Memory effect in composites of liquid crystal and silica aerosil

Aerosil silica nanoparticles dispersed in a liquid crystal (LC) possess the interesting property of keeping memory of an electric- or magnetic-field-induced orientation. Two types of memory have been identified: thermally erasable memory arising from the pinning of defect lines versus a “permanent” memory where the orientation persists even after thermal cycling the samples up to the isotropic phase. To address the source of the latter type of memory, solid-state nuclear magnetic resonance spectroscopy and conventional x-ray diffraction (XRD) were first combined to characterize the LC orientational order as a function of multiple in-field temperature cycles. Microbeam XRD was then performed on aligned gels of different concentrations to gain knowledge of the structural properties at the origin of the memory effect. No detectable anisotropy of the gel or significant breaking of silica strands with heating ruled out the formation of an anisotropic silica network as the source of the permanent memory as previously proposed. Instead, support for a role of the surface memory effect, well known for planar substrates, in stabilizing the permanent memory was deduced from “training” of the composites, that is, optimizing the orientational order through the thermal in-field cycling. The ability to train the composites is inversely proportional to the strength of the random-field disorder. The portion of thermally erasable memory also decreases as the silica density increases. We propose that the permanent memory originates from the surface memory effect operating at points of intersection in the silica network. These areas, where the LC is strongly confined with conflicted surface interactions, are trained to achieve an optimized orientation and subsequently act as sites from which the LC orientational order regrows after zero-field thermal cycling up to the isotropic phase.

Figure 1

Symmetrized NMR spectra [thin (black) line] and their fits [thick (red) line]: (a) Pake pattern obtained for the sample of 8CB with 0.035 g cm${}^{-3}$ of aerosil before any in-field temperature cycle, $\Delta \nu$/2 = 33.2892 kHz, an intensity factor of 96.8, and a linewidth ${\sigma }_f=$ 0.010$\nu$; (b) sample of 8CB with 0.059 g cm${}^{-3}$ of aerosil after 22 training cycles, $\Delta \nu$/2 = 33.1597 kHz, an intensity factor of 95.2, a Gaussian fraction $\alpha =$ 1, and linewidths ${\sigma }_f=$ 0.021$\Delta \nu$ and ${\sigma }_{\theta }=$ 28.3${}^{\circ }$. (c) An x-ray mosaic scan [thin (black) line] and its fit [thick (red) line] for a sample of 8CB with 0.035 g cm${}^{-3}$ of aerosil after 12 cycles and a full width at half maximum equal to 7.77${}^{\circ }$.

Figure 2

Angular spreads obtained through NMR (HWHM, squares) and XRD (FWHM, crosses) fits: (a) 8CB with 0.035 g cm${}^{-3}$ of aerosil, (b) 8CB with 0.059 g cm${}^{-3}$ of aerosil (the angular spread as determined by XRD after the zero-field temperature cycle is indicated by a triangle at cycle No. 26), and (c) 8CB with 0.091 g cm${}^{-3}$ of aerosil. Points connected by solid lines indicate that sample did not leave the NMR chamber.

Figure 3

Image recorded on the CCD detector, showing the scattering from a (20 $\times$ 20)-$\mu {\text{m}}^2$ region of 8CB with 0.059 g cm${}^{-3}$ of aerosil at 25${}^{\circ }$C, before the zero-field temperature cycle. The white lines are contour plots of the fit by a 2D Lorentzian with a FWHM in the parallel direction of $1/{\xi }_{\parallel }=$ 7.51 $\times$ 10${}^{-5}$ Å${}^{-1}$ and in the perpendicular direction of $1/{\Gamma }_{\phi }=$ 1.48${}^{\circ }$. The white lines indicate the FWHM, one-quarter, one-eighth, and one-sixteenth of the peak. The speckled appearance of the image is linked to the coherence of the beam. The speckles are elongated in the $q$ direction as the aspect ratio of the image is not preserved here.

