Nonlinear optical hyperbranched polyaspartimide/montmorillonite nanocomposites based on reactive fluorine- or phosphorous-containing organoclays

Article abstract of DOI:10.1016/j.polymer.2013.05.040

A series of novel hyperbranched polyaspartimide/organoclay nanocomposites were developed for second-order nonlinear optics. The hyperbranched polyaspartimides were synthesized from an azobenzene chromophore bis(4-aminophenyl(4-(4-nitrophenyl)-diazenyl)phenyl)amine (DAC; A₂ type monomer) and a 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide- containing trimaleimide (Dopo-trimaleimide; B₃ type monomer) via Michael addition reaction. Moreover, reactive fluorine- and phosphorous- containing organoclays (F-montmorillonite, i.e. F-MMT and P-montmorillonite, i.e. P-MMT) which had been prepared by modifying the high aspect ratio silicate layers of MMTs with two different swelling agents 1,1,1-tris[4-(4-amino-2- trifluoromethylphenoxy)-phenyl]ethane (F-triamine) and (9,10-dihydro-9-oxa-10- phosphaphenanthrene-10-yl)tris(4-aminophenyl)methane (P-triamine) were respectively incorporated into the nonlinear optical hyperbranched polyaspartimides. The organoclays were exfoliated and dispersed in the hyperbranched polyaspartimide films. The incorporation of exfoliated organoclay F-MMT could effectively enhance the electro-optical (EO) coefficients and temporal stability of the hyperbranched polyaspartimides. The hyperbranched polyaspartimide/F-MMT nanocomposite exhibited temporal stability (r ₃₃(t)/r₃₃(0) = 73%) at 100 C, EO coefficient r ₃₃ of 20.9 pm/V, and optical loss of 3.0 dB/cm at 1310 nm.

Full text of DOI:10.1016/j.polymer.2013.05.040

Polymer 54 (2013) 3850e3859
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Nonlinear optical hyperbranched polyaspartimide/montmorillonite
nanocomposites based on reactive fluorine- or
phosphorous-containing organoclays
,
a
Ya-Yu Siao a, Shi-Min Shau a, Shu-Hui Hu a, Rong-Ho Lee a , Ching-Hsuan Lin ,
*
,
Jeng-Yue Wu a, Ru-Jong Jeng b
**
a Department of Chemical Engineering, National Chung Hsing University, Taichung 402, Taiwan
b Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel hyperbranched polyaspartimide/organoclay nanocomposites were developed for
second-order nonlinear optics. The hyperbranched polyaspartimides were synthesized from an azo-
benzene chromophore bis(4-aminophenyl(4-(4-nitrophenyl)-diazenyl)phenyl)amine (DAC; A2 type
Received 1 March 2013
Received in revised form
11 May 2013
Accepted 15 May 2013
Available online 23 May 2013
monomer) and
a 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide-containing trimaleimide
(Dopo-trimaleimide; B3 type monomer) via Michael addition reaction. Moreover, reactive fluorine- and
phosphorous-containing organoclays (F-montmorillonite, i.e. F-MMT and P-montmorillonite, i.e. P-MMT)
which had been prepared by modifying the high aspect ratio silicate layers of MMTs with two different
swelling agents 1,1,1-tris[4-(4-amino-2-trifluoromethylphenoxy)-phenyl]ethane (F-triamine) and (9,10-
dihydro-9-oxa-10-phosphaphenanthrene-10-yl)tris(4-aminophenyl)methane (P-triamine) were respec-
tively incorporated into the nonlinear optical hyperbranched polyaspartimides. The organoclays were
exfoliated and dispersed in the hyperbranched polyaspartimide films. The incorporation of exfoliated
organoclay F-MMT could effectively enhance the electro-optical (EO) coefficients and temporal stability
of the hyperbranched polyaspartimides. The hyperbranched polyaspartimide/F-MMT nanocomposite
exhibited temporal stability (r33(t)/r33(0) ¼ 73%) at 100 ꢀC, EO coefficient r33 of 20.9 pm/V, and optical
loss of 3.0 dB/cm at 1310 nm.
Keywords:
Polyaspartimide
Organoclay
Nonlinear optics
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
devices [5], organic solar cells [6], supramolecular assembly [7],
and nanoscale catalysis [8].
Hyperbranched polymers with three-dimensional (3D) den-
dritic architectures have attracted great attention because of their
unique properties including low viscosity, good solubility, and
multiple functionality, and greater availability relative to the den-
drimers [1,2]. Hyperbranched polymers prepared by one-pot
polymerization of ABn monomers, difunctional monomer (A2) and
trifunctional monomer (B3), or other systems are suitable for large-
scale production. These hyperbranched polymers have been
intensively explored in many areas, including intercalating agent
for layered materials [3], drug delivery [4], organic light-emitting
Recently, the research field of hyperbranched polymers has also
been extended to second-order nonlinear optical (NLO) polymer
field due to their 3D dendritic architectures [9e11]. In general, high
chromophore contents and the dipoleedipole strong intermolec-
ular electrostatic interactions would bring about the decreased
poling efficiency of the NLO polymers. The 3D spatial separation of
the NLO chromophores endows the hyperbranched polymers with
favorable site-isolation effect [12], which could minimize the
strong intermolecular electrostatic interactions among the NLO
chromophores with high dipole moment, and thus enhance the
optical nonlinearities. Moreover, these hyperbranched polymers
with globular shape conformation can be used to improve the
solubility and processability, and facilitate the alignment of NLO
chromophores during the electric-field poling process [13]. Apart
from that, their void-rich topological structure could minimize the
optical losses in the NLO polymeric systems [14]. Earlier in 1997,
Zhang et al. first prepared a hyperbranched polymer with a second
* Corresponding author. Tel.: þ886 4 22854308; fax: þ886 4 22854734.
