Tunable Dielectric Properties in Polyacrylonitrile/ Multiwall Carbon Nanotube Composites Stefania Palade, 1 Aida Pantazi, 1 Silviu Vulpe, 1 Ciceron Berbecaru, 1,2 Viorica T¸ucureanu, 3 Ovidiu Oprea, 4 R. Florentina Negrea, 5 Daniela Dragoman 1,6 1 Faculty of Physics, University of Bucharest, P.O. Box MG-11, 077125 Bucharest, Romania 2 Romanian Materials Science-Crystal Growth Society, 077125 Bucharest, Romania 3 National Research and Development Institute in Microtechnologies, Str. Erou Iancu Nicolae 126 A, 077190 Bucharest, Romania 4 Faculty of Applied Chemistry and Materials Science, University Politehnica Bucharest, 1-7 Polizu Str 011061, Bucharest, Romania 5 National Institute of Materials Physics, Str. Atomistilor 105 bis, P.O. Box MG-7, 077125 Bucharest, Romania 6 Academy of Romanian Scientists, Splaiul Independentei 54, 050094 Bucharest, Romania Composites containing polyacrylonitrile (PAN) and dif- ferent mass contents of multiwall carbon nanotubes (MWCNTs) were prepared and structurally investigated. X-Ray Diffraction of pristine PAN reveals the presence of crystalline and amorphous phases which change their ratio under thermal annealing and addition of MWCNT. For as prepared samples, thermal analysis reveals two glass transition temperatures, which support the hypothesis that unoriented PAN is a two-phase material. Infrared spectrum of as prepared PAN suggests that the polymer is not stabilized. Dielectric investiga- tions of PAN/MWCNT composites show that permittivity has a strong increase as the MWCNT mass content increases, while the dielectric losses are comparable in all samples. These results suggest that PAN/MWCNT composites could find important applications in elec- tronics. POLYM. COMPOS., 00:000–000, 2015. VC 2015 Society of Plastics Engineers INTRODUCTION Polyacrylonitrile (PAN) is a relatively insoluble and hard material, with a high melting temperature, and is used generally in copolymers [1]. In the form of fibers, PAN is at the heart of acrylic fiber industry [2], while carbon nanofibers can be obtained by carbonization of aligned electrospun PAN nanofiber bundles [3]. PAN composites modify greatly the properties of the polymer matrix. For example, addition of Fe 3 O 4 nanopar- ticles [4] influences the crystallinity and imparts magnetic properties on Fe 3 O 4 -PAN nanocomposite fibers, whereas carbon nanotube (CNT) fillers, with excellent mechanical, thermal, and electrical properties [5] are expected to improve the respective characteristics of PAN/CNT com- posites. Some recent experiments showed that composites containing relatively aligned PAN nanofibers and single- walled CNTs wrapped by PAN [6], or dried PAN and vapor grown carbon nanofibers coated by PAN [7], show enhanced mechanical properties caused by the strong interfacial interaction between PAN and CNTs as well as a significant increase in electrical conductivity above a percolation threshold. Similar considerable enhancement of mechanical properties, as well as a major increase in glass transition temperature, was reported in solution spun PAN/single-walled CNT fibers [8], while the influence of multiwall carbon nanotubes (MWCNTs) on the morphol- ogy and electrical conduction of nano-web sheets of PAN were studied in [9]. PAN/CNT films [10] or pyro- lyzed PAN-S-MWCNT composites [11] could be used as supercapacitors, whereas MWCNTs embedded in PAN nanofiber paper show large anisotropic electrical conduc- tivity [12]. Correspondence to: C. Berbecaru; e-mail: berbecaru2ciceron@yahoo.com Contract grant sponsor: National Authority for Scientific Research under the Core Project PN09-450102 (to R.F.N.). DOI 10.1002/pc.23744 Published online in Wiley Online Library (wileyonlinelibrary.com). VC 2015 Society of Plastics Engineers POLYMER COMPOSITES—2015