A study on hydrogen-bonded network structure of polybenzoxazines

Article abstract of DOI:10.1021/jp010606p

The hydrogen-bonded network structure for polybenzoxazines is investigated by Fourier Transform Infrared Spectroscopy (FT-IR) with model dimer systems. Comparing the FT-IR spectra of the polybenzoxazines and model dimers, it is shown that the simpler structures of asymmetric dimers well simulate the hydrogen-bonded network structure between polymer chains while the structures of symmetric dimers reflect the hydrogen bonding scheme related to the end-groups of polymer chains. It is confirmed that the amine functional group in the Mannich bridge is greatly responsible for the distribution of hydrogen bonding species. Bisphenol A/methylamine-based polymer (BA-m) mainly consists of -OH...N intramolecular hydrogen bonding while bisphenol A/aniline-based polymer (BA-a) has a large amount of intermolecular hydrogen bonding and relatively weak hydrogen bonding groups in the polymer network structure. The possible network structure, in the sense of hydrogen bonding, for BA-m and BA-a polymers is proposed and a generalized explanation for the structure-property relationships in polybenzoxazines is also discussed.

Full text of DOI:10.1021/jp010606p

J. Phys. Chem. A 2002, 106, 3271-3280
3271
ARTICLES
A Study on Hydrogen-Bonded Network Structure of Polybenzoxazines†
Ho-Dong Kim and Hatsuo Ishida*
NSF Center for Molecular and Microstructure of Composites (CMMC),
Department of Macromolecular Science and Engineering,
Case Western ReserVe UniVersity, CleVeland, Ohio 44106
ReceiVed: February 15, 2001
The hydrogen-bonded network structure for polybenzoxazines is investigated by Fourier Transform Infrared
Spectroscopy (FT-IR) with model dimer systems. Comparing the FT-IR spectra of the polybenzoxazines and
model dimers, it is shown that the simpler structures of asymmetric dimers well simulate the hydrogen-
bonded network structure between polymer chains while the structures of symmetric dimers reflect the hydrogen
bonding scheme related to the end-groups of polymer chains. It is confirmed that the amine functional group
in the Mannich bridge is greatly responsible for the distribution of hydrogen bonding species. Bisphenol
A/methylamine-based polymer (BA-m) mainly consists of -OH‚‚‚N intramolecular hydrogen bonding while
bisphenol A/aniline-based polymer (BA-a) has a large amount of intermolecular hydrogen bonding and relatively
weak hydrogen bonding groups in the polymer network structure. The possible network structure, in the
sense of hydrogen bonding, for BA-m and BA-a polymers is proposed and a generalized explanation for the
structure-property relationships in polybenzoxazines is also discussed.
Introduction
Recently, polybenzoxazines having a number of unique
properties have been actively studied. This class of polymers
1
,2
has many advantages such as excellent mechanical properties,
2
high char yield, near zero volumetric shrinkage/expansion upon
polymerization, low water absorption despite the large amount
3
of hydroxyl groups in the backbone structure, excellent resis-
4
5
6
tance to chemicals and UV light, and high Tg. For several
years, these properties have been extensively studied to achieve
a good molecular understanding of the polymer, as well as to
meet the industrial interests.
1
According to Ishida and Allen, it has been determined that
the network structure of polybenzoxazine is supported by the
interaction of polymer chains due to strong hydrogen bonding,
as well as chemical cross-linking. Therefore, taking into account
the low cross-linking density in polybenzoxazines, the role of
hydrogen bonding in the polymer system is of great importance
in interpreting structure-property relationships. Despite the
similar chemical structure of BA-m polymer and BA-a polymer,
they have different FT-IR spectra in the region of the hydroxyl
stretching frequencies as shown in Figure 1. This has been
simply explained by the fact that the strength of hydrogen
bonding is dependent on the electronegativity of the side group
that is attached to the nitrogen atom (Scheme 1). That is, the
electron cloud density around the nitrogen atom attached to the
benzene ring is lower than that of the BA-m polymer because
the electrons on the nitrogen atom in BA-a polymer are more
delocalized.
Figure 1. Comparison of the FTIR spectra for BA-m and BA-a
polybenzoxazines in the region of hydroxyl stretching frequency.
This network structure in polybenzoxazines, which is sup-
ported by hydrogen bonding, might be a possible and important
contribution to forming a rigid network structure. However,
despite the importance of hydrogen bonding in polybenzox-
1
,7,8
azines, only a few papers
have been reported because it is
difficult to analyze the structure of polybenzoxazines due to
the instrumental limitations for thermoset resins. In addition,
since the polybenzoxazine structure can form several different
hydrogen bonding species between hydroxyl groups or between
hydroxyl group and tertiary nitrogen atom, it has been even
more difficult to study the structure. Furthermore, although it
was postulated in previous papers that the physical properties
of polybenzoxazines including chemical and UV stability may
†
Part of the special issue “Mitsuo Tasumi Festschrift”.
Author to whom correspondence should be addressed.
*
1
0.1021/jp010606p CCC: $22.00 © 2002 American Chemical Society
Published on Web 06/07/2001

3272 J. Phys. Chem. A, Vol. 106, No. 14, 2002
Kim and Ishida
SCHEME 1: Difference of Electronegativity around the
Nitrogen Atom in Mannich Bridge (A)
Methylamine-Based Structure, and (B) Aniline-Based
Structure
SCHEME 2: Bis-phenol A-Based Benzoxazine Monomer
Ar-H). 13C NMR (50.1 MHz, CDCl3, 298 K) δ: 15.74, 20.41
(
1
4
4C, Ar-C), 41.06 (1C, N-CH3), 59.25 (2C, Ar-C-N), and
21.80-152.00 (12C, Ar). Anal. Found: C, 76.40; H, 8.54; N,
.60. Calcd. For C19H25NO2: C, 76.22; H, 8.42; N, 4.68.
