Processing, structure, and properties of fibers from polyester/carbon nanofiber composites
Introduction
Single [1], [2], [3] and multiwall carbon nanotubes [4], [5], [6], [7], [8] as well as vapor-grown carbon nanofibers [9] are promising candidates for reinforcing various polymer matrices. Vapor grown carbon nanofibers which typically have diameters in the 50–200 nm range are also referred to as multiwall carbon nanotubes. By comparison, the diameters of single wall carbon nanotubes (SWNTs) are of the order of 1 nm and a multiwall carbon nanotube (MWNT) diameter can be upwards of a few nanometers. Carbon nanotubes as well as nanofibers exhibit good thermal [10] and electrical conductivity [11] and possess excellent mechanical properties [12]. Thermoplastics such as polypropylene [13], [14], [15], [16], [17], [18], polycarbonate [19], [20], [21], [22], [23], nylon [15], [24], poly(ether sulfone) [25], poly (phenylene sulfide) [26], acrylonitrile-butadiene-styrene [25], thermosets such as epoxy [26], [27] as well as thermoplastic elastomers such as butadiene-styrene diblock copolymer [28] have been reinforced with carbon nanofibers (CNFs). Carbon nanofibers have been blended into polymer matrices using conventional mixing methods such as use of twin-screw extruder [13], [20], [21], [22], high shear mixer [16], [24], as well as two-roll mill [26]. To improve CNF dispersion and nanofiber/polymer interfacial strength, carbon nanofibers have been purified [16], ball-milled [15], [26], functionalized [16], and surface treated with plasma [15], [26].
Ball-milled CNF/nylon [15] composites have slightly improved tensile strength and double the modulus of unreinforced material, while ball milled CNF/PP [15] composites have double the tensile strength and quadruple the modulus of unreinforced material. CNF/PP composites made by air-etched fiber, CO2-etched fibers, and fibers covered with low concentrations of aromatics possessed significantly better mechanical properties than did fibers whose surface was heavily coated with aromatics [15]. In studying ball milled and plasma treated carbon nanofibers in epoxy and poly(phenylene sulfide) (PPS) matrices, Patton et al. [26] found flexural strength to increase by up to 68% over neat resin at 19.2% reinforcement.
Along with the efforts towards improvement of matrix polymer mechanical properties, research on the preparation of multifunctional materials has also been carried out [14], [17], [26], [29], [30], [31]. Lozano et al. [17] prepared CNF/PP composites and observed a percolation threshold for electrical conduction of 9–18 wt.% CNF. With addition of 1 and 5 wt.% HDPE, Wu et al. [31] found the percolation threshold of PMMA/CNF composites was reduced from 8.0 to 4.0 phr (parts per hundred parts) and that it could be further reduced to 1.5 phr after annealing. The drastic decrease in percolation threshold was attributed to the selective adsorption of HDPE in PMMA/CNF composites.
The rheological [17], [22], [23], crystallization [16], and thermal degradation behavior [16] of polymer/CNF composites have also been studied. Lozano et al. showed that the incorporation of 30 wt.% CNF into PP raised the working temperature of the resin by 100 °C [16]. They also showed that addition of CNF increased the rate of PP crystallization [16]. Increased polypropylene crystallization rates have also been reported with addition of single wall carbon nanotubes (SWNT) [32].
In this study, different grades of CNFs have been melt blended into poly(ethylene terephthalate). Melt blended PET/CNF composites have been spun into fibers. Processing, properties, and morphology results of these studies are reported herein.
Section snippets
Experimental
Various grades of carbon nanofibers (CNFs) used in this study are listed in Table 1 and were obtained from Applied Sciences Inc., Cedarville, OH. Elemental analyses of the carbon nanofibers were carried out by Atlantic Microlab, Inc. Surface composition of carbon nanofibers was determined by X-ray photoelectron spectroscopy (XPS) using a Surface Science's SSX-100 ESCA spectrometer employing Al Kα X-rays. The structure of the CNFs was also studied by Raman and FTIR spectroscopies. The Raman
Results and discussion
Various grades of carbon nanofibers along with their bulk and surface elemental compositions are listed Table 1. The various fiber designations used below are explained in the table footnote. PR-21 has a larger fiber diameter (about 200 nm) than does PR-24 (about 100 nm). The bulk oxygen analyses of PR-24-AG, PR-24-PPO, and PR-24-ISO show comparable contents, while the surface oxygen content in PR-24-PPO and PR-24-ISO are higher than the other fibers tested. The sulfur content in PR-24-ISO was
Conclusions
Poly(ethylene terephthalate) (PET) resin was compounded with several grades of carbon nanofibers, each at 5 wt.% filler loading. Compounding methods included ball-milling, high shear mixing in the melt, as well as extrusion using a twin-screw extruder. PET/CNF composite resins were then melt-spun into fibers using conventional PET fiber spinning conditions. Morphology and mechanical properties of these composite fibers have been studied and show that CNFs can be incorporated into PET matrix
Acknowledgements
Financial support from KoSa and National Science Foundation, carbon nanofibers and useful discussions with Drs. R.L Jacobsen and D.G. Glasgow of Applied Science Incorporation are gratefully acknowledged. Raman spectroscopy was carried out by Dr. Sreekumar T. Veedu and XPS study by Professor Brent Carter. We are thankful to Drs S. Ran and Benjamin S. Hsiao for the X-ray diffraction and Ann Hiltner for the AFM image.
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