RESEARCH NEWS

April 2004 19

The next generation of carbon fibers

could consist of a composite of the

copolymer polyacrylonitrile (PAN) and

single-walled carbon nanotubes

(SWNTs) [Sreekumar et al.,

Adv. Mater. (2004) 16 (1), 58].

The team from Georgia Institute of

Technology and Rice University

dispersed purified high-pressure

carbon monoxide (HiPco) SWNTs in a

solution of PAN in a 99:1 PAN/SWNT

weight ratio. Carbon fibers were then

dry-jet wet spun on a small-scale

spinning machine at a draw ratio of

4.3. The PAN/SWNT fibers show a

100% increase in tensile modulus at

room temperature and an order of

magnitude increase at 150°C. Current

PAN-based fibers, which are used by

the aerospace industry, have a

modulus of 220-280 GPa.

“There is sufficient evidence to suggest

that the PAN/SWNT composite will

lead to a carbon fiber with a modulus

higher than 500 GPa, without

sacrificing compressive strength,” says

Satish Kumar of Georgia Tech. The

composite fibers also show a

significant reduction in thermal

shrinkage and polymer solubility, as

well as a 40°C increase in glass

transition temperature. These

observations indicate that there is an

interaction between PAN and SWNTs.

The improved carbon fibers have a

number of potential advantages.

Reduced shrinkage could be important

for processing, for example. Their

improved high temperature stiffness

could also lead to new applications.

The researchers expect to improve the

modulus of the composite still further

by increasing the nanotube orientation.

“We are also pursuing this process to

make electrically conducting fibers with

the touch, feel, an dye-ability of a

typical textile,” says Kumar.

Cordelia Sealy

The future offibers?COMPOSITES

Polyimides are frequently used as the insulatinglayer in microelectronics applications, while aphotoresist is used as the mask to provide thephotolithographic pattern in the manufacture ofcomputer chips and other microelectronic devices.However, this requires complex and costlyprocessing technology. Using photosensitivepolyimides (PSPI) would allow simplified, lessexpensive, and safer processing. The development of PSPI materials with highthermal stability, low coefficient of thermalexpansion, and improved mechanical and electricalproperties is very desirable for microelectronicapplications. To accomplish this, researchers fromShanghai Jiao Tong University and ShanghaiResearch Institute of Synthetic Resins synthesizedpolyimide/clay nanocomposites with

photolithographic properties by polymerization of4,4’-diamino-3,3’-dimethyldiphenylmethane (MMDA)and benzophenone-3,3’,4,4’-tetracarboxylicdianhydride (BTDA) [Liang et al., Eur. Polym. J.(2004) 40 (2), 307]. The clay used ismontmorillonite. Excellent photolithographic patterns are obtainedwhen the montmorillonite content is below 2 wt.%.The clay increases the tensile strength of thepolymer matrix, while the elongation at break is notobviously affected. The presence of montmorillonitealso results in improved thermal stability, a markeddecrease in coefficient of thermal expansion,decreased solvent uptake, a small increase in glasstransition temperature, and improved mechanicalproperties such as tensile strength.John K. Borchardt

Photosensitive polyimidesPOLYMERS

Blending provides benefitsCOMPOSITES

Carbon nanotubes (CNTs) are increasinglybeing studied as fillers for polymernanocomposites. Researchers at theInstitute of Materials Research andEngineering, Singapore increase the polarityof the surfaces of multiwalled CNTs(MWNTs) by placing them in nitric acid[Zhang et al., Macromolecules (2004) 3377(2), 256]. After melt compounding thetreated MWNTs with nylon 6 in a twin-screwmixer at 250°C for ten minutes, 1% MWNTswere incorporated into the polymer. Thin filmsamples were prepared by compressionmolding and studied using transmissionelectron microscopy. Analysis indicates thatthe MWNTs are uniformly distributedthrough the nylon matrix. Tensile studiesindicate that most of the MWNTs onfracture surfaces break apart, while a fewMWNTs are pulled out of the polymer matrixintact. This suggests strong interfacialadhesion between the nylon 6 matrix and theCNTs. Comparison of the tensile modulus andtensile strength of the MWNT-filled nylon 6and an unfilled nylon 6 control confirms theresults; both tensile modulus and strengthare much greater in the composite. Thetensile modulus is 852 MPa compared with396 MPa for the unfilled control, whiletensile strength is 40 MPa and 18 MPa,respectively. Elongation at break of thecomposite is 125%, compared with more

than 150% for the unfilled control. The presence of only 1 wt.% MWNTs thushas a significant effect on the mechanicalproperties. This indicates both the utility ofMWNTs as fillers and the usefulness of thecompounding method to prepare MWNT-fillednanocomposites. If CNTs improve the properties of polymers,why can’t polymers be used to improve theproperties of CNTs? The poor solubility ofCNTs in common solvents makes themdifficult to process, but attempts to improveprocessability have met with limited success.Now US and Italian researchers havephysically coated MWNTs with electricallyconductive poly(ortho-toluidine) [Bavastrelloet al., Langmuir (2004) 2200, 969]. This isaccomplished by in situ oxidativepolymerization of ortho-toluidine in thepresence of MWNTs. The nanocompositesare soluble in chloroform and can be readilymade into electrochemically stable Langmuir-Schaefer films at the air-chloroforminterface in the presence of hydrochloricacid. The films are quite electrochemicallystable and UV-visible spectroscopy indicatesthat the ππ-ππ* electron transition of thepoly(ortho-toluidine) is not altered by thepresence of MWNTs. This suggests thatMWNTs do not dope the polymer but simplywrap around the poly(ortho-toluidine) chains.John K. Borchardt

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