Simple synthetic route to soluble polyimides via nitro-displacement reaction and their second-order nonlinear optical properties

Article abstract of DOI:10.1039/a902230i

Nonlinear optical (NLO) functionalized polyimides were directly synthesized using the nitro-displacement reaction between an alkanediol monomer and a diimide monomer without a thermal curing step. The polyimides were soluble in aprotic polar solvents such

Full text of DOI:10.1039/a902230i

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J O U R N A L O F
Simple synthetic route to soluble polyimides via nitro-displacement
reaction and their second-order nonlinear optical properties
Yeong-Beom Lee, Han Young Woo, Chong-Bok Yoon and Hong-Ku Shim*
Department of Chemistry, Korea Advanced Institute of Science and Technology,
Taejon 305-701, Korea. Tel: 82-42-869-2827; Fax: 82-42-869-2810;
E-mail: hkshim@sorak.kaist.ac.kr
C H E M I S T R Y
Received 22nd March 1999, Accepted 15th June 1999
Nonlinear optical (NLO) functionalized polyimides were directly synthesized using the nitro-displacement
reaction between an alkanediol monomer and a diimide monomer without a thermal curing step. The
polyimides were soluble in aprotic polar solvents such as DMSO, DMAc, DMF and NMP, and showed glass
transitions at temperatures between 172 and 198 ³C. We observed good thermal stability up to around 300 ³C
(at 5% weight loss) for the polymers. The weight average molecular weights of the resulting polymers were
determined to be 10 400±15 100 (Mw/Mn~1.84±2.00). The poled polymer ®lms showed good nonlinearity
(d33~76 pm V21) in the Maker fringe method for the second harmonic generation (SHG).
1
In recent years, nonlinear optical (NLO) polymers have
become a potential candidate material for applications such
as frequency modulators, linear electro-optic devices and
optical storage because of advantages such as good processa-
bility, large nonlinearity, facile molecular design and low
price.1,2 Much research in NLO polymers to realize new
advanced polymeric systems with better thermal and temporal
stabilities of the dipole alignment has been carried out for
polymer systems such as side chain polymers,3 crosslinked
polymers4 and organic±inorganic hybrid systems.5 Polyimides
in NLO polymers have high thermal stabilities and can
maintain the dipole alignment of NLO chromophore pendants
attached on the polymer backbone at high temperatures.6b
However, most aromatic polyimides require processing at high
temperatures for the imidization from a precursor polyamic
acid and show limited solubility in common organic solvents.1
Polyetherimides with an ether linkage are a better polymeric
system with enhanced processability and solubility while
maintaining the meritorious properties of polyimides.7 Since
the one-step polymerization of polyimides by nitro-displace-
ment reaction without thermal imidization was ®rst intro-
duced,8 some groups lately reported poly(etherbenzimida
zole)s9 with heat-resistance and poly(arylene ether ketone)s10
containing tri¯uoromethyl groups. However, efforts to pre-
pare polyimides by nitro-displacement using alkyl oxide
instead of aryl oxide have not been reported yet.
were characterized by H-NMR spectra on a Bruker AM 200
or AM 300 spectrometer. FT-IR spectra were obtained with a
Bomem Michelson series FT-IR spectrophotometer (on KBr
plates) and UV±visible spectra were measured on a Shimadzu
UV-3100S. Differential scanning calorimetric (DSC) analysis
and thermogravimetric analysis (TGA) were performed under
nitrogen atmosphere at a heating rate of 10 ³C min21 with a
Dupont 9900 analyzer. The number and weight average
molecular weights of polymers were estimated by a gel
permeation chromatography (GPC) instrument, Waters 2690
model, using dimethylacetamide (DMAc) as eluant. The setup
for performing the corona poling was designed to heat up to
300 ³C and to apply the poling voltage in the range of 0±10 kV
under nitrogen atmosphere. The second harmonic generation
coef®cients (d33) of these polymer samples were measured using
the Maker fringe method for second harmonic generation
(SHG) from 1064 nm laser radiation.11b The polarized Q-
switched Nd : YAG laser (Lumonics HY750) with a 10 ns pulse
width and a 10 Hz repetition rate was used as the light source.
The second harmonic signal was detected by the photomulti-
plier tube (Hamamatsu R-928) and averaged over 300 pulses in
a boxcar integrator (Stanford SRS250) to increase the signal to
noise ratio. Film thickness was determined using a Tencor
Alpha-Step 500 surface pro®ler.
