Colloid-in-liquid crystal gels formed via spinodal decomposition

Abstract

We report that colloid-in-liquid crystal (CLC) gels can be formed via a two-step process that involves spinodal decomposition of a dispersion of colloidal particles in an isotropic phase of mesogens followed by nucleation of nematic domains within the colloidal network defined by the spinodal process. This pathway contrasts to previously reported routes leading to the formation of CLC gels, which have involved entanglement of defects or exclusion of particles from growing nematic domains. The new route provides the basis of simple design rules that enable control of the microstructure and dynamic mechanical properties of the gels.

Figure 1

(A) Polarized light and (B) corresponding brightfield micrographs of a 13.3 wt% PS-colloid/E7 film at the indicated temperatures (cooling at a rate of 0.2°C/min). (C) Brightfield (top row) and fluorescence (bottom row) micrographs of a 13.3 wt% PS-colloid/E7 (doped with 0.1% Nile Red) film at the indicated temperatures (cooling at a rate of 0.2°C/min). The scale bars correspond to 20 μm.

Figure 2

Schematic illustrations of two distinct pathways that lead to formation of CLC gels upon cooling of colloids dispersed initially in an isotropic phase of mesogens (the temperature decreases left to right) (A) The pathway proposed by Terentjev, Poon and coworkers for the PMMA colloids in 5CB. (B) The pathway identified in this study for PS-colloids in E7. (Gray: isotropic phase of the nematogens, black: colloids, red: nematic phase of the nematogens, T: temperature of the system, T_NI: nematic-to-isotropic phase transition temperature of the pure nematogen, T_PS: colloidal phase separation temperature.)

Figure 3

Phase behavior of PS-colloids in E7. Lines are drawn to guide the eye. All samples were sandwiched between two glass slides. Thickness of the samples: ~4 μm. See text for discussion of the effect of sample thickness on the system.

Figure 4

Microstructural characterization of a film of 13.3 wt% PS-colloids in E7 following a quench from 71.0°C to 67.0°C. (A) Change in the structure factor, S(Q,t) with time plotted as a function of Q. (B) Structural peak position, Q_max, plotted as a function of time. The thickness of the sample was ~4 μm.

Figure 5

The average size of the E7-rich domains measured as a function of the PS-colloid concentration following a thermal quench at 10°C/min from 71°C to 67°C (see Methods for details). The thickness of the sample was ~4 μm. (n=3)

Figure 6

Dependence of Q_max² on quench depth ΔT for a sample containing 13.3 wt% PS-colloid in E7 (thickness of ~4 μm). (n=4)

Figure 7

Plot of t₁ as a function of L³, as predicted by eqn. (5). See text for discussion.

Figure 8

Structural peak position, Q_max, plotted as a function of time for 13.3 wt% PS-colloid/E7 films of indicated thicknesses. The temperature of each system was 67°C.

Figure 9

(A) Temperature (at 2 Hz, 2% strain) and (B) frequency (at 2% strain and indicated temperature) dependence of storage (filled symbols) and loss (open symbols) modulus of 13.3 wt% PS-colloids in E7. The inset in B shows an illustration of a typical frequency-response of a colloidal gel; fluidization for ω<ω₁, elastic response for ω₁<ω<ω₂, viscous dissipation for ω>ω_2.

Figure 10

Bright-field micrographs of 13.3 wt% PS-colloid in E7 at the indicated temperatures (A) Sample after equilibration for 2 hours at 61.0°C following a rapid quench from 80.0°C. Subsequently, the sample was cooled to 59.0°C then 30.0°C at a rate of 0.2°C/min. (B) Sample after equilibration for 2 days at 61.0°C following a rapid quench from 80.0°C. Subsequently, the sample was cooled to 59.0°C then 30.0°C at a rate of 0.2°C/min.