7536
Macromolecules 2008, 41, 7536-7542
Effect of Carbon Nanotube Functionalization on the Structural and Mechanical Properties of Polypropylene/MWCNT Composites Anton A. Koval’chuk,*,† Vitaliy G. Shevchenko,‡ Alexander N. Shchegolikhin,§ Polina M. Nedorezova,† Alla N. Klyamkina,† and Alexander M. Aladyshev† N.N. SemenoV Institute of Chemical Physics, Russian Academy of Sciences, Moscow, Russia; N.S. EnikolopoV Institute of Synthetic Polymer Materials, Russian Academy of Sciences, Moscow, Russia; and N.M. Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Moscow, Russia ReceiVed July 16, 2008; ReVised Manuscript ReceiVed August 22, 2008
ABSTRACT: Multiwall carbon nanotubes (MWCNTs) have been functionalized via addition of undecyl (C11-) radicals generated by thermal decomposition of lauroyl peroxide. The functionalized MWCNTs (C11-MWCNTs) were characterized by Raman, IR spectroscopy, and thermogravimetric analysis (TGA). Isotactic polypropylene (iPP) and syndiotactic polypropylene (sPP) nanocomposites containing both purified and alkyl-functionalized MWCNTs have been synthesized via in situ polymerization method with the use of C2- and Cs-symmetry zirconocenes activated by MAO in liquid propylene medium. The effect of incorporating C11-MWCNTs on structural, electrical, and mechanical properties of the polypropylene-based nanocomposites has been studied. Analysis of electrical properties in the microwave range elucidates interfacial transformations in the composites induced by MWCNT functionalization and evaluates relative aspect ratios for the filler particles in different systems. The synthesized materials demonstrate improved filler dispersion and mechanical characteristics as a result of nanotube chemical functionalization. The MWCNT functionalization leads to markedly improved nanocomposite plasticity and noticeable enhancement of sPP tensile modulus.
Introduction Polyolefins (mainly, polypropylene and polyethylene) constitute the family of the most influential and versatile polymer materials of primary commercial importance due to their attractive set of properties and low cost.1,2 The continuous development of polyolefin-based materials aims at obtaining new properties for achieving further expansion toward new application areas. Great expectations are presently connected with the modification of polymer properties by employing carbon nanotubes (CNTs) as reinforcement filler particles for obtaining novel multifunctional composites combining improved mechanical and thermal properties, electric conductivity, and reduced flammability at low nanotube contents.3-6 Recent advances in CNT synthesis have enabled their large-scale production with low commercial prices comparable to conventional carbon fibers7 that opens up possibilities of nanocomposite mass production. However, the problem of CNT aggregation and limited compatibility between nanotubes and polymer matrices, especially polyolefins that have nonpolar chemical structure, makes quite challenging the obtaining polymer composites containing carbon nanotubes. The in situ polymerization approach is regarded in the literature8-14 as the most promising for efficient CNT dispersion and compatibilization between polyolefins and carbon nanotubes. Metallocene catalysts are the most efficient and versatile tool for obtaining polyolefin-based nanocomposites via in situ polymerization method. Homogeneous metallocenes are soluble in hydrocarbons, and therefore, they can perfectly cover the surface of nanoparticles and fibers.14,15 Moreover, metallocene catalysts provide unmatched opportunities for the precision synthesis of stereoregular polyolefin architectures and tailoring polymer matrix properties.16,17 Recently, we have introduced novel method for obtaining polypropylene/CNT nanocomposites that is appropriate for high* Corresponding author. E-mail:
[email protected]. † N.N. Semenov Institute of Chemical Physics. ‡ N.S. Enikolopov Institute of Synthetic Polymer Materials. § N.M. Emanuel Institute of Biochemical Physics.
throughput production of the nanocomposites.18 However, the obtained results have demonstrated that the optimization of the composite interfaces is needed toward achieving enhanced compatibilization between PP matrix and nanotubes and attaining better mechanical performance of the materials. CNT sidewall covalent functionalization is considered as a versatile tool for obtaining improved nanotube dispersion and better compatibility between polymers and nanotubes19-23 due to the intermolecular interactions between polymer chains and the functional groups grafted to CNTs. In compliance with nonpolar chemical structure of PP macromolecules, attachment of alkyl chains to CNT surface is expected to provide the most efficient interfacial interaction in the respective nanocomposites. In the present work we use the radical mechanism for attaching -(CH2)10CH3 alkyl chains generated by thermal decomposition of lauroyl peroxide (radical initiator) to multiwall carbon nanotubes (MWCNTs). The MWCNT functionalization has been carried out according to the protocol originally described by Umek et al. for single-wall carbon nanotubes (SWCNTs).24 The advantage of this chemical functionalization approach over different alkylation methods lies in its simplicity, absence of air- and moisture-sensitive compounds, and high degree of functionalization provided, which makes it suitable for largescale CNT functionalization. The effect of MWCNT functionalization on the properties of isotactic (iPP) and syndiotactic polypropylene (sPP) nanocomposites synthesized by in situ polymerization method described in the present paper has not been previously reported in the literature. Experimental Section Materials. Pristine CVD-grown MWCNTs (purity g95%, average diameter