Materials
In this paper, we introduce a simple synthetic route to NLO-
functionalized polyetherimides using the nitro-displacement
reaction. Nitro-displacement reaction using alkyl oxide as a
nucleophile has been carried out at a mild temperature without
a thermal curing step as in Scheme 2. The aromatic carbon to
which the nitro group is attached is very electron de®cient
because of the electron-withdrawing properties of the nitro
group and the imide group, and as a result, the aromatic
nucleophilic attack by the nucleophile gives the step poly-
merization by a nitro-displacement. We synthesized monomers
with a dialkylaminonitrostilbene (DANS) unit to give second-
order NLO activity to the resulting polymers.
3-Nitrophthalic anhydride (98%), 4-nitrophthalic anhydride
(92%), diethyl azodicarboxylate (DEAD) (97%), 4,4'-methyle-
nedianiline (MDA) (97%), anhydrous N,N-dimethylformamide
(DMF) (99.8%), 1-methylpyrrolidin-2-one (NMP) (99.5%) and
sodium hydride (60% dispersion in oil) were purchased from
Aldrich Co. and used without further puri®cation. Triphenyl-
phosphine (99%) was purchased from Fluka Co. and N-
phenyldiethanolamine (min. 95%) was purchased from TCI
Co. (Japan) and both of them were used without further
puri®cation. Anhydrous tetrahydrofuran (THF) was obtained
by distillation using sodium as drying agent. All other solvents
and reagents were analytical-grade quality, purchased com-
mercially and used as received unless otherwise stated.
Experimental
Analytical instrumentation
Synthesis of monomers
Melting points were uncorrected and were measured with an
Electrothermal 9100 model (England). Synthesized compounds
Compound 2 comes from the protection of alkanediol of N-
phenyldiethanolamine (1) with benzoyl chloride (Scheme 1).
J. Mater. Chem., 1999, 9, 2345±2350
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Scheme 2 Synthetic route to the polyetherimides.
1H-NMR (200 MHz, CDCl3): d 8.02 (t, 4H, ArH), 7.54 (m, 2H,
ArH), 7.41 (t, 4H, ArH), 7.30 (t, 2H, ArH), 6.90 (d, 2H, ArH),
6.75 (t, 1H, ArH), 4.52 (t, 4H, ±NCH2CH2O±), 3.83 (t, 4H,
±NCH2CH2O±). Calcd. for C24H23NO4: C, 74.02; H, 5.95; N,
3.60; Found: C, 74.06; H, 6.02; N, 3.61%.
Scheme 1 Synthetic route to the monomers.
2-[(4-Formylphenyl)(2-benzoyloxyethyl)amino]ethyl benzoate
(3). 2-[Phenyl(2-benzoyloxyethyl)amino]ethyl benzoate (10 g,
25.7 mmol) and DMF (92 mL, 123.4 mmol) were put into a 2-
necked 250 mL round-bottomed ¯ask. Phosphorus oxychloride
(47 g, 30.9 mmol) was added dropwise and the resultant
mixture was stirred for 3 h at 80 ³C. The solution color
changed to dark green. Water (20 mL) was poured into the
mixture and it was extracted with ethyl acetate three times.
Then the pH of the solution mixture was adjusted to basic using
NaOH solution. After removing the residual solvent by
evaporator, a white solid was obtained and the solid was
recrystallized from methanol to yield a white powder product
(8.4 g, 20.0 mmol, 78%). UV±vis (CH2Cl2) lmax/nm: 318, 236;
1H-NMR (200 MHz, CDCl3): d 9.74 (s, 1H, ±CHO), 7.95 (d,
4H, ArH), 7.72 (d, 2H, ArH), 7.51 (t, 2H, ArH), 7.39 (t, 4H,
ArH), 6.90 (d, 2H, ArH), 6.75 (t, 1H, ArH), 4.54 (t, 4H,
±NCH2CH2O±), 3.90 (t, 4H, ±NCH2CH2O±). Calcd. for
C24H23NO4: C, 71.93; H, 5.55; N, 3.36; Found: C, 71.86; H,
5.65; N, 3.38%.
Compound 3 was obtained by Vilsmeier reaction from 2.
Compound 5 was synthesized by Horner±Emmons±Wittig
reaction between 3 and 4, and by deprotection to the alcohol.
