Kinetics study of carbon nanotube alignment under shear force via real-time polarized Raman spectroscopy

Y. He, A.I. Taub
University of Michigan-Ann Arbor,
United States

Keywords: carbon nanotube, polymer composite, Raman spectroscopy, real time

Summary:

Carbon nanotubes (CNTs) have been studied extensively since their discovery by Iijima in 1991. They have a unique quasi one-dimensional nanoscale structure with remarkable mechanical, electrical and thermal properties. Numerous investigations have been done on various CNT/polymer composites and significant property enhancement is observed due to the addition of CNTs. However, the commercial application of CNTs have not seen widespread 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. 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. Efforts have been made to investigate various CNT/polymer systems and alignment methods, but the data reported are highly variable. We have recently developed a polarized Raman spectroscopy method to characterize CNT alignment under an applied electric field in real time. In this work, a rotational shear flow is applied as the driving force for alignment. A shearing stage (from Linkam Scientific, UK) has been integrated to the Raman spectroscopy microscope. Casey et al. aligned CNTs in this shearing stage and studied the length distributions from the end-state alignment via polarized photoluminescence. Our real-time observations indicate CNTs align in the direction of the flow and the degree of alignment reaches a plateau in a few minutes. Upon the removal of the shear force, the alignment gradually becomes randomized due to the thermal energy. The shear rate has been found to affect the speed as well as the degree of alignment.