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High transparency and thermal stability of alicyclic polyimide with
crosslinking structure by triallylamine
Juo-Chen Chena, 1, Wen-Yen Tsengb, 2, I-Hsiang Tsengb, 3, Mei-Hui Tsaib, 4,*
aDepartment of Electronic Engineering, National Chin-Yi University of Technology, No.35, Lane215,
Sec.1, Chung-Shan Rd., Taiping City, Taichung County 41101
bDepartment of Chemical and Materials Engineering, National Chin-Yi University of
Technology, No.35, Lane215, Sec.1, Chung-Shan Rd., Taiping City, Taichung County 41101
Taiwan, R.O.C.
[email protected], [email protected], [email protected],
Key word: alicyclic polyimide, crosslinkable, transparency, dimensional stability
Abstract. Colorless alicyclic polyimides (ALPIs) were synthesized from an alicyclic dianhydride,
bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BCDA) and an aromatic diamine,
3,4'-oxydianiline (3,4'-ODA). For comparison, a series of crosslinkable ALPI membranes with
different content of crosslinkable reagents were prepared. The crosslinkable PI reacts with the
crosslinkers and forms covalent bond to create the crosslink structure between PI backbones by free
radical reaction. Almost of the crosslinkable PIs exhibit excellent dimensional stability and higher
transparency because of the crosslink structure and non-conjugate alicyclic chain. All of the
crosslink ALPIs could be coated into flexible and tough films. They had a UV-Vis cut-off at 297 nm
and a transmittance of higher than 80% in near ultraviolet region. These PIs show low coefficient of
thermal expansion ranging from 57.36 to 47.53 ppm/oC, the glass transition temperature in the
range of 336.2-333.0 oC, the decomposition temperature in the range of 433.7-440.0
oC. The
crosslinkable ALPIs show excellent optical properties with the excited wavelength ranging from
340 to 328 nm and stronger emission intensity than linear PI, the haze lower than 0.7, the refractive
index about 1.6 and the abbe numbers over 165.
Introduction
Aromatic polyimides (PIs) are a kind of high performance polymer with excellent thermal
stability, chemical resistance and electric property that are widely used for aerospace, transportation
and electronic industry in the form of films and moldings [1,2,3]. However, wholly aromatic PIs are
difficult to fabricate because of their insolubility in common solvents and high glass transition
temperature [4,5]. Besides, the mostly well-known PI (Kapton) synthesized from pyromellitic acid
dinanhydride (PMDA) and 4,4'-diaminodiphenyl ether (ODA), has strong colorlation from
yellowish-brown to blackish-brown. The characteristic absorption tailings in the visible region was
due to the intra- or inter-molecular charge transfer (CT) between highly conjugated aromatic
structure of the PI backbones [6-11]. Those problems greatly limit the usage of PI in the area where
colorlessness or transparency is the basic requirement.
Advanced Materials Research Vols. 287-290 (2011) pp 1388-1396Online available since 2011/Jul/04 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.287-290.1388
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 128.42.202.150, Rice University, Fondren Library, Houston, USA-20/11/14,16:41:56)
Many approaches have been made through the replacement of aromatic structure with aliphatic
structure [12] or the incorporation of flexible linkages [13,14], bulky pendant groups [15-17],
noncoplanar biphenylene moieties [3,18-20] and trifluoromethyl (-C(CF3)3) [21,22] into the
polymer backbone. The main purpose of these methods is to increase the steric hindrance, decrease
the crystallinity and reduce the intra- and/or inter-molecular interactions [4]. The alicyclic
polyimide (ALPIs) with unconjugated polycyclic structure [12] in main chain are transparent and
colorless to remedy the problems of wholly aromatic polyimides without excessively sacrificing
their thermal properties [4,30].
