Nematic Nanotube Gels

We report the creation of nematic nanotube gels containing large domains of isolated, oriented, half-micron-long, single-wall carbon nanotubes (SWNTs). We make them by homogeneously dispersing surfactant coated SWNTs at low concentration in an $N$-isopropyl acrylamide gel and then inducing a volume-compression transition. These gels exhibit hallmark properties of a nematic: birefrigence, anisotropy in optical absorption, and disclination defects. We also investigate the isotropic-to-nematic transition of these gels, and we describe the physical properties of their ensuing nematic state, including a novel buckling of sample walls. Finally, we provide a simple model to explain our observations.

Figure 1

(a),(b) Capillary tubes containing SWNT-NIPA gels before and after shrinking. NIPA gels prepared with surfactant only shrank by almost the same ratio. (c) The surfaces of the shrunken SWNT-NIPA gels can undulate and buckle upon compression. (d) A schematic representation of the nanotube gels with $c_{\mathrm{S}\mathrm{W}\mathrm{N}\mathrm{T}}\ge 3.30\mathrm{m}\mathrm{g}/\mathrm{m}\mathrm{l}$.

Figure 2

A summary of the effects of time and concentration on the alignment of nanotubes in NIPA gels.

Figure 3

(a) Birefringence images of a nematic nanotube gel in different orientations with respect to the input polarizer pass axis. (b) Liquid crystalline defects [15] observed near the sample edges. (b1) An array of disclinations with strength $+1$ or $-1$ was found in the same sample. (b2) An enlarged image of one of the defects shows four lobes. The dark lines within the bright lobes arise from cracks in the shrunken gel. (b3) In the same sample but at a different location we observe defects with two bright lobes, most likely strength $1/2$ disclinations.

Figure 4

(a) Order parameter ($S$) plotted against nanotube concentration in the shrunken SWNT-NIPA gels ($c_{\mathrm{S}\mathrm{W}\mathrm{N}\mathrm{T}}$). Error bars were derived from birefringent domains in multiple samples, and from uncertainty in our estimates of $N$ and $L$. The inset shows typical images in transmission obtained at the center of the sample for a SWNT-NIPA gel with nanotube concentration $8.25\mathrm{m}\mathrm{g}/\mathrm{m}\mathrm{l}$. The illuminated cross section for these measurements was $128\mu \mathrm{m}$ in diameter, and the illumination wavelength was $647\pm 5\mathrm{n}\mathrm{m}$. Position-dependent absorption measurements did not detect tube concentration gradients on the $50–100\mu \mathrm{m}$ scale. $I_{\parallel }$ and $I_{\perp }$ were derived from the average pixel value of each image. Note, if the order parameter of our shear aligned samples were less than unity (as assumed), then the curves would shift vertically and uniformly to lower values, but shapes of these curves and hence the crossover would remain. (b) Light extinction at the edge and the center of gel for $c_{\mathrm{S}\mathrm{W}\mathrm{N}\mathrm{T}}=8.25\mathrm{m}\mathrm{g}/\mathrm{m}\mathrm{l}$ showed an exponential decay, whereas for $c_{\mathrm{S}\mathrm{W}\mathrm{N}\mathrm{T}}=0.82\mathrm{m}\mathrm{g}/\mathrm{m}\mathrm{l}$, the extinction was exponential only near the edge. This led us to believe that the nanotube alignment in gels with low nanotube concentration is surface induced.