Multivalent photo-crosslinkable coumarin-containing polybenzoxazines exhibiting enhanced thermal and hydrophobic surface properties

Article abstract of DOI:10.1039/c5ra27705a

In this study, mono-, bi-, and trivalent coumarin-containing benzoxazine monomers (mono-, di-, and tri-coumarin BZ) were synthesized in high yield and purity by facile Mannich reactions of 4-methyl-7-hydroxycoumarin and paraformaldehyde with aniline, bisphenol A-NH₂, and 1,3,5-tri(4-aminobenzene), respectively, in 1,4-dioxane. ¹H and ¹³C nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR) and high resolution mass spectroscopy support the chemical structures of these three benzoxazine monomers. Differential scanning calorimetry (DSC) and FTIR spectroscopy were used to investigate the curing polymerization behavior and photodimerization ([2π + 2π] cycloaddition) of the coumarin units of mono-, di-, and tri-coumarin BZ to form poly(mono-coumarin BZ), poly(di-coumarin BZ), and poly(tri-coumarin BZ), respectively. DSC measurement revealed that the thermal polymerization temperature of coumarin-containing benzoxazine monomers was lower than that of the model compound 3-phenyl-3,4-dihydro-2H-benzooxazine (263°C) which was attributed to the catalytic effect of the coumarin moiety and a strong electron withdrawing electron conjugated CC bond in the coumarin unit. In addition, the glass transition and thermal decomposition temperatures of poly(tri-coumarin BZ) (T_g = 240°C; T_d5 = 370°C) were higher than poly(di-coumarin BZ) and poly(mono-coumarin BZ), consistent with the former's higher crosslinking density. In addition, the water contact angles of poly(tri-coumarin BZ) polymers prepared with and without photo-dimerization prior to thermal curing (112 and 110°, respectively) were higher than the corresponding poly(mono-coumarin BZ) and poly(di-coumarin BZ), presumably because of greater degrees of intramolecular hydrogen bonding between the CO units of the coumarin moieties and the phenolic OH units of the benzoxazine rings, resulting in lower surface free energies. Thus, the presence of multivalent photo-crosslinkable coumarin units enhanced the thermal and hydrophobic surface properties of these polybenzoxazines.

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Multivalent photo-crosslinkable coumarin-
containing polybenzoxazines exhibiting enhanced
Cite this: RSC Adv., 2016, 6, 10683
thermal and hydrophobic surface properties†
Ruey-Chorng Lin,a Mohamed Gamal Mohamed,a Kuo-Chih Hsu,a Jia-Yu Wu,a
a
ab
*
Yu-Ru Jheng and Shiao-Wei Kuo
In this study, mono-, bi-, and trivalent coumarin-containing benzoxazine monomers (mono-, di-, and tri-
coumarin BZ) were synthesized in high yield and purity by facile Mannich reactions of 4-methyl-7-
hydroxycoumarin and paraformaldehyde with aniline, bisphenol A–NH2, and 1,3,5-tri(4-aminobenzene),
respectively, in 1,4-dioxane. 1H and 13C nuclear magnetic resonance (NMR), Fourier transform infrared
(FTIR) and high resolution mass spectroscopy support the chemical structures of these three
benzoxazine monomers. Differential scanning calorimetry (DSC) and FTIR spectroscopy were used to
investigate the curing polymerization behavior and photodimerization ([2p + 2p] cycloaddition) of the
coumarin units of mono-, di-, and tri-coumarin BZ to form poly(mono-coumarin BZ), poly(di-coumarin
BZ), and poly(tri-coumarin BZ), respectively. DSC measurement revealed that the thermal polymerization
temperature of coumarin-containing benzoxazine monomers was lower than that of the model
compound 3-phenyl-3,4-dihydro-2H-benzooxazine (263 ꢀC) which was attributed to the catalytic effect
of the coumarin moiety and a strong electron withdrawing electron conjugated C]C bond in the
coumarin unit. In addition, the glass transition and thermal decomposition temperatures of poly(tri-
coumarin BZ) (Tg ¼ 240 ꢀC; Td5 ¼ 370 ꢀC) were higher than poly(di-coumarin BZ) and poly(mono-
coumarin BZ), consistent with the former's higher crosslinking density. In addition, the water contact
angles of poly(tri-coumarin BZ) polymers prepared with and without photo-dimerization prior to thermal
curing (112 and 110ꢀ, respectively) were higher than the corresponding poly(mono-coumarin BZ) and
poly(di-coumarin BZ), presumably because of greater degrees of intramolecular hydrogen bonding
between the C]O units of the coumarin moieties and the phenolic OH units of the benzoxazine rings,
resulting in lower surface free energies. Thus, the presence of multivalent photo-crosslinkable coumarin
units enhanced the thermal and hydrophobic surface properties of these polybenzoxazines.
Received 25th December 2015
Accepted 18th January 2016
DOI: 10.1039/c5ra27705a
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materials, which generally exhibit high melting and curing
temperatures.14–16
Introduction
The physical characteristics of PBZs can be modied in
several ways (e.g., introduction of nitrile, allyl, acetylene, or
propargyl side groups, or strong hydrogen bonding moieties, on
the benzoxazine monomers) to form interesting materials dis-
playing varied thermal and mechanical properties and resis-
tance to solvents.17–20 The physical properties of PBZs (e.g., their
hydrophobic surface properties) are strongly dependent on the
degrees of intramolecular hydrogen bonding between the
phenolic hydroxyl (OH) groups and N atoms in the Mannich
bridge and intermolecular OH/O hydrogen bonding (aer ring
opening of the benzoxazine monomers).21
Light-responsive groups are attractive materials that can
change the behavior of their chromophores upon photo-
irradiation, either through changes in their molecular struc-
tures or through the effects of different light sources.22–25 Readily
accessible photoresponsive molecules include spiropyran, dia-
rylethene, stilbene, azobenzene, anthracene, cinnamic acids,
Polybenzoxazines (PBZs) are phenolic resin polymers that have
attracted much interest because of their ready synthesis
(through thermal polymerization of benzoxazine monomers
without the need for any catalysts)1–5 and excellent properties
(high thermal stability, good molecular design exibility,
chemical resistance, mechanical robustness, low ammability,
and low surface free energy).6–13 Nevertheless, many challenges
must be overcome to enhance the processibility of these
aDepartment of Materials and Optoelectronic Science, National Sun Yat-Sen
University, Kaohsiung, Taiwan. E-mail: kuosw@faculty.nsysu.edu.tw
bDepartment of Medicinal and Applied Chemistry, Kaohsiung Medical University,
Kaohsiung, Taiwan
† Electronic supplementary information (ESI) available: FTIR spectra, DSC traces,
high resolution mass spectroscopy and WCAs of mono-coumarin BZ, di-coumarin
BZ, and tri-coumarin BZ. This material is available free of charge via the internet.