Synthesis of an Aniline-Based Polybenzoxazine Model
Dimer. N,N-Bis(3,5-dimethyl-2-hydroxybenzyl)aniline (Scheme
d) was also synthesized by the method described in the
3
1
1
previous paper. The product was recrystallized from hexane
until white crystals were obtained and subsequently purified by
column separation. White needlelike crystal. H NMR (200
1
MHz, CDCl3, 298 K) δ: 2.16, 2.17 (12H, Ar-CH3), 4.26 (4H,
Ar-CH2-N), 6.65, 6.82 (4H, Ar-H), and 7.00-7.31 (5H, Ar-
1
3
also be related to the nature of the amines,4,5 the distribution of
H). C NMR (50.1 MHz, CDCl3, 298 K) δ: 15.84, 20.48 (4C,
Ar-C), 55.68 (2C, Ar-C-N), and 121.60-151.70 (18C, Ar).
Anal. Found: C, 79.89; H, 7.69; N, 3.91. Calcd. For
C24H27NO2: C, 79.74; H, 7.53; N, 3.87.
hydrogen bonding species in polybenzoxazine has not been
evaluated. In addition, the reason two typical polybenzoxazines
have significantly different properties is not clear.
Synthesis of an Asymmetric Methylamine-Based Model
Dimer. N-Benzyl-N-(3,5-dimethyl-2-hydroxylbenzyl)methyl-
amine (Scheme 3b) was synthesized using 2,4-dimethylphenol,
p-formaldehyde, and N-methylbenzylamine as follows. A solu-
tion of 2,4-dimethylphenol (0.05 mol), p-formaldehyde (0.05
mol), and N-methylbenzylamine (0.05 mol) was azeotropically
distilled in n-hexane until 0.9 mL of water had been collected.
The hexane solution was washed several times with distilled
water, and dried over sodium sulfate. The residual products were
purified by column chromatography on silica gel using hexane/
In this paper, we will, therefore, investigate the hydrogen-
bonded network structure of polybenzoxazines using well-
designed polybenzoxazine model dimer molecules. Additionally,
this paper intends to establish the generalized explanation
between the physical properties of polybenzoxazines and the
network structure.
Experimental Section
All chemicals were used as received. Bisphenol A (97%),
p-formaldehyde (95%), aniline (99%), methylamine (40% in
water), 2,4-dimethylphenol (98%), benzylbromide (98%), sali-
cylaldehyde (98%), N-methylbenzylamine (97%), and N-phen-
ylbenzylamine (99%) were obtained from Aldrich Chemical Co.
Formaldehyde (37% in water) was purchased from Fisher
Scientific.
1
tetrahydrofuran (10:1) as the eluent. Pale yellow liquid. H NMR
(
200 MHz, CDCl3, 298 K) δ: 2.20, 2.21 (9H, -CH3), 3.58,
3
7
1
.69 (4H, Ar-CH2-N), 6.65, 6.86 (2H, Ar-H), and 7.24-
1
3
.38 (5H, Ar-H). C NMR (50.1 MHz, CDCl3, 298 K) δ:
5.64, 20.38 (2C, Ar-C), 41.05 (1C, N-C), 61.03, 61.50 (2C,
Ar-C-N), and 120.73-153.47 (12C, Ar). Anal. Found: C,
8
8
0.07; H, 8.15; N, 5.56. Calcd. For C17H21NO: C, 79.96; H,
.29; N, 5.49.
Synthesis of Benzoxazine Monomers and Polymers. Two
typical bifunctional benzoxazine monomers were synthesized
9
and purified according to the procedure of Ning and Ishida or
Synthesis of an Asymmetric Aniline-Based Model Dimer.
Ishida.10 One was 2,2-bis(3,4-dihydro-3-methyl-2H-1,3-ben-
zoxazine)propane (abbreviated as BA-m) based on methylamine
and the other was 2,2-bis(3,4-dihydro-3-phenyl-2H-1,3-benzox-
azine)propane (abbreviated as BA-a) based on aniline. The purity
of the compounds was determined using 200 MHz proton
N-Benzyl-N-(3,5-dimethyl-2-hydroxylbenzyl)aniline (Scheme
3e) was synthesized using 2,4-dimethylphenol, p-formaldehyde,
and N-phenylbenzylamine as follows. A solution of 2,4-
dimethylphenol (0.05 mol), p-formaldehyde (0.05 mol), and
o
N-phenylbenzylamine (0.05 mol) was reacted at 105 C for 1
1
nuclear magnetic resonance ( H NMR) spectra. These benzox-
h. The products were then dissolved in chloroform, washed
several times with distilled water, and dried over sodium sulfate.
