Synthesis, curing behavior, and thermal properties of fluorene-based benzoxazine-endcapped copoly...

Synthesis, curing behavior, and thermal properties of fluorene-based benzoxazine-endcapped copoly(ether ketone ketone)s

Hui Wang • Jun Wang • Tian-tian Feng •

Noureddine Ramdani • Yue Li • Xiao-dong Xu •

Wen-bin Liu

Received: 5 September 2014 / Accepted: 4 November 2014

� Akademiai Kiado, Budapest, Hungary 2014

Abstract A series of fluorene-based benzoxazine-end-

capped copoly(ether ketone ketone)s (co-PEKKs) were

successfully synthesized, in which the co-PEKKs were

prepared with different molar ratios of 2,7-dihydroxy-9-

fluorenone to bisphenol-A. The obtained phenol-ended co-

PEKKs reacted with formaldehyde and n-butylamine to

yield the benzoxazine-endcapped co-PEKKs (B-PEKKs).

The structure of these oligomers was studied by Fourier

transform infrared spectroscopy and 1H nuclear magnetic

resonance spectroscopy, and the molecular mass of these

phenol-ended co-PEKKs was detected by gel permeation

chromatography. The thermal behaviors of these B-PEKKs

were determined by differential scanning calorimetry. In

addition, the thermal and dynamic mechanical properties of

the polymer films were investigated by thermogravimetric

analysis and dynamic mechanical analysis. The results

indicate that the incorporation of co-PEKKs within the

benzoxazine backbone considerably improves the tough-

ness of the fluorene-containing polybenzoxazines without

much sacrifice of their thermal properties. Furthermore, as

the content of 2,7-dihydroxy-9-fluorenone increases, the T5

values decrease, while the Tg values and char yields of the

prepared polymers increase to reach their highest values at

256 �C and 64 %, respectively.

Keywords Fluorene-based polybenzoxazines �Copoly(ether ketone ketone) � Polymerization behavior �Thermal properties

Introduction

Over the past few decades, polybenzoxazines, as recently

developed phenolic resins, have received much interest in

academia and industries. They provide various attractive

properties that cannot be afforded by traditional phenolic

resins and epoxies, such as high thermal stability, high char

yields, high glass transition temperature (Tg), low flam-

mability, low absorption of water, and low dielectric con-

stant [1–7]. Another attractive aspect of benzoxazine

compounds is that they can be prepared from inexpensive

raw materials like phenol, primary amine, and formalde-

hyde. Therefore, the wide ranges of these raw materials

allow tremendous molecular-design flexibility for the

benzoxazine precursor [8–13]. Polybenzoxazines are

polymerized via a thermal ring-opening polymerization

reaction of the corresponding benzoxazine monomers without

the addition of any extra catalysts and without generating any

byproduct or volatiles, and thus offer an excellent dimensional

stability for their cured products [14–17].

Various studies have been carried out to enhance the

outstanding thermal properties of polybenzoxazines. The

incorporation of rigid groups or other thermosetting resins

can improve the thermal properties of polybenzoxazines,

and the introduction of fluorene moieties into benzoxazine

chains is one of these approaches. Fluorene contains two

benzene rings linked with a five-membered ring and pro-

vides high overlaps of p-orbitals. Polymers containing a

fluorenyl structure in their backbone (so-called ‘Cardo-type

polymers’) have many excellent improved properties, such

H. Wang � J. Wang (&) � T. Feng � N. Ramdani �Y. Li � X. Xu � W. Liu (&)

Polymer Materials Research Center, Key Laboratory of

Superlight Material and Surface Technology of Ministry of

Education, College of Materials Science and Chemical

Engineering, Harbin Engineering University, Harbin 150001,

People’s Republic of China

e-mail: [email protected]

W. Liu

e-mail: [email protected]