See DOI: 10.1039/c5ra27705a
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and cinnamic esters and coumarin derivatives. As known the chemical structures using differential scanning calorimetry
photo-dimerization of coumarin unit and its derivatives in (DSC), thermogravimetric analysis (TGA), Fourier transform
solution and solid state under photo-irradiation (350 nm) has infrared (FTIR), nuclear magnetic resonance (NMR) and high
been reported.23,24 For example, the photo-irradiation of resolution mass spectroscopy. In addition, we used DSC and
coumarin molecule in non-polar benzene afforded a mixture of FTIR spectroscopy to monitor the thermal curing polymeriza-
syn-head-head, anti-head-head, syn-head-tail and anti-head-tail of tion of the monomers and the subsequent photo-dimerization
cyclobutane dimers through [2p + 2p] cycloaddition. In addition, of the coumarin units. Finally, we used water contact angle
the photocycloaddition reaction of coumarin unit is reversible (WCA) measurements to examine the surface properties of these
when irradiated at a shorter wavelength (250 nm).26–28 For novel coumarin-functionalized benzoxazines before and aer
example, Saegusa et al. investigated the reversible gelation of thermal curing at various temperatures.
polyoxazolines with coumarin units.29 Yagci et al. were the rst to
report the incorporation of coumarin units into benzoxazine
structures, which exhibited high thermal stability aer photo-
dimerization of their coumarin moeities.30 In addition, we
Experimental section
Materials
previously synthesized a bifunctional benzoxazine monomer
featuring pyrene and coumarin moieties; the glass transition
temperature (Tg) increased aer photo-dimerization of coumarin
units, with the pyrene moieties enhancing the dispersion of
single-walled carbon nanotubes through p–p stacking.31
In this present study, we prepared three new mono-, bi-, and
trivalent coumarin/benzoxazine derivatives and converted them
into photo-crosslinkable coumarin-containing PBZs in high
yield and high purity (Scheme 1). We characterized these
Paraformaldehyde (96%), sodium bicarbonate (NaHCO3), potas-
sium carbonate (K2CO3), sodium hydroxide (NaOH), bisphenol A,
resorcinol, ethyl acetoacetate, and ethyl acetate (EA) were
purchased from Acros. 4-Chloro-4-nitrobenzene, 4-uore-4-
nitrobenzene, pyridine, and phloroglucinol (1,3,5-trihydroxy-
benzene) were purchased from Scharlau. Palladium on activated
carbon (Pd/C) and hydrazine monohydrate (98%) were purchased
from Alfa Aesar. Tetrahydrofuran (THF), N,N-dimethylformamide
(DMF), 1,4-dioxane, chloroform (CHCl3), dichloromethane
Scheme 1 Synthesis of (a) mono-coumarin BZ, (b) di-coumarin BZ, and (c) tri-coumarin BZ, and their thermal curing to form (d) poly(mono-
coumarin BZ), (e) poly(di-coumarin BZ), and (f) poly(tri-coumarin BZ).
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(CH2Cl2), and ethanol (EtOH) were purchased from Acros. 4- DMSO-d6, d, ppm): 5.75–6.73 (m, CH aromatic), 4.97 (NH). 13C
Methyl-7-hydroxycoumarin (coumarin–OH) was synthesized NMR (125 MHz, DMSO-d6, d, ppm): 98.35–161.84 (aromatic).
according to a previously reported procedure.31
6-Methyl-3-phenyl-3,4-dihydrochromeno[6,7-e][1,3]oxazin-
2,2-Bis(4-(4-nitrophenoxy)-phenyl)propane (bisphenol A– 8(2H)-one (mono-coumarin BZ). A solution of aniline (1.16 g,
NO2) (Scheme S1†). A mixture of bisphenol A (3.00 g, 13.1 mmol), 12.5 mmol) and paraformaldehyde (0.716 g, 23.9 mmol) in 1,4-
1-chloro-4-nitrobenzene (4.35 g, 27.6 mmol), and K2CO3 (7.27 g, dioxane (100 mL) was stirred for 1 h in an ice bath and then
ꢀ
52.6 mmol) in DMF (150 mL) was stirred under reux (130 C) a solution of coumarin–OH (2.00 g, 11.4 mmol) in 1,4-dioxane
under a N2 atmosphere. Aer 48 h, the mixture was cooled and (50 mL) was added. The mixture was heated with stirring for 12
ltered the insoluble salts. The ltrate was evaporated to dryness h under reux. Aer cooling to room temperature, the solution
under reduced pressure and the residue was then extracted with was concentrated under reduced pressure to obtain a viscous
CH2Cl2 (3 ꢁ 80 mL). The combined extracts were washed several oil, which was extracted into CH2Cl2 (3 ꢁ 100 mL). The
times with distilled water, dried (MgSO4, 30 min), and concen- combined extracts were washed several times with 1% NaHCO3
trated. The solid product was recrystallized (EtOH), giving yellow and nally with distilled water, dried (MgSO4, 1 h), and
crystals (2.62 g, 87%): mp (DSC): 122 ꢀC. FTIR (KBr, cmꢂ1): 1587 concentrated under reduced pressure to give a yellow solid,
1
and 1350 (NO2 stretch). H NMR (500 MHz, DMSO-d6, d, ppm): which was puried through column chromatography (SiO2; n-
7.10–8.25 (m, CH aromatic). 13C NMR (125 MHz, DMSO-d6, d, hexane/EA, 1 : 1) to obtain a white solid (1.3 g, 65%); mp
ppm): 30.68 [C(CH3)2], 117.90–163.71 (aromatic).
