Examination of Mechanical Stretching to Increase Alignment in Carbon Nanotube Composites

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Virginia Tech

Individual carbon nanotubes have been theoretically and experimentally proven to be the strongest and stiffest materials discovered to date with tensile strengths ranging from 1-5 TPa and elastic modulus values as high as 150 GPa. In this work, the recent development of continuous sheets of CNTs, produced by Nanocomp Technologies Inc ., are investigated for their potential as reinforcement in polymer matrix composite (PMC) materials. The potential of these nanotube-based PMC materials have been reported by researchers at Florida State University (FSU). Through the use of mechanical stretching procedures to increase the alignment of the nanotubes within the CNT sheets, the tensile strength and Young's modulus of the composites in the FSU study averaged 3081 MPa and 350 GPa, respectively. These values are for composites fabricated from 40% stretched CNT sheets and are 48% and 107% improvements over composites fabricated from the pristine, unstretched CNT sheets. However, the test specimens used in the FSU study consisted of a single CNT ply and each coupon was individually stretched and cured for testing. Therefore, the process used to generate the coupons which exhibited these high mechanical properties would be difficult to scale to a usable size for aerospace structural components. In the current study, a scalable process has been developed in which 2-ply, 3" x 3" panels of CNT and resin composites are fabricated. An apparatus and methodology for mechanically stretching the CNT sheets used in these composite panels has also been developed. After initial testing was conducted with the CNT composites and the coupons exhibited significant elongation at failure, along with the absence of a linear elastic region, conventional test standards for material testing were deemed impractical. For this reason, new mechanical testing methodologies have been developed to determine the mechanical properties of specific strength and specific modulus of CNT-polymer composites.

In order to obtain the maximum benefits of a fiber in any matrix in terms of stiffness and strength, it is preferable to align the high strength and stiffness fibers in the direction of loading. Given that these CNT sheets essentially consist of billions of short, discontinuous CNTs of 2-3mmin length, the process of mechanical stretching is used in an attempt to align these tubes in the direction of the applied tensile load. Here we have explored methodologies for stretching, fabricating, and mechanical testing. Having identified a process which seems viable, an examination into the effect of the mechanical stretching to increase the alignment of the nanotubes within the CNT sheets, and thus to increase the material properties of the 2-ply composites constructed from them, is conducted. In order to correlate the enhancements in the mechanical properties with the increased alignment of the CNTs, polarized Raman spectroscopy techniques have been used. Lastly, Scanning Electron Microscopy (SEM) is used to examine the effect of stretching on the pristine CNT sheet, as well as examine the fracture surfaces of failed test coupons to better characterize the failure modes.

In this report, polarized Raman spectroscopy has been used to confirm the enhancedalignment of nanotubes within the CNT sheets through the used of a nematic order parameter. Unstretched sheets exhibit an order parameter of 0.07 and 0.09 for untreated and Acetone treated sheets, respectively. Upon stretching the untreated sheets to 45%, the order parameter increases to 0.1409 and, when stretched to 30%, Acetone treated sheets have an order parameter of 0.1518.

During the mechanical testing of 2-ply composites fabricated from stretched CNT sheets, the effect of this increased alignment is made apparent. Untreated CNT sheets are used to fabricate 2-ply composites after being stretched and are compared to baseline values of panels fabricated using sheets which are not stretched. In the panels fabricated with PEI resin and 43% stretched, untreated CNT sheets, a 137% increase in average specific strength and a 44% increase in average specific modulus over the baseline panel is observed. For panels fabricated with BMI and 33% stretched, untreated CNT sheets, a 169% increase in average specific strength and 105% increase in average specific modulus is observed when compared to the baseline panel. These increases are evidence for the potential of mechanical stretching to align the nanotubes within the CNT sheets and bolster the mechanical properties of resulting CNT-polymer composites.

Carbon Nanotube, Composite Fabrication, Mechanical Stretching, Composite Tensile Testing, Raman Spectroscopy