123

J Therm Anal Calorim

DOI 10.1007/s10973-014-4294-1

as good heat-resistant, high char yield, high limited oxygen

index, good flame-retardance, and excellent solubility in

common organic solvents [18, 19]. Recently, various

researches on synthesis, polymerization behavior, curing

kinetics, and thermal properties of fluorene-based ben-

zoxazines have been reported [20, 21]. However, their

further applications were still limited mainly due to the

brittleness of the formed polymers, despite the advanta-

geous thermal properties. A numerous studies have been

conducted aiming to improve the toughness of the poly-

benzoxazine thermosets. The first one is related to the

preparation of novel benzoxazine monomers containing

aliphatic soft segments which can afford much tougher

polybenzoxazines [22–24]. The second one is based on the

incorporation of some elastomeric or thermoplastic poly-

mers into the polybenzoxazine structure such as the use of

rubber [25], polyurethane [26], polysiloxane [27], poly-

sulfone [28], and poly(epsilon-caprolactone) [29] as the

toughening agent blending with benzoxazine monomers.

Moreover, main-chain type poly(benzoxazine-co-ure-

thane)s [30] and poly(benzoxazine-co-sulfones)s [31] have

been successfully synthesized to modify the brittleness of

polybenzoxazines. The telechelic polybenzoxazines with

thermoplastic oligomers as a backbone were also found to

be more efficient in improving the toughness of this kind of

thermoset [32–35].

Poly(ether ketone ketone) (PEKK), as a representative of

poly(aryl ether ketone) (PAEK), is known as a high perfor-

mance engineering thermoplastic. The PAEK is used in a wide

range of fields, such as polymer films, molding resins, coating,

and insulating materials. Their excellent mechanical, electri-

cal, and chemical properties promote them to be a better

candidate for advanced materials [36–38]. The incorporation

of PAEK into polybenzoxazines has been proved its capability

to overcome the brittleness of these polymers. Ishida and

coworkers successfully synthesized PAEK with benzoxazine

end groups [39]. They calculated the chain length of the oli-

gomer using 1H NMR and studied the tensile strength, the

probability of reaction, the degree of swelling, and the gel

fraction at different degree of end-capping. However, the

descriptions of the synthesis, curing behavior, and thermal

properties about fluorene-based polybenzoxazines containing

PEKK oligomers have not been reported up to now.

In the present work, novel series of fluorene-based

benzoxazine-endcapped co-PEKKs were successfully syn-

thesized from phenol-ended co-PEKKs, formaldehyde, and

n-butylamine, in which the phenol-ended co-PEKKs were

prepared with different molar ratios of 2,7-dihydroxy-9-

fluorenone to bisphenol-A. The chemical structures of the

prepared co-PEKKs and B-PEKKs were confirmed by

FTIR and 1H NMR spectroscopies. The curing behavior of

B-PEKKs and the thermal properties of the cured polymers

were investigated using DSC and TG, respectively. The

dynamic mechanical properties of the prepared polymer

films were evaluated by DMA. This work aims to develop a

new approach to improve the toughness of fluorene-con-

taining polybenzoxazines without much sacrifice of their

advantageous thermal properties through incorporating of

co-PEKKs into benzoxazine backbone.

Experimental

Materials

2,7-dihydroxy-9-fluorenone (BHK) and bisphenol-A (BPA)

were obtained from Shanghai Aladdin Reagent Co., Ltd

(China). 4,40-difluorobenzophenone was purchased from

Chengdu Xiya Reagent Co., Ltd (China). Formaldehyde

(37 mass% in water) is purchased from Tianjin fengchuan

Chemical Reagent Co., Ltd (China). All the other reagents

and solvents were purchased from Tianjin Kermel Chem-

ical Reagent Co., Ltd (China), and used without further

purification.

