Synthesis and characterization of poly(ethylene glycol) diacrylate copolymers containing azobenzene groups prepared by frontal polymerization

Article abstract of DOI:10.1002/pola.24765

A novel polymer matrix containing amino-nitro substituted azobenzene groups was obtained by frontal polymerization. (E)-2-(Ethyl(4-((4-nitrophenyl) diazenyl)phenyl)amino)ethyl methacrylate (MDR-1) was copolymerized with poly(ethylene glycol) diacrylate (PEGDA) using this easy and fast polymerization technique. The effect of the amount of the incorporated azo-monomer into the polymer matrix was studied in detail and correlated to front velocity, maximum temperature, initiator concentration, and monomer conversion. The obtained materials were characterized by infrared spectroscopy (Fourier transform infrared), and their thermal properties were studied by thermogravimetric analysis and differential scanning calorimetry. Moreover, the optical properties of the polymers were studied by absorption spectroscopy in the UV-Vis region. Absorption spectra of the copolymers exhibit a significant blue shift of the absorption bands with respect to the azo-monomer, due to the presence of H-aggregates. Cubic nonlinear optical (NLO) characterizations of the PEGDA/MDR-1 copolymers were performed according to the Z-Scan technique. It has been proven that samples with higher MDR-1 content (0.75 mol %) exhibited outstandingly high NLO-activity with negative NLO-refractive coefficients in the promising range of n₂ = -8.057 × 10^-4 esu.

Full text of DOI:10.1002/pola.24765

Synthesis and Characterization of Poly(ethylene glycol) Diacrylate
Copolymers Containing Azobenzene Groups Prepared by Frontal
Polymerization
JAVIER ILLESCAS,1,2 YESSICA S. RAMIREZ-FUENTES,1 ERNESTO RIVERA,1 OMAR G. MORALES-SAAVEDRA,3
3
ANTONIO A. RODRIGUEZ-ROSALES, VALERIA ALZARI,2 DANIELE NUVOLI,2 SERGIO SCOGNAMILLO,2 ALBERTO MARIANI2
´
1Instituto de Investigaciones en Materiales, Universidad Nacional Auto´ noma de Me´xico. Circuito exterior S/N. Ciudad
Universitaria C.P. 04510 Me´xico D.F.
2Dipartimento di Chimica, Universita` di Sassari, and local INSTM Unit, Via Vienna 2, 07100 Sassari, Italy
3Centro de Ciencias Aplicadas y Desarrollo Tecnolo´ gico, Universidad Nacional Auto´ noma de Me´xico, Circuito exterior S/N.
Ciudad Universitaria C.P. 04510 Me´xico D.F.
Received 7 April 2011; accepted 7 May 2011
DOI: 10.1002/pola.24765
Published online 23 May 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: A novel polymer matrix containing amino–nitro sub-
stituted azobenzene groups was obtained by frontal polymeriza-
tion. (E)-2-(Ethyl(4-((4-nitrophenyl)diazenyl)phenyl)amino)ethyl
methacrylate (MDR-1) was copolymerized with poly(ethylene gly-
col) diacrylate (PEGDA) using this easy and fast polymerization
technique. The effect of the amount of the incorporated azo-
monomer into the polymer matrix was studied in detail and cor-
related to front velocity, maximum temperature, initiator concen-
tration, and monomer conversion. The obtained materials were
characterized by infrared spectroscopy (Fourier transform infra-
red), and their thermal properties were studied by thermogravi-
metric analysis and differential scanning calorimetry. Moreover,
the optical properties of the polymers were studied by absorption
spectroscopy in the UV–Vis region. Absorption spectra of the
copolymers exhibit a significant blue shift of the absorption
bands with respect to the azo-monomer, due to the presence of
H-aggregates. Cubic nonlinear optical (NLO) characterizations of
the PEGDA/MDR-1 copolymers were performed according to the
Z-Scan technique. It has been proven that samples with higher
MDR-1 content (0.75 mol %) exhibited outstandingly high NLO-
activity with negative NLO-refractive coefficients in the promising
range of n2 ¼ ꢀ8.057 ꢁ 10ꢀ4 esu. 2011 Wiley Periodicals, Inc. J
C
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Polym Sci Part A: Polym Chem 49: 3291–3298, 2011
KEYWORDS: azo polymers; frontal polymerization; nonlinear op-
tical; poly(ethylene glycol) diacrylate; radical polymerization
INTRODUCTION In the last 20 years, frontal polymerization
