Polymers in aligned carbon nanotube arrays

Abstract

In the present experimental study we investigate polymer behavior in aligned carbon nanotube (CNT) arrays. We analyze the polymer filling mechanism in composite materials containing highly ordered and vertically aligned carbon nanotubes. CNTs are obtained by a template assisted chemical vapor deposition (CVD) method. Porous aluminum oxide (PAOX) is used as template. PAOX is obtained by electrochemical anodization of aluminum substrates and contains arrays of hexagonally arranged pores. Different forms of the CNT arrays are studied: fully and partially closed tubes having one or two carbon layers on one or both sides of the array, and open tubes in a freestanding CNT arrays. Investigation is performed by Small Angle X-ray Scattering (SAXS) in combination with transmission and scanning electron microscopy (TEM and SEM) and atomic force microscopy (AFM). The tubes being used are of 40 mkm length and 40/90 nm diameter. Different polymers are infiltrated into CNT arrays - glassy polystyrene (PS), polymethylmethacrylate (PMMA) and semicrystalline polydimethylsiloxane (PDMS) of different molecular weight. Infiltration is performed in polymer solution and in melt. It was found that the original order of the template is only locally preserved in the CNT array. Imbibition of polymer melts is achieved in the interior of CNTs as well as in between tubes, modifying the local order in the array. We compare structural changes of different CNT arrays caused by polymer infiltration. The filling kinetics is followed with time-resolved SAXS in the whole array and in tube interior only. We find that the polymer flow into CNT arrays deviates from the Lucas-Washburn equation and can be described with compressed exponentials. During the imbibition of polymers into CNT arrays a depletion layer is formed in vicinity of the CNT surface. To investigate the influence of the CNT surface on the polymer behavior we apply differential scanning calorimetry (DSC) and study polymers at the glass transition as well as melting and crystallization. We fill arrays with two different polymers - PS in tube interior and PDMS in interstices between tubes. The glass transition for inner and outer polymer is analyzed and compared. We also investigate the crystallization behavior of PDMS in bulk and in CNT arrays. Isothermal and nonisothermal crystallization kinetics are considered. Both experiments show the enhanced crystallization of the polymer in the presence of CNTs. The dynamical aspects of the polymer behavior are further probed by Brillouin light spectroscopy (BLS). We observe Brillouin scattering of acoustic excitations in two porous systems: randomly distributed parallel pores in ion-track-etched polycarbonate (ITPC) and highly ordered parallel pores in the PAOX template. ITPC is produced by irradiating the polycabonate film with an ion beam and subsequent wet-chemical etching of the ion beam tracks. Both systems have a pore radius of R=20 nm and differ with respect to order and density of pores. In the experiment we follow the changes of BLS spectra depending on the angle between the template surface and the incident laser beam. BLS spectra of emtpy porous templates and templates filled with PDMS are compared. We observe two types of Brillouin modes: bulk modes of PDMS and the templates as well as surface modes.