(DSC): 142 ꢀC. FTIR (KBr, cmꢂ1): 1263 (asymmetric COC
4-(4-(2-(4-(4-Aminosphenoxy)phenyl)propan-2-yl) phenoxy) stretching), 1341 (CH2 wagging), 937 and 1490 (vibrations of
1
benzenamine (bisphenol A–NH2) (Scheme S1†). A stirred trisubstituted benzene ring), 1733 (C]O stretching). H NMR
mixture of bisphenol A–NO2 (2.50 g, 5.32 mmol) and 10% Pd/C (500 MHz, DMSO-d6, d, ppm): 6.79–7.54 (m, CH aromatic), 6.21
(0.033 g, 0.314 mmol) in absolute EtOH (60 mL) was heated (s, 1H, CH), 5.55 (s, 2H, OCH2N), 4.75 (s, 2H, CCH2N), 2.36 (s,
under reux (80–90 ꢀC) under N2 for 2 h. Hydrazine mono- 3H, CCH3). 13C NMR (125 MHz, DMSO-d6, d, ppm): 108.96–
hydrate (3.33 mL, 68.4 mmol) was added dropwise carefully and 157.89 (aromatic), 160.60 (C]O), 79.81 (OCH2N), 44.85
then reuxing of the mixture was continued for 48 h. Aer (CCH2N), 18.22 (CH3). High resolution FT-MS [M]+ m/z for
cooling, charcoal was added and the mixture was ltered. The
solution was stirred at 0 C to form a solid product, which was
C
18H16NO3: 294.12: calcd: 294 (Fig. S1†).
4-Dihydrochromeno[6,7-e][1,3]oxazin-8(2H)-one (di-coumarin
ꢀ
ltered off under suction. The crude product was dried under BZ) (Scheme S1†). A solution of bisphenol A–NH2 (0.800 g, 1.95
ꢀ
vacuum at 30 C to give semitransparent white crystals (1.05 g, mmol), coumarin–OH (0.721, 4.10 mmol), and para-
42%), mp: 132 ꢀC (DSC). FTIR (KBr, cmꢂ1): 3200–3400 (NH formaldehyde (0.246 g, 8.19 mmol) in 1,4-dioxane (70 mL) was
stretch). 1H NMR (500 MHz, DMSO-d6, d, ppm): 6.57–7.12 (m, CH heated under reux (90–100 ꢀC) with stirring under N2. Aer 24
aromatic), 4.94 (NH2), 1.57 [d, 6H, C(CH3)2]. 13C NMR (125 MHz, h, the solution was cooled to room temperature and concen-
DMSO-d6, d, ppm): 115.42–157.51 (aromatic), 30.83 [C(CH3)2].
trated under reduced pressure. The solid residue was extracted
1,3,5-Tris(4-nitrophenoxy)benzene (Scheme S2†). K2CO3 into CH2Cl2 (3 ꢁ 100 mL); the combined extracts were washed
(40.0 g) was added to a solution of phloroglucinol (10.1 g, 0.0800 several times with 1 N NaOH and then with distilled water, dried
mol) in DMF (200 mL) and H2O (40 mL) under a N2 atmosphere (MgSO4, 40 min), and then concentrated under pressure to
and then the mixture was heated under reux (140 C) for 6 h. obtain a yellow solid (1.00 g, 57%). FTIR (KBr, cmꢂ1): 1727 (C]O
ꢀ
Aer cooling to 70 ꢀC, 4-uoro-4-nitrobenzene (44.7 g, 0.310 stretch), 1383 (CH2 wagging), 1239 (asymmetric COC stretching),
mol) was added and then the mixture was again heated under 1059 (symmetric C–O–C stretching), 932 (vibration of trisubsti-
1
reux (150–160 ꢀC) for 12 h under N2. The mixture was cooled to tuted benzene ring) (Fig. S1†). H NMR (500 MHz, DMSO-d6, d,
room temperature and concentrated under reduced pressure; ppm): 6.77–7.53 (m, CH aromatic), 6.20 (d, 1H, CCH), 5.50 (s, 2H,
the residue was poured into 5% NaOH (500 mL) to afford OCH2N), 4.70 (s, 2H, CCH2N), 2.35 (d, 3H, CH3), 1.55 [d, 6H,
a brown solid, which was ltered off, washed several times with C(CH3)2]. 13C NMR (125 MHz, DMSO-d6, d, ppm): 160.58 (C]O),
distilled water, and dried under vacuum at 100 ꢀC for 24 h. 108.83–157.85 (aromatic carbon resonances), 80.25 (OCH2N),
Recrystallization (pyridine/H2O) afforded yellow crystals (9.56 g, 45.29 (CCH2N), 30.74 (C(CH3)2), 18.21 (CH3). High resolution FT-
87%); mp (DSC): 200 ꢀC. FTIR (KBr, cmꢂ1): 1584, 1341 (NO2). 1H MS [M]+ m/z for C51H43N2O8: 811.30: calcd: 811.10 (Fig. S3†).
NMR (500 MHz, DMSO-d6, d, ppm): 6.97–8.26 (m, aromatic). 13
NMR (125 MHz, DMSO-d6, d, ppm): 109.57–162.56 (aromatic).
