Thermotropic side-chain liquid crystalline copolymers containing both mono- And bisazobenzene mesogens: Synthesis and properties

Article abstract of DOI:10.1021/ma051455h

The synthesis of a novel series of copolymers containing push-pull bisazobenzene (BAz) and monoazobenzene (MAz) groups with various N-alkyl donor groups and with spacers of two and four methylene units is presented, and the effects of the structural variables on the mesomorphic properties of the polymers are investigated. The copolymer with very short spacer (n = 2) and bulky N-ethyl group exhibits liquid crystalline (LC) properties when the BAz content is at least 30 mol %. Replacing the N-ethyl group with N-methyl increases the tendency toward smectic mesomorphism and expands the mesophases stability. If the flexible spacer is extended to four methylene units, the glass transition temperature (T_g) decreases strongly, whereas the isotropization temperature (T_i) is only slightly affected. X-ray diffraction investigations reveal an orthogonal partial bilayer smectic phase in which the mesogens are interdigitated. Annealing the BAz polymer films above T_g brings about the out-of-plane (homeotropic) orientation of chromophores and the formation of H-aggregates.

Full text of DOI:10.1021/ma051455h

9526
Macromolecules 2005, 38, 9526-9538
Thermotropic Side-Chain Liquid Crystalline Copolymers Containing
Both Mono- and Bisazobenzene Mesogens: Synthesis and Properties
Cristina Cojocariu† and Paul Rochon*,‡
Department of Chemistry, Queen’s University, Kingston, Ontario, Canada K7L 3N6, and
Department of Physics, Royal Military College, Kingston, Ontario, Canada K7K 5L0
Received July 5, 2005; Revised Manuscript Received September 13, 2005
ABSTRACT: The synthesis of a novel series of copolymers containing push-pull bisazobenzene (BAz)
and monoazobenzene (MAz) groups with various N-alkyl donor groups and with spacers of two and four
methylene units is presented, and the effects of the structural variables on the mesomorphic properties
of the polymers are investigated. The copolymer with very short spacer (n ) 2) and bulky N-ethyl group
exhibits liquid crystalline (LC) properties when the BAz content is at least 30 mol %. Replacing the N-ethyl
group with N-methyl increases the tendency toward smectic mesomorphism and expands the mesophases
stability. If the flexible spacer is extended to four methylene units, the glass transition temperature (Tg)
decreases strongly, whereas the isotropization temperature (Ti) is only slightly affected. X-ray diffraction
investigations reveal an orthogonal partial bilayer smectic phase in which the mesogens are interdigitated.
Annealing the BAz polymer films above Tg brings about the out-of-plane (homeotropic) orientation of
chromophores and the formation of H-aggregates.
Introduction
storage applications.19-21 BAz chromophores are distin-
guished by high anisotropy of molecular polarizability
that can lead to large photoinduced orders when com-
pared to MAz-based materials. The birefringence per
azo structural unit in a copolymer with 11 mol % BAz
is 5 times larger than that corresponding to a MAz-
based copolymer having similar azo content.19 In a
series of side-chain LC (SCLC) copolymers with 4-alky-
loxy-BAz moieties, Stumpe et al.20 observed the largest
and the most stable anisotropy for the copolymer
containing 40 mol % BAz. Furthermore, annealing the
oriented films above Tg amplified the photoinduced
anisotropy.
The presence of both MAz and BAz chromophores in
the polymer chemical structure is of considerable inter-
est for the creation of new promising multifunctional
materials.22 In such systems, both chromophores can
undergo photoisomerization and consequently orienta-
tion under polarized light exposure, thus increasing the
efficiency of photoalignment. Moreover, the occurrence
of LC order, expected as a result of highly anisotropic
chromophores, could enhance the photoinduced anisot-
ropy. In this respect, we report here the synthesis,
thermotropic behavior, and optical properties of a novel
series of copolymers having both MAz and BAz in their
side chain (Chart 1). The properties of the corresponding
BAz-homopolymers are also presented for the first time.
The polymer’s abbreviations (PnR)x stand for the poly-
methacrylate backbone (P), the number n of methylene
carbons in the side chain spacer, the specific alkylamino
functionality R (E and M for ethyl and methyl, respec-
tively), and the concentration of BAz units (x). The
chromophores are substituted with electron-donor (amino)
and electron-acceptor (nitro) groups, an important
consideration in the development of large photoinduced
anisotropy.11 While detailed studies on the thermotropic
properties of SCLC copolymers containing MAz meso-
gens of various chemical structures have been system-
atically reported,23-26 to the best of our knowledge there
is no such study concerning copolymers having both
MAz and BAz mesogens in their side chains. The trends
The possibility of creating molecular oriented struc-
tures by light in azobenzene (Az)-containing polymer
films has stimulated a considerable research activity
from both fundamental and applied standpoints.1 The
photoresponsiveness of such systems is based on the
readily induced and reversible isomerization between
the trans and cis isomers of Az, which triggers signifi-
cant changes in the physical and chemical properties
of polymeric materials. For instance, irradiation with
linearly polarized light (LPL) induces reversible bire-
fringence and linear dichroism in initially isotropic
polymer films as a result of the orientation of Az dipole
axis perpendicular to the light polarization direction.2-5
Also, massive displacement of the polymeric material,
creating micrometer-deep surface relief gratings (SRG),6,7
and photoinduced chirality in achiral azopolymer films8
have been reported. All these properties are of great
interest for photonic applications, making the Az-
containing polymers suitable for optical data storage
applications, photoswitching, alignment of liquid crys-
tals, optical elements, and other photonic devices.9
To fully understand the relationship between the
structure and physical property, various systems with
Az moieties either in the side or main chain have been
synthesized, and their photoresponsive behavior in
amorphous4,10,11 or liquid crystalline3,5,12,13 (LC) polymer
films has been investigated. It was shown that the
photoinduced order depends strongly on various factors
such as the molecular structure of Az, the spacer length,
the chromophore aggregation, and the content of pho-
tochromic groups in sample.1,10-16 Recently, there has
been a growing interest in the photochemical behavior
of azo block copolymers17 and dendrimers.18
Although chromophores having only one azo group in
their chemical structure (monoazobenzenes (MAz)) have
been primarily studied, systems with bisazobenzene
moieties (BAz) have also been investigated for optical
† Queen’s University.
‡ Royal Military College.
10.1021/ma051455h CCC: $30.25 © 2005 American Chemical Society
Published on Web 10/19/2005