The residual products were purified by column chromatography
on silica gel using hexane/ethyl acetate (5:1) as the eluent. Pale
azine monomers were polymerized without added initiator or
catalyst according to the method which was shown in the
4
previous paper. R in the structure denotes the substituent of
1
the primary amine (Scheme 2).
yellow liquid. H NMR (200 MHz, CDCl3, 298 K) δ: 2.20,
2
.22 (6H, Ar-CH3), 4.27, 4.31 (4H, Ar-CH2-N), 6.69, 6.86
Synthesis of a Methylamine-Based Polybenzoxazine Model
Dimer. N,N-Bis(3,5-dimethyl-2-hydroxybenzyl)methylamine
1
3
(
2H, Ar-H), and 6.68-7.35 (10H, Ar-H). C NMR (50.1
MHz, CDCl3, 298 K) δ: 15.62, 20.48 (2C, Ar-C), 55.19, 57.68
(2C, Ar-C-N), and 112.81 ∼ 152.84 (18C, Ar). Anal. Found:
C, 83.49; H, 7.12; N, 4.63. Calcd. For C22H23NO: C, 83.24;
H, 7.30; N, 4.41.
(
Scheme 3a) was synthesized according to a previous study
1
1
using 2,4-dimethylphenol, formaldehyde, and methylamine.
White, irregular crystals were obtained by recrystallization from
ethyl ether for the methylamine-based dimer. White crystal. H
1
NMR (200 MHz, CDCl3, 298 K) δ: 2.22 (12H, Ar-CH3), δ:
Synthesis of Dibenzylmethylamine. N,N-Dibenzylmethyl-
amine (Scheme 3c) was synthesized using benzylbromide and
2
.24 (3H, N-CH3), 3.66 (4H, Ar-CH2-N), and 6.73, 6.89 (4H,

Hydrogen-Bonded Network of Polybenzoxazines
J. Phys. Chem. A, Vol. 106, No. 14, 2002 3273
SCHEME 3: Polybenzoxazine Model Dimers. (A) Methyl-dimer, (B) Asymmetric Methyl-dimer, (C) Benzylic Methyl-
dimer, (D) Aniline-dimer, (E) Asymmetric Aniline-dimer, and (F) Benzylic Aniline-dimer
methylamine as follows.12 A stoichiometric amount of reactants
benzylbromide: methylamine ) 2:1) were reacted in ethyl ether
Fourier transform infrared (FT-IR) spectra were obtained on
a Bomem Michelson MB110 FT-IR spectrophotometer which
was equipped with a liquid nitrogen cooled, mercury cadmium
telluride (MCT) detector with a specific detectivity, D*, of 1
(
at room temperature for 10 min and the viscous product
dissolved in ethyl ether. The ether solution was washed several
times with distilled water, and dried over sodium sulfate. The
residual products were purified by column chromatography on
silica gel using hexane/methylene chloride (3:2) as eluent. Pale
1
0
1/2
-1
× 10 cm Hz W . Coaddition of 128 scans was recorded at
-
1
a resolution of 4 cm after 20 min purge with dry nitrogen.
FT-IR spectra of the BA-m and BA-a polymers were taken from
the thin films cured on potassium bromide (KBr) plates, using
1
yellow liquid. H NMR (200 MHz, CDCl3, 298 K) δ: 2.17 (3H,
4
-
CH3), 3.51 (4H, Ar-CH2-N), and 7.19-7.38 (10H, Ar-H).
the same procedure in a previous paper. The crystalline dimer
1
3
C NMR (50.1 MHz, CDCl3, 298 K) δ: 42.24 (1C, N-C),
spectra for Methyl-dimer and Aniline-dimer were taken as a
thin powder layer between two KBr plates. The spectra for the
benzylic Methyl-dimer, benzylic Aniline-dimer, asymmetric
Methyl-dimer, and asymmetric Aniline-dimer in pure liquid
phase were collected from a thin layer between two KBr plates.
The KBr pellet technique was not used to avoid water from
interfering with the OH stretching region of interest. The solution
spectra for the dimers in CCl4 were obtained in a barium fluoride
liquid cell with a 0.5 mm thickness for 10 mM and 50 mM
concentrations and with a 5 mm thickness for 1 mM concentra-
tion. The spectrum of the liquid cell with spectrophotometric
grade carbon tetrachloride (CCl4) was subtracted from the
solution spectra.
6
1.85 (2C, Ar-C-N), and 126.87-139.31 (12C, Ar). Anal.
Found: C, 85.06; H, 7.98; N, 6.44. Calcd. For C15H17N: C,
8
5.26; H, 8.11; N, 6.63.
Synthesis of Dibenzylaniline. N,N-Dibenzylaniline (Scheme
f) was synthesized using benzylbromide and aniline as follows.
3
A solution of benzylbromide (0.04 mol) and aniline (0.06 mol)
in chloroform was reacted at room temperature with vigorous
stirring for 1 h. An excess amount of aniline was used as an
acid-trap as well as reactant. The mixture was then dissolved
in chloroform and the organic layer was separated, dried over
sodium sulfate, and evaporated. The residual products were
purified by column chromatography on silica gel using hexane/
1
methylene chloride (5:1) as eluent. Pale yellow liquid. H NMR
Heavily overlapped bands in the hydroxyl stretching region
-1
(
7
5
200 MHz, CDCl3, 298 K) 4.65 (4H, Ar-CH2-N), and 6.65-
of FT-IR spectrum from 2000 to 4000 cm were curve-resolved
using a mixed Lorentzian-Gaussian function in GRAMS
software. It was calculated until the least-squares curve-fitting
converged.
.35 (15H, Ar-H). 13C NMR (50.1 MHz, CDCl3, 298 K) δ:
4.15 (2C, Ar-C-N), and 112.39-138.54 (18C, Ar). Anal.
