Maleimide copolymers containing azobenzene moieties-synthesis and study of liquid crystalline and optical properties

Article abstract of DOI:10.1039/c6ra21072d

New reactive copolymers have been synthesized by copolymerization of two functional N-substituted-maleimides with styrene. These copolymers, with aldehyde and carboxyl functional groups, were further chemically modified with p-aminoazobenzene. After verification of the chemical structures by IR and ¹H-NMR spectroscopy, the copolymers were characterized by solubility, X-ray diffraction measurements and their thermal stability was controlled by thermogravimetrical analysis (TGA). The thermal and thermotropic properties were investigated using polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). A comparative study of the photoisomerization in solution and thermal recovery characteristics of azo copolymers vs. the corresponding maleimide model compound has been performed.

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Maleimide copolymers containing azobenzene moieties – Synthesis and study
of liquid crystalline and optical properties.
∗
Vasile Cozan , Camelia Hulubei, Anton Airinei, Simona Morariu
“
Petru Poni” Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda 41A,
Iasi 700487, Romania
ABSTRACT
New reactive copolymers have been synthesized by copolymerization of two functional Nꢀ
substitutedꢀmaleimides with styrene. These copolymers, having aldehyde and carboxyl
functional groups, were further chemically modified with pꢀaminoazobenzene. After
1
verification of the chemical structures by IR and HꢀNMR spectroscopy, the copolymers
were characterized by solubility, Xꢀray diffraction measurements and their thermal
stability was controlled by thermogravimetrical analysis (TGA). The thermal and
thermotropic properties were investigated using polarizing optical microscopy (POM) and
differential scanning calorimetry (DSC). A comparative study of the photoisomerization in
solution and thermal recovery characteristics of azo copolymers vs. the corresponding
maleimide model compound has been performed.
Keywords: styreneꢀmaleimide copolymer; azobenzene; liquid crystal, photochromism
Introduction
Azobenzene is a wellꢀknown photochromic molecule, being isomerizable from the trans
(
E) to the cis (Z) form by UV irradiation and reverses upon thermal heating or visible light
1
irradiation. The reversible trans-cis photoisomerization of azobenzene chromophore acts
∗
Corresponding author Eꢀmail address: vcozan@icmpp.ro
1

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as a molecular switch, being responsible for photoinduced optical dichroism and
2
ꢀ4
birefringence. Also, the azobenzene systems allow the optical control of photoinduced
5
birefringence – fact which has not been observed in other materials. Important
development of the scientific research was ascertained in the area of azobenzene
containing polymers, starting from early efforts to covalently bind azobenzene units into
6
the polymeric chains. until nowadays when new applications have emerged: lightꢀinduced
7
surface patterning, colloidal spheres, tunable diffraction gratings, supramolecular selfꢀ
8,9
10
assembly and chirality transfer, liquid crystalline elastomers, sideꢀchain azoꢀpolymers
1
1,12
useful for encoding information.
Nꢀsubstituted maleimides, which belong to the 1,2ꢀ
disubstituted ethylenic compounds, are attractive monomers for design of special vinyl
copolymers. The maleimide ring affords, besides predefined structure distribution by
1
3
radical polymerization, useful carrier features for many functional groups. Maleimide
copolymers, having good engineering properties like: tractability, high thermal stability
and durability, good water, fire and radiation resistance, have many applications in
1
4
electronics for aircraft and aerospace industry. It was noted that most of the works on
photoresponsive azobenzeneꢀcontaining materials are based on polymeric matrices. In this
respect, the interactions between chromophore units are better suppressed in nonpolar
1
1
polystyrene matrices than in poly(methyl methacrylate) ones. For this reason, a series of
styrene/maleimide copolymers containing sideꢀchain azobenzene chromophore units have
been reported and the tests of their properties showed the accomplishment of demands
15ꢀ18
related to electroꢀoptic coefficient, thermal stability and optical loss.
The aim of our
paper is the synthesis of new maleimide/styrene copolymers containing sideꢀchain
azobenzene chromophore units and the investigation of their thermal and optical
properties.
2

