Self-assembly of azobenzene based side-chain liquid crystalline polymer and n-alkyloxybenzoic acids

Article abstract of DOI:10.1007/s12039-011-0083-6

Liquid crystalline pendant polymeric complexes have been obtained by supramolecular assembly of two mesogenic components namely, poly[4-(10- acryloyloxydecyloxy)-4'- phenylazobenzonitrile] (P10) and 4-alkyloxybenzoic acids (A7-A12). Hydrogen bond formed between carboxylic acid and cyano moiety served as molecular bridge. The polymeric complexes acquitted as undivided liquid crystalline properties exhibited stable and enantiotropic mesophases. The precursor, monomer and polymer were analysed by ¹H-NMR and ¹³C-NMR spectroscopy. The hydrogen bonding interaction in polymer complexes (P10-A7 to P10-A12) was investigated by FT-IR spectroscopy. The thermal behaviours and textural analysis were studied by differential scanning calorimetry and polarized optical microscopy respectively. Indian Academy of Sciences.

Full text of DOI:10.1007/s12039-011-0083-6

c
J. Chem. Sci. Vol. 123, No. 3, May 2011, pp. 255–263. ꢀ Indian Academy of Sciences.
Self-assembly of azobenzene based side-chain liquid crystalline polymer
and n-alkyloxybenzoic acids
KUMARASAMY GAYATHRI, SUBRAMANIAN BALAMURUGAN and
PALANINATHAN KANNAN∗
Department of Chemistry, Anna University, Chennai 600 025, India
e-mail: pakannan@annauniv.edu
MS received 11 September 2010; revised 4 January 2011; accepted 28 February 2011
Abstract. Liquid crystalline pendant polymeric complexes have been obtained by supramolecular assembly
of two mesogenic components namely, poly[4-(10-acryloyloxydecyloxy)-4ꢁ- phenylazobenzonitrile] (P10) and
4-alkyloxybenzoic acids (A7-A12). Hydrogen bond formed between carboxylic acid and cyano moiety served
as molecular bridge. The polymeric complexes acquitted as undivided liquid crystalline properties exhibited
1
stable and enantiotropic mesophases. The precursor, monomer and polymer were analysed by H-NMR and
13C-NMR spectroscopy. The hydrogen bonding interaction in polymer complexes (P10-A7 to P10-A12) was
investigated by FT-IR spectroscopy. The thermal behaviours and textural analysis were studied by differential
scanning calorimetry and polarized optical microscopy respectively.
Keywords. Self-assembly; H-bonding; liquid crystals; nematic phase; smectic-A phase.
1. Introduction
could be used to stabilize and enhance mesomorphic
property and these interactions are much stronger than
dipole–dipole interaction.17–19 This type of interaction
possesses utility in the area of miscibility of liquid
crystalline blends and in design of novel guest–host
liquid crystalline systems as well. Photoisomerization
of azobenzene has been used to design photochromic
liquid crystals, as rod-like trans-isomer of azobenzene
is known to stabilize LC phase, whereas bent shaped
cis-isomer destabilizes this LC phase.20–24
Present work deals with the mixing of one indepen-
dent liquid crystalline component and polymer to form
complex with new mesogen (figure 1). The polymer
and n-alkyloxybenzoic acid were reported in the lit-
erature.25–27 The magnitude of mesophase stabilization
is hitherto unreported in the literature for conventional
binary mixtures of this kind of mesogens. Here, H-
bonding between fixed polymeric acceptor with varied
donors units of 4-alkyloxybenzoic acids were prepared,
characterized and discussed.
Supramolecular liquid crystals obtained via intermolec-
ular H-bonding fascinated attention during the last
decade.1–7 Molecular interactions received significant
influence on the ordering of liquid crystalline state.
