Kinetics of Carbon Nanotube Alignment Under Shear Force via Real-time Polarized Raman Spectroscopy Y. He * and A. I. Taub * * University of Michigan-Ann Arbor, MI, USA, oliverhe@umich.edu and alantaub@umich.edu ABSTRACT Carbon nanotube (CNT) alignment under shear force is studied via in situ, real-time polarized Raman spectroscopy. This technique enables the real-time detection of changes in CNT orientation in a CNT/epoxy suspension. In a sheared fluid, the CNT particles were found to align with the flow direction and they align faster and reach higher degrees of alignment at high shear rates. These results provide guidance for the fabrication of highly aligned CNT/polymer composites. Keywords: Carbon nanotube, Raman spectroscopy, real-time characterization, shear force alignment, composite processing INTRODUCTION Carbon nanotubes (CNTs) have been studied extensively since their discovery by Iijima in 1991 [1]. They have a unique quasi one-dimensional nanoscale structure with remarkable mechanical, electrical and thermal properties [2]. Numerous investigations have been done on various CNT/polymer composites and significant property enhancement is observed due to the addition of CNTs [3]. However, CNTs have not seen widespread commercial adoption due to (1) composite properties falling below the strengthening expected based on individual CNT measurements, (2) manufacturing hurdles at high volume, and (3) the high price of the CNT’s [4]. Further advancement in composite properties can be achieved by manipulating the CNT alignment in composites. It has been shown that the alignment of CNTs in a polymer matrix significantly improves strength and stiffness of the composites [5]. Efforts have been made to investigate various CNT/polymer systems and alignment methods, but the data reported are highly variable [6]. CNTs have been shown to align in the shear force direction [7-9]. But these studies focused mainly on end- state alignment of a CNT suspension [7] or cured CNT composite [8]. Real-time optical observations have been reported for individual microscopic multi-walled CNTs or CNT bundles [9]. Since most molding processes induce a shear force on the composite, an understanding of alignment kinetics is key for scaling up this process. In this work, shear-induced real-time changes of CNT alignment were monitored using real-time polarized Raman spectroscopy [10]. The degree of alignment was directly and quantitatively measured through the changes of CNT’s G- band intensity. Raman spectra was taken at different polarization angles to find the alignment direction. Shear rate was varied to examine its influence on the speed and degree of alignment. EXPERIMENTAL Single-walled carbon nanotubes (AP-SWNT, davg=1.4 nm, l=1-5 µm) were obtained from Carbon Solutions (Riverside, CA) and dispersed in a difunctional bisphenol A/epichlorohydrin (DGEBA) liquid epoxy resin (EPON 828) from Miller-Stephenson Chemical Company. The CNT concentration was 0.01% throughout this study and samples were prepared as previously reported [10]. A shearing stage (CSS450, from Linkam Scientific, UK) generates a shear force between two parallel quartz disks by rotating one of them while keeping the other stationary (torsional flow). The CNT/epoxy suspension was confined between the disks with a gap of 1000 µm, at a temperature of 21 o C. The shearing stage is integrated with a WITec alpha300R Confocal Raman microscope (Spectra Physics Excelsior 532-60). A 532 nm green laser was focused at a depth of 400 µm from the surface of the sample, with a power of 5.00 ± 0.05 mW. The incident and scattered light were polarized by two wave plates. The polarizations are denoted by HH or VV, where the first letter refers to the polarization of the incident light and the second letter denotes the scattered light, where H is parallel to the shear force direction and V is perpendicular to it. The integration time for each Raman spectrum is 5 seconds. Other details of the real-time polarized Raman spectroscopy are documented previously [10]. RESULTS Raman spectroscopy is a widely used method to study carbon-based materials. The intensity of CNT’s G-band were found to be a function of the angle of polarization [11]. The G-band shows strongest intensity when the incident light is parallel to CNT’s longitudinal axis and are suppressed when light is polarized perpendicularly. Thus the G-band intensity from polarized Raman spectra, serve as a probe for the degree of alignment of CNT [10]. 69 Advanced Materials: TechConnect Briefs 2017