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Results and Discussion
Synthesis and characterization
Nꢀsubstituted maleimides are considered as polymerizable electronꢀpoor ethylenic
monomers and readily produce alternating copolymers with electronꢀrich monomers such
styrene and vinyl ether derivatives [24]. In view to prepare new maleimide/styrene
copolymers containing sideꢀchain azobenzene chromophore units, three functional
maleimide monomers: 4ꢀmaleimidoꢀbenzoic acid (M1), 4ꢀmaleimidoꢀbenzoyl chloride
(M2) and Nꢀ(4ꢀformylꢀphenoxyꢀ4’ꢀcarbonylphenyl)maleimide (M3) were synthesized as
reported [20,23]. These compounds have been used further to synthesize chromophore
functionalized maleimide model compounds M4 and M5 (Scheme 1).
Scheme 1.
1
The chemical structure of all the resulting maleimides was confirmed by FTIR, Hꢀand
1
3
CꢀNMR spectroscopy. Suggestive FTIR spectra of chromophore model compounds M4
and M5 are presented in Fig. 1. Spectra revealed the presence of both common and specific
ꢀ1
absorption bands. Some common absorption bands follows (cm ): υ =C–H (vinyl and
aromatic) 3100 (M4), 3070 (M5); υ C=O (imide II) 1700 (M4), 1720 (M5); υ C–N (imide
III) 1380 (M4), 1405 (M5); outꢀofꢀplane 1,4ꢀphenylene ring 810 (M4), 845 (M5); bending
ꢀ1
C=O (imide IV) 750 (M4), 758 (M5). As specific absorption bands (cm ): υ N–H 3410
(
(
M4); υ C=O (–NH–CO–) 1640 (M4); υ C=O (–COO–) 1627 (M5); υ –CH=N– 1625
M5); υ C–O asym. (–COO–) 1260 (M5).
Figure 1
1
Fig. 2 shows the HꢀNMR spectrum for the maleimide M4 in DMSOꢀd at room
6
temperature. The singlet at δ 10.67 ppm was assigned to the proton of –CONH– linkage,
the multiplet at 7.45ꢀ8.23 ppm was assigned to the resonance of aromatic protons while the
singlet at 7.24 was due to the protons of ꢀCH=CHꢀ linkage belonging the maleimide ring.
3

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Figure 2
The molecular design of photochromic polymers can be realized mainly either by blending
dye molecules with the polymer matrix, or by covalent binding the dye molecule via
polymerization or chemical modification route. The second option gives polymers having
chromophores chemically linked to the main chain thus avoiding the phase separation [25].
In a first step, two styreneꢀmaleimide copolymers P1, P3 having different sideꢀchain
reactive functional groups: ꢀCOOH and –CH=O, respectively were prepared. This was
done by radical copolymerization in solution of styrene with Nꢀ(4ꢀcarboxyphenyl)ꢀ
maleimide (M1) or Nꢀ(4ꢀformylphenoxyꢀ4’ꢀcarbonylphenyl)ꢀmaleimide (M3) (Scheme 2).
These two intermediate copolymers were further chemically modified with pꢀ
aminoazobenzene (4), leading to the final azoꢀcopolymers P2 and P4, containing, besides
the common azo (–N=N–), different chemical linkages: –CO–NH–; (P2) and –COO–; –
CH=N–; (P4) (Scheme 2).
Scheme 2
An alternative method to prepare such copolymers by copolymerization of styrene with the
maleimide model compounds M4 and M5, failed, showing both low yields and poor
molecular weight. The imide structure (Fig. 3) was confirmed by the presence of four
ꢀ1
specific absorption bands (cm ) at: imide I at 1782 (P2), 1775 (P4), imide II at 1714 (P2,
P4), imide III at 1383 (P2), 1382 (P4), imide IV at 758 (P2), 757 (P4). Supplementary
ꢀ1
absorption bands illustrated the presence of particular chemical linkages (cm ): amide at
1648 (P2), ꢀCH=Nꢀ at 1626 (P4), ester at 1207 and 1157 (P4).
Figure 3
1
Fig. 4 shows the HꢀNMR spectrum in CDCl
3
of the copolymer P4 with the corresponding
assignments. The absence of any signal at δ 9.8 ppm due to CHO protons and the presence
of a singlet at δ 8.27 ppm due to azomethine (ꢀN=CHꢀ) protons proves the chemical
4

