Benzoxazine-based thermosets with autonomous self-healing ability

Article abstract of DOI:10.1021/ma5025126

A self-healing strategy for poly(propylene oxide)s bearing benzoxazine units (PPO-Benz) through supramolecular attractions is described. Poly(propylene oxide) bisamine (PPO) with a molecular wieght of 2000 Da was reacted with formaldehyde and bisphenol A to yield desired PPO-Benz with 12 360 Da. The cross-linked polymer films were then prepared by solvent casting of suitable compositions of PPO-Benz and carboxlic acid containing benzoxazine monomer (Carb-Benz) in chloroform followed by thermal ring-opening reaction of benzoxazine groups at 200 °C. Thermal curing and thermal stability of the film and final products were investigated. It was demonstrated that the self-healing capacity of the films was improved by employing Carb-Benz in the formulation.

Full text of DOI:10.1021/ma5025126

Article
pubs.acs.org/Macromolecules
Benzoxazine-Based Thermosets with Autonomous Self-Healing
Ability
†
,†
,†,‡
Mustafa Arslan, Baris Kiskan,* and Yusuf Yagci*
†
Department of Chemistry, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey
‡
Center of Excellence for Advanced Materials Research and Chemistry Department, Faculty of Science, King Abdulaziz University,
80203, Jeddah 21589, Saudi Arabia
*
S Supporting Information
ABSTRACT: A self-healing strategy for poly(propylene
oxide)s bearing benzoxazine units (PPO-Benz) through
supramolecular attractions is described. Poly(propylene
oxide) bisamine (PPO) with a molecular wieght of 2000 Da
was reacted with formaldehyde and bisphenol A to yield
desired PPO-Benz with 12 360 Da. The cross-linked polymer
films were then prepared by solvent casting of suitable
compositions of PPO-Benz and carboxlic acid containing
benzoxazine monomer (Carb-Benz) in chloroform followed by
thermal ring-opening reaction of benzoxazine groups at 200 °C. Thermal curing and thermal stability of the film and final
products were investigated. It was demonstrated that the self-healing capacity of the films was improved by employing Carb-Benz
in the formulation.
INTRODUCTION
Scheme 1. Thermally Activated Ring-Opening
Polymerization of a Bisbenzoxazine Monomer
■
Polybenzoxazines are relatively new phenolic resins and offer a
superior alternative to novolac and resole type systems. Thus,
the interest in polybenzoxazines is continuously increasing in
the polymer science field as reflected by the scientific
publication and patents. Most of the superior properties of
these addition-cure resins are stemming from the Mannich base
bridges −CH −N(R)−CH − and the intra- and intermolecular
2
2
hydrogen bonds between phenolic −OH and amine groups.
These noncovalent attractions are the reason for the low
humidity uptake and high glass transition temperatures (T ) of
these systems. Moreover, upon ring-opening polymerization
g
primary amines, and formaldehyde as starting materials
20−26
(
Scheme 2).
(
ROP) the cyclic structure of the monomers generally inhibits
The other widespread approach to regulate the properties is
shrinkage of the resin which is a common problem for many
curable systems. Noncatalytic polymerization, high char yield,
and limited release of byproducts during curing are additional
benefits of the polybenzoxazine-based resins. The synthesis of
polybenzoxazines from its corresponding 1,3-benzoxazine
monomers can simply be achieved by thermally activated
27−34
based on the preparation of polybenzoxazine prepolymers.
Such polymeric precursors generate high cross-linking degree,
reduce cold flow, and increase the toughness of the resin.
Various chemistries were successfully used to synthesize
Scheme 2. Synthesis of a Monofunctional 1,3-Benzoxazine
Monomer from a Phenol and a Primary Amine
1
ROP with or without a catalyst (Scheme 1). In noncatalytic
conditions the ROP temperatures vary in a range of 160−250
2
−6
°C.
The properties of polybenzoxazines resins can be controlled
by three common methodologies that are (i) synthesis of
monomers, (ii) forming their composites by adding fillers or
additives, and (iii) synthesis of polybenzoxazine prepoly-
7
−11
mers.
A great number of benzoxazine monomers
12
13
14
15,16
containing hydroxyl, alkyl, phenyl, maleimide,
prop-
Received: December 14, 2014
Revised: February 16, 2015
1
7
18
19
argyl, nitrile, allyl, and carboxyl groups were synthesized.