Synthesis of phenol-ended co-PEKKs

These phenol-ended co-PEKKs were prepared by 4,40-difluorobenzophenone and bisphenol compounds under the

presence of dried potassium carbonate, and the molar ratio of

4,40-difluorobenzophenone to the mixed bisphenol com-

pounds was 2:3. The synthesis of phenol-ended co-PEKK

with a 2:1 molar ratio of BHK:BPA was chosen as a repre-

sentative example. 4,40-difluorobenzophenone (4.36 g,

0.02 mol), 2,7-dihydroxy-9-fluorenone (4.24 g, 0.02 mol),

bisphenol-A (2.28 g, 0.01 mol), dried potassium carbonate

(4.5 g, 0.033 mol), 100 mL N, N-dimethylacetamide, and

15 mL toluene were added to a 250-mL three neck flask

equipped with a condenser, nitrogen inlet, a Dean Stark trap,

and magnetic stirrer. The mixture was first heated at 160 �C

for 3 h to remove the resulting water, and then the temperature

was raised up to 170 �C and kept for another 3 h to complete

the reaction. Once the reaction completed, the solution was

cooled to room temperature and filtered to remove the solid

portions. The filtered liquid was poured into distilled water

containing 1 mass% acetic acid to precipitate the oligomer.

The oligomer was then filtered and washed several times with

distilled water until neutral, followed by drying in a vacuum

oven to afford a brown powdery phenol-ended co-PEKKs,

abbreviated as PEKK21 (60 % yield, Mn = 2,135 g mol–1,

Mw = 3,612 g mol–1, PDI = 1.69). FT-IR (KBr, cm–1):

3,600–3,300 (–OH stretching), 2,965 (C–H stretching), 1,658,

1,718 (C=O stretching), 1,231 (C–O–C asymmetric stretch-

ing). 1H NMR (500 MHz, CDCl3, ppm): 1.65–1.71 (Ar–C–

CH3), 6.76–7.79 (Ar–H).

H. Wang et al.

123

The same synthesis pathway was applied to prepare

other phenol-ended co-PEKKs: PEKK01 (57 % yield,

Mn = 2,596 g mol–1, Mw = 4,201 g mol–1, PDI = 1.62),

PEKK18 (57 % yield, Mn = 2,468 g mol–1, Mw =

3,973 g mol–1, PDI = 1.61), PEKK14 (59 % yield,


PEKK12 (60 % yield, Mn = 2,365 g mol–1, Mw =

4,001 g mol–1, PDI = 1.69), PEKK11 (60 % yield,


and PEKK10 (62 % yield, Mn = 1,814 g mol–1, Mw =

2,918 g mol–1, PDI = 1.61).

Synthesis of B-PEKKs

Formaldehyde (4.86 g, 0.06 mol) and 1,4-dioxane (10 mL)

were added to a 100-mL three neck round-bottomed flask

equipped with a magnetic stirrer and reflux condenser. The

mixture was firstly cooled below 5 �C, and then the n-

butylamine (2.19 g, 0.03 mol) in 1,4-dioxane (10 mL) was

added drop-wise and reacted for 2 h. After that, the

PEKK21 (6.5 g, 0.003 mol) was added to the mixture and

reacted at 110 �C for 6 h. The solution was cooled to room

temperature and poured into ethanol. The precipitate

powder was isolated by filtration and washed thoroughly

with ethanol. The obtained product was dried under vac-

uum oven to afford a brown powder B-PEKK21 (76 %

yield, Mn = 2,335 g mol–1, Mw = 3,923 g mol–1, PDI =

1.68). FT-IR (KBr, cm–1): 1,231 (C–O–C asymmetric

stretching), 1,160 (C–N–C asymmetric stretching), 1,080

(C–O–C symmetric stretching), 928 (C–H out-of-plane

bending). 1H NMR (500 MHz, CDCl3, ppm): 0.93 (–CH3),

1.36, 1.54, 2.75 (–CH2–CH2–CH2–), 1.66–1.71 (Ar–C–CH3),

3.95–4.02 (Ar–CH2–N), 4.83–4.89 (O–CH2–N), 6.71–7.81

(Ar–H).