(FP) has attracted the attention of the scientific community. In
general, it begins with a localized reaction ignited by an exter-
nal heating or UV-light source. FP was carried out for the first
time in 1972 by Chechilo et al.,1 who polymerized methyl
methacrylate (MMA) using benzoyl peroxide (BPO) as initia-
tor. They studied the effects of pressure and initiator concen-
tration over the propagating front speed. Since this early
work, FP has evolved in such a way that many research
groups have gotten involved in this field. Davtyan et al.2 pub-
lished a first review about FP covering all the literature up to
1984 and studied the gel effect on the frontal radical polymer-
ization of MMA.3 Later, Pojman et al.4 got implicated in this
research field and published a review covering the most im-
portant trends in FP up to 1996. Among other aspects, they
investigated the formation of simultaneous-interpenetrating
polymer networks,5 the influence of the reactor geometry and
the spin modes in FP,6 the use of a microencapsulated initia-
tor,7,8 the FP of various acrylic monomers,9–13 urethane-acryl-
ates,14 poly(dicyclopentadiene),15 epoxy resins, and more
recently, their frontal cationic curing.16,17 Finally, some ionic
liquids were also studied.18 Our research group has used FP
to obtain unsaturated polyester/styrene resins,19 diurethane
acrylates,20 and polyurethanes,21,22 polymer-based nanocom-
posites with montmorillonite,23 and polyhedral oligomeric sil-
sesquioxanes,24 interpenetrating polymer networks,25 the
consolidation of porous materials,26–29 and different kinds of
hydrogels.30–33 Recently, we proposed FP as a novel method
to obtain stimuli responsive hydrogels containing partially
exfoliated graphite34 and for the preparation of hybrid inor-
ganic/organic epoxy resins.35
Chen and coworkers have worked on this field using FP for
the polymerization of vinylpyrrolidone,36 2-hydroxyethyl ac-
rylate,37 N-methylolacrylamide,38 epoxy resin/polyurethane
networks,39 polyurethane-nanosilica hybrid nanocompo-
sites40 and quantum dot polymer nanocomposites.41 Lately,
they have also synthesized hydrogels of N-vinylimidazole for
adsorption of metals42 and amphiphilic gels.43 Crivello and
Correspondence to: E. Rivera (E-mail: riverage@iim.unam.mx) or A. Mariani (E-mail: mariani@uniss.it)
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Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 3291–3298 (2011) 2011 Wiley Periodicals, Inc.
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transform infrared (FTIR) spectroscopy; their thermal prop-
erties were determined by thermogravimetric analysis (TGA)
and differential scanning calorimetry (DSC), and their optical
properties were studied by absorption spectroscopy in the
UV–Vis range. Finally, the cubic NLO-characterizations of
the PEGDA/MDR-1 copolymers were performed according to
the Z-Scan technique.
RESULTS AND DISCUSSION
The results of the FP of PEGDA performed under various
reaction conditions are summarized in Table 1. The data
refer to BPO concentrations ꢂ0.6 mol %, which is the mini-
mum value that allows the front to self-sustain. It was found
that at higher BPO concentrations a faster Vf is observed,
ranging from 0.68 cm minꢀ1 for 0.6 mol % BPO to 1.90 cm
minꢀ1, when the initiator concentration was 7.1 mol %. In
FIGURE 1 Structures of PEGDA (left) and MDR-1 (right).
coworkers reported a new strategy to monitor propagating
fronts in the photo-initiated cationic FPs, focusing their
research mainly on epoxide monomers.44–47
In a very recent article, we demonstrated that FP can be a
technique exploitable to obtain materials that cannot be pre-
pared by the classical method, i.e., graphene-containing nano-
composite hydrogels of poly(N-isopropylacrylamide) were
synthesized. At variance to what happened during the classi-
cal polymerization occurrence, because of the fast monomer
into polymer conversion, graphene did not reaggregate to
graphite flakes, thus allowing obtaining a homogeneously
dispersed nanocomposite.48
ꢃ
the same range, Tmax slightly increases from 150 to 168 C.