Matsumoto and Feger [12] synthesized ALPIs from an alicyclic dianhydride, bicycle
[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BCDA) and an aromatic. The polyalicyclic
PIs are colorless and transparent when heated up to 300 o
C in air or 400oC in N2 [12]. However,
large dimensional change was observed from most colorless PI due to the decrease in crystallinity
and molecular interaction. Jin et al. [22] synthesized maleic anhydride-terminated polyimides (PMI)
and copolyimides by free radical reaction. This crosslinked PI system exhibit very high thermal
stability and excellent manufacturing performance. Choi and Chang et al. [23] synthesized PI from
bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BCDA) and bis[4-(3-aminophenoxy)
phenyl] sulfone (BAPS) followed by heat-treated from 250oC to 350
oC to enhance the degree of
crosslink from 85% to 93% [23]. In this work, ALPI was prepared via one step method of alicyclic
dianhydride and aromatic diamine. Adequate amount of crosslink reagent was mixed with the
semi-aromatic PIs to create the crosslink structure between PI backbones by free radical reaction in
order to reduce the dimensional change. There is no literature that make use of triallylamine to link
together with polymer chains. Besides, the crosslinked PIs maintained thermal stability and
exhibited improved optical transparency and mechancial strength .
Experimental
Materials
Bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BCDA) and 3,4'-oxydianiline
(3,4’-ODA) were purchased from Aldrich. γ-butyrolactone (GBL) and dimethylacetamide (DMAc)
were provided by TEDIA. The catalyst, isoquinoline, and crosslinkable reagent, triallylamine, were
purchased from TCI and Alfa Aesar, respectively.
Sample preparation
Alicyclic polyimides (ALPIs) with 20w% of solid content was synthesized form
bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BCDA), 3,4'-oxydianiline (3,4'-ODA)
by one step method and the synthesize process was shown in Fig. 1. In a 100-ml three-necked
round-bottomed flask, 0.8930 g ODA and 1.1070 g BCDA and were dissolved and mixed complete
in 8 g co-solvent (GBL and DMAc) by mechanical stirrer and purging with N2 to insulate the air
and water at the room temperature. After stirring for 2 h, the catalyst (isoquinoline) was mixed with
the above solution at 170~180oC for 12~15 h to form ALPI solution by thermal cyclization reaction.
Advanced Materials Research Vols. 287-290 1389
The procedure of preparing of the crosslinkable ALPIs was similar to linear ALPIs. 0.8930 g
ODA, 1.1070 g BCDA and some amounts of AIBN and various content of triallylamine were
dissolved and homogeneously mixed with 8 g co-solvent at room temperature. After reacted for 2 h,
stoichiometry of isoquinoline was mixed at 170~180 oC for 12~15 h to form crosslinkable ALPI
solutions. The linear and crosslinkable ALPI solutions were coated on glass plate and thermal cured
at 80 o
C, 170 o
C and 230 o
C each for 1.5 h in the oven. The concentrations of triallylamine in the
resultant ALPIs films were 0, 20%, 63% mol%, respectively and the thickness of ALPI films were
ranging from 22-30 µm.
Measurement
The attenuated total reflection–Fourier transform infrared (ATR-FTIR, Nicolet-380) spectra
were obtained with 64 scans per spectrum at a 2 cm-1
resolution. The ultraviolet-visible (UV-Vis)
spectra were obtained using a Shimadze UV-1800 spectrophotometer optimized with a spectral
width of 200-800 nm. The fluorescent-emssion spectra of ALPIs films were charaterized using a
Shimadze RF-5301 spectrofluorophotometer with a resolution of 1 nm and scanning in the range of
300-700 nm. The refractive index was measured by using an Atago Abbe refractometer and haze
property was characterized from Nippon Denshoku NDH 200. Dielectric measurements were
carried out by means of dielectric analyzer (Agilnet-4294A). The thermal and mechanical properties
were measured by using a thermogravimetric analyzer (TGA-Q500), a dynamic-mechanical
analyzer (DMA-2980) and a thermo mechanical analyzer (TMA-Q400) from TA instruments. The
values of DTA and coefficient of thermal expansion (CTE) were measured at a heating rate of
10oC/min under nitrogen flow. The mechanical property was measured at a heating rate of 3
oC/min
and preloading force with 0.1 N by DMA.
+
O
OO
O
NN
O
OO
O
NN
R.T., 2 hrs.
170~180oC, 12~15 hrs.
