Twin liquid crystals and segmented thermotropic polyesters containing azobenzene-effect of spacer length on LC properties

Article abstract of DOI:10.1002/pola.26062

We report systematic studies on a homologous series of twin liquid crystalline (LC) molecules based on phenyl and naphthyl azobenzene (PnP and NpnNp) as well as segmented copolyesters based on them. The twin series had the structure azobenzene-oligooxyethylene-azobenzene, where the ethyleneoxy length was varied from 2 to 6 units. The LC properties of the twin series depended on the chemical structure of the azochromophore and also the length of the central oligooxyethylene segment. The PnP series exhibited smectic LC properties for n > three oligooxyethylene units. Conversely, NpnNp series exhibited spherulitic phases only for the shortest member -Np2Np. One non-LC short spacer twin (P2P) and one LC long spacer twin (P6P) were incorporated as part of a main chain polyester composed of fully aliphatic segments of sebacate and di or tetraethylene glycol (DEG/TEG) units by melt polycondensation. Non-LC P2P formed LC polymers even at low (5 mol %) incorporation in DEG-based copolymers, whereas the LC-P6P could do so only at 30 mol % incorporation. The LC properties of the twin molecules as well as copolymers were studied using differential scanning calorimetry, polarized light microscopy (PLM) along with variable temperature wide angle X-ray diffraction.

Full text of DOI:10.1002/pola.26062

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Twin Liquid Crystals and Segmented Thermotropic Polyesters Containing
Azobenzene—Effect of Spacer Length on LC Properties
Chinmay G. Nardele, S. K. Asha
Polymer and Advanced Material Laboratory, Polymer Science and Engineering Division, National Chemical Laboratory (CSIR-
NCL), Pune 411008, Maharashtra, India
Correspondence to: S. K. Asha (E-mail: sk.asha@ncl.res.in)
Received 25 January 2012; accepted 12 March 2012; published online 5 April 2012
DOI: 10.1002/pola.26062
ABSTRACT: We report systematic studies on a homologous series
of twin liquid crystalline (LC) molecules based on phenyl and
naphthyl azobenzene (PnP and NpnNp) as well as segmented
copolyesters based on them. The twin series had the structure
azobenzene–oligooxyethylene–azobenzene, where the ethyle-
neoxy length was varied from 2 to 6 units. The LC properties of
the twin series depended on the chemical structure of the azo-
chromophore and also the length of the central oligooxyethylene
segment. The PnP series exhibited smectic LC properties for n >
three oligooxyethylene units. Conversely, NpnNp series exhib-
ited spherulitic phases only for the shortest member –Np2Np.
One non-LC short spacer twin (P2P) and one LC long spacer twin
(P6P) were incorporated as part of a main chain polyester com-
posed of fully aliphatic segments of sebacate and di or tetraethy-
lene glycol (DEG/TEG) units by melt polycondensation. Non-LC
P2P formed LC polymers even at low (5 mol %) incorporation in
DEG-based copolymers, whereas the LC-P6P could do so only at
30 mol % incorporation. The LC properties of the twin molecules
as well as copolymers were studied using differential scanning
calorimetry, polarized light microscopy (PLM) along with vari-
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KEYWORDS: azobenzene; azo polymers; main chain copolyester;
liquid crystalline polymers (LCP); melt polycondensation; poly-
condensation; twin liquid crystal
INTRODUCTION Azobenzene as a chromophore has been
studied widely for its most important property of photoiso-
merization due to which it finds application in a variety of
areas like optical data storage, molecular switches, display
devices, nonlinear optical devices, photomechanical systems,
and so forth.1–6 Liquid crystalline (LC) azobenzenes (side
chain or main chain) have generated lot of interest because
of their large photoinduced anisotropy and photochemical
phase transitions.7–9 In general, main chain polymers are
mostly condensation polymers which can be synthesized by
melt condensation or by the solution route. Solution poly-
merization has been the method of choice for most of the
reported main chain azobenzene polymers mainly due to its
low degradation temperatures which precluded a melt con-
densation route.10–16 However, the molecular weights
reported using the solution polymerization are generally
very low corresponding to less than 10 unit incorporation
on an average.13–14,17 The melt condensation route has the
advantage that it is devoid of solvent and also helps to build
the molecular weight. However, it is only applicable to mono-
melt polycondensation route. These twin azobenzene units
were designed having the structure azobenzene-spacer-azo-
benzene, where the spacer was oligooxyethylene of varying
length and two types of azobenzene chromophores—one
based on simple phenol (PnP) and the other on naphthol
(NpnNp) were used. The terminal azobenzene moieties were
functionalized with AC(O)OMe units so that transesterifica-
tion could be done with diols to form polyesters.
Twin molecules having the structure mesogen-spacer-meso-
gen or spacer-mesogen-spacer have long been considered as
ideal representatives for ‘‘segmented main chain liquid crystal
polymers’’ [-(flexible segment-rigid cores)n-].18–20 Although
started out as model systems to understand the behavior of
main chain liquid crystalline polymers, it was soon evident
that these twin liquid crystals were interesting systems on
their own. A vast majority of the studies comparing homolo-
gous twin systems and the related main chain polymers dealt
with polymethylene chains as the flexible units. The reason
for this being the interesting ‘‘odd–even’’ oscillation observed
in the isotropization temperatures and entropies as a function
of the even or odd parity of the number of atoms in the meth-
ylene spacer length.18,21 Conversely, the poly(oxyethylene)
units preferred a ‘‘gauche’’ conformation and the twins with
ꢀ
mers having low melting temperatures (<150 C).
In this article, we report our design of twin azobenzene sys-
tems which could directly be used as AA type monomer in a
Additional Supporting Information may be found in the online version of this article.
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poly(oxyethylene) central spacer generally did not show odd–
even oscillation of the isotropic-LC transition entropy.22,23 In
the design of our azobenzene twin molecules, the choice of
oligooxyethylene as the central spacer was very important, as
it served two purposes, first, it reduced the melting transition
of the resulting twin molecules so that melt polycondensation
route could be adopted, and second, it improved their solubil-
ity considerably. Another important aspect was that the polar
oligooxyethylene spacer units are usually incompatible with
the rigid rod-like azobenzene mesogen resulting in segrega-
tion into separate domains leading to mesophase formation.
Not many reports are available for azo-based twin systems,
where the central spacer segment is based on oligooxyethy-
lene A(CH2CH2O)n.24,25
flight (MALDI-TOF) in combination with size-exclusion chro-
matography (SEC). Elemental analysis was done by Thermo-
finnigan flash EA 1112 series CHNS analyzer. The HRMS mass
spectral analysis was done using a Autoconcept mass spec-
trometry instruments (MSI) UK in EI mode. The MALDI-TOF
analysis was done on Voyager-De-STR MALDI-TOF (Applied
Biosystems, Framingham, MA) equipped with 337-nm pulsed
nitrogen laser used for desorption and ionization. One micro-
molar solution of sample was premixed with 2,5 dihydroxy
benzoic acid matrix in CHCl3 and mixed well before spotting
on 96-well stainless steel MALDI plate by dried droplet
method for MALDI analysis. For small molecules, SEC was
performed using polystyrene standards for the calibration in
CHCl3 as eluent. The flow rate of CHCl3 was maintained as 1
lL/min throughout the experiments and the sample solutions
at concentrations 3–4 mg/mL were filtered and injected for
recording the chromatograms at 30 ꢀC. The molecular weights
of the polymers were determined by gel permeation chroma-
tography (GPC), which was performed using a Viscotek VE
1122 pump, Viscotek VE 3580 RI detector and Viscotek VE
3210 UV/vis detector in THF using polystyrene as standards.
Infrared spectra were recorded using Bruker FT-IR (ATR
Copolymerization is a versatile means to fine-tune the meso-
phase stability and sometimes the nature of the mesophase
as well. Segmented chain copolymers can be synthesized
starting from two premesogenic comonomers or one meso-
genic and one nonmesogenic monomer. The twin azobenzene
molecules were incorporated in varying mole ratios in a
main chain polyester based on dimethyl sebacate (DMS) and
DEG/TEG. This choice of the polyester scaffold for introduc-
ing the azobenzene moieties were also interesting as DEG
and sebacate formed a semicrystalline polyester, whereas
TEG and sebacate resulted in a completely amorphous poly-
mer. The questions that we addressed in this systematic
study were the following (1) the effect of spacer length on
the liquid crystalline properties of the two twin series, (2)
the effect of the chromophore – phenylazo versus naphthy-
lazo on the liquid crystalline characteristics of twin mole-
cules having the same spacer length, and (3) the effect of
copolymerization of mesogenic and nonmesogenic twin mol-
ecules with nonmesogenic comonomers. The mesophase
behavior of the twin molecules and copolyesters were inves-
tigated by differential scanning calorimetry (DSC), polarizing
light microscopy (PLM) in combination variable temperature
wide angle X-ray diffraction studies (VT-WXRD).