To obtain a polymer material having possible relevance for
future NLO applications, various copolymers of PEGDA and
MDR-1 were prepared. It is very well known that azo-poly-
mers bearing a noncentrosymmetric molecular structure, as
well as electron-donor and electron-acceptor groups, may ex-
hibit NLO properties such as SHG52 when ordered in macro-
scopic polar structures (for instance in electrically poled or
Z-type Langmuir-Blodgett films). In this respect, a concentra-
tion of MDR-1 that does not exceed 1.2 mol % was consid-
ered adequate. A series of preliminary runs were performed
to determine the initiator concentration range that allows
the front to self sustain for a MDR-1 content up to 1.2 mol
%. Because of the moderate reactivity of azo-monomers such
as MDR-1 toward polymerization,62 the minimum amount of
BPO necessary for the FP to occur was higher than that used
for the homopolymerization of PEGDA (i.e., 2.4 instead of 0.6
mol %). Figure 2 shows the data obtained by increasing the
MDR-1 content up to 0.75 mol %, keeping the BPO concen-
tration equal to 2.4 mol %. As it can be seen, both Vf and
Tmax decrease as the concentration of MDR-1 augments, thus
confirming the aforementioned moderate reactivity of the
azo-monomer. Specifically, Vf goes from 1.32 to 0.62 cm
minꢀ1 as the MDR-1 concentration increases from 0 to 0.75
mol %. As well, Tmax decreases from 161 to 140 ꢃC in the
same range of concentrations.
Poly(ethylene glycol) diacrylate (PEGDA; Fig. 1) is a mono-
mer that has been primarily used in biomaterials science for
the elaboration of phase-separation membranes for proteins,
as adsorbent of metallic ions in different kinds of solutions
or in drug delivery applications. Particularly, this monomer
has been of tremendous interest because of its suitability to
prepare hydrogels able to respond to external stimuli such
as pH or temperature changes.49,50
On the other side, azopolymers have been considered as
highly versatile materials due to the photo-induced motions
which occur on them, when they are irradiated with laser
polarized light.51 Besides, these polymers exhibit nonlinear
optical (NLO)-properties of second and third order; for
instance, second harmonic generation (SHG) and third har-
monic generation, which make them attractive prospects for
the elaboration of optoelectronic devices.52 Several reviews
covering most of the implications of azobenzene in polymer
structures have been published.51–56 In the last years, vari-
ous azo-polymers bearing amino-nitro substituted azoben-
zene units have been synthesized and characterized.57,58 In
general, they exhibit absorption maximum wavelengths close
to those reported for similar push–pull azo-compounds.59,60
In these materials, both J- and H-type aggregations have
been observed in cast films.58,61 This phenomenon can be
exploited for optical applications such as optical storage and
photolithography.
All samples were characterized by DSC and TGA. In particu-
lar, by DSC, it was found that conversions were almost
TABLE 1 Vf, Tmax, Conversion, and Tg for the FP of PEGDA,
Varying the Initiator Concentration
BPO
Vf (cm
Tmax
(ꢃC)
Conversion
(%)
Tg
(ꢃC)
Sample
(mol %)
minꢀ1
)
In this article, we report the FP of PEGDA, using BPO as
initiator. In addition, we have incorporated (E)-2-(ethyl
(4-((4-nitrophenyl)diazenyl)phenyl)amino)ethyl methacrylate
(named here MDR-1) as a model comonomer. FP was chosen
in that it is much easier and faster if compared with the clas-
sical routes. The polymers were characterized by Fourier
1
2
3
4
5
0.6
1.2
2.4
4.7
7.1
0.68
1.00
1.32
1.40
1.90
150
158
161
158
168
98
94
85
95
84
ꢀ21
ꢀ24
ꢀ20
ꢀ25
ꢀ26
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The FTIR-spectra of MDR-1 monomer and the polymers are
shown in Figure 4. The characteristic bands of the MDR-1
are indicated in the experimental section. The FTIR spectrum
of the PEGDA matrix exhibits characteristic bands at m ¼
2896 (s, CH2), 1743 (s, C¼¼O), 1267 (s, CAO esther), and
1116 (s, OACH2) cmꢀ1
.