Compound 6 was obtained by Mitsunobu reaction of a diol
and a phthalimide.12 Compound 7 was obtained by acid-
catalyzed imidization of 3-nitrophthalic anhydride and 4,4'-
methylenedianiline with acetic acid using a Dean±Stark
apparatus for removal of the residual water.
2-[Phenyl(2-benzoyloxyethyl)amino]ethyl benzoate (2). N-
Phenyldiethanolamine (20 g, 110 mmol) and benzoyl chloride
(27.3 mL, 242.7 mmol) were put into a 2-necked 250 mL
round-bottomed ¯ask with methylene chloride (19.5 mL,
112.2 mmol). Triethylamine (32.8 mL, 242.7 mmol) was
added dropwise and the resultant mixture was stirred for 3 h
in an ice bath (at 0 ³C). Water (20 mL) was poured into the
mixture and it was extracted with methylene chloride three
times. After removing the residual solvent by evaporator, a
yellowish liquid was obtained and the solvent was removed
under vacuum. The liquid changed to a white solid. The solid
was recrystallized in methanol to yield a white powder as
product (37 g, 95 mmol, 86%). UV±vis (CH2Cl2) lmax/nm: 231;
Diethyl 4-nitrobenzylphosphonate (4). 1-(Bromomethyl)-4-
nitrobenzene (20 g, 93.5 mmol) was put into a 2-necked
100 mL round-bottomed ¯ask with triethyl phosphate
2346
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(19.5 mL, 112.2 mmol). This solution was stirred at 90 ³C for
10 h. The resulting mixture was vacuum distilled after which a
transparent yellowish liquid was obtained as product (21.3 g,
78.01 mmol, 83.5%). The product was preserved in a refrig-
erator. 1H-NMR (200 MHz, CDCl3): d 8.15 (d, 2H, ArH),
7.44 (dd, 2H, ArH), 4.00 (m, 2H, ±OCH2CH3), 3.25 (d, 2H,
ArCH2±), 1.21 (t, 6H, ±OCH2CH3). Calcd. for C11H16NO5P:
C, 48.36; H, 5.90; N, 5.13; Found: C, 48.15; H, 6.33; N,
5.13%.
phthalimide), 8.24 (d, 2H, phthalimide), 8.09 (t, 2H, phthal-
imide), 7.43±7.37 (m, overlapped 8H, ArH), 4.08 (s, 2H,
ArCH2Ar). Calcd. for C29H16N4O8: C, 63.51; H, 2.94; N,
10.22; Found: C, 63.42; H, 2.86; N, 10.02%.
Synthesis of polyetherimides
The synthesis of polyimides was performed by step polymer-
ization using an aromatic nucleophilic substitution of a 3- or 4-
positioned activated nitro group of the phthalic imides as
shown in Scheme 2. The reaction conditions of the polymer-
ization were mild (70 ³C) and sodium hydride, which is used as
a base in aprotic polar solvents with high boiling point, was
used. The synthesis of PEI 1-DANS and PEI 2-DANS followed
the synthetic route of PEI.
4-[Bis(2-hydroxyethyl)amino]-4'-nitrostilbene
(5)6a. 2-[(4-
Formylphenyl)(2-benzoyloxyethyl)amino]ethyl benzoate (3)
(6 g, 14.38 mmol) and sodium hydride (0.69 g, 17.26 mmol)
were put into a 2-necked 250 mL round-bottomed ¯ask with
anhydrous tetrahydrofuran (ca. 100 mL). To this mixture,
diethyl 4-nitrobenzylphosphonate (4) (3.94 g, 14.38 mmol) was
added dropwise. The solution was stirred at 70 ³C for 16 h. The
solution changed to dark red as the nitrostilbene moiety was
formed. After removing the solvent using the evaporator, the
resulting solution was extracted with methylene chloride three
times. The dried organic extracts were added to methanol
(80 mL) and sodium hydroxide (2.53 g, 63.27 mmol) and the
solution was stirred at 80 ³C for 12 h. After completion of the
reaction, some solvent was removed and water was poured into
the reaction ¯ask. After ®ltering, a red solid was obtained and
dried over P2O5 at 80 ³C for 24 h in vacuo. The yield of
compound 5 was 5.61 g (69%). Mp: 182±183 ³C; UV±vis
(CH2Cl2): lmax/nm: 434; FT-IR (KBr) ~nmax/cm21: 3320 (O±H),
1583 (CLC), 1338 (NO), 1108 (C±O); 1H-NMR (200 MHz,
DMSO-d6): d 8.16 (d, 2H, ArH), 7.74 (d, 2H, ArH), 7.45 (d,
2H, ArH), 7.39 (d, 1H, CHLCH), 7.06 (d, 1H, CHLCH), 6.70
(d, 2H, ArH), 4.80 (br s, 2H, ±OH), 3.53 (d, 4H, NCH2CH2O),
3.46 (d, 4H, NCH2CH2O). Calcd. for C18H20N2O4: C, 65.84;
H, 6.14; N, 8.53; Found: C, 65.50; H, 6.10; N, 7.82%.
