Thermal stabilization of polyimides with boric acid esters

Article abstract of DOI:10.1134/S1070427211090205

New and previously synthesized boric acid esters were tested as thermal stabilizing additives to polyimides.

Full text of DOI:10.1134/S1070427211090205

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2011, Vol. 84, No. 9, pp. 1582−1586. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © V.I. Grachek, E.T. Krut’ko, L.Yu. Osmolovskaya, A.I. Globa, 2011, published in Zhurnal Prikladnoi Khimii, 2011, Vol. 84, No. 9,
pp. 1533−1536.
MACROMOLECULAR COMPOUNDS
AND POLYMERIC MATERIALS
Thermal Stabilization of Polyimides
with Boric Acid Esters
V. I. Gracheka, E. T. Krut’kob, L. Yu. Osmolovskayab, and A. I. Globab
aInstitute of Physical Organic Chemistry, National Academy of Sciences of Belarus, Minsk, Belarus
bBelarussian State University of Technology, Minsk, Belarus
Received December 30, 2010
Abstract—New and previously synthesized boric acid esters were tested as thermal stabilizing additives to
polyimides.
DOI: 10.1134/S1070427211090205
The resistance of polymeric materials to various
kinds of aging in the course of processing and service
determines the possible limits of their application.
Therefore, enhancement of the heat resistance of
polymeric materials and thus prolongation of their
reliable service life is an urgent problem of polymer
chemistry [1]. Of much interest are multifunctional
stabilizers containing in one molecule several functional
groups acting by different mechanisms [2]. Organoboron
compounds are among such stabilizers. Along with
enhancing the heat resistance, they act as fungicidal
additives to polymeric materials [3–5].
heteroazeotropic distillation of the water formed in
the reaction. The BAE structures were proved by IR
and H NMR spectroscopy, mass spectrometry, and
1
elemental analysis.
The bands at 1350 ± 9 and 1040 ± 8 cm–1 in the
IR spectra of BAEs characterize the symmetric and
asymmetric vibrations of В–О groups, and the bands
at 1240 ± 7 and 1090 ± 10 cm–1, the symmetric and
asymmetric vibrations of С–О groups. The bands at
1410 ± 4 cm–1 belong to the vibrations of the С–N=О
group. The absorption band at 1642 cm–1 characterizes
the vibrations of the СН=N azomethine group, and that
at 1607 ± 5 cm–1, the vibrations of the С=N bond in
the ring. The band at 1757 cm–1 belongs to stretching
vibrations of the ester carbonyl group [6].
In this study we examined how boric acid esters
affect the heat resistance of polymeric materials. With
the aim to develop promising thermal stabilizers for
polyimides, we tested previously synthesized boric acid
esters and prepared new representatives of this class.
1
The Н NMR and mass spectra of the compounds
synthesized are presented in Table 1.
1
The Н NMR spectra were recorded on a Tesla BS-
As seen from Table 1, pyrocatechol ring protons give
signals at 6.21 ± 0.03 and 5.62 ± 0.02 ppm. The proton
of the azomethine group gives a signal at 8.16 ppm.
567 spectrometer (100 MHz) from solutions in СDСl3,
with HMDS as internal reference. The IR spectra were
measured with a Protégé 460 Fourier spectrometer
(Nicolet) from KBr pellets. The melting points of the
compounds were determined with a Kofler bench.
The mass spectra of BAEs contain medium-
intensity molecular peaks. The most characteristic and
the strongest peaks are those at m/z 135, 108, 92, and
65. Their origin can be associated with the C6H4O3B
fragment. As seen from Table 1, the intensity of
these peaks depends on the compound structure. The
Boric acid esters (BAEs) were prepared in one step
by heating boric acid with pyrocatechol and hydroxy
compounds at 70–80°С in benzene, with simultaneous
1582