mode) spectrophotometer in the range of 4000–600 cmꢁ1
.
UV–Vis spectra were recorded using a Perkin Elmer Lambda-
35 UV–Vis spectrometer. The thermal stability of all the model
compounds and azo copolyesters were analyzed using Perkin-
Elmer: STA 6000 thermogravimetric analyzer (TGA) under
nitrogen atmosphere from 40 to 800 ꢀC at 10 ꢀC/min. DSC
was performed using a TA Q10 model. About 2–3 mg of the
samples had taken in aluminium pan, well-sealed and
scanned at 10 ꢀC/min. The instrument was calibrated with in-
dium standards before measurements. The phase behaviours
of the molecules were analyzed using LIECA DM2500P polar-
ized optical microscope equipped with Linkam TMS 94 heat-
ing and cooling stage connected to a Linkam TMS 600 tem-
perature programmer. The transition from isotropic to liquid
crystalline phase was monitored by the evolution of charac-
teristic textures. WXRD were recorded by a Philips analytical
diffractometer using Cu Ka emission and the spectra were
recorded in the range of (2y) 3–50ꢀ and analyzed using X’pert
software. Powder X-ray diffraction of all the samples were
carried out in a PANalytical X’pert Pro dual goniometer dif-
fractometer. An X’celerator solid-state detector was used in
wide-angle experiments. The radiation used was Cu Ka (1.54
Å) with a Ni filter and the data collection was carried out
using a flat holder in Bragg-Brentano geometry. Care was
taken to avoid sample displacement effects. Variable tempera-
ture in situ XRD experiments were carried out in an Anton-
Paar XRK900 reactor.
EXPERIMENTAL
Materials
Diethylene glycol (DEG), tri, tetraethylene glycol (TEG), hexa-
ethylene glycol, DMS, titaniumtetrabutoxide (Ti(Obu)4), p-
toulene sulfonyl chloride, and a-naphthol were purchased
from Aldrich Company and were used as such. P-amino ben-
zoic acid, phenol, sodium nitrite, potassium carbonate, and
potassium iodide were purchased from Merck Chemicals. Di-
methyl formamide (DMF), tetrahydrofuran (THF), and metha-
nol were purchased locally and were purified using standard
procedures.
Synthesis of Azo Dye
The hydroxy azobenzoic acid and the corresponding ester
were synthesized as described in our earlier report.14
Measurements
1H and 13C NMR spectra were recorded on a Bruker-AVENS
200 MHz spectrometer. Chemical shifts are reported in ppm
at 25 ꢀC using CDCl3 or CDCl3/TFA as solvent containing
small amount of tetramethylsilane (TMS) as internal standard.
The purity of the compounds was determined by elemental
analysis as well as high resolution mass spectrometry
(HRMS)/matrix-assisted laser desorption/Ionization-time of
Synthesis of Azo Model Compounds
Synthesis of 4-(40-Methoxy-phenylazo)-Benzoic
acid methyl ester (P0)
4-(40-hydroxy-phenylazo)-benzoic acid (1 g, 4 mmol), 0.5 mL
sulfuric acid, and 20 mL dry methanol were refluxed
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overnight. The reaction mixture was then poured into cold
water, neutralized with base and the precipitate was filtered,
washed with water and dried. The crude product was puri-
fied by column chromatography using hexane-ethyl acetate
(v/v 95/5) mixture. Yield ¼ 0.9 g (86%). mp: 169 ꢀC. 1H
NMR (200 MHz CDCl3): d (ppm): 8.18 (2H, d, Ar), 7.92 (4H,
m, Ar), 7.02 (2H, d, Ar), 3.94 (3H, s, AC(O)OCH3), 3.90 (3H,
s, OCH3). 13C NMR (CDCl3) d ppm: 52.10, 55.57, 114.37,
122.34, 125.16, 130.55, 131.30, 147.19, 155.49, 162.76,
166.57. FTIR (KBr) (cmꢁ1): 3004, 2949, 2842, 1718, 1602,
1583, 1499, 1434, 1313, 1281, 1282, 1153, 1106, 1024, 959,
866, 840, 776, 698, 556. HRMS m: 270.29. HRMS mþ1:
271.15. ELEM. ANAL calculated for C15H14N2O3: C, 66.66; H,
5.22; N, 10.36. Found: C, 66.32; H, 5.18; N, 9.97.
Synthesis of Polymers: Melt Polycondensation
SDP2P-5%
DMS (0.412 g, 1.788 mmol (x mol)), P2P (0.0548 g, 0.094
mmol (1–x mol)), DEG (0.200 g, 1.884 mmol) were taken in
a test tube-shaped polymerization apparatus and melted by
placing in oil bath at 100 ꢀC with constant stirring to melt
the solid. Once a homogenous mixture was formed, the reac-
tion mixture was cooled to room temperature and 1 mol %
of titaniumtetrabutoxide (Ti(Obu)4) was added as a catalyst.
The polycondensation apparatus was made oxygen and mois-
ture free by nitrogen purge. The polymerization tube was
immersed in the oil bath at 150 ꢀC and the polymerization
was carried out with slow nitrogen purge for 4 h. The result-
ant viscous mass was further condensed by applying high
ꢀ
vaccum (0.01 mm of Hg) at 150 C for 2 h. The polymer was
Synthesis of 4-(40-Methoxy-naphthalen-1-ylazo)-Benzoic
acid methyl ester (Np0)
dissolved in THF, filtered to remove catalyst and precipitated
in cold methanol to obtain the azo copolyester.
A similar procedure as above was adopted using 4-(40-
hydroxy-naphthalen-1-ylazo)-benzoic acid. The crude product
was purified by column chromatography using hexane-ethyl
acetate (v/v 95/5) mixture. Yield ¼ 1 g (90%). mp: 141 ꢀC.
1H NMR (200 MHz CDCl3): d (ppm): 8.99 (1H, d, Ar), 8.34
(1H, d, Ar), 8.23 (2H, m, Ar), 8.00 (3H, m, Ar), 7.72 (1H, d,
Ar), 7.62 (1H, d, Ar), 6.93 (1H, d, Ar), 4.11 (3H, s,
AC(O)OCH3), 3.97 (3H, s, OCH3) 13C NMR (CDCl3) d ppm:
52.28, 55.93, 103.78, 113.74, 122.17, 122.58, 123.02, 125.54,
125.88, 127.87, 130.61, 131.18, 132.88, 141.57, 155.59,
159.34, 166.65. FTIR (KBr) (cmꢁ1): 3053, 2953, 2843, 1717,
1602, 1579, 1468, 1433, 1392, 1247, 1192, 1095, 1014, 955,
922, 870, 815, 758. HRMS m: 320.12. ELEM. ANAL calculated
for C19H16N2O3: C, 71.24; H, 5.03; N, 8.74. Found: C, 71.45;
H, 5.16; N, 8.35.