PEI. N-Phenyldiethanolamine (0.52 g, 2.74 mmol) was put
into a 2-necked 100 mL round-bottomed ¯ask with anhydrous
DMF (10 mL). To this, sodium hydride (0.274 g, 6.85 mmol)
was added. Then, bis[4-(3-nitrophthalimido)phenyl]methane
(1.5 g, 2.74 mmol) was added to the ¯ask and the ¯ask was
maintained at 70 ³C for 48 h. The crude polymer was extracted
with a Soxhlet extractor for 2 days to remove low molecular
weight polymers. The yield of a yellowish polymer was 1.16 g
(66.6%). UV±vis (CH2Cl2) lmax/nm: 227 and 333; FT-IR (KBr)
1
~nmax/cm21: 1784, 1730 (CLO), 1284 (ArC±O±CH2); H-NMR
(200 MHz, DMSO-d6): d 7.9±7.5 (br m, 4H, ArH), 7.5±7.2 (br
d, 8H, ArH), 7.2±6.8 (br s, 4H, ArH), 6.8±6.6 (br m, 2H, ArH),
6.6±6.4 (br m, 1H, ArH), 4.35 (s, 2H, ArCH2Ar), 4.2±3.6 (br d,
8H, ±NCH2CH2± and ±NCH2CH2±); Mn: 5200; Mw: 10 400;
Tg/³C: 172.4; Td5/³C: 303.
PEI 1-DANS. The yield of reddish solid polymer was 3.20 g
(81.0%). UV±vis (thin ®lm on ITO) lmax/nm: 433; FT-IR (KBr)
~nmax/cm21: 1774, 1716 (CLO), 1335 (NO), 1283 (ArC±O±CH2);
1H-NMR (300 MHz, DMSO-d6): d 8.2±8.0 (br s, 2H, ArH),
7.9±7.5 (br s, 2H, ArH), 7.5±7.2 (br s, 8H, ArH), 7.2±6.9 (br s,
2H, ±CHCH±), 6.9±6.6 (br s, 2H, ArH), 4.5±4.2 (br s, 2H,
±PhCH2Ph±), 4.2±3.6 (br d, 8H, ±NCH2CH2O± and
±NCH2CH2O±); Mn: 8300; Mw: 15 200; Tg/³C: 197.3; Td5/³C:
295.
4-{Bis[2-(4-nitrophthalimido)ethyl]amino}-4'-nitrostilbene
(6). 4-[Bis(2-hydroxyethyl)amino]-4'-nitrostilbene
(3 g,
9.13 mmol), 4-nitrophthalimide (3.72 g, 19.17 mmol), and
triphenylphosphine (5.18 g, 19.17 mmol) were added to
60 mL of anhydrous tetrahydrofuran in a 2-necked round-
bottomed ¯ask and diethyl azodicarboxylate (DEAD) (3.34 g,
19.17 mmol) was added dropwise into the ¯ask at room
temperature. While DEAD was being added, thick brown-
colored solids precipitated from the solution. The mixture was
stirred for 24 h at room temperature. The resulting solid was
recrystallized in methanol and the product was dried under
vacuum to yield 3.34 g of reddish brown solid (80.4%). Mp:
259±260 ³C; UV±vis (CH2Cl2) lmax/nm: 420; FT-IR (KBr)
~nmax/cm21: 1773, 1715 (CLO), 1583 (CLC), 1332 (NO);
1H-NMR (200 MHz, DMSO-d6): d 8.57 (d, 2H, phthalimide),
8.44 (s, 2H, phthalimide), 8.17 (d, 2H, ArH), 8.08 (d, 2H,
phthalimide), 7.70 (d, 2H, ArH), 7.37 (d, 2H, ArH), 7.21 (d,
1H, CHCH), 7.04 (d, 1H, CHCH), 6.83 (d, 2H, ArH), 3.78
(br d, 4H, NCH2CH2N), 3.64 (br s, 4H, NCH2CH2N).