O
OO
O
OO
O
H2N
NH2
O
O
n
O
OO
O
NN
O
OO
O
NNO
O
n
O
OO
O
NN
O
OO
O
NNO
O
n
thermal curing at
80oC, 1.5 hr
170oC, 1.5 hr
230oC, 1.5 hr
N
triallyl amine
+ N NNN
AIBN
+
N
Crosslinkable PI
BCDA3,4'-ODA
N
Fig 1. Reaction scheme for preparing crosslinkable polyimide
1390 Applications of Engineering Materials
Results and discussion
FT-IR spectra of crosslinkable ALPI films
During the membrane preparation, the polymization of allyl group of triallylamine and the
double bond of BCDA was expected with the presence of the initiator AIBN and formed
crosslinking structure between PI backbones. Fig. 2 show the ATR-FTIR spectra of linear and
crosslinkable ALPI films with different content of triallylamine. The sample code, PIN0, PIN20,
andPIN63 indicated the the content of triallylamine was 0, 20, and 63 mol%. The FTIR spectra of
all ALPI membranes showed the absorption bands at 1775 cm-1
and 1699 cm-1
due to the
asymmetric and symmetric C=O stretching vibrations of the imide groups. The C-N function group
absorption at 1371 cm-1
was assigned to stretching vibration of the imide structure. The chemical
structure of ALPIs was unchanged after free radical polymerization. The major difference between
the linear ALPI and crosslinkable ALPIs was the area ratio of characteristic peaks C=C to C=O
shown in Table 1. The literature [23] indicated when the crosslink degree of PI is increased, the
intensity of C=C (705 cm-1
) of dianhydride beccomes weak. The area ratio (C=C to C=O) of PIN0
and PIN20 were 0.158 and 0.127 indicating the concentration of C=C decreased significantly after
free radical polymerization. However, the intensity of C=C became stronger when more
triallylamine was mixed into PI. It is believed that the self-linkage of triallylamine may form and
hinder the connection between triallyamine and PI. Large amounts of C=C remained when the
triallylamine was not able to crosslink the PI network that the C=C to C=O area ratio of PIN63 was
similar to PIN0.
2000 1800 1600 1400 1200 1000 800
Wavenumbers (cm-1)
PIN63
PIN20
Ab
sorb
ance
(a.
u.)
PIN0
Fig 2. FTIR spectra of preparing ALPI films
Table 1. The area ratio of ALPI films
Sample
name
PIN0 0.158
PIN20 0.127
PIN63 0.146 Characterization absorbance of C=O at 1699 cm-1 Characterization absorbance of C=C at 705 cm-1
Optical properties
Wholly aromatic PIs strongly absorb visible lights because of their high conjugated structure
and the intermolecular or intramolecular CTCs formed between or within polymer backbones. It
usually presents pale yellow to deep brown. Fig. 3(a) and 3(b) shows UV-vis spectra of PIN0,
PIN20, and PIN63 derived from alicyclic BCDA, aromatic 3,4'-ODA and (amount) crosslinkable
reagent of triallylamine. Most of these samples with a thickness of 22-29 µm show high optical
transparency (>74%, at 400 nm) and colorless when the concentration of crosslinkable reagent was
below 63 mol%. It remained excellent transparency in near ultraviolet region, especially for PIN63
(> 80%). It is believed that the alicyclic PI structure without the conjugate bonds and the separation
Advanced Materials Research Vols. 287-290 1391
of chromophores between inter-molecular by crosslinking of triallylamine avoid the π-π∗ transition
and weaken the CTCs effect [24]. From Table 2, the cut-off wavelength of ALPI (PIN0) was 322
nm and then bule shifted to below 300 nm for PIN20 and PIN63. The reasons of blue shift are due
to the decrease in the amounts of double bonds of alicyclic dianhydride and the reduction in CTCs
effect. These results indicated the initiator AIBN could make ALPIs more transparent by free
radical reacting with double bonds of dianhydride. Notably, the energy gap of films increased from
3.4 to 3.7 eV as shown in Table 2. It also demonstrate the bule-shifted phenomenon of cut-off
wavelength.
