Chiral polyesters with azobenzene moieties in the main chain, synthesis and evaluation of nonlinear optical properties

Article abstract of DOI:10.1039/a900567f

The synthesis, characterization and evaluation of nonlinear optical properties of a new series of chiral copolyesters containing push-pull electronic azobenzene mesogenic groups in the main chain are reported. The copolyesters showed liquid crystalline behaviour with high glass transition temperatures (~200 °C), stable liquid crystalline mesophases (up to 360 °C) and high second harmonic generation (SHG) efficiency. The second order nonlinear susceptibility values were measured as a function of the percentage of the chiral building unit and the temporal stability was found to be significant over a long duration of time.

Full text of DOI:10.1039/a900567f

J O U R N A L O F
Chiral polyesters with azobenzene moieties in the main chain,
synthesis and evaluation of nonlinear optical properties
Damodaran Bahulayan† and Krishnapillai Sreekumar*
Department of Chemistry, University of Kerala, Kariavattom Campus, Trivandrum 695 581,
Kerala, India
C H E M I S T R Y
Received 21st January 1999, Accepted 27th April 1999
The synthesis, characterization and evaluation of nonlinear optical properties of a new series of chiral copolyesters
containing push–pull electronic azobenzene mesogenic groups in the main chain are reported. The copolyesters
showed liquid crystalline behaviour with high glass transition temperatures (~200 °C), stable liquid crystalline
mesophases (up to 360 °C) and high second harmonic generation (SHG) efficiency. The second order nonlinear
susceptibility values were measured as a function of the percentage of the chiral building unit and the temporal
stability was found to be significant over a long duration of time.
electronic charges across the entire length or parts or segments
of the polymer molecule under the influence of an external
field (optical or electric).14 The use of chiral building units
can prevent crystallization into centrosymmetric space groups.
Long polymers in solution often crystallise into a hexagonal
columnar phase. When the polymers contain chiral groups,
this close packing into a triangular lattice competes with the
tendency of the polymers to twist macroscopically. Similarly
to the twist grain boundary phase of chiral smectics, macro-
scopic chirality can proliferate when screw dislocations enter
the crystal.15 In addition to this, cooperative chiral order plays
a vital role in the self assembly of ordered supramolecular
structures. The present paper describes the synthesis, charac-
terization and evaluation of the nonlinear optical properties
of a set of chiral polyesters containing azobenzene mesogenic
groups.
Introduction
Much interest has been directed at the use of organic
compounds as materials for nonlinear optical applications in
recent years.1–3 There is a myriad of possibilities among
organic small molecules and polymers, which allow consider-
able flexibility in the molecular design. One of the formidable
challenges associated with present day chemical synthesis is
the construction of multimolecular assemblies with tailor made
properties. This is possible by engineering interactions/spatial
relationships between pre-selected constituent molecular build-
ing blocks.4,5 An emerging field of interest is nonlinear optics.