A powerful tool toward designing of highly orga-
nized soft matter arises from strong cohesive forces
such as H-bonding between identical or complemen-
tary molecules.8,9 A number of such systems have been
investigated following the reports of Kato and Frechet
Jean10,11 and reviewed extensively.12–14 Liquid crys-
talline polymers (LPCs) possess functional materials as
electro-optical devices or high-strength fibres. A wide
variety of self-organized molecular systems, such as
liquid crystals attracted to produce artificial biologi-
cal systems and other functional materials.15 Shandryuk
et al.16 studied the orientation behaviour of liquid crys-
tal networks having the SmCA structure stabilized by
H-bonds. The hydrogen bonds are responsible for the
elastic properties of the liquid crystal polymer network
and the mechanism of its plastic flow above the thresh-
old strain. The anisotropy of the mechanical proper-
ties of the H-bonded polymer materials is observed
in monodomain LC networks. If polymer containing a
pendant group capable of forming hydrogen bonding
2. Experimental
2.1 Materials
Cyanoaniline, sodium hydroxide, phenol, decanediol,
acrylic acid, triethylamine and 2, 2ꢁ-azobisisobutyro-
nitrile (AIBN) (Merck, Germany), p-hydroxybenzoic
∗For correspondence
255

256
Kumarasamy Gayathri et al.
ing stage and a TP -93 temperature programmer. Small
quantity of samples were placed between two thin glass
cover slips and heating and cooling carried out at the
rate of 5◦C min−1. The photographs were taken using
Nikon FM10 camera and exposed on Kodak 200 film.
2.3 Synthesis of 4-(heptyloxy) benzoic acid (A7)
Alkylated products of 4-hydroxybenzoic acid were syn-
thesized based on the Williamson aryl ether synthe-
sis conditions. A typical procedure for the synthesis
of 4-(heptyloxy) benzoic acid (A7) is as follows: 4-
hydroxybenzoic acid (0.01 mol, 1.52 g) dissolved in dry
DMF (250 mL), potassium carbonate (4.95 g, 0.05 mol)
added and stirred. Then 1-bromoheptane (3.36 mL,
0.03 mol) was added drop-wise to the reaction mixture
for 24 h at 90◦C. At the end of reaction, the mixture
was cooled, poured over crushed ice, extracted with
diethyl ether and dried over anhydrous sodium sulphate.
Then, solvent was evaporated under vacuum and puri-
fied by recrystallization using ethanol to get colour-
less crystalline solid (yield 63%). A similar procedure
was adopted for the synthesis of remaining O-alkylated
compounds (A8–A12).
Anal. Calcd. for C13H18O3: C, 70.24; H, 8.16; O,
21.59; found: C, 70.80; H, 8.24; O, 21.67. FT-IR
(KBr pellet, cm−1): 2919, 2848, 1682, 1256, 945,771.
1H NMR (CDCl3, 400 MHz), δ (ppm): 11.02 (s, 1H,
–COOH), 8.12 (d, 2H, Ar–H), 6.96 (d, 2H, Ar–H), 4.01
(t, 2H, –OCH2), 1.81 (q, 2H, –CH2), 1.24-.1.39(m, 6H,
–CH2), 0.93(t, 3H, –CH3). 13CNMR (75 MHz, CDCl3):
δ = 172.2, 163.7, 132.3, 121.4, 114.18, 68.2, 31.6, 29.0,
25.8, 22.5.
Figure 1. Supramolecular assembly of liquid crystal com-
plexes through H-bonding interactions.
acid and bromoalkanes, these chemicals are AR grade
and used as received. Acetone, dichloromethane, THF,
diethyl ether, chloroform, dimethylformamide (SRL,
India) methanol and other solvents were purified by
usual procedures.28 Silica gel (100–200 mesh) (SRL,
India) was dried in a hot air oven at 100◦C for 2 h and
cooled before use.