Calcd. for C34H24N5O8: C, 64.76; H, 3.84; N, 11.11; Found:
C, 64.36; H, 3.54; N, 11.68%.
PEI 2-DANS. The yield of reddish solid polymer was 0.34 g
(61.15%). UV±vis (thin ®lm on ITO) lmax/nm: 438; FT-IR
(KBr) nmax/cm21: 1767, 1709 (CLO), 1335 (NO), 1286 (ArC±O±
~
CH2); 1H-NMR (300 MHz, DMSO-d6): d 8.3±7.9 (br s, 4H,
ArH), 7.9±7.7 (br s, 4H, ArH), 7.7±7.2 (br m, 10H, ArH and
±CHCH±), 7.2±6.9 (br s, 4H, ArH), 6.9±6.6 (br s, 4H, ArH),
4.5±4.0 (br s, 4H, ±NCH2CH2O±), 4.0±3.7 (br s, 4H,
±NCH2CH2N±), 3.7±3.4 (br d, 8H, ±NCH2CH2O± and
±NCH2CH2N±); Mn: 9000; Mw: 11000; Tg/³C: 172.0; Td5/³C:
300.
Film preparation for SHG measurements
The puri®ed polymers were dissolved in anhydrous NMP
solvent at a concentration of 10 wt%. The polymer solutions
were ®ltered through a 0.45 mm Te¯on ®lter (Whatman) and
the ®ltrates were spin coated on the precleaned (with sonication
in isopropyl alcohol and acetone) indium-tin oxide (ITO)
coated glass substrates to give ®lms whose thickness was
around 300 nm. The ®lms obtained were put in a vacuum oven
at 100 ³C under reduced pressure for 24 h for the removal of
residual solvent. After drying the polymer ®lms, they were
poled at around Tg for about 20 min and cooled to room
temperature under an electric ®eld of 3.5±4.5 kV. The distance
between the corona electrode and polymer ®lm was 1 cm and
the thickness of the tungsten wire used as the corona electrode
was 25 mm.
Bis[4-(3-nitrophthalimido)phenyl]methane (7). 3-Nitrophtha-
lic anhydride (5.00 g, 26 mmol) and 4,4'-methylenedianiline
(2.50 g, 13 mmol) were put into a 2-necked 250 mL round-
bottomed ¯ask with acetic acid (100 mL). The mixture was
stirred at 55 ³C for about 30 min and then a Dean±Stark
separator was used to remove the by-product, water, over
120 ³C for 24 h. The resulting mixture was precipitated by
addition of methanol and after ®ltering, recrystallized from
methanol. The puri®ed product was a yellowish white solid and
the yield was 3.32 g (6.05 mmol, 46.5%). Mp: 291±293 ³C; UV±
vis (CH2Cl2) lmax/nm: 225; FT-IR (KBr) ~nmax/cm21: 1784, 1720
(CLO); 1H-NMR (200 MHz, DMSO-d6): d 8.31 (d, 2H,
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Results and discussion
The synthetic routes of the monomers and the ®nal polyether-
imides are shown in Schemes 1 and 2. In Scheme 1, N-
phenyldiethanolamine (1) was protected with benzoyl chloride
in methylene chloride solvent in the presence of an equivalent
amount of triethylamine as a catalyst. Following Vilsmeier
formylation in DMF solvent, compound 2 yielded compound
3. A subsequent reaction of 3 and phosphonate 4 by Horner±
Emmons±Wittig reaction and deprotection with sodium
hydroxide gave the diol monomer.13b Compound 5 and 4-
nitrophthalimide produced the dinitroimide monomer
6
including a DANS moiety by Mitsunobu reaction using diethyl
azodicarboxylate (DEAD) and triphenylphosphine as reagents
in anhydrous tetrahydrofuran.12 Bis[4-(3-nitrophthalimido)-
phenyl]methane (7) was produced from the imidization of 4,4'-
methylenedianiline (MDA) and 3-nitrophthalic anhydride.8 All
intermediates, including the monomers, were characterized by
elemental analysis, and the measurement of melting points and
1
common spectroscopic techniques such as H-NMR, FT-IR,
and UV±visible spectroscopies. Their results are in good
agreement with the structures obtained in each step of the
synthetic routes. The polyetherimides (PEIs), PEI, PEI 1-
DANS and PEI 2-DANS, were synthesized by step polymer-
ization between alkanediol and diimide monomers using
sodium hydride as a base in an anhydrous DMF solvent at
70 ³C for 48 h. Then, the ®nal PEIs were puri®ed by Soxhlet
extraction in methanol for 2 days. The yields of the PEIs were
67% (a yellowish powder), 81% (a reddish powder) and 61% (a
reddish powder) for PEI, PEI 1-DANS and PEI 2-DANS,
respectively. Using this method it is possible to polymerize the
3,3'-isomer and 4,4'-isomer by the substitution of 3- or 4-
nitrophthalimide. The ®nal PEIs were readily soluble in aprotic
polar solvents such as DMF, DMSO, DMAc, and NMP due to
their alkoxide linkages. Their solubility was especially good in
NMP.