The use of organic compounds with high second order nonlin-
ear optical properties allows full exploitation of their desirable
qualities by allowing long interaction lengths, large power
densities with modest power input, ultra-fast response times
and damage resistance to optical radiation.6–8 Nonlinear
optical phenomena, typically, frequency doubling (infrared
into visible) require molecular structures with donor–acceptor
and/or conjugated functionalities which will not crystallise
into centrosymmetric space groups.9,10 Many organic mol-
ecules and polymers which can exhibit high second harmonic
generation (SHG) activity, require special techniques like
poling under an electric field to remove the centre of symmetry
and orient themselves in a directional fashion so that they are
practically applicable for device fabrication.11,12 The main
approaches in this regard centre on attempts to design mol-
ecules with structural features that favour packing in an
acentric crystal structure and incorporating donor–acceptor
moieties. These molecules are doped into polymer matrices,
cast into films and poled under a large electric field. This
process is advantageous because of good mechanical properties
and ease of device fabrication. But, there are limitations
regarding loading of the chromophore molecules. Also, when
the external field is removed, the films relax back to random
orientation and become SHG inactive. A novel and facile way
to overcome this difficulty is to use main chain liquid crystalline
polymers incorporating chiral units and/or donor–acceptor
building blocks. The contribution of p-electrons in determining
SHG activity has been well established. We have tried to
couple the dual effects of molecular chirality and macroscopic
chirality with donor–acceptor p-electron systems to design
polymers with inbuilt directional orientation, thereby giving
good SHG activity.13 The incorporation of donor–acceptor p-
electron systems provides a pathway for the redistribution of
Experimental
Materials and methods
Infrared spectra were recorded on a Perkin-Elmer Model 882
IR spectrometer. UV–Vis spectra were recorded on a Shimadzu
UV-2100 spectrophotometer. 1H NMR spectra were recorded
on a JEOL 90 MHz NMR spectrometer with TMS as internal
standard. Optical rotations were measured using an ELICO
Model polarimeter PA-21 at a wavelength of 535 nm in a
cuvette of 10 cm length. Thermogravimetry and differential
scanning calorimetric measurements were made on a TGA V5
OD Dupont 200 TGA instrument and an MDSC VI. IA TA
Inst. 2000 instrument respectively. Inherent viscosities were
measured with an automated Ubbelohde viscometer, thermo-
statted at 25 °C. Molecular weights were determined by GPC
measurements.
Second harmonic generation activities were measured by the
Perry and Kurtz method.16 Measurements were done both by
the thin film method and the powder method using a Spectra
Physics GCR-R series Nd5YAG laser (1064 nm, 11 ns,
400 mJ pulse−1). For comparison, the standards used were
urea and 2-methyl-4-nitroaniline (MNA). Thin films of thick-
ness 1 mm were prepared by a spin coating technique. For
powder measurements, the samples were ground and graded
by use of standard sieves (100–150 mm) and loaded on a
microscopic slide with a sample thickness of 1 mm. Urea and
MNA samples used as standards were also powdered and
sieved (100–150 mm) after drying under high vacuum. They
were also mounted with the same thickness as the polyester
samples. The laser beam was directed unfocused on to the
†Present address: Clays and Clay Minerals Division, Regional
Research Laboratory (CSIR), Trivandrum 695 019, India.
J. Mater. Chem., 1999, 9, 1425–1429
1425

sample kept at a 45° angle to the laser beam; the emission was
collected from the front face of the sample at a 90° angle. The
second harmonic signal at 532 nm was detected by a photomul-
tiplier and stored on a Philips model PM 3323 digital
oscilloscope.
dried (~14 g). Crystallization of this product from dioxane–
alcohol mixture afforded 4,4∞-diamino-2,2∞-dinitrodi-
phenylmethane as orange–yellow flakes (mp 206–208 °C)18
[Analysis, found: C 54.11, H 4.18, N 19.33%; required for
C H N O : C 54.16, H 4.16, N 19.44%]. 4,4∞-Diamino-
13 12 4 4
3,5,3∞,5∞-tetramethyl-2,2∞-dinitrodiphenylmethane (mp 226 °C):
Preparation of monomers
[Analysis, found: C 59.33, H 5.62, N 16.34%; required for
C H N O : C 59.30, H 5.8, N 16.28%].
17 20 4 4
1,453,6-Dianhydro-
D-sorbitol
(isosorbide).