Macromolecules, Vol. 38, No. 23, 2005
Thermotropic Side-Chain LC Copolymers 9527
Chart 1
(Fluka, 99.9% anhydrous) were all used as received. Reagent
grade tetrahydrofuran (THF) and 1,4-dioxane were dried by
distillation from purple sodium benzophenone ketyl under
argon. Triethylamine (Et3N) (99.5%) and N,N-dimethylforma-
mide (DMF) were vacuum-distilled off from calcium hydride
just prior to use. N-Methylaniline (98%) and methacryloyl
chloride (Fluka, 97%) were distilled under reduced pressure
and kept in sealed tubes under argon gas. 2,2′-Azobis(iso-
butyronitrile) (AIBN, 98%) was recrystallized twice from
absolute methanol. All other reagents and solvents were
commercially available and used as received.
Monomer Synthesis. The MAz monomers 4′-{(2-meth-
acryloyloxyethyl)methylamino}-4-nitroazobenzene,14 4′-{(2-
methacryloyloxyethyl)ethylamino}-4-nitroazobenzene,4 and 4′-
{(4-methacryloyloxy butyl)methylamino}-4-nitroazobenzene14
were prepared as previously reported. The synthetic route for
the target BAz monomers is shown Scheme 1.
Synthesis of N-(n-Hydroxyalkyl)-N-methylaniline
(1-n, n ) 2, 4). These compounds were prepared by a modified
literature procedure28 as follows: 2-chloro-1-ethanol (8.05 g,
100 mmol) and sodium iodide (45.0 g, 300 mmol) were
dissolved in acetone (60 mL) and refluxed for 8 h under argon.
The resulting mixture was evaporated to dryness, and the solid
was dissolved in 60 mL of DMF. Freshly distilled N-meth-
ylaniline (21.5 g, 200 mmol) and NAHCO3 (16.8 g, 200 mmol)
were added to this solution, and the mixture was stirred at
70 °C for 20 h. The reaction mixture was cooled to room
temperature, poured into water, and extracted several times
with ethyl ether. After the organic layer was dried over
anhydrous MgSO4, the solvent was evaporated off and the
residue oil was purified over silica gel column chromatography
using a mixture of ethyl acetate:hexane (2:1 v/v) as eluent.
Yield: 68%.
N-(2-Hydroxyethyl)-N-methylaniline (1-2). 1H NMR
(CDCl3), δ (ppm): 7.32 (m, 2H, Ph-H), 6.90 (m, 3H, Ph-H),
3.80 (t, 2H, CH2OH), 3.49 (t, 2H, NCH2), 3.00 (s, 3H, CH3),
2.6 (broad t, 1H, OH). 13C NMR (CDCl3), δ (ppm): 149.6 (Ph:
C1), 129.0 (Ph: C3, C5), 116.8 (Ph: C4), 112.6 (Ph: C2, C6),
59.2 (CH2OH), 55.0 (NCH2), 38.5(NCH3).
N-(4-Hydroxybutyl)-N-methylaniline (1-4). Yield: 30%.
1H NMR (CDCl3), δ (ppm): 7.26 (m, 2H, Ph-H), 6.75 (m, 3H,
Ph-H), 3.68 (t, 2H, CH2OH), 3.36 (t, 2H, NCH2), 2.95 (s, 3H,
CH3), 1.85 (broad t, 1H, OH), 1.65 (m, 4H, CH2). 13C NMR
(CDCl3), δ (ppm): 148.7 (Ph: C1), 130.1 (Ph: C3, C5), 116.2
of the thermodynamic phase transition parameters of
our polymers will be discussed with reference to the
content of BAz units, the size of the amine function, and
the length of the spacer. These parameters should
influence the polymers optical storage capabilities. The
potential of these materials as optical storage media was
investigated by birefringence measurements and SRG
formation and will constitute the subject of a forth-
coming paper.27
Experimental Section
Materials. Unless otherwise specified, the reagents were
obtained from Aldrich. N-(2-Hydroxyethyl)-N-ethylaniline (99%),
2-chloroethanol (99%), 4-chloro-1-butanol (g90%), 4-amino-4′-
nitroazobenzene (Disperse Orange 3, 90%), sodium iodide
(99.5%), sodium nitrite (99.99%), sodium hydrogen carbonate
(Sigma, 99.5%), hydrochloric acid (EMD, 36-38%), and acetone
Scheme 1