Moreover, the FTIR spectra of MDR-1/PEGDA copolymers
with different azobenzene contents were recorded. If we an-
alyze the spectrum of the MDR-1/PEGDA 0.75 mol % copoly-
mer, we can observe a series of bands at 2910 (s, CH2, CH3),
1741 (s, C¼¼O), 1652 (s, C¼¼C aromatic), 1268 (s, CAO
esther), and 1112 (s, OACH2) cmꢀ1. Since the content of
MDR-1 is very low with respect to the polymer matrix, it is
very difficult to visualize the bands corresponding to the
amino (R2N), nitro (NO2), and azo (N¼¼N) groups. However,
the presence of the band at 1652 cmꢀ1, due to the phenyl
rings of the azobenzene moieties, which is not present in the
FTIR spectrum of the polymer matrix, confirms that the azo-
benzene chromophore has been incorporated in this
copolymer.
FIGURE 2 Vf and Tmax for the frontal copolymerization of MDR-
1 with PEGDA [BPO: 2.4 mol %] as functions of the MDR-1
concentration.
quantitative, and Tg values were always between ꢀ26 and
ꢃ
The optical properties of the polymers were studied by UV–
Vis spectroscopy and the absorption spectra of the copoly-
mers bearing different MDR-1 contents are shown in Figure
5. As it could be expected, all polymers exhibited a maxi-
mum absorption band around 450–470 nm, whose intensity
increases with the azobenzene content. To get a deeper
insight in the optical properties of the polymers, we com-
pared the absorption spectrum of the PEGDA/MDR-1 (0.75
ꢀ20 C, thereby suggesting that neither BPO nor MDR-1 con-
centrations affect these parameters in a remarkable way (Ta-
ble 1).
Figure 3 shows the TGA of the PEGDA matrix and of two
copolymers with different MDR-1 content. Actually, since the
polymerizations occur only with low MDR-1 concentrations,
the TGA curves of the obtained copolymers are very similar
to that of PEGDA. Indeed, the incorporation of MDR-1 does
not affect significantly the thermal stability of the PEGDA
matrix, even though the amount of the azo-monomer was
increased up to 0.75 mol %. In fact, PEGDA matrix shows a
ꢃ
T10 value of 363 C, slightly lower than that observed for the
ꢃ
copolymer (T10 ¼ 385 C), which can be attributed to a plas-
ticizing effect of MDR-1. All polymers exhibited good thermal
stabiꢃlity showing drastic degradation between 400 and
500 C.
FIGURE 4 (a) FTIR analysis of PEGDA polymer matrix and the
copolymers with different MDR-1 concentrations [0.075 and
0.75 mol %] [BPO: 2.4 mol %]. (b) For better analysis, an ampli-
fication of the FTIR spectrum of PEGDA/MDR1 0.75% is also
included.
FIGURE 3 TGA of PEGDA polymer matrix and the copolymers
with different MDR-1 concentrations [0.075 and 0.75 mol %]
[BPO: 2.4 mol %].
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linear absorption coefficients evaluated within the visible
range for this sample. Indeed, the Lambert–Beer law applies
for such semitransparent film allowing an adequate data
analysis and making this copolymer a potential candidate for
some optical applications due to its appropriate transpar-
ency. It is evident from Figure 6(a) that the highest absorp-
tive properties occur within the 400–580 nm spectral range,
which may indicate additional conjugation of delocalized p-
electrons provided by the higher content of MDR-1
FIGURE 5 UV spectra of the azo-monomer (MDR-1) and of co-
polymer PEGDA/MDR-1 with the highest azo-monomer concen-
tration [0.75 mol %] [BPO: 2.4 mol %].
mol %) with those of the MDR-1 monomer and the oligomer
isolated during the soxhlet extraction. As we can see, the
absorption spectrum of the MDR-1 monomer in CHCl3 solu-
tion shows a well defined maximum absorption band at kmax
¼ 473 nm as other amino–nitro substituted azobenzenes,
belonging to the pseudostilbenes category according to Rau’s
classification.60 This kind of azobenzenes exhibits a total
overlap of the p–p* and n–p* bands, which are inverted in
the energy scale.56 Similarly, the oligomer exhibited a well
defined band at kmax ¼ 452 nm, which is 21 nm blue shifted
with respect to the corresponding monomer. This hypsochro-
mic effect reveals the presence of H-aggregates (parallel
interactions) between the azobenzene groups in the
oligomer. Further, 1H NMR experiments (not shown) con-
firmed that such oligomers possess relatively high azoben-
zene content.