Similar procedure was used to synthesize other B-PEKKs,

and they are abbreviated with B-PEKK01, B-PEKK18,

B-PEKK14, B-PEKK12, B-PEKK11, and B-PEKK10,

respectively. B-PEKK01 (71 % yield), B-PEKK18 (74 %

yield), B-PEKK14 (74 % yield), B-PEKK12 (75 % yield),

B-PEKK11 (76 % yield), and B-PEKK10 (79 % yield).

Curing of B-PEKKs

All the fluorene-containing benzoxazine-endcapped co-

PEKKs were polymerized without any initiator or catalyst

according to the following schedule: 150 �C/2 h, 180 �C/

3 h, and 210 �C/2 h under a pressure of 0.1 MPa. The

following abbreviations will be used for this series of pol-

ybenzoxazines: PB-PEKK01, PB-PEKK18, PB-PEKK14,

PB-PEKK12, PB-PEKK11, PB-PEKK21, and PB-PEKK10,

respectively.

Preparation of polybenzoxazine films

Solventless method was used to prepare the polybenzox-

azine films. All the synthesized benzoxazine monomers

were melted and cast on a glass plate, and their resulting

viscous prepolymers were degassed in a vacuum oven. The

films were finally cured at 150 �C for 2 h, 180 �C for 3 h,

and 210 �C for 2 h.

Characterization

Fourier transform infrared (FTIR) spectra were recorded by a

Perkin-Elmer Spectrum 100 spectrometer in the range of

4,000–650 cm-1, which was equipped with a deuterated

triglycine sulfate (DTGS) detector and KBr optics. Trans-

mission spectra were obtained at a resolution of 4 cm-1 after

averaging two scans by casting a thin film on a KBr plate for

monomers and cured samples. 1H NMR characterizations

were performed by a Bruker AVANCE-500 NMR spec-

trometer using deuterated chloroform (CDCl3) as solvent

and tetramethylsilane (TMS) as internal standard at 25 �C.

Molecular mass and polydispersities were determined by gel

permeation chromatography (GPC) equipped with a Waters

2414 refractive-index detector using tetrahydrofuran as the

eluent at a flow rate of 1 mL min–1. DSC measurements were

evaluated by a TA Q200 differential scanning calorimeter

under a 50 mL min-1 constant flow of nitrogen. The

instrument was calibrated with a high-purity indium stan-

dard, and a-Al2O3 was used as the reference material. About

5 mg of sample was put into a hermetic aluminum sample

pan at 25 �C, which was then sealed and tested immediately.

The dynamic scanning experiments ranged from 30 to

350 �C at a heating rate of 20 �C min-1. The thermogravi-

metric measurements were carried out by a TA Instruments

Q50 at a heating rate of 20 8C min-1 from 30 to 800 8Cunder nitrogen atmosphere at a flow rate of 50 mL min-1.

The dynamic mechanical thermal properties of the poly-

benzoxazine films were performed by a TA Q800 dynamic

mechanical analyzer at a frequency of 1 Hz and amplitude of

10 lm.

Results and discussion

Synthesis and characterization

The synthesis of B-PEKKs was conducted in two steps. In

the first step, phenol-ended co-PEKKs were prepared by

varying the molar ratio of BHK to BPA. The average

number molecular mass of these co-PEKKs is from 1,814

to 2,596 g mol-1, and the polymerization degree of them is

from 3 to 4. In addition, the molecular mass of co-PEKKs

decreases with the increasing content of BHK. These

Synthesis, curing behavior and thermal properties of co-PEKKs

123

results indicate that the lower content of BPA is, the con-

tent of hydroxyl group is higher in per unit mass. At the

second step, these B-PEKKs were prepared by the usual

monomer synthesis reaction of phenol-ended co-PEKKs

with n-butylamine, and formaldehyde [40]. The synthesis

routes of these co-PEKKs and B-PEKKs are shown in

Scheme 1.