1,453,6-
Dianhydro--sorbitol (isosorbide) was prepared by the
dehydration of -sorbitol (E. Merck, Germany) using concen-
trated sulfuric acid and removing the water and excess sulfuric
acid with a rotary vacuum flash evaporator. White needle
shaped crystals (mp 61 °C) were obtained and used without
further purification. Terephthaloyl chloride was obtained from
E. Merck (Germany).
Bis(4-hydroxyphenylazo)-2,2∞-dinitrodiphenylmethane. The
diazonium chloride prepared by reacting an ice-cold solution
of 4,4∞-diamino-2,2∞-dinitrodiphenylmethane (10 g) in hydro-
chloric acid (6 M, 20 ml) with aqueous sodium nitrite solution
(6 g in 15 ml water), on coupling with phenol at 0–5 °C gave
a brown solid (the pH of the solution was maintained at 7.5–8
by adding solid sodium acetate before the addition of phenol).
It was collected, washed with water and dried. Crystallization
of the product from dioxane–alcohol mixture afforded bis(4-
hydroxyphenylazo)-2,2∞-dinitrodiphenylmethane (mp 257 °C).
[Analysis, found: C 60.18, H 3.58, N 16.81%; required for
4,4∞-Diaminodiphenylmethane.
Diaminodiphenylmethanes
(DDM) were prepared by the condensation of the correspond-
ing aniline with formaldehyde following Scanlan’s procedure.17
In a typical experiment, 40% formaldehyde solution (4 g) was
added drop-wise with effective shaking to a cold solution of
aniline (9.4 g) in a minimum quantity of dil. HCl (0.1 M).
The yellow solution obtained was heated in a water bath at
65–70 °C for 6 h. The reaction mixture was neutralized by
adding solid sodium carbonate. It was steam distilled to
remove any unreacted amine. The solid residue was dissolved
in dil. HCl and decolorized with charcoal and then neutralized.
The product precipitated was collected, washed well with water
and dried (~10 g). On crystallization from ethanol, colourless
flakes of 4,4∞-diaminodiphenylmethane were obtained (mp
88–93 °C).
C H N O : C 60.24, H 3.61, N 16.86%]. IR(KBr) cm−1:
25 18 6 6
1540, 1320 (NO ), 1625 (–NLN–), 3400 (OH); UV, l
(nm):
2
max
270, 415. 1H NMR (d): 9.8, [2H(s) (OH)], 8.1–8.4, [8H(m)
(phenyl)], 7.3–6.9 [2H(m) (phenyl)], 3.5 [2H(s) (CH )]. Bis(4-
hydroxyphenylazo)-2,2∞-dinitro-3,5,3∞,5∞-tetramethyldiamino-
2
diphenylmethane (mp 271 °C). [Analysis, found: C 62.75, H
4.58, N 15.20; required for C H N O : C 62.81, H 4.69, N
29 26 6 6
15.16%]. UV, l
(nm): 268, 410. 1H NMR: (d) 1.3 [12H(d)
max
(CH )], 9.8 [2H(s) (OH)], 8.1–8.4 [8H(m) (phenyl)], 7.3–6.9
3
[2H(m)(phenyl)], 3.5 [2H(s) (CH )].
2
Polycondensation
4,4∞-Diamino-3,5,3∞,5∞-tetramethyldiphenylmethane
(mp
126 °C) was prepared by adopting the same procedure for the
condensation of 2,6-dimethylaniline with formaldehyde.
The polyesters were prepared by the condensation of the
monomers, terephthaloyl chloride (2 equiv.), isosorbide (1
equiv.) and azobiphenol (1 equiv.) in a mixture of dimethylac-
etamide and 1,2-dichlorobenzene (154 v/v) at 160 °C for 18 h.
The mole percentages of isosorbide and the azobiphenols were
varied from 0 to 100% to obtain the polyesters 5a–g and 6a–g.
After the reaction, the products were precipitated by adding
the cooled reaction mixture into methanol.