2.2 Measurements
Infrared spectra were recorded on a Thermo Electron
Corporation Nicolet 380 FT-IR spectrometer. High res-
1
olution H and 13C-NMR spectra were recorded on a
Bruker AM-400 spectrometer in CDCl3 with TMS as an
internal standard. Elemental analysis was carried out on
2.4 Synthesis of 4-(4-hydroxyphenylazo)benzonitrile
a Carlo-Erba 1106 system. The weight-average molec- 4-(4ꢁ-Hydroxyphenylazo)benzonitrile was prepared
ular weight (Mw), number average molecular weight using a common procedure as follows: 4-cyanoaniline
ꢀ
(Mn), polydispersity (Mw Mn) of the polymers were (6.8 g, 0.07 mol) was dissolved in concentrated
obtained on a PL-GPC model 210 chromatograph using hydrochloric acid (16 mL) and water (16 mL) contained
DMF as eluent at 25◦C and calibrated with polystyrene in a conical flask. The dissolved 4-cyanoaniline was
(molecular weight 1,84,300) standard. The absorption diazotized by slow addition of sodium nitrite solution
spectra of polymers in spectroscopic grade chloroform (0.07 mol, 4.83 g in 20 mL of water) at below 5◦C.
solution were recorded on a Shimadzu UV-2450 UV- Phenol (6.15 g, 0.07 mol) was dissolved in sodium
Visible spectrophotometer, barium sulphate was used hydroxide solution (10%; 45 mL) in a 250 mL beaker
as a standard. Differential scanning calorimetry (DSC) and cooled the solution to 5◦C. The phenolic solution
was conducted on a Perkin-Elmer model DSC Pyris was vigorously stirred and cold diazonium salt solution
1 system calibrated with indium and zinc standards added drop-wise, then, the mixture was kept in an ice
with heating rate of 10◦C min−1. Polarizing micro- bath for 30 min with occasional stirring. The mixture
scopic studies were performed with a Euromex polariz- was filtered through a Buchner funnel with gentle suc-
ing microscope equipped with a Linkem HFS 91 heat- tion, washed well with water. The precipitate was dried

Self-assembly of azobenzene based side-chain liquid crystalline polymer and n-alkyloxybenzoic acids
257
in vacuum oven at 60◦C for 2 days. Resultant product for 12 h. Precipitated amine hydrochloride was filtered
was recrystallized from chloroform to get orange-red and solvent evaporated under vacuum. Crude product
coloured solid, (yield: 10.8 g, 68%).
was dissolved in ethyl acetate, washed with water
Anal. Calcd. for C13H9N3O: C, 69.95; H, 4.06; N, (3 × 100 mL), brine solution (3 × 100 mL) and dried
18.82; O, 7.17; found: C, 69.90; H, 4.00; N, 18.75; over anhydrous sodium sulphate. Solvent was removed
O, 7.21. FT-IR (KBr pellet, cm−1): 3619, 2235, 1582, under vacuum then residue purified by column chro-
1
1475. H NMR (CDCl3, 400 MHz), δ (ppm): 7.92 (d, matography in silica–gel, hexane and ethyl acetate (9:
2H, Ar-H), 7.80 (d, 4H, Ar-H), 7.26(s, 1H, Ar-H), 6.98 1 v/v) used as a eluent to give 2.21 g of monomer as
(d, 2H, Ar-H) 13CNMR (75 MHz, CDCl3): δ = 159.5, yellow–orange coloured solid (yield 92%).
154.7, 146.9, 133.1, 125.6, 123.1, 118.1, 116.0, 113.2.
Anal. Calcd. for C24H27N3O3: C, 71.09; H, 6.71;
N, 10.36; O, 11.84,; found: C, 71.01; H, 6.74; N,
10.40; O, 11.84. FT-IR (KBr pellet, cm−1): 2916, 2850,
2235, 1600, 1747, 1583, 1475, 1154. 1H NMR (CDCl3,
400 MHz), δ (ppm): 7.87 (d, 4H, Ar–H), 7.71(d,
2H, Ar–H), 6.94 (d, 2H, −Ar−CH), 6.05 (d, 1H,
acryloyl-CH2), 5.72 (d, 1H, acryloyl-CH2), 4.07 (t, 2H,
−OCH2), 3.97 (t, 2H, −OCH2), 2.09 (s, 1H, acryloyl-
CH), 1.77 (q, 2H, −CH2), 1.72 (q, 2H, −CH2), 1.17–
1.42 (m, 10H, –CH2). 13CNMR (75 MHz, CDCl3): δ =
164.9, 161.3, 153.4, 145.2, 131.8, 131.7, 129.0, 127.2,
124.0, 123.2, 121.9, 121.6, 117.2, 113.4, 111.7, 67.0,
63.2, 28.0, 27.9, 27.8, 27.7, 27.2, 24.5, 24.4.
