Interfacial fracture energy measurements for multi-walled carbon nanotubes pulled from a polymer matrix Asa H. Barber a , Sidney R. Cohen b , Shmuel Kenig c , H. Daniel Wagner a, * a Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel b Chemical Research Support, Weizmann Institute of Science, Rehovot 76100, Israel c Department of Plastics Engineering, Shenkar College of Engineering and Design, Ramat-Gan 52526, Israel Received 19 January 2004; accepted 26 January 2004 Available online 21 July 2004 Abstract Pullout experiments were performed at the nanoscale using an atomic force microscope, to assess the interfacial adhesion be- tween multi-walled carbon nanotubes and a matrix of polyethylene–butene. Fracture energy for the nanotube–polymer interface was calculated from the measured pullout forces and embedded lengths. The results suggest the existence of a relatively strong interface, with higher fracture energy for smaller diameter nanotubes. Ó 2004 Elsevier Ltd. All rights reserved. Keyword: Nanotubes 1. Introduction Carbon nanotubes can be considered as a potentially new class of reinforcement for polymer composites. A carbon nanotube is a cylindrical tubule containing sp 2 hybridized carbon–carbon bonds along its length. Both single-walled carbon nanotubes and the outer wall of multi-walled carbon nanotubes may be viewed as single molecules and, because of their nanosize, are potentially defect-free structures. Since the defect density in a fiber determines its mechanical properties [1], the scale ad- vantage over current fibers such as glass or carbon is an understandable attraction. While recent studies have questioned whether the structure of carbon nanotubes is actually perfect [2], direct mechanical testing of individual nanotubes [3], as well as other indirect approaches such as Raman spectroscopy [4,5], frag- mentation of nanotubes in a polymer due to compres- sion [6–8], observation of nanotube freestanding vibrations [9] and computer simulation work [10,11], indicate that properties such as Young’s modulus, tensile strength and strain to failure are all in excess of those of conventional engineering fibers. Efforts to produce carbon nanotube reinforced poly- mer composites initially showed disappointing results [12,13], probably due to poor dispersions of nanotubes within a polymer matrix. As better dispersions of na- notubes have been produced within the composites, such as through shear melt processing [14,15] or via chemical modification of the nanotube surface to improve dis- persion [16], the mechanical properties of nanotube- based composites have improved dramatically. The degree of interfacial adhesion between nanotubes and polymers is a key parameter in both the production and physical properties of carbon nanotube composites, and is important in understanding the surface behavior of nanoscale materials. Adequate interfacial stress transfer from the matrix to the reinforcement is only possible when the interface has not failed during com- posite loading. Failure of the interface effectively neu- tralizes the efficiency of the reinforcement. The structure of carbon nanotubes apparently shows little promise for forming a strong interface with a polymer matrix. A perfect sp 2 hybridized carbon structure has limited * Corresponding author. Tel.: +972-8-934-2594; fax: +972-8-934- 4137. E-mail address: cpwagner@wis.weizmann.ac.il (H.D. Wagner). 0266-3538/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2004.01.023 Composites Science and Technology 64 (2004) 2283–2289 COMPOSITES SCIENCE AND TECHNOLOGY www.elsevier.com/locate/compscitech