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modification reaction of the aldehyde pendent groups of the copolymer P3. The region
from δ 6.73 to 7.85 ppm is due to the 22 aromatic protons. The multiplet at δ 3.2–4.1 ppm
showed the presence of the new formed [>CH–CH<]
n
linkage. Depending upon the
substitution in maleimide, this signal has been reported in the δ range of 3.0–5.0 ppm, [23,
2 n
26ꢀ28]. The ethylidene protons of the styrene moiety (>CHꢀCH ꢀ) were assigned at δ 3.02ꢀ
3
.07 ppm.
Figure 4
Figure 5
13
A representative CꢀNMR spectrum of the copolymer P2 is plotted in Figure 5. The two
types of carbonyl carbon atoms appeared around 177 ppm (succinimide) and at 167 ppm
(
amide). The aromatic carbons ranged between 154 ppm and 120 ppm, and the aliphatic
carbons are present in the range 55ꢀ20 ppm. The copolymers showed good solubility in
common aprotic dipolar solvents like DMF, NMP, DMAc, DMSO and also in THF. This
behavior could be the result of the presence of voluminous sideꢀchain units which, by
steric hindrance, prevents the close packing of the polymer chains. Their molecular
weights have been determined by GPC measurements on THF solutions. The obtained
results are listed in Table 1. As can be seen from Table 1, reasonable molecular weights
were obtained, even little higher than other sideꢀchain azobenzene containing polymers
[
29a].
Table 1
Thermal stability and liquid crystalline properties
The thermal stability of the copolymers was evaluated by TGA under nitrogen atmosphere.
The obtained results are summarized in Table 1 and Figure 5. The temperature for 10 %
o
weight loss (T10), comprised between 329 and 355 C was selected as criterion of thermal
stability of copolymers. The lowest T10 value of the copolymer P1 could be explained by
5

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the poor thermal stability of aldehyde functional group. The azobenzene containing
copolymers P2 and P4 have T values intermediary between copolymers P1 and P4. The
1
0
highest T10 value of copolymer P4 could be due to an extension of thermal stability in the
initial step due to hidrogen bonds. Also, literature data showed that copolymers having
o
pendant carboxyl groups exhibited T10 values above 450 C [29b].
Figure 6
Two stages were observed during decomposition process. The first stage, ranging between
o
3
53 and 365 C, was assigned to the gaseous nitrogen release due to cleavage of –N=N–
o
linkages [30]. The second stage comprised between 492 and 517 C incorporates the
decomposition of the rest of chemical moieties. The thermotropic liquid crystalline
behavior of the copolymers was studied by polarizing optical microscopy (POM) of thin
layer specimen between glass slides, as qualitative observation of textures, and also by
differential scanning calorimetry (DSC) to record temperature of phase transitions. The
obtained results are presented in Table 2 and Figure 7. The transition temperatures listed in
Table 2 were recorded only during the first heating cycle, because of possible affectation
of the mesophase by the occurring of the thermal degradation.
Table 2
All the investigated samples exhibited enantiotropic liquid crystalline behavior, excepting
sample P1, which was isotropic. In Fig. 7 the DSC thermograms are related with the
corresponding POM microphotographs of the observed texture. Two endothermal peaks
have been recorded in all cases and were assigned: first to crystalꢀnematic phase transition
and second to nematicꢀisotropic phase transition. In all cases the second endothermal peak
is more or less affected by an exothermal peak which follows. Only the sample M4 has the
o
isotropization peak blended with the exothermal one with maximum at 266 C. This
overlapping phenomenon is known in case of low molecular compounds, especially of
6

