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Carbon nanoparticles in cholesteric liquid crystals

Stefan Schymura, Jan Lagerwall

Martin Luther University Halle-Wittenberg, Institute of Chemistry Physical Chemistry,

Muehlpforte 1, 06108 Halle, Germany

Introduction

Since the discovery of new carbon modifications in the late 20thcentury, the fullerenes in

1985 [1] large, closed cage carbon clusters and the carbon nanotubes (CNTs) in 1991 [2]

cylindrical tubes of sp2-hybridized carbon the research on these fascinating materials and

the search for applications is going on. In recent years the introduction of said materials in

liquid crystalline (LC) phases, both thermotropic as well as lyotropic, gained particular

interest [3]. There are two principle approaches in the research on such composites:

1.

The ordering of the nanoparticles in a complex manner using the inherently orderedLC as template [4-8], especially important in using the in shape as well as in their

physical properties highly anisotropic CNTs [9].

2. The fine tuning of the properties of the liquid crystal, in particular concerning its

performance in displays (switching times, ion content or aging effects [10-15]), but

also the actual phase sequence can be drastically altered [16].

This work is focused on the incorporation of fullerene C60and single walled carbon nanotubes

in thermotropic cholesteric phases. We follow both approaches described above, investigating

the changes of the properties of the cholesteric host, such as pitch and phase behavior, as well

as the influence of the helical structure on the dispersion quality and stability of the resulting

composites.

Experimental

The carbon nanoparticles used in this study are the fullerene C60(SES, USA; purity 99,9 %)

and single walled CNTs (CNI, USA; purity < 15 wt% ash content) produced by high-pressure

carbon monoxide disproportionation (HiPCO-process). As cholesteric hosts a mixture of 46

wt% cholesteryl nonanoate, 44 wt% cholesteryl oleyl carbonate and 10 wt% cholesteryl

benzoate (mixture 1) was chosen for studying the influence on the pitch and phase behavior,

and the commercial nematic mixture RO-TN-403/015S (Hoffmann-La Roche) doped with

varying concentrations of cholesteryl nonanoate (mixture series 2) was employed for

investigating the influence of the helical structure on the dispersion stability.

The dispersion was achieved by immersing a Dr. Hielscher UIS250L sonotrode into the

sample, placed in an ice-water bath, operating it at 90 % amplitude with a 0.5 s sonication

cycle for half an hour. Pitch investigations were performed by measuring the selective

reflection wavelength rof the LC in planar aligning E.H.C. liquid crystal cells with a cell gap

of 4 microns using a Photo Research PR-702A spectral scanner mounted on a Nikon Eclipse

LV100 microscope in reflection mode equipped with an Instec STC200 hot stage. The helix

pitch is obtained by dividing rby the average refractive index, typically about 1.5.

Differential scanning calorimetry (DSC) was done with a Perkin Elmer Pyris 1 DSC.

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Results

Liquid crystal properties:

First, the influence of carbon nanoparticle doping on the cholesteric pitch was investigated.We found that both C60and CNTs lead to a shift of the region of selective reflection to lower

temperatures (Fig. 1). Thus for a given temperature the carbon nanoparticle doping lowers the

pitch, an observation that is in contrast to a reported increase of the cholesteric pitch for

platinum nanocrystals as dopant [17]. The fullerene causes a distinctly larger shift (3.5 C at

0,014 wt%) than CNTs of the same mass fraction (1.5 C at 0.016 wt%), probably due to the

higher particle number of C60in similar masses of material. The effect can however not be

strictly colligative because a rough estimate of the mass of a typical CNT reveals that the

difference in particle number is some five orders of magnitude, hence any colligative effect

should for CNTs be negligible in comparison with the corresponding C60experiment. The

shift is combined with a decrease of the melting point of roughly the same magnitude (2.3 C

and 1.0 C, respectively). Changes of the clearing point also occur but seem not be correlatedto the pitch shift; while the CNTs induce a slight increase of the clearing point, C60as dopant

causes a decrease.

Fig. 1: Influence of carbon nanoparticle doping on the selective reflection wavelength of mixture 1

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Fig. 2: Dependence of the inverse selective reflection wavelength at a certain temperature on the mass

fraction of C60

As investigated for the case of C60the tightening of the helix at a certain temperature

correlates with the concentration of dopant (Fig. 2). The relative displacement of the r(T)curve caused by certain mass fractions of dopant is also similar to the relative shift of the

melting point in the same mixture. A mixture with 0.016 wt% C60 shows a 2.3 times largerr(T) displacement than a mixture with 0.008 wt% (3.5C and 1.5C, respectively), cf. Fig. 3,which is identical to the ratio of the reductions of the melting point (2.3C and 1.0C). At

least in the system studied here the effect on the helical pitch from the nanoparticle doping

seems to be a secondary effect, the fundamental being the particle-induced freezing point

depression.

Fig. 3: Selective reflection wavelength in dependence of C60mass fraction, the lines are guide to the

eye

CNT dispersion stability:

The influence of the pitch on the dispersion of the CNTs was investigated by comparing the

stability of 0.1 mg CNTs dispersed in 1 ml nematic liquid crystal with different amounts of

chiral dopant added (mixture series 2). The pitch of such a mixture decreases with higher

amounts of chiral additive. The used chiral dopant, cholesteryl nonanoate, was found to have

a stabilizing effect on a CNT-suspension in a reference experiment with an isotropic toluene

hexadecane mixture as host for the nanotubes. In the LC however a faster aggregation and

sedimentation can be seen with higher amounts of cholesteryl nonanoate. A mixture doped

with 50 mg of the chiral additive per ml LC showed aggregation visible by eye after one nightwhile a mixture with only 10 mg/ml showed such aggregation only after one full day. The

undoped liquid crystal maintained reasonable dispersion quality for three days. Because the

dopant molecule itself has a stabilizing effect, as seen from the reference experiment, we

conclude that the destabilization is mainly due to the decreasing pitch of the helical director

modulation. However the nanotubes in these experiments were not optimally dispersed,

meaning that after the ultrasound treatment there were still bundles of CNTs visible by optical

microscopy. Such bundles distort the director field thus causing elastical stresses in the liquid

crystal, which can be minimized by building greater aggregates. Experiments on more

optimally dispersed (ideally single dispersed) CNTs are part of ongoing investigations.

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Conclusions

We found that by adding carbon nanoparticles to cholesteric liquid crystals the properties of

the LC can be influenced such that the pitch-temperature dependence curve is shifted to lower

temperatures. Also the melting point of the mixture decreases by about the same amount.

Lower concentrations of particles lead to a smaller shift with C60causing a larger shift thansimilar weight fractions of CNTs. Because the effect on the helix seems to be driven mainly

by the reduction of crystallization temperature we may anticipate that a cholesteric LC with

temperature range extending well below room temperature should allow nanoparticle doping

with only a negligible effect on the helical modulation.

Regarding dispersion stability we can conclude that the helical superstructure of the

cholesteric LC enhances the tendency of the host to cause large bundles of CNTs to be

expelled from the LC phase analogous to other colloidal particles [18] - and to further

aggregate to minimize the interacting areas between LC and CNT-bundles.

Acknowledgments

We thank Bettina Flting and Eva Enz for the help in acquiring the DSC-data. Also financial

support from the Exzellenzscluster Nanostrukturierte Materialien of the Land Sachsen-

Anhalt is gratefully acknowledged.

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