Carbon nanotechnology

Melt mixing method and coagulation method for preparing carbon nanotube / polymer composites

Carbon nanotubes have shown exceptional stiffness and strength and remarkable thermal and electrical properties, which make them ideal candidates for the development of multifunctional material systems. Previous research has demonstrated the fabrication and characterization of carbon nanotube/polymer composites. However, although remarkable properties of single carbon nanotubes have been reported, nanotube composites have not yet demonstrated their theoretical potential. One of the biggest challenges is obtaining a uniform dispersion of nanotubes in a polymer matrix because carbon nanotubes tend to bundle together even after attempts are made to disperse them. Uniform dispersion of nanotubes in materials is absolutely critical to harnessing their strength, electrical conductivity and thermal stability.

Recently, Winey’s group at Penn developed two processing methods to effectively mix carbon nanotubes evenly into a polymer matrix. In their work, poly(methyl methacrylate) (PMMA) was chosen as matrix for its good solubility in dimethylformamide (DMF).

The first method is called melt mixing method [1]. In this method, nanotubes were first sonicated for 1 h in DMF. A 10 wt% solution of PMMA in DMF was prepared, which was combined with the dispersed nanotube and sonicated for 3 h. Subsequently, the mixtures were poured into Teflon dishes and dried at 20°C. The as-cast composite films were then folded and broken into pieces of 1–1.5 cm2 and stacked between two polished metal plates. This stack was then hot pressed at 180°C and 3000 lb for 3 min. The resulting ~50–100 m thick films were again broken into pieces, stacked between the metal plates, and hot pressed. This melt mixing procedure was repeated as many as 25 times. Subsequently, the films were dried under vacuum at 180°C for 20 h. Finally, the films were melt mixed five more times at 180°C and 3000 lb for 3 min. The composites obtained using this method exhibit fairly good nanotube dispersion under optical microscope and SEM. The elastic modulus and electrical conductivity are also enhanced.

The other method is called “coagulation method” [2]. In this method, nanotubes and PMMA are first mixed into a solvent, creating a fine suspension, and then plunged into distilled water. The PMMA rapidly precipitates out of this mixture, dragging the nanotubes with it and preventing them from clumping. More specifically, nanotubes were added to DMF to give a concentration of 0.25 mg of nanotubes/mL of DMF. A sonication bath was used for 24 h to disperse the nanotubes in DMF. On the basis of the desired weight fraction of nanotubes in the final composite, an appropriate quantity of PMMA was dissolved in the nanotubes and DMF mixture. The suspension was then dripped into a large amount of distilled water (VDMF/Vwater = 1:5) in a blender. PMMA precipitated immediately because of its insolubility in the DMF/water mixture. The precipitating PMMA chains entrapped the nanotube and prevented nanotube from bundling again. After filtration and drying in vacuum at 120 °C for 24 h, the raw nanotube/PMMA composites were obtained. The final product showed very good nanotube dispersion in PMMA, also strength and conductivity gains over ordinary PMMA. Furthermore, the composites demonstrated improved thermal stability relative to PMMA, indicating promise as a fire-retardant additive.

Their detailed characterization on these composite samples showed that the coagulation method is more effective than melt mixing method in terms of dispersing nanotubes uniformly in a polymer matrix. Further improvement can be expected by chemical functionalization of nanotube surface since 1) functionalized nanotubes can have better solubility in chemical solvent, and 2) functionalization may greatly improve the nanotube/polymer interfacial properties.

References:

1. R. Haggenmueller, H. H. Gommans, A. G. Rinzler, J. E. Fischer and K. I. Winey, Chem. Phys. Lett., 330(3-4), p219-225 (2000)
2. F. Du, J. E. Fischer, K. I. Winey, Journal of Polymer Science Part B: Polymer Physics, 41(24), p3333 – 3338 (2003)