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maleimide type [33]. This exo peak is due to the polymerization process of ethylenic
maleimide double bond. For the remaining samples there is an overlap between the
polymerization and thermal degradation.
Figure 7
As can be seen from Fig. 5, only the sample P3 showed a banded texture under shear
stress, while the other samples presented a fine grainy texture, as reported for other sideꢀ
chain liquid crystalline polymers [29,34ꢀ36]. Noël explained that such fine grainy textures
have been investigated by transmission electron microscopy, which revealed that they
correspond to regions of uniform orientation correlation within nematic structures [37].
The reason for the generation of such texture was supposed to be the low level of
interchain attraction [29]. We observed that, since our copolymers had the side chain
mesogenic moieties directly coupled to the main chain, without any flexible spacer, the
interchain attraction became disfavored. In case of samples M3, M4 and M5 we tried to
correlate the thermotropic behavior with their molecular features like: ovality (O
v
) [38],
molecular polarizability (α) [31] and mesogenic index (MI) [32]. As VillanuevaꢀGarcia
pointed out [38], in case of cylinderꢀshape (calamitic) molecules like ours, the ovality (O
can be related with the aspect ratio (length/diameter) of the molecule, by the equation:
v
)
1/3
2/3
v
O = (3/54) ∙ (2x+1)/(x)
where: x = length/diameter ratio. Looking at the O
could be described:
v
values, the following shape domains
O << 1 oblate spheroid; O = 1 perfect sphere; O > 1 prolate spheroid;
v
v
v
By using Hyperchem [39] the geometry optimization of compounds M3, M4 and M5 was
performed (Fig. 8).
Figure 8
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The calculated values of O
polarizability ( ), calculated with program ACD/Chemsketch and the mesogenic index MI
32]. The last one represents an empirical method for predicting the capability of forming
v
are presented in Table 3, together with values of molecular
α
[
mesophase for copolymers in relation with their chemical composition [32] but we
observed that it works also for low molecular compounds. If the condition MI > 10 is
satisfied, then thermotropic behavior is expected. As can be observed from Table 2 all the
O values are greater than 1 which indicate a prolate spheroid shape, as a consequence of
v
the aspect ratio values depicted in Fig. 6. The MI values of all the compounds are greater
than 10 which suggested a thermotropic behavior as expected (Table 3).
Table 3
Considering both the CrꢀN and NꢀI phase transitions, the following series of increasing
transition temperatures can be written: M5<M3<M4. Although sample M5 has greater
polarizability value than samples M3 and M4, the greater transition temperatures in case of
sample M4 can be explained by the high content of hydrogen bonds of amide linkage. The
o
mesophase range, comprised between 18 and 43 C, increases with the increasing of all
discussed parameters, polarizability, ovality and mesogenic index. By comparing the
transition temperatures, the mesophase range and the texture aspect of the model
compounds M4, M5 with those of corresponding pair copolymers P2, P4, respectively, one
can observe that transition temperatures of copolymers are higher than those of model
compounds, as expected. The mesophase range is equal in case of pair M5ꢀP4, while
copolymer P2 had a better mesophase stability than corresponding model M4, probably
due to hydrogen bonds. No significant differences of texture features were observed for
both models M4,M5 and copolymers P2, P4. The copolymer P1, having no mesogenic
pendant groups, showed no mesophase, while P2ꢀP4 having pendant mesogenic groups of
variable length, exhibited nematic phases. As can be seen from Fig. 5, the samples
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containing azobenzene moiety showed textures orangeꢀred colored. The copolymer P3 and
maleimide M3, both without azobenzene moiety, exhibited different colored textures: a
blackꢀwhite banded texture for sample P3 but orange grainy texture for sample M3.
Analyzing the transition temperatures of the copolymers P3ꢀP4 one can obtain the
increasing temperature order series: P3 < P4 < P2. While the copolymers P2 and P4 had
similar molecular weights, the lower transition temperatures of sample P3 can be explained
by its lower molecular weight. The copolymer P2 had superior transition temperatures to
those of P4, probably due to the same reason of hydrogen bonds. An investigation of
copolymers by powder Xꢀray diffraction at room temperature revealed an amorphous
behavior. Fig. 9 showed two suggestive diffractograms of samples P2 and P4.
Figure 9
One can observe that, although both samples exhibited crystallineꢀlike melting
endothermal peaks in DSC thermogram, however their Xꢀray diffraction patterns are
o
almost amorphous, with a broad reflection around 17ꢀ18 . Similar results were reported for
liquid crystalline copolymers having side chain azobenzene mesogenic units [36, 40, 41].
Optical properties of the polymers in solution
UVꢀVIS absorption spectroscopy was used to characterize the trans-cis-trans
isomerization process of azobenzene chromophores in maleimide derivatives. Fig. 10
exhibits the electronic absorption spectra of M4 in DMF solution after exposure at
different times of UV irradiation at 365 nm. The strong absorption band at about 360 nm is
*
assigned to the π – π transition of the trans form of the azo bond and the band at 440 nm is
*
due to n ꢀ π transition of the cis form of the azo bond. [42, 43]. As shown, the absorption
band at around 360 nm decreases gradually in intensity, while the maximum absorbance at
440 nm increased with light irradiation time, indicating formation of the cis isomers in
system. Two isosbestic points are seen at about 320 and 422 nm suggesting that the trans-
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cis photoisomerization is the only photochemical modification and no side reactions are
involved. The cis isomer content in the photostationary state was found to be 0.86 for M4.
The photoisomerization process in the polymer containing azo chromophore P2 was also
*
followed spectrally monitoring the π – π absorption band located at about 360 nm. The
spectral pattern was similar to the azomaleimide derivative M4 including the isosbestic
points at 323 and 421 nm, respectively, in DMF solution. The photoisomerization kinetics
of azobenzene chromophore in M4 and P2 was determined by monitoring the absorbance
which corresponding to absorption maximum of trans isomer. Kinetic data were fitted to
the following equation: ln[(A
o
ꢀ A
∞
)/( A
t
ꢀ A
∞
)] = k
1
t, were A
o
, A
t
and A
∞
are the
absorbance at time 0, t and at the photostationary state, respectively, and the k
1
is the rate
constant of photoprocess [44, 45].
Figure 10
o ∞ t ∞
The plots of ln[(A ꢀ A )/( A ꢀ A )] versus irradiation time are linear for both
sample M4 and sample P2, as expected (Fig. 11). The corresponding firstꢀorder rate
ꢀ2
ꢀ2 ꢀ1
constants k
1
were of 5.77x10 and 4.95x10 s for M4 and P2 derivatives, respectively.
Figure 11
Fig. 12 illustrates the changes in absorbance during cis-trans thermal isomerization
o
in DMF solution at 50 C for P2. As a result, the absorption band at 360 nm increases in
intensity and the spectrum was restored to that value of the nonirradiated sample after 200
min.
Figure 12
The kinetics of thermal recovery of azo chromophores was described by a firstꢀ
∞ o ∞ t 2 2
order equation ln[(A ꢀ A )/(A ꢀ A )] = k t, where k is the cis-trans isomerization rate
constant. Firstꢀorder plots for cis-trans relaxation for M4 and P2 are given in Fig. 13. It is
1
0