On the other hand, the PEGDA/MDR-1 (0.75 mol %) copoly-
mer exhibited a broad absorption band centered at kmax
¼
460 nm, which is 13 nm blue shifted when compared with
that of the monomer. However, the presence of an additional
blue shifted band at 415 nm as well as the presence of a dis-
crete shoulder at 510 nm, reveals the presence of H-aggre-
gates and traces of J-aggregates, respectively. It is very well
known that polymers bearing donor-acceptor substituted
azobenzenes tend to form antiparallel pairs to reach certain
neutrality and stability. This behavior was previously
reported in the literature for other azo-polymers.63 Accord-
ing to the UV–Vis spectra shown in Figure 5, the PEGDA/
MDR-1 (0.75 mol %) copolymer shows the presence of H-
aggregation, which was not observed for the MDR-1
monomer.
FIGURE 6 Linear and NLO measurements obtained for the pris-
tine PEGDA hosting matrix and the PEGDA/MDR-1 copolymer
sample with higher MDR-1 content (0.75 mol %; prepared film
samples): (a) Absorption coefficients of the PEGDA/MDR-1 co-
polymer film evaluated within the visible range, (b) closed
aperture Z-Scan data and theoretical fitting (dashed line)
obtained at kZ-Scan ¼ 632.8 nm for the PEGDA/MDR-1 film sam-
ple (an estimated experimental error below 5% is also consid-
ered for the Z-Scan data: error bars), and (c) closed aperture Z-
Scan data obtained for a pristine PEGDA reference sample
under similar experimental conditions.
Regarding the NLO properties and given the amorphous na-
ture of the obtained materials, the PEGDA/MDR-1 copolymer
sample with highest MDR-1 content (0.75 mol %) was
selected for cubic NLO-characterization. The thickness of the
studied sample (sandwiched PEGDA/MDR-1 film prepared
within two glass slices) was ꢄ27 lm. Figure 6(a) shows the
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chromophores within this sample. This suggestion was
explored by means of NLO Z-Scan experiments as explained
below. Under this framework, the available laser excitation
line for Z-Scan experiments (kZ-Scan ¼ 632.8 nm) is also
depicted in this figure (vertical dashed line). At this wave-
length, lowest absorption conditions occur, allowing nonreso-
nant NLO-characterizations of the sample which is a critical
point when working with low Tg-based organic materials. In
fact, a small linear absorption coefficient in the order of
a0 ꢅ 59.13 mꢀ1 was evaluated for the studied copolymer at
clearly exhibit a peak-to-valley transmittance asymmetry,
NLO absorption effects are expected.69 The fitting parame-
ters are in this case the induced phase shifts DU or DW,
respectively. In the former case, the phase shift is given by
DU ¼ 2pcI0Leff/k,67 from which the NLO refractive index
(c-coefficient) can be obtained. In the latter case, the phase
shift is provoked by the NLO-absorption and is given by DW
¼ bI0LEff/2,65 allowing the evaluation of the NLO absorption
(b-coefficient), either due to two photon (or multiphoton)
absorption and/or saturable absorption. In these equations,
k is the laser wavelength, I0 is the input beam intensity (at
focal spot: z ¼ 0), and Leff is the effective thickness of the
kZ-Scan. This value is very useful for the determination of the
nonlinear refraction and absorption coefficients according to
the Z-Scan technique. Additionally, the linear refractive index
of this material was estimated to be n0 ꢅ 1.5738.
film sample, defined as Leff ¼ [1ꢀ(eꢀa L )](a0)ꢀ1, where a0
represents the linear absorption coefficient. All these equa-
tions are well established and have been proven in early
Z-Scan works.64–69 The theoretical restrictions imposed by
these formulas to apply such expressions at optimal condi-
tions (|DU0| < p, S ꢅ 20%, etc.) are not always fully satisfied
in our experimental result due to the large phase shifts and
huge nonlinearities. Nevertheless, in most cases (mainly in
the case of well defined c > 0 or c < 0 curves); our results
nearly satisfy these conditions and can be conveniently fitted
according to these theoretical formulas. Thus, for comparison
purposes and to be consistent with the estimation of the c-
values and b-values, we assumed their applicability and used
them in our experimental results. The TF allowed us to eval-
uate a positive NLO-refractive coefficient of c ¼ ꢀ2.14 ꢁ
10ꢀ10 m2 Wꢀ1 (or n2 ¼ ꢀ8.057 ꢁ 10ꢀ4 esu) and a NLO
absorption coefficient of b ¼ þ8.69 ꢁ 10ꢀ5 m Wꢀ1. The
obtained c/n2-values are very large, many orders of magni-
tude larger than those observed for typical glass substrates
or for the classical CS2 standard reference material: þ1.2 ꢁ
10ꢀ11 esu (Z-Scan at k ¼ 10.6 lm) or 6.8 ꢁ 10ꢀ13 esu
(degenerate four wave mixing at k ¼ 532 nm).64–68 On the
other hand, the positive sign obtained for the b-coefficient
reveals the nature of the NLO absorptive phenomena of
our sample, indicating strong multiphoton absorption
effects.64–69 This fact indicates the onset of thermal effects
during Z-Scan experiments due to long cw-laser irradiation
and low Tg-values of the samples. Indeed, Z-Scan experiments
were performed at extremely low laser energy conditions
(ꢄ3 mW) to avoid photodegradation effects on the sample,
which started at power regimens in the order of ꢄ5 mW.
