Examining the effect of hydroxyl groups on the thermal properties of polybenzoxazines: Using molecular design and Monte Carlo simulation

Article abstract of DOI:10.1039/c8ra02033g

The influence of methylol and phenolic hydroxyl on the thermal properties of polybenzoxazines has been studied using two monofunctional benzoxazine monomers synthesized from para methylol-/ethyl- phenol, aniline and paraformaldehyde. The chemical structur

Full text of DOI:10.1039/c8ra02033g

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PAPER
Examining the effect of hydroxyl groups on the
thermal properties of polybenzoxazines: using
molecular design and Monte Carlo simulation
Cite this: RSC Adv., 2018, 8, 18038
a
b
a
b
Kan Zhang, Lu Han, Yijing Nie, Matthew Louis Szigetib and Hatsuo Ishida
*
*
The influence of methylol and phenolic hydroxyl on the thermal properties of polybenzoxazines has been
studied using two monofunctional benzoxazine monomers synthesized from para methylol-/ethyl- phenol,
aniline and paraformaldehyde. The chemical structures of the synthesized monomers are confirmed by 1H
nuclear magnetic resonance (NMR), 13C NMR and Fourier transform infrared spectroscopy (FT-IR).
Polymerizations are monitored by differential scanning calorimetry (DSC). The glass transition
temperature (Tg) of each polybenzoxazine is measured by DSC as well as dynamic mechanical analysis
(DMA), indicating the greatly increased Tg via incorporation of methylol functionality into benzoxazine
moiety. Monte Carlo simulations are also applied to further investigate the underlying structure-property
relationship between intermolecular hydrogen-bonding network originating from different types of
hydroxyl groups and thermal properties of polybenzoxazines. The agreement between the experimental
and simulation results provide us with a fundamental understanding of the designing roles in highly
thermally stable polybenzoxazines.
Received 7th March 2018
Accepted 11th May 2018
DOI: 10.1039/c8ra02033g
rsc.li/rsc-advances
Benzoxazines containing hydroxyl group have been exten-
sively studied due to their lower polymerization temperature
and the modiable character of hydroxyl functionality.24–27
Introduction
In recent years, needs for development of polymers with highly
thermally stable properties have increased drastically due to
technological advancements that require materials for use in
harsher environments. Polybenzoxazines are a class of ther-
mosetting phenolic resins that gained great interest because of
their unusual set of competitive properties.1–6 They combine
various outstanding properties of high performance thermosets
like high thermal stability,7 high char yield,8 high glass transi-
tion temperature (Tg),9 near-zero volumetric change during
polymerization,10 good mechanical11 and dielectric proper-
ties,12,13 low water absorption14 and low ammability.15 The
most attractive characteristic of polybenzoxazines is their
extraordinarily rich molecular design exibility and versatility
in molecular structure. Thus, polybenzoxazine can satisfy
specic requirements for different applications such as high
temperature resistant adhesives, coatings, membranes and
aerospace materials. For example, the thermal stability of pol-
ybenzoxazines can be achieved by incorporation of various
functionalities such as amide,16,17 imide,17,18 allyl,19,20 nitrile21
and furfuryl22 or increasing the number of oxazine rings in
molecular backbone.23
Among these hydroxylated compounds, methylol functional
benzoxazines have been reported as exible precursors for
chemical modication and their improved polymerization
behavior and polymer properties.28,29 For example, a meth-
acryloyl-benzoxazine-containing monomer from methylol-
functional benzoxazine was synthesized and its photo-
polymerization behavior as well as thermal polymerization was
studied by Jin et al.28 Besides, the hydrogen-bonding interac-
tions and polymerization mechanism of methylol functional
benzoxazine isomers have been systematically investigated.30–33
It has been found that intermolecular hydrogen-bonding exists
in para-methylol functional benzoxazines. However, intra-
molecular hydrogen-bonding system is present only in the
ortho-isomer.32 In addition, both condensation reaction
between the methylol and phenyl groups, in a manner similar to
the traditional phenolic resins, and the usual cationic ring-
opening polymerization of the oxazine group during thermal
treatment of ortho-methylol functional benzoxazine were found
to occur.33 Unlike smart reactions reported for ortho-functional
benzoxazines that further undergo structural transformation
forming benzoxazole structure at elevated temperatures, only
phenolic moieties can be formed for para-isomer upon heating.