The FTIR spectra of these co-PEKKs and B-PEKKs are

shown in Fig. 1a, b, respectively. As shown in Fig. 1a, the

absorption bands at 3,600–3,300 cm-1 are assigned to the

stretching vibration of –OH group. The bands at 2,965 cm-1

are attributed to the asymmetric –CH3 stretching vibration of

bisphenol-A. The absorptions located at 1,658 cm-1 are

corresponded to the C=O stretching vibration [39]. Mean-

while, the sharp bands at 1,718 cm-1 are ascribed to the

stretching vibration of C=O attached with fluorene ring for

BHK. The absorption peaks at 1,596, 1,502, and 1,460 cm-1

are attributed to the vibration of benzene skeleton. The

strong bands at 1,231 cm-1 are due to the aromatic ether

stretching vibration, and the absorption bands at 1,158 and

1,012 cm-1 are in-plane deformation bands of aromatic

hydrogen. Moreover, the bisphenyl ketone band appears at

925 cm-1, and the bands at 833 cm-1 are assigned to the

para-substituted benzene ring [41]. Additionally, as shown

in Fig. 1b, the absorption bands located at 927–928 cm-1

are assigned to the characteristic peaks of the benzene with

an attached oxazine ring [42]. Also the asymmetric and

symmetric stretching vibrations of C–O–C are located at

1,226–1,240 and 1,061–1,080 cm-1, respectively. And the

asymmetric stretching vibrations of C–N–C located at

1,159–1,161 cm-1 [43]. The FTIR results indicate that the

characteristic absorption bands of these two oxazine rings

have slight difference but not obviously, which is not only

attributed to the overlap of characteristic peaks for co-PE-

KKs and benzoxazine rings but also due to the dilution

effect as higher PEKK content contains less number of

benzoxazines.

Figure 2 shows the 1H NMR spectra of these B-PEKKs.

The aromatic protons are observed at 6.76–7.79 ppm. The

chemical shift of 1.65 ppm is assigned to the methyl group

resonance of bisphenol-A at the chain end, and the chemical

shift of 1.71 ppm is assigned to the methyl group resonance

of bisphenol-A in the main chain [39]. The signals of the

methylene protons of –CH2– groups give doublets at 3.95

and 4.02 ppm for Ar–CH2–N, 4.83 and 4.89 ppm for O–

CH2–N, in which the chemical shifts of 4.02 and 4.89 ppm

are attributed to the oxazine ring from BHK, while the

chemical shifts of 3.95 and 4.83 ppm belong to the oxazine

ring from BPA. From the result of 1H NMR spectrum, there

exist two types of oxazine rings, one is attributed to BHK

O

O

O

OOC

OO

OO

OH HO

O

OO

m/n = 2/3, 0≤ x ≤1

O

O

O

OHDMAc, Toluene

K2CO3

O

OO

O

O

O

O

N

OHNH2

(CH2O)nn(1–x)–1nx–1

nx–1

OO

C

CO

C

CC

HO

N

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3HOnxF

Om F

CH3

CH3

n(1–x)–1

n(1–x)

Scheme 1 The synthesis route of benzoxazine-endcapped co-PEKKs

3600

B-PEKK10

B-PEKK21

B-PEKK11

B-PEKK12

B-PEKK14

B-PEKK18

B-PEKK01

3200 1600 1200 800

3600 3200 1600 1200 800

Wavenumber/cm–1

Wavenumber/cm–1

Tran

smitt

ance

/%Tr

ansm

ittan

ce/%

PEKK10

PEKK21

PEKK11

PEKK12

PEKK14

PEKK18

PEKK01 2965 1658

17181231

1158

1012925

833

1658

1718

1240~12261161~1159

1080~1061928~927

(a)

(b)

Fig. 1 FTIR spectra of the co-PEKKs (a) and B-PEKKs (b)

H. Wang et al.

123

and the other is assigned to BPA. Moreover, the 1H NMR

spectrum also shows that the real oxazine ring ratios of BHK

to BPA for B-PEKK18, B-PEKK14, B-PEKK12,

B-PEKK11, and B-PEKK21 are 2:7, 1:3, 5:8, 2:1, and 10:3,

respectively, which are higher than that of their starting

ratios. For BHK, the larger steric hindrance of cardo group

hinders the formation of PEKK oligomers with longer chain

segments. Consequently, the number of BPA in the chain

ends is less than that of BHK in co-PEKKs, which is in

accordance with the result of GPC.