** Corresponding author. Tel.: þ886 2 33665884; fax: þ886 2 33665237.
E-mail addresses: rhl@dragon.nchu.edu.tw (R.-H. Lee), rjong@ntu.edu.tw
(R.-J. Jeng).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.05.040

Y.-Y. Siao et al. / Polymer 54 (2013) 3850e3859
3851
harmonic coefficient, d33 of 2.8 pm/V, which might be due to the
poor poling efficiency of the NLO chromophores under electric field
[15]. The rigid NLO chromophores played the role of branching unit,
which resulted in the poor poling efficiency of the NLO polymers. A
series of guest-host polymers based on the blend of a crosslinkable
hyperbranched NLO oligomer and poly(ether sulfone) were re-
ported by Wang et al. [16]. A large d33 value (65 pm/V) and good
temporal stability at 85 ꢀC were obtained for these poled and
crosslinked polymer blends. Tang et al. developed an azo-
functionalized hyperbranched poly(aryleneethynylene) exhibiting
large d33 values (177 pm/V at 1064 nm) and excellent thermal
stability (no decay in d33 when heated to 152 ꢀC) [17]. In addition,
Chao et al. reported a series of A2 þ B3 hyperbranched NLO poly-
mers through the ring-opening addition reactions of azetidine-2,4-
dione that showed better NLO properties than their corresponding
linear analogs [18]. Shi and co-workers synthesized a rather rigid
A2 þ B3 hyperbranched polytriazole using click chemistry. The
effective relaxation temperature (Tert) of this polytriazole was ca.
165 ꢀC [19]. Moreover, Li et al. reported two new azobenzene dye-
containing hyperbranched polytriazoles derived from AB2 mono-
mers via click chemistry [20]. The Terts of the polytriazoles via dy-
namic temperature scanning were lower than 120 ꢀC. Later on, the
Tert was improved up to as high as 153 ꢀC for a hyperbranched
polyaryleneethynylene prepared through an A2 þ B3 approach via
Sonogashira coupling reaction [21]. Apart from that, several efforts
have been made to develop the NLO hyperbranched polymers with
large NLO coefficients [22e25]. It was found that the presence of 3D
dendritic structure was favorable for the enhancement of EO co-
efficients of the hyperbranched polymers. However, there is still
room for improvement for the long-term NLO stability of the
hyperbranched polymeric systems at elevated temperatures.
In order to enhance the temporal stability of NLO polymers,
Odobel and co-workers designed and synthesized two hyper-
branched polymers end-capped with azobenzene chromophores
through post-functionalization [26]. The synthetic strategy pro-
vided a new way to covalently attach the NLO chromophores to the
polymers, which could feasibly tune the thermal and NLO proper-
ties of the materials through chemical structure modifications. A
crosslinkable hyperbranched polyimide which combines the ben-
efits of a high Tg, good solubility, and high poling efficiency was
obtained for a post-poling crosslinking strategy [27]. Another
approach is to introduce an inorganic component such as the SiO2
nanoparticles [28e31], montmorillonites (MMTs) [32e34], or
phosphine oxides into the organic NLO materials, yielding organic/
inorganic hybrid materials [35]. Previously, we reported a series of
polyimide/MMT [32,33] and poly(amide-imide)/MMT [34] nano-
composites comprising NLO moieties, which showed much better
long-term stability at elevated temperatures than those of the
pristine polymers. In addition, the NLO-active arylphosphine ox-
ides were incorporated into the polymer matrices to obtain an NLO-
active guest-host system with low optical losses [35]. Polymers
comprising arylphosphine oxides with excellent thermal stability
would prevent the decomposition of materials during the poling
process at elevated temperatures.
In this study, a series of novel hyperbranched polyaspartimides
were prepared through the Michael addition reaction between
amine- and maleimide-containing compounds, which exhibited
more favorable properties in the aspect of processing than did the
condensation-type polyimides. The NLO hyperbranched poly-
aspartimides were synthesized in one-pot polymerization between
a
difunctional azobenzene dye, bis(4-aminophenyl(4-(4-nitro
phenyl)-diazenyl)-phenyl)amine (DAC; A2 type monomer) and a
trifunctional 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-ox
ide-containing trimaleimide (DOPO-trimaleimide; B3 type mono-
mer) via A2þB3 approach, using p-toluenesulfonic acid (p-TSA) as
catalyst. The thermal stabilities of the hyperbranched poly-
aspartimides would be further enhanced via the incorporation of
the DOPO moieties [36]. The DOPO-trimaleimide was synthesized
from the reaction between maleic anhydride and DOPO-containing
triamine (P-triamine) and subsequent imidization. Moreover, the P-
triamine with multi-functional groups was chosen as the inter-
calating agent for MMTs. A reactive organoclay P-MMT was pre-
pared by the intercalation of P-triamine into MMTs. In addition,
another reactive organoclay F-MMT was also prepared by the
intercalation of
a fluorine-containing triamine, 1,1,1-tris[4-(4-
amino-2-trifluoromethylphenoxy)-phenyl]ethane (F-triamine) in
to MMTs [32]. Furthermore, the hyperbranched polyaspartimide/
organoclay based nanocomposites were prepared through the
incorporation of F-MMT or P-MMT into the polymeric matrices.
One of the amino groups from the intercalating agent would form
(a)
CH3
C
CF3
F3C
CH3
C
F3C
CF3
CH3
C
HO
OH
O2N
O
O
NO2
NH2
H2N
O
O
THPE
OH
K2CO3
125 oC, DMAc
10% Pd/C, Hydrazine
80 oC, EtOH
O
O
CF3
+
CF3
F3C
Cl
NO2
Yield: 82.9%
NO2
NH2
Yield: 87.7%
F-trinitro
2-chloro-5-nitrobenzo-trifluoride
F-triamine
(b)
O
N
Maleic anhydride
H
P O
O
O
P O
O
O
P
O
Et3N
DOPO
O
O
NH2
O
NH2
NH2
+
(CH3CH2O)2O, CH3COON a
70 oC
O
H2N
DMAc, RT, 18hr
N
O
ClN
N
NH2
O
O
Yield: 78.5%
%
Yield: 92.5
P-triamine
DOPO-trimaleimide
Pararosaniline Chloride
Scheme 1. Synthesis of the F-triamine, P-triamine, and DOPO-trimaleimide.