The synthesis can easily be facilitated by using suitable phenols,
©
XXXX American Chemical Society
A
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polybenzoxazine prepolymers including coupling reactions,
min load speed. The tensile strength and percentage elongation at
break were recorded.
Synthesis of Poly(propylene oxide)benzoxazine (PPO-Benz).
16
alternating copolymerization of donor−acceptor monomers,
3
5
36
37
Diels−Alder, Mannich reactions, polyesterification, poly-
3
8
39,40
In a 250 mL round bottomed flask, paraformaldehyde (10.0 mmol,
etherification, hydrosilylation,
etc. These synthetic
0
.30 g), bisphenol A (2.6 mmol, 0.60 g), and poly(propylene oxide)
approaches essentially yield diverse polybenzoxazines with
designed properties. Although polybenzoxazines were used in
many applications, the use of polybenzoxazines in the synthesis
of self-healing materials has scarcely been investigated. In this
direction, recent publications from the authors’ laboratory
reported the first usage of polybenzoxazine precursors as a self-
bisamine (PPO) (2.5 mmol, 5 g) were dissolved with 50 mL of
toluene and 25 mL of ethanol mixture. The reaction mixture was
refluxed for 24 h. The solvent was evaporated under vacuum, and a
blondish oily product was precipitated in cold n-hexane. The
precipitation process has been done two times. Final oily product
was dried at room temperature in a vacuum for 1 day.
41
Synthesis of Carboxyphenylbenzoxazine (Carb-Benz). Carb-
healing additive for polysulfones and coumarin functional
55
42
Benz was synthesized according to the procedure in the literature: In
a 500 mL round bottomed flask, paraformaldehyde (105.0 mmol, 3.2
g), phenol (53.0 mmol, 5 g), and 4-aminobenzoic acid (51.0 mmol, 7
g) were dissolved in 200 mL of 1,4-dioxane and refluxed for 5 days.
After cooling at room temperature, the solvent removed at reduced
pressure. Ethyl ether (150 mL) was added to the oil with magnetic
stirring, and a yellow precipitate was obtained. The yellow solid was
filtered and dried under reduced pressure. For spectral and thermal
characterization data see Figures S1−S3.
polybenzoxazines. In these systems, heat- and light-induced
intrinsic self-healing was accomplished through covalent bond
formations. In contrast to such healing mechanism, supra-
molecular networks that generally associate with reversible
supramolecular interactions exhibit a higher dynamic healing
behavior. It appears that these materials display autonomous
healing upon damage and have numerous potential applica-
4
3,44
tions.
So far, such self-healing has been reported in a clay−
45 46,47
Film Preparations. To obtain polybenzoxazine films, PPO-Benz
dendrimer mixture, metal ion−polymer systems,
molecular gels,
supra-
disulfide−thiol
4
8,49
50,51
was mixed with Carb-Benz monomer in CHCl in three different w/w
3
π−π stacking networks,
ratios: 1:0.1, 1:0.05, and 1:0.025. These solutions were charged into a
Teflon mold. The solvent was evaporated at room temperature for 5
days. After the solvent removal, films were exposed to thermal curing
at 200 °C for 1 h in oven. Finally brownish, transparent
polybenzoxazine cross-linked films were obtained.
5
2
53
exchange, and multicomponent systems. Additionally, self-
healing polymers based on hydrogen bonding can also generate
supramolecular network systems that contain highly dynamic
noncovalent bonds. These bonds show a reversible “sticker-
like” behavior enabling connection and reconnection between
several hydrogen bonding regions. Typical examples include
ureido−pyrimidone bonding, nucleobases and similar systems,
Leibler bond, butylurea of guanosine and 2,7-diamido-1,8-
naphtyridine attractions, polystyrene grafted with polyacryla-
RESULTS AND DISCUSSION
■
In general, the elementary steps of supramolecular self-healing
systems are interdiffusion and entanglement of polymer chains.
Accumulation of hydrogen bonding sites on the cut edge,
surface approach, and wetting are important. For spontaneous
repair of material as a response to damage a balanced dynamic
system for formation of a network should be chosen. In other
words, a sufficiently high molecular flexibility must be
established to form dynamic networks. Strong interacting
functional groups in the supramolecular system can result in
high mechanical stability but at the same time slow down or
reduce the healing capacity. Weak interactions enhance
molecular dynamics, but in such cases can generate only soft
materials that could not meet required mechanical properties.