Curing behavior of these B-PEKKs

The curing behavior of the prepared B-PEKKs was studied

by DSC. Figure 3 shows the non-isothermal DSC curves of

B-PEKKs. As can clearly be seen from Fig. 3, each fluo-

rene-containing benzoxazine-endcapped co-PEKKs shows

two main thermal transitions. The stepwise decreases in

heat capacity of the samples from 91 to 196 �C are the

glass transition temperatures (Tg) of the co-PEKKs back-

bones. However, the thermal transitions located at

241–275 �C are attributed to the thermal curing charac-

teristic of oxazine ring opening. The DSC curves indicate

that the exothermic temperatures of these B-PEKKs are

slightly higher than that of conventional fluorene-contain-

ing benzoxazine due to the dilution effect of the reactive

ring by the non-reactive co-PEKKs chain and the restricted

mobility of the oligomers [34, 39, 40]. Moreover, the

exothermic temperature decreases as the content of BHK in

B-PEKKs increases, which indicates that the oxazine ring

attached with BHK cures preferentially, and then the

hydroxyl groups generated catalyze the ring-opening of the

remaining oxazine ring till the formation of the final net-

work structure. Notably, the enthalpy values for the syn-

thesized B-PEKKs are extremely lower than that of other

benzoxazines, which can be attributed that the higher

molecular mass monomer contains less number of oxazine

rings per unit mass [44].

Figure 4 illustrates the non-isothermal DSC curves of

B-PEKK21 at each curing stage. As shown in Fig. 4, the

degree of curing gradually increases with the heat treat-

ment temperature increasing, and the polymerization

reaction is almost completed at 210 �C. At each curing

stage, the step changes of the curves are attributed to the Tg

values of each oligomers.

8 7

B-PEKK10

B-PEKK21

B-PEKK11

B-PEKK12

B-PEKK14

B-PEKK18

B-PEKK01

6 5 4

δ/ppm3 2 1 0

Fig. 2 1H NMR spectra of the benzoxazine-endcapped co-PEKKs

50 100 150 200

Temperature/°C

Hea

t flo

w/W

g–1

End

oE

xo

250 300

56.6 J/g

40.4 J/g

33.7 J/g

32.5 J/g

29.9 J/g

26.5 J/g

20.8 J/g

275 °C

260 °C

258 °C

257 °C

246 °C

246 °C

241 °C

194 °C (I)

134 °C (I)

133 °C (I)

120 °C (I)

119 °C (I)

118 °C (I)

91 °C (I)B-PEKK01

B-PEKK18

B-PEKK14

B-PEKK12

B-PEKK11

B-PEKK21

B-PEKK10

Fig. 3 DSC curves of the benzoxazine-endcapped co-PEKKs

Hea

t flo

w/W

g–1

End

oE

xo

50 100 150

134 °C (I)

140 °C (I)

171 °C (I)

214 °C (I)

210 °C

180 °C

150 °C

B-PEKK21

4.2 J/g

31.5 J/g

40.4 J/g

200

Temperature/°C

250 300

Fig. 4 DSC curves of B-PEKK21 at each curing stage


123

Glass transition temperature of these PB-PEKKs

The Tg values of these PB-PEKKs obtained by DSC and

DMA are summarized in Table 1. The Tg values of the

prepared oligomers at different curing stages are depicted

in Fig. 5, which increase with the increasing of heat

treatment temperature, and the Tg values reach the maxi-

mum as the formation of infinite network structure. As also

can be seen in Fig. 5, as the content of BHK increases, the

Tg values of the oligomers increase. In addition, it can be

clearly seen that the Tg value of PB-PEKK01 is the lowest

one in this series of polymers, but still remains much

higher than that of typical bisphenol-A-n-butylamine-based

polybenzoxazine (135 �C) [45]. However, the Tg value of

PB-PEKK10 is the highest one, which is 41 �C higher than

that of traditional fluorene-n-butylamine-based poly-

benzoxazine [40].