3852
Y.-Y. Siao et al. / Polymer 54 (2013) 3850e3859
an ionic bond with negatively charged silicates, whereas the rest of
the two amino groups in the swelling agent are available for further
reaction with the maleimide end groups of the hyperbranched
polymer. As a result, the layers of organoclays would be exfoliated
and dispersed in the polymeric matrices. Better temporal stability
was obtained for these NLO-active hyperbranched polyaspa
rtimide/organoclay based nanocomposites.
N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF),
tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), acetone, chlo-
roform, methanol (MeOH), etc. were available from Tedia Chemical
Co. All reagents and solvents are reagent grade. DMAc was purified
through distillation under reduced pressure over CaH2 and then
þ
ꢀ
stored over 4 A molecular sieves. Montmorillonite, Na -MMT was
supplied by Nanocor Co., is a sodium type with a cationic exchange
capacity (CEC) of 1.20 mequiv gꢁ1 and a surface area of 750 m2 gꢁ1
.
2. Experimental
Synthesis of the F-triamine, P-triamine, DOPO-trimaleimide, NLO-
active hyperbranched polyaspartimides, and polymer/organoclay
nanocomposites are presented in Schemes 1 and 2. NLO chromo-
phore DAC and F-triamine were synthesized according to our pre-
vious studies [32,37]. Details of the synthetic procedures are shown
as follows:
2.1. Materials
All chemicals were purchased and used as received unless
otherwise stated. All reactions were carried out under nitrogen. N,
O
N
NH2
O
P
O
O
N
+
O
N
N
NH2
N
O
O2N
N
O
O
DAC
(A2 monomer)
DOPO-trimaleimide
(B3 monomer)
p-TSA 120 oC, DMAc
HB11
R:
HB32
HB21
Hyperbranched polymers
P
O
O
P-trimine
+
NH3
or
p-TSA
120 oC, DMAc
R
F3C
O
CF3
O
CH3
C
NH2
NH2
F-MMT or P-MMT
O2N
O
CF3
N
N
H
N
F-triamine
N
NO2
+ H3N
NH
R
NH
HN
O
N
N
O
O
O
N
N
O
O
O
+
O
NH3
R
N
N
O
N
P
N
NH
R
N
H
NH
N
HN
P
O
O
O
O
N H
O
NH
O
O
N
+
N
NH3
O
O
O
O
HN
HN
HN
O
O
H
N
P
O
N
N
N
N
O
N
NO2
N
N
O2N
Scheme 2. Synthesis of the NLO-active hyperbranched polymers and corresponding nanocomposites.

Y.-Y. Siao et al. / Polymer 54 (2013) 3850e3859
3853
2.2. Monomer synthesis
2.4. MMT intercalated with F-triamine and P-triamine
2.2.1. Synthesis of P-triamine
An example of a preparative procedure for preparing layered
silicates with wide d-spacings is shown as follows: Naþ-MMT
(1 g, 1.20 mequiv gꢁ1) was placed in a 500 mL flask and dispersed
vigorously into 100 mL of de-ionized water at room temperature.
In a separate vessel, the precursor intercalating agent (F-tri-
amine; 6.04 g, 1.20 mmol) was acidified with hydrochloric acid
(37% in water, 0.18 g, 1.20 mmol, acidification ratio of
Hþ/NH2 ¼ 1/3) in 100 mL of ethanol. The solution was poured
into the flask containing the swelled Naþ-MMT slurry. The
mixture was stirred vigorously at 60 ꢀC for 11 h and then allowed
to cool to room temperature. The resulting agglomerated
precipitate was collected and washed thoroughly with de-
ionized water to remove any residual ions or free intercalating
agents. MMT/F-triamine was dried in a vacuum oven at 70 ꢀC.
Organoclay MMT/P-triamine was prepared in a similar manner.
The MMTs were intercalated by fluorine- or phosphorous-
containing triamine that are designated as F-MMT and P-MMT,
respectively.
Triethylamine (15.15 g, 0.15 mol) was added to a solution of
pararosaniline chloride (16.20 g, 0.05 mol) in ethanol (100 mL).
The solution was stirred at room temperature under a N2 atmo-
sphere for 2 h. Subsequently, DOPO (10.9 g, 0.15 mol) was added.
The mixture was heated under reflux for 12 h and then cooled to
room temperature. The resulting solution was then poured slowly
into a mixture of water and MeOH (1:1)ꢀ. The precipitate was
filtered off and dried under vacuum at 60 C to yield the purple
product P-triamine (78.5%). IR (KBr): 1193 cmꢁ1 (P]O), 3448 and
3332 cmꢁ1 (eNH2). 1H NMR (400 MHz, d-DMSO):
d
(ppm) ¼ 8.01e
8.11 (dd, 1H, P-Ar-H), 7.88e7.90 (d, 1H, Ph-Ar-H), 7.66e7.71 (t, 1H,
Ar-H), 7.29e7.36 (m, 2H, Ar-H), 7.22e7.26 (t, 1H, Ar-H), 7.08e7.13
(t, 1H, Ar-H), 6.96 (br, 6H, Ar-H), 6.90e6.93 (d, 1H, O-Ar-H), 6.35e
6.38 (d, 6H, H2N-Ar-H), 5.01 (s, 6H, NH2). Anal. Calcd. for
C31H26N3O2P (502.97): C, 73.96%; H, 5.17%; N, 8.35%; O, 6.36%.
Found: C, 74.12%; H, 5.23%; N, 8.27%; O, 6.45%. Td (TGA in
N2) ¼ 345 ꢀC. Tm (DSC) ¼ 324 ꢀC.
