A comparative study of melt spun polyamide-12 fibres reinforced with carbon nanotubes and nanofibres
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.
Section snippets
Materials and experimental details
The arc-grown nanotubes (AGNT) [25], aligned catalytically-grown nanotubes (aCGNT) [26], and entangled catalytically-grown nanotubes (eCGNT) [27] were produced using previously reported techniques. The catalytically-grown nanofibres (CNF) were purchased from Applied Sciences Inc., USA, grade PR-19-PS. The average outer diameters of the nanomaterials were 15, 43, 10 and 155 nm, respectively. All materials were multi-walled and used as produced. All of the catalytically-grown materials were
Results
Macroscopically, all of the as-spun polyamide-12 nanocomposite fibres appeared to be of generally good quality, with a reasonable surface finish and uniform diameter, regardless of catalytic nanotube filler type and loading fraction. The entangled multi-wall carbon nanotubes led to a slightly rougher fibre surface compared to the carbon nanofibres and aligned multi-wall carbon nanotubes. Only the nanocomposite fibres containing the arc-grown multi-wall carbon nanotubes showed nonuniformity in
Discussion
It is important that the new results on nanotube composites are set in the context of other, more established, nanocomposites. Clay-containing, nanocomposites, which are frequently based on polyamides, have received widespread attention due to the possibility of tailoring their strength/stiffness/toughness balance as well as improving the thermal stability, fire retardance, and gas barrier characteristics. Recently, nanoclay-reinforced polyamide-6 fibres have been produced, in this case for
Conclusions
This study has successfully explored the potential of various multi-wall carbon nanotubes and nanofibres as mechanical reinforcements in polyamide-12 composite fibres. Using as-produced nanomaterials, filler loading fractions of up to 15 wt% were realised, by following thermoplastic processing techniques to produce melt-spun nanocomposites. The vapour-grown carbon nanofibres and CVD-grown multi-wall carbon nanotubes dispersed well into the matrix and provided linear increases in initial
Acknowledgements
The authors would like to thank the National Centre for Biomedical Sciences in Ireland and DSM for the use of their equipment and Prof. J.-B. Nagy (Namur) for the supply of nanotube material. Financial support from the EC Thematic Network ‘CNT-NET’ [G5RT-CT-2001-05026], the EPSRC, and the Cambridge European Trust is gratefully acknowledged.
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