9528 Cojocariu and Rochon
Macromolecules, Vol. 38, No. 23, 2005
(Ph: C4), 112.2 (Ph: C2, C6), 61.5 (CH2OH), 57.3 (NCH2),
41.7 (NCH3), 29.8 (CH2), 26.7 (CH2).
4-{(2-Methacryloyloxyethyl)methylamino}-4′-(4-nitro-
phenylazo)azobenzene (4-2). Dark red crystals; yield 69%;
1
Synthesis of 4-{N-Methyl-N-(n-hydroxyalkyl)amino}-
4′-(4-nitrophenylazo)azobenzene (2-n, n ) 2, 4) and
4-{N-Ethyl-N-(2-hydroxyethyl)amino}-4′-(4-nitrophen-
ylazo)azobenzene (3-2). The alcohols were prepared in 50-
60% yield as in the following example: 4-amino-4′-nitroa-
zobenzene (5.00 g, 20.6 mmol) was dissolved in a solution of
concentrated hydrochloric acid (10 mL) in DMF (40 mL). The
mixture was cooled to 0 °C in an ice-water bath, and then
sodium nitrite (1.45 g, 21.0 mmol) dissolved in a small amount
of water was added dropwise. The reaction mixture was stirred
for 2 h, and then 1-2 (3.50 g, 23.1 mmol) in 15 mL of acetic
acid was added slowly. The resultant purple solution was
vigorously stirred for 4 h at 0-5 °C and for an additional 16
h at room temperature. The precipitate formed was collected
by vacuum filtration, washed with saturated NaCO3, and
finally dried. The crude product was purified by silica gel
chromatography using toluene:acetone (1:1 v/v), followed by
recrystallization from THF:hexane to yield 4.67 g of purple
2-2 crystals. Yield: 56%.
4-{N-Methyl-N-(2-hydroxyethyl)amino}-4′-(4-nitro-
phenylazo)azobenzene (2-2). Mp 207 °C (by DSC). 1H NMR
(DMSO-d6), δ (ppm): 8.46 (d, 2H, Ph-H ortho to NO2), 8.18-
7.80 (3d, 8H, Ph-H), 6.89 (d, 2H, Ph-H ortho to NR2), 4.82
(t, 1H, OH), 3.60 (m, 4H, NCH2 and CH2OH), 3.11 (s, 3H, CH3).
13C NMR (DMSO-d6), δ (ppm): 155.2, 154.7, 152.4 (Ph:
C1′,C4′,C1′′ linked to -NdN-), 151.5 (Ph: C4′′ linked to
NO2), 148.3 (Ph: C1 linked to NR2), 142.8 (Ph: C4 linked to
-NdN-), 125.2, 124.7, 124.0, 123.2 (8C, ortho to -NdN-),
122.6 (2C, ortho to NO2), 111.4 (2C, ortho to NR2), 58.1 (CH2-
OH), 53.9 (NCH2), 38.6 (NCH3). UV-vis (THF), λmax: 348, 514
nm.
mp 173 °C (by DSC). H NMR (CDCl3), δ (ppm): 8.42 (d, 2H,
Ph-H ortho to NO2), 8.20-7.80 (m, 8H, Ph-H), 6.86 (d, 2H,
Ph-H ortho to NR2), 6.10 (s, 1H, CdCH2, cis), 5.59 (s, 1H,
CdCH2, trans), 4.42 (t, 2H, CH2O), 3.82 (t, 2H, NCH2), 3.18
(s, 3H, NCH3), 1.94 (s, 3H, dC-CH3). 13C NMR (CDCl3), δ
(ppm): 167.2 (CdO), 155.8 (Ph: C1′,C4′,C1′′ linked to
-NdN-), 152.3 (Ph: C4′′ linked to NO2), 148.7 (Ph: C1 linked
to NR2), 143.7 (Ph: C4 linked to -NdN-), 135.8 (-CdCH2),
126.3 (-CdCH2), 126.6, 124.8, 124.5, 123.5 (8C, ortho to
-NdN-), 123.0 (2C, ortho to NO2), 111.9 (2C, ortho to NR2),
61.6 (CH2O), 51.0 (NCH2), 39.2 (NCH3), 18.3 (dCCH3). UV-
vis (THF) λmax
:
346, 496 nm. Elem. Anal. Calcd for
C25H24N6O4: C, 63.54%; H, 5.11%; N, 17.78%. Found: C,
63.19%; H, 5.06%; N, 17.15%.
4-{(4-Methacryloyloxybutyl)methylamino}-4′-(4-nitro-
phenylazo)azobenzene (4-4). Purple crystals; yield 81%;
1
mp 168 °C (by DSC). H NMR (CDCl3), δ (ppm): 8.40 (d, 2H,
Ph-H ortho to NO2), 8.15-7.90 (m, 8H, Ph-H), 6.80 (d, 2H,
Ph-H ortho to NR2), 6.12 (s, 1H, CdCH2 cis), 5.58 (s, 1H,
CdCH2 trans), 4.23 (t, 2H, CH2O), 3.52 (t, 2H, NCH2), 3.11
(s, 3H, NCH3), 1.97 (s, 3H, dCCH3), 1.78 (m, 4H, CH2). 13C
NMR (CDCl3), δ (ppm): 167.4 (CdO), 155.9, 155.2, 152.4 (Ph:
C1′,C4′,C1′′ linked to -NdN-), 152.1 (Ph: C4′′ linked to NO2),
148.7 (Ph: C1 linked to NR2), 143.9 (Ph: C4 linked to
-NdN-), 136.4 (-CdCH2), 125.5 (-CdCH2), 126.2, 124.8,
124.5, 123.5 (8C, ortho to -NdN-), 123.1 (2C, ortho to NO2),
111.7 (2C, ortho to NR2), 64.1 (CH2O), 52.4 (NCH2), 38.8
(NCH3), 26.3 (NCH2CH2), 23.7 (CH2CH2CH2), 18.3 (dCCH3).
UV-vis (THF) λmax
: 348, 510 nm. Elem. Anal. Calcd for
C27H28N6O4: C, 64.78%; H, 5.63%; N, 16.79%. Found: C,
64.49%; H, 5.68%; N, 16.68%.
4-{N-Methyl-N-(4-hydroxybutyl)amino}-4′-(4-nitro-
phenylazo)azobenzene (2-4). Mp 169 °C (by DSC). 1H NMR
(DMSO-d6), δ (ppm): 8.46 (d, 2H, Ph-H ortho to NO2), 8.12-
7.76 (3d, 8 H, Ph-H), 6.88 (d, 2H, Ph-H ortho to NR2), 4.27
(t, 1H, OH), 3.48 (m, 4H, NCH2 and CH2OH), 3.07 (s, 3H, CH3),
1.49-1.64 (m, 4H, CH3). 13C NMR (DMSO-d6), δ (ppm): 155.2,
154.8, 152.1 (Ph: C1′,C4′,C1′′ linked to -NdN-), 151.6 (Ph:
C4′′ linked to NO2), 148.4 (Ph: C1 linked to NR2), 142.8 (Ph:
C4 linked to -NdN-), 125.3, 124.7, 124.1, 123.3 (8C, ortho
to -NdN-), 122.6 (2C, ortho to NO2), 111.3 (2C, ortho to NR2),
60.4 (CH2OH), 51.4 (NCH2), 37.9 (NCH3), 29.4 (CH2CH2OH),
23.1 (NCH2CH2). UV-vis (THF), λmax: 350, 512 nm.
4-{(2-Methacryloyloxyethyl)ethylamino}-4′-(4-nitro-
phenylazo)azobenzene (5-2). Red crystals; yield 56%; mp
162 °C (lit. 198 °C).19 1H NMR (CDCl3), δ (ppm): 8.40 (d, 2H,
Ph-H ortho to NO2), 8.12-7.92 (m, 8H, Ph-H), 6.85 (d, 2H,
Ph-H ortho to NR2), 6.12 (s, 1H, CdCH2 cis), 5.61 (s, 1H,
CdCH2 trans), 4.39 (t, 2H, CH2O), 3.75 (t, 2H, NCH2), 3.56
(q, 2H, NCH2CH3), 1.96 (s, 3H, dC-CH3), 1.28 (s, 3H, CH3).
13C NMR (CDCl3), δ (ppm): 168.2 (CdO), 156.7, 155.5, 153.3
(Ph: C1′,C4′,C1′′ linked to -NdN-), 152.0 (Ph: C4′′ linked
to NO2), 149.6 ((Ph: C1 linked to NR2), 144.7 (Ph: C4 linked
to -NdN-), 136.8 (-CdCH2), 127.2 (-CdCH2), 127.4, 125.7,
125.4, 124.4 (8C, ortho to -NdN-), 124.0 (2C, ortho to NO2),
112.7 (2C, ortho to NR2), 62.6 (CH2O), 49.8 (NCH2), 46.7(NCH2-
CH3), 19.3 (dCCH3), 13.2 (CH2CH3). UV-vis (THF), λmax: 340,
502 nm. Elem. Anal. Calcd for C26H26N6O4: C, 64.18%; H,
5.38%; N, 17.27%. Found: C, 64.01%; H, 5.61%; N, 17.34%.
Polymerization. The synthesis of (P2M)30 (Table 1), as a
typical polymerization procedure, is discussed below.
4-{N-Ethyl-N-(2-hydroxyethyl)amino}-4′-(4-nitro-
phenylazo)azobenzene (3-2). Mp 227 °C (lit. 171 °C).19 1
H
NMR (DMSO-d6), δ (ppm): 8.46 (d, 2H, Ph-H ortho to NO2),
8.13-7.81 (3d, 8H, Ph-H), 6.89 (d, 2H, Ph-H ortho to NR2),
4.85 (t, 1H, OH), 3.60 (m, 4H, CH2), 1.16 (s, 3H, CH3). 13C NMR
(DMSO-d6), δ (ppm): 155.4, 155.0 (Ph: C1′, C4′ linked to -Nd
N-), 151.5 (Ph: C1′′, C4′′), 148.6 (Ph: C1 linked to NR2), 142.8
(Ph: C4 linked to -NdN-), 125.8, 124.5, 123.6 (8C, ortho to
-NdN-), 122.9 (2C, ortho to NO2), 111.5 (2C, ortho to NR2),
58.5 (CH2OH), 52.3 (NCH2CH2), 45.3 (NCH2CH3), 12.3 (CH3).
UV-vis (THF) λmax: 346, 502 nm.
Into a baked 100 mL two-necked round-bottom flask were
placed 1.12 g (2.37 mmol) of 4-2, 0.88 g (2.4 mmol) of 4′-{(2-
methacryloyloxyethyl)methylamino}-4-nitroazobenzene, and
39 mL of distilled 1,4-dioxane. After stirring under argon for
30 min at 40 °C, the reaction mixture was transfer to a baked
Pyrex ampule using a dry cannula. A solution of 0.16 g of AIBN
(0.97 mmol) in 1 mL of THF was added, and immediately the
mixture was frozen in liquid nitrogen and vacuum was applied.
Following three freeze-pump-thaw cycles, the ampule was
sealed under argon and placed in an oil bath preheated at 80
°C; the reaction lasted 72 h. The polymerization mixture was
poured into vigorously stirred hot ethanol. The solid precipitate
was filtered, dissolved in DMF, and reprecipitated into a
mixture of hot ethanol-acetone (10% acetone). This procedure
was repeated several times until the unreacted monomers were
completely removed. Finally, the polymer was dried in a
vacuum oven at 80 °C for 2 days. Yield: 46%.
Synthesis of BAz Monomers. The synthesis of 4-{(2-
methacryloyloxyethyl)methylamino}-4′-(4-nitrophenylazo)a-
zobenzene (4-2), as an example, is given below: 2-2 (1.62 g,
4.00 mmol) was dissolved in 25 mL of anhydrous THF, and
freshly distilled triethylamine (1.12 mL, 8.04 mmol) was added
to this solution. The reaction mixture was then cooled to 0-5
°C. Methacryloyl chloride (0.80 mL, 8.1 mmol) in THF (5 mL)
was injected gradually to the above solution via a glass syringe
while the solution temperature was kept below 5 °C. The
formation of precipitate of triethylammonium salt was ob-
served immediately after the methacryloyl chloride addition.
The reaction mixture was stirred overnight at room temper-
ature. The resulting precipitate was filtered off, and the filtrate
was evaporated to dryness. The remaining purple solid was
then dissolved in CHCl3, washed with 5% aqueous NaHCO3
and water to neutrality, and finally dried over MgSO4.
Purification was performed by column chromatography using
chloroform as eluent followed by recrystallization from 50:50
toluene:ethanol.
1
The H NMR spectrum of (P2M)30 is presented in Figure 1
together with the monomers spectra. Table 1 summarizes the
reaction conditions and the characterization of the polymers.
We should add that BAz homopolymers purification was
carried out from pyridine/ethanol-acetone mixtures.
Characterization Methods. 1H NMR (400 MHz) and 13
C
NMR (100 MHz) spectra were recorded on a Bruker AVANCE