2.5 Synthesis of 4-(10-hydroxydecyloxy)-4ꢁ-
phenylazobenzonitrile
4-(10-Hydroxydecyloxy)-4ꢁ-phenylazobenzonitrile was
synthesized by adopting a similar procedure reported
elsewhere.26 A suspension of anhydrous K2CO3 (11 g
0.08 mol), pinch of KI and 4-(4ꢁ-hydroxyphenyl-
azo)benzonitrile (4.5 g, 0.02 mol) in dry DMF (80 mL)
was refluxed with stirring for 1 h. 10-Bromodecanol
(0.024 mol, 4.78 mL) was added drop-wise to the reac-
tion mixture and refluxed additionally for 48 h. After
completion of reaction, the mixture was filtered and
washed with excess of DMF. The filtrate was poured
in ice water, extracted using diethyl ether and dried
with anhydrous sodium sulphate. Solvent was removed
under vacuum and purified by column chromatography
(silica gel, chloroform) to afford 5.63 g of compound as
orange coloured solid (yield 75%).
Anal. Calcd. for C21H25N3O2: C, 71.77; H, 7.17; N,
11.96; O, 9.10,; found: C, 71.70; H, 7.20; N, 11.96;
O, 9.14. FT-IR (KBr pellet, cm−1):3591, 2916, 2850,
2235, 1600, 1583, 1475. 1H NMR (CDCl3, 400 MHz), δ
(ppm): 7.85 (d, 4H, Ar–H), 7.71(d, 2H, Ar–H), 7.19(s,
1H, −OH), 6.94(d, 2H, Ar–H), 3.98 (t, 2H, –OCH2),
3.56 (t, 2H, −OCH2), 1.75 (q, 2H, −CH2), 1.25–1.51
(m, 12H, –CH2). 13CNMR (75 MHz, CDCl3): δ =
162.7, 154.8, 146.7, 133.1, 125.4, 123.0, 118.6, 114.8,
113.1, 68.4, 63.0, 32.7, 29.5, 29.4, 29.3, 29.1, 25.9,
25.7.
2.7 Synthesis of poly[4-(10-acryloyloxydecyloxy)-4ꢁ-
phenylazobenzonitrile] (P10)
4-(10-Acryloyloxydecyloxy)-4ꢁ-phenylazobenzonitrile
(400 mg, 0.67 mmol) and AIBN (2 wt %) were dis-
solved in dry THF and gentle steam of nitrogen purged
into solution in polymerization tube. The tube was
kept in an oil bath at 60◦C for 48 h. Then the solution
was cooled and poured into excess of dry methanol to
precipitate the product. Crude polymer thus obtained
was reprecipitated twice using chloroform and dried at
45◦C under vacuum for 48 h to afford orange coloured
powder (yield: 256 mg, 56%).
M. P. 52–94◦C. FT-IR (KBr pellet, cm−1): 2916,
1
2851, 2235, 1600, 1747, 1582, 1475, 1153. H NMR
(CDCl3, 400 MHz), δ (ppm): 7.82(m, 4H, Ar–H),
6.94(m, 4H, Ar–H), 3.83 (m, 4H, −OCH2), 0.87–1.75
(m, 16H, −CH2). 13CNMR (75 MHz, CDCl3): δ =
161.4, 161.1, 147.0, 146.8, 124.3, 114.6, 114.1, 55.5,
29.5, 29.2, 28.1, 26.0.
2.6 Synthesis of 4-(10-acryloyloxydecyloxy)-4ꢁ-
phenylazobenzonitrile
4-(10-Hydroxydecyloxy)-4ꢁ-phenylazobenzonitrile
(2.39 g, 4.4 mmol) and triethylamine (1.16 mL,
8.0 mmol) were dissolved in dry THF under nitro-
gen atmosphere and stirred around 5–10◦C. Acryloyl
2.8 Preparation of hydrogen bonded polymer
complexes
chloride (0.78 mL, 8.0 mmol) in dry THF solution was All polymer complexes investigated in the present
added drop-wise over a period of 30 min, then the study were prepared by evaporation technique from
mixture was warmed to room temperature and stirred THF solution containing equimolar amounts of donor

258
Kumarasamy Gayathri et al.