Despite lack of the structural transformation unique to the
ortho-isomers, it is worth noticing that excellent thermal
stability of the polybenzoxazines derived from para-methylol
functional benzoxazine monomers compared with other
aSchool of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013,
China. E-mail: nieyijing@ujs.edu.cn
bDepartment of Macromolecular Science and Engineering, Case Western Reserve
University, Cleveland, OH, 44106, USA. E-mail: hxi3@cwru.edu
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Scheme 1 Synthesis of methylol and ethyl functional benzoxazine monomers.
traditional polybenzoxazines based on non-hydroxyl benzox- received from Sigma-Aldrich. Toluene, ethyl acetate, hexane,
azine monomers can be obtained. It is hypothesized that the chloroform, sodium hydroxide, and sodium sulfate were ob-
existence of additional hydroxyl from methylol groups in poly- tained from Fisher Scientic and used as received.
benzoxazine network is the key factor causing their outstanding
thermal properties.
Synthesis of (3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazin-6-
yl)methanol (abbreviated as pHBA-a)
Dynamic Monte Carlo simulations are usually applied to
investigate microscopic structural changes or segmental
dynamics, such as polymer crystallization,34–36 structural relax-
ation37,38 and glass transition.39,40 Previously, the correlation
between dynamic heterogeneity and conformational transition
during glass transition41 has been revealed by using dynamic
Monte Carlo simulations. Such conformational change is the
key factor controlling the effect of chain topology on dynamic
heterogeneity and fragility.42,43 In addition, we also demon-
strated that local segmental density and interfacial interaction
are the key factors controlling the distribution of local Tgs in
ultrathin polymer lms.44 In short, many experimental ndings
can be theoretically supported, and, more importantly, under-
lying molecular mechanisms can be unearthed based on
molecular simulations where experimental verication is
difficult.
This work forms part of our ongoing research into examining
and understanding the role of hydroxyl functionalities on
improving the thermal properties of polybenzoxazines. Here, we
synthesize two benzoxazine monomers, pHBA-a (a hydroxyl-
containing benzoxazine) and pEPH-a (ethyl-functional benzox-
azine) as shown in Scheme 1 to investigate the existence of
different roles of hydroxyl on the thermal properties of poly-
benzoxazines. pEPH-a is designed as a control by replacing the
hydroxyl in pHBA-a by methyl for comparison. We correlate
analytical measurements carried out on very pure benzoxazine
samples with Monte Carlo simulations in order to better
understand the underlying structure-property relationship
between different types of hydroxyl interactions and thermal
properties of polybenzoxazines.
Into a 50 mL round ask were added 20 mL of toluene, aniline
(1.4 g, 15 mmol), pHBA (1.83 g, 15 mmol), and para-
formaldehyde (0.96 g, 30 mmol). The mixture was reuxed for
6 h. Then the product was concentrated using a rotary evap-
orator and redissolved in chloroform followed by base-wash
using 1 N NaOH aqueous solution three times and subse-
quent wash with deionized water. The chloroform solution
was dried over sodium sulfate anhydrous and recrystallized
1
from chloroform to yield white crystals (yield: 75%). H NMR
ꢀ
(600 MHz, DMSO-d6, 25 C) d, ppm: 4.34 (d, 2H, –CH2–OH,),
4.62 (s, 2H, Ar–CH2–N, oxazine), 5.00 (t, 1H, –OH), 5.40 (s, 2H,
O–CH2–N, oxazine), 6.65–7.21 (m, 8H, Ar). FT-IR spectra
(KBr), cmꢁ1: 3250 (stretching of OH of methylol), 1497
(stretching of trisubstituted benzene ring), 1236 (Ar–O–C
asymmetric stretching), 944 (oxazine ring related mode). Anal.
calcd for C15H15NO2: C, 74.69%; H, 6.22%; N, 5.84%. Found: C,
74.13%; H, 6.37%; N, 5.69%.
Synthesis of 6-ethyl-3-phenyl-3,4-dihydro-2H-benzo[e][1,3]
oxazine (abbreviated as pEPH-a)
Into a 50 mL round ask were added 20 mL of toluene, aniline
(1.4 g, 15 mmol), pEPH (1.86 g, 15 mmol), and paraformaldehyde
(0.96 g, 30 mmol). The mixture was reuxed for 6 h. Then the
product was cooled to room temperature and washed with 1 N
NaOH solution and then water to eliminate the unreacted residual
starting materials and byproducts. The solution was dried over
sodium sulfate anhydrous and followed by fractionating with
column chromatography (eluent: hexanes and ethyl acetate, in
a volume ration of 4 : 1) to obtain nal products (yield: 58%). 1H
NMR (600 MHz, CDCl3, 25 ꢀC) d, ppm: 1.18 (m, 3H, –CH3), 2.55 (q,
2H, –CH2), 4.61 (s, 2H, Ar–CH2–N, oxazine), 5.33 (s, 2H, O–CH2–N,
oxazine), 6.64–7.25 (m, 8H, Ar). FT-IR spectra (KBr), cmꢁ1: 1499
Experimental
Materials
4-Hydroxybenzyl alcohol (pHBA) (98%), 4-ethylphenol (pEPH) (stretching of trisubstituted benzene ring), 1228 (Ar–O–C asym-
(98%), aniline, and paraformaldehyde (96%) were used as metric stretching), 949 (oxazine ring related mode). Anal. calcd for
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C15H15NO2: C, 80.30%; H, 7.16%; N, 5.85%. Found: C, 80.09%; H,
7.22%; N, 5.78%.