The molecular weights were obtained from gel permeation
chromatography (GPC) analysis using polystyrene as standard
and DMAc as eluant. The weight average molecular weights of
the polymers were determined to be 10 400 (Mw/Mn~2.00) for
PEI, 15 200 (Mw/Mn~1.84) for PEI 1-DANS and 11 000
(Mw/Mn~1.26) for PEI 2-DANS (using DMF solvent as
eluant).
Fig. 1 1H-NMR spectra of (a) PEI (200 MHz), (b) PEI 1-DANS
(300 MHz) and (c) PEI 2-DANS (300 MHz) in DMSO-d6.
1
Fig. 1 is the H-NMR spectra of the polymers, which show
signal broadening due to polymerization, but the chemical
shifts are well consistent with the proposed polymer structures.
After puri®cation of the polymers, we could observe neither the
amide proton nor the proton (dy10 ppm) of the aldehyde that
could have been generated from the rupture of the imide ring
by sodium hydride, in all polymers. In the NMR spectra of PEI
and PEI-DANS 1, small peaks above d 8.2 originate from the
protons of the nitrophthalic imide group in the polymer end
group.14 The peak at 2.49 ppm comes from DMSO and the
sizable peak at 3.3 ppm originates from H2O molecules in
DMSO-d6 solvent. The FT-IR spectra of PEI also showed the
formation of alkyl aryl ether bonds in all of PEI, PEI 1-DANS
and PEI 2-DANS. After polymerization, a new medium peak
around 1285 cm21 appeared and it was due to the asymmetric
stretching of ArC±O±CH2 in the polymer backbone.14 From
this observation, we could deduce that the nitro-displacement
reaction was successfully completed. In addition, the strong
peak around 1715 cm21 and the medium peak around
1775 cm21 were due to the carbonyl asymmetric stretching in
the imide ring. In the UV±visible spectra, PEI 1-DANS and
PEI 2-DANS showed absorption maxima at lmax~433 and
438 nm (thin ®lm on ITO glass substrate) respectively and these
absorption peaks originated from the p±p* transition of the
conjugated 4,4'-aminonitrostilbene moiety.
®rst decomposition (at 5% weight loss) of all polymers occurred
around 300 ³C under nitrogen atmosphere (Table 1 and Fig. 2).
In DSC thermograms, the polymers did not show melting
endothermic peaks, indicating that they represent a noncrys-
talline phase, but showed relatively high glass transition
temperatures; Tg values are 172.4 ³C for PEI, 197.3 ³C for
PEI 1-DANS and 172.0 ³C for PEI 2-DANS. In the case of PEI
1-DANS, the glass transition occurred at a higher temperature
(by ca. 25 ³C) than others because of the rigidity of the NLO-
functionalized chromophore and the aromatic group on the
imide backbone of the polymer chain.