Thus, for efficient healing, we have deliberately selected
poly(propylene oxide) with a molecular weight of 2000 Da as
the matrix material in polybenzoxazine precursor synthesis so as
to introduce sufficient dynamics to the healable system. Such
propylene oxide amines have high chain mobility and could
54
mide, and quadruple hydrogen bonding. In the view of
supramolecular attraction concept, the intra- and intermolecular
hydrogen bonding in polybenzoxazines can be utilized to
design a novel self-healing system. To demonstrate the
feasibility of this strategy, we report poly(propylene oxide)-
based polybenzoxazine networks with embedded carboxylic
acid containing benzoxazines as the stimuli to form efficient
autonomous healing though both covalent and supramolecular
network.
EXPERIMENTAL SECTION
■
9
Materials. 4,4′-Isopropylidenediphenol (bisphenol A) (Aldrich,
7%), paraformaldehyde (Acros, 96%), poly(propylene glycol) bis(2-
aminopropyl ether) (M ∼ 2000 Da, Aldrich), 4-aminobenzoic acid
n
(
9
≥99%, Aldrich), ethanol (≥99.5%, Aldrich), toluene (Carlo Erba,
9.5%), chloroform (Acros, 99+%), hexane (Aldrich, 95%), 1,4-
provide softness to the networks that they were incorpo-
dioxane (Riedel-de Haen 99.5%), and diethyl ether (≥98%, Aldrich)
56,57
rated.
Hence, a successful synthesis of poly(propylene
were used as received.
Characterization. H NMR spectra were recorded in CDCl with
1
oxide)-based polybenzoxazines (PPO-Benz) were performed
3
using poly(propylene oxide) bisamine (PPO) (M : 2000) and
n
Si(CH ) as internal standard, using a Bruker AC250 (250.133 MHz)
3
4
bisphenol A as difunctional phenol source (Scheme 3). A
toluene/ethanol mixture (2:1, v:v) was used as the solvent in
instrument. FT-IR spectra were recorded on a PerkinElmer FT-IR
Spectrum One spectrometer. Differential scanning calorimetry (DSC)
was performed on PerkinElmer Diamond DSC from 30 to 320 °C
with a heating rate of 20 °C min under nitrogen flow. Thermal
gravimetric analysis (TGA) was performed on a PerkinElmer
Diamond TA/TGA with a heating rate of 10 °C min under nitrogen
flow. Molecular weights were determined by gel-permeation
chromatography (GPC) instrument equipped with a Waters styragel
column (HR series 2, 3, and 5E) with THF as the eluent at a flow rate
of 0.3 mL/min and a Waters 410 differential refractometer detector.
Uniaxial elongation measurements were performed on polymeric film
28
order to prevent possible gelation during precursor synthesis.
It is known that depending on the solvent used, formaldehyde
with primary amines can readily form 1,3,5-dihydrotriazines.
The chemical structure of the PPO-Benz was confirmed by
1
H NMR and FT-IR spectral analysis. As can be seen from
Figure 1 where the 1H NMR spectrum of PPO-Benz is
presented, the appearance of the protons resonating at 4.92
ppm (O−CH −N) and 4.01 ppm (Ar−CH −N) is clear
2
2
2
evidence for the benzoxazine ring formation on poly(propylene
) discloses the
samples (approximately 16.4 mm length and 8.6 mm cross-section
area). Measurements were carried out using a PerkinElmer Pyris
Diamond DMA (SII Nanotechnology Inc.) at 25 °C under 50 mN/
oxide). Moreover, the peak at 1.59 ppm (−CH
3
bisphenol A moiety. The integration ratios of the characteristic
B
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Scheme 3. Synthesis of Benzoxazine Containing
Poly(propylene oxide)s (PPO-Benz)s
precursor obtained. These results confirm successful synthesis
of benzoxazine functional poly(propylene oxide).
The GPC trace of the precursor polymer is depicted in
Figure S4. Clearly, successive condensation reactions resulted
in the formation of PPO with relatively high molecular (M =
n
−1
1
2 360 g mol , PDI = 1.43). The shoulders observed in the
lower molecular weight region are typical consequences of the
step-growth polymerization.
It is known that depending on the functionalities present,
benzoxazines have the ring-opening polymerization temper-
ature generally between 150 and 260 °C. Figure 3 shows the
Figure 3. DSC thermogram of PPO-Benz under a N atmosphere.