C
3,30,300-(4,40,400-(Benzene-1,3,5-triyltris(oxy))tris(benzene-4,1-
diyl))tris(6-methyl-3,4-dihydrochromeno[6,7-e][1,3]oxazin-
1,3,5-Tris(4-aminophenoxy)benzene (Scheme S2†). A mixture 8(2H)-one) (tri-coumarin BZ) (Scheme S2†). A solution of 1,3,5-
of 1,3,5-tris(4-nitrophenoxy)benzene (3.00 g, 6.14 mmol) and tris(4-aminophenoxy)benzene (1.50 g, 3.76 mmol), coumarin–
10% Pd/C (0.300 g, 2.82 mmol) in anhydrous EA (50 mL) was OH (2.12 g, 12.0 mmol), and paraformaldehyde (0.688 g, 22.9
stirred for 3 days under a H2 atmosphere. The mixture was mmol) in 1,4-dioxane (60 mL) was heated under reux (80–90
ltered and the ltrate concentrated under reduced pressure. ꢀC) with stirring under N2. The reaction was monitored using
The solid was extracted with CH2Cl2 (3 ꢁ 80 mL). The combined thin layer chromatography (TLC). Aer 48 h, the solution was
extracts were washed several times with distilled water, dried cooled to room temperature and ltered. The ltrate was
(MgSO4, 30 min), and then concentrated under reduced pres- concentrated under reduced pressure to obtain a yellow solid,
ꢀ
sure to give a white solid (2.00 g, 75%), mp: 90 C (DSC). FTIR which was extracted into CH2Cl2 (3 ꢁ 100 mL); the combined
(KBr, cmꢂ1): 3200–3400 (NH stretch). 1H NMR (500 MHz, extracts were washed several times with 1 N NaOH, dried
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(MgSO4, 1 h), and concentrated under reduced pressure to
obtain a yellow solid (0.50 g, 34%); FTIR (KBr, cmꢂ1): 1383 (CH2
wagging), 1260 (asymmetric COC stretching), 1059 (C–O–C
stretching), 932 (vibration of trisubstituted benzene ring), 1727
(C]O stretch) (Fig. S2†). 1H NMR (500 MHz, DMSO-d6, d, ppm):
6.79–7.53 (aromatic), 6.20 (t, 1H, CCH), 5.48 (t, 2H, OCH2N),
4.70 (t, 2H, CCH2N), 2.35 (t, 3H, CH3). 13C NMR (125 MHz,
DMSO-d6, d, ppm): 160.56 (C]O), 101.07–157.83 (aromatic),
80.10 (OCH2N), 45.13 (CCH2N), 18.20 (CCH3). High resolution
FT-MS [M]+ m/z for C60H46N3O12: 1000.31: calcd: 1000.00
(Fig. S5†).
Thermal curing of mono-, di-, and tri-coumarin BZ
Desired amounts of (mono, di-and tri-coumarin BZ) were put
onto aluminum pans and polymerized in stepwise manner: at
150 ꢀC, 180 ꢀC, 210 ꢀC, and 240 ꢀC for 2 h. Each thermal curing
sample has the red color and more dark as curing temperature
increasing. Aer completing the curing process, samples were
cooled to room temperature. The same procedures were done
with (mono, di-and tri-coumarin BZ) aer photo-dimerization
of coumarin unit.
Photo-dimerization of coumarin-functionalized benzoxazine
(Scheme 2)
Fig. 1 (a) 1H and (b) 13C NMR spectra of mono-coumarin BZ.
An appropriate amount of coumarin BZ (0.1 g) was dissolved in
dichloromethane (20 mL) was placed in quartz tube. The tube
Fig. 2 (A) DSC thermograms and (B) FTIR spectra of mono-coumarin BZ, recorded after each curing stage.
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was irradiated in the Carousel equipped with 15 Philips lamps glass transition temperatures (Tg), degradation temperatures
emitting light nominally at 365 nm for 50 min at room (Td), char yields, and hydrophobic surface properties. In
temperature.
a previous study, we synthesized 4-methyl-7-hydroxycoumarin
(coumarin–OH) through an acid-catalyzed reaction of resor-
cinol and ethyl acetoacetate.31 Here, we used coumarin–OH as
a common phenolic heterocyclic material to prepare our three
different coumarin-containing monomers to provide PBZs as
photoresponsive crosslinking materials (Scheme 1).
Samples of coumarin-functionalized benzoxazines for contact
angle measurements
A solution of coumarin Bz (0.06 g) was dissolved in 2 mL of
dichloromethane (DCM), ltered at room temperature through
a 0.2 mm syringe lter before and aer UV exposure and then
spin-coated onto a glass substrate with rotating speeds 1100
rpm for 15 s. The samples were thermally polymerized in an
We synthesized 6-methyl-3-phenyl-3,4-dihydrochromeno
[6,7-e][1,3]oxazin-8(2H)-one (mono-coumarin BZ) through the
reaction of paraformaldehyde, aniline, and coumarin–OH in
1,4-dioxane. We synthesized 4-dihydrochromeno[6,7-e][1,3]oxa-
zin-8(2H)-one (di-coumarin BZ) in two steps (Scheme 1 and
Fig. S2†): reduction of 2,2-bis(4-(4-nitrophenoxy)-phenyl)
propane (bisphenol–NO2) over Pd/C in hydrazine mono-
hydrate afforded bisphenol A–NH2; subsequent reaction of
bisphenol–NH2, coumarin–OH, and paraformaldehyde in 1,4-
dioxane provided di-coumarin BZ. We synthesized tri-coumarin
BZ in high purity in three steps (Scheme 1 and Fig. S4†): the
reaction of phloroglucinol with 1-uoro-4-nitrobenzene in the
presence of K2CO3 in DMF produced 1,3,5-tris(4-nitrophenoxy)
benzene, which we reduced over Pd/C under H2 at ambient
temperature to afford 1,3,5-tris(4-aminophenoxy)benzene,
ꢀ
oven at 150, 180, 210, and 240 C for 2 h for each step.