E ¼ cEc + lELJ + bEb
(1)
where Ec is the bending energy for two consecutive bonds along
the chain, reecting the chain semiexibility.34 Besides,
hꢀ ꢁ
ꢀ ꢁ i
ELJ ¼ 43
12 ꢁ
is the Lennard-Jones potential for non-
Polymerization of benzoxazine monomers
6
s
s
r
r
Each benzoxazine monomer was polymerized using different
polymerization cycles. Polymerizations of pHBA-a was carried
out in an oven by heating at 160, 180, 200 and 220 C for 2 h
bonded segments (3 is the characteristic energy well of the
potential, s is the lattice spacing in consideration of the
excluded volume, and r is the distance between two non-bonded
segments), Eb is the mixing-interaction energy for one nearest-
neighbor pair of hydroxyl groups, reecting the hydrogen-
bonding interactions, c is the net number of non-collinear
connection pairs of bonds along the chain, l is the net
number of Lennard-Jones interaction pairs between non-
bonded segments within the cut-off distance, and b is the net
change of the nearest-neighbor hydroxyl pairs. In the current
simulations, 43/Ec is xed at 2, Eb/Ec is set to ꢁ1, s is 1 lattice
site, the cut-off distance for the Lennard-Jones potential is 3
lattice sites, and kT/Ec (k is the Boltzmann's constant and T is
the temperature) represents the reduced system temperature
(abbreviated as T*). The initial chains with regular arrange-
ments were relaxed for 106 MC cycles (one MC cycle represents
the step when all segments have a move trial on average) to
obtain an equilibrium melt state at T* ¼ 8.0. Subsequently, the
system was cooled gradually from T* ¼ 8.0 to T* ¼ 0.05 at
a cooling rate of 0.01 unit of T* per 100 MC cycles.
ꢀ
each step, thus obtaining poly(pHBA-a). In addition, polymeri-
zation ofꢀpEPH-a was done by heating at 160, 180, 200, 220, 240
and 260 C for 2 h each step, thus forming poly(pEPH-a).
Characterization. Varian Oxford AS600 (600 MHz) nuclear
magnetic resonance (NMR) spectrometer was used to obtain 1H
and 13C NMR spectra using the number of transient of 64 and
1024, respectively. 1H–1H nuclear Overhauser effect spectros-
copy (NOESY) was conducted on Bruker AVANCE II 400 MHz
NMR. A Bomem Michelson MB100 Fourier transform infrared
(FT-IR) spectrophotometer was used to obtain FT-IR spectra.
The spectrophotometer was equipped with a deuterated trigly-
cine sulfate (DTGS) detector and a dry air purge unit. Sixty four
scans were coadded at a resolution of 4 cmꢁ1 in the frequency
range of 4000–400 cmꢁ1. All benzoxazine monomers were nely
ground with potassium bromide powder and pressed into a disk
and the spectrum was taken as the transmission mode. A TA
Instruments differential scanning calorimeter (DSC) model
2920 was used with a temperature ramp rate of 10 ꢀC minꢁ1 and
a nitrogen ow rate of 60 mL minꢁ1 for all tests of DSC study. All
samples were sealed in hermetic aluminum pans with lids.
Viscosity was tested by Anton Paar GmbH Rheometer MCR 301
with constant shear rate at 10 sꢁ1 with a temperature ramp rate
Result and discussion
Synthesis of benzoxazine monomers
of 5 ꢀC minꢁ1. Dynamic mechanical analysis (DMA) was con- Monofunctional benzoxazine monomer containing methylol
ducted on a TA Instruments Model Q800 DMA applying functionality was successfully synthesized using a primary
controlled strain tension mode with amplitude of 10 mm and amine (aniline), formaldehyde, and 4-hydroxybenzyl alcohol
a temperature ramp rate of 3 C minꢁ1. Strain sweep was rst (pHBA). In addition, a counterpart, pEPH-a, has also been
ꢀ
performed to determine the linear viscoelastic limit. Ther- designed. pEPH-a is a benzoxazine compound with para func-
mogravimetric analyses (TGA) were performed on a TA Instru- tionality of ethyl group, while the functionality is methylol for
ments Q500 thermogravimetric analyzer that was purged with pHBA-a at the same position. The synthesis routes involving
nitrogen at a ow rate of 40 mL minꢁ1. A heating rate of structure of the reactants used, and nal benzoxazine mono-
10 C minꢁ1 was applied.
mers are shown in Scheme 1.