2.2.2. Synthesis of DOPO-trimaleimide
2.5. Synthesis of hyperbranched polyaspartimide/organoclay
nanocomposites
P-triamine (1.51 g, 0.3 mmol) was first dissolved in 20 mL
DMAc in a 100 cm3 flask. After the compound was completely
dissolved, maleic anhydride (1.77 g, 1.8 mmol) was added. The
solution was stirred at room temperature for 18 h, and then
acetic anhydride (2.86 g, 2.8 mmol) and sodium acetate (0.41 g,
0.5 mmol) were added to the solution at 70 ꢀC. The solution was
stirred for another 1.5 h. The product was purified by
re-precipitations into methanol, and dried under vacuum.
The precipitate was filtered off and dried under vacuum at 60 ꢀC
to yield the yellowish white product (92.5%). IR (KBr):
1213 cmꢁ1 (P]O), 1716 and 1772 cmꢁ1 (eC]O of maleimide),
3094 cm-1 (eC]CeH of maleimide). 1H NMR (400 MHz,
Hyperbranched polymer/organoclay nanocomposites consisting
of azobenzene dyes were prepared as the following manner: a
suspension solution of MMT was obtained by stirring the modified
MMT (F-MMT or P-MMT; 0.15 g or 0.23 g) in DMAc (0.5 mL) for 12 h.
The suspension was then added to hyperbranched polymer (HB11;
1 g) in DMAc to form hyperbranched polymer/organoclay samples
with different weight ratios (1, 3 and 5 wt% of organoclay P-MMT).
These mixtures were heated to 120 ꢀC and reacted under a dry
nitrogen atmosphere for 78 h, using p-TSA (5 wt%) as catalyst. The
product was obtained by re-precipitations of the mixture into
methanol, and purified and dried in vacuum at 60 ꢀC for 12 h. The
HB11/F-MMT samples with 1, 3 and 5 wt% of F-MMT are designated
as HBF1, HBF3 and HBF5, respectively. In addition, the HB11/P-MMT
samples with 1, 3 and 5 wt% of P-MMT are designated as HBP1,
HBP3 and HBP5, respectively.
d-DMSO):
d
(ppm) ¼ 8.03-8.05 (dd, 1H, P-Ar-H), 7.72e7.79 (m,
2H, Ph-Ar-H), 7.40e7.52 (m, 3H, Ar-H), 7.34-7.37 (t, 1H, Ar-H),
7.17e7.22 (d, 6H, Ar-H), 7.13e7.17 (d, 6H, maleimide; CH]CH),
6.90-6.98 (d, 1H, Ar-H). Anal. Calcd. for C43H26N3O8P (742.97): C,
69.45%; H, 3.50%; N, 5.65%; O, 17.23%. Found: C, 70.28%; H,
3.42%; N, 5.78%; O, 16.95%. 13C NMR (DMSO-d6,
d)：62.6 (d,
J ¼ 92.3 Hz), 118.9, 120.7, 121.2 (d, J ¼ 110.6 Hz), 123.6, 124.4,
2.6. Instruments
125.6, 126.3, 128.0 (d, J ¼ 12.2 Hz), 130.0 (d, J ¼ 4.8 Hz), 130.2,
130.9, 131.9 (d,
150.7 (d, J ¼ 10.9 Hz), 169.7 (d, J ¼ 9.2 Hz). Td (TGA in
J
¼
9.2 Hz), 133.9, 134.7, 138.8, 141.9,
IR measurements were performed on a Fourier transform
infrared (FTIR) spectrometer (PerkinElmer Spectrum). 1H NMR
spectra were obtained with a Varian Gemini-400 using d-DMSO as
solvent. Elemental analysis was performed on a Heraeus CHN-OS
Rapid Analyzer. Gel permeation chromatography (GPC) was per-
formed in N,N-dimethylformamide (DMF) as eluant with a Waters
Apparatus equipped with Waters Styrogel columns with a refrac-
tive index (RI) detector and polystyrene calibration. UVevis
spectra were recorded on a Perkin Elmer Lambda 2S spectropho-
tometer to measure the dye contents. Differential scanning calo-
rimetry (DSC) and thermogravimetric analysis (TGA) were
performed on a Seiko SII model SSC/5200 at a heating rate of
10 ꢀC/min under nitrogen. Thermal degradation temperature (Td)
is taken at the position of 5% weight loss. Tg and melting tem-
perature (Tm) were measured at the second heating. The d-spacing
of the intercalated MMT was analyzed by an X-ray powder
diffractometer (XRD, Shimadzu SD-D1 using a Cu target at 35 kV,
30 mA). The d-spacing of the intercalated MMT was analyzed by
N2) ¼ 448.5 ꢀC.
2.3. Synthesis of hyperbranched polyaspartimides
Hyperbranched polymers HB11, HB23 and HB12 comprising
different contents of DAC were synthesized by reacting the
difunctional azobenzene dye DAC with DOPO-trimaleimide at
different molecular ratios. The trimaleimide was dissolved in
20 mL of DMF. After complete dissolution, the difunctional
compound DAC was added to the solution. The concentrations of
all of the reactants were kept well below 0.1 M to avoid the
gelation of reaction mixture. These mixtures were heated to
120 ꢀC and reacted under dry nitrogen atmosphere for 78 h,
using p-TSA (10 wt%) as catalyst. The product was obtained by re-
precipitations in methanol, and purified and dried in vacuum at
60 ꢀC for 12 h 1H NMR analysis of HB12 is shown as follows: 1H
NMR (400 MHz, d-DMSO):
d
(ppm) ¼ 8.38e8.40 (d, NO2-Ar-H),
using Bragg’s equation (n
l
¼ 2 d sin ). The value for n ¼ 1 was
q
7.72e7.86 (m, eN]N-Ar-H), 7.68e7.70 (m, -Ar-H), 7.10e7.60 (m,
-Ar-H), 6.80e7.02 (m, -N-Ar-H, -Ar-H), 6.58e6.65 (m, -N-Ar-H).