Figure 4

Integrated intensity in $q$ and $\phi$ at 25${}^{\circ }$C as a function of the horizontal position for 8CB with 0.059 g cm${}^{-3}$ of aerosil before the zero-field temperature cycle [(black) squares], after heating the sample to the $N$ phase (37${}^{\circ }$C) [(red) crosses], and after heating the sample to the isotropic phase (46${}^{\circ }$C) [(blue) triangles].

Figure 5

Integrated volume $A{\xi }_{\parallel }{\Gamma }_{\phi }^2$ versus ${\xi }_{\parallel }$ for the no-gel sample [(black) squares], 8CB with 0.035 g cm${}^{-3}$ of aerosil [(red) crosses], 8CB with 0.059 g cm${}^{-3}$ of aerosil [(blue) triangles], and 8CB with 0.091 g cm${}^{-3}$ of aerosil [(green) circles] at 25${}^{\circ }$C (a) before and (b) after zero-field temperature cycles into the nematic or isotropic phase (except for the no-gel sample, which was cycled in field). Each symbol corresponds to one of the 200 ($20\times 20$)-$\mu$m${}^2$ regions with the exclusion of the invalid data points. The line displays the relation $A{\xi }_{\parallel }{\Gamma }_{\phi }^2=$ 1 $\times$ 10${}^{-4}$ ${\xi }_{\parallel }^3$.

Figure 6

Correlation lengths ${\Gamma }_{\phi }$ versus ${\xi }_{\parallel }$ for the no-gel sample [(black) squares], 8CB with 0.035 g cm${}^{-3}$ of aerosil [(red) crosses], 8CB with 0.059 g cm${}^{-3}$ of aerosil [(blue) triangles], and 8CB with 0.091 g cm${}^{-3}$ of aerosil [(green) circles] at 25${}^{\circ }$C before zero-field temperature cycles. Each symbol corresponds to one of the 200 ($20\times 20$)-$\mu$m${}^2$ regions with the exclusion of the invalid data points. The line displays the relation ${\Gamma }_{\phi }=1.8\times {10}^{-5}$ ${\xi }_{\parallel }^{1.5}$.

Figure 7

Integrated intensity versus ${\Gamma }_{\phi }$ for (a) 8CB with 0.035 g cm${}^{-3}$ of aerosil, (b) 8CB with 0.059 g cm${}^{-3}$ of aerosil, and (c) 8CB with 0.091 g cm${}^{-3}$ of aerosil at 25${}^{\circ }$C before the first zero-field temperature cycle [(black) squares], after the first zero-field temperature cycle (up to 37${}^{\circ }$C for 8CB with 0.035 and 8CB with 0.059 g cm${}^{-3}$ of aerosil and up to 46${}^{\circ }$C for 8CB with 0.091 g cm${}^{-3}$ of aerosil) [(red) crosses], and after a second zero-field temperature cycle (up to 46${}^{\circ }$C for 8CB with 0.035 and 8CB with 0.059 g cm${}^{-3}$ of aerosil and up to 80${}^{\circ }$C for 8CB with 0.091 g cm${}^{-3}$ of aerosil) [(blue) triangles].

Figure 8

(a) Static scattering versus $q$ for 8CB with 0.059 g cm${}^{-3}$ of aerosil at 25${}^{\circ }$C. The five azimuthal directions analyzed here correspond to binning by 50${}^{\circ }$, from 50${}^{\circ }$ to 300${}^{\circ }$, where 0${}^{\circ }$ is the vertical direction and 180${}^{\circ }$ is the LC alignment direction. (b) Corresponding geometry.

Figure 9

Intensity integrated over the $q$ range 0.0016–0.025 Å${}^{-1}$ as a function of vertical position for 8CB with 0.059 g cm${}^{-3}$ of aerosil. The two lower lines correspond to the data taken at 25${}^{\circ }$C before [thin (black) line] and after [thick (red) line] the zero-field temperature cycle. The higher thick (blue) line (with a cross at $z=-0.6$) corresponds to the data taken at 46${}^{\circ }$C.