(46 mmol) and acceptor (46 mmol) moieties. The sol- tion maximum of polymer. Structures of precursors and
vent was evaporated slowly at room temperature fol- 4-alkyloxybenzoic acids (donors) were confirmed by
lowed by drying in vacuum at 40◦C.
FT-IR, H and 13C-NMR spectroscopy and elemen-
tal analysis. Spectral values are in accordance with
assigned structures (see Supporting information, fig-
ure S1). The carboxylic acid proton resonated at
11.02 ppm of 4-alkyloxybenzoic acids. The resonance
around 6.96–8.12 ppm and 0.93–4.01 ppm are ascribed
to aromatic and aliphatic regions respectively. In poly-
mer, aromatic proton resonated around 6.94–7.82 ppm,
alkyl proton linked with ether resonated at 3.83 ppm
and disappearance of acrylic protons at 6.05, 5.72,
2.09 ppm confirmed polymerization reaction (see Sup-
porting information, figure S2). The ester group res-
onated at 161.4 ppm, C ≡ N signal of nitrile group
at 114.6 ppm, together with carbon signal for aromatic
ring around 114.1–161.1 ppm. The carbon in aliphatic
chains resonated around 26.0–55.5 ppm. Structures of
1
3. Results and discussion
3.1 Synthesis and characterization
The synthetic route for the preparation of various alky-
loxybenzoic acids, polymer and its hydrogen bonded
complexes are shown in scheme 1. The monomer
was polymerized by free radical addition polymer-
ization method in the presence of AIBN at 60◦C in
THF. Polymer is soluble in DMF, CHCl3, CH2Cl2 and
acetone where as insoluble in methanol, ethanol, 2-
propanol, benzene and toluene. Table 1 shows ther-
mal properties, molecular weights and UV-vis absorp-
Scheme 1. Synthesis of hydrogen bonded polymer complexes.

Self-assembly of azobenzene based side-chain liquid crystalline polymer and n-alkyloxybenzoic acids
259
Table 1. DSC, GPC and UV data of polymer.
Thermal properties (◦C) TGA
GPC
UV
Tg
-
Tm
52
Ti
Td
(Mn)
(Mw/Mn)
λ max(nm) CHCl3
Polymer
94
336a, 418b
1,33,000
1.28
360
a
b
Tg, glass transition; Tm, melting; Ti, isotropic; Td, decomposition; First decomposition; Second decomposition; (Mw),
Weight average molecular weight; (Mn), number average molecular (Mw/Mn), molecular weight distribution
multifunctional H-bonding molecules used in the
present study are shown in scheme 1. Here, P10 con-
taining pendant nitrile group acts as an H-bond accep-
tor and 4-alkyloxybenzoic acids (A7–A12) act as H-
bonding donors, both donor and acceptor components
show liquid crystalline property at different tempera-
tures. H-bonded complexes were prepared by maintain-
ing 1:1 stoichiometry of nitrile and carboxylic acids.
Infrared spectra of 4-decyloxybenzoic acid (A10),
polymer (P10) and polymer complex (P10–A10) are
depicted in figure 2. The spectra of free 4-decyloxy-
benzoic acid shows sharp band at 1678 cm−1 ascribed
C = O stretching, a strong intense band at 3449 cm−1
assigned to OH- stretching of the carboxylic acid
group and fermi resonances are observed at 2664 and
2556 cm−1 in A10.29 The IR spectrum of polymer
(P10) exhibits an intense band at 2221 cm−1 attributed
to C = N stretching. The infrared absorbance of com-
plexes display a sharp band at 1722 cm−1ascribed
to C = O stretching of benzoic acid moiety. When
compared to 4-decyloxybenzoic acid spectrum, the
polymer complexes indicate higher frequency shifts
(∼44 cm−1) in the C = O stretching of benzoic acid
moiety and cyano (∼11 cm−1) group of polymer. These
shifts strongly suggest the formation of intermolec-
ular H-bonding between carboxylic acid and cyano
groups. The H-bonded network structures have been
obtained by predominant formation of thermodynam-
ically favoured intermolecular H-bonds between indi-
vidual components.30 It is interesting to note that
the stable liquid crystalline behaviour maintained
for H-bonded networks built from multifunctional
compounds.