Macromolecules, Vol. 38, No. 23, 2005
Table 1. Synthesis and Characterization of the Polymers Used in This Study
BAz (mol %)
in copolymc
Thermotropic Side-Chain LC Copolymers 9529
d
d
sample
n
R
in feed
10-3Mn
Mw/Mn
yield (%)
PDR1Ma
(P2E)10
2
2
2
2
2
2
2
2
2
4
4
4
C2H5
C2H5
C2H5
C2H5
C2H5
CH3
CH3
CH3
CH3
CH3
CH3
CH3
0
17
50
66
100
0
50
66
100
0
0
10
32
49
100
0
30
50
100
0
6.0
5.6
4.5
4.0
e
5.0
4.3
3.8
e
1.90
1.76
1.32
1.48
e
1.40
1.27
1.42
e
68
53
46
40
31
60
48
37
28
77
46
36
(P2E)30
(P2E)50
(P2E)100
P2MANb
(P2M)30
(P2M)50
(P2M)100
P4MANb
(P4M)50
(P4M)100
4.4
3.6
e
1.60
1.34
e
67
100
50
100
a PDR1M is the MAz homopolymer with n ) 2 and R ) C2H5 (Chart 1); synthesis reported in ref 4. b P2MAN and P4MAN are the MAz
homopolymers with R ) CH3 and n ) 2 and n ) 4, respectively (Chart 1); synthesis reported in ref 14. c Determined from 1H NMR
spectra. d By GPC in THF. e Not measured; the polymer is not soluble in THF.
400 spectrometer using CDCl3, DMSO-d6, or trifluoroacetic
acid as solvents. Chemical shifts are in ppm from the internal
standard tetramethylsilane (TMS).
X-ray diffraction analysis was carried out with Cu KR
radiation (λ ) 0.154 06 nm) on a Scintag X1 Advanced
Diffraction system equipped with a θ-θ wide-angle goniom-
eter. The measurements were performed at room temperature
on thick polymer films (about 8 µm). The samples for XRD
measurements were prepared as follows: first, the polymer
films were annealed at their texture forming temperature for
several days and then rapidly placed on a cooled metal surface
to freeze the molecular arrangements in the liquid crystalline
states. The layer spacing d was determined according to
Bragg’s relation: d ) λ/(2 sin θ).
UV-vis measurements in the 700-300 nm spectral region
were performed at room temperature in solutions (for mono-
mers) or in films (for polymers) with a Shimadzu TR-280
spectrometer. Films were prepared by spin-coating solutions
of polymers in pyridine onto glass microscope slides and
subsequently dried in a vacuum oven. To evaluate the out-of-
plane orientation of the azochromophores in polymer films,
polarized absorption spectra were taken at different incident
angles (θi) of the probe light. s- and p-linearly polarized light
were obtained by passing the spectrometer light through a
calcite Glan-Taylor prism that was mounted in a rotating
holder and placed in front of the film sample. Reference spectra
at various incident angles were subtracted from sample spectra
at different θi to discard the effect of the additional multiple
reflection on polymer films. To account for the difference in
optical path length at each incident angle, the observed
absorbance (Aobs) was corrected (Acorr) according to eq 1:16
The calculated length (lc) of a hypothetical single-layer
period was estimated using Hyperchem 3 (Hypercube Inc.) for
the lowest energy conformation of the side-chain with all-trans
methylene units.
Results and Discussion
Monomers Synthesis and Characterization. The
general strategy for the synthesis of push-pull substi-
tuted BAZ monomers is outlined in Scheme 1 and
involves three main steps that will be discussed suc-
cessively.
Acorr ) Aobs cos[sin-1(sin θi/nf)]
(1)
The first step comprises the nucleophilic displacement
of the halide of an n-haloalkanol with N-methylaniline
to yield the tertiary amines 1-n. Although the prepara-
tion of tertiary amines by direct alkylation of alkyl
halides with secondary amines is rarely a useful syn-
thetic procedure, due to of the polyalkylation reactions,
Robello28 reported high yields of tertiary amines by
reacting alkyl chlorides or bromides with N-methyla-
niline. The reaction was carried out in refluxing butanol
in the presence of K2CO3 and KI as acid absorbent and
catalyst, respectively. The main drawback of this pro-
cedure is that it is too slow to be practical: it requires
4-5 days for completion. We therefore employed a
slightly modified approach to prepare 1-n. First, to
increase the nucleophilicity of alkyl halides, 2-chloro-
1-ethanol and 4-chloro-1-butanol were converted to their
iodine analogues. Second, the alkylation with N-meth-
ylaniline was carried out in DMF at 70 °C in the
presence of NaHCO3. Under these conditions, the reac-
tion was completed after 20 h as indicated by thin-layer
chromatography. This modified procedure was particu-
larly useful in the preparation of 1-4 because the
milder conditions used reduced the intermolecular cy-
clization of 4-chloro-1-butanol to form THF, thus in-
creasing the amine yield.
where nf is the refractive index of the polymer film.
The out-of-plane order parameter, Sh,15 was derived from
the absorbance of the initial as-cast film and the absorbance
of the annealed film to
Sh ) 1 - P with P ) Aannealed/Ainitial
(2)
The average molecular weights and polydispersity indices of
the polymers were determined by gel permeation chromatog-
raphy (GPC) using a Waters system equipped with six 5µ-
Styragel columns (100, 500, 103,104, 105, and 106 Å pore sizes),
a Waters 440 UV/vis detector, and a Waters 410 differential
refractometer. The measurements were made at 35 °C with
THF as eluent at a flow rate of 1.0 mL/min. GPC columns were
calibrated using monodisperse polystyrene standards. Elemen-
tal analyses of monomer and copolymer samples were per-
formed by Elemental Analysis Service (Montreal University)
using the dynamic flash combustion method and a Fisons EA
1108 analyzer.
Differential scanning calorimetry (DSC) measurements were
performed with a Perkin-Elmer DSC 6 instrument under
nitrogen at a heating/cooling rate of 10 °C/min. Indium was
used as calibration standard. First-order transition tempera-
tures are given by the peak maximum value and glass
transitions by the midpoint of the heat capacity jump.
Optical microscopy observations were performed on polymer
films between glass slides by means of a Nikon Labophot-2
POL polarized optical microscope equipped with an Instec
HS1-I programmable hot stage and a Nikon FX-35 color
camera.
The second step involves the coupling reaction be-
tween the diazonium salt of 4-amino-4′-nitroazobenzene,
which was prepared in situ, and 1-n or the com-

9530 Cojocariu and Rochon
Macromolecules, Vol. 38, No. 23, 2005
Figure 1. 1H NMR spectra of (P2M)30 in DMSO-d6 and its corresponding comonomers.
mercially available N-(2-hydroxyethyl)-N-ethylaniline to
afford the mesogenic azo alcohols 2-n and 3-2, respec-
tively. In the last step, the methacrylate monomers 4-n
and 5-2 were obtained in 55-80% yields by reacting
the azo alcohols with methacryloyl chloride in THF in
the presence of triethylamine as nucleophilic catalyst
troscopy as well as the elemental analysis confirmed the
chemical structure and purity of the monomers. All
monomers are easily soluble at ambient temperature
in organic solvents such as chloroform, THF, DMF, and
DMSO.
DSC measurements revealed for 4-2, 4-4, and 5-2
only a single endothermic transition in the heating scan
1
and hydrochloric acid acceptor. H and 13C NMR spec-