Moreover, 1H NMR spectra of HB11 and HB23 were similar to that
of HB12.
calculated from the observed values for n ¼ 2, 3, 4, etc. Trans-
mission electronic microscopy (TEM) was performed on a Zeiss EM
902A and operated at 80 kV and samples of approximately 70 nm
were microtomed at room temperature.

3854
Y.-Y. Siao et al. / Polymer 54 (2013) 3850e3859
2.7. Thin film preparation and EO properties measurements
These hyperbranched polyaspartimide/organoclay nano-
composites were respectively dissolved in DMF (concentration
w10 wt%). The polymer solution was stirred at room temperature
(a) F-trinitro
Ph-O-Ph
NO
2
for 3 h and filtered through a 0.45 mm syringe filter. Thin films were
prepared by spin-coating the filtered polymer solution onto indium
tin oxide (ITO) glass substrates. Prior to the poling process, these
thin films were dried in vacuum at 60 ꢀC for 24 h. The poling
process for the second-order NLO polymer films was carried out
using an in situ contact poling technique (poling voltage: 100 V;
kept at approximately 10 ꢀC lower than the Tg of the sample). EO
coefficients (r33) of the poled samples were measured at 830 nm
using the simple reflection technique [38]. For the optical loss
measurement, a laser beam (830 nm) passed through a linear
polarizer and a polarizing beam splitter. Subsequently, the TE
polarized light was focused onto a prism coupler that was mounted
on a rotation stage. The scattered light was imaged with an
infrared-sensitive charge-injection-device camera system. A sta-
tistical linear fit of the data to the logarithm of the scattered light
intensity versus the distance propagated down the waveguide
yielded a slope as the optical loss.
(b) F-triamine
NH
2
(c) P-triamine
P=O
NH2
C=CH
(d) DOPO-trimaleimide
maleimide
P=O
4000 3500 3000 2500 2000 1500 1000
500
-1
Wavenumber (cm )
Fig. 1. FTIR spectra of (a) F-trinitro, (b) F-triamine, (c) P-triamine, and (d) DOPO-
trimaleimide.
3. Results and discussion
d
¼ 5.01 ppm was absent for DOPO-trimaleimide. Furthermore, the
chemical structures of these monomers are also confirmed by
elemental analysis. 13C NMR spectrum of the DOPO-trimaleimide is
shown in Figure S1 (Supporting Information).
3.1. Synthesis and characterization of monomers
As shown in Scheme 2, a series of NLO hyperbranched poly-
aspartimides and polyaspartimide/F-MMT (or P-MMT) nano-
composites were synthesized via Michael addition reaction. In
comparison with the preparation of dendrimers via a convergent or
divergent route, the one-pot synthesis of hyperbranched polymers
is drastically simplified. The difunctional NLO chromophore DAC
was synthesized by the coupling of p-nitrosonitrobenzene and
tris(4-aminophenyl)amine according to the methods described in
the literature [37]. This chromophore possesses a Tm at 240 ꢀC and a
high Td at 300 ꢀC.
To serve as the intercalating agent, we synthesized the fluorine-
and phosphorus-containing triamine (F-triamine and P-triamine)
according to Scheme 1. According to the literature [32], 1,1,1-
trishydroxy phenyl ethane (THPE) was reacted with 2-chloro-5-
nitrobenzotrifluoride in the presence of K2CO3 and DMAc to yield
the corresponding trinitro compound (F-trinito), which we then
utilized Pd/C as catalyst to produce the fluorine-containing tri-
amine (F-triamine; Scheme 1(a)). FTIR spectra of F-trinitro and F-
triamine are shown in Fig. 1(a) and (b), respectively. As for
phosphorus-containing triamine, P-triamine was prepared by the
reaction of DOPO and pararosaniline chloride in the presence of
ethylene triamine (Scheme 1(b)) [36]. In Fig.1(c), the spectrum of P-
triamine displayed strong characteristic absorption for phosphine
oxide (1193 cmꢁ1) and amino group (3432 and 3448 cmꢁ1; eNH2)
In this work, the P-triamine was further imidized with maleic an-
hydride to provide a B3 monomer, i.e. DOPO-trimaleimide with an
excellent yield (>90%). Fig. 1(d) shows the FTIR spectrum of DOPO-
trimaleimide. The performance of the imidization was confirmed
by the formation of imide groups with characteristic absorption
bands at 1716 and 1772 cmꢁ1. The disappearance of the eNH2 ab-
sorption peak and the presence of the C]CeH absorption peak at
3094 cmꢁ1 also provided the evidence of imidization. In addition,
Fig. 2 showed the 1H NMR spectra of P-triamine and DOPO-
trimaleimide. The amine absorption peak was observed at
3.2. Synthesis and characterization of hyperbranched
polyaspartimides and their corresponding polyaspartimide/MMT
nanocomposites
In this study, compound DAC with diamine group was utilized as
the chromophore that is capable of performing Michael addition
reaction toward trimaleimide at high temperatures, using p-TSA as
the catalyst. We prepared amine-terminated hyperbranched poly-
aspartimide by employing diamine-to-trimaleimide with feed
molar ratios of 3:2, 1:1, and 2:1. In Scheme 2, these three hyper-
branched polyaspartimides (HB11, HB32 and HB21) were obtained
via the reaction of DAC (A2 type monomer) and DOPO-trimaleimide
(B3 type monomer). Chemical structures of the hyperbranched
polyaspartimides were investigated using FTIR spectroscopy
(Fig. 3). The absorption of primary amine was located at 3250e
3500 cmꢁ1 for DAC, while a broad band corresponding to the ab-
sorption of secondary amine was appeared at 3250e3700 cmꢁ1 for
HB11, HB32 and HB21. Moreover, the absorption peaks of the imide
(C]O; 1772, 1716 cmꢁ1) and P]O (1193 cmꢁ1) groups were also
observed. Table 1 summarizes the average molecular weights of the
hyperbranched polymers as analyzed by GPC. The weight average
molecular weights (Mw) of the hyperbranched polymers were in
the range 9510e10,706 g molꢁ1, with polydispersity (PDI) values in
the range of 1.8e1.9. Basically, the functional groups ratio of 1:1
would lead to the highest Mw among the hyperbranched polymers,
and possibly gelation. In order to avoid the gelation of the hyper-
branched polymer, the concentration of the reactants was kept well
below 0.1 M. Consequently, the difference of the Mw values is
insignificant for HB32, HB11, and HB21. We also used TGA to
measure the Tds and char yields of these hyperbranched polymers
(Table 2). The DOPO-trimaleimide exhibited excellent thermal
stability at about 448.5 ꢀC. This is because DOPO-trimaleimide
would form a densely crosslinked network while the sample was
heated to the temperatures higher than 220 ꢀC in the TGA thermal
chamber [28,39], indicating that the observed high Td (448 ꢀC) was
actually derived from the densely crosslinked maleimide polymer.
