matrices is influenced not only by the processing history but also by the presence of nanoparticles. The addition of various types of carbon nanotubes and nanofibres to thermoplastics has already been observed to influence the crystallisation kinetics and resulting morphology [14], [15], [16], [17], [18], [19]. Such changes in matrix morphology need to be considered when evaluating the nanocomposite performance with regard to the intrinsic filler properties [20], [21]. The effects of carbon nanotubes or nanofibres on such oriented polymer systems, although significant [21], have not yet been fully established. Finally, it should be noted that the presence of additives such as colouring pigments has been shown to influence matrix morphology during fibre spinning [22], whilst there is the whole technology of nucleating agents which are deliberately added to influence crystalline microstructure.

To date, only relatively modest improvements in melt-blended nanotube composite strength and stiffness have been observed. These experimental results are probably symptomatic of the early stages of development of nanotube composite systems and have been explained in terms of a low level of dispersion, the use of intrinsically defective carbon material, and possibly poor intra-tube or interfacial shear strengths. A further critical issue is the shape of carbon nanotubes embedded in the polymer matrix. Carbon nanotube waviness may significantly reduce their reinforcement capability [23], [24]. Since results to date, for large-scale CVD carbon nanotubes and nanofibres, do not yet indicate that the intrinsic mechanical properties are superior to conventional fillers, there is particular interest in using nanotubes in fine structures, such as spun fibres or micro-injection mouldings, where conventional chopped fibres cannot be physically accommodated. The approach used in this study explores this idea and was aimed at evaluating the reinforcement potential of a range of multi-wall carbon nanotubes in comparison to vapour-grown carbon nanofibres in as-spun polyamide-12 fibres. This approach allowed a range of filler loading fractions to be investigated using relatively small volumes of the various nanomaterials. Special emphasis was placed on the characterisation of the fillers' influence on the matrix crystallisation kinetics and the resulting polymer morphology.