Figure 2. FT-IR spectra of A10, P10 and P10–A10.
Transition temperature of these mesophase is Cr–SmA
at 52◦C, SmA–N at 58◦C and N–I at 94◦C (figure 3).
3.2 Mesomorphic property of polymer (P10)
Formation of nematic mesophase was identified by
The liquid crystalline property of polymer was eval- schlieren texture with observation of flashlight while
uated with polarized optical microscopy (POM) and applying small mechanical stress on the liquid crys-
differential scanning calorimetry (DSC) measurements. talline melt.29 DSC thermogram confirms the formation
Optical texture of polymer exhibited nematic and Sm– of liquid crystalline phases for polymer. Azobenzene
A mesophases28 while cooling from isotropic liquid. containing polymer was established two endothermic

260
Kumarasamy Gayathri et al.
Figure 3. POM photographs of polymer (a) nematic at 80◦C, (b) Sm-A at 55◦C on cooling from isotropic.
Figure 4. POM photographs of (a) A8 - nematic droplet; (b) A10 - nematic;
(c) A12 - schlieren nematic; and (d) A12 - schlieren nematic.
transitions in between 52 and 94◦C. Two endother- 3.3 Mesomorphic property of 4-(alkyloxy)benzoic
mic transitions noticed correspond to crystalline-liquid acids (A7-A12)
crystalline (Tm) and liquid crystalline-isotropic (Ti)
Carboxylic acids were the first compounds discovered
to exhibit liquid crystal behaviour due to H-bonding.
The mesogenicity of those compounds attributed to
H-bonded dimerization leading to lengthening of
rigid core moiety that promotes liquid crystallinity.
The homologous series of 4-(alkyloxy)benzoic acids
transition temperatures respectively. In addition to
endothermic transitions, azobenzene containing poly-
mer exhibited one small hump just below Tm cor-
responding to glass transition temperature. The ꢀT
(ꢀT = Ti − Tm) data indicated that polymer possesses
stable mesophases.

Self-assembly of azobenzene based side-chain liquid crystalline polymer and n-alkyloxybenzoic acids
261
Table 2. Phase transition temperature of A7–A12.
Compound
Phase transition temperature (◦C)
Cr
SmA
N
I
A7
A8
A9
A10
A11
A12
*
*
*
*
*
*
92
101
97
94
94
*
*
98
108
*
*
*
*
*
*
146
147
150
147
138
136
*
*
*
*
*
*
*
*
117
121
77
(A7–A12) were exhibited liquid crystalline phases
(figure 4). LC property of all compounds was evalu-
ated with DSC measurements. Phase transition data of
4-alkyloxybenzoic acids are listed in table 2. In this
series of compounds A7, A8, A10, and A11 exhibited
smectic and nematic phases; on the other hand, A9 and
A12 rendered nematic mesophases.
3.4 Mesomorphic property of H-bonded complexes of
P10–A7 to P10–A12
All the complexes examined in the present study were
prepared by evaporation technique from THF solu-
tion containing equimolar amounts of polymer and
4-alkyloxybenzoic acids followed by drying in room
temperature slowly. Phase transition temperature is dif-
fering from that of an individual component indicates
the formation of H-bonding between acid and cyano
group in the polymer complex. The complexes from
P10–A7 to P10–A12 behaved as undivided polymeric
components and exhibited liquid crystallinity. All the
complexes displayed nematic phase and some of them
exhibited SmA as well. The POM image of complexes
is depicted in figure 5. Traces of DSC thermogram of
complexes P10–A7 to P10–A12 is observed in second
heating cycle (see supporting information figure S3).