Macromolecules, Vol. 38, No. 23, 2005
Thermotropic Side-Chain LC Copolymers 9531
Table 2. Elemental Analysis of BAz Copolymers
C
H
N
sample
formula
mol wt
calcd
found
calcd
found
calcd
found
(P2E)10
(P2E)30
(P2E)50
(P2M)30
(P2M)50
(P4M)50
(C20.6H22.4N4.2O4)n
(C21.9H23.3N4.6O4)n
(C22.9H23.9N5O4)n
(C20.8H21.2N4.6O4)n
(C22H22N5O4)n
(392.8)n
(415.7)n
(433.2)n
(399.6)n
(420.4)n
(448.5)n
62.98
63.27
63.48
62.51
62.84
64.26
62.56
63.11
64.25
63.12
62.26
65.10
5.74
5.65
5.56
5.35
5.27
5.84
5.50
5.84
5.45
5.58
5.34
5.63
14.97
15.50
16.17
16.12
16.66
15.61
15.31
15.67
16.35
15.87
16.31
15.24
(C24H26N5O4)n
at 173 °C (∆H ) 98 J g-1), 168 °C (∆H ) 116 J g-1),
and 162 °C (∆H ) 76 J g-1), respectively. The large
enthalpy changes are typical of a transition between a
crystalline phase and an isotropic phase. POM observa-
tions confirmed that the monomers melt directly into
the isotropic phase. The crystalline forms obtained from
the melt are less stable than those from solution.
However, the most thermodynamically stable crystalline
forms can also be obtained from the melt by annealing
at the appropriate temperature. There is little depen-
dence of the transition temperatures on alkyl spacer
length (in comparison, the precursors have melting
points that do depend on the spacer length: 207 and
169 °C for 2-2 and 2-4). This suggests that the
monomer melting point is dominated by thermodynamic
changes involving interactions of the rigid BAz cores and
not the alkyl chains.
Synthesis and Structural Characterization of
the Polymers. The homopolymerization of 4-n and
5-2 as well as the copolymerization with the corre-
sponding MAz monomers4,14 were carried out in 1,4-
dioxane at 80 °C with AIBN as radical initiator.
Methacrylic monomers with pendant Az moieties that
also contain polar groups such as -NO2 are weakly
reactive in free radical polymerization because both the
Az moiety and the nitrobenzene group can act as
retarders.29 Consequently, large amounts of initiator
(g2 wt %) and long reaction times are usually needed
to prepare MAz polymers in high yields. Since the
retardation effect is expected to be even more pro-
nounced when BAz monomers are involved in polym-
erization, we used about 8 wt % initiator and stopped
the polymerization reaction after 72 h. Despite these
conditions, the polymerization yields were rather mod-
erate ranging from 28% to 53%. The yields decrease with
increasing BAz content in reaction mixture likely due
to the stronger retardation effect of these moieties. The
results of polymerization are summarized in Table 1.
For comparison, the MAz homopolymers, the so-called
PDR1M, P2MAN, and P4MAN, were prepared as previ-
ously described.4,14
The copolymer structures and compositions were
determined by 1H NMR spectroscopy and confirmed by
elemental analysis. Figure 1 displays the 1H NMR
spectrum for one of the copolymer samples (i.e., (P2M)30))
along with that of its corresponding comonomers. The
spectral assignments clearly support the proposed struc-
ture: the sharp and clearly separated proton resonances
of monomers disappear after copolymerization, while
broad and overlapped resonances of azochromophores
bound to polymeric chains appeared at nearly the same
positions. Furthermore, the vinylidene protons, which
resonate in the 6.3-5.6 ppm region for monomers,
disappear in the (P2M)30 spectrum, whereas the R--
methyl resonances are shifted upon polymerization from
∼1.90 ppm to higher field (1.15 ppm). All of these
confirm the consumption of the double bond in co-
polymerization. The copolymer composition can be
readily determined from the ratio of the integrated peak
areas of the protons ortho to amino group (g, g′)
resonating at ∼6.7 ppm and the total number of
aromatic protons resonating in the 7.5-8.6 ppm range
(h-j and h′-l′). The results are listed in Table 1.
Elemental analysis of the copolymers closely matched
the values calculated from NMR data (Table 2). Al-
though one should exercise caution in interpreting
copolymer composition at relatively high conversions,
it appears that BAz monomers are less reactive than
the MAz monomers on the basis of the fact that their
content is smaller in copolymers than in the correspond-
ing feeds.
The resulting number-average molecular weight (Mn)
values of the copolymers, as evaluated by GPC analysis,
range from 3600 to 5600 g/mol, with polydispersity
indices between 1.27 and 1.76. Compared to Mn of other
BAz polymers,30 for which values of 9100 up to 45 000
were reported, the Mn of our samples are yet low. This
might be attributed to the presence of nitrobenzene, a
well-known chain transfer agent. Moreover, an in-
creased extent of termination reactions, as a result of
high concentration of growing chain radicals (due to the
high AIBN/monomer ratio used), should also be taken
into account. The molecular weight of BAz homopoly-
mers (P2E)100, (P2M)100, and (P4M)100 could not be
determined by GPC because these samples are insoluble
in THF. Actually, in terms of solubility, the homopoly-
mers are readily soluble only in pyridine, strong acid
solvents (i.e., trichloroacetic acid, methanesulfonic acid),
and mixtures of hexafluoro-2-propanol:chloroform. Con-
versely, the copolymers are easily soluble in conven-
tional laboratory solvents as THF, DMSO, DMF, and
NMP. It is very likely that the “dilution” of polymer
chains with the less polar MAz groups diminishes the
strong dipole-dipole interactions among the BAz side
chains sufficiently to increase the solubility in common
organic solvents.
Mesomorphic Properties of the Polymers. Phase
transition temperatures and relevant thermodynamic
parameters of the polymers were determined by a
combination of DSC measurements and POM observa-
tions. As previously mentioned, the monomers 4-2,
4-4, and 5-2 melt directly into the isotropic phase with
no indication of LC character. The same holds for MAz
monomers.4,14 However, the BAz homopolymers and the
copolymers with MAz, with few exceptions, exhibit
thermotropic LC properties. The thermograms of most
samples display a LC phase melting endotherm on
heating and a LC phase formation exotherm on cooling
from the isotropic phase. Thus, as observed for other
systems,14,28,31 the “polymer effect” is functioning: the
macromolecular chains string the mesogenic pendants
together in a comblike fashion, correlating their move-
ments and stabilizing them in LC mesophases.
Figure 2 displays the second DSC heating curves for
the copolymer series with a very short spacer (n ) 2)
and ethylamino functionality. At this point we should

9532 Cojocariu and Rochon
Macromolecules, Vol. 38, No. 23, 2005
Table 3. Thermodynamic Parameters of the Polymers
phase transitionsa (°C)
BAz
MAz
(corresponding enthalpy changes,
sample (mol %) (mol %)
J g-1) heating/cooling
PDR1M
(P2E)10
(P2E)30
(P2E)50
(P2E)100
P2MAN
(P2M)30
(P2M)50
(P2M)100
P4MAN
(P4M)50
(P4M)100
0
10
32
49
100
0
30
50
100
0
100
90
68
51
0
100
70
50
0
g 120 i
g 103 i
g 105 n 125bi/i 98 g
g 108 s 146 (4.2) i/i 140 (- 4.0) s 101 g
g 114 s 159 (5.9) i/i 154 (- 5.5) s 110 g
g 110 s 134 (2.1) n 166 (2.6) i
g 97 s 128 (3.6) i/i 123 (3.1) s 93 g
g 100 s 159 (5.8) i/i 153 (5.4) g 96
g 102 s 174 (7.2) i/i 167 (6.9) g 95
g 85 s 170c (3.6) i
100
50
0
50
100
g 79 (6.2) s 156 i/i 149 (5.7) g 73
g 77 s 171 (7.6) i/i 163 (7.3) g 70
Figure 2. DSC heating curves (second scan) for the PDR1M-
(P2E)x series.
a Data from second heating and first cooling scans. b Determined
by POM. c With thermal decomposition. Abbreviations: g ) glass;
n ) nematic; s ) smectic; PDR1M is the MAz homopolymer with
n ) 2 and R ) C2H5; P2MAN and P4MAN are MAz homopolymers
with R ) CH3 and n ) 2 and 4, respectively (Chart 1).
note that the rather low molecular weights of the
investigated polymers might place the polymers thermal
behavior in the regime in which the transition temper-
atures are strongly molecular weight dependent.32 This,
however, does not prevent a comparison of the transi-
tional properties of the polymers here as being directly
related to the different structural features of the samples
because the polymers have similar molecular weights
(see Table 2). From a structural standpoint, both homo-
polymers of this particular series, namely PDR1M and
(P2E)100, possess the typical features of SCLC polymers,
i.e., a linear backbone containing the rigid-rod-like
mesogenic groups, the principles of which have been
reviewed elsewhere.32 However, because of the bulky
N-ethyl group, which reduces the axial-to-diameter ratio
(A/D) of MAz mesogen and accordingly its anisometry,
and the short alkyl spacer, which restricts the mobility
of side groups, the occurrence of LC phase is impeded
in the former homopolymer. As a result, its DSC
thermogram reveals only a second-order transition
corresponding to the glass transition at 120 °C. In the
case of (P2E)100, however, the effect of bulky N-ethyl
substituent on the A/D ratio of the more elongated BAz
mesogen is less pronounced; therefore, the tendency to
form LC phases is not suppressed. Besides, it is known
that three-ring mesogens are more favored in a LC
environment.32 Accordingly, (P2E)100 does exhibit on
heating a first-order transition at 159 °C, in addition
to the glass transition at 114 °C (Figure 2).
The copolymers propensity toward thermotropic self-
organization increases, as expected, with increasing the
proportion of BAz side groups. In fact, a relatively low
proportion of such units (about 30%) proves to be enough
to overcome the energetically preferred random coil
conformation of the polymethacrylate backbone and
allow the mesogens to pack in the parallel arrangement
needed to form a LC phase. Thus, (P2E)10 is completely
amorphous, as indicated by the lack of first-order
transitions in DSC and the lack of birefringence in
POM, whereas the remaining copolymers (P2E)30 and
(P2E)50 form mesophases above Tg. The phase transition
temperatures and the enthaply changes (∆H) associated
with the LC-isotropic phase transition are collected in
Table 3. Several observations are worth mentioning.
First, one notices a significant depression of Tg from 120
°C (PDR1M) to 103 °C when 10 mol % BAz groups are
introduced in the macromolecular chain. Such decrease
could be ascribed to the plasticization effect of polymer
backbone by the elongated BAz mesogen. Interestingly,
a further increase in BAz content does not continue to
diminish Tg, but rather enhances it so that, at 100 mol
% BAZ, the sample show a Tg at 114 °C. As the increase
in Tg is concurrent with the observation of liquid
crystallinity in the copolymer series, one could rational-
ize this behavior in terms of the reduction in the specific
free volume on passing from the isotropic glass phase
to the LC phase. Hence, the stabilization of mesogens
in LC phases compensates to some extent the plastici-
zation effect of BAz groups on Tg.
By copolymerization of mesogenic groups with non-
mesogenic units, the distance between the side meso-
gens increases, which in turn causes a reduction of the
mesophase stability and the extent of mesophase struc-
tural order.32 As compared to (P2E)100, the decrease of
isotropization temperatures (Ti) and enthaply changes
with increasing the proportion of MAz comonomer in
copolymers is in accord with this view (Table 3). In fact,
the dilution with a very large fraction of MAz units as
in (P2E)30 renders the arrangement of BAz cores more
difficult to develop so that the endothermic transition
cannot be detected in DSC. However, a Ti at 125 °C was
determined by optical microscopy observations.
In POM, freshly cast films of (P2E)30, (P2E)50, and
(P2E)100 are dark at ambient temperature, indicative
of a glassy state, but display birefringent domains when
heated above Tg. To promote formation and identifica-
tion of optical textures typical of low molar mass LC,
the samples were brought into the isotropic phase and
cooled very slowly (0.1 °C/min) into their mesophases,
where they were annealed for long times (24-72 h).
Generally, the lower the BAz content the longer the
annealing period. The homopolymer (P2E)100 and the
copolymer with 50% BAz developed an optical pattern
consisting of very fine grained, confocal textures (Figure
3a), which are closely reminiscent of the smectic A
phases illustrated in refs 23 and 33. Such fine grain
textures can be interpreted as the result of a relatively
high nucleation density that results in ordered domains
of very small direction. Attempts to enlarge the bire-
fringent domains either by increasing the annealing
time or by reducing the cooling rate from the isotropic
phase were unsuccessful, possibly because the bulky
N-ethyl group partially distorts the lateral packing
arrangement of the mesogens and impedes the growth
of LC domains. Yet, after several heating to/cooling from
melt cycles, large and diffuse zones of homeotropic
alignment were developed, a result implying an ortho-
gonal arrangement of the director with respect to the
layer planes and which points to the existence of a

