Elsevier

Polymer

Volume 47, Issue 5, 22 February 2006, Pages 1704-1714
Polymer

Morphology and properties of melt-spun polycarbonate fibers containing single- and multi-wall carbon nanotubes

https://doi.org/10.1016/j.polymer.2006.01.003Get rights and content

Abstract

Polycarbonate fibers based single wall and multi-wall nanotubes (SWNT and MWNT) were prepared by first dispersing the nanotubes via solvent blending and/or melt extrusion followed by melt spinning the composites to facilitate nanotube alignment along the fiber axis. Morphological studies involving polarized Raman spectroscopy and wide angle X-ray scattering using a synchrotron radiation source show that reasonable levels of nanotube alignment are achievable. Detailed transmission electron microscopy (TEM) investigations on the polymer-extracted composite fibers reveal that MWNT more readily disperse within the PC matrix and have higher aspect ratios than do SWNT; extraction of the polymer from the composite prior to TEM imaging helps overcome the common issue of poor atomic contrast between the CNT and the organic matrix. Stress–strain analysis on the composites fibers show that MWNT, in general, provide greater stiffness and strength than those based on SWNT. Despite significant reinforcement of the polycarbonate, the level of reinforcement is far below what could be achieved if the nanotubes were completely dispersed and aligned along the fiber axis as predicted by composite theory.

Introduction

Carbon nanotubes (CNTs) are a new class of lightweight materials that posses extraordinary mechanical, electrical, and thermal properties. For example, the tensile modulus and strength of single and multiwalled nanotubes (SWNT and MWNT) have been estimated experimentally and theoretically to be of the order of 1 TPa and 30 GPa, respectively [1], [2], [3], [4]. The combination of CNTs' low density, mechanical performance, and high aspect ratio make them excellent candidates for their use as fillers in high performance polymer nanocomposites. Such nanocomposites are attractive for their potential uses in aerospace and defense-related applications as well as in more conventional applications, e.g. automotive parts. However, full exploitation of the mechanical properties of CNTs in polymer composite applications will require exceptional control of dispersion and attainment of high degrees of alignment of individual tubes within the polymer matrix.

Significant progress is being made to address the issues of CNT dispersion and alignment. For example, surfactants and amphiphilic polymers, e.g. sodium dodecyl sulfate [5], sodium dodecylbenzene sulfonate [6], [7], octylphenolethoxylate [8], poly(vinyl pryrollidone) [9], and Gum Arabic [10], are capable of dispersing low concentrations of SWNTs in solvents. Unfortunately, the low concentrations (≪1%) and difficultly in removing the surfactant from the final product inhibit commercialization of this approach. More recent efforts in our laboratories [11], [12] and elsewhere [13] have used moderately to highly acidic solutions, respectively, to disperse relatively high concentrations of SWNTs (∼1–10 wt%) without the need of surfactants. A downside to strong acids is the tendency to degrade the nanotubes. The need for a more practical and environmentally conducive means of dispersing CNT within polymers have led researchers to evaluate direct melt processing of CNT–polymer mixtures, e.g. via twin screw extrusion, which has yielded promising results [14], [15], [16], [17], [18]. With regard to alignment, a number of approaches ranging from external magnetic fields [19], [20] to electrospinning [21], [22], [23], [24] to more conventional fiber spinning techniques [16], [25], [26], [27], [28], [29] have been used to orient CNTs. Specifically, Hagenmuller et al. [25], [30], Sennett et al. [16], and Sreekumar et al. [27] reported high degrees of orientation of CNTs in melt-spun PMMA and PC fibers and dry–wet spun PAN fibers, respectively, as determined by polarized Raman spectroscopy, infrared spectroscopy, and transmission electron microscopy (TEM).

The aim of the present work is (i) to achieve good nanotube dispersion throughout the polymer matrix using solution and melt compounding methodologies, (ii) to promote nanotube alignment using the technique of fiber melt spinning, and (iii) to develop an understanding of the relationship between nanocomposite morphology and properties. Commercially available grades of SWNTs and multi-wall carbon nanotubes (MWNTs) have been incorporated into polycarbonate. Polycarbonate was chosen because of its good combination of stiffness and ductility, its ability to disperse CNTs via melt compounding, and because of its amorphous structure. An amorphous polymer was desired since fillers are well known to alter the crystalline morphology in semi-crystalline polymers which can complicate the interpretation of nanocomposite structure–property relationships.

Section snippets

Materials

Table 1 lists the materials used in this work. A high molecular weight grade of PC was chosen to generate high shear forces that could aid in the breakup and dispersion of the carbon nanotube bundles during melt compounding and to facilitate alignment during melt spinning of fibers. The higher molecular weight material also provides good melt strength and high levels of ductility which are needed when drawing fibers during melt spinning.

Melt processing

Fig. 1 shows the different processing routes used to form

Nanocomposite morphology

Various analytical methods can be used to determine the microstructure of the composites. For SWNTs, polarized Raman spectroscopy works well, while wide angle X-ray scattering can be used only on the MWCNTs. TEM is a challenge for both types of samples. Fig. 2 shows polarized Raman spectra for PC fibers containing SWNT-soln and SWNT-dry obtained at various fiber angles, ψ, relative to the polarization direction. Each set of spectra exhibits peaks at frequencies between ∼160–265 cm−1 and at ∼1310

Discussion

The above results indicate that reasonable levels of CNT dispersion and alignment can be achieved using solution and melt compounding methodologies combined with fiber melt spinning. With regard to nanotube alignment, analyses on Raman spectra and WAXD patterns confirm quantitatively that intermediate levels of CNT orientation are achieved along the fiber axis. However, because SWNTs do not give a recognizable wide angle X-ray signal, and MWNTs do not provide useful Raman data for determining

Conclusions

Polycarbonate composite fibers containing SWNT and MWNT were fabricated by first dispersing the nanotubes by solvent blending and/or melt extrusion followed by melt spinning the composites to promote tube alignment along the fiber axis. MWNT were found to more readily disperse within the PC matrix and have larger particle aspect ratios than SWNT as determined by transmission electron microscopy; careful extraction of the PC matrix prior to TEM investigations helped circumvent the issue of poor

Acknowledgements

The authors Dr Vaughn Samuelson and James O'Connor of DuPont for their assistance with the initial setup of the melt press spinner, and Prof Robert E. Cohen of MIT for the use of the Haake Mini-Lab extruder. This material is based upon work supported by, or in part by, the US Army Research Laboratory and the US Army Research Office under contract DAAD-19-02-D-0002.

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    Present address: Lord Corporation, Chemical R&D, 110 Lord Dr., Cary NC 27512, USA.

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