Reinforcement of epoxy resins with multi-walled carbon nanotubes for enhancing cryogenic mechanical properties Zhen-Kun Chen a, b , Jiao-Ping Yang a , Qing-Qing Ni c , Shao-Yun Fu a, * , Yong-Gang Huang d a Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China b Graduate School, Chinese Academy of Sciences, Beijing 100039, China c Department of Functional Machinery & Mechanics, Shinshu University, 3-15-1 Tokida, Ueda, Japan d Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208, USA article info Article history: Received 16 January 2009 Received in revised form 30 April 2009 Accepted 1 August 2009 Available online 5 August 2009 Keywords: Epoxy resins Carbon nanotubes Nanocomposites abstract Epoxy resins are widely applied in cryogenic engineering and their cryogenic mechanical properties as important parameters have to be improved to meet the high requirements by cryogenic engineering applications. Carbon nanotubes (CNTs) are regarded as exceptional reinforcements for polymers. However, poor carbon nanotube (CNT)–polymer interfacial bonding leads to the unexpected low reinforcing efficiency. This paper presents a study on the cryogenic mechanical properties of multi-walled carbon nanotube reinforced epoxy nanocomposites, which are prepared by adding multi-walled carbon nanotubes (MWCNTs) to diglycidyl ether of bisphenol-F epoxy via the ultrasonic technique. When the temperature decreases from room temperature to liquid nitrogen temperature (77 K), a strong CNT–epoxy interfacial bonding is observed due to the thermal contraction of epoxy matrix because of the big differences in thermal expansion coefficients of epoxy and MWCNTs, resulting in a higher reinforcing efficiency. Moreover, synthetic sequence leads to selective dispersion of MWCNTs in the brittle primary phase but not in the soft second phase in the two phase epoxy matrix. Consequently, the cryogenic tensile strength, Young’s modulus, failure strain and impact strength at 77 K are all enhanced by the addition of MWCNTs at appropriate contents. The results suggest that CNTs are promising reinforcements for enhancing the cryogenic mechanical properties of epoxy resins that have potential applications in cryogenic engineering areas. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Epoxy resins have wide engineering applications due to their low cost, easy processability, good thermal, mechanical and elec- trical properties, etc. With the rapid developments in spacecraft and superconducting cable technologies, and large cryogenic engineering projects such as the International Thermonuclear Experimental Reactor (ITER), etc., epoxy resins have been increas- ingly employed in cryogenic engineering applications as impreg- nating materials, adhesives and matrices for advanced composites [1–8]. However, pure epoxy resins normally have poor crack resis- tance at room temperature [9–13] and could be more brittle at cryogenic temperatures [10–13], which makes them unsuitable for some cryogenic engineering applications that demand epoxy resins to have high cryogenic mechanical properties. For example, during service of epoxy resins in ITER, temperature might change from room temperature (RT) to cryogenic temperatures (like liquid nitrogen temperature 77 K etc.) or vice versa [14–16], this would induce thermal stresses within epoxy resins and thus demands epoxy resins to have high cryogenic mechanical properties to stand for internal thermal stresses. Therefore, it is of great importance to improve the cryogenic mechanical properties of epoxy resins so that they can be gainfully used in cryogenic engineering applica- tions. Recent publications from our group have reported on epoxy toughening and strengthening of epoxy resins for enhancing cryogenic mechanical properties using flexible diamines [12], thermoplastic poly(ethersulfone) [13], silica nanoparticles [17], exfoliated montmorillonite (MMT) [18], hyperbranched polymer [19], polyurethane [20] and n-butyl glycidyl ether [21]. Carbon nanotubes (CNTs) are long cylinders of covalently bonded carbon atoms and have a diameter from a few angstroms to several tens of nanometers across. CNTs have exceptional mechanical properties [22–25] and thus extensive research work has been carried out on carbon nanotube (CNT) reinforced polymer composites [26–35]. However, weak interfacial bonding between CNTs and polymers leads to poor stress transfer and thus has limited the full realization of CNTs as reinforcements for polymers. Therefore, chemical functionalization of CNTs has been conducted * Corresponding author. Tel./fax: þ86 10 82543752. E-mail addresses: syfu@mail.ipc.ac.cn, syfu@cl.cryo.ac.cn (S.-Y. Fu). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2009.08.001 Polymer 50 (2009) 4753–4759