200 300 400 500 600 700 800
0
20
40
60
80
100
Tra
nsm
itta
nce
(%
)
Wavelength (nm)
PIN0
PIN20
PIN63
(3a)
300 400 500 600 700 8000.0
0.2
0.4
0.6
0.8
1.0
Ab
sorb
ance
Wavelength (nm)
PIN0
PIN20
PIN63
(3b)
Fig 3. UV-vis transmission spectra (a) and absorption spectra (b) of ALPI films
Fig. 4 showed the emission spectra of PIN0 to PIN63 films with excitation wavelength at 329
nm. The emission intensity of PIN20 and PIN63 were all higher than PIN0 because the stack
phenomenon was broken by crosslinked structures. The separation of fluorophores to weaken intra-
and/or inter-molecular interactions can be achieved by swelling the polymer backbones [25]. In
Table 2 shows the excitation wavelength, the emission wavelength and the Stoke shift of films PIN0
to PIN63. The excitation peak of PIN0 appeared at 340 nm showed bule-shifted compared to that of
PIN20 and PIN63 (333, and 328 nm). The Stokes shift of PIN20 was 0.37, which was lower than
that of PIN0 was 0.40. The main chain became rigid due to the limited crosslinking structure. When
mixing with more triallylamine, such as PIN63, the linking chain may become longer and more
flexible that larger Stokes shifts than PIN0 were observed. It is believed that the distance between
polymer chains of PIN63 was longer than PIN0 [26].
350 400 450 500 550 600
0
100
200
300
400
500
600
Inte
nsi
ty
Wavelength (nm)
PIN0
PIN20
PIN63
Fig 4. Fluorescent emission spectra of PI films
1392 Applications of Engineering Materials
Table 2. Transparency, cut-off wavelength, energy gap, fluorescence and
Stoke shift properties of ALPI films
Sample
code
T%
[400
nm]
Cut-off
wavelength
[nm]
Eg
[eV]
Ex
[nm]
Em
[nm]
Stoke shift
[eV]
PIN0 76.8 322 3.43 340 382 0.40
PIN20 74.0 297 3.75 333 370 0.37
PIN63 80.1 297 3.77 328 373 0.46
Refractive index, haze and dielectric properties
Fig. 5 shows the refractive indices of the films measured by Abbe refractometer. All of the
refractive indices of ALPIs were around 1.60 and the values became less and less gradually after
crosslinking. The swelled backbone structure by crosslinking decreased the density of PI and
lowered the refractive index [21,22]. Abbe numbers (νD) has been frequently used as a measure of
the degree of dispersion of refractive index. In general, larger values of ν are expected because
smaller dispersion of refractive index is preferable for conventional optical applications such as lens
and waveplates. The Abbe number of ALPIs measured at the wavelengths of 486, 589 and 656 nm
representing the three primary colors (blue, green, and red) shown in Fig. 5 were listed in Table 3.
The most transparent PIN20 film exhibited the highest Abbe number (νD =165.1) as expected. Haze
is also an important factor for optical materials and commonly used to estimate the phenomenon of
scattering within a matrix. Samples with larger value of haze will lose waves and reduce the
transparency. The haze value of PIN20 was the lowest and the one of other films containing more
triallylamine was increased. The self-assembly of triallylamine by free radical reaction may form
large particles and increase the haze values [27]. The dielectric property (Dk) of PIN0-PIN63 could
observed at Table 3. The values of dielectric constant decreased when ALPIs contained crosslinkers
to limit the moiety of backbones. Here, the decrease in mobility was due to both the increase of the
viscosity and the hindrances of crosslinking structure [28].
PIN0 PIN20 PIN63
1.598
1.600
1.602
1.604
1.606
1.608
1.610
1.612
1.614
Ref
ract
ive
ind
ex
Symbol
486(F)nm
589(D)nm
656(C)nm
Fig. 5 Refractive indices of ALPI films measured at 486, 589 and 656 nm
Table 3. Refractive indices, Abbe number, haze, dielectric properties of ALPI films
Sample
code
486 (F) nm
589 (D) nm
656 (C) nm
Abbe No. haze Dk
PIN0 1.6093 1.6130 1.6138 136.22 2.38 3.9
PIN20 1.6078 1.6109 1.6115 165.11 0.68 3.7
PIN63 1.5985 1.6022 1.6033 126.50 1.77 3.6
Advanced Materials Research Vols. 287-290 1393
Thermal and mechanical properties
The thermogravimetric analysis (TGA) of ALPI films was done at a heating rate of 10K per
minute in nitrogen atmosphere. Differential TG curve of the representative ALPI films were
displayed in Fig. 6 and Table 4. A series of ALPIs showed good thermal stability with few weight
loss up to 430 oC. A slight weight loss in the temperature range of 200-350
oC was observed for
crosslinkable ALPIs. The crosslinked triallylamine may be decomposed in this temperature
range[27], however, the ALPIs matrix remain excellent thermal stability. Fig. 7 showed the tan delta
of PIN0 to PIN63 films measured by dynamic mechanical analysis. The peak of tan delta was
assigned as glass transition temperature (Tg) and the values were listed in Table 4.
