Stabilizing Thin Film Polymer Bilayers against Dewetting Using Multiwalled Carbon Nanotubes Jaseung Koo, † Kwanwoo Shin,* ,| Young-Soo Seo, ⊥ Tadanori Koga, ‡ Seongchan Park, † Sushil Satija, # Xuming Chen, § Kyunghwan Yoon, § Benjamin S. Hsiao, § Jonathan C. Sokolov, † and Miriam H. Rafailovich* ,‡ Department of Materials Science and Engineering, Chemical and Molecular Engineering Program, and Department of Chemistry, State UniVersity of New York at Stony Brook, Stony Brook, New York 11794, Department of Chemistry, Sogang UniVersity, Seoul 121-742, South Korea, Department of Nano Science & Technology, Sejong UniVersity, Seoul 143747, South Korea, and Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 ReceiVed July 12, 2007; ReVised Manuscript ReceiVed October 9, 2007 ABSTRACT: We have investigated the effect of the length of multiwalled carbon nanotubes (MWNT) on the dewetting dynamics of thin polymer films. The results indicated that long nanotubes were much more effective than short nanotubes in stabilizing the films against dewetting. The diffusion of polymer chains in the filled matrices was measured using neutron reflectivity, and the result indicated no significant effect on the diffusion coefficient by either short or long nanotubes. We therefore proposed a model whereby the nanotubes did not interact with the individual polymer chains. On the other hand, the long nanotubes formed an effective entangled network, which prevented long-range motion of the polymer films upon dewetting. This model was supported by rheological experiments on bulk samples where the nanotubes had a strong effect on G′ of the polymer and only a negligible effect on G′′. Introduction Thin polymer multilayer films are increasingly being used for numerous applications ranging from dielectric and optical coatings to electronic packaging. 1-3 Because most polymers are immiscible, the interfacial tension between the layers can be unfavorable leading to dewetting. 4 Recently, several research groups have demonstrated that the addition of nanoparticles into the matrix can control dewetting. 5-8 Barnes et al. 5 have shown that the addition of fullerene (C 60 ) fillers can stabilize ho- mopolymer films against dewetting due to surface interactions between the silicon substrates and the particles that can pin the polymer chains. Xavier et al. 6 demonstrated that dewetting can also be controlled even in the case when particles/matrix interactions were unfavorable simply by controlling the relative size of the particles and the polymer chains. Here, we demonstrate that the aspect ratio of the filler can also affect dewetting by changing the rheology of the dewetting layer. Fullerene pipes or carbon nanotubes (CNTs), consisting of the same chemical structure as the C 60 , have received consider- able attention due to their unique rheological properties. 9-11 Du et al. 10 investigated the influence of the single-walled nanotubes (SWNT) on the viscoelastic properties of the composites. They reported that at low frequencies, they found an increase in the storage modulus (G′) for concentrations as low as 0.5% (by weight), while at high frequencies, G′ was unaffected even by the addition of 2% (by weight) SWNTs. They attributed this behavior to network formation, which yielded a gel-like rheological response and hence was able to explain the enhance- ment in the flame retardant behavior they observed. 11 Here, we propose that the network formation may also enhance the stability of thin films containing nanotubes against dewetting. In this work, therefore, we study the effect of nanotube length and nanotube loading on the dewetting behavior of polymer thin films and correlate the results with diffusion measurements where single chain dynamics are measured. To probe the hypothesis of network formation, we also investigate the differences between films containing long entangled MWNTs (l-MWNTs) and short straight MWNTs (s-MWNTs), which cannot form networks. We show that the l-MWNTs are more effective at stabilizing the films, while neither short nor long MWNTs affect the diffusion of individual polymer chains. To compare with bulk phenomena, rheological measurements were also performed on melt blended samples containing s-MWNTs and l-MWNTs. The results confirm the previously suggested model 10,12 for formation of a nanotube network where the mesh size can be an important factor in the polymer dynamics. Experimental Section Materials. Commercially available multiwall carbon nanotube (MWNT) (Nanostructured & Amorphous Materials, Inc., USA), and polystyrene (PS) and poly(methyl methacrylate) (PMMA), as well as their deuterated analogues, were used in these experiments. The specifications regarding the polymers used are listed in Table 1. Purified MWNT. MWNTs were produced by the chemical vapor deposition (CVD) method. The nanotubes have an average diameter d c ) 25 ( 5 nm. The MWNTs were purified by the oxidizing acid treatment method developed by Smalley et al. 13 MWNTs (100 mg) were immersed in 100 mL of concentrated 3:1 solutions of sulfuric acid (H 2 SO 4 ) and nitric acid (HNO 3 ). The MWNT suspended solutions were kept in an ultrasonicator (Bran- * Corresponding authors. E-mail: kwshin@sogang.ac.kr (K.S.); mrafailovich@notes.cc.sunysb.edu (M.H.R.). † Department of Materials Science and Engineering, State University of New York at Stony Brook. ‡ Chemical and Molecular Engineering Program, State University of New York at Stony Brook. § Department of Chemistry, State University of New York at Stony Brook. | Sogang University. ⊥ Sejong University. # NIST. 9510 Macromolecules 2007, 40, 9510-9516 10.1021/ma071550l CCC: $37.00 © 2007 American Chemical Society Published on Web 11/30/2007