Supplementary Material
Enhancement of electrical conductivity of a liquid crystal-gold nanoparticle
composite by a gel network of aerosil particles
B. Kamaliya, M. Vijay Kumar, C.V. Yelamaggad and S. Krishna Prasad*
Centre for Nano and Soft Matter Sciences, Bangalore 560013, INDIA
1.
LC-NPs composites and Cell Preparation
The host liquid crystalline compound (LC) 4-pentylphenyl 2-chloro-4-(4-pentylbenzoyloxy)
benzoate (PCPBB for short) was synthesized in our laboratory using the method reported
elsewhere [1]. Spherical Gold nanoparticles (GNP), capped with 1-dodecanethiol and having
a diameter of 3 nm, were purchased from NanopartzTM.
The details of the particle
characterization, provided by the manufacturer along with the Certificate of analysis are,
Particle size from DLS and TEM: 3 nm; UV-Vis Peak: 516 nm and Concentration: 2.49 x
1014 nps/mL. Both hydrophilic (Aerosil A- 300) and hydrophobic (Aerosil R812) nanosilica
particles (Aerosil®) were from Degussa Corporation, provided as a gift by Mr. Vikas Rane of
d-hindia Ltd, Mumbai. Toluene was obtained from a local source which was used as a
solvent while mixing nanoparticles in LC.
The mixtures were prepared by weighing the required weight fractions of the NPs and the
host LC, transferring the materials to a 5 ml vial filled with toluene, followed by
ultrasonication for 20 minutes to ensure uniform dispersion of all constituents in the solvent.
Then, the vial was kept in a hot air oven at 1100C for two hours and vacuum dried for about
14 hours to achieve the complete evaporation of the solvent. The concentration of the Gold
NPs was 3wt%, and aerosil NPs was 1.5wt% in the appropriate composites. Polarizing
Optical Microscopy images display the uniform distribution of the nanoparticles throughout
the host material in all the four composites. Differential Scanning Calorimeter scans were
obtained in an inert atmosphere using Perkin Elmer DSC 8000 in the cooling mode at a rate
5K/min.
2. Procedure for electrical measurements
The electrical measurements were performed with samples sandwiched between two low sheet
resistivity (< 10 ohm per square) indium tin oxide coated glass plates. Mylar strips, placed
outside the electrically active area, defined the thickness of the cell, with the nominal
thickness being ~ 0.05 mm. The sample cells were placed inside a home-made copper block
heater, which in turn was kept between the pole-pieces of an electromagnet (Bruker B-20) that
provided a uniform magnetic field of 0.8 Tesla over the entire sample region. Owing to the
positive diamagnetic nature of the sample, this field, which is much above the Freedericksz
threshold, was used to orient the LC molecules, and fix the probing direction to be parallel or
perpendicular to the nematic director. The electrical conductivity data were obtained using an
Impedance analyser (HP 4194A) at a frequency of 1 kHz with a probing voltage of 0.5V.
3. Explanation for the thermal behaviour of  for LCPhil composite
Here we provide the reason for the reversal in the thermal trend at low temperatures observed
for the LCPhil composite. The hydrophilic aerosil particles which are entirely decorated with
siloyl groups provide a strong anchoring of the LC molecules at the particle surfaces. Thus,
the spherical shape of the particles would lead to a 3-dimensional “powder-like” alignment of
the LC molecules present in the vicinity of the aerosil surface despite the fact that the bulk of
the LC molecules have the orientation dictated by the substrate surface or applied field. This
results in a certain number of molecules being in the homeotropic orientation and thus being
influenced by the director relaxation. This feature is not seen for pure LC sample because
there is no opposing influence for the surface-dictated orientation. The absence of the step for
the LCG composite shows that the anchoring at the GNP surfaces (with alkyl thiol capping) is
much weaker than at the aerosil surfaces. The GNP influence could perhaps be dominating
when both (GNP and aerosil) kinds of nanoparticles are present, as seen for the LCGPhil and
LCGPhob composites.
References
[1].
J. P. Van Meter and B. H. Klanderman, Mol. Cryst. Liq. Cryst., 22, 285 (1973).