Third Day Poster Session, 17 June 2010 - NanoTR-VI

Determination of Dielectric Anisotropy Properties of Poly(N-Vinylimidazole) Based Hydrogen Bonded
Side Chain Liquid Crystalline Polymer by 4-Cyano-4'-Pentylbiphenyl Nematic Liquid Crystal
Yeşim H. Gursel a , B. Filiz Senkal a , Fahrettin Yakuphanoglu b*
a Istanbul Technical University, Department of Chemistry, 34469, Maslak, İstanbul, Turkey
b Fırat University, Department of Physics, 23119 Elazığ, Turkey
Abstract— The liquid crystal and dielectric anisotropy properties of poly(n-vinylimidazole) based hydrogen bonded side chain
liquid crystalline polymer (HB-PLC) doped 5CB and pure 5CB liquid crystals have been investigated by polarized optical
microscopy and dielectric spectroscopy method. The polarized optical microscopy results show that HB-PLC exhibited an
enantiotropic nematic phase on heating and cooling cycles. The real and imaginary parts of the dielectric constant dependence of the
voltage applied shows that HB-PLC dopant changes the dielectric parameters of the LCs. The dielectric behaviour of the LCs shows
only a relaxation process. Dielectric anisotropy () properties of the LCs changes from the positive type to negative type. It is
evaluated that the dielectric anisotropy and relaxation properties of 5CB and 5CB/ HB-PLC LCs can be controlled by HB-PLC
dopant.
Recently, side chain liquid crystalline polymers (SCLCPs),
which combine the unique properties of low-molar mass liquid
crystals and polymers, have been the subject of intensive
research mainly due to their interesting electrical and optical
properties which makes them good candidates for applications
in microelectronic devices ranging from optical data storage
and non-linear optics to being the stationary phase in gas
chromatography and high performance liquid
chromatography. Side-chain liquid-crystalline polymers
(LCPs) are usually prepared by covalently linking rigid
mesogens to a polymer backbone through flexible spacers. In
recent years, self-assembly through specific interactions, such
as hydrogen-bonding ionic, ionic-dipolar and charge transfer
interactions 13 , has been recognized as a new strategy for
constructing side-chain liquid crystal polymers. Selfassembled
materials formed by non-covalent bonding have
attracted much attention because these materials are good
candidates for the next generation of materials for which
dynamic function, environmental compatibility, and low
energy processing are required. The dielectric-spectroscopy
technique (DST) is a powerful technique successfully applied
for understanding the molecular details [13]. The dielectric
anisotropy is expressed as ||
, where
||
and
are the parallel and the perpendicular components of the
electric permittivity, respectively. Regarding the dielectric
constant, there are two structure types. One is named as
positive dielectric anisotropy (p-type) and its dielectric
constant along the director axis is larger than that along the
axes perpendicular to the director. is greater than zero in
this case. The other type is named as negative dielectric
anisotropy (n-type), is less than zero. Variation of with
respect to the spot frequencies reveals that LC orientation has
p-type property at low frequencies, and as the frequency
increases the dielectric anisotropy character shifts to n-type. In
present study, we have investigated dielectric anisotropy
properties of poly(n-vinylimidazole) based hydrogen bonded
side chain liquid crystalline polymer (HB-PLC) doped 5CB
and pure 5CB liquid crystals.
To confirm the liquid crystalline nature of the HB-PLC and
identify the mesophase, hot-stage polarized optical microscope
(POM) was used. The POM results showed that HB-PLC
exhibited an enantiotropic nematic phase on heating and
cooling cycles. As it was heated to 97 °C, the typical nematic
schlieren texture appeared (Fig.1).
Fig.2 Schlieren texture of polymer HB-PLC at 97 °C (400x)
The plots of the capacitance-voltage (C-V)of (HB-PLC) doped
5CB and pure 5CB liquid crystals for >0 and