0
S
Z-Scan measurements were performed at room conditions on
the PEGDA/MDR-1 prepared film. The observed nonlocal
effect of this sample is shown in Figure 6(b). A rigorous the-
oretical fitting was performed to evaluate both the nonlinear
absorptive and refractive properties of this sample. The
NLO-response of the developed copolymer was characterized
by varying the polarization input planes of the He–Ne laser
system to explore microscopic material asymmetries or ani-
sotropies throughout the sample structure. In general, since
all NLO-measurements were systematically performed with
different laser input polarization states (from 0 to 90ꢃ: s-
polarization to p-polarization, respectively) and the obtained
curves are quite similar in each sample, the film structures
do not seem to show any significant anisotropic behavior,
thus confirming their amorphous nature. On the other hand,
the Z-Scan curve for a pristine, highly transparent, PEGDA
reference sample [Fig. 6(c)] exhibits negligible nonlinear
refraction and absorption when compared with the curves
obtained for the PEGDA/MDR-1 system at the same laser
power regime. Taking into account the theory developed by
Sheik-Bahae and coworkers and Liu et al.,64–69 it is observed
from our measurements that the nonlinear refractive
response of the studied sample can be unambiguously deter-
mined by a typical peak-to-valley transmittance curve. Hence,
one can immediately observe that the PEGDA/MDR-1 copoly-
mer sample (0.75 mol % MDR-1) exhibits a negative NLO-
refraction coefficient (c < 0).
The respective theoretical fit (TF) to the obtained Z-Scan
transmission data (solid lines) are also shown in Figure 6(b).
To perform the TF according to previous theoretical studies,
the normalized Z-Scan transmittance TN can be determined
as a function of the dimensionless sample position (x ¼
z/z0), where z0 is the Rayleigh range and z is the Z-Scan
sample position. Hence, the TF was obtained according to
the following equation, considering both nonlinear refraction
and absorption effects.69
EXPERIMENTAL
Materials
PEGDA (Mn % 575, d ¼ 1.12 g mLꢀ1), triethylamine (TEA,
formula weight (FW) ¼ 101.19, bp ¼ 88.8 ꢃC, d ¼ 0.726 g
mLꢀ1), and Disperse Red-1 dye (DR-1, FW ¼ 314.34, mp ¼
160–162 ꢃC) were purchased from Sigma-Aldrich. BPO (FW
¼ 242.23, mp ¼ 102–105 ꢃC) and methacryloyl chlorꢀid1e
(MAcC, FW ¼ 104.53, bp ¼ 95–96 ꢃC, d ¼ 1.07 g mL
)
TN ꢅ 1 þ ½4x=ð1 þ x2Þð9 þ x2ÞꢆDU
were purchased from Fluka. All reagents were used as
received, without further purification.