Macromolecules, Vol. 38, No. 23, 2005
Thermotropic Side-Chain LC Copolymers 9533
Figure 4. DSC heating curves (second scan) for the P2MAN-
(P2M)x series.
P2MAN-(P2M)x series affects significantly the transition
temperatures and the mesophases stability and, in some
cases, the type of mesophase formed. The DSC heating
curves of the P2MAN-(P2M)x series are shown in Figure
4. All samples, but P2MAN, display a step in baseline
due to glass transition and an endothermic transition
that appears to be better defined than in the PDR1M-
(P2E)x series. Comparison of the thermodynamic pa-
rameters of the two series (Table 3) reveals that the
glass transitions of N-methyl polymers are 10-12 °C
lower than those of N-ethyl counterparts, whereas the
Ti are higher. This brings about a rather extended
mesophase (Ti-Tg) in N-methyl systems. For instance,
the mesophase of (P2M)50 spans ∼60 °C while its
structural cousin (P2E)50 displays the smectic phase
only over 38 °C.
Such behavior can be rationalized as follows: on one
hand, the smaller methyl group is expected to exert
weaker steric constraints than the bulky ethyl group
on intra- and intermolecular movements of chain seg-
ments as well as on the conformational changes of the
backbones. This would decrease the energy required for
cooperative segmental motion needed in going from the
isotropic glassy to anisotropic LC state. As a result, a
reduction in glass transition would occur, as in fact was
observed. On the other hand, the effect of diminished
steric hindrance caused by the methyl group strength-
ens the lateral interactions among the side mesogens,
thereby promoting a more compact packing of the
molecules to stabilize the LC phase. Accordingly, the
methyl-substituted systems will tend to exhibit more
ordered phases, at higher temperatures, than their ethyl
analogoues. This is clearly indicated in our polymers by
the larger enthalpy changes involved in the mesophasic
transitions as well as by the higher Ti of methyl systems
(Table 3). Furthermore, (P2M)30 displays a higher order
of mesomorphism (i.e., smectic order) than its closely
related counterpart (P2E)30, which forms nematic struc-
tures. The first-order transition (not apparent in (P2E)30)
can now be distinguished in the DSC scan of (P2M)30.
The more effective packing of mesogens also leads to
LC order in P2MAN, with nematic and smectic phases,14
in contrast to its related PDR1M.
Figure 3. Polarizing optical micrographs of (a) (P2E)50 at 120
°C and (b) (P2E)30 at 115 °C (magnification 400×).
smectic A mesophase. This view is also supported by
X-ray diffraction data analysis (see the following) as well
as by the order of magnitude of the isotropization
enthalpies (i.e., 4.2 J g-1 for (P2E)50 and 5.9 J g-1 for
(P2E)100, which are typical of smectic A phases).23,34,35
Generally, the nature of the mesophase is not influ-
enced by the addition of nonmesogenic comonomers.23,32
However, changes have been reported.36,37 In a copoly-
mer series containing biphenyl and MAz mesogens,
Ikeda et al.37 observed that the copolymers with high
fraction of azo groups display smectic phases similar to
homopolymers. By contrast, the copolymers with low
content of azo groups (i.e., 6, 15, and 35 mol %) exhibit
nematic phases. A mesophase change occurs as well in
our system. When the concentration of BAz units is
reduced to 30 mol %, because of the backbone dilution
with many non-LC comonomers, the stabilizing effect
of the polymer backbone is no longer strong enough to
promote the ordered smectic phase as in (P2E)50 and
(P2E)100. Instead, a nematic organization is formed, in
which the mesogens are oriented more or less parallel
with the backbone. The nematic phase of (P2E)30 was
identified by univocal observations of schlieren features
with typical disclinations of integer and half-integer
order38 (Figure 3b).
The mesophases of N-methyl polymers, observed on
POM, revealed significant differences as compared to
N-ethyl systems, in both the LC texture and the time
required for the texture to develop. Thus, whereas the
smectic (P2E)50 and (P2E)100 reveal fine-grain textures
(Figure 3a) after very long annealing times (up to 72
h), (P2M)30, (P2M)50, and (P2M)100 display better devel-
oped mesomorphic textures such as fanlike areas after
Substitution of an N-ethyl moiety in the PDR1M-
(P2E)x series with an N-methyl group to obtain the