4,4∞-Diamino-2,2∞-dinitrodiphenylmethane. Nitration of the
DDM’s was accomplished by using a mixture of anhydrous
potassium nitrate and 98% sulfuric acid at 0 °C.18 To an ice-
cold solution (0–5 °C) of 4,4∞-diaminodiphenylmethane
(0.05 M, 9.9 g) in conc. sulfuric acid (18 M, 40 ml), a solution
of potassium nitrate (10.1 g) in conc. sulfuric acid (18 M,
15 ml) was added during a period of 1 h. The stirring was
continued for another 3 h, keeping the reaction mixture below
5 °C. The reaction mixture was poured into crushed ice and
neutralized with ice-cold aqueous ammonia. The yellow solid
was collected on a filter, washed thoroughly with water and
Results and discussion
The polyesters 5a–g and 6a–g were prepared by the
polycondensation of terephthaloyl chloride (1) with 1,453,6-
dianhydro--sorbitol (isosorbide) (2), which acts as the chiral
1426
J. Mater. Chem., 1999, 9, 1425–1429

Table 1 Yields and properties of polyesters
building unit, and an azobiphenol, bis(4-hydroxyphenylazo)-
2,2∞-dinitrodiphenylmethane (3) or bis(4-hydroxyphenylazo)-
2,2∞-dinitro-3,5,3∞,5∞-tetramethyldiphenylmethane (4). The
polycondensation reaction was conducted in a solvent mixture
of dimethylacetamide and 1,2-dichlorobenzene (154 v/v).
Preliminary studies on the condensation reaction in solvents
like dioxane, tetrahydrofuran and dimethylacetamide resulted
in polyesters with low inherent viscosity. This was attributed
to the possibility of side reactions, especially, formation of
ether linkages and exchange reactions leading to cyclic bypro-
ducts. Hydrogen chloride, which is produced during the reac-
tion is soluble in the above solvents, and it catalyses the side
reactions. But when an aromatic solvent like 1,2-dichloro-
benzene is used, this problem can be avoided, since hydrogen
chloride, being insoluble, is evolved and swept out of the
reaction mixture without interfering in the course of the
reaction.19 The easy removal of hydrogen chloride drives the
reaction in the desired direction, to the desired extent and
polyesters with comparably high molecular weight can be
obtained. There is the possibility of controlling the reaction
by adjusting the temperature and duration of the reaction so
that polyesters with comparable molecular weights and good
mechanical properties together with good solubility are
obtained. Temperatures around 160 °C can be used, since
isosorbide is highly thermally stable and it does not undergo
racemization at this temperature.20,21 Comparatively high mol-
ecular weight polymers have been obtained and the 1H NMR
spectrum indicated the incorporation of all the reactive mon-
omeric units. But the sequence of the copolymer or the
copolymer composition was not obtainable from the 90 MHz
NMR spectrum. Also, no information about the relative
reactivities of the different monomers is available. The
1H NMR spectrum of the polyesters showed a closely spaced
Isosorbide
Polyester content (%) (%)
Yield
g
a
M
9
M /M
[a] 25b
9
inh
n
w
n
D
5a
5b
5c
5d
5e
5f
100
80
60
50
40
20
0
100
80
60
50
40
20
0
95
90
93
90
85
85
95
90
92
87
92
85
87
91
0.45 51 000 1.3
0.51 45 000 1.7
0.82 47 000 1.3
0.81 42 000 1.5
1.13 32 000 2.0
0.71 35 000 1.8
0.85 48 000 1.4
0.46 45 000 1.4
0.54 48 000 1.7
0.71 52 000 1.5
0.85 51 000 1.7
0.71 42 000 1.6
0.81 48 000 1.8
1.01 53 000 1.7
−210.0
−190.2
−162.1
−152.5
−116.1
−75.1
0
−199.2
−185.2
−160.5
−142.2
−100.1
−65.2
0
5g
6a
6b
6c
6d
6e
6f
6g
aConcentration, 2 g l−1 in acetonitrile–trifluoroacetic acid (451) at
25 °C. bConcentration, 5 g l−1 in acetonitrile–trifluoroacetic acid (451)
at 25 °C.
on the extent of the biphenol unit; the higher the percentage
of biphenol, the higher the value of the decomposition tempera-
ture. For the polyesters 5a–g and 6a–g, these varied in the
range 410–450 °C for the first stage (10–15% mass loss) and
540–570 °C for the second stage. After the second stage, a
residue of 43–57% was observed for the polyesters.