THERMAL STABILIZATION OF POLYIMIDES WITH BORIC ACID ESTERS
1583
Table 1. Mass spectra and 1H NMR spectra of the synthesized boric acid esters I–Va
1H NMR spectrum, δ, ppm
СН1 СН2 СН3 СН4 СН5 СН6 СН7
6.18 5.60 5.67 5.48
Mass spectrum, m/z (I, %)
Compound
С6Н4О3В+ С6Н4О2+ С6Н4О+
С5Н5
+
М+
135
53
85
76
48
108
75
98
84
97
92
45
71
90
40
65
59
89
71
75
I
251 (30)
II
6.24 5.62 7.62
6.23 5.63 5.69 5.43 8.16 6.76 6.71 381 (36)
6.20 5.64 5.68 5.64 241 (61)
7.68 291 (67)
III
IV
a Compounds V and VI were described in [7].
PAA, which makes the film formation technologically
feasible because of the absence of gelation. We obtained
colorless and orange transparent BAE-containing films.
The films were subjected to cyclization on heating to
400°С. The temperature was elevated from 100 to 300°С
stepwise over a period of 4 h, because at continuous
heating the BAEs partially volatilize. The degree of
imidization of the films, according to the IR data, is no
less than 96% [8].
mass spectrum contains a strong peak with m/z = 65,
corresponding to the С5Н5 ion which can be formed only
by a rearrangement with the transfer of the hydrogen
atom from one part of the molecule to another. The peak
at m/z = 135 corresponds to the С6Н4О3В ion; that at
m/z = 108, to the С6Н4О2 ion; and that at m/z = 92, to the
С6Н4О ion. The physicochemical characteristics of the
synthesized new BAEs are given in Table 2.
To the polyimide prepolymer, polyamido acid
(PAA) synthesized by polycondensation of pyromellitic
dianhydride with 4,4'-diaminodiphenyl oxide and
dissolved in dimethylformamide, we added BAE in an
amount of 1–5 wt % relative to PAA. From the composite
obtained, we cast films on a glass support. The BAEs
introduced into PAA do not react with active groups of
ThephysicomechanicalpropertiesofBAE-containing
polyimide films were studied with a UMIV-3 device. The
results are given in Table 3. For descriptiveness sake,
instead of σ (Pa) and ε (%) we present the quantities Kσ
and Kε defined as
Kσ = σ/σ0, Kε =ε/ε0,
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 9 2011

1584
GRACHEK et al.
Table 2. Yields, melting points, and analytical data for I–V
Found, %/Calculated, %
Compound
Yield, %
Тm, °С
Formula
C
H
N
В
61.91
61.95
3.60
3.57
11.17
11.11
4.31
4.29
I
64
73
75
72
298–299
149–150
173–174
94–95
С13Н9BN2O3
С16Н10 ВNO4
C23H16BNO4
C12H8BNO4
66.08
66.04
3.49
3.44
4.92
4.81
3.69
3.71
II
72.58
72.50
4.23
4.20
3.71
3.67
2.89
2.83
III
IV
59.87
59.80
3.41
3.32
5.75
5.81
4.46
4.49
Table 3. Effect of additions of boric acid esters on the physicomechanical properties of the polyimide films
Component content, wt %
Physicomechanical properties of films
Compound
Film color
BAE
PAA
Kσ
Kε
I
3.0
5.0
1.0
3.0
5.0
3.0
5.0
3.0
5.0
3.0
5.0
3.0
5.0
–
97.0
95.0
99.0
97.0
95.0
97.0
95.0
97.0
95.0
97.0
95.0
97.0
95.0
100.0
1.02
1.04
1.00
1.01
0.97
0.96
0.96
1.00
1.01
0.98
0.96
1.01
1.01
1.00
1.00
0.98
1.05
1.04
0.98
1.07
1.08
1.06
1.09
1.02
1.04
1.06
1.08
1.00
Colorless
"
II
III
IV
Orange
Colorless
Orange
"
V
VI
No BAE
Yellowish
introduction of a BAE containing a conjugated bond
system leads to extension of the conjugation system
present in the polyimide materials and thus to coloration
of the material.
where σ0 is the strength of the polyimide film without
additive; σ, strength of the polyimide film with BAE; ε0,
elasticity of the film without additive; and ε, elasticity of
the film with BAE.
The heat resistance of the films was studied with
an OD-108 derivatograph (MOM, Hungary) in the
dynamic mode in the temperature interval 20–1000°C
at a heating rate of 5 deg min–1. The weighed portions
of 20–30-μm-thick polyimide films were 100 mg. The
results are given in Table 4.
As seen from Table 3, introduction of 1–5 wt % BAEs
intoPAAdoesnotnoticeablyalterthephysicomechanical
properties of the polyimide films. It should be noted that
BAEs containing an azomethine bond impart orange
color to the films. Coloration of films with boric acid
esters is characteristic of only polyimide polymers,
because in previous studies concerning introduction of
similar BAEs into other polymeric materials, e.g., into
polyvinyl chloride, no coloration was observed [9].
Azomethine dyes are well known [10]. Presumably,
The degradation temperature Тd characterizing the
degradation of the polyimide backbone increases on
introduction of a boric acid ester by 16–60°C compared
to the polyimide film without stabilizer. The service
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 9 2011