1
Yield ¼ 0.350 g (48 %). H NMR (200 MHz CDCl3): d (ppm):
8.17 (4H, d, Ar), 7.90 (8H, m, Ar), 7.04 (4H, d, Ar), 4.50 (4H,
s, ArAC(O)AOCH2ACH2O), 4.23–3.67 (22H, oligooxyethylene
region), 2.31 (4H, t, Alph-C(O) OCH2ACH2), 1.59 (4H, m,
AC(O)OACH2ACH2ARACH2ACH2AC(O)OA), 1.28 (8H, m,
AC(O)OACH2ACH2A (CH2)4ACH2ACH2AC(O)OA).
SDNpnNp-x
DMS, naphthyl twin azobenzene (NpnNp), and DEG were
taken as monomers, adopting the same polymerization
procedure.
STPnP-x
DMS, phenyl twinazobenzene (PnP), and TEG were taken as
monomers, adopting the same polymerization procedure.
a,x-Bis(4-Diethyleneoxyphenyl-40-azophenyl)Methyl-
benzoate (P4P)
RESULTS
The synthesis of the twin azobenzenes along with P0 and
Np0—model compounds without ethylene glycol units is
given in Scheme 1. The detailed procedure for the synthesis
of the diazo dyes and their ester has already been reported
earlier.14 The twin series having the structure—azo dye-(oli-
gooxyethylene spacer)n-azodye were synthesized by coupling
the ditosylate of different oligooxyethylene with the azo dye
esters. P0 and Np0 were synthesized by refluxing the azodye
in the presence of acid in methanol as solvent. Normally
introduction of ether linkage at phenolic position requires
the reaction of alkyl halide in the presence of base. However,
in these push–pull azo systems carrying out of esterification
for longer time resulted in the esterification of carboxyl
group as well as introduction of ether linkage at the phenolic
position. This was confirmed by the appearance of two
4- (4-Hydroxy-phenyl-1-ylazo)benzoic acid methyl ester (0.3
g, 1.17 mmol), TEG ditosylate (0.4 g,0.97 mmol), anhydrous
potassium carbonate (0.29 g, 0.58 mmol), a catalytic amount
of KI were dissolved in 10 mL of dry DMF. The mixture was
ꢀ
stirred at 80 C for 48 h under nitrogen. The resulting solu-
tion was cooled to room temperature, poured into water.
The product was filtered, washed with water and dried. The
crude product was purified by column chromatography using
CHCl3-methanol (v/v 96/4) mixture. Yield ¼ 0.265 g (34%).
1
ꢀ
mp:140 C. H NMR (200 MHz CDCl3): d (ppm): 8.17 (4H, d,
Ar), 7.89 (8H, m, Ar), 7.01 (4H, d, Ar), 4.20 (4H, t,
AOCH2ACH2O), 3.94 (6H, s, AC(O)OCH3), 3.90 (4H, m,
AOCH2ACH2OACH2A), 3.76 (4H, m, AOCH2ACH2OACH2A).
13C NMR (CDCl3) d ppm: 52.27, 67.74, 69.57, 70.67, 70.86,
114.86, 122.34, 125.12, 130.55, 131.14, 147.00, 155.26,
161.82, 166.60. FTIR (KBr) (cmꢁ1): 3017, 2932, 2866, 1721,
1602, 1499, 1437, 1407, 1285, 1257, 1191, 1145, 1108,
1060, 957, 867, 840, 774, 697, 553. ELEM. ANAL calculated for
1
AOCH3 peaks of ester and ether linkage in the H NMR spec-
tra. The structures of all the azo dye molecules are shown in
Scheme 1. The structural characterization of the azo twin se-
ries were done by H NMR and 13C NMR spectroscopy (Sup-
1
C36H38N4O9: C, 64.47; H, 5.71; N, 8.35; Found C, 64.61; H,
porting Information). The purity of all molecules were con-
firmed by SEC (Supporting Information), high-resolution
mass spectroscopy or matrix-assisted laser desorption ioni-
zation–time of flight (MALDI-TOF) (Supporting Information)
and along with elemental analysis. The SEC analysis showed
5.82; N, 8.82. MALDI-TOF mþ1: 693
A similar procedure was adopted for the synthesis of the
naphthylazo twin molecules as well. Synthetic and character-
ization details are given in Supporting Information.
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SCHEME 1 Synthesis of phenyl and naphthyl twin azobenzenes.
single peak for all molecules confirming their purity. In the
HRMS or MALDI-TOF analysis, molecular ions m, or mþ1
radicals were obtained. Scheme 2 shows the incorporation of
the twin azobenzenes into a main chain polyester backbone
by melt polycondensation. The most widely used reactions
for the preparation of polyesters are direct esterification of
diacid/diacid chloride with diol or transesterification of die-
ster with diol which are usually performed at high tempera-
ture in the melt. Generally, the degradation temperature of
the azobenzene chromophore is lower than the melting tem-
perature; thereby making it impossible to carry out melt
polycondensation. Conversely, solution polymerization generally
result in very low molecular weight oligomers, as the poor
solubility leads to its precipitation from the polymerization
medium before build-up of high molecular weight occurs.16
These difficulties were overcome in the polymerization
reported here by designing the azobenzene dimethyl ester as
a twin molecule with oligooxyethylene units which not only
lowered the melting temperatures but also improved the sol-
ubility and the miscibility in the molten reaction medium.
DMS and DEG/TEG were used as the AA and BB monomers,
respectively, into which varying amounts of the different azo
twin dimethyl esters were introduced as the A0A0 monomer.
Two homopolyesters of DMS with DEG named as (SD-homo)
and with TEG named as (ST-homo) were synthesized. For
the copolymers, the total mole equivalents of AA (DMS) and
A0A0 (dimethylester of azotwin) were kept equal to that of
0
the BB (diol), that is, sum of AA and A0A : BB ¼ 1:1. Scheme
3 shows the general structure of the random azo copoly-
mers. The incorporation of the azo molecule into the ali-
phatic polyester backbone was determined from the proton
NMR spectra. Table 1 gives the list of different copolymer
compositions that were carried out with the actual incorpo-
ration of the azo content determined from 1H NMR analysis
given in brackets. The polymers based on sebacate/DEG
were named as SDPnP-x% and SDNpnNp-x%, respectively,
where the n represented the number of central ethylene gly-
col units in the azo twin segment and x represented the mol
% incorporated. Similarly, the polymers based on sebacate/
TEG were named as STPnP-x%. Two sets of twin molecules
were chosen to be incorporated into polyester; P2P (the
smallest spacer twin which was not liquid crystalline) and
P6P (the largest spacer twin which was liquid crystalline).
For the copolymers, the highest azo solid content that could
be taken in the feed was ꢂ30 mol % for the higher spacer
twin. For the shorter spacer twins, the poor solubility/misci-
bility of the azo molecule in the other molten components
limited the feed to ꢂ15 mol %. Thus, in the P2P polymer se-
ries, 5 and 15 mol % incorporation of P2P could be achieved
with both DEG and TEG. Although a 30 mol % P2P in the
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SCHEME 2 Synthesis of random azo copolymers from Phenyl and Naphthyl twin azobenzenes.
feed was attempted, the polymerization could not be practi-
cally carried out due to the large solid content of P2P which
was immiscible in the other two components, namely 100
mol % DEG/TEG and 70 mol % DMS. In the P6P polymer se-
ries, 5 and 30 mol % incorporated polymers were synthe-
sized using both DEG and TEG. The proton NMR signals of
the copolymers were identified by comparison with the NMR
spectra of the corresponding twin molecule with the homo-
polymer. The formation of the polymer was confirmed by the
appearance of the new peak at 4.5 ppm (indicated by circle
in Fig. 1) corresponding to the new aromatic ester linkage.