d
¼ 5.01 ppm for P-triamine (Fig. 2(a)). The characteristic peak at
d
¼ 7.13 ppm, assigned to the proton absorption of C]CeH of the
maleimide ring, was observed in the 1H NMR spectrum of DOPO-
trimaleimide (Fig. 2(b)). The amine absorption peak at

Y.-Y. Siao et al. / Polymer 54 (2013) 3850e3859
3855
H2O
c
a
d
f
(a) P-triamine
e
b
g
h
i
j
k
k
j
DMSO
c, d
i
g
h
e
b
a
8
f
9
5
4
ppm
0
7
6
3
2
1
f
(b) DOPO-trimaleimide
e
g
d
c
h
k
b
a
j
i
H2O
i, j, k
c, d, g
DMSO
f
h
a e,b
9
5
4
ppm
0
8
7
6
3
2
1
Fig. 2. 1H NMR spectra of (a) P-triamine and (b) DOPO-trimaleimide.
Table 1
Compositions and average molecular weights of the NLO-active hyperbranched
polymers.
DAC
Sample
Feed molar ratio
Mw
Mn
PDI
(DAC/DOPO-trimaleimide)
-NH
HB11
HB32
HB21
1/1
3/2
2/1
9510
9397
10,706
5056
5130
5702
1.9
1.8
1.9
2
HB11
The molecular weight distribution was determined by gel permeation chromatog-
raphy with polystyrene standards in DMF.
HB21
HB32
On the other hand, with the introduction of DAC, the Tds of the
resultant hyperbranched polymers were observed at the range of
272e308 ꢀC with the char yields greater than 40% at 700 ꢀC
(Figure S2, Supporting Information). As shown in Figure S3
(Supporting Information), similar Tg values were vaguely
observed for HB11 (202.1 ꢀC) and HB21 (194 ꢀC), whereas the Tg of
HB32 was not detectable from DSC. On the other hand, the UVevis
absorption spectra of hyperbranched polyaspartimides (HB11,
HB32 and HB21) are shown in Figure S4 (Supporting Information).
The maximal absorption wavelengths of the NLO polymers were
observed at ca. 501 nm whereas the cut-off wavelengths of the
polymers were located at ca. 780 nm.
-NRH
C=O
NO
P=O
2
4000 3500 3000 2500 2000 1500 1000
500
-1
Wavenumber
(
cm
)
Fig. 3. FTIR spectra of DAC, HB11, HB21, and HB32.

3856
Y.-Y. Siao et al. / Polymer 54 (2013) 3850e3859
Table 2
Thermal properties of the hyperbranched polymers and corresponding
nanocomposites.
a
b
Sample
Intercalation
agent
Contents of
organoclay (wt%)
Tg (ꢀC)
Td (ꢀC)
HB11
HB32
HB21
HBF1
HBF3
HBF5
HBP1
HBP3
HBP5
e
e
e
e
1
3
5
1
3
5
202.1
N.D.
194.0
212.1
230.7
219.0
155.2, 205.7
176.3, 239.7
181.5, 228.5
308.2
308.5
272.3
369.9
372.6
346.8
349.4
349.0
367.5
e
e
F-MMT
F-MMT
F-MMT
P-MMTc
P-MMTc
P-MMTc
a
Baseline shift in the second heating DSC traces, with a heating rate of 10 ꢀC/min
in N2.
b
Temperatures at which 5% weight loss were recorded by TGA at a heating rate of
10 ꢀC/min in N2.
c
Two glass transitions were observed for the HB11/P-MMT samples.
As shown in Scheme 2, organoclays F-MMT and P-MMT were
prepared by intercalating MMTs with the swelling agents F-tri-
amine and P-triamine, respectively. By adjusting the acidification
ratio of Hþ/NH2 (1/3), reactive organoclays were formed using F-
triamine or P-triamine as the swelling agent for the silicate layers of
MMTs. One of the functional groups of the swelling agents formed
an ionic bond with the negatively charged silicates, whereas the
remaining functional groups were available for further reaction
with maleimide groups of hyperbranched polymer HB11. The
organoclays F-MMT and P-MMT were further respectively
embedded into the hyperbranched polyaspartimide HB11 to form
the NLO-active nanocomposites. The hyperbranched poly-
aspartimide HB11 was chosen for further study in preparing the
nanocomposites because of its abundant presence of peripheral
maleimide groups. By using the swelling agents with trifunctional
groups, the organoclays could be easily exfoliated and dispersed in
the hyperbranched polymers. Good thermal properties and optical
transparency were observed for these hyperbranched poly-
aspartimide/organoclay nanocomposites. X-ray diffraction patterns
of the pristine MMT, swelling-agent-modified MMTs (F-MMT and
P-MMT), and hyperbranched polymer HB11/F-MMT (or P-MMT)
nanocomposites (HBF1, HBF3, HBF5, HBP1, HBP3, and HBP5) are
presented in Fig. 4. For the pristine MMT sample, a strong X-ray
Fig. 4. XRD patterns of (a) Naþ-MMT, (b) F-MMT, (c)HBF1, (d) HBF3, (e) HBF5, (f)
P-MMT, (g) HBP1, (h) HBP3, and (i) HBP5.