Also it appears that the Tg values of these crosslinked

polymers shift to higher temperatures as the content of

BHK increases. These results are mainly attributed to the

higher rigidity of fluorene skeleton in the chain backbone,

which restrains the internal rotations and thermal motion of

polymer segments [46].

Film forming and mechanical properties of PB-PEKKs

These films were prepared by solventless method, and the

flexibility of these PB-PEKK films was also demonstrated. It

should be pointed out that the films with higher BHK con-

tent (PB-PEKK10 and PB-PEKK21) were too brittle to form

their casting successfully. The photographs of PB-PEKK01,

PB-PEKK18, PB-PEKK14, PB-PEKK12, and PB-PEKK11

are shown in Fig. 6. It is seen that PB-PEKK01 and PB-

PEKK18 are completely bendable without any difficulty.

Dynamic mechanical properties of the cured films were

examined by DMA. Figure 7a, b shows the temperature

dependency of the storage modulus (E) and tan d for PB-

PEKK01, PB-PEKK18, PB-PEKK14, PB-PEKK12, and PB-

PEKK11, respectively. It is observed that the storage mod-

ulus maintains at lower value by increasing the BPA con-

tent, indicating the improvement in the toughness of these

polymers. It has been reported that the brittleness may arise

from the high rigidity of the chain itself [39]. Therefore, the

introduction of fluorenyl group increased the stiffness of

these polymers, but the fluorene-containing polybenzoxa-

zines based on co-PEKKs still exhibit a significantly

improved toughness comparing to the typical bisphenol-A

and the fluorene-based polybenzoxazine [47, 48].

As can be seen from Fig. 7b, the peak values of tan d,

associated with the Tg values of these PB-PEKKs, shift to

higher temperatures as the content of BHK rises, which is

in accordance with the previous DSC results.

Table 1 Thermal properties of the cured polymers

Sample

code

Tg/oCa Tg/oCb E/

MPacT5/oC Char yield at

800 �C/%

PBPEKK01 167 178 2,038 427 38

PBPEKK18 179 188 2,039 426 49

PBPEKK14 186 191 2,121 424 52

PBPEKK12 196 214 2,212 414 53

PBPEKK11 201 227 2,591 413 58

PBPEKK21 214 – – 401 60

PBPEKK10 256 – – 363 64

a The Tg values were measured by DSCb The Tg values were examined by DMAc E, The storage modulus were recorded at 50 �C