ꢀ½2ðx2 þ 3Þ=ð1 þ x2Þð9 þ x2ÞꢆDW: ð1Þ
Here, the first term is related to NLO-refractive effects,
whereas the second one is associated to NLO-absorptive phe-
nomena. Indeed, as the obtained Z-Scan data [Fig. 6(b)]
Synthesis of MDR-1
DR-1 (0.72 g, 2.28 mmol) was dissolved in freshly distilled
THF (12 mL) under argon atmosphere; then TEA (0.33 g,
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3.32 mmol) was added to the solution with a syringe. The
mixture was cooled in an ice bath and MAcC (0.29 g, 2.76
mmol), dissolved in THF, (4 mL) and was added drop wise
by means of an addition funnel. The reaction mixture was
stirred for 24 h at room temperature. The resulting product
was extracted with chloroform, dried with anhydrous MgSO4,
and concentrated at reduced pressure. Then, the crude prod-
uct was purified by flash column chromatography on silica
gel, using an appropriate mixture hexane–chloroform as elu-
ent. Pure MDR-1 monomer (Fig. 1) was obtained as a dark
red solid with good yield (75%). The structure of MDR-1
was confirmed by FTIR and 1H NMR spectroscopies and the
observed signals correspond to those previously reported in
the literature.70,71
was determined from the first thermal scan, whereas Tg val-
ues were determined from the second scan (Table 1). TGA
measurements were performed using a TA Instrument ther-
ꢃ
mobalance TGA 2050, under air flow, from 25 to 500 C at a
ꢀ1
ꢃ
heating rate of 10 C min
.
FTIR spectra of the samples in KBr pressed pellets were
recorded in a Fourier transform infrared spectroscope (JAS-
COFT 480 spectrometer), carrying out 16 scans at a resolu-
tion of 4 cmꢀ1. UV–Vis spectra of the polymers were carried
out in a Hitachi U-2010 spectrometer (1 cm quartz cell). The
MDR-1 concentration of all samples was calculated by
absorption spectroscopy and the extinction coefficient for
this monomer was estimated to be 46,700 M cmꢀ1 in CHCl3.
Finally, due to the amorphous structure of the resulting
PEGDA/MDR-1 copolymers, these systems were also studied
as active media for cubic v(3)-NLO effects such as nonlinear
refraction and absorption via Z-Scan measurements.72 The
experimental Z-Scan set-up was implemented using an unpo-
larized laser beam from a 35-mW He–Ne laser system work-
ing at 632.8 nm (THORLABS, HRR170-1). Its energy was
carefully monitored and kept constant during long Z-Scan
measurements. The spatial mode of the laser beam was close
to Gaussian TEM00. The polarization plane of the He–Ne
laser beam was adjusted and controlled by means of a linear
polarizer mounted on a rotation stage. The polarized laser
beam was focused on the sample by means of a positive lens
(f ¼ 5 cm), so that a light power density of approximately
8.53 ꢁ 106 W mꢀ2 reached the studied sample at the focal
spot. At last, the samples were mounted on a motorized
translation stage (25 mm length travel in steps of 2 lm) to
perform Z-Scan experiments within the optical focal range. A
large area Si-photodetector (EOT ET-2040) was located at
ꢄ0.96 m from the focusing lens, after a 2.5-mm diameter
(20% transmittance) diaphragm-aperture. All NLO-signals
captured from photodetectors were measured with a digital
oscilloscope (Tektronix TDS, 744A), and all motion systems
and set-up management were automated via a LabView con-
trol program.
IR (KBr): m ¼ 3090 (s, CAH aromatic and vinylic), 2961
(s, CH2), 2924 (s, CH2 and CH3), 1727 (s, C¼¼O), 1603 (s,
C¼¼C aromatic), 1516 (s, NO2), 1447 (s, N¼¼N), 1378, 1337
(s, CAO of the esther), 1261(s, CAN), 1099 (s, OACH2), 856
(out of plane, ¼¼CH2 vinylic), 802 (out of plane, ¼¼CAH aro-
matic) cmꢀ1
.
1H-NMR (CDCl3, 400 MHz): d ¼ 8.33 (d, 2H, J ¼ 9.06 Hz,
H4); 7.93 (d, 2H, J ¼ 9.06 Hz, H3); 7.91 (d, 2H, J ¼ 8.81 Hz,
H2); 6.83 (d, 2H, J ¼ 9.32 Hz, H1); 6.12 (s, 2H, H5 J1 ¼ 1.76);
5.60 (s, 1H, H6, J1 ¼ 1.51); 4.38 (t, 2H, J1 ¼ 6.29 Hz, J2
¼
6.29 Hz, COOACH2); 3.74 (t, 2H, J1 ¼ 6.29 Hz, J2 ¼ 6.29,
NCH2), 3.56 (t, 2H, J1 ¼ 7.05 Hz, J2 ¼ 7.05 NCH2CH3); 1.95
(s, 3H, CH3 of methacrylate); 1.27 (t, 3H, J1 ¼ 7.05 Hz, J2
7.05, NCH2ACH3) ppm.