2
DSC profile of PPO-Benz. The precursor is curable, and its
curing temperature is in accordance with many classical
bisbenzoxazine monomers or main chain polybenzoxazine
precursors. PPO-Benz showed an exotherm with an onset at
1
Figure 1. H NMR spectra of PPO and the corresponding PPO-Benz.
2
21 °C, end-set at 271 °C, and the curing maximum was
peaks agree with the proposed structure. FT-IR spectra of the
correponding polymers also give evidence for the formation of
benzoxazine functional PPO-Benz precursor (Figure 2). The
consumption of amino groups of poly(propylene)amines can
be detected by the disappearances of N−H stretching
vibrations in the spectra. Moreover, the stretching vibrations
detected as 252 °C. The total amount of curing exotherm is 20
J/g. After curing of PPO-Benz, the second run did not exhibit
any curing exotherm, indicating the consumption of the
benzoxazine groups in the first thermal treatment. Glass
transition temperature (T ) of polybenzoxazine films were
g
detected by DSC with a fast scanning method (40 °C/min).
−1
of aromatic C−H (3022−3055 cm ) and aromatic CC
Two T s were observed at 32 and 151 °C. These values reveal
−
1
g
(
1511−1621 cm ) as well as the out-of-plane bending
−1
two distinct domains in a typical film.
vibrations of aromatic C−H at 963 cm are detected for the
Autonomous healing of thermally cured PPO-Benz polymer
films was investigated. For this purpose, the films were prepared
first by mixing PPO-Benz and Carb-Benz with the structure
presented in Chart 1 in three different w/w ratios: 1:0.1, 1:0.05,
Chart 1. Structure of Carb-Benz
and 1:0.025. The abbreviations Carb-Benz10%-PPO-Benz,
Carb-Benz5%-PPO-Benz, and Carb-Benz2.5%-PPO-Benz rep-
resent the respective mixing ratios. The maximum weight of
Carb-Benz kept 100 mg in 1 g of PPO-Benz because excess
amount of Carb-Benz generates voids in the film and phase
separation takes place. While the onset of the ring-opening
reaction of pristine PPO-Benz is 221 °C (Figure 3), the curing
is conducted at 200 °C because ring-opening polymerization of
Carb-Benz commences at around 160 °C (see Figure S3) and
triggers the curing of PPO-Benz easily. In principle, the films
Figure 2. FT-IR spectra of PPO-Benz (a) and PPO (b).
C
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could heal themselves without an external stimulant through
hydrogen bonding with the existing amino phenols. However,
only inefficient healing was attained without addition of Carb-
Benz. A possible drawback of carboxylic acid group could be
the recovery is presented in Figure 5, and a video as Supporting
Information is present.
Table 1
19
decarboxylation of acid groups; it has been reported that the
weight loss at around 300 °C increases with the carboxylic
benzoxazine content. We have also observed this effect for the
film which was prepared with the ratio of 1:0.1 (w/w) as the
bubbles were formed after curing. However, the films with less
amount of Carb-Benz were smooth (Figure S5).
Basically, healing of a polymeric material can be regarded as
the recovery of its properties, and polymers exhibit various
properties that can be a measure of the extent of healing. In
general, recovery of the properties such as tensile strength,
fracture toughness, etc., is measured to quantify the extent of
healing. However, such quantification could not reflect the
exact amount of the recovery, since pure mechanical properties
cannot cover the overall properties of a material. Even though,
in a practical manner measuring the degree of healing of
polymeric systems for a defined mechanical property is
commonly used as the basic method. Accordingly, the “healing
efficiency” of a self-healing system the can be expressed as η
Young modulus
healing efficiency
(η) (%)
polymer
(E) (kPa)
CarbBenz%10 + PPO-Benz
CarbBenz%10 + PPO-Benz
CarbBenz%5 + PPO-Benz
182.3
96.4
96
41
26
a
163.7
157.5
136.5
190.1
a
CarbBenz%5 + PPO-Benz
CarbBenz%2.5 + PPO-Benz
CarbBenz%2.5 + PPO-Benz
a
a
Cut healed sample.