150

100

120

140

160

180

200

220

240

260

280

300B-PEKK01B-PEKK18B-PEKK14B-PEKK12B-PEKK11B-PEKK21B-PEKK10

160 170 180

Temperature/°C

Tg/

°C

190 200 210

Fig. 5 Variation of Tg versus temperature by DSC

Fig. 6 Photographs of thin films: a PB-PEKK01, b PB-PEKK18,

c PB-PEKK14, d PB-PEKK12, and e PB-PEKK11

H. Wang et al.

123

Thermal stabilities of the prepared PB-PEKKs

The thermal stabilities of these PB-PEKKs were evaluated

by TG under nitrogen atmosphere. The TG and DTG

curves of them at various compositions are shown in

Fig. 8a, b, respectively. The values of 5 % mass loss

temperatures (T5) as well as the char yield (Yc) at 800 �C

are collected in Table 1. The results clearly indicate that

two main processes occur during the thermal degradation

of co-PEKKs functional polybenzoxazines. The first deg-

radation below 400 �C is due to the volatilization of amine

fragments from the benzoxazine rings at the chain end,

whereas the second degradation between 400 and 600 �C is

related to the decomposition of both polybenzoxazines and

co-PEKKs [39, 49, 50]. Moreover, as the content of BHK

rises, the volatilization of amine fragments increases, but

the degradation of polybenzoxazines and co-PEKKs

decreases. This is due to the higher content of BHK in the

polymers shortens the length of co-PEKK chain segments,

thus the number of benzoxazine at the chain end would be

much higher. In addition, the T5 values of all the synthe-

sized polymers are above 360 �C, which are much higher

than that of the traditional fluorene-containing and the bi-

sphenol-A-based polybenzoxazines, abbreviated as poly(B-

bbf) (329 �C) and PB-a (310 �C) [16, 51], respectively.

Furthermore, it appears that the char yields of the prepared

PB-PEKKs increase with the increasing of BHK content in

these polymers. It is worthy to note that the char yields of

PB-PEKK21 and PB-PEKK10 exceed 60 %, which are

extremely higher than those of traditional polybenzoxa-

zines and PAEKs [52].

Conclusions

Several fluorene-based benzoxazine-endcapped co-

poly(ether ketone ketone)s with different molar ratios of

100

0.0

0.2

0.4

0.6

0.8

1.0

40

50

60

70

80

90

100

200 300 400

Temperature/°C

Der

iv. m

ass

% °

C–1

Mas

s/%

500 600 700 800

100 200 300 400

Temperature/°C

a

b

c

d

efg

500 600 700 800

a PB-PEKK01b PB-PEKK18c PB-PEKK14d PB-PEKK12e PB-PEKK11f PB-PEKK21

g PB-PEKK10

a PB-PEKK01b PB-PEKK18c PB-PEKK14d PB-PEKK12e PB-PEKK11f PB-PEKK21

g PB-PEKK10

g

fedcb

a

(a)

(b)

Fig. 8 TG (a) and DTG (b) curves of these cured polymers

100

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0

500

1000

1500

2000

2500

3000

150 200

Temperature/°C

Tan

ΔS

tora

ge m

odul

us/M

Pa

250

10050 150 200

a PB-PEKK01b PB-PEKK18c PB-PEKK14d PB-PEKK12e PB-PEKK11

a b c d e

Temperature/°C250

a PB-PEKK01b PB-PEKK18c PB-PEKK14d PB-PEKK12e PB-PEKK11

a b

cd

e

(a)

(b)

Fig. 7 Temperature dependence curves of storage modulus (a) and

tan d (b) for PB-PEKK01, PB-PEKK18, PB-PEKK14, PB-PEKK12,

and PB-PEKK11


123

2,7-dihydroxy-9-fluorenone to bisphenol-A were success-

fully synthesized and characterized. The produced oligo-

mers were further crosslinked through the thermal

activated ring-opening polymerization of the benzoxazine

rings to form the cured network structure. The results

indicate that the molecular mass of co-PEKKs decreases

with increasing the content of BHK. The thermal stabilities

of the obtained thermosets are better than those of typical

fluorene-containing and bisphenol-A-based polybenzoxa-

zines. In addition, the obtained thermosets exhibit a higher

glass transition temperature (Tg) and still maintain at better

toughness. Also, as the content of BHK in co-PEKKs

increases, both the Tg values and the char yields of the

synthesized polymers are enhanced, while their T5 values

relatively decrease.

Acknowledgements The authors greatly appreciated the financial

supports from National Natural Science Foundation of China (Project

Nos 50973022), Specialized Research Funds for the Doctoral Pro-

gram of Higher Education (Project No. 20122304110019), Funda-

mental Research Funds for the Central Universities (Project Nos.

HEUCFT1009 and HEUCF201310006), and the open fund of Key

Laboratory of Superlight Material and Surface Technology of Min-

istry of Education, Harbin Engineering University.

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