The integrated area of the aromatic protons of azobenzene
SCHEME 3 Structure of random azo copolymers.
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TABLE 1 Molecular Weight of the Random Azo Copolymers
Polymer (% Azo Incorporation
from 1H NMR)
a
a
Twin
Mn
Mw
PDI
LC
DEG series
P2P
SDP2P-5 (4.5)
SDP2P-15 (14)
SDP6P-5 (5.4)
SDP6P-30 (25)
SDNp2Np-5 (5)
SDNp6Np-5 (5)
SDNp6Np-30 (30)
8300
14,400
13,800
4000
13,000
26,200
19,000
5030
1.57
1.80
1.37
1.24
1.44
1.59
1.86
H
H
X
H
X
X
X
P6P
Np2Np
Np6Np
46,800
16,300
7400
67,800
25,900
13,800
TEG series
P2P
STP2P-5 (5)
25,300
12,700
13,660
18,900
38,000
22,720
19,540
32,900
1.50
1.78
1.43
1.73
X
H
X
X
STP2P-15 (17)
STP6P-5 (6)
P6P
STP6P-30 (31)
Homopolymers
DMS/DEG
DMS/TEG
8700
12400
28400
1.42
1.57
X
X
18000
Molecular weights as determined by gel permeation chromatography in THF at 30 ꢀC using polystyrene standards for calibration.
was compared with the A(OAC(O) ACH2)A protons of the
sebacate to determine the mol incorporation of the azoben-
zene into the polymer backbone. Figure 1 shows the repre-
sentative proton NMR spectra of the twin molecule P6P and
its copolymer with sebacate and DEG–SDP6P-5%. All the
random copolymers thus developed have good solubility in
common organic solvents. The molecular weight and polydis-
persity of the polymers were determined by GPC and the
FIGURE 1 1H NMR of phenyl twin azobenzene P6P, homopolymer SD-homo, and random azo copolymer SDP6P5.
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TABLE 2 Absorption and Photoisomerization Data of the
Phenyl and Naphthyl-based Twin Azobenzenes Series in THF
Compound kmax Abs (e, Mꢁ1, cmꢁ1
)
Photoisomerization (%)
P0
253, 355, 445
253, 355, 447
253, 355, 447
253, 355, 447
253, 356, 447
280, 415
92
80
91
90
90
42
42
47
52
P2P
P3P
P4P
P6P
Np0
Np2Np
Np4Np
Np6Np
280, 415
280, 415
280, 415
values are given in Table 1. In general, it was observed that
the molecular weight of the DEG-based copolymers were
lower as compared to that of the TEG copolymers. In the
DEG copolymer series, higher (30 mol %) incorporation of
azo dye resulted in a drastic reduction of the molecular
weight. The TEG-based copolymers had reasonably high mo-
lecular weights in the range of Mn 12,700–25,300. This is
quite remarkable considering the fact that earlier reports on
homopolymer of the single chromophore 4,40-azodibenzoyl
chloride with TEG resulted in polymers with very low molec-
ular weight (Mn ꢂ2300) corresponding to only five repeat
units.16
FIGURE 2 Normalized UV absorption spectrum of P0 and Np0
in THF.
either a 360 nm (for Phenylazo series) or a 450 nm (for the
naphthylazo series) Oriel bandpass filter. The samples in
THF (1 ꢃ 10ꢁ6 M) had kept in the dark for overnight was
irradiated and the absorption was recorded at every 5–10 s
interval to trace the trans to cis photoisomerization. The
change in absorption spectra of P0 and Np0 as
Photophysical Properties
The absorption spectra of the twin molecules and polymers
were recorded in THF and the peak absorption values are
given in Table 2. The absorption spectra of azobenzenes con-
sist of three major bands. The lowest energy transition
occurs at ꢂ430–440 nm and is assigned to the n–p* transi-
tion. The second transition occurs in the UV region around
320 nm and is assigned to the p–p* transition for trans azo-
benzene (for cis azobenzene the p–p* band occurs around
280 nm). The peak maximum of this band is sensitive to the
polarity and also to the presence of substituents. The third
energy transition around 230–240 nm is considered to arise
from the p–p* transition in the phenyl rings. In push–pull
substituted aromatic azobenzene systems, there is an overall
red shift of the p–p* transitions.26 Thus, the simple phenol-
based azobenzene molecules, for example, P0 with the ester
group at one end and methoxy group at the other end had
the n–p* transition at ꢂ445 nm, the p–p* transition at 355
nm and the aromatic phenyl p–p* transition at ꢂ253 nm.
Conversely, in the more conjugated naphthol-based azoben-
zene series the n–p* band was almost wholly overlain by the
bathochromically shifted p–p* band, which appeared at 411
nm. The aromatic phenyl p–p* transitions was observed at
ꢂ279 nm. Figure 2 compares the absorption spectra of P0
and Np0 in THF. The twin molecules as well as copolymers
had the absorption spectra similar to that of the respective
model compounds. The twin azobenzenes were irradiated
with a 50-W short arc mercury vapor lamp with an output
wavelength in the range 280–450 nm in combination with
FIGURE 3 Photoisomerization studies of model compounds (a)
P0 and (b) Np0 in THF.
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TABLE 3 Thermal Data of Twin Azobenzenes (PnP, NpnNp), and Random Azo Copolymers (SDPnP-x%, STPnP-x%)
a
a
a
a
b
b
b
b
Tm
DHm
Tcl
DHcl
Tc
DHc
Tc
DHc
e
(C-LC)
(ꢀC)
(C-LC)
(J/g)
(C/LC-I)
(ꢀC)
(C/LC-I)
(J/g)
(I/Lc-LC/C)
(ꢀC)
(I/LC-LC/C)
(J/g)
(LC/C-C)
(ꢀC)
(LC/C-C)
(J/g)
TD
Sample
(ꢀC)
P0
–
–
–
–
–
–
169
215
173
129.2
114.5
84.1
163
205
*1781
172
69.5
132
106
106
123
–
ꢁ114.8
ꢁ115.9
ꢁ4.8
149
ꢁ6.49
ꢁ54.9
241
304
315
P2P
P3P
158.13
ꢁ10.8
P4P
–
–
140
119
141
190
139d
34
47.2
95.4
54.3
78.0
87.34
6.97
3.9
ꢁ6.9
118
87
ꢁ32.9
ꢁ55
ꢁ93.7
ꢁ63.0
–
304
339
267
331
325
331
385
360
P6P
–
–
ꢁ14.4
Np0
128
25.4
–
–
–
–
101
113
–
Np2Np
Np4Np
Np6Np
SDP2P5
SDP2P15
–
19c
ꢁ6c
34
24
–
–
–
–
44.6
14.5
66.5
90
*25
75
–
ꢁ5.2
ꢁ13.6
46.6
ꢁ13.3
8.4
47
ꢁ9.3
SDP6P30
45
64
79.5
8
0.5
1.7
7.8
0.7
117
99
0.48
8.7
90
–
64
25
ꢁ13.5
ꢁ11
335
364
STP2P15
*80
ꢁ
a
e
During second heating cycle with 10 ꢀC/min.
During second cooling cycle with 10 ꢀC/min.
Glass Transition (Tg) during second heating cycle.
During first heating cycle.
TD—10 % weight loss under N2 atmosphere in TGA. Tm—melting tem-
perature. Tcl—Clearing temperature. Tc—cooling cycleNote- ‘‘*’’ transi-
tions observed in PLM.
b
c
d
representative samples of the PnP and NpnNp series on UV
irradiation is shown in Figure 3 and the inset in the figure
shows the % conversion as a function of photoirradiation
time. Table 2 shows the % trans to cis conversion at the
photostationary state for both series. The phenylazo series
reached the photostationary state with more than 80% con-
version within 15 s of irradiation. The phenylazo twin series
exhibited increasing extent of trans to cis photoisomerization
with increase in the central oligooxyethylene spacer length.