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observed that the values of cis-trans isomerization constants are too close and thus the
polymer matrix have a little influence on the relaxation of azoaromatic moieties. The rate
ꢀ4
ꢀ1
ꢀ4 ꢀ1
constants of thermal isomerization were 3.22x10 s for M4 and 3.13x10 s for P2.
Figure 13
The photobehavior of M5 was different by comparison with M4 derivative. During UV
irradiation in DMF solution the absorption band at about 366 nm decreases progressively
in intensity but the cis isomer content was much smaller (40%) after 200 s, when the
photostationary state was achieved. Further irradiation does not modify the spectral
o
pattern. Thermal recovery at 70 C of M5 solution determines the return to the starting
spectrum after 105 min. The same behavior was also applicable to polymer P4. In this case
the transformation degree is only 0.34 after 400 sec. However, back isomerization occurs
faster, in 80 min, relating to low weight molecular compound M5.
Conclusions
New maleimide/styrene copolymers bearing side chain azobenzene moieties as well
as functionalized maleimide model compounds were synthesized. The polarizing optical
microscopy and DSC measurements were shown important and interesting characteristics
of liquid crystalline properties. The azo maleimide derivatives exhibit enatiotropic
crystalline behavior. The azo maleimide/styrene copolymers and their corresponding
maleimide model compounds were investigated for trans-cis isomerization under UV light
irradiation and their recovery was studied thermally. The results revealed that the azo
model compound M4 and copolymer P2 have a fast response to UV irradiation by
comparison with M5 and P4 derivatives.
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Legend of figures
Fig. 1. FTIR spectra with assignments of maleimide model compounds M4, M5.
1
Fig. 2. HꢀNMR spectra with assignments of maleimide model compound M4.
Fig. 3. FTIR spectra with assignments of copolymers P2 and P4.
1
Fig. 4. HꢀNMR spectra with assignments of copolymer P4.
13
Fig. 5. CꢀNMR spectra with assignments of copolymer P2.
Fig. 6. TGA curves of copolymers P1ꢀP4.
Fig. 7. DSC thermograms of maleimide monomer M3, model compounds M4, M5 and
copolymers P2 and P4.
Fig. 8. Hyperchem models of M3ꢀM5
Fig. 9. Xꢀray powder diffraction curves of copolymers P2 and P4 at room temperature.
Fig. 10. Change of the electronic absorption spectra of M4 in DMF on UV irradiation;
irradiation time: 0; 2; 5;10; 20; 25; 35; 45; 60; 120; 240 s.
Fig. 11. Firstꢀorder kinetic plots for photoisomerization process in M4 and P2
Fig. 12. Variation of UVꢀVis absorption spectra during thermal recovery for P2 in DMF
o
solution at 50 C; recovery time: 0; 3; 6; 19; 25; 35; 50; 80; 110; 230 min.
o
Fig. 13. Cis-trans isomerization kinetics of azoaromatic chromophores at 50 C for M4 and
P2
Scheme 1. Synthesis of maleimide monomers (M1ꢀM3) and azo maleimide model
compounds (M4, M5).
Scheme 2. Synthesis of copolymers P1ꢀP4.
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FTIR spectra with assignments of maleimide model compounds M4, M5.
4
11x527mm (96 x 96 DPI)