THERMAL STABILIZATION OF POLYIMIDES WITH BORIC ACID ESTERS
Table 4. Data on heat resistance of polyimide films
1585
Specific
viscosity of 0.5%
solution of PAA
in DMF
Component content,
wt %
Temperature, °C
Service life, min
Compound
BAE
PAA
Т5%, °С
Т10%, °С
Тd, °С
450°C
500°C
I
3.0
5.0
1.0
3.0
5.0
97.0
95.0
99.0
97.0
95.0
2.2
2.2
2.1
2.1
2.1
502
505
520
520
521
528
528
560
560
565
516
520
550
550
560
97
38
47
79
83
90
105
400
430
468
II
III
IV
V
3.0
5.0
97.0
95.0
2.3
2.3
515
517
535
537
524
529
247
289
51
69
3.0
5.0
3.0
5.0
3.0
5.0
97.0
95.0
97.0
95.0
97.0
95.0
2.1
2.1
2.3
2.3
2.2
2.1
505
507
518
520
510
512
525
535
538
540
532
537
521
527
528
531
530
534
212
243
223
268
249
249
40
61
44
57
48
67
VI
No BAE
–
100.0
2.1
497
520
500
15
13
prevents thermal oxidative degradation of the polyimide.
Introduction of BAE into the films does not alter the
main physicomechanical properties of the polymer:
strength and elasticity.
life of the polyimide film at 450 and 500°С (defined
as the period in which the tensile strength decreases to
500 kg cm–2) was determined. As seen from Table 4,
introduction of BAE prolongs the service life of the
polyimide with the additive from 15 to 97–468 min at
450°C and from 13 to 22–90 min at 500°C. Table 4 shows
that all the BAEs tested enhance the heat resistance of
the polyimide films relative to the material without BAE.
However, the thermal stabilizing performance of BAEs
dependsontheirstructure.Theboricacidestercontaining
a heterocyclic benzimidazole group is least active. The
presence of an azomethine group in BAEs enhances
their performance and imparts color to polyimide films.
The most effective thermal stabilizer for polyimide films
is the BAE containing a nitroso group and a naphthalene
ring (compound II). Specifically the nitroso group in
the naphthalene ring ensures high thermal stabilizing
properties of the additive. With the naphthalene ring
replaced by the benzene ring (compound IV), the thermal
stabilizing effect is lower (Table 4). Aging of polymeric
materials involves degradation and rearrangement of
bonds, which initiate thermal oxidative degradation of
the chains. The effective BAE contains an electron-
withdrawing boron atom which, by accepting electrons
on vacant orbitals, blocks radicals formed in the course
of polymer aging; furthermore, it contains the nitrogen
atom of the nitroso group, capable of chain termination,
and the naphthalene ring. Apparently, this combination
Thus, boric acid esters can be used as stabilizers
for preparing polyimide materials with enhanced heat
resistance and prolonged service life, without affecting
the physicomechanical characteristics. Furthermore, the
esters containing the azomethine group can be used as
coloring thermal stabilizers for polyimide materials. It
is known [11, 12] that organoboron compounds protect
polymeric materials from fouling with mold fungi,
i.e., prevent biodegradation of the polymers. It can
be expected that introduction of BAE into polyimide
materials will prolong the time of their reliable operation.
CONCLUSIONS
(1) The boric acid esters studied enhance the heat
resistance of polyimide materials.
(2) The boric acid esters containing an azomethine
bond act simultaneously as thermal stabilizers and as
additives imparting orange color to polyimides.
REFERENCES
1. Zaikov, G.E., Kauchuk Rezina, 2008, no. 4, pp. 10–13.
2. Zemskii, D.N. and Dorofeeva, Yu.N., Kauchuk Rezina,
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 9 2011

1586
2009, no. 6, pp. 12–13.
GRACHEK et al.
Poliimidy – klass termostoikikh polimerov (Polyimides:
a Class of Heat-Resistant Polymers), Leningrad: Nauka,
1983.
3. Puchkov, A.F. and Turenko, S.V., Zh. Prikl. Khim., 2005,
vol. 78, no. 8, pp.11–14.
9. Grachek, V.I., Naumova, S.F., Motol’ko, G.R., et al.,
Dokl. Akad. Nauk Bel. SSR, 1988, vol. 32, no. 3, pp. 235–
238.
4. Chem, T.S.S., Chang, C., and Floss, H.G., J. Org. Chem.,
1981, vol. 46, no. 13, pp. 2661–2665.
5. Grachek, V.I., Bukanova, N.N., Smolyakov, A.V., et al.,
Zh. Prikl. Khim., 2001, vol. 74, no. 1, pp. 147–150.
10. Borodkin, V.F., Khimiya krasitelei (Dye Chemistry),
Moscow: Khimiya, 1981.
6. Bellamy, L.J., The Infra-Red Spectra of Complex Mol-
ecules, New York: Wiley, 1957.
11. Meilakhs, L.A., Gorelik, R.A., and Dorokhov, V.A.,
Kauchuk Rezina, 1986, no. 2, pp. 42–43.
7. Grachek, V.I. and Lukashik, A.N., Zh. Prikl. Khim., 2006,
vol. 79, no. 5, pp. 830–834.
12. Fedortsova, E.V., Idlis, G.S., Shvarts, E.M., et al., Zh.
Obshch. Khim., 1994, vol. 64, no. 1, pp. 139–141.
8. Bessonov, M.I., Koton, M.M., Kudryavtsev, V.V., et al.,
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 9 2011