ꢀ
Simulation details. Dynamic Monte Carlo simulations were
The chemical structures of pHBA-a and pEPH-a are
carried out on lattice space. First, a lattice box containing 30 ꢂ conrmed by 1H NMR, 13C NMR, FT-IR and elemental analysis.
1
30 ꢂ 30 cubic lattice cells with periodic boundary conditions Fig. 1 shows the H NMR spectra of pHBA-a and pEPH-a. The
was set up. Then, 390 model chains, each containing 60 characteristic resonances2 attributed to the oxazine ring struc-
segments, were regularly put into the simulation box. In the ture, Ar–CH2–N– and –O–CH2–N– for pHBA-a are observed at
lattice space, one lattice site could be only occupied by one 4.60 and 5.40 ppm, respectively. The corresponding resonances
segment, and double occupations were not allowed due to the for pEPH-a are observed at 4.61 and 5.33 ppm, respectively. The
volume exclusion of polymer chains. In this condition, the 1H NMR spectrum also indicates the presence of the methylol
polymer volume fraction was 0.867, and the remaining vacancy group (–CH2OH) in pHBA-a based on the observation of the
lattice sites were treated as free volume for the trial moves of resonances of –CH2–O and hydroxyl attached to a methylol
polymer chains. The bonds could be oriented either along group at 4.34 and 5.00 ppm, respectively. Moreover, the reso-
lattice axes or along diagonals, and therefore the coordination nances due to the presence of ethyl group in pEPH-a are
number of each segment was 26. During chain movements, observed at 1.18 and 2.55 ppm.
a segment could jump from an occupied site into a neighboring
unoccupied site.
13C NMR analyses were also performed to further conrm
the structures of pHBA-a and pEPH-a. The spectra are shown in
The conventional Metropolis sampling algorithm is used to Fig. 2. The two singlets at 49.70 and 79.85 ppm for pHBA-a and
decide whether a motion can be accepted with the potential 50.49 and 79.33 ppm for pEPH-a are typical carbon resonances2
energy penalty by eqn (1):
of Ar–CH2–N– and –O–CH2–N– of the oxazine ring, respectively.
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Fig. 1 1H NMR spectra of pHBA-a (in DMSO-d6) and pEPH-a (in CDCl3).
1
2D H–1H NMR NOESY analysis of benzoxazine monomer
The resonance at 63.25 ppm is assigned to the carbon of
methylol group for pHBA-a while the resonances at 15.76 and
28.05 ppm are due to the presence of ethyl in pEPH-a.
Prior to this study, the presence of hydrogen bonding systems
in methylol functional benzoxazines has been investigated by
FT-IR spectra of pHBA-a and pEPH-a using the KBr pellet
method are shown in Fig. 3. The FT-IR spectrum of pHBA-
a shows a broad band around 3250 cmꢁ1, corresponding to
the stretching vibrational mode of the –OH group. Besides,
characteristic bands of asymmetrically trisubstituted benzene
ring are observed at 1497 and 1499 cmꢁ1 for pHBA-a and pEPH-
a, respectively, conrming the presence of functionalities as
a constitutive part of both benzoxazine monomers. In addi-
tion, the existence of a benzoxazine ring aromatic ether in the
monomers is indicated by the bands centered at 1236 and
1228 cmꢁ1 for pHBA-a and pEPH-a, respectively, which is due
to the C–O–C asymmetric stretching modes.45 Furthermore,
the characteristic bands at 944 and 949 cmꢁ1 for pHBA-a and
pEPH-a, which are mainly related to the oxazine ring, the
vibration of O–C2 vibration with a minor contribution from the
phenolic ring (C–H out of plane bending, C–C–C–C torsion).46
Moreover, elemental analysis was also performed and the
excellent agreement between the calculated and observed data
for the puried sample, showing that the targeted compounds,
pHBA-a and pEPH-a were obtained in high purity. This last
result along with all the previous spectroscopic information
are consistent with the successful synthesis of the target
compounds.
1
1D H NMR, and only intermolecular hydrogen bonding was
observed in pHBA-a due to the existence of the clear depen-
dence of the OH chemical shi upon concentration.32 Herein,
1
2D H–1H NOESY NMR of pHBA-a in DMSO-d6 has been per-
formed and its results are shown in Fig. 4. The OH proton from
methylol interacts through space with no other protons in the
same molecule, further indicating no intramolecular
hydrogen bonding is formed in pHBA-a. Therefore, only
intermolecular hydrogen bonds could be formed through
methylol–methylol or methylol-oxazine ring interaction in
pHBA-a, resulting in stronger driving force for crystal forma-
tion and higher melting temperature.