9534 Cojocariu and Rochon
Macromolecules, Vol. 38, No. 23, 2005
Figure 5. Polarizing optical micrographs of (P2M)50 at 130
°C (magnification 200×).
Figure 7. Polarizing optical micrographs of (P4M)100 at 135
°C (magnification 400×).
the other hand, increasing the spacer length also serves
to dilute the mesogen-mesogen interactions, thereby
depressing the Ti. Moreover, the mesogens with a longer
spacer have a greater degree of freedom in their
isotropic phase and hence a larger ∆Si. As Ti is
determined by the ratio of ∆Hi/∆Si, the result would be
also a decrease in Ti. The decrease of Ti with the spacer
length was reported for 4-metoxybiphenyl-based poly-
acrylates,41 polyacetylenes with side-chain biphenyl
cores,35 or polynorbornene systems with 4′-methoxy-4-
biphenyl mesogenic groups.42 For the systems investi-
gated here, it appears that the two opposite effects
somehow balance so that no significant change in Ti is
observed when the alkyl spacer is expanded.
Figure 6. DSC heating curves (second scan) for the P4MAN-
(P4M)x series.
When viewed on POM, (P4M)50 and (P4M)100, an-
nealed in their mesophase temperature for several
hours, develop the elongated sharp-pointed particles
(baˆtonnets) (Figure 7). This indicates the smectic A
phase.
only 12 h of annealing. The fan-shaped texture, which
is revealing of a smectic A phase, is shown in Figure 5
for (P2M)50.
Analysis of the isotropization enthalpies reveals that
∆H follows an increasing trend with increasing the
fraction of BAz mesogens (Table 3). If we simply assume
that Ti reflects largely the strength of the interaction
between the mesogenic units, then it is immediately
apparent that BAz core interactions are much stronger
than MAz core interactions. Increasing the proportion
of BAz groups intensifies these interactions and hence
enhances ∆H.
The DSC traces for the series with a longer spacer
(n ) 4) and methylamino functionality (P4MAN-(P4M)x)
are shown in Figure 6. Similar to previous polymers, a
glass transition (at lower temperature) and a first-order
transition (at higher temperature) are distinguished.
Lengthening the spacer from two to four methylene
units reduces the Tg by about 20 °C, an effect often
observed in SCLC polymers and which is ascribed to
the internal plasticization action of longer alkyl spacer.
By contrast to Tg, the change in spacer length [(P2M)50
vs (P4M)50 and (P2M)100 vs (P4M)100] has little influence
on Ti. For instance, Ti of (P2M)50 is 159 °C, whereas that
of (P4M)50 is 156 °C. The flexible spacer can affect Ti in
two ways:34 on one hand, extending the spacer length
enhances the molecular anisometry of the side chains,
resulting in better liquid crystalline properties and
hence higher Ti. Such a dependence was reported for
nitroazobenzene chromophores with a styrene back-
bone,34 side-chain copolymers containing carbazolyl-
methylene-anilineandnitrobenzylidene-anilinegroups,39
and 4-cyanoazobenzene-based polymethacrylates.40 On
X-ray Analysis. To complement DSC and POM
observations and gain more information on the molec-
ular arrangements, modes of packing, and types of order
in mesophases, X-ray diffraction analysis was carried
out on polymer films. The X-ray profiles under ambient
conditions are shown in Figure 8, and the associated
data derived from Bragg equation are listed in Table 4.
In all samples, but (P2E)30, the diffraction patterns
display two sharp and intense reflections at low angles
(2° < 2θ < 6°), a result indicative of a periodic lamellar
structure corresponding to smectic layers. The reciprocal
spacings of the sharp diffractions are in 1:2 ratio (Table
4). In the wide-angle region, the diffractograms revealed
only broad halos centered at about 2θ ) 19-21°, which
correspond to an average intermolecular distance of
approximately 4.6-4.2 Å. The broad halo (showed here
only for (P4M)50 in Figure 8b) suggests that the lateral
packing of the molecules within the smectic layer is
disordered, i.e., a liquidlike arrangement of the meso-
genic groups in the layers. Such arrangement is con-
sistent with a disorder mesophase of the smectic A or
C type. Given the homeotropic regions, fan textures, or
baˆtonnets structures observed under POM, the smectic
phases in our samples were assigned as being of A-type.
The lack of diffraction peaks in the low-angle region
confirms the occurrence of nematic phase in (P2E)30, in
accordance with the schlieren texture observed under
POM. For the remaining samples, several qualitative

Macromolecules, Vol. 38, No. 23, 2005
Thermotropic Side-Chain LC Copolymers 9535
Figure 9. Schematic representation of SAd phase exhibited
by BAz polymers; the filled and empty ellipsoids represent the
BAz and MAz mesogens, respectively; d1 ) layer spacing
determined from X-ray data; lc ) extended molecular length
calculated from molecular modeling.
phases) but smaller than 2lc (representing orthogonal
bilayer SA2 phases). This implies an orthogonal partial
bilayer structure (SAd) in which the mesogenic cores are
interdigitated in an antiparallel fashion, as shown in
Figure 9. Such partial bilayer phases are commonly
observed for low molar mass compounds and SCLC
polymers possessing strong dipoles where the antipar-
allel pairs serve to minimize the dipolar energy.34,39,41
Although all samples exhibit similar SAd-type orga-
nization, the degree of overlapping of side chains,
measured as the difference 2 - d1/lc, is rather different.
For a given copolymer composition and a given spacer
length, the degree of interdigitation increases from 0.50
to 0.64 on going from (P2E)50 to (P2M)50 following the
reduction in the bulkiness of amino functionality. In
other words, the methyl systems overlap to a greater
extent than does the ethyl systems, and consequently
there is a higher packing density in the former poly-
mers. It is this increased packing density that enhances
Ti and ∆H in the P2MAN-(P2M)x series when compared
to their structural analogues (Table 3).
Extending the spacer is known to reinforce the
interactions between the alkyl parts, and this in turn
drives the system toward a monolayer smectic struc-
ture.35,44 Tang and co-workers35 showed that an increase
of the spacer from 2 to 4 methylene units in SCLC
polyacetylene results in the growth of the monolayer
structure at the expense of partially bilayer one. A
further increment in the spacer length completely
eliminated the SAd phase. In our samples no transition
toward a monolayer smectic phase occurred in (P4M)50
or (P4M)100, except a slight increase in the degree of
interdigitation. This result implies that the dipolar
interactions, mainly due to the nitro groups, are stron-
ger than the energetically favored segregation forces,
driving thus the system toward a partially bilayer
structure. The d1 spacing of (P4M)50 and (P4M)100 is
about 2.6 Å longer than the d1 spacing of (P2M)50 and
(P2M)100. As this value is very close to 2.9 Å, which
represents the theoretical increase from an ethyl to
butyl spacer assuming an extended conformation, it
indicates that the butyl spacer exist mainly in the all-
trans conformation.
Figure 8. Ambient temperature X-ray diffractograms: (a) in
the low-angle region for BAz polymers; (b) in the 1°< 2θ <
30° region for (P4M)50.
Table 4. X-ray Diffraction Data of BAz Polymers
sample
lc (Å)
d1 (Å)
d2 (Å)
2 - d1/lc
(P2E)50
(P2E)100
(P2M)30
(P2M)50
(P2M)100
(P4M)50
(P4M)100
24.4
24.4
24.2
24.2
24.2
27.1
27.1
36.4
35.8
32.8
32.6
31.9
35.3
35.1
17.2
17.4
16.1
16.0
15.9
18.0
18.3
0.50
0.53
0.64
0.65
0.68
0.69
0.70
observations can be made. First, we noticed that the
peaks sharpness and intensity relative to the broad halo
vary with the size of amino functionality employed and
the aliphatic spacer length, likely reflecting differences
in mesogens packing. Thus, (P2E)50 and (P2E)100 yield
the weakest and the widest reflection peaks presumably
due to the bulkiness of an ethyl group that hinders an
efficient packing of side mesogens. When the ethyl group
is replaced by the methyl moiety and the spacer length
is increased by two more methylene units, the reflection
peaks become sharper and more intense (see (P4M)50
and (P4M)100 peaks), further evidence that the more
anisotropic mesogens favors improved organization. It
is noteworthy that in all samples the first-order reflec-
tion is the most intense one, which may suggest that
the polymethacrylate backbone is not strictly phase
separated from the mesogens layers. For a phase-
separated system consisting of distinct polymer backbon
layers and mesogen layers, the second-order reflection
is strong and the first-order reflection vanishes.43
The thickness of smectic layer d1 determined from the
first-order reflection was compared with the calculated
molecular length lc of the mesogens in their fully
extended all-trans conformation. In all samples d1 is
larger than lc (representing orthogonal monolayer SA1
Optical Studies of Polymer Films by Polarized
UV-vis Spectroscopy. Electronic spectra of the poly-
mer films show two absorption bands: a peak in UV
region, attributed to the benzene rings, and a more
intense band in the visible range, which is assigned to
the vibronic coupling between n-π* and π-π* electronic
transitions of trans-azochromophores. The latter spec-
tral absorption is due to the high charge-transfer
interaction between the electron-donor group (amino)

