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S. Dineshkumar et al. / Journal of Molecular Structure 1128 (2017) 730e740
(ꢀ0.326, ꢀ0.298) > HMBE (ꢀ0.326, ꢀ0.297), which is same as the
conductivity of the respective polymers. So, the order of conduc-
tivity of the polymers are proved with the help of electron density
distributions of monomers. DC conductivity studies confirm that
the polymers can be used as semi-conductive materials in elec-
tronic, opto-electronic and photovoltaic applications.
3.4.2. Dielectric properties
The frequency and temperature dependent dielectric constant
and dielectric loss of polymers were measured. The plots of varia-
tion of dielectric constant and dielectric loss with logarithmic fre-
quency at various temperature are shown in Fig. 9. The dielectric
constant of the material was calculated using the formula [25].
ε r ¼ Cd/ε0A
(2)
where, C is the capacitance and d is the pellet thickness, A is the
cross sectional area of the pellet and ε0 is the free space permittivity
of the pellet. The dielectric constant of PHEBE, PHMBE and PHNAE
decreases with increase in frequency and beyond certain frequency,
it has attained constant value. The synthesized polymers have high
dielectric constant at low frequency as the dipoles have sufficient
time to align with the field before it changes the direction. At high
frequency, polymers have low dielectric constant because the di-
poles do not have sufficient time to align before the field direction
changes. When the frequency is low polymers exhibit high
dielectric constant values with increase in temperature because the
intermolecular forces between polymer chains are minimized and
also enhances thermal agitation of polymer chains. Whereas, at low
temperature, the dielectric constant is less because the segmental
motion of the chain is practically freezed. The temperature
dependent dielectric constant of polymers at a fixed frequency
50 Hz was analyzed and the plot shown in Fig. 10. At this constant
frequency when the temperature increases dielectric constant in-
creases. The increase in dielectric constant is very high for PHNAE
and very low for PHEBE. The PHNAE polymer has naphthalene
Fig. 8. Charge density distribution views of (a) HEBE, (b) HMBE and (c) HNAE.
moiety which is having more number of
two polymers with phenyl rings. These
p
bonds than the other
bonds are loosely
conductivity of the samples was calculated using the given formula
[24].
p
attached so, easily polarized, resulted in high dielectric constant.
So, the PHNAE can be used to make passive component like ca-
pacitors, resistors etc. [26], The variation of dielectric loss at various
temperature with increase in frequency is shown in Fig. 9 (b, d, f).
The value of dielectric loss is high at lower frequencies and it is low
at higher frequency regions. The low dielectric loss in high fre-
quency region suggested that the synthesized polymers are suit-
able for electro-optical device applications [27].
s
¼ [(IXL)/(VXA)]
(1)
where, I is the current, V is the voltage, L is the thickness of the
pellet and A is the cross sectional area of the pellet. A graph is
plotted between time and solid state conductivity values measured
at air atmosphere. On doping with iodine the electron emitting
imine nitrogen and electron pulling iodine coordinate and the
formation of radical cation (polaron) structure in polymer chain (on
imine nitrogen) is enabled. The electron vacancy formed due to this
polaron facilitates the electron flow, and this causes increase in
electrical conductivity. The electrical conductivity increases with
increase in iodine vapour contact time. After 120 h of contact time
the measured electrical conductivity of polymers are in the order
PHNAE > PHEBE > PHMBE. Polymer PHNAE is having higher elec-
trical conductivity (around 10ꢀ3 S cmꢀ1) than the other two poly-
mers (10ꢀ6 S cmꢀ1). In general, the electrical conductivity depends
on the charge density on the nitrogen coordinating sites of the
polymer chains. When the charge density on the coordinating site
increases, the extent of iodine coordination with the coordinating
site increases and they by conductivity increases [18]. By calcu-
lating the charge density of imine nitrogens of monomers by
Huckel calculation method, the charge density imine nitrogens of
polymers are compared [18]. The distribution of electron density of
monomers are shown in Fig. 8. The charge density of imine nitro-
gens of monomers are in the order HNAE (ꢀ0.451, ꢀ0.287) > HEBE
3.5. Thermal properties
Thermal stability of polymers was studied by thermogravi-
metric analysis. TGA traces of polymers are shown in Fig. 11. Tem-
perature corresponding to 10, 30, 50% weight loss and char yield are
given in Table 4. The initial weight loss (2e3%) observed in the
range of 30e110 ꢁC is attributed to the presence of occluded
moisture in polymers. All the polymers are undergoing single step
degradation. The degradation may be mainly due to breaking of
CeOeC etheric bond present in polymer. The polymers are slightly
differing in their weight loss at various temperatures and char
yield. This may be due to the propagation of polymer chain with
more number of radical sites leading to the formation of polymer
with more free ends (-OH group) which may be degrade easily. The
intensity of eOH signal in 1H-NMR is relatively high in PHNAE in-
dicates more number of free ends. The results described that the
polymers with long conjugation increases the delocalization of
electrons, leading to a higher resistance against temperature. The
p