Phase transition temperatures and enthalpy changes
were measured from DSC are tabulated in table 3.
Tm was observed in the range of 61.9–110.3◦C in
these series of compounds. Polymer complex P10–
A7 establishes SmA phase between the temperatures
of 103.2 and 112.0◦C and further heating it displayed
nematic phase around 112–134.3◦C. This mesophase
Figure 5. POM photographs of (a) P10-A7-nematic
droplet; (b) P10-A8-nematic; (c) P10-A9-schlieren nematic;
(d) P10-A10 - schlieren nematic; (e) P10-A11-smectic-A;
and (f) P10-A12-smectic-A.
transition trends was exhibited by P10–A11 and P10– tion was increased with increasing the terminal spacer
A12, whereas P10–A10 exhibited solely nematic phase. length of 4-alkyloxybenzoic acids. The ꢀT values of
The P10–A7 to P10–A12 renders mesophase duration these series are calculated in POM study was in accor-
(ꢀT) at different ranges of temperatures (table 3). Here, dance with DSC measurements. The melting tempera-
P10–A11 possesses maximum duration 72.0◦C. P10– ture (Tm) and isotropic temperature (Ti) also increased
A7 and P10–A8 possess minimum mesophase duration with respect to increasing terminal chain length of 4-
28.1◦C and 30.1◦C respectively. This mesophase dura- alkyloxybenzoic acids. This trend was maintained up

262
Kumarasamy Gayathri et al.
Table 3. Phase transition temperature and associated transition enthalpy values (ꢀH, J/g) in parentheses
for polymer complexes.
Complexes
DSC DATA (◦C)
N
Cr
SmA
*
I
ꢀT (ꢀC)
P10-A7
P10-A8
P10-A9
P10-A10
P10-A11
P10-A12
*
*
*
*
*
*
103.2 (12.8)
107.6 (15.9)
110.3 (22.1)
96.8 (14.0)
63.5 (20.7)
61.9 (14.3)
112.0 (9.9)
*
*
*
*
*
*
134.3 (11.9)
137.7 (19.2)
143.0 (26.4)
148.5 (8.5)
136.1 (24.3)
127.6 (11.7)
*
*
*
*
*
*
28.1
30.1
36.8
49.4
72.0
66.8
*
*
108.3 (5.2)
101.1 (13.6)
Figure 6. Graphical representations of the phase transition temperatures of P10–A7 to P10–A12.
to the carbon chain length nine after that this transi- 13C-NMR spectroscopy and in good agreement with
tion steeply decreased with increasing chain lengths. that of corresponding structures. The liquid crys-
The transitions observed under POM were found to be talline behaviour of polymer, 4-alkyloxybenzoic acids,
agreed with DSC results. The correlation between the dimers and hydrogen bonded polymer complexes were
series of complexes and transition temperatures (Tm confirmed by POM and DSC analysis. The poly-
and Ti) noticed from DSC measurement is shown in mer exhibited smectic-A and nematic textures. 4-
figure 6. The isotropic temperature of P10–A7 to P10– Alkyloxybenzoic acids (A7–A12) displayed nematic
A12 were decreased with respect to individual com- and smectic phases at various temperatures. The poly-
pounds A7–A12 respectively, moreover, the mesomor- mer complexes evidenced nematic mesophase, but
phic range also drastically increased. The hydrogen P10–A7, P10–A11 and P10–A12 exhibited nematic as
bonded complex also exhibit absorption maximum at well as Sm-A mesophases. The P10–A11 and P10–A7
366 nm and the absorption obtained as broader due to render maximum and minimum mesophase durations
tight packing of molecule in the solid state. The azoben- of 72.6◦C and 28.1◦C respectively. This mesophase
zene containing polymer in chloroform solution shows duration was increased with increasing terminal spacer
strong absorption band in the range of 366 nm cor- length of 4-alkyloxybenzoic acids. This type of inter-
responds to the strong symmetry allowed π–π∗ elec- action has usefulness in the area of miscibility of liq-
tronic transition and the small absorption band in the uid crystalline blends in the design of novel guest–host
range of 440–473 nm corresponds to the weak symme- liquid crystalline systems.
try forbidden n–π∗ transition of the trans-configuration
of azobenzene units (see supporting information figure
Supplementary materials
S4).