9536 Cojocariu and Rochon
Macromolecules, Vol. 38, No. 23, 2005
Table 5. UV-vis Characteristics of Thin Filmsa
as-cast film
λmax (nm)
annealed film
sample
Ainitial
Tannealing (°C)
∆λmax (nm)
Aanealed
Sh
(P2E)10
(P2E)30
(P2E)50
(P2E)100
(P2M)30
(P2M)50
(P2M)100
(P4M)50
(P4M)100
466
472
482
510
474
490
512
466
483
0.86
0.45
0.91
0.79
0.73
0.67
0.81
0.90
0.76
115
118
138
145
112
148
166
145
168
0
-3
0.86
0.38
0.66
0.43
0.58
0.45
0.40
0.57
0.29
0
0.18
0.27
0.38
0.21
0.30
0.45
0.35
0.48
-6
-8
-4
-7
-10
-8
-13
a λmax ) maximum wavelength; Ainitial ) initial absorbance maximum; ∆λmax ) the shift in maximum wavelength after annealing;
Aannealed ) absorbance maximum after annealing; Sh ) out-of-plane order parameter.
Figure 10. UV-vis spectra of PDR1M-(P2E)x in polymer films.
and the electron-acceptor group (nitro). Figure 10 shows
as an example the electronic spectra for selected mem-
bers of the PDR1M-(P2E)x series. It can be seen that
(P2E)100 exhibits a maximum absorbance (λmax) at longer
wavelength (510 nm) in comparison with MAz ho-
mopolymer (460 nm). This is due to the larger conju-
gated system of BAz chromophore that increases the
π-electron delocalization and lowers thus the energy of
the π-π* transition. The λmax of the copolymers is
affected by composition, a bathochromic shift in their
absorption maximum being observed as the fraction of
BAz groups is increased. The λmax of BAz samples is
listed in Table 5.
Annealing the as-cast polymer films above Tg results
in significant changes of the electronic spectra. As may
be seen in Figure 11a for one of the copolymer samples
Figure 11. Polarized UV-vis spectra of fresh and annealed
(i.e., (P2E)50) the maximum absorbance, recorded at
films of (P2E)50 taken at θi ) 0° (a) and θi ) 50° (b) with
normal incidence, fell by 27% of the initial value. Such
drop of absorbance may arise from the partial homeo-
s-polarized (solid line) and p-polarized (dotted line) probe light.
tropic (out-of-plane) alignment of the chromophores, as
has been observed for other azochromophores.15,16,45 To
gain insight into the spatial orientation of the chro-
mophores, including the in-plane and out-of-plane modes,
measurements of absorption spectra with s- and p-
polarized probe light of the spectrometer were taken at
various incident angles θi. p-polarized light with the
electric vector parallel to the plane of incidence is
sensitive to the out-of-plane orientation, whereas s-
polarized light, with the electric vector perpendicular
to the plane of incidence, is relatively inert to that.
Three parameters, namely Ap/As, p-λmax, and s-λmax were
employed, where Ap and As represent the absorbances
at λmax taken by p- and s-polarized light, and p-λmax and
s-λmax are the maximum wavelengths observed with p-
and s-polarized light. A dependence of the first param-
eter on the incident angle reveals the out-of-plane
orientation of the chromophores, whereas a dependence
of the last two parameters provides information on the
orientation of the aggregated chromophores.
At normal incidence (θi ) 0°), the copolymer spectra
recorded before and after annealing are superimposed
as the polarization of the probe beam is changed from
p to s (Figure 11a). That means the chromophores are
optically isotropic in the film plane (Ap/As ) 1). However,
when the annealed film is tilted toward the measuring
beam at an angle of 50°, the s- and p-electronic spectra
are no longer superimposed (Figure 11b). Instead, a
significant anisotropy in aborbance is observed. As the
incident angle is increased, p-polarized light, being

Macromolecules, Vol. 38, No. 23, 2005
Thermotropic Side-Chain LC Copolymers 9537
Figure 12. Changes in Ap/As as a function θi for (P2E)50.
sensitive to the homeotropic alignment, “sees” more and
more chromophores having such orientations. This
brings about a rise in absorbance with respect to normal
incidence absorbance. On the other hand, the absor-
bance recorded with s-light does not significantly with
θi. Accordingly, a plot of Ap/As vs θi, with θi varying from
-50° to 50°, displays in the case of annealed film a
distinct minimum at θi ) 0° (Figure 12). On the
contrary, no clear θi dependence of Ap/As ratio is
observed for the as-cast film. These results reveal that
the annealing process causes a spontaneous self-
organization of the side chains to form a smectic phase
in which the chromophores tend to align homeotropi-
cally to the film surface. The homeotropic order param-
eter Sh calculated using eq 2 (see Experimental Section)
is 0.27.
Simultaneous with the absorbance drop, the anneal-
ing process leads also to a blue shift of λmax of about 6
nm. This blue shift can be related to the formation of
H-aggregates (“card-pack” or “face-to face” aggregates),
an assembly with linear chromphores arranged in such
a way that their transition dipole moments are parallel
to each other and ordered perpendicular to the stacking
direction. The occurrence of such aggregates, caused by
the ππ stacking of the aromatic cores, is frequently
encountered in ordered systems such as LC poly-
mers16,45 and Langmuir-Blodgett films.46
The molecular aggregation in (P2E)50 appears to occur
selectively for the chromophores oriented homeotropi-
cally to the film surface. p-polarized spectra of the
annealed film display significant dependence on θi: the
larger θi, the more p-λmax is blue-shifted, accompanied
by the increment of absorbances (Figure 13a). Con-
versely, s-polarized spectra do not change considerably
with θi, which implies no marked occurrence of H-
aggregation in the film plane. Hence, the aggregates
formed during annealing are homeotropically oriented
to the film plane.
Except for (P2E)10, which does not exhibit LC order,
all other BAz samples show similar changes of electronic
spectra, albeit of different extent, as (P2E)50 on anneal-
ing. Table 5 summaries the results. The spontaneous
orientation of chromophores in BAz homopolymers leads
to the largest decrease of absorbance, when measured
at normal incidence, and consequently results in the
highest order parameter Sh. By copolymerization, the
homeotropic order is reduced so that, ultimately, no
homeotropic alignment is observed in the LC MAz
homopolymers.13
Figure 13. Polarized UV-vis spectra of (P2E)50 annealed film
at various incident angles (θi) of (a) p- and (b) s-polarized light.
giving rise to optical anisotropy. The propensity of
thermally induced homeotropic orientation of chromo-
phores in BAz polymers could play an important role
in the photoinduced anisotropy process. For poly-
methacrylates containing 4-methoxyazobenzene chro-
mophores and different alkyl spacers (CnMeO, n ) 2,
6, 12 methylene units), Ichimura et al.16 observed an
exclusive in-plane photoorientation at both room and
elevated temperatures for C2MeO and C6MeO. In
contrast, in C12MeO, which is the only sample exhibit-
ing homeotropic alignment upon heating, both in-plane
and out-of-plane photoorientation occurred simulta-
neously at room temperature as well as elevated tem-
peratures. Furthermore, the out-of-plane orientation
was predominant when the film was annealed in the
temperature region of the LC phase.
The photoinduced orientational behavior of chromo-
phores in our samples will be the subject of a forth-
coming paper.27 In the synthesized polymers, to produce
any motion of the chromophore with respect to its initial
orientation, both NdN groups in bisazochromophores
have to photoisomerize. If just one of the azo groups is
photoisomerized to its cis configuration, the probability
of returning to a trans configuration in the same
position is overwhelmingly high. Thus, the process of
orientation perpendicular to pump polarization was
slower than in monoazo-based polymers as it requires
many more cycles of trans-cis-trans photoisomeriza-
tion. However, the photoinduced orientation was more
stable than in monoazo polymers based on the fact that,
again, two thermal cis-trans isomerizations should be
Irradiation of isotropic azopolymer films with LPL
induces a perpendicular orientation of chromophores
with respect to the polarization direction of actinic light,

9538 Cojocariu and Rochon
Macromolecules, Vol. 38, No. 23, 2005
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Conclusions
In this study, a series of novel SCLC copolymers
containing push-pull BAz and MAz side groups were
synthesized and studied in regard to the effect of BAz
content, the bulkiness of amino donor group, and the
length of the spacer on the mesomorphic properties of
the polymers. The copolymers with a very short spacer
(two methylene units) and ethyl amino functionality
show liquid crystallinity at a relatively low content
(30%) of BAz units. Replacing the bulky ethyl group
with methyl moiety increases the tendency toward
smectic mesomorphism and expands the mesophases
stability by lowering the Tg and raising the Ti. For the
same methylamino functionality, extending the flexible
spacer from two to four methylene units affects signifi-
cantly the Tg but has only a little effect on Ti.
With few exceptions, most of BAz polymers exhibit
smectic A mesophases as revealed by DSC, POM, and
X-ray diffraction analysis. The SA phase involves a
partial bilayer structure in which the side mesogens are
interdigitated in an antiparallel fashion.
Annealing the polymer films above Tg brings about a
partial out-of-plane (homeotropic) orientation of the
azochromophores, leading to the formation of H-ag-
gregates. The findings reported here are anticipate
helping the design of novel azopolymers for digital and
holographic storage applications.
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Acknowledgment. We thank Mrs. Lyn Kerns (RMC)
for X-ray diffraction experiments. Funding from the
Natural Sciences and Engineering Research Council of
Canada (NSERC) and the Department of Defense
Canada is gratefully acknowledged. C.C. acknowledges
the PGS B Scholarship granted by NSERC and Ontario
Government. The authors dedicate this paper to the
memory of Professor Almeria Natansohn.
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