η = 100 × K
healed
K
virgin
(1)
Figure 5. Photograph of cut healed specimen (1) and stretching of the
same sample (2) (Carb-Benz%5 + PPO-Benz).
where Khealed and Kvirgin are the fracture toughness (the area of
stress−strain curve) of the healed and virgin specimens,
respectively. Thus, tensile tests were performed to measure
the ability of the damaged cured Carb-Benz and PPO-Benz
mixture (Carb-Benz + PPO-Benz) to recover their strength. To
do so, the damage generated by cutting the film into two
separate parts. Then, the damaged specimens were kept in
contact from the edges of the cut with applied pressure using a
glass slip for 12 h at ambient temperature. Thereafter, the
healed specimens were subjected to tests for healing efficiency,
and the ratio of tensile strengths of the healed and virgin
specimens were found. Figure 4 shows that tensile strengths of
cross-linked films of Carb-Benz + PPO-Benz mixtures can be
restored to some extent and the efficiency of the healing for cut
specimens are tabulated in Table 1. A visual demonstration of
In polybenzoxazines, two types of hydrogen bonding can be
considered: (i) intermolecular hydrogen bonding between two
phenolic hydroxyl groups of polybenzoxazine and (ii) intra-
molecular hydrogen bonding between phenolic hydroxyl
groups and nitrogen atoms on the Mannich bridge. These
hydrogen bonding attractions can easily be detected in IR
spectrum of polybenzoxazines as broadening of phenolic O−H
stretching vibration band. Addition of Carb-Benz into PPO-
Benz during film formation could generate increased number of
hydrogen binding sites (Chart 2). Binding of −COOH to
Chart 2. Demonstration of Possible Hydrogen Bonding Sites
Highlighted as Circles in Carb-Benz Added PPO-Benz
Network
phenolic −OH and tertiary amine would affect the hydrogen
bonding in polybenzoxazine resin and results in a shift of −OH
band toward the low-energy region in IR. Likewise, this shift is
clearly detectable in Figure 6, evidencing the supramolecular
healing mechanism. As stated previously, without addition of
Carb-Benz, sufficient healing was not observed, indicating the
importance of hydrogen bonding for successful healing.
Figure 4. Stress−strain (%) behavior of virgin specimens Carb-Benz
%
%
10 + PPO-Benz (a), Carb-Benz%5 + PPO-Benz (b), and Carb-Benz
2.5 + PPO-Benz (c) and cut healed specimens (a′), (b′), and (c′),
respectively.
D
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polybenzoxazine networks for autonomous self-healing pro-
cesses. In the approach, benzoxazine bearing poly(propylene
oxide) (PPO-Benz) was prepared through conventional main
chain polybenzoxazine precursor synthesis methodology using
poly(propylene oxide) amines, bisphenol A, and formaldehyde.
Carboxylic acid containing benzoxazine monomer mixed with
PPO-Benz, the film prepared from these components at
suitable composition were then cross-linked by thermally
activated ring-opening reactions of the benzoxazine groups.
The autonomous self-healing property of the film was
demonstrated on the cured films. Current work is devoted to
the design and production of self-healable polymers containing
polybenzoxazine networks by supramolecular attractions. The
process is useful for further expanding the use of benzoxazines
in high performance materials, and the chemistry shown here
may be extended to other polymers apart from poly(propylene
oxide)s.
Figure 6. FT-IR spectra of cured Carb-Benz5%-PPO-Benz film (a)
and cured PPO-Benz film (b).
ASSOCIATED CONTENT
Supporting Information
NMR spectrum, DSC thermogram, FT-IR spectrum of Carb-
Benz, GPC trace of PPO-Benz, and images of the films; a visual
demonstration of the recovery is presented in a video. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Thermal stability of the PPO-Benz polymers was explored by
■
*
S
thermogravimetric analysis (TGA) under a N atmosphere. The
2
TGA and derivative TGA curves of the cured PPO-Benz and
Carb-Benz + PPO-Benz are shown in Figure 7, and the results
AUTHOR INFORMATION
Corresponding Authors
(Y.Y.) E-mail yagci@itu.edu.tr; Tel +90 2122853241; Fax +90
12 2856386.
(B.K.) E-mail kiskanb@itu.edu.tr; Tel +90 2122853241; Fax
90 212 2856386.
■
*
2
*
+
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Baris Kiskan acknowledges financial support of the Science
Academy, Turkey, by means a BAGEP Research Award.
■
Figure 7. TGA and derivative TGA of cured PPO-Benz (a, a′) and
cured Carb-Benz5% + PPO-Benz (b, b′).
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DOI: 10.1021/ma5025126
Macromolecules XXXX, XXX, XXX−XXX