In the DSC curves, for all the polyesters upon heating, a
weak step-down in heat capacity corresponding to the glass
transition (T ) was observed at a temperature of 200–250 °C.
g
There was a gradual decrease in T values with a decrease in
g
the amount of isosorbide units. After the glass transition, a
triplet signal at d 4.6 (CH ), a doublet at 5.3 (CH) and 5.7
broad endothermic peak was observed for the polyesters
associated with the mesophase–isotropic transition analogous
to the liquid crystalline behaviour. It seems that higher mole
fractions of the biphenol stabilize the LC phase and the
isotropization temperature rises above 350 °C. On cooling, the
transitions were reversible with a supercooling of few degrees.
Due to the non-availability of a hot-stage polarized micro-
scope, optical photographs of the polyesters could not be
obtained. They would have provided a clear picture of the
texture of the LC phase and helped us in characterizing it.
The dynamic mechanical behaviour of the polyesters was
studied in the linear viscoelasticity range at 1 Hz frequency.
The transition temperatures of the dynamic mechanical pro-
cesses were evaluated as a function of temperature and copoly-
mer composition. For the different polyesters, there was a
gradual decrease in the storage modulus (G∞) with increase in
temperature. A pronounced decrease in G∞ values was observed
at regions corresponding to b and a relaxation processes.
Later, in the regions of isotropization relaxations, a complex
viscoelastic behaviour was observed involving a decrease in G∞
values followed by an increase and finally reaching a plateau.
This corresponded almost identically to the isotropization
region in the DSC curve. This peculiar viscoelastic behaviour
has been ascribed to the interplay of two conflicting tendencies:
the usual decrease in modulus with increasing temperature
and an unusual modulus increase due to an increase in the
constraints on the mesogens as a transition from a locally
organized region to a random chain arrangement occurs.23
For the loss modulus (G◊) also, similar changes were observed
with increase in temperature. The b-relaxation process which
is more concerned with the relaxation modes of the mesogenic
units, was seemingly more sensitive to the copolymer composi-
tion. Typical DSC curves of the polyester 5d are given in
Fig. 1. Fig. 2 represents the dynamic mechanical transitions of
the polyester 5d. The thermal properties of the polyesters 5a–g
and 6a–g as evaluated by DSC and DMA are summarised
in Table 2.
2
(CH bridge) indicating the presence of isosorbide units. The
spectra also showed signals at d 3.5 (s, CH ), 1–1.13 (d, CH
2
3
for 6) and an aromatic A B pattern at 8.1–8.4 (phenyl linked
2 2
to NLN) and 7.6–7.8 (phenyl linked to –COO–). These are
indicative of the azobiphenol and terephthaloyl units.
UV spectra of all the polyesters were obtained in DMF at
room temperature. The spectra exhibited characteristic bands
of phenyl groups (220–240 nm), nitro groups (270 nm, n–p*)
and azo groups (390–420 nm, p–p*). The UV spectra were
also recorded after cooling from the isotropic liquid state.
There was no change in the absorption maxima. This showed
that all the building units remained intact even in the molten
state. In many cases of azo polymers, some extent of decompo-
sition of the polymer or thermal rearrangement of the azo
groups was observed on heating to high temperatures.22 The
high temperature stability of the polyesters was revealed from
the IR spectra also. There was no change in the vibrational
frequency values after cooling from the isotropic liquid state.
The polyesters showed almost 100% optical purity even in
the molten state. Relatively high values of specific rotation (a)
have shown that no extensive racemization occurred during
the polymerization stage or when the polyesters were heated
to melt. When the optical rotation was measured after cooling
from the liquid phase, no change in the value of a was
observed. The value of a decreased with a decrease in the
amount of isosorbide units. The isosorbide unit has proved to
be a highly stable building block which could impart molecular
level chirality to copolymers and retain it at high temperatures.