HBP1-HBP5) are shown in Table 2. Moreover, DSC and TGA ther-
mograms of the polymers and corresponding nanocomposites are
shown in Figs. 6 and 7, respectively. The Tg and Td values of the
hyperbranched polyaspartimides were observed in the range 190e
205 ꢀC and 272e308 ꢀC, respectively. The Tg values of HB11/F-MMT
samples first increased with increasing F-MMT content and then
reached a maximum point at the inorganic ratio of 3 wt% as
compared to the neat polymer sample. The highest value of Tg
(230.7 ꢀC) was observed for the HBF3. This is due to the restriction
of the polymer chains by the exfoliated clay platelets. However,
further increase of the clay loading (e.g. 5 wt%) was found to be
incapable of further enhancing the Tg of the sample [40]. As shown
in Fig. 6(a), the glass transitions were observed clearly at temper-
atures higher than 210 ꢀC for the HB11/F-MMT samples. This im-
plies that the F-MMT was dispersed uniformly in the polymer
matrices of HB11. On the other hand, the Tgs were indistinct at
temperatures from 150 to 250 ꢀC for the HB11/P-MMT samples
(Fig. 6(b)). Two broad transition bands were observed at about
150e190 and 200e250 ꢀC, respectively. This implies that a rather
broad range of distinct local compositions were present in the
HB11/P-MMT nanocomposites [40e42]. In addition, Fig. 7 shows
TGA thermograms of HB11 and its corresponding nanocomposites
(HBF1-HBF5 and HBP1-HBP5). Two weight loss stages were
observed in the TGA thermograms. The first degradation stage was
observed at 350e450 ꢀC, resulted from the decomposition of the
side-chain NLO dyes and phosphorus-containing moieties [32,43].
In addition the second degradation stage was observed at tem-
peratures higher than 500 ꢀC, resulted from the decomposition of
the imide linkages. The TGA traces of nanocomposites were found
diffraction peak at 2
q
¼ 6.8ꢀ was caused by the diffraction of the
(001) crystal surface of layered silicates, equaling a d-spacing of
ꢀ
12 A. The d-spacing values of F-MMTand P-MMT were about 25 and
ꢀ
15 A, respectively. For all of the HB11/F-MMT (or P-MMT) nano-
composites with different organoclay contents, no diffraction peak
in the range 2e10ꢀ (2
q) was observed in the XRD patterns, implying
the presence of disordered form or exfoliation morphology in the
samples (Fig. 4). This indicates that the polymer chains could easily
permeate into the spaces between the layered structures of modi-
fied MMTs. To confirm these results, we further used TEM to
visualize the spatial distributions of these nanocomposites (Fig. 5).
TEM images indicate that the morphologies of the organoclays
changed from intercalation to exfoliation during the in situ poly-
merization process. The hyperbranched polymer chains would
permeate into the galleries of MMTs. The inter-spacings were
further expanded and subsequently the MMTs were eventually
exfoliated to form individual nanoplatelets. The TEM images of the
nanocomposites incorporated with F-MMTor P-MMT indicates that
the silicate layers were dispersed in the polymer matrices, exhib-
iting exfoliated morphology. This is consistent with the XRD study
mentioned earlier.
Thermal properties of the hyperbranched polyaspartimides
(HB11, HB32, and HB21) and nanocomposites (HBF1-HBF5 and

Y.-Y. Siao et al. / Polymer 54 (2013) 3850e3859
3857
Fig. 5. TEM micrographs of (a) HBF3, (b) HBF5, (c) HBP3, and (d) HBP5.
to shift toward higher temperature ranges than that of the pristine
hyperbranched sample HB11. This is because of the presence of the
barrier effect of the MMT layered structures, as well as the strong
interaction between the organoclays and hyperbranched polymer.
Moreover, higher values of char residues were observed for the
HB11/F-MMT and HB11/P-MMT samples as compared to that of the
pristine hyperbranched polyaspartimide HB11.
These hyperbranched polyaspartimides (HB11, HB32, and HB21)
are soluble at room temperature in polar aprotic solvents such as
DMF, DMAc and DMSO, and partially soluble in THF, acetone and
chloroform. In addition, the organoclays can be easily swelled and
dispersed in aprotic solvents by simple mixing. For all of the
nanocomposites, the organoclays could be well-dispersed in DMF,
DMAc, and DMSO.
lower dye content of HB11. In addition, the optical losses of HBF1,
HBF3, and HBF5 were 2.8, 3.0, and 4.1 dB/cm, respectively, while
3.3, 3.3, and 4.9 dB/cm for HBP1, HBP3, and HBP5, accordingly. The
optical loss value increased with increasing content of organoclay
for the nanocomposites. Nevertheless, the thermal stability was
improved as a result of the organoclay addition. Large optical losses
are generally referred to the second harmonic (overtone) vibra-
tional absorption of CeH bond in the near-infrared region, scat-
tering losses from structure defects (voids), and absorption losses
from charge-transfer interactions. The introduction of fluorine- or
phosphorus-containing functionalities would reduce the number
density of CeH, OeH and NeH bonds in the intrinsic structures,
which plays a key role in lowering the optical losses of the materials
[30,44]. In addition, the optical losses for HB11/P-MMT nano-
composites were larger than those of HB11/F-MMT nano-
composites. This is possibly because the organoclay P-MMT was not
distributed uniformly in HB11. As shown in DSC thermograms
(Fig. 7(b)), the thermal transitions were much broader when
compared to those of the HB11/F-MMT samples [40e42]. The non-
uniform distribution of the P-MMT in the polymer matrices of HB11
resulted in larger optical losses of the HB11/P-MMT
nanocomposites.