Polymerization behavior of benzoxazine monomer
DSC was used to study the polymerization behavior of pHBA-
a and pEPH-a as shown in Fig. 5. The thermograms show that
the ring-opening polymerization of pEPH-a started at 250 ꢀC
with its maximum centered at 267 ꢀC. However, the onset of
polymerization is shied in the presence of methylol group to as
low as 175 C for pHBA-a. In addition, the peak is broadened
and the maximum is also reduced to 216 ꢀC. In general,
electron-withdrawing substituents lead to a more acidic phenol
ꢀ
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Fig. 2 13C NMR spectra of pHBA-a (in DMSO-d6) and pEPH-a (in CDCl3).
Fig. 3 FT-IR spectra of benzoxazine monomers.
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Fig. 4 2D 1H–1H NOESY NMR spectra of pHBA-a in DMSO-d6 at 25 ꢀC (left) and NOE interactions observed (right).
species, resulting in a stronger catalytic effect taking place.47 polymerization at the range from 242 ꢀC to 283 ꢀC, which is due
However, the methylol group has neither a strong electron- to the cleavage of zwitterionic intermediate, forming very
withdrawing nor acidic character. The existence of intermolec- unstable phenolic species and N-methyleneaniline.23 The
ular hydrogen-bonding involving the methylol group and the weight reduction at this high temperature might also come
oxazine ring seems to accelerate the ring-opening polymeriza- from the water evaporation due to the partial occurance of the
tion in an intermolecular manner. Besides, pHBA-a shows an traditional methylene bridge formation of methylol reacting
endothermic melting peak at 100 ꢀC while the melting with the benzene ring, and/or the condensation of methylol
temperature of pEPH-a centers at 44 ꢀC. The appearance of OH groups to form aliphatic ether bonds.ꢀHowever, the initial
in benzoxazine moiety signicantly increases the melting weight loss of pHBA-a starts around 141 C, and the monomer
temperature.
evaporation has been signicantly reduced only to 3.0% before
Fig. 5 also shows a comparison of TGA and DSC thermo- starting polymerization due to the presence of methylol group.
grams of benzoxazine monomers. As shown in Fig. 5a, the The weight loss during the polymerization of pHBA-a is also
initial weight loss of pEPH-a starts at 104 ꢀC, and evaporation of much lower compared with that of pEPH-a. Furthermore, the
an excessive amount, as high as 62.5% of monomer is observed dependence of viscosity of pHBA-a on temperature was also
before even starting polymerization, reecting the nature of investigated. In general, the required viscosity for resin transfer
weak intermolecular interaction. The heat of melting value was molding (RTM) processing technology is roughly below 1 Pa s at
166.2 J gꢁ1 for pHBA-a while a much lower heat value (24.1 J gꢁ1
)
the processing temperature. Interestingly, the viscosity of
was found for pEPH-a, resulting in the easy evaporation of pHBA-a is less than 0.05 Pa s during 140 to 215 ꢀC at the heating
pEPH-a during the thermal treatment. A further ꢃ35.2% of rate of 5 ꢀC minꢁ1 as shown in Fig. 6, which exhibits the
weight loss can be observed during the ring-opening advantage of being monofunctional benzoxazine with very low
Fig. 5 TGA vs. DSC plots of benzoxazine monomers (heating rate: 10 ꢀC minꢁ1, under a N2 atmosphere): (a) pEPH-a and (b) pHBA-a.
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viscosity. It should be cautioned that the transducer used for the
DMA was applied to further conrm the glass transition
viscosity measurement had the sensitivity limit in the vicinity of temperature of polybenzoxazines. DMA samples of poly(pEPH-
0.1 Pa s and, therefore, this observed value should be taken as a) was difficult to prepare due to the very high amount
only the indication of low viscosity that easily satises the (97.7%) of monomer evaporation before and during the poly-
viscosity requirement for an RTM resin. All the result above merization as shown in Fig. 5, thus only poly(pHBA-a) was
highlights the advantage of the presence of methylol in ben- measured. The DMA results of poly(pHBA-a) are shown in Fig. 8
zoxazine moiety which is combined with a small monofunc- for the storage modulus (E0), loss modulus (E00) and tan d. As
tional benzoxazine structure.
shown in the gure, the Tg determinedꢀby the peak temperature
of both E00 and tan d is as high as 274 C, which is in relatively
good agreement with that measured by DSC at 261 ꢀC in Fig. 6.
The signicant improvement of Tg for poly(pHBA-a) seems to be
attributed to the formation of rich intermolecular hydrogen-
bonding network endowed by methylol functionality along
with phenolic Mannich bridge network. The weight reduction
centered at 220 ꢀC could also be partially attributed to this
weight loss of water as a consequence of condensation reaction.