Table 4. Thermal and mechanical propertied of ALPI films
Sample
name Deriv. Weight
[%/min]
Tg
[℃]
Young’s
Modulus
[GPa]
Elongation at
break
[%]
C.T.E.
[ppm/℃]
PIN0 440.4 336.2 0.94 24.3 57.36
PIN20 434.5 338.5 1.24 14.8 47.53
PIN63 433.7 333.0 1.09 138.4 51.49 The C.T.E. values were measured in the range of 100-200℃
The Tg value of these films was all over 330 oC, on the other hand, the damping values of
PIN20 and PIN63 were all smaller than PIN0. The PIN0 film showed a Young’s modulus of 0.94
GPa and 24.3% elongation at break as shown in Fig. 8 and Table 4. Notably, the PIN20
crosslinkable film with shorter crosslinking chain length had a higher crosslinking density and
higher modulus and consequently were more rigid and brittle [29]. When the content of
triallylamine was higher than 20 mol%, the crosslinking chain became longer and flexible. Hence,
the improved tensile property of those crosslinkable ALPIs was revealed in the stress–strain curves.
The coefficient of thermal expansion (C.T.E.) of PIN0-PIN63 was in the range of 47.53-57.36
ppm/oC determined from TMA. After crosslinking, the crosslinkable structure limited the mobility
of polymer backbone that ALPIs (PIN20-PIN63) exhibited lower C.T.E. than PIN0. The lowest
C.T.E. obtained in the study was from PIN20, 47.53 ppm/oC, which was 17 % smaller than that of
PIN0.
100 200 300 400 500 600
0.0
0.5
1.0
1.5
2.0
2.5
Der
iv.
Wei
gh
t (%
/oC
)
Temperature (oC)
PIN0
PIN20
PIN63
Fig. 6. Differential TG curve of ALPI films
255 270 285 300 315 330 345
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Tan
Del
ta
Temperature (oC)
PIN0
PIN20
PIN63
Fig. 7. Tan delta of ALPI films
1394 Applications of Engineering Materials
0 20 40 60 80 100 120 1400
20
40
60
80
100
Str
ess
(MP
a)
Strain (%)
PIN0
PIN20
PIN63
Fig. 8. Stress-strain curve of ALPI films
Conclusions
Novel transparent alicyclic polyimides with crosslinkable structures were synthesized. All ALPI
films, especially for PIN63, was entirely colorless and showed excellent optical properties,
including high transparency (> 80%) in near ultraviolet region, short cut-off wavelength (< 300 nm)
and low haze (0.68). The refractive index of ALPI films was about 1.61 and the Abbe number was
in a range from 117 to 165. The intensity of fluorescence of PIN63 was 9-fold higher than PIN0.
The increase in Stoke shift and the decrease in refractive indice of PIN63 films confirmed the
polymer chains were swelled. The ALPI had good thermal stability with few weight loss up to
400oC, and possessed a Tg ranging from 333 to 338
oC. The elongation of break was 138% and
Young’s modulus was 1.09 GPa for PIN63. In particular, PIN63 was a colorless, tough and flexible
PI film with a low C.T.E. of 47 ppm/ o
C and was a promising candidate for the flexible substrate.
Acknowledgements
The authors would like to thank Taiwan’s Ministry of Economic Affairs for financially
supporting this research on the flexible polymeric materials for electronic usages
(99-EC-17-A-07-S1-120).
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Applications of Engineering Materials 10.4028/www.scientific.net/AMR.287-290 High Transparency and Thermal Stability of Alicyclic Polyimide with Crosslinking Structure by
Triallylamine 10.4028/www.scientific.net/AMR.287-290.1388
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