3.3. Optical properties of the hyperbranched polyaspartimide/F-
MMT (or P-MMT) nanocomposites
The optical losses of the hyperbranched polyaspartimides and
nanocomposites were measured based on TE guide modes at
1310 nm, ranged from 2.7 to 4.9 dB/cm. It was reported that the
hyperbranched polymers exhibited lower optical loss than the
linear ones do due to the void-containing dendritic structure effect
[14]. The optical losses of HB11, HB32, and HB21 are 2.7, 3.8, and
3.8 dB/cm, respectively. The HB11 possessed relatively low optical
loss value than did HB32 and HB21, which might be due to the
The dye contents, EO coefficients (r33), and temporal stabilities
of the poled polyaspartimide HB11 and nanocomposites HBF3 and
HBP3 are summarized in Table 3. The dye contents were deter-
mined by UVevis spectroscopic methods [32]. The maximum

3858
Y.-Y. Siao et al. / Polymer 54 (2013) 3850e3859
Table 3
(a)
NLO properties of the hyperbranched polymers and corresponding nanocomposites.
HB11
HBF1
HBF3
HBF5
Sample
Dye contenta
(wt%)
EO coefficient,
r33 (pm/V)
at 830 nm
Optical loss
(dB/cm)
at 1310 nm
Temporal
stability (%),
r33(t)/r33(0)b
HB11
HBF3
HBP3
38.0
36.9
36.7
15.3
20.9
12.8
2.7
3.0
3.3
28
73
44
a
Dye content was determined by UVevis.
Determined at 100 ꢀC for 100 h.
b
absorption wavelength (lmax) of polymer appeared at 500 nm due
to the * transition of the NLO chromophore. To generate stable
pep
oriented dipoles and large EO coefficients, NLO-active polymers
should be aligned and annealed under an electric field. For the
hyperbranched polymer HB11 with dye content 38%, an EO coeffi-
cient of 15.3 pm/V was resulted from the in situ poling process
(measured at 830 nm). Moreover, the r33 value (20.9 pm/V) of the
nanocomposite HBF3 was larger than that of the pristine poly-
aspartimide HB11 (15.3 pm/V). This indicates that the existence of
exfoliated silicate platelets did not interfere with the alignment of
NLO dyes under the electric field, yet effectively minimized the
aggregation of NLO moieties. The chromophore-containing hyper-
branched polymer with site-isolation effect tethered on the silicate
platelets would arrange in a noncentrosymmetric state under
electric field, which is favorable for the enhancement of EO co-
efficients [32,33]. This site-isolation effect might be more pro-
nounced for the nanocomposite incorporated with the organoclay
F-MMTs due to the low surface energy of fluorine containing sub-
stituents (eCF3). Therefore, the r33 value of the nanocomposites
HBF3 (20.9 pm/V) was much larger than that of nanocomposite
HBP3 (12.8 pm/V) despite their similar dye contents (36e38%). The
r33 value of HB11 was reduced by the incorporation of organoclay P-
MMTs. This is attributed to the non-uniform distribution of the P-
MMTs in the polymer matrices of HB11, resulting in poor poling
efficiency of HBP3.
75
100 125 150 175 200 225 250 275
o
Temperature
(
C)
(b)
HB11
HBP1
HBP3
HBP5
3.4. Temporal stability of the NLO hyperbranched polyaspartimide/
F-MMT (or P-MMT) nanocomposites
50
75 100 125 150 175 200 225 250 275 300
o
Temperature
(
C
)
To investigate the long-term NLO stability of the poled hyper-
branched polymers and nanocomposites, the dipole reorientation
of the film was observed by measuring EO coefficient as a function
of time at 100 ꢀC. Fig. 8 shows the r33 temporal characteristics for
Fig. 6. DSC thermograms of the hyperbranched polyaspartimide/organoclay (F-MMT
or P-MMT) nanocomposites.
HB11
100
80
60
40
20
HB11
HBF1
HBF3
HBF5
HBP1
HBP3
HBP5
1.0
0.8
HBF3
HBP3
0.6
0.4
0.2
0.0
100
200
300
400
500
oC
600
700
800
0
20
40
60
80
100
Temperature
(
)
Time (h)
Fig. 7. TGA thermograms of the hyperbranched polyaspartimide/organoclay (F-MMT
or P-MMT) nanocomposites.
Fig. 8. Temporal stability of EO coefficients for the poled hyperbranched
polyaspartimide/organoclay (F-MMT or P-MMT) nanocomposites samples at 100 ꢀC.

Y.-Y. Siao et al. / Polymer 54 (2013) 3850e3859
3859
the poled HB11, HBF3 and HBP3 samples. A fast decay of the EO
References
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samples with an optimum amount of well-dispersed silicate
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elevated temperatures. This indicates that the mobility of the
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We have developed a series of novel and thermally stable
hyperbranched polyaspartimides consisting of azobenzene chro-
mophores. To further improve the thermal stability, reactive orga-
noclays were incorporated into the hyperbranched polya
spartimides. These reactive organoclays were obtained using fluo-
rine- or phosphorus-containing triamine as the intercalating agent
for MMTs. One of the amino groups of the intercalating agents
formed an ionic bond with the negatively charged silicates, whereas
the remaining functional groups were available for further reaction
with the hyperbranched NLO polymer. Consequently, the
chromophore-containing hyperbranched polymer with site-
isolation effect tethered on the silicate platelets would be ar-
ranged in a noncentrosymmetric state under electric field. Better EO
performance was observed for the polyaspartimide/F-MMT com-
posites as compared to that of the pristine hyperbranched poly-
aspartimide. Moreover, the presence of chemical bonds between the
organoclays and the polyaspartimide chains could enhance the
thermal stability of the NLO polymer. The mobility of the aligned
DAC chromophores in polymer matrices was restricted by the
exfoliated layered silicates. Better temporal stability of EO coeffi-
cient at 100 ꢀC was achieved for the NLO-active hyperbranched
polyaspartimide/organoclay (F-MMT or P-MMT) nanocomposites
than that of the pristine hyperbranched polyaspartimide.
Acknowledgments
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Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2013.05.040.