The longer members like the P4P and P6P had the highest
extents of isomerization of 90% (Supporting Information,
Fig. S11) similar to the isolated chromophore P0 (92%).
Conversely, the maximum extent of trans to cis photoisome-
rization exhibited by the NpnNp twin series was ꢂ40% in
the photostationary state (ꢂ30 s). No noticeable trend of %
conversion with spacer length was apparent in this series.
The reversibility of the photoisomerization was confirmed by
following the back conversion to the trans state on keeping
in the dark for overnight. Molecules of both series attained
the original trans absorption spectra fully on keeping in the
dark. The reduced isomerization efficiency of the naphthyl
azo twins in comparison with the phenyl azo series is due to
its bulky nature which hindered the motion. The photoiso-
merization in the copolyesters were similar to those
observed for the corresponding twin molecules. One would
have expected the covalent linkage of azobenzene group to
the polymer main chain backbone would result in fewer
degrees of freedom and hence to a lower extent of photoiso-
merization compared to the twin molecule. However, no
such difference in extent of photoisomerization was observed
indicating that the polymer backbone provided enough flexi-
bility for the alignment of azo groups in the main chain.27,28
Thermal Mesophase Characteristics of Twin Molecules
The thermal stability of the azotwins were determined by
TGA under nitrogen atmosphere (Supporting Information,
Figures S12 and S13). No weight loss was observed until
ꢀ
they were heated up to 200 C. Table 3 compares the 10 wt
% decomposition temperature of the single azochromophore
(P0 and Np0) with that of the respective twin series. The
twin molecules had much higher decomposition tempera-
tures compared with the single chromophore. The connec-
tion of two mesogens to form a dimeric or twin structure is
known to afford better thermal as well as mesogenic stabil-
ity to the latter compared to the single molecule. The ther-
motropic liquid crystalline tendency of the azo twin series
were studied using DSC analysis coupled with PLM and tem-
perature dependent wide angle X ray diffraction (WXRD)
studies. In the phenylazo twin series, monotropic meso-
phases were exhibited by members having spacer length
greater than 2. Thus, P3P, P4P, and P6P exhibited smectic A
(SA) phases on cooling, with P3P additionally exhibiting ne-
matic phase as well. Figure 4 shows the second heating and
cooling scans in the DSC thermogram of the phenyl twin
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FIGURE 4 DSC thermograms of (a) Phenyl twin azobenzenes P3P, P4P and P6P and (b) Naphthyl twin azobenzenes Np0 and
Np2Np.
azochromophores P3P, P4P, P6P. The enthalpy values and
phase transition temperature are given in Table 3. The melt-
ing (heating cycle) and isotropic to LC transition (cooling
cycle) were decreased with increase in spacer length. Model
compound P0 was not liquid crystalline and exhibited a
sharp melting at 168 ꢀC in the heating cycle. In the cooling
cycle, two transitions were observed at 163 ꢀC and 149 ꢀC,
which corresponded to the isotropic to crystal and crystal to
crystal transitions, which were confirmed by the PLM. The
twin molecule P2P also did not exhibit a mesophase during
the heating or cooling cycle and had a melting transition at
this temperature (Fig. 6, plot f). The overall LC window
exhibited by P3P was thus ꢂ20 ꢀC (from 178 to 158 ꢀC).
Figure 6 shows the VT-WXRD data as a plot of intensity ver-
sus 2y recorded from 2y ¼ 3–35ꢀ at various temperature
intervals, whereas cooling from the isotropic phase. The plot
‘‘a’’ at 185 ꢀC corresponded to the isotropic phase, plot ‘‘b’’
ꢀ
at 178 C in Figure 6 showed a broad peak around 2y ¼ 20ꢀ,
which could be assigned to the lateral distance between
mesogens of about
4 Å, characteristic of the nematic
phase.29 Plot ‘‘c’’ at 172 ꢀC corresponded to the SA phase
and plot ‘‘d’’ corresponded to the colored grainy LC phase
as observed under the PLM. Although WXRD did not show
any characteristic features of smectic phase until 164 ꢀC,
room temperature WXRD recorded for the low angle
region of 2y ¼ 0.5–10ꢀ (Supporting Information, Fig. S22)
clearly showed the existence of a peak at low angle corre-
sponding to molecular length of 33.5 Å around 2y ¼ 2.8ꢀ.
ꢀ
215 C. Conversely, P3P was monotropic and exhibited mul-
tiple transitions in the cooling cycle, which were confirmed
to be liquid crystalline phases based on observation under
the PLM. There was only one melting transition in the sec-
ond heating cycle at 173 ꢀC with an enthalpy of 84 J/g.
Although cooling, sharp transitions were observed at 172,
166, 158, and at 155 ꢀC. Although DSC did not show any
ꢀ
P4P exhibited monotropic behavior with two transitions
observed only during the cooling cycle. Figure 7a,b shows
the focal conic fan-shaped textures typical of smectic A
phases observed at 125 ꢀC on cooling from the isotropic
melt. The LC window was around 14 ꢀC. P6P also exhibited
similar behavior as P4P with a monotropic transition corre-
sponding to broken fan-like smectic texture and an LC win-
dow of 20 ꢀC. The variable temperature WXRD plot of P4P
and P6P are given in the Supporting Information. A surpris-
ing feature observed in the variable temperature WXRD plot
of the three molecules P3P, P4P, and P6P were the appear-
ance of sharp crystalline-like d spacings during the meso-
phase in the 2y region from 20 to 25ꢀ, which corresponded
to the packing of oligooxyethylene units. Similar observation
has been reported for azoxy-based liquid crystalline polyest-
ers, where this has been attributed to the retention of order
of the solid in the mesophase also.17 In a highly ordered
smectic mesophase, the layered order of the solid state
would be maintained. P4P and P6P also exhibited peaks at
transition above 172 C (while cooling), typical nematic tex-
ꢀ
tures were observed at 178 C in the PLM as shown in Fig-
ure 5a on cooling from the isotropic melt. This nematic
ꢀ
threads remained only for 2–3 C, which was too weak to be
detected by DSC and then batonnets started appearing at
ꢀ
173 C, which coalesced into the typical focal conic textures
of smectic A (SA) (Fig. 5 b,c). The focal conic textures
remained until 169 ꢀC then suddenly the texture was
changed into multicolored grainy features, which is corre-
sponded to the second transition observed in DSC with peak
maxima at 166 ꢀC (Fig. 5d). Beyond this temperature, DSC
showed transitions at 158 and 155 ꢀC associated with huge
enthalpy change of 54.9 J/g (compared to the smaller values
of 10.8 and 4.8 J/g of the LC transitions); but under the
PLM, sharp changes were not discernible. This transition at
158 ꢀC was confirmed to be crystallization based on the
large enthalpy change as well as from the variable tempera-
ture WXRD measurements, which showed the appearance of
multiple sharp peaks corresponding to crystallization around
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FIGURE 5 Polarized Light microscope images of Phenyl twin azobenzene P3P (a) at 178 ꢀC (schlieren texture), (b) at 172 ꢀC (Baton-
nets), (c) at 170 ꢀC (focal conic texture-SA), and (d) at 166 ꢀC (unidentified SX phase).