Found: C, 87.69; H, 7.19; N, 5.02. Calcd. For C20H19N: C,
8
7.87; H, 7.01; N, 5.12.
The purity of the dimers was examined using a Varian XL200
Results
1
13
nuclear magnetic resonance spectrometer ( H NMR and
C
NMR) and a Hewlett-Packard 6890 Gas Chromatograph equipped
with a 5973 Mass Selective Detector (GC-MS). For simplicity,
N,N-bis(3,5-dimethyl-2-hydroxybenzyl)methylamine, N,N-bis(3,5-
dimethyl-2-hydroxybenzyl)aniline, N,N-dibenzylmethylamine,
N,N-dibenzylaniline, N-benzyl-N-(2-hydroxylbenzyl)methy-
lamine, and N-benzyl-N-(3,5-dimethyl-2-hydroxylbenzyl) will
be referred to as Methyl-dimer, Aniline-dimer, asymmetric
Methyl-dimer, asymmetric Aniline-dimer, benzylic Methyl-
dimer, and benzylic Aniline-dimer in this paper, respectively.
Symmetric Dimers. To simulate the distribution of hydrogen-
bonded species in polybenzoxazines, the FT-IR spectra for
several benzoxazine model dimers were investigated. Since
benzylic dimers do not have hydroxyl stretching bands, they
show flat baselines in the hydrogen-bonded hydroxyl stretching
regions as shown in Figures 2c and 3c. Since the chemical
structures of the symmetric and asymmetric dimers are similar
to benzylic dimmers, except for the hydroxyl groups, the
hydroxyl stretching regions of FT-IR spectra for the symmetric

3274 J. Phys. Chem. A, Vol. 106, No. 14, 2002
Kim and Ishida
Figure 2. FT-IR spectra for (a) Methyl-dimer in solution state in CCl
50 mM), (b) Methyl-dimer in crystal state, and (c) benzylic Methyl-
dimer in liquid phase.
4
Figure 3. FT-IR spectra for (a) Aniline-dimer in solution state in CCl4
(50 mM), (b) Aniline-dimer in crystal state, and (c) benzylic Aniline-
dimer in liquid phase.
(
and asymmetric dimers will reflect the distribution of hydrogen
bonding in the dimers. In the study of hydrogen bonding for a
7
benzoxazine model dimer using methylamine by Dunkers, it
was shown that it is possible to form several kinds of hydrogen-
bonded species, such as -OH‚‚‚O intermolecular hydrogen
bonding, -OH‚‚‚O intramolecular hydrogen bonding, and
-
OH‚‚‚N intramolecular hydrogen bonding. The band centered
-
1
at 3400 cm in Figure 2b shows the existence of -OH‚‚‚O
intermolecular hydrogen bonding, and the broad band between
-
1
2
500 and 3300 cm
might be designated for -OH‚‚‚O
intramolecular hydrogen bonding and/or -OH‚‚‚N intramo-
lecular hydrogen bonding. Consequently, this result has been
used to explain the network structure and good physical
properties of polybenzoxazines reported in many papers.
However, despite the understanding that the hydrogen bond-
ing in the Mannich bridge is strongly affected by the basicity
of amine functional group which is attached to the nitrogen
atom, the actual difference between the Methyl-dimer and
Aniline-dimer was overlooked with regard to hydrogen bonding.
The analysis of the aniline-based model system is essential in
order to interpret the more realistic hydrogen bonding structure
in the BA-a polymer. Therefore, the model Aniline-dimer for
the BA-a polymer is synthesized and the FT-IR spectrum is
investigated.
4
Figure 4. FT-IR spectra for Methyl-dimer in CCl solution; (a) 50
mM, (b) 10 mM, and (c) 1 mM concentration.
Surprisingly, despite the large difference in basicity in the
corresponding amines, the FT-IR spectra for the two model
dimer crystals show very similar features (Figures 2b and 3b).
Taking into account the fact that the nature of hydrogen bonding
in Mannich bases is closely related to the acidity of the parent
phenol or basicity of the amine functional group, it was
expected that the distribution of hydrogen bonding species in
Aniline-dimer should be different. However, from the similar
spectra for both model dimer crystals, it seems that the basicity
of the amine functional group has little influence on the
hydrogen bonding structure in the crystal lattice. Since the
molecules in crystal form have an order in a certain lattice cell
and are closely arranged to each other, the hydrogen bonding
structure is highly affected by the physical packing of molecules.
Therefore, it is possible that there is no significant difference
in the spectrum of the crystal state for both model dimers if
both model dimers have similar crystal structures.
1
3
In solution, however, the dimer molecules can freely change
their relative positions and have wide spaces between them, thus
allowing the intermolecular and intramolecular hydrogen bond-
ing to be distinguished by examining CCl4 solution spectra at
various concentrations. Figure 4 shows the FT-IR spectra of
Methyl-dimer at various concentrations in CCl4. In Figure 4a,
the spectrum for a concentration of 50 mM shows significant

Hydrogen-Bonded Network of Polybenzoxazines
J. Phys. Chem. A, Vol. 106, No. 14, 2002 3275
Figure 6. FT-IR spectra for asymmetric Methyl-dimer (a) in liquid
4
Figure 5. FT-IR spectra for Aniline-dimer in CCl solution; (a) 50
mM, (b) 10 mM, and (c) 1 mM concentration.