The ®nal polyetherimides were dissolved in NMP and were
processed into optically good quality ®lms by spin coating with
a heated polymer solution. For induced second order optical
nonlinearity, the polymer ®lms were poled at around Tg for
about 20 min and cooled to room temperature under an
applied electric ®eld. After the corona discharge poling, the
UV±visible spectra of PEI 1-DANS and PEI 2-DANS
exhibited a slight blue shift and a decrease in absorption
because of the birefringence caused by dipole alignment. The
absorption changes are directly related to the orientation of the
chromophores and can be used as a means of evaluating the
degree of orientation. If one assumes that the molecular
electronic transition moment is parallel to the permanent
Thermal characterization of the PEIs was carried out using
TGA and DSC instruments. TGA studies indicated that the
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Table 1 The physicochemical properties of the resulting polyetherimides
a
Mn
a
Polymers
Mw
Mw/Mn
Tg/³C
Td5/³Cb
lmax/nm
PEI
PEI 1-DANS
PEI 2-DANS
5200
8300
(9000)
10400
15200
(11000)
2.00
1.84
(1.26)
172.4
197.3
172.0
303
295
300
227, 333 (CH2Cl2)
433.4 (on ITO glass)
438.1 (on ITO glass)
aMolecular weight of the ®nal polymers is estimated by gel permeation chromatography using DMAc (DMF) as eluant. bOnset of 5% weight loss on
TGA thermograms at a heating rate of 10 ³C min21 under nitrogen atmosphere.
Table 2 The optical properties of the ®nal polyetherimides
Calcd.
chromophore
contents/wt%
Film
thicknessa
/mm
f
a2v at
b
d33/pm V21
d33(`)
Polymers
W
532 nm/mm21
(d33/d31
)
/pm V21
PEI 1-DANS
PEI 2-DANS
30.43
52.31
0.106
0.350
0.16
0.27
0.64
0.75
76 (3)
54 (3)
21
14
aFilm thickness was measured on the alpha step surface pro®ler. bAbsorption coef®cient a2f v at 532 nm after poling.
ground-state dipole moment, one can obtain the order
parameter W by measuring the absorbance A0 of an unpoled
system and Ap of a poled sample with light polarized
perpendicular to the poling direction.2b Owing to the
absorbance change, the order parameter W of the poled ®lm,
which is related to the poling ef®ciency, was estimated to be
0.27 (W~12Ap/A0) for PEI 2-DANS. The optical measure-
ments were taken after the removal of residual charges caused
by the electronic poling ®eld, by leaving it at room temperature
overnight. The second harmonic generation (SHG) measure-
ments were performed at a fundamental wavelength of
1064 nm using the mode-locked Nd : YAG laser. In order to
determine the microscopic second-order susceptibility of the
material, the angular SHG dependence was recorded and then
compared with the values obtained from a 4.65 mm thick,
Y-cut quartz plate (Fig. 3). The SHG coef®cient, d33, was
calculated using a model for the ®tting derived from that of
Fig. 3 Maker fringe patterns of (a) a reference, Y-cut quartz plate, (b)
PEI 1-DANS ($), the ®tting curve (#).
Herman and Hayden,11a including corrections of the absorp-
tion and birefringence of the ®lm sample with re¯ections. The
SHG measurement showed large d33 values of 76 pm V21 for
PEI 1-DANS and 54 pm V21 for PEI 2-DANS. As the polymer
®lms showed non-negligible absorption at 532 nm, we calcu-
lated the non-resonant values (d33(`)) of these polymers by the
approximation of two level model.15 The d33(`) values of PEI
(98 pm V21 in d33 value) used in practical devices.16 The study
in temperature ranges and conditions for the stability of the
poled polymers is needed to satisfy the conditions required in
real applications.2a
1-DANS and PEI 2-DANS were 21 pm V21 and 14 pm V21
,
Conclusions
respectively. As compared with our previous study through a
different synthetic approach, these order parameter values were
lower than those, but these non-resonant values were higher
than those.13c
We have directly prepared PEI and two NLO-functionalized
PEIs through a simple synthetic route using the nitro-
displacement reaction between an alkanediol and a dinitro-
imide monomer without any side reaction such as the breaking
of the imide ring. The resulting polymers showed good second-
order NLO activity with d33 values of 76 pm V21 (PEI 1-
DANS) and 54 pm V21 (PEI 2-DANS). We are working on the
preparation of a series of poly(etherimide)s with various
substituents and a further study of the thermal and temporal
stability of a second-order NLO activity will be carried out.
Our polymer ®lms had lower d33 values than that of LiNbO3
Acknowledgements
This work was supported by KAIST and the Center for
Advanced Functional Polymers (CAFPoly) through the Korea
Science & Engineering Foundation (KOSEF). We gratefully
thank Mr J. H. Moon, Professor C. S. Yoon (KAIST) for their
help in measuring SHG activities of our samples.
References
Fig. 2 TGA and DSC (inset) thermograms of PEI, PEI 1-DANS and
PEI 2-DANS (at a heating rate of 10 ³C min21 under nitrogen
atmosphere).
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