multiple intervals indicating layered structure in the LC
phase. For instance, in the LC phase at 125 ꢀC, P4P showed
two reflections at 2y ¼ 4.53ꢀ and 9.31ꢀ which corresponded
to the d-spacing values of 19.74 and 9.50 Å, respectively,
(Supporting Information, Fig. S18). Similarly, P6P also
showed peaks at 2y ¼ 6.02ꢀ and 12.05ꢀ which corresponded
to d-spacing values of 14.80 and 7.36 Å, respectively (Sup-
porting Information Fig. S19). In all these molecules, exis-
tence of the low angle peak corresponding to molecular
length gave support for the smectic LC Phase (Supporting In-
formation, Fig. S22).30 Supporting Information, Figure S22
compares the low angle X-ray measurements for the PnP se-
ries at room temperature, which clearly indicated a shift in
the peak position to lower angle with an increase in the oli-
gooxyethylene segment indicating longer periodicity for the
higher spacer members. Noteworthy was the observation of
the sharp peak around 2y ꢂ25ꢀ corresponding to a d
spacing of ꢂ3.5 Å in these PnP twin molecules, which in ar-
omatic molecules is usually attributed to the p–stacking
interaction.31,32 This feature was a clearly distinguishing one,
while comparing the packing in the NpnNp twin series
(described below).
cause fall in clearing temperature.33 In the naphthylazo twin
series, multiple transitions were observed in the DSC ther-
mogram of Np0 and Np2Np. Figure 4b shows the second
heating and cooling scans in the DSC thermogram of Np0
and Np2Np of the naphthyl series and their enthalpy values
Compared with the phenylazo twin series, the naphthylazo
twin series had lower clearing temperatures. This is
expected as a lateral substitution on the core is known to
FIGURE 6 Variable temperature wide angle X-ray diffraction of
Phenyl twin azobenzene P3P.
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FIGURE 7 Polarized Light microscope images of phenyl and Naphthyl twin azobenzenes (a) P4P-136 ꢀC (focal conic texture-SA,
40X magnification) (b) P4P-128 ꢀC (focal conic texture-SA) (c) P6P-109.7 ꢀC (fan like texture-SA) (d) P6P-104.8 ꢀC (focal conic tex-
ture-SA, 40X magnification) (e) Np0- 30 ꢀC (Spherulitic texture) (f) Np2Np- 30 ꢀC (Spherulitic texture).
and phase transitions are given in Table 3. Model compound
at various intervals during cooling from the isotropic melt.
Compared with the PnP series, Np0 and Np2Np were crys-
talline and had several sharp reflections both in the low
angle and wide angle region. However, the oligooxyethylene
segment packing that was observed in PnP series ꢂ2y ¼
20–25ꢀ as well as the p stacking interaction observed ꢂ 2y
¼ 25ꢀ were not observable in Np2Np. Np4Np and Np6Np,
Conversely, exhibited glass transition Tg during heating and
cooling (Supporting Information, Fig. S15). Np4Np also
exhibited a melting transition at 139 ꢀC during the first
Np0 ꢀexhibited two transitions both in the heating (126 and
ꢀ
141 C) and cooling cycle (106 and 101 C) and observation
under the PLM indicated spherulitic textures as shown in
Figure 7e. The spherulitic phase remained frozen until room
temperature and the texture was stable even after a month.
The twin molecule Np2Np also exhibited sperulitic texture
at 123 ꢀC on cooling, (Fig. 7f), which remained stable until
room temperature. The Supporting Information shows the
variable temperature WXRD data of Np0 and Np2Np taken
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polymers become amorphous. When aromatic units in the
form of aromatic dicarboxylic acids are introduced into the
polyester backbone, they not only increase the melting tran-
sition but also induce thermotropic liquid crystallinity to the
polymer. In addition, p bond stabilization in the form of azo
linkages have been shown to contribute to the rigidity of the
ring, which also stabilizes the mesophase formation.31 For
instance, the literature reports the mesophase characteristics
of a series of low-molecular weight polyesters formed by
4,40-azodibenzoyl chloride with DEG, triethylene glycol, and
TEG.17,34 The DEG-based azo copolymers exhibited meso-
phase, whereas the tri and TEG-based azo copolymers were
not liquid crystalline.
The homopolymers of DMS with DEG and TEG – SD-homo
and ST-homo were characterized for their thermal stability
and crystallinity using TGA and DSC measurements. The
homo as well as copolyesters were thermally stable till
300 ꢀC (Supporting Information, Fig. S14). SD-homo was
semicrystalline and exhibited cold crystallization and double
melting peaks while heating, whereas sharp crystallization
was observed at 8 ꢀC while cooling (Fig. 8). Such double
melting transitions have been observed for the crystallization
of other polymers also where it has been shown that it is an
after effect of annealing during the DSC scan which results
in crystallites with different degrees of perfection.35 The
lower endotherm transition (at 34 ꢀC) is due to the melting
of initial crystallites produced during the crystallization pro-
cess, whereas the higher temperature endotherm (47 ꢀC)
corresponds to crystal reorganization during melting (melt-
ing of more perfect crystals). Figure 9a shows the spherulitic
crystals of SD-Homo observed under PLM after annealing the
sample at 25 ꢀC for 1 h. Conversely, increasing the chain
length from DEG to TEG resulted in a completely amorphous
homopolymer–ST-homo. The DEG-based copolymers incorpo-
rating varying mol % of the twin molecules P2P (short non-
LC twin) and P6P (long LC twin) were analyzed for their
thermal characteristics. Figure 8 compares the second heat-
ing and cooling cycles of twin molecule P2P and homopoly-
mer SD-homo along with the respective copolymers SDP2P-
5 and SDP2P-15. P2P was not liquid crystalline and had a
FIGURE 8 DSC thermograms of P2P, SD-homo, SDP2P-5 and
SDP2P-15.
heating cycle. Subsequent cooling and heating cycles showed
ꢀ
only a Tg ꢂ 19 C. In the case of Np6Np, a melting transition
was observed in the first heating cycle following which it
showed a Tg ꢂ ꢁ6 ꢀC in the cooling cycle. In the second
heating cycle, in addition to Tg a large cold crystallization
exotherm and a sharp small endotherm were observed at 26
ꢀC and 36 ꢀC, respectively, which could be attributed to the
crystallization of the oligooxyethylene component. The lower
glass transition temperature for Np6Np implied increased
plasticization due to increased flexibility of the hexaethylene
glycol spacer compared with TEG. From the above observa-
tion, it was clearly evident that in Np4Np and Np6Np the
thermal properties were determined mostly by the spacer
than by the aromatic units.
ꢀ
ꢀ
melting transition at 215 C, which crystallized at 205 C in
the cooling cycle. The 5 mol % incorporation of P2P
ꢀ
(SDP2P-5) exhibited a sharp melting transition at 34 C (en-
thalpy of 44.6 J/g) with another broad weak transition
ꢀ
around 65 C (3.9 J/g). While cooling, a broad exotherm was
observed (30 to ꢁ13 ꢀC) with a sharp peak centered at ꢁ5
ꢀC. Annealing the sample at 25 ꢀC and observation under
PLM showed the typical thread-like pattern of the nematic
phase as shown in Figure 9b. An isothermal crystallization
experiment was carried out by annealing the sample at 25
Thermal Mesophase Characteristics of Azobenzene
Copolymers
ꢀ
ꢀC for 30 min, followed by rapid cooling to ꢁ5 C. The sam-
ple was then heated at a rate of 10 ꢀC/min, which is also
plotted (in red) along with the second heating cycle in Fig-
ure 8. It showed two endothermic transitions at 33 and
45 ꢀC, respectively, similar to that of the SD-Homo. The iso-
thermal annealing in the nematic phase followed by rapid
cooling allowed for the reorganization of the crystals which
Linear aliphatic polyesters are generally crystalline and have
low melting points (less than 100 ꢀC) with the melting
points increasing with the number of methylene groups
between the ester groups. When the diols are polyethylene
glycols, the melting transition as well as the percentage crys-
tallinity of the polymer decreases and for higher glycols, the
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FIGURE 9 Polarized light microscope images of homopolymer and random azo copolymer (a) SD-homo- 30 ꢀC (Spherulitic crystal-
line phase) (b) SDP2P5-30 ꢀC (Nematic texture) (c) SDP2P15- 77 ꢀC (Nematic texture) (d) SDP2P15- 45 ꢀC (Smectic phase) (e)
SDP6P30-91 ꢀC (Nematic droplets) (f) STP2P15- 30 ꢀC (Nematic droplets).