As discussed earlier that some of the methylol groups might
condense to form crosslinks based on the aliphatic ether group,
which will aid in increasing the Tg. Additional complication
arises due to the possibility of the traditional methylene bridge
formation in the phenolic resin by the aromatic methylol group
reacting with the neighboring benzene rings. Further detailed
analysis is needed to investigate this cause of weight reduction
around 220 ꢀC to be either as a result of aliphatic ether
formation, methylene bridge formation, or degradation of the
zwitterionic intermediates.
Thermal properties of polybenzoxazines
Fig. 7 shows the comparison of the DSC thermosgrams of pol-
ybenzoxazines. In general, the Tg of polybenzoxazines based on
monofunctional benzoxazine monomers are much lower than
the polybenzoxazines derived from difunctional benzoxazine
monomers, which are due to the low crosslink density of mono-
functional benzoxazines despite some expected reactivity on the
aromatic ring of the phenolic and aniline moiety.8 As shown in
Fig. 7, the Tg of poly(pEPH-a) is observed at 98 ꢀC, which is even
lower than the polybenzoxazine based on PH-a (phenol and
aniline based monobenzoxazine).48 The incorporation of ethyl
functionality into benzoxazine decreases the rigidity of the
molecule itself, resulting in a lower Tg. However, poly(pHBA-a)
shows a Tg as high as 261 ꢀC, which is unexpectedly higher
than many other polybenzoxazines based on even difunctional
benzoxazine monomers.12,49 An interesting observation for the
glass transition behaviors iꢀs that the existence of great differ-
ence in Tg (as high as 163 C) between two polybenzoxazines,
although only slight difference exists from the view point of
their chemical structures.
In order to better understand the effect of hydroxyl groups on
the glass transition behavior of polybenzoxazines, coarse-
grained dynamic Monte Carlo simulations were performed in
this study. It has been well-known that the phenolic structure
can be formed upon ring-opening polymerization of
Fig. 6 Viscosity versus temperature of pHBA-a.
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Fig. 7 DSC thermograms of poly(pHBA-a) and poly(pEPH-a).
benzoxazine, thus the ring-opening polymerization of pHBA- the blue spheres represent the remaining parts (benzene,
a and pEPh-a can straightforwardly be described as shown in methylene, benzyl amine and methyl, respectively) in both
Scheme 2, neglecting the other possible crosslink reactions for polybenzoxazines. Due to the demand for the simplicity of
simplicity. In addition, two model chains for poly(pHBA-a) and molecular simulations, the size difference between these
poly(pEPH-a) are also designed as shown in Scheme 2 (right groups was ignored.
part). Herein the red spheres denote the hydroxyl groups, while
Fig. 8 Dynamic mechanical spectra of poly(pHBA-a).
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Scheme 2 Ring-opening polymerization of pHBA-a and pEPH-a. The right part displays the schematic representation of model chains of
polybenzoxazines.
One of the advantages of molecular simulations is that behavior of the polymer glass transition. As shown in Fig. 9, the
molecular motions can be tracked directly. Fig. 9 shows the MSDs of segments for both polybenzoxazines decrease at the
evolutions of mean square displacements (MSDs) of segments initial stage during cooling. But the MSDs level off below
during each cooling step:
a turning point, and the corresponding values are close to 0 at
very low temperatures, indicating that segment motions have
been completely frozen. In experiments, the glass transition is
usually dened by the temperature at which relaxation time
reaches 100 s.51 However, the time scale of molecular simula-
tions is much shorter (the simulation time unit is about 1 ps)
compared with the experiments. Therefore, it is difficult for
simulation time to reach the time scale of the experiments. In
the current paper, we dene the temperatures of the turning
points in the MSD curves (namely, the crossing points of two
extrapolation line in Fig. 8) as the glass transition temperatures.
Again as seen in Fig. 9, poly(pHBA-a) exhibits much higher glass
transition temperature (T* ¼ 1.05) than that of poly(pEPH-a) (T*
¼ 0.65), which is consistent with the experimental results
shown above. Since poly(pHBA-a) contains methylol function-
ality, more proposed types of intermolecular hydrogen-bonding
systems (Fig. 10) can be formed in polymeric network, which
restricts the motions of polymer chains, resulting in the
increase of its glass transition temperature. In the following
sections, we further probe the details of the effects of the
different segments on the polymer glass transition at the
molecular scale.