¼
FP Experiments
In a glass test tube (16 cm length, 16 mm inner diameter),
suitable amounts of PEGDA, MDR-1, and BPO were mixed
until all BPO was completely dissolved (Table 1). Caution: FP
experiments carried out with more than 7.1 mol % BPO
resulted in test tube explosion!
The test tubes containing the mixture were locally heated at
the top level of the solution, using a soldering iron as heat-
ing source, until the formation of a propagating front was
observed. The heat released during the conversion of the
monomer into polymer was responsible for the formation of
a hot polymerization front, able to self-sustain, and propa-
gate throughout the whole tube. Front velocity (60.05 cm
minꢀ1) and front maximum temperature (615 ꢃC) were
recorded.
CONCLUSIONS
For the first time, the frontal copolymerization of an azo-
monomer was carried out. MDR-1 was copolymerized with
PEGDA. The influence of the amounts of MDR-1 and initiator
was studied to determine the optimum concentration range
in which FP can occur. Azo-polymers containing up to 0.75
mol % of MDR-1 were successfully obtained. Such concentra-
tion is good enough to confer their interesting NLO proper-
ties. The UV–Vis spectra of the polymers exhibit a significant
blue shift of the absorption bands with respect to MDR-1
monomer, due to the formation of H-aggregates. Outstanding
cubic NLO effects were measured in the PEGDA/MDR-1 co-
polymer sample with higher MDR-1 content (0.75 mol %)
via the Z-Scan technique, where high NLO-refractive coeffi-
cients in the order of 10ꢀ4 esu were found. This remarkable
NLO activity was mainly due to the MDR-1 azo-compound as
the reference PEGDA sample did not exhibited any NLO-
response at same experimental conditions, acting only as an
Characterization
Temperature profiles were measured using a K-type thermo-
couple placed into the monomer mixture at 2 cm (60.5 cm)
from the bottom of the tube. It was connected to a digital
thermometer (Delta Ohm 9416) for temperature reading and
recording. The position of the front, easily visible through
the glass walls of the tube, was measured as a function of
the time.
DSC measurements were conducted in a DSC Q100 Waters
TA Instrument. For each sample, two consecutive scans were
carried out from ꢀ80 to þ300 ꢃC with a heating rate of 10
ꢃC minꢀ1, under argon atmosphere. Monomer conversion
3296
WILEYONLINELIBRARY.COM/JOURNAL/JPOLA

ARTICLE
17 Scognamillo, S.; Bounds, C.; Luger, M.; Mariani, A.; Pojman,
adequate mechanical hosting matrix. In addition, the NLO-
absorptive response of the studied material was established
as a multiphoton process. However, more NLO investigations
should be performed in these materials to further under-
stand the electronic and thermal contributions to the cubic
nonlinearities. Additionally, complementary studies on the
MDR-1 chromophore loading in these kinds of PEGDA-based
copolymers and other systems should also be performed
order to improve both the NLO-response and thermal prop-
erties for stable NLO-applications (including quadratic NLO-
effects for eclectically poled film samples). Some of these
experiments are currently underway and will be presented
in a future work.
J. A. J Polym Sci Part A: Polym Chem 2010, 48, 2000–2005.
18 Jimenez, Z.; Pojman, J. A. J Polym Sci Part A: Polym Chem
2007, 45, 2745–2754.
19 Fiori, S.; Malucelli, G.; Mariani, A.; Ricco, L.; Casazza, E.
e-Polymers 2002, 057, 1–10.
20 Mariani, A.; Fiori, S.; Bidali, S.; Alzari, V.; Malucelli, G.
J Polym Sci Part A: Polym Chem 2008, 46, 3344–3351.
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2003, 36, 2674–2679.
22 Mariani, A.; Bidali, S.; Fiori, S.; Malucelli, G.; Sanna, E.
e-Polymers 2003, 044, 1–9.
We thank Miguel Angel Canseco and Gerardo Cedillo for their
assistance with absorption and NMR spectroscopies, respec-
tively. Javier Illescas and Yessica S. Ramı´rez-Fuentes are grate-
ful to CONACyT for scholarship. We also thank Henkel Co., the
Instituto de Ciencia y Tecnologı´a del Distrito Federal (ICyTDF),
and the Italian Ministry of University and Scientific Research
for financial support.
23 Mariani, A.; Bidali, S.; Caria, G.; Monticelli, O.; Russo, S.;
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