was observed as the double melting in the following heating
cycle.35 On increasing the incorporation of P2P from 5 to 15
mol %, biphasic nature was evident in the DSC transitions
with coexistence of two distinct phases. The first one corre-
sponded to the crystalline regions of the sebacate and DEG
polymer and the second corresponded to the liquid crystal-
line phase of the aromatic unit. In the first heating cycle,
transition corresponding to the melting of the reorganized
aromatic units at 92 ꢀC. In the second heating cycle, three
endothermic transitions were observed at 10 ꢀC, 24 ꢀC en-
thalpy (14.5 J/g) and 90 C enthalpy (8.4 J/g). On cooling, a
ꢀ
grainy texture (Fig. 9c) was observed under PLM around
100 ꢀC, which produced typical nematic threads on further
cooling. Correspondingly, in DSC a weak transition enthalpy
ꢀ
(0.44 J/g) was observed around 92 C. Variable temperature
WXRD of SDP2P-15 is given in Figure 10. The XRD pattern
of the polymer was broad and had lesser number of peaks
compared to that of the twin molecule indicating the lower
crystallinity. Plots ‘‘a’’ and ‘‘b’’ at 140 and 100 ꢀC,
ꢀ
crystallites at 45 C was visible along with two more transi-
tions at lower temperatures 22 and 7 ꢀC; and a higher one
corresponding to the nematic to isotropic transition of the
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A series of copolymers were developed using TEG as the BB
monomer instead of DEG. Because the naphthylazo copoly-
mers did not exhibit any LC tendencies with DEG, the proba-
bility of mesophase formation with a more flexible TEG
could be ruled out. Therefore, only P2P and P6P were incor-
porated into copolyesters with DMS and TEG. 5 and 15 mol
% incorporated P2P, namely, STP2P-5 and STP2P-15 were
analyzed for their thermal properties. The DEG-based copoly-
mers were all obtained as solid powders, whereas STP2P-5
was a viscous liquid. The DSC thermogram of the ST-homo
with that of its 5 and 15 mol % azo copolymers STP2P-5
and STP2P-15 have given in Supporting Information Figure
S17. The ST homopolymer did not show any transition in the
heating or cooling cycles. STP2P-5 exhibited a cold crystalli-
zation followed by melting at 6 ꢀC in the first heating cycle
but in the following cycles no transitions were observed. Af-
ter 15 mol % incorporation of P2P, new transitions were
observed in the DSC thermogram of STP2P-15. Two transi-
tions were observed at 8 and 99 ꢀC while heating but only
one transition was observed at around 25 ꢀC while cooling.
Annealing the sample at 80 ꢀC resulted in the formation of
nematic droplets (Fig. 9f) under the PLM. The copolymers
STP6P-5 and STP6P-30 did not exhibit any mesophases
when observed under PLM and their DSC also was devoid of
any transitions.
FIGURE 10 Variable temperature wide angle X-ray diffraction
of random azo copolymer SDP2P15.
respectively, corresponded to isotropic phase, whereas plots
‘‘c’’ and ‘‘d’’ recorded at 88 and 85 ꢀC, respectively, corre-
sponded to the nematic phase. DSC showed a broad transi-
tion hꢀaving a large enthalpy of 9.3 J/g with a peak centered
at 47 C. Observation under PLM showed a change in texture
around this temperature indicating formation of a more or-
dered smectic phase (Fig. 9d).36,37 This was further con-
firmed by the appearance of more reflections in the wide
angle region of the variable temperature WXRD recorded at
this temperature (Fig. 10, plot ‘‘e’’). This ordered phase was
DISCUSSION AND CONCLUSION
Two series of twin azobenzene molecules were synthesized
and studied for the effect of varying central oligooxyethylene
spacer segment on the liquid crystalline characteristics.
These twin azobenzene moieties were also introduced as
part of main chain copolyester composed of sebacate and
DEG or TEG.
ꢀ
retained until room temperature of 25 C as observed under
PLM as well as WXRD. Higher incorporation of P2P in the
DEG copolymer series was not possible due to practical rea-
sons; however, the longer spacer twin molecule P6P could
be incorporated up to 30 mol %. SDP6P-5 did not exhibit a
LC phase but only a melting and crystallization in the heat-
ing and cooling cycles, respectively. Supporting Information,
Figure S16 compares the second heating and cooling cycles
of P6P, SD-Homo along with that of SDP6P-5 and SDP6P-
30 copolymers. Here also, at higher incorporation biphasic
regions were distinguishable in the DSC thermogram, corre-
sponding to the crystalline sebacate/DEG domains and liquid
crystalline azoaromatic domains. Thus, multiple transitions
were observed in the heating cycle of SDP6P-30 and while
cooling, nematic droplets were observed at around 90 ꢀC
under the PLM (Fig. 9e).
The following observations and conclusions were drawn
based on the experimental analysis.
(a) Main chain azobenzene-based polymers are normally
synthesized by the solution polymerization method, as
the melt condensation route would result in degradation
of azo chromophores. The strategy of incorporating the
azobenzene chromophore as a part of a twin molecule
having the structure azobenzene-oligooxyethylene-azo-
benzene could circumvent this problem by reducing the
melting temperature of the twin molecule and also
improving their solubility as well as miscibility in the
molten reaction medium. Thus, main chain azobenzene
copolymers incorporating upto 30 mol % azo twin chro-
mophores with reasonably high molecular weights in the
range Mn ꢂ 12,700 to 25,300 could be achieved.
Three copolymers of the naphthylazo twin series were syn-
thesized – SDNp2Np-5, SDNp6Np-5, and SDNp6Np-30 and
were analyzed for their thermal characteristics. They did not
exhibit any thermotropic liquid crystalline behavior. Com-
pared with the phenylazo twin series, only the lowest mem-
ber of the napththylazo twin molecule—Np2Np exhibited LC
behavior; with the higher members already having lost the
flexible/rigid balance required for mesophase formation. The
introduction of more flexible segments via copolymerization
would be counter intuitive in inducing mesophase abilities in
the NpnNp copolymer series.
(b) The linking of a polar oligooxyethylene unit with a non-
polar rigid aromatic azo chromophore is expected to
result in stable mesophases due to the mutual non solu-
bility.38 The twin series based on phenylazo (PnP) and
naphthylazo (NpnNp) had a combination of nonpolar
(N) azobenzene units with polar (P) oligooxyethylene
units linked as triphilic N-P-N system leading to incom-
patibility. If the incompatibility between the flexible
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surements. CN thanks CSIR-New Delhi, India for Senior
Research Fellowship.
oligooxyethylene units and the rigid azobenzene units
results in segregation into separate domains, then, a
smectic phase is preferred as in the twins P3P, P4P, and
P6P. However, this flexibility is lost in the shorter twin
molecule P2P which in spite of having a N-P-N triphilic
character did not exhibit any LC phase. Considering the
naphthylazo twin series, even though they also formed
triphilic N-P-N systems, the naphthyl ring on the azoben-
zene expanded the breadth of the rigid aromatic core
making the chromophore lose out on the axial to equato-
rial ratio and only the lowest spacer Np2Np exhibited
spherulitic phase. The NpnNp twin series of molecules
had lower melting transitions than the PnP series of the
same spacer length and also had better solubility. The
increased solubility also implied lesser microphase sepa-
ration. The higher (n ¼ 4 and 6) members were so flexi-
ble that they exhibited only a glass transition in their
DSC thermograms.
