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Viscoelastic rheology of colloid-liquid crystal composites
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Image of FIG. 1.
FIG. 1.

Schematic view of the piezorheometer. The scale bar indicates approximate size dimensions.

Image of FIG. 2.
FIG. 2.

Calibration measurements of Newtonian liquids 2000AW and 100000BW. Graph (a) shows the frequency dependency of the loss modulus of 100000BW at three different temperatures together with the fit according to with as the dynamic viscosity (straight lines with a slope of 1 Pa s). In the left graph (b) the viscosity is plotted against the temperature for both compounds together with the calibration curves (solid lines) provided by the supplier DKD.

Image of FIG. 3.
FIG. 3.

Temperature dependence of storage and loss moduli (left axis) at 50 Hz in course of network formation for sample PDMS-10. The cooling rate was set to 0.1 K/min, the sample thickness to . The right axis gives the scaling for the loss angle . of pure 5CB at is shifted to lower temperatures of about due to the presence of impurities like alkanes (Ref. 35). Due to resolution limits the values for and above cannot be resolved.

Image of FIG. 4.
FIG. 4.

Frequency sweeps for samples containing 5% and 10% of PHSA and PDMS-stabilized PMMA colloids dispersed in 5CB. The mixtures were cooled at 0.1 K/min down to and kept constant for several hours before starting the frequency sweeps. Filled symbols represent and open symbols .

Image of FIG. 5.
FIG. 5.

(a) Microscopic images of the network structure of sample PHSA-10 at a sample thickness of and various cooling rates. For increasing cooling rate the size of the nematic droplets decreased. (b) Image analysis: for each data point the average droplet size and its variance are calculated on the base of more than 50 droplets. Besides PHSA-10 the samples PHSA-05 and PDMS-05 were analyzed. The data are consistent with a power-law dependency (straight lines as guide to the eye). Areas with different hatchings separate parameter constellations for which the domain size exceeds the sample thickness and vice versa.

Image of FIG. 6.
FIG. 6.

Dependency of the shear moduli on the cooling rate for sample PHSA-10. The transition from a 2D structure to a 3D one in Fig. 5(b) is reflected in a convergence of the mechanical spectra. For cooling rates of 0.2 and 0.5 K/min nearly no difference in the spectra could be seen.

Image of FIG. 7.
FIG. 7.

The sample PDMS-05 was kept inside the rheometer for successive heating-cooling cycles. (a) The temperature dependent storage modulus is plotted for three different frequencies and for the first nine subsequent cycles. A pronounced hysteresis with respect to cooling and heating is revealed. The maximal cooling rate between the turning points was 0.5 K/min. (b) Different shaded symbols represent the relative storage modulus at for maximal temperatures of 29.5 and , respectively, in course of 20 cycles. The temperatures are held constant for 50 min before each measurement.

Image of FIG. 8.
FIG. 8.

Aging behavior of the network exemplary shown for PDMS-10 for stopping temperatures of about 32 and . The filled and open symbols represent the storage and loss modulus, respectively. The data were normalized to the initial moduli.

Image of FIG. 9.
FIG. 9.

a) Frequency dependence of the shear moduli for four different temperatures. Filled symbols represent and open symbols , respectively. Characteristic features of the single curves, such as crossing points of and , are common to all curves. Shifts in frequency correspond to different sample temperatures and are indicated by arrows. (b) Temperature dependence of the frequency of the phase maximum for colloids stabilized with PDMS and PHSA at a weight fraction of 10%. For each temperature this frequency was determined by a parabolic fit to the data (see inset for a temperature of ). The dashed curve serves as a guide to the eye suggesting an exponential dependency.

Image of FIG. 10.
FIG. 10.

Superposition of the frequency spectra of Fig. 9(a) according to the shift factors of Fig. 9(b). Along the whole frequency axis the overlap of adjacent spectra is reasonably good. Deviations are most apparent for the loss angle at low and intermediate frequencies and in the moduli for the highest frequencies.


Generic image for table
Table I.

Particle parameters: for each synthesis method one single batch was used to prepare samples at PMMA contents of 5 and . Radii and standard deviations were determined by SEM on the basis of 50 particles. Both radii and densities were comparable for the two types of colloids.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Viscoelastic rheology of colloid-liquid crystal composites