4
phase, (b) in solution state in CCl (50 mM), and (c) in solution state
in CCl (1 mM).
4
intermolecular hydrogen bonding as demonstrated by the strong
state, became liberated in solution and turned into the intramo-
lecular association. Again, a more detailed investigation for this
intramolecular association will be done later in this paper by
examining the FT-IR spectra for asymmetric dimers. One more
interesting point is that the overall FT-IR spectrum of the BA-a
polymer is similar to that of the Aniline-dimer spectrum in CCl4,
which is logical, taking into account that the present polyben-
zoxazines are thermoset polymers and are, therefore, amorphous.
As a result, the FT-IR spectrum of the hydroxyl stretching region
of Aniline-dimer in CCl4, which is not restricted by a physically
ordered structure, is suitable to simulate the BA-a polymer
spectrum.
Asymmetric Dimers. Although the results from symmetric
dimers can be used to hypothesize the hydrogen bonding
structures of polybenzoxazines, they show complex hydrogen-
bonded structures. As is shown in Schemes 2 and 7, when one
benzoxazine ring is opened, only one Mannich bridge is formed,
and thus, a polybenzoxazine has one hydroxyl group per one
Mannich bridge. However, since the symmetric models have
two hydroxyl groups for each Mannich bridge, one extra
hydroxyl group makes the distinction between the -OH‚‚‚O
intramolecular hydrogen bonding and -OH‚‚‚N intramolecular
hydrogen bonding difficult. In addition, since half of the -OH
groups in the dimers may be involved in intermolecular
hydrogen bonding, the realistic contribution of hydroxyl group
to intermolecular hydrogen bonding in polymers cannot be
simulated accordingly. To eliminate these possibilities, asym-
metric model dimers having only one -OH group in the
structure were synthesized. As is shown in Schemes 3b and 3e,
only two kinds of hydrogen bonding can be formed in these
asymmetric dimers; one is -OH‚‚‚O intermolecular hydrogen
bonding, and the other is -OH‚‚‚N intramolecular hydrogen
bonding. Based on these asymmetric dimers, the actual contri-
bution of each hydrogen-bonded species to the FT-IR spectrum
can be examined.
band at 3401 cm- as well as free hydroxyl groups (3615 cm )
due to the dilution of the dimer molecules. This assignment for
intermolecular hydrogen bonding is confirmed by the fact that
1
-1
-
1
the intensity of this band at 3401 cm is systematically
decreased by the decrease in concentration. Figure 4c displays
the spectrum of 1 mM of Methyl-dimer in CCl4. If intramo-
lecular hydrogen bonding is not present, all the hydroxyl groups
are expected to be free at this concentration as has been shown
in studies on hydrogen bonding in phenolic dimers.1
4,15
Obvi-
ously, two evident bands for the free hydroxyl group (3615
-
1
-1
cm ) and the intramolecular hydrogen bonding (3207 cm )
are shown and better resolved at 1 mM concentration. However,
it is not clear whether these bands originate from -OH‚‚‚N
intramolecular hydrogen bonding or from -OH‚‚‚O intramo-
lecular hydrogen bonding. Besides, the existence of a broad band
-
1
around 3400 cm , which is a similar frequency for -OH‚‚‚O
intermolecular hydrogen bonding, is also observed in the
spectrum of 1 mM. Since the possibility of -OH‚‚‚O intermo-
lecular association is low in 1 mM concentration, this broad
band might originate from another intramolecular association,
such as -OH‚‚‚O intramolecular hydrogen bonding. This
complexity will be investigated in detail using the asymmetric
dimers which have a simpler hydrogen bonding schemes. In
addition, it is interesting that the band centered at 3559 cm ,
which is independent of concentration, is observed. Although
the assignment for this band cannot be precisely determined
from these spectra, it is clear that this band originates from the
intramolecular association. The nature of this band will be
discussed later.
-
1
Features similar to the Methyl-dimer are shown in the FT-
IR spectrum of Aniline-dimer in CCl4 (Figure 5). However, the
amount of intermolecular hydrogen bonding, which is repre-
-
1
sented by the intensity of the band centered at 3421 cm , is
relatively small compared to the spectrum of Methyl-dimer in
-
1
5
0 mM concentration. Instead, the band at 3549 cm , which
From the FT-IR spectrum of the asymmetric Methyl-dimer
(Figure 6), intramolecular hydrogen bonding is observed in the
liquid phase. No intermolecular hydrogen bonding exists even
in the liquid phase of asymmetric Methyl-dimer. Since the
is suspected to be an intramolecular association, is significantly
increased. This means that the large amount of OH groups,
which formed intermolecular hydrogen bonding in the crystal

3276 J. Phys. Chem. A, Vol. 106, No. 14, 2002
Kim and Ishida
SCHEME 4: Proton-Transfer Equilibrium in Mannich
Bases
In addition, it is interesting that the stretching band for
protonated nitrogen exists, which is demonstrated by the broad
-1
-1
tail around 2700 cm to 2400 cm in both asymmetric dimers.
The existence of proton-transfer equilibrium (OH‚‚‚N T
-
+
O ‚‚‚H N) has been reported for the hydrogen bonding system
in Mannich base which is a direct analogue of phenol-amine
1
6-20
complex (Scheme 4).