REFERENCES AND NOTES
1 Hafiz, H. R.; Nakanishi, F. Nanotechnology 2003, 14, 649–654.
2 Jousselme, B.; Blanchard, P.; Gallego-Planas, N.; Delauney,
J.; Allain, M.; Richomme, P.; Levillain, E.; Roncali, J. J. Am.
Chem. Soc. 2003, 125, 2588–2889.
3 Mativetsky, J. M.; Pace, G.; Elbing, M.; Rampi, M. A.; Mayor,
M.; Samori, P. J. Am. Chem. Soc. 2008, 130, 9192–9193.
4 Southerland, R. L. Handbook of Nonlinear Optics; Marcel
Dekker: New York, 1996.
5 Wu, Y.; Kanazawa, A.; Shiono, T.; Ikeda, T.; Zhang, Q. Poly-
mer 1999, 40, 4787–4793.
6 Lim, L. S.; Li, N.; Lu, J.; Ling, Q.; Zhu, C. X.; Kang, E.; Neoh,
K. G. Appl. Mater. Interfaces 2009, 1, 60–71.
7 Kumar, G. S.; Neckers, D. C. Chem. Rev. 1989, 89, 1915–1925.
8 Natansohn, A.; Rochon, P.; Pezolet, M.; Audet, P.; Brown, D.;
To, S. Macromolecules 1994, 27, 2580–2585.
(c) The short (n ¼2) and long (n ¼ 6) spacer twins in both
PnP and NpnNp series were incorporated in varying
mole ratios into an aliphatic copolyester composed of
sebacate and DEG as well as TEG. At higher incorpora-
tion of the PnP units, biphasic regions were observed in
the DSC thermograms of both DEG and TEG copolymers
and they exhibited liquid crystalline phases. Conversely,
Np2Np and Np6Np failed to induce mesogenicity even
on 30 mol % incorporation in DEG copolymers.
9 Zettsu, N.; Ogasawara, T.; Arakawa, R.; Nagano, S.; Ubukata,
T.; Seki, T. Macromolecules 2007, 40, 4607–4613.
10 Heldmann, C.; Warner, M. Macromolecules 1998, 31,
3519–3531.
11 Howe, L. A.; Jaycox, G. D. J. Polym. Sci. Part A: Polym.
Chem. 1998, 36, 2827–2837.
12 Irie, M.; Hirano, Y.; Hashimoto, S.; Hayashi, H. Macromole-
cules 1981, 14, 262–267.
13 Kumaresan, S.; Kannan, P. J. Polym. Sci. Part A: Polym.
Chem. 2003, 41, 3188–3196.
The analysis of the LC behavior of the two series of twin
molecules and their main chain copolyesters thus showed
that they were dependent on the length of the oligooxyethy-
lene linkers as well as the nature of the azo chromophore
(phenylazo vs. naphthylazo). Incorporation of ‘‘rigid azoaro-
matic twin’’ units into a nonmesogenic polymer helped to
induce mesogenicity in the copolymer. Although the twin
molecules exhibited smectic LC phases, their incorporation
in the copolymer at low incorporation resulted mostly in ne-
matic LC phase. The randomness of the sequence along the
copolymer chain would impose restrictions on the transla-
tional mobility of the monomeric units of the polymer,
thereby making it unfavorable for the formation of the lay-
ered smectic phases. Another noteworthy point is that the
rigid azoaromatic twin units cannot be considered as the
‘‘mesogen’’ as that would imply that the liquid crystalline
properties of the polymer was related only to the presence
of these aromatic units. The studies presented here clearly
showed that two polymers (DEG and TEG copolymers) con-
taining the same ‘‘mesogen’’ unit behaved differently; one
was mesogenic, whereas the other was not. The observation
of mesogenicity is a complex phenomenon, especially in a
random copolymer and both the ‘‘rigid’’ and the ‘‘flexible’’
spacer would be expected to have almost equal contribution
in deciding the final outcome.
14 Smitha, P.; Asha, S. K. J. Phys. Chem.
B 2007, 117,
6364–6373.
15 Xu, C.; Wu, B.; Becker, M. W.; Dalton, L. R.; Ranon, P. M.;
Shi, Y.; Steier, H. Chem. Mater. 1993, 5, 1439–1444.
16 Xu, Z.; Drnoyan, V.; Natansohn, A.; Rochon, P. J. Polym.
Sci. Part A: Polym. Chem. 2000, 38, 2245–2253.
17 Iimura, K.; Koide, N.; Ohta, R.; Takeda, M. Makromol. Chem.
1981, 182, 2563–2568.
18 Griffin, A. C.; Britt, T. R. J. Am. Chem. Soc. 1981, 103,
4957–4959.
19 Nieri, P.; Ramireddy, C.; Wu, C. N.; Munk, P.; Lenz, R. W.
Macromolecules 1992, 25, 1796–1800.
20 Volksen, W.; Yoon, D. Y.; Cotts, P. M. Macromolecules 1989,
22, 3846–3850.
21 Imrie, C. T.; Henderson, P. A. Chem. Soc. Rev. 2007, 36,
2096–2124.
22 Creed, D.; Gross, J. R. D.; Sullivan, S. L.; Griffin, A. C.;
Hoyle, C. E. Mol. Cryst. Liq. Cryst. 1987, 149, 185–193.
23 Hiejima, T.; Seki, K.; Kobayashi, Y.; Abe, A. J. Macromol.
Sci. Part B: Phys. 2003, B42, 431–439.
24 Kitazawa, S.; Kimura, K.; Shono, T. Bull. Chem. Soc. Jpn.
1983, 56, 3253–3257.
25 Semenova, N. V.; Morosanova, E. I.; Pletnev, I. V.; Mak-
simova, M.; Zolotov, Y. A. Mikrochim. Acta 1995, 119,
81–93.
26 Zollinger, H. Azo and Diazo Chemistry, Aliphatic and Aro-
matic Compounds; Interscience Publisher: New York, 1961.
ACKNOWLEDGMENTS
The authors thank the DST project GAP 277826 and CSIR net-
work project NWP0054 for financial support. The authors
thank Mangesh Mahajan for variable temperature XRD mea-
27 Chen, D. L.; Morawetz, H. Macromolecules 1976, 9,
463–468.
28 Tabak, D.; Morawetz, H. Macromolecules 1970, 3, 403–410.
2784
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 2770–2785

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
29 Castell, P.; Galia, M.; Serra, A. Macromol. Chem. Phys.
2001, 202, 1649–1657.
30 Although low angle peak from 2y ¼ 0.5–10ꢀ was recorded
only at room temperature (25 ꢀC), its existence could be
expected in the smectic LC phase temperature also.
33 Imrie, C. T. Liq. Cryst. 1989, 6, 391–396.
34 Koide, N.; Ohta, R.; Iimura, K. Polym. J. 1984, 16, 505–510.
35 Carpaneto, L.; Marsano, E.; Valenti, B.; Zanardi, G. Polymer
1982, 33, 3865–3872.
36 Sepelj, M.; Lesac, A.; Baumeister, U.; Diele, S.; Nguyen, H.
L.; Bruce, D. W. J. Mater. Chem. 2007, 17, 1154–1165.
31 Braun, D.; Cherdron, H.; Rehahn, M.; Ritter, H.; Voit, B. Polymer
Synthesis: Theory and Practice: Fundamentals, Methods, Experi-
ments, Springer-Verlag: Berlin, Heidelberg, Germany, 2005.
37 Han, D.; Tong, X.; Zhao, Y.; Galstian, T.; Zhao, Y. Macromo-
lecules 2010, 43, 3664–3671.
32 Goel, M.; Jayakannan, M. J. Phys. Chem. B 2010, 114,
12508–12519.
38 Florjanczyk, E. B. J. Phys. Chem. 2006, 110, 2582–2592.
WWW.MATERIALSVIEWS.COM
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