D
~
MSD ¼ ½riðt ¼ 100 MC cycles; T*Þ
E
2
ꢁ~riðt ¼ 0 MC cycles; T*Þꢄ
(2)
~
where ri is the position of segments at the simulation time of t
and the temperature of T* (the reduced system temperature),
and h i denotes an average over all segments. As is well-known,
MSD of segments is dominated by diffusion ability of segments,
which is inversely proportional to viscosity.50 Therefore, we can
use the changes of MSD of segments to reect the molecular
Fig. 11 displays the evolutions of MSDs of different segments
(the phenolic hydroxyl directly linked to the backbone; the
hydroxyl from methylol groups on the side chains; and the
remaining segments for poly(pHBA-a), and the phenolic hydroxyl
directly linked to the backbone, the methyl from ethyl groups on
the side chains and the rest segments for poly(pEPH-a)) during
cooling. As shown in Fig. 11, the value of the MSDs for both
hydroxyl groups of poly(pHBA-a) is rstly dropped to 0, indicating
Fig. 9 The mean square displacements of segments during cooling
process for poly(pHBA-a) and poly(pEPH-a), respectively.
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Fig. 10 Proposed intermolecular hydrogen-bonding interactions in poly(pHBA-a) and poly(pEPH-a).
that the hydroxyl groups in poly(pHBA-a) can be completely groups on the side chains for poly(pHBA-a), respectively. The
frozen prior to other segments in both polybenzoxazines. mixing parameter of hydroxyl groups is dened as the mean
However, poly(pEPH-a) exhibits different changes of segmental fraction of neighboring sites of each hydroxyl occupied by other
mobility. The MSDs of the hydroxyl and the remaining segments hydroxyl groups. It can be found that the mixing parameters of
reach 0 before that of the methyl. This result suggests that the both types of hydroxyl groups of poly(pHBA-a) increase gradually
only structural difference, the hydroxyl and methyl in para- during the glass transition process, indicating the local aggrega-
substitution of poly(pHBA-a) and poly(pEPH-a), playing the tion of the hydroxyl groups. In addition, the mixing parameters for
opposite role for affecting the Tg of polybenzoxazine. The incor- the hydroxyl groups of poly(pEPH-a) also increase during the glass
poration of methylol can signicantly increases the Tg while ethyl transition. However, for poly(pHBA-a), the hydroxyl from methylol
decreases.
groups on side chains exhibits the higher plateau values compared
In addition, the intermolecular interactions formed among with that directly linked to the backbone. For poly(pEPH-a), the
different types of hydroxyl groups in polybenzoxazines should also corresponding plateau values are the lowest. Interestingly, the
be considered. Fig. 12 depicts the variations of mixing parameters hydroxyl from methylol groups on side chains of poly(pHBA-a)
of the phenolic hydroxyl groups directly linked to the backbone for possess the highest mobility and rather smaller steric
poly(pHBA-a) and poly(pEPH-a), and the hydroxyl from methylol
Fig. 12 Evolutions of mixing parameters of the phenolic hydroxyl groups
directly linked to the backbone for poly(pHBA-a) and poly(pEPH-a), and
Fig. 11 The mean square displacements of different segments during the hydroxyl from methylol groups on the side chains for poly(pHBA-a),
cooling process for poly(pHBA-a) and poly(pEPH-a), respectively.
respectively.
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hindrances at relatively high temperatures as demonstrated via the glass transition temperature. However, it is much harder for the
highest values of the MSDs as shown in Fig. 11, thus they are more hydroxyl groups in poly(pEPH-a) to aggregate during cooling,
prone to aggregate during cooling under the effect of the inter- and thus they would be frozen at lower temperature compared
molecular interactions, resulting in the highest mixing parameters with those in poly(pHBA-a). Furthermore, the presence of the
at low temperatures. This behavior is reected on the curve being methyl on the side chains with relatively stronger mobility can
quickly reducing the MSD as the temperature is reduced than also contribute to the decreasing of the glass transition
other groups, such as phenolic OHs, as shown in Fig. 11. However, temperature of poly(pEPH-a).
due to the relatively high steric hindrances, the aggregation
It is important to point out that the above dynamic Monte
process of the phenolic hydroxyl groups directly linked to the Carlo simulation examined the role of intermolecular hydrogen
backbone of poly(pHBA-a) during cooling will somewhat hindered, bonding of the various OH groups, disregarding the possible
leading to the lower values of mixing parameters than the methylol covalent bond formation. However, judging from the rubbery
OH groups. Similarly, the phenolic hydroxyl groups directly linked plateau not being completely horizontal, the covalent bond
to the backbone of poly(pEPH-a) have relatively low mobility as also formation may not be complete. Slightly above the Tg, weak-
demonstrated by the lower values of the MSDs in Fig. 10 than the ening of the intermolecular hydrogen bonding is likely taking
phenolic OH groups of poly(pHBA-a). In the meantime, the adja- place which is reected on the reduction of the E0 values as the
cent main chain acts as large steric hindrances to block the temperature increase. Therefore, evaluating the role of inter-
aggregation of hydroxyl groups. As a result, it is most difficult for molecular hydrogen bonding is essential in understanding the
the hydroxyl groups in poly(pEPH-a) to aggregate in local regions, properties of the polybenzoxazines of current study.
leading the lowest values of the mixing parameter among three
different types of OH groups examined.
