Carbon Nanotube Alignment: Electromagnetic Field and Shear Force

Abstract

Carbon nanotube (CNT)/polymer composites are promising structural materials because of their excellent strength and stiffness. However, the reported data of their strengthening effect is highly scattered. This variability and the high material cost are important inhibitors for wide application of CNT/polymer composites. We aim to improve the cost-property relationship by improving the consistency of strengthening polymer composites by addressing the CNT alignment. CNT alignment has been found to be crucial to the mechanical properties of the composite, e.g. modulus and strength, analogous to the role of carbon fibers in polymer composites.

Previous studies focused on discrete measurements and limited data on alignment kinetics has been reported. We developed an in situ, real-time characterization method using Raman spectroscopy to study CNT alignment in a polymer matrix. Alignment methods include electric field, magnetic field and shear force. CNT alignment is characterized in CNT-Polystyrene Sulfonate (PSS)/Polyvinyl Alcohol (PVA) film (discrete measurement) and in CNT/EPON 828 suspension (real-time study), with a focus on the latter. For real-time experiments, the indicator of alignment behavior (changes of G-band intensity) is tracked over time in the alignment direction.

The technique has been successfully applied to study alignment behavior in an AC field. CNT alignment is confirmed through distinct Raman spectra of unaligned (CNTs randomly oriented) and aligned CNT-PSS/PVA films. Real-time alignment behavior is captured in the CNT/EPON 828 mixture. Higher electric field strength leads to faster alignment and a degree of alignment.

Magnetically-induced alignment has been observed in a CNT-PSS/PVA film. For real-time experiments, alignment is not obvious and the Raman signals are more scattered than observations from electric field experiments. This is due to both the low magnetic susceptibility of our pristine CNTs and the translational motion of CNTs. Further studies are needed using higher field magnets or high susceptibility CNTs. For example, CNTs coated or infiltrated with magnetic nanoparticles.

CNT alignment has also been successfully characterized in a shear flow. Alignment behaviors are similar to the observations in electric field. Higher shear rate results in faster alignment and a greater degree of alignment. Higher temperature lowers the degree of alignment but does not affect the alignment speed. These two effects agree with the P´eclet number and our model of driving force for alignment. Moreover, the alignment is reversible and reproducible in response to changes of shear rate and temperature. Unusual alignment behaviors were found under high shears and at low or high temperatures. Further investigation is needed to determine the CNT behavior in these situations. A comparison between shear force alignment and its electric field counterpart shows similar alignment kinetics but different driving forces. In both cases, thermal randomization occurs immediately when the applied field is discontinued, suggesting the need to maintain the field in order to capture the full potential of alignment for composite manufacturing.