Figures S1–S4 are given as supplementary information
(see the website: www.ias.ac.in/chemsci).
4. Conclusions
Acknowledgements
The precursors, monomer, polymer and polymer com-
plexes were characterized by FT-IR, H-NMR and
The authors sincerely acknowledge the University
Grants Commission (UGC), New Delhi, Government
1

Self-assembly of azobenzene based side-chain liquid crystalline polymer and n-alkyloxybenzoic acids
263
of India [F. 31–110/2005 (SR)] and S B gratefully 13. Kato T, Mizoshita N and Kishimoto K 2006 Angew. Int.
Ed. 45 38
acknowledges the Council of Scientific and Industrial
Research (CSIR), New Delhi, India, for the award of
Senior Research Fellowship.
14. Paleos C M and Tsiourvas D 2001 Liq. Cryst. 28 1127
15. Gray G W and Goodby J W 1984 Smectic Liquid Crytals
(Glasgow, UK: Leonard Hill)
16. Shandryuk G A, Kuptsov S A, Shatalova A M, Plate N A
and Talroze R V 2003 Macromolecules 36 3417
17. Xu J, Toh C L, Liu X, Wang S, He C and Lu X 2005
Macromolecules 38 1684
References
18. Han X, Zhang S, Shanks R A and Pavel D 2008 React.
Funct. Polym. 68 1097
19. Grimsdale A C and Müllen K 2005 Angew. Chem. Int.
Ed. 44 5592
1. Kumar U and Frechet Jean M J 1992 Adv. Mater. 4
665
2. Saravanan C, Deepa M, Kannan P and Senthil S 2008 Z.
Naturforsch. 63b 571
20. Serpe M J and Craig S L 2007 Langmuir 23 1626
3. Bladdon P and Griffin A C 1993 Macromolecules 26 21. Delphia Shalini Rosalyn P, Senthil S, Kannan P, Vinitha
6604
G and Ramalingam A 2007 J. Phys. Chem. Solids 68
1812
22. Kumaresan S and Kannan P 2003 J. Polym. Sci: Part A:
Polym. Chem. 41 3188
4. Saravanan C, Ambili V and Kannan P 2010 React. Func.
Polym. 70 217
5. Toh C L, Xu J, Lu X and He C 2008 Liq. Cryst. 35 241
6. Thote A J and Gupta R B 2003 Ind. Eng. Chem. Res. 42 23. Kawatsuki N, Unisuga S, Neko T, Uchida E and Kondo
1129 M 2009 React. Funct. Polym. 69 836
7. Li J, Wisner J A and Jennings M C 2007 Org. Lett. 9 24. Sakthivel P and Kannan P 2005 Polymer 46 9821
3267
25. Shibaev V P, Kostromin S G and Plate N A 1982 Eur.
Polym. J. 18 651
8. Paraschiv I, Delforterie P, Giesbers M, Posthumus M A,
Marcelis A T M, Zuilhof H and Sudholter E J R 2005 26. Zentel R, Schmidt G F, Mayer J and Benalia M 1987a
Liq. Cryst. 32 977 Liq. Cryst. 2 83
9. Kohlmeier A, Nordsieck A and Janietz D 2009 Chem. 27. Bennett G M and Jones B 1939 J. Chem. Soc. 420
Mater. 21 491
28. Perrin D D and Armarego W L F 1988 Purification
of laboratory chemicals (New York: Pergamon Press)
pp. 1–378
10. Kato T and Frechet Jean M J 1989 Macromolecules 2
3818
11. Kato T and Frechet Jean M J 1989 J. Am. Chem. Soc. 29. Itahara T and Yokogawa Y 2007 J. Mol. Struct. 827 95
111 8533
30. Saravanan C, Ambili V and Kannan P 2010 React.
Funct. Polym. 70 217
12. Tschierske C 2001 J. Mater. Chem. 11 2647