Especially, Rospenk and Zeegers-
1
8
-
+
Huyskens reported that the stretching band for -O ‚‚‚H N
intramolecular hydrogen bonding is observed between 2800 and
700 cm using 2-(N,N-dodecylaminomethyl)-3,6-dichloro-4-
-1
2
nitrophenol and its -OD deuterated analogue. From the curve-
fitting results of the spectra in 1 mM solution shown in Figure
8
, it is obvious that the existence of protonated nitrogen is
Figure 7. FT-IR spectra for asymmetric Aniline-dimer (a) in liquid
phase, (b) in solution state in CCl (50 mM), and (c) in solution state
in CCl (1 mM).
-
1
demonstrated by the bands at 2750 cm (asymmetric Methyl-
dimer) and at 2830 cm (asymmetric Aniline-dimer).
4
-
1
4
asymmetric dimer has only one hydroxyl group in the structure,
if the stretching band for the -OH‚‚‚O intermolecular hydrogen
bonding is not shown, it can be concluded that only the
intramolecular hydrogen bonding exists in the asymmetric
Methyl-dimer. Furthermore, no evidence of free hydroxyl groups
is observed even at 1 mM concentration. This strongly supports
that -OH‚‚‚N intramolecular hydrogen bonding in the Methyl-
dimer is very stable and all hydroxyl groups in the asymmetric
Methyl-dimer are participating in the -OH‚‚‚N intramolecular
hydrogen bonding.
On the other hand, a significant amount of intermolecular
hydrogen bonding is found in the spectrum of asymmetric
Aniline-dimer in the liquid phase (Figure 7a). This means that
the hydroxyl groups in the asymmetric Aniline-dimer are
competitively forming the -OH‚‚‚N intramolecular hydrogen
bonding or -OH‚‚‚O intermolecular hydrogen bonding. Fur-
thermore, as shown in Figure 7, parts b and c, the free hydroxyl
stretching mode observed in the FT-IR spectrum of the
asymmetric Aniline-dimer in CCl4 suggests that a considerable
amount of the hydroxyl groups, which formed the intermolecular
hydrogen bonding in the liquid phase, are freed by the dilution.
From the evaluation of FT-IR spectra of the asymmetric
Figure 8. Curve fitting for FT-IR spectra of symmetric dimers in CCl
solution (1 mM); (a) Methyl-dimer and (b) Aniline-dimer.
4
From the comparison of the curve-resolved band for the
protonated nitrogen, it is noticeable that the intensity of the band
-1
at 2750 cm for the asymmetric Methyl-dimer is much stronger
than that for the asymmetric Aniline-dimer. This means that
-
1
dimers, the concentration-independent bands at 3113 cm (for
-
1
-
+
the asymmetric Methyl-dimer) and 3168 cm (for the asym-
metric Aniline-dimer) can be assigned to -OH‚‚‚N intramo-
lecular hydrogen bonding. The stretching frequency for this
the proton-transfer equilibrium (OH‚‚‚N T O ‚‚‚H N) in the
asymmetric Aniline-dimer is shifted to the left-hand side, and
thus, the polarizability of a proton in Mannich base is affected
by the amine functional group. Consequently, this difference
in the proton-transfer equilibrium can also be used for explaning
why the asymmetric Aniline-dimer shows the bands for free
hydroxyl group and intermolecular hydrogen bonding while the
asymmetric Methyl-dimer has only intramolecular hydrogen
bonding even in a very diluted concentration.
-OH‚‚‚N intramolecular hydrogen bonding can be more clearly
resolved in the spectra of 1 mM CCl4 solution. This assignment
agrees well with the typical chelated hydrogen bonds in Mannich
7
,16
bridges reported by several authors.
It can also be assigned
-1
the band at 3417 cm to the -OH‚‚‚O intermolecular hydrogen
bonding because this band in the asymmetric Aniline-dimer in
the liquid phase shows concentration dependency. However, in
the case of symmetric dimers, a broad band still exists around
-OH‚‚‚π Hydrogen Bonding. As mentioned in the sym-
-
1
metric dimers section, although the shoulder band at 3559 cm
-
1
-1
3
400 cm even at the dilute concentration of 1 mM. Hence, it
in the Methyl-dimer and strong band at 3547 cm in the
Aniline-dimer have the nature of intramolecular association
because those are not dependent on concentration, the origin
of this stretching band cannot be precisely determined. The
implies the possibility of overlapped intermolecular and in-
tramolecular hydrogen-bonded OH bands near 3400 cm- in
symmetric dimers.
1

Hydrogen-Bonded Network of Polybenzoxazines
J. Phys. Chem. A, Vol. 106, No. 14, 2002 3277
SCHEME 5: Hydrogen Bonding in Asymmetric Aniline-dimer. (A) -OH‚‚‚N Intramolecular Hydrogen Bonding, (B)
-
OH‚‚‚π Intramolecular Hydrogen Bonding, (C) -OH‚‚‚π Intermolecular Hydrogen, and (D) -OH‚‚‚O Intermolecular
Hydrogen Bonding
assignment for this band in the Methyl-dimer was attempted
by Dunkers et al., but they could not explain this band clearly.
SCHEME 6: Hydrogen Bonding in Aniline-dimer. (A)
-OH‚‚‚N Intramolecular Hydrogen Bonding, (B)
-OH‚‚‚π Intramolecular Hydrogen Bonding, and (C)
-OH‚‚‚O Intermolecular Hydrogen Bonding
7
However, using simple hydrogen bonding structure of asym-
metric dimers, this band can be investigated in more detail.