Characterization
1H and 13C NMR spectra were recorded using an INOVA 500
instrument with DMSO-d6 and CDCl3 as solvents. FTIR spectra
were recorded using a Bruker Tensor 27 spectrophotometer and
the conventional KBr disk method, with 32 scans at a spectral
resolution of 4 cmꢂ1. The polymer lms were sufficiently thin to
obey the Beer–Lambert law. Elevated-temperature FTIR spectra
were recorded in the temperature-controlled compartment of
the spectrometer. The molecular weights of (mono-, di-, and tri-
coumarin BZ), were recorded using a Bruker Solarix high reso-
lution Fourier Transform Mass spectroscopy system FT-MS
(Bruker, Bremen, Germany). Dynamic curing kinetics and
glass transition temperatures were measured using a TA Q-20
DSC apparatus operated under a N2 atmosphere. The sample
(ca. 5–10 mg) was placed in a sealed aluminum sample pan. The
dynamic thermal curing behavior was recorded during the ꢂrs1t
heating scan from 30 to 350 ꢀC at a heating rate of 20 ꢀC min
;
the glass transition temperature behavior was recorded during
the second heating scan from ꢂ90 to 350 ꢀC at a heating rate of
20 ꢀC minꢂ1 from the sample quenched at 300 ꢀC. The thermal
decomposition and char yield of PBZs were determined using
a TA Q-50 TGA analyzer operated under a N2 atmosphere. The
uncured and cured PBZs samples (ca. 5–10 mg) were placed in
a Pt cell and heated from 30 to 800 ꢀC under a N2 atmosphere at
a heating rate of 20 ꢀC minꢂ1. Photo-dimerization of coumarin
units was performed directly under a UV-Vis lamp at 365 nm
(power: 90 mW cmꢂ1); corresponding UV-Vis spectra were
recorded by using a Shimadzu mini 1240 spectrophotometer.
Water contact angles were measured using an FDSA Magic
Droplet 100 contact angle goniometer interfaced with image
capture soware; droplets (5 mL) of deionized water at room
temperature were placed on the PBZs samples before and aer
photo-dimerization of the coumarin units.
Results and discussion
Preparation for three different functionalities of coumarin
moieties functionalized benzoxazine monomers
In this study, we synthesized benzoxazine monomers present-
ing one, two, and three coumarin units, respectively. We sus-
pected that, aer thermal curing of these benzoxazine
monomers, photodimerization, through [2p + 2p] cycloaddi-
1
Fig. 3 (a–c) H and (d–f) 13C NMR of (a and d) bisphenol A–NO2, (b
tion, of the coumarin units in the PBZs would enhance their and e) bisphenol A–NH2, and (c and f) di-coumarin BZ.
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which we reacted with coumarin–OH and paraformaldehyde compound 3-phenyl-3,4-dihydro-2H-benzooxazine (Pa) (263 ꢀC)
through the Mannich reaction in 1,4-dioxane to afford tri- lacking the coumarin unit; its curing peak appeared at 263.0 ꢀC,
coumarin BZ.
implying that the coumarin unit acted as a catalyst that lowered
the curing temperature and a strong electron withdrawing elec-
tron conjugated C]C bond in coumarin unit.30,31 Aer thermal
Characterization of mono-coumarin BZ
ꢀ
curing at 150 C, the melting point disappeared and theꢀcuring
We used FTIR and NMR spectroscopy to conrm the chemical
structures of our coumarin-functionalized benzoxazines.
Fig. 1(a) presents the H NMR spectrum of mono-coumarin-BZ
exotherm peak and the reaction heat decreased to 213.8 C and
101.7 J gꢂ1, respectively. The curing exotherm peak disappeared
completely aer thermal curing at 180 ꢀC; glass transition
temperatures of 170.2 and 196.4 ꢀC appeared aer thermal curing
1
in d-DMSO. The signals ranging from 6.22 to 7.54 ppm repre-
sent the aromatic protons; the peaks at 6.20 and 2.36 ppm
correspond to the CH]C and CH3 groups of the coumarin unit.
The benzoxazine ring of mono-coumarin BZ is characterized by
signals at 4.75 (ArCH2N) and 5.55 (OCH2N) ppm; the relative
intensity of these two signals is 1 : 1, conrming the high purity
of this benzoxazine monomer. Fig. 1(b) displays the corre-
sponding 13C NMR spectrum, with signals at 159.76 and 18.13
ppm for the C]O and CH3 groups of the coumarin unit and
characteristic signals for the ArCH2N and OCH2N groups of the
ꢀ
at 210 and 240 C, respectively. FTIR spectra revealed the main
characteristic absorption peaks of the uncured mono-coumarin
BZ monomer at 937 cmꢂ1 (out-of plane C–H bending) and 1263
cmꢂ1 (C–O–C stretching);32–37 both of these signals decreased
gradually upon increasing the curing polymerization tempera-
ture, disappearing almost completely aer curing at 240 ꢀC.
Characterization of di-coumarin BZ
benzoxazine ring at 44.61 and 79.38 ppm, respectively. These Fig. 3 presents the NMR spectra of bisphenol A–NO2, bisphenol
spectra conrmed the chemical structure of mono-coumarin BZ. A–NH2, and di-coumarin BZ in d-DMSO. The 1H NMR spectrum
We employed DSC and FTIR spectroscopy to investigate the of bisphenol A–NO2 [Fig. 3(a)] features signals at 7.10–8.25 ppm
thermal polymerization behavior of mono-coumarin BZ (Fig. 2). for the aromatic protons and 1.57 ppm for the C(CH3)2 protons;
The meltꢀing point of the uncured mono-coumarin BZ monomer a signal at 4.95 ppm appears in Fig. 3(b) for the NH proton of
was 142 C, conrming its high purity, with a curing exotherꢂm1 bisphenol A–NH2. The 1H NMR spectrum [Fig. 3(c)] of
peak centered at 229.4 ꢀC and a reaction heat of 294.4 J g
di-coumarin-BZ features signals in the range from 6.73 to
[Fig. 2(A)-(a)]. The curing exotherm temperature of mono- 7.54 ppm corresponding to the aromatic protons, and signals at
coumarin BZ was much lower than that of the model 6.17 and 2.34 ppm for CH]C and CH3 groups, respectively, of
Fig. 4 (A) DSC thermograms and (B) FTIR spectra of di-coumarin BZ, recorded after each curing stage.