Despite this weakening of the interaction in the rubbery
plateau, the E0 at Tg + 50 ꢀC is much greater than the cross-
Fig. 13 directly displays the spatial distribution of the linked polybenzoxazine derived from bisphenol A and aniline,
hydroxyl groups (the red spheres) in poly(pHBA-a) at two which is usually abbreviated in the literature as BA-a. The G0 of
different temperatures (T* ¼ 8.0 and T* ¼ 0.5; one above the poly(Ba-a) at Tg + 50 ꢀC was reported to be 4.5 MPa.14
glass transition temperature and the other below the glass Considering that the Poisson's ratio of polybenzoxazines at
transition temperature). It can be clearly seen in Fig. 13a that this temperature is not far from 0.50, it is equivalent to around
these hydroxyl groups distribute homogenously at T* ¼ 8.0. E0 ¼ 10 MPa. The E0 of poly(pHBA-a) under current study is
However, at T* ¼ 0.5, some local aggregation areas can be 30 MPa, indicating that this mono-oxazine benzoxazine has
observed as shown in Fig. 13b, further demonstrating that the unexpectedly higher effective crosslink density than bisox-
intermolecular interactions can drive the hydroxyl groups in azine like the one derived from BA-a.
poly(pHBA-a) to aggregate during cooling. Apparently, the
TGA was used to study the thermal stability of poly-
hydroxyl groups of poly(pHBA-a) aggregate under the effects of benzoxazines and results are shown in Fig. 14. A summary of the
the intermolecular hydrogen-bonding during glass transition main thermal properties, Td5 and Td10, dened as the temper-
process. The intermolecular interactions and the aggregation atures at which a weight loss of 5 and 10%, respectively,
structures make the hydroxyl groups be frozen at a higher occurred, and char yield, dened as the residual weight at
ꢀ
temperature compared with other segments. In addition, the 800 C under N2 atmosphere, are presented in Table 1.
freezing of the hydroxyl groups also restricts the motions of
For polybenzoxazines based on monofunctional benzoxazine
other segments. Thus, poly(pHBA-a) exhibits a much higher monomers, the initial decomposition temperatures are lower
Fig. 13 Snapshots for the spatial distribution of the hydroxyl groups (the red spheres) in t poly(pHBA-a) at two different temperatures (T* ¼ 8.0
and T* ¼ 0.5).
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Fig. 14 Thermogravimetric analysis of poly(pHBA-a) and poly(pEPH-a).
Table 1 Summary of TGA thermogram results
fundamentally study the structure-property relationship
between molecular interactions and thermal properties of pol-
ybenzoxazines. The combination of molecular design with
experiments and simulations clearly demonstrated that the
hydroxyl functionality in the monomer serves two roles to
improve the remarkable thermal properties of poly-
benzoxazines, such as hydrogen-bonding sites and additional
crosslinking sites (phenolic Mannich bridge network, aliphatic
ether linkages).
Tg (ꢀC)
TGA (N2)
Sample
DSC
DMA (tan d)
Td5 (ꢀC)
Td10 (ꢀC)
(%)
poly(pEPH-a)
poly(pHBA-a)
98
261
—
274
351
395
404
436
47
60
than for the polybenzoxazines derived from difunctional ben-
zoxazine monomers. The initial degradation of monofunctional
benzoxazine based polybenzoxazines are mostly taken place
from the terminal Schiff base and secondary amides as struc-
tural defects.52 However, the decomposition temperaturesꢀ of
poly(pHBA-a) reected on the Td5 and Td10 are 395 and 436 C,
respectively, which are signicantly higher than poly(pEPH-a).
Moreover, poly(pHBA-a) presents a char yield of 60% at
800 ꢀC, which is considerably higher than that of poly(pEPH-a)
(47%). TGA results suggest an enhanced thermal stability of the
polybenzoxazines with additional hydroxyls by incorporation of
methylol functionality.
Conflicts of interest
There are no concts to declare.
Acknowledgements
We thank the nancial support of the National Natural Science
Foundation of China (NSFC) (Nos. 51603093 and 21404050) and
the Jiangsu Provincial Science and Technology Agency (No.
BK20160515).
Conclusions
References
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with methylol functionality was found to be unexpectedly
higher than its counterpart without additional hydroxyls.
Dynamic Monte Carlo simulations were applied to
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