Despite the simple hydrogen bonding scheme in the asymmetric
Aniline-dimer, the FT-IR spectrum of the asymmetric Aniline-
dimer in the liquid phase shows that there is a new band other
than -OH‚‚‚N intramolecular or -OH‚‚‚O intermolecular
hydrogen bonding (Figure 7a). According to Pimentel and
McClellan,21 hydrogen bonding exists between the proton of
acids and aromatics because aromatics act as good electron
14
donors. And, according to Cairns and Eglinton, it has been
reported that -OH‚‚‚π intramolecular hydrogen bonding is
-1
observed at 3516 cm in the infrared spectrum of well-designed
novolac dimers. They also concluded that this -OH‚‚‚π
intramolecular hydrogen bonding is highly affected by the
conformation of molecules.
A similar explanation can be applied to the benzoxazine
model dimers. In the asymmetric Aniline-dimer, the hydroxyl
groups which are not intramolecularly associated can form either
-
OH‚‚‚O intermolecular hydrogen bonding (Scheme 5d) or
OH‚‚‚π hydrogen bonding (Scheme 5b and 5c) as long as
-
the conformational proximity is allowed. However, since most
of the hydroxyl groups in the asymmetric Methyl-dimer are
involved in -OH‚‚‚N intramolecular hydrogen bonding, there
are no available hydroxyl groups to form -OH‚‚‚π hydrogen
bonding.
More compelling evidence that -OH‚‚‚π intramolecular
hydrogen bonding exists can be found in the Aniline-dimer. That
is, since one extra hydroxyl group is available in the symmetric
dimers (Scheme 6), the formation of -OH‚‚‚π intramolecular
hydrogen bonding is possible as well as relatively free hydroxyl
group. Also, the existence of aniline in the Aniline-dimer
structure makes the -OH‚‚‚π intramolecular hydrogen bonding
conformation more favorable. The observation of a concentra-
tion-independent band at 3549 cm- in Figure 5 supports this
bond formation.
1
BA-m and BA-a polymers has been explained by the results
obtained from the experiments using aliphatic amine model
dimerssmainly the Methyl-dimer. However, from this system-
Polybenzoxazines and Hydrogen Bonding. As was men-
tioned above, the hydrogen-bonded network structure of the

3278 J. Phys. Chem. A, Vol. 106, No. 14, 2002
Kim and Ishida
SCHEME 7: Proposed Hydrogen-Bonded Network Structure for Two Typical Polybenzoxazines
TABLE 1: Vibrational Assignments for Hydrogen Bonding
Species in Model Dimers and Polybenzoxazines
a
b
c
d
e
state
V
Free
V
OH-π
V
OH-O
V
OH-N
V
O-HN
Methyl-dimer crystal
3559
3401 3207
3401 3207
2768
2768
2768
2831
2831
2831
2750
2750
2750
2830
2830
2830
2749
2830
5
1
0 mM 3615 3559
mM
3615 3559 *3476 3207
Aniline-dimer crystal
3549
3421 3219
3421 3217
5
1
0 mM 3614 3549
mM
3614 3549 *3421 3181
asym.
Methyl-dimer 50 mM
mM
liquid
Aniline-dimer 50 mM 3617
mM 3617
liquid
3113
3113
3113
1
asym.
3549
3550
3417 3168
3417 3168
3170
3112
3417 3171
1
BA-m
BA-a
polymer
polymer
a
Wavenumber (cm-1) of relatively free hydroxyl group. Wavenum-
b
Figure 9. Curve fitting for FT-IR spectra of polybenzoxazines; (a)
BA-m polymer and (b) BA-a polymer.
-1
c
ber (cm ) of -OH‚‚‚π intramolecular hydrogen bonding. Wavenum-
-1
ber (cm ) of -OH‚‚‚O intermolecular hydrogen bonding. (* assigned
d
-1
-1
to -OH‚‚‚O intramolecular hydrogen boning). Wavenumber (cm
)
)
atic study for hydrogen bonding in the model dimers, a more
clear explanation for the relationship between the properties of
polybenzoxazines and the network structures can be given. To
determine the fraction of each hydrogen bonding species in
polybenzoxazines, the FT-IR spectra for polymers are curve-
resolved by using the corresponding frequencies of each
hydrogen bonding species, as illustrated in Figure 9 and the
results are summarized in Table 1. Although the differences in
the specific absorptivities for each hydrogen bonding species
must be taken into account, a qualitative assessment for the
distribution of hydrogen bonding species can be made by the
comparison of each of the fraction. Therefore, the relationship
between the properties of the polymer and the distribution of
hydrogen bonding species will be discussed in this section.
While the network structure of the BA-m polymer mainly
e
of -OH‚‚‚N intramolecular hydrogen bonding. Wavenumber (cm
-
+
of -O ‚‚‚H N intramolecular hydrogen bonding.
ring-opening reaction of the benzoxazine rings, the cross-linking
density is too low to explain the very high modulus of
polybenzoxazines. Therefore, it is believed that the physical
interaction of polymer chains due to hydrogen bonding plays
an important role in fostering the excellent properties of
polybenzoxazines. In addition, the difference in the distribution
of hydrogen bonding in the polymer presents more supporting
evidence for the higher volumetric expansion value of the BA-m
polymer (Table 2), by the molecular curling due to the
preferential -OH‚‚‚N intramolecular hydrogen bonding. Since
the BA-m polymer chains are highly curled, rather than
extended, due to the -OH‚‚‚N intramolecular hydrogen bonding,
it is difficult to tightly pack the BA-m polymer chains, thus
leading to volumetric expansion. On the other hand, a large
amount of the hydroxyl groups in the BA-a polymer are involved
in intermolecular hydrogen bonding or in the form of relatively
weak intramolecular hydrogen bonds. As a result, since the
intermolecular hydrogen bonding has a significant effect on the
packing of polymer chains, the BA-a polymer chain may have
a chance to be more extended during the ring opening
polymerization and, therefore, show low volumetric expansion.