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the coumarin unit. Characteristic signals of the benzoxazine
ring appear at 4.71 and 5.50 ppm for the ArCH2N and OCH2N
groups, respectively. Moreover, the integration area of the two
proton signals ArCH2N and OCH2N was close to 1 : 1; con-
rming the high purity of this benzoxazine monomer. Fig. 3(d)–
(f) present the 13C NMR spectra of bisphenol A–NO2, bisphenol
A–NH2, and di-coumarin-BZ. The successful reduction of
bisphenol A–NO2 to bisphenol A–NH2 was characterized by the
disappearance of the signal at 120.0 ppm [Fig. 3(d)] for the
ArCNO2 group and the appearance of a signal at 117.3 ppm for
the ArCNH2 group [Fig. 3(e)]. We observed characteristic signals
at 160 (C]O), 79.8 (OCH2N), 45.1 (ArCH2N), and 18.1 (CH3)
ppm for di-coumarin BZ [Fig. 3(f)], conrming the successful
introduction the coumarin units into the benzoxazine rings.
Again, we employed DSC to monitor the behavior of di-
coumarin BZ before and aer thermal curing at 150, 180, 210,
ꢀ
and 240 C. Fig. 4(A) presents the DSC analyses for this mono-
mer recorded at a heating rate of 20 C minꢂ1. An exothermic
ꢀ
ꢀ
peak for di-coumarin BZ was centered at 251.4 C, with a reac-
tion heat of 89.5 J gꢂ1. We observed two interesting phenomena
for this system: (a) a glass transition temperature was evident
for all curing teꢀmperatures, with the value of Tg increasing, from
126.3 to 207.8 C, upon increasing the thermal curing temper-
ature; (b) the glass transition temperature (207.8 ꢀC) of poly(di-
coumarin BZ) was higher than that of poly(mono-coumarin BZ)
(196.4 ꢀC) aer thermal curing at 240 ꢀC, consistent with
a higher crosslinking density for poly(di-coumarin BZ). Fig. 4(B)
presents corresponding FTIR spectra of di-coumarin BZ recor-
ded before and aer thermal curing. Characteristic absorption
peaks of the benzoxazine ring structure of the monomer, for
C–O–C and C–N–C stretching and the benzene ring mode
appeared at 1239, 1186, and 932 cmꢂ1, respectively. Aer
Fig. 5 (a–c) 1H and (d–f) 13C NMR of (a and d) 1,3,5-tris(4-nitophe-
noxy)benzene, (b and e) 1,3,5-tris(4-aminophenoxy)benzene, and (c
and f) tri-coumarin BZ.
thermal curing at 180, 210, and 240 ꢀC, the characteristic signals at 160 and 18.1 ppm representing the C]O and CH3
absorption peak at 932 cmꢂ1 (out-of plane C–H bending) dis- groups, respectively, of the coumarin units.
appeared from the spectrum of poly(di-coumarin BZ), with the
We used DSC and FTIR spectroscopy to study the thermal
signal of the C]O groups at 1733 cmꢂ1 shiing to lower curing polymerization of tri-coumarin BZ. The DSC thermogram
wavenumber, due to hydrogen bonding between the C]O of uncured tri-coumarin BZ features a sharp exothermic peak
groups of the coumarin units and phenolic OH groups.
centered at 241.4 ꢀC, with a reaction heat of 146.1 J gꢂ1 [Fig. 6(A)].
The reaction heat of the exotherm peak decreased gradually
upon increasing the curing temperature, disappearing
Characterization of tri-coumarin BZ
ꢀ
completely aer thermal curing at 240 C. Compared with the
Fig. 5 displays NMR spectra of 1,3,5-tris(4-nitrophenoxy) behavior of mono-coumarin-BZ and di-coumarin-BZ, the curing
benzene, 1,3,5-tris(4-aminophenoxy)benzene, and tri- peak shied to higher temperature upon increasing the
coumarin BZ. Three signals appeared for the aromatic temperature because the relatively higher crosslinking density of
protons of 1,3,5-tris(4-nitrophenoxy)benzene at 6.94, 7.30, and tri-coumarin-BZ inhibited its thermal curing ability, with
8.28 ppm [Fig. 5(a)]; a signal at 4.90 ppm corresponding to the complete thermal curing occurring only at 240 ꢀC, much higher
NH protons and signals at 5.91–6.74 ppm for the aromatic than the complete thermal curing temperatures of mono-
protons were present in the spectrum of 1,3,5-tris(4- coumarin BZ and di-coumarin BZ (180 ꢀC). Because of the rela-
aminophenoxy)benzene [Fig. 5(b)]. The 1H NMR spectrum of tively higher crosslinking density arising from the greater
tri-coumarin-BZ [Fig. 5(c)] features signals at 6.17 and 2.34 ppm number of aromatic heterocyclic groups iꢀn poly(tri-coumarin
corresponding to the CH]C and CH3 groups of the coumarin BZ), its glass transition temperature (240.5 C) was higher than
units, with characteristic signals for the benzoxazine ring at those of poly(mono-coumarin BZ) (196.4 ꢀC) and poly(di-
ꢀ
4.70 and 5.55 ppm (ArCH2N and OCH2N groups, respectively). coumarin BZ) (207.8 C). Fig. 6(B) presents FTIR spectra of tri-
The 13C NMR spectrum of tri-coumarin BZ [Fig. 5(f)] conrmed coumarin BZ before and aer thermal curing, revealing
its chemical structure, with signals at 79.88 and 45.05 ppm phenomena similar to those for mono-coumarin BZ and di-
ꢀ
corresponding to the characteristic resonances of the OCH2N coumarin BZ. Aer thermal curing at 240 C, the characteristic
and ArCH2N groups, respectively, in the oxazine ring, and absorption signals for the monomer at 1502, 1379, 1238 and 932
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Fig. 6 (A) DSC thermograms and (B) FTIR spectra of tri-coumarin BZ, recorded after each curing stage.