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1
H-NMR spectra with assignments of maleimide model compound M4.
4
22x183mm (96 x 96 DPI)

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FTIR spectra with assignments of copolymers P2 and P4.
4
22x500mm (96 x 96 DPI)

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1
H-NMR spectra with assignments of copolymer P4.
3
14x177mm (96 x 96 DPI)

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1
3C-NMR spectra with assignments of copolymer P2.
4
22x169mm (96 x 96 DPI)

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TGA curves of copolymers P1-P4.
2
67x172mm (96 x 96 DPI)

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DSC thermograms of maleimide monomer M3, model compounds M4, M5 and copolymers P2 and P4.
15x610mm (96 x 96 DPI)
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3
98x208mm (96 x 96 DPI)

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X-ray powder diffraction curves of copolymers P2 and P4 at room temperature.
19x144mm (96 x 96 DPI)
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Change of the electronic absorption spectra of M4 in DMF on UV irradiation; irradiation time: 0; 2; 5;10;
0; 25; 35; 45; 60; 120; 240 s.
2
2
17x169mm (96 x 96 DPI)

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First-order kinetic plots for photoisomerization process in M4 and P2
1
92x152mm (96 x 96 DPI)

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Variation of UV-Vis absorption spectra during thermal recovery for P2 in DMF solution at 50 oC; recovery
time: 0; 3; 6; 19; 25; 35; 50; 80; 110; 230 min.
1
93x153mm (96 x 96 DPI)