The properties of the copolyesters are summarised in Table 1.
Thermal properties of the polyesters
Thermogravimetric studies were conducted for the polyesters
from 30 to 600 °C. The polyesters were stable up to 400 °C.
After that decomposition occurred in two stages. The tempera-
ture of initiation of both the decomposition stages depended
J. Mater. Chem., 1999, 9, 1425–1429
1427

Table 3 Optical properties of the polyesters
SHG activitya
Polyester
Urea
MNA
bb/10−30 esu
Refractive indexc
5a
5b
5c
5d
5e
5f
0.82
0.87
0.90
0.95
0.68
0.55
0.45
0.85
0.87
0.92
0.97
0.80
0.65
0.57
0.87
0.90
1.05
1.35
0.85
0.65
0.58
0.90
0.92
1.11
1.47
0.93
0.72
0.61
2.359
2.441
2.826
3.655
2.301
1.764
1.576
2.430
2.486
2.998
3.979
2.531
1.954
1.647
1.8257
1.8107
1.8012
1.7570
1.8011
1.7725
1.8152
1.8211
1.8005
1.7771
1.7910
1.8151
1.7952
1.7815
5g
6a
6b
6c
6d
6e
6f
Fig. 1 DSC curves (1, heating and 2, cooling) of 5d.
6g
aSHG of urea and MNA=1. bStatic hyperpolarizability based on
MNA (b of MNA=2.7 10−30 esu). cRefractive index at 535 nm.
value of b was determined from eqn. (1),
3e2 h2 fDm
ge
b=
(1)
2m DE 3
ge
where E is the energy of transition, m , the dipole moment
ge ge
change and f, the oscillator strength of the first excitation. The
energy of transition is found from the band maximum of the
UV–Vis absorption spectrum and the oscillator strength, f of
the transition from the area under the band. The change in
dipole moment was determined by a solvatochromic method.25
All the polyesters showed moderately good efficiency in exhibit-
ing SHG ability. Even those samples with zero mole percentage
of isosorbide units (5g and 6g) showed considerable ability. It
is not fully clear how the polyesters 5g and 6g could show
good SHG ability even without the presence of isosorbide
units. Some of the samples (5d and 6d) were even better than
MNA. This was more conspicuous when the thin film tech-
nique was used. It was observed that when semicrystalline
polymer films were deposited on ordered polymer substrates
like Teflon, Teflon surfaces showed excellent ability to induce
orientation in polymer thin films grown on them.26 The
semicrystalline grains of the polyester exhibit a greater degree
of alignment along one particular direction. This is attenuated
by the directional property of the azomesogenic group, due to
a preferred trans orientation.27 The directional orientation of
the polyester molecules is highly enhanced by the helical
organization of the macromolecular chains. The centre of
symmetry is necessarily broken when the preferred helicity is
achieved in the supramolecular organization.28 The extent of
chiral order is dependent on the enantiomer concentration.
The fact that the polyesters 5 and 6 show no racemization,
even in the isotropic liquid state reveals that absolute chiral
order prevails. Here, the polymer backbone itself is
chiral, supplementing the directional order of the azomesogenic
group and the donor–acceptor p-electron system; there is a
preference for one sense of the helix, which leads to the high
SHG ability. Probably, this is the reason why even polyesters
5g and 6g with no isosorbide units or polyesters 5a and 6a
with no azobiphenol units exhibit comparably good SHG
ability.
Fig. 2 Plots of dynamic mechanical transitions vs. temperature of 5d.