cmꢂ1 disappeared from the spectrum of poly(tri-coumarin BZ), increasing the thermal curing temperature for each of these
indicating that completely ring opening of benzoxazine ring.
three coumarin-functionalized BZ monomers. We attribute the
weight loss for these three different monomers to amine evap-
oration through Mannich base cleavage and phenol and
coumarin degradation.30 Second, the thermal decomposition
temperature and char yield of poly(tri-coumarin BZ) were higher
TGA of mono-, di-, and tri-coumarin BZ
Fig. 7 presents TGA thermograms of the mono-, di-, and tri-
ꢀ
(370 C and 52.43 wt%, respectively) than those of poly(mono-
coumarin BZ monomers before and aer thermal curing at
coumarin BZ) (336 ꢀC and 51.39 wt%, respectively) and
poly(di-coumarin BZ) (366 ꢀC and 46.29 wt%, respectively), due
to it having the network with the highest crosslinking
density.30,31 In addition, we also examined the thermal stability
of the model poly(Pa BZ) lacking the coumarin unit; its char
yield was 43 wt%. Thus, the thermal stabilities our three
coumarin-functionalized BZ derivatives were better than that of
the Pa-type benzoxazine aer thermal curing.
ꢂ1
ꢀ
ꢀ
150, 180, 210, and 240 C at a heating rate 20 C min under
N2. We observed two phenomena. First, the thermal decompo-
sition temperature and char yield both increased upon
Photodimerization through [2p + 2p] cycloaddition of
coumarin moieties
Coumarin units can undergo photochemical dimerization
through [2p + 2p] cycloaddition in direct and sensitized reac-
tions via singlet and triplet excited states.38,39 Fig. 8(A) displays
the UV-Vis spectrum of mono-coumarin BZ aer photo-
dimerization [Scheme 2(a)] of its coumarin unit under UV
irradiation (350 nm) at a concentration of 10ꢂ4 M. The typical
absorption peaks of coumarin and its derivatives appear
between 250 and 300 nm, corresponding to p–p* transitions,
with another p–p* absorption signal, due to the pyrone unit,
appearing between 310 and 340 nm.28c,31a,40,41 As revealed in
Fig. 8(A), UV irradiation for 30 min led to a decrease in the
Fig. 7 TGA analyses of (A) mono-coumarin BZ, (B) di-coumarin BZ,
and (C) tri-coumarin BZ, recorded after each curing stage.
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Fig. 8 (A) Changes in UV-Vis absorbance of mono-coumarin BZ solutions in THF after irradiation at 365 nm. (B) DSC traces of mono- and di-
coumarin BZ before and after photodimerization of their coumarin units. (C and D) 1H NMR spectra of (C) mono-coumarin BZ and (D) di-
coumarin BZ before and after photodimerization.
intensity of the absorption signal at 318 nm, conrming that photodimerization of each monomer, due to destruction of the
dimerization through [2p + 2p] cycloaddition of the olenic benzoxazine surface units during irradiation.30,31 Yagci et al.
bond destroyed the delocalized p system. We calculated the reported that various isomers, with different syn, anti, head-to-
degree of photodimerization of the 4-methylcoumarin moieties head, and head-to-tail congurations for the CH3 groups, can
1
of mono-coumarin BZ using the equation28c,31a
form aer UV irradiation.30 Fig. 8(C) and (D) displays H NMR
spectra of mono- and di-coumarin BZ before and aer UV
irradiation for 30 min in CDCl3. Aer their photodimerization,
as presented in Schemes 2(a) and (b), two new characteristic
signals appeared at 3.7 and 1.4 ppm, representing the protons
of the CH group (peak e) of the cyclobutane and the CH3 group
(peak f), respectively. In addition, the intensity of the signal at
6.20 ppm for the CH]C group of the coumarin unit decreased
to approximately 35% [Fig. 8(C)], consistent with the behavior
Photodimerization degree ¼ (1 – At/A0) ꢁ 100%
where A0 and At are the absorbance of the coumarin unit at 318
nm initially and aer irradiation for time t, respectively. Aer
irradiation for 30 min, the degree of dimerization for this
system was approximately 35%. Fig. 8(B) and S6–S8† present
DSC thermograms of mono-, di-, and tri-coumarin BZ before
and aer [2p + 2p] dimerization of their coumarin moieties. As
mentioned above in our discussion of Fig. 2(A) and 3(A), the
exothermic peaks of mono-coumarin BZ and di-coumarin BZ
1
observed in the UV-Vis spectrum [Fig. 8(A)]. Thus, the H NMR
spectra provided strong evidence for dimerization of the
coumarin moieties in these coumarin-functionalized BZ
monomers. Fig. 9 displays TGA traces (under N2) of the uncured
di-coumarin BZ, the poly(di-coumarin BZ) obtained aer
thermal curing alone, and the poly(di-coumarin BZ) obtained
aer both photodimerization and thermal curing. Here, we
ꢀ
appeared at 229 and 251 C, respectively. The curing exotherm
peak shied to lower temperature and the reaction heat
decreased (for mono-coumarin BZ: Tp, 204 ꢀC; DH, 210 J gꢂ1; for
di-coumarin BZ: Tp, 234 ꢀC; DH, 78
J
gꢂ1
)
aer
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Scheme 2 Photodimerization of (a) mono-coumarin BZ, (b) di-coumarin BZ, and (c) tri-coumarin BZ, with subsequent thermal curing to form
PBZ matrices of high crosslinking density.
