Raman Scattering Test of Mechanical and Sensor Properties of Advanced Nanocomposites

University of Houston • University of Houston-Clear Lake • ISSO Annual Report Y2006 • 83-84

Viktor G. Hadjiev, Leonard Yowell, Sivaram Arepalli

ABSTRACT—In this report on the project completed in 2004, we briefly demonstrated that single wall carbon nanotubes (SWNT) embedded in polymer matrix can be used as distributed, nano-scale, strain gauges measured remotely by Raman spectroscopy.

In the course of the project, we measured load transfer in various SWNT nanocomposites^1,2 using the Raman scattering micromechanical test.³ As a result, it has been well documented that the G-mode frequency at 1590 cm^-1 varies linearly with nanotube's axial strain e_axial. Specifically, the fractional G-mode frequency change is proportional to the axial strain

where A~1 follows from an empirical model for chiral SWNT,³ and A~1-2 is found using the Local Density Approximation (LDA) calculations.⁴ In a nanocomposite containing SWNTs, any external load is transferred to the nanotubes through the interfaces between the matrix and nanotubes. This process can also be described as a strain transfer S ≤ 1 given by

, e_m is the matrix strain.

The strain transfer is deduced from the Raman scattering test in which we simultaneously measure the changes in both the G-mode frequency and the surface strain e_S≈ e_m (for low SWNT concentration nanocomposites), close to the laser spot, with varying external load.

We measured a nanocomposite sample containing 0.001% SWNTs dispersed in Epon 682 and cured with W-agent in the presence of ~16 T magnetic field. Typically nanocomposites of SWNTs prepared in a strong magnetic field contained well oriented nanotubes along the field direction.

To quantitatively assess the degree of the orientation of SWNTs, we measured a series of Raman spectra for incident laser polarization directions at various angles, 0 ≤ Y ≤ 180^o, with respect to the director (direction of the magnetic field). It is well known that the G-mode intensity of a SWNT varies with the angle Y between the incident laser polarization and the nanotube axes as cos⁴Y. If all nanonotubes in the nanocomposite sample are perfectly aligned in one direction, the resulting G-mode intensity should follow cos⁴Y as well.

Figure 1. G-mode Raman spectra of 0.001% SWNT/epoxy nanocomposite measured at various angles between the laser polarization and the direction of magnetic field applied during the curing process.

Figure 2. Variation of the G-mode intensity with angle (open circles) and the best fit to experimental points (solid line) using the expression 1.

Figure 3. G-mode Raman line in 0.001% SWNT/epoxy nanocomposite at zero stress (1592.4 cm^-1) and an external load (the line shifted upward by 5.6 cm^-1) that creates matrix strain.

Figures 1 and 2 show that the G-mode intensity change with Y in the sample under study resembles that of cos⁴Y. These data, however, allow more quantitative treatment, and we find the average degree of nanotube misalignment Q from the fit to the dependence given in Fig. 2 with the expression⁵

where

are the second- and fourth-order orientation parameters. We obtained for the second-order parameter

which corresponds to misalignment of the nanotubes of no more than 25 degrees.

In Fig. 3 we present the change in the G-mode Raman spectra under external compressive load along the director. The load created a matrix strain e ≈ -0.004 measured by a strain gauge placed close to the laser spot. Given the measured fractional frequency change,

one obtains very good strain transfer S ≈ 0.88 in a 0.001 percent SWNT/epoxy sample. This value of S is characteristic for well dispersed nanotubes and for those that adhere to the matrix. In addition to the frequency shift, the G-mode Raman line in the sample under load becomes inhomogeneous broadened. This effect comes in part from the contribution of those nanotubes that are misaligned and therefore subjected to a reduced load. On the other hand, the nanocomposite contained different diameter nanotubes, which, although having approximately the same G-mode frequency, shifted differently under the load due to the different strength of coupling to epoxy. Currently, we are working on a refined approach to resolve these problems of misalignment.

In this study, we demonstrated that in addition to reinforcement, well-dispersed nanotubes and those that adhere to the matrix can serve also as nano-scale strain gauges. Further improvement of the performance of the nano-gauges can be achieved by using nanotubes with narrow diameter distribution.

References
¹V. G. Hadjiev D. C. Lagoudas, E-S. Oh, P. Thakre, D. Davis, B. S. Files, L. Yowell, S. Arepalli, J. L. Bahr, and J. M. Tour, "Buckling Instabilities of Octadecylamine Functionalized Carbon Nanotubes Embedded in Epoxy," Composites Science and Technology 66 (2006): 128.
²V. G. Hadjiev C. A. Mitchell, S. Arepalli, J. L. Bahr, J. M. Tour, and R. Krishnamoorti, "Thermal Mismatch Strains in Sidewall Functionalized Carbon Nanotubes/Polystyrene Nanocomposites," J. of Chem. Phys. 122.12 (2005): 124708.
³V. G. Hadjiev, M. N. Iliev, S. Arepalli, P. Nikolaev, B. S. Files, "Raman Scattering Test of Single-wall Carbon Nanotube composite," Appl. Phys. Lett. 78 (2001): 3193.
⁴Wu G., J. Zhou, and J. Dong "Raman Modes of the Deformed Single-wall Carbon Nanotubes," Phys. Rev. B 72, 1(2005): 15411.
⁵Liu T. and S. Kumar, "Quantitative Characterization of SWNT Orientation by Polarized Raman Spectroscopy," Chem. Phys. Lett. 378.3 (2003): 257.

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