Second harmonic generation
The second harmonic generation efficiency of the polyesters
was examined by the powder reflection technique of Kurtz
and Perry16 and by reflection from a thin film deposited on
Teflon tapes.24 A laser beam from an Nd5YAG pulsed laser
(1064 nm, 11 ns pulses) was used. The measurements were
recorded in comparison with urea and 1-methyl-2-nitroaniline
(MNA). The results are shown in Table 3. The second har-
monic signal was observed at 532 nm. As the absorption
maxima of the polyesters were quite different from that of the
SH signal, there was no possibility of reabsorption of this
signal by any of the samples and therefore no resonance
enhancements were observed. Hence the static hyperpolariz-
ability (excitation energy=0 eV) was determined. The absolute
Table 2 Thermal properties of polyesters
Polymer
T /°C
T /°Ca
T /°Cb
T /°Cb
T /°Cb
g
i
b
a
i
5a
5b
5c
5d
5e
5f
250
242
230
215
208
205
198
261
255
240
233
225
213
203
280–310
290–322
295–325
302–330
311–342
320–348
330–357
281–314
288–325
297–333
311–348
321–354
333–367
345–375
180
188
195
201
214
221
230
178
188
198
210
219
228
237
220
229
232
238
244
251
260
215
220
231
240
251
268
282
295
314
320
325
335
340
351
300
310
321
338
347
355
365
5g
6a
6b
6c
6d
6e
6f
The temporal stability of the SH signal was monitored at
100 and 200 °C. The polyester samples except 5f, 5g, 6f and
6g retained 100% SH signal intensity at 200 °C. But for the
samples 5f and 6f, SH signal intensities were reduced to 90%
of their original values when the samples were heated at 200 °C
for 25 h. For samples 5g and 6g which did not contain any
isosorbide units, the decrease in SH signal intensity was more
catastrophic. The variation in the temporal stability of the SH
6g
aFrom DSC measurements, heating rate 10° min−1 in nitrogen. bFrom
DMA studies, heating rate 5° min−1 at 1 Hz in nitrogen.
1428
J. Mater. Chem., 1999, 9, 1425–1429

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g
7
8
lowest T values. They also did not contain isosorbide units.
g
It could therefore be assumed that the presence of isosorbide
units was essential in stabilizing the SHG ability at high
temperature. The variation of temporal stability of the SH
signal with temperature is presented in Fig. 3.
9
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Conclusions
A series of polyesters containing an azomesogenic group and
a chiral building block in the main chain has been prepared
and characterized. They exhibited good thermal properties,
have high T values. Some of the dynamic mechanical trans-
g
itions were peculiar; additional directional order as a result of
the presence of the azomesogenic group and macroscopic
chirality resulted in an unusual increase in storage modulus
(G∞) values at high temperatures. All the polyesters showed
moderately good SHG capability. One could have expected a
dramatic enhancement in SHG ability for the polyesters where
there was a cumulative influence of the directional property
of the azomesogenic groups and the prevailing chiral order of
the polymer chains. But the results were not encouraging.
There may be loss of macroscopic chirality due to some
unforeseen reasons, probably due to steric compulsions. The
polymer chains characterized by helical structures are noncen-
trosymmetric at the molecular level. But in randomly oriented
polymer films obtained by solvent evaporation, noncentrosym-
metry may be lost. Alignment of the molecular helices by
application of an electric field across the solution with simul-
taneous evaporation of the solvent could give high SHG
activity. Poling the polymer films at temperatures above the
glass transition temperature can induce macroscopic chirality
and the resulting directional order can be locked. Better results
may be obtained by shifting to the Maker–Fringe or HRS
methods. Measurement of the SH signal by the thin film
technique proved to be promising because of the added
organizational order induced by the Teflon substrate. The
temperature stability of the SH signal was also significantly
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Acknowledgement
Paper 9/00567F
The authors sincerely thank Professor C. P. Joshua for the
inspiration he has rendered them to pursue research in polymer
photochemistry.
J. Mater. Chem., 1999, 9, 1425–1429
1429