used the 10 wt% weight loss temperature (Td) 321, 381, 398, and as a result of destruction of the surface during irradiation,
365 ꢀC, respectively-as a proxy for the thermal stability. The whereas the char yield increased slightly to 48 wt% because of
value of Td of poly(di-coumarin BZ) gradually increased upon the relatively higher crosslinking density aer UV irridation.31
increasing the curing temperature, with a char yield of 46 wt% We employed DSC to monitor the effect of photo-crosslinking of
aer thermal curing at 240 ꢀC. Interestingly, both photo- the coumarin units under UV irradiation (300 nm) on the glass
dimerization and thermal curing gave a poly(di-coumarin BZ) transition temperatures of the PBZ matrices formed from our
displaying a degradation temperature that decreased, possibly three different coumarin-functionalized PBZs (Fig. 10). The
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glass transition temperature increased for each of our
coumarin-functionalized benzoxazine monomers aer photo-
dimerization. For example, the values of Tg increased from
170.0 to 203.4 ꢀC, from 198.8 to 210.1 ꢀC, and from 218.8 to
260.0 ꢀC for poly(mono-coumarin BZ), poly(di-coumarin BZ),
and poly(tri-coumarin BZ), respectively, aer combining pho-
todimerization with thermal curing at 210 ꢀC. We attribute
these increases in glass transition temperature to the increased
crosslinking density of the PBZs aer photodimerization of
their coumarin units. The polymer formed from the trivalent
coumarin/benzoxazine displayed the highest glass transition
and thermal decomposition temperatures and char yield, as
would be expected from the highest crosslinking density for this
compound, the presence of methyl group in the repeating unit
and the additional crosslinking reaction through the trans-
esterication of COO group in coumarin unit and OH groups
which generated aer ring opening thermal polymerization of
benzoxazine as displayed in Scheme 2(c).31b
Fig. 9 TGA analyses of di-coumarin BZ: (a) uncured monomer; (b and
c) after thermal curing at (b) 210 and (c) 240 ꢀC, and after both pho-
todimerization and thermal curing at 240 ꢀC.
Fig. 10 DSC analyses of (a and b) poly(mono-coumarin BZ), (c and d) poly(di-coumarin BZ), and (e and f) poly(tri-coumarin BZ) after (a, c and e)
thermal curing at 210 ꢀC and (b, d and f) sequential photodimerization and thermal curing at 210 ꢀC.
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Fig. 11 WCAs of poly(mono-coumarin BZ), poly(di-coumarin BZ), and poly(tri-coumarin BZ) prepared through thermal curing at 210 ꢀC before
and after UV exposure at 365 nm.
respectively). In addition, the WCA values of the poly(tri-
coumarin BZ) obtained aer both photodimerization and
thermal curing was higher than those of the corresponding
poly(mono-coumarin BZ) and poly(di-coumarin BZ) polymers,
due to greater intramolecular OH/N hydrogen bonding (from
the Mannich bridges) and additional intramolecular OH/O]C
hydrogen bonding (between the C]O groups of the coumarin
units and the phenolic OH groups) of the poly(tri-coumarin BZ).
Water contact angles of photocrosslinked coumarin-
functionalized PBZs
The physical properties of PBZs are dependent on the degrees of
intra- and intermolecular hydrogen bonding between the
phenolic OH groups and the N atoms in the Mannich bridges
aer ring opening of the benzoxazine monomers.18,21 In this
present study, we observed evidence, using FTIR spectroscopy,
for additional intramolecular OH/O]C hydrogen bonding
between the C]O groups of the coumarin units and the OH
groups. Fig. 11 and S6† summarize the WCAs for poly(mono-
coumarin BZ), poly(di-coumarin BZ), and poly(tri-coumarin
Conclusions
BZ) before and aer photodimerization and then thermal We have synthesized three different photo-crosslinkable
ꢀ
polymerization at 210 C for 2 h, and aer applying different coumarin-containing benzoxazine monomers through facile
thermal curing conditions (Fig. S9†), respectively. Interestingly, Mannich reactions of coumarin–OH and paraformaldehyde
the WCAs of the poly(mono-coumarin BZ), poly(di-coumarin with aniline, bisphenol A–NH2, and 1,3,5-tri(4-aminophenoxy)
BZ), and poly(tri-coumarin BZ) polymers obtained aer photo- benzene, respectively, in 1,4-dioxane. FTIR, NMR and high
dimerization of the coumarin units of their monomers and resolution mass spectra conrmed the chemical structures of
subsequent thermal curing were 108, 108, and 122ꢀ, respec- these three benzoxazine monomers. DSC and TGA analyses
tively; they were higher than those of the poly(mono-coumarin revealed that the glass transition temperatures, thermal
BZ), poly(di-coumarin BZ), and poly(tri-coumarin BZ) poly- decomposition temperatures, and char yields of the resulting
mers obtained aer thermal curing alone (105, 100, and 110ꢀ, polymers were strongly dependent on the valency of the
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monomers and the degrees of photodimerization of the
coumarin units, both of which affected the crosslinking densi-
M. Arslan, B. Kiskan and Y. Yagci, Macromolecules, 2015,
48, 1329–1334.
ties. In addition, the WCAs of the poly(tri-coumarin BZ) poly- 17 C. F. Wang, S. F. Chiou, F. H. Ko, J. K. Chen, C. T. Chou,
mers obtained with and without photodimerization possessed
highly hydrophobic surfaces because strong intramolecular
C. F. Huang, S. W. Kuo and F. C. Chang, Langmuir, 2007,
23, 5868–5871.
hydrogen bonding; these highly thermally stable and highly 18 C. F. Wang, F. C. Chang and S. W. Kuo, in Handbook of
hydrophobic surfaces could be good candidates for use in high-
performance applications and polymer coatings.
Benzoxazine Resins, ed. H. Ishida and T. Agag, Elsevier,
Amsterdam, 2011, ch. 33, p. 579.
19 M. R. Vengatesan, S. Devaraju, K. Dinakaran and M. Alagar,
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20 A. Sudo, R. Kudoh, H. Nakayama, K. Arima and T. Endo,
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Acknowledgements
This study was supported nancially by the Ministry of Science
and Technology, Taiwan, Republic of China, under contracts 21 H. Ishida, in Handbook of Benzoxazine Resins, ed. H. Ishida
MOST103-2221-E-110-079-MY3 and MOST102-2221-E-110-008-
MY3.
and T. Agag, Elsevier, Amsterdam, 2011, ch. 1, p. 1.
22 N. Gogoi, D. Rastogi, M. Jassal and A. K. Agrawal, J. Text.
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