-
+
consists of -OH‚‚‚N and -O ‚‚‚H N intramolecular hydrogen
bonding, the BA-a polymer has a relatively small amount of
intramolecular hydrogen bonding. Instead, the large portion of
hydroxyl groups in the BA-a polymer forms either weak
-
OH‚‚‚π intramolecular hydrogen bonding or -OH‚‚‚O inter-
molecular hydrogen bonding (Scheme 7). Consequently, this
supports the previous hypothesis that the BA-m polymer chains
3
are more highly curled due to intramolecular hydrogen bonding.
Although polybenzoxazines are chemically cross-linked by the

Hydrogen-Bonded Network of Polybenzoxazines
J. Phys. Chem. A, Vol. 106, No. 14, 2002 3279
SCHEME 8: Penetration of Water or Acetic Acid Molecules
TABLE 2: Comparison of Physical Properties for the BA-a
and BA-m Polymer
water molecules. However, the hydroxyl groups in the BA-a
polymer such as in the relatively free hydroxyl group and
-OH‚‚‚π intramolecular hydrogen bonding might be involved
in the interaction with water molecules because those hydrogen
bondings can be easily interrupted which, in turn, lead to the
higher saturation value.
BA-m polymer
+3.2%
BA-a polymer
-0.4%
vol. expansion(%)3
water absorption1
diffusion coef.
saturation
9
2
9
2
3.6 × 10 cm /s
0.5 × 10 cm /s
1.3%
1.9%
Additionally, this can be supported by the explanation for
the excellent chemical stability of the BA-a polymer in
cross-link density1
3
3
3
3
<1.1 × 10 mol/cm
1.1 × 10 mol/cm
4
carboxylic acid that was proposed in a previous paper. That
resistance to
4
is, carboxylic acid molecules can penetrate into the BA-m
polymer more easily due to the bulkier network structure, as
well as the affinity of carboxylic acid for the Mannich bridge,
resulting in macroscopic stress cracking by solvents. On the
other hand, since the BA-a polymer may have a tighter network
structure, the relatively large carboxylic acid molecules have
difficulty penetrating into the polymer structure.
carboxylic acid
poor
poor
excellent
excellent
5
UV light
In addition, since the BA-a polymer has additional reaction sites
2
2,23
on the aniline functional group in Mannich bridge,
it is
obvious that the BA-a polymer is more chemically cross-linked
and has tighter packing than the BA-m polymer.
The other interesting behavior is the low water absorption of
these polybenzoxazines having large numbers of hydroxyl
groups. Despite the belief that the BA-m polymer has a bulkier
structure than the BA-a polymer, the percentage of weight gain
at saturation for the BA-m polymer is smaller than that of the
BA-a polymer, even though the BA-m polymer has a larger
diffusion coefficient. The larger diffusion coefficient of the
BA-m polymer is responsible for the bulkier network structure,
which results in a greater free volume for water to penetrate.
However, the fact that the BA-m polymer has smaller water
uptake at saturation is somewhat conflicted at a first glance with
the result of higher diffusion coefficient for the BA-m polymer.
Although the larger free volume provides greater diffusion
coefficient, the environment within the free space is more
hydrophobic in the BA-m polymer, as the majority of the
hydroxyl groups form intramolecular hydrogen bonding with
the nitrogen atoms, thus leading to an expected lower water
uptake (Scheme 8). As discussed previously in the model dimer
study, the -OH‚‚‚N intramolecular hydrogen bonding in the
BA-m polymer is stable, and so is difficult to be disturbed by
Conclusions
The hydrogen bonding architectures in polybenzoxazines have
been investigated utilizing the FT-IR spectra of model dimers
and polymers. From the comparison of the FT-IR spectra of
the polybenzoxazines and model dimers, it has been shown that
the simpler structure of the asymmetric dimers well simulates
the hydrogen-bonded network structure between polymer chains
while the structure of symmetric dimers reflects the hydrogen
bonding scheme related to the end-groups of polymer chains.
Furthermore, it has been shown that the BA-m polymer consists
mainly of -OH‚‚‚N and -O ‚‚‚H N intramolecular hydrogen
bonding, while the BA-a polymer has a large amount of
intermolecular hydrogen bonding and relatively weak intramo-
lecular hydrogen bonding in the polymer network structure.
From the results of the model dimer study, it has been found
that the amine functional group in the Mannich bridges is greatly
responsible in influencing the distribution of the hydrogen
bonding species. Possible hydrogen bonding network structures
for the BA-m and BA-a polymers were proposed. Consequently,
-
+

3280 J. Phys. Chem. A, Vol. 106, No. 14, 2002
Kim and Ishida
generalized explanations for the relationships between the
hydrogen-bonded network structure and the excellent properties
of polybenzoxazines, such as the volumetric expansion, water
absorption, and resistance to carboxylic acid have also been
discussed.
(9) Ning, X.; Ishida, H. J. Polym. Sci., Polym. Chem. Ed. 1994, 32,
1
121.
(
(
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(
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(