High Performance Self-Healing Bismaleimide/Diallylbisphenol A/Poly(phenylene oxide) MicrocapsulesComposites With Low Temperature Processability

Chao Lin,1,2 Li Yuan,1,2 Aijuan Gu,1,2 Guozheng Liang,1,2 Jianyuan Wu11Department of Materials Science and Engineering, College of Chemistry, Chemical Engineering and MaterialsScience, Soochow University, Suzhou, Jiangsu 215123, People’s Republic of China

2Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of PolymerScience and Engineering, College of Chemistry, Chemical Engineering and Materials Science, SoochowUniversity, Suzhou 215123, People’s Republic of China

Novel high performance self-healing 4,40-bismaleimido-diphenylmethane (BDM)/diallylbisphenol A(BA)/poly(phenylene oxide) microcapsules filled with epoxy resin(PPOMCs) systems with low temperature processabilitywere developed. The effects of PPOMCs on the reac-tivity of BDM/BA resin system were investigated; theproperties of cured BDM/BA/PPOMCs systems such asfracture toughness, dynamic mechanical property,dielectric property, and self-healing ability were dis-cussed. The morphologies of the cured resin systemswere characterized using scanning electronic micro-scope and light microscopy. Results reveal that theaddition of PPOMCs can catalyze the polymerizationreaction of BDM/BA resins. BDM/BA systems withappropriate PPOMCs content cured at low temperaturepossess excellent fracture toughness, high glass tran-sition temperature (Tg), and low dielectric property. Theself-healing ability of BDM/BA can be realized by theintroduction of PPOMCs owing to the polymerization ofthe released core materials from PPOMCs. The self-healing efficiency of healed BDM/BA/PPOMCs systemscan be influenced by the size and content of PPOMCsand the contact areas between the crack surfaces. PO-LYM. COMPOS., 34:335–342, 2013. ª 2013 Society of PlasticsEngineers

INTRODUCTION

Bismaleimides (BMI) resins containing unsaturated

groups (C¼¼C) can be thermally polymerized without the

formation of volatile byproducts, which provides consid-

erable advantages in processing over conventional

condensation-type polyimides. The cured BMI resins pos-

sess excellent mechanical properties, high temperature sta-

bility, excellent electric properties, low water absorption,

and so on, their outstanding properties of cured BMI

allow BMI resins to be used for multilayer printed circuit

boards, electrical insulation, semiconductor industries,

advanced composites for the aerospace industry, and

structural adhesives [1, 2]. However, the cured BMI resins

are extremely brittle, during the processing of BMI com-

posites, damages such as microcrack and delamination are

always caused by heat, force load, and so on, leading to a

significant reduction in strength, stiffness, and stability

[3–8]. References have been reported that polymeric

microcapsules containing healing agent can heal micro-

cracks within polymer matrix to maintain the properties

of polymer composites as well as toughen polymer com-

posites [9–11]. The realization of self-healing function in

polymer matrix is attributed to the release of healing

agent core materials into the cracks by capillary action

and the polymerization of the healing agent. In our previ-

ous work, poly(urea-formaldehyde) (PUF) microcapsules

containing epoxy resins have been introduced to BMI sys-

tem, unfortunately, the thermal stability of cured BMI/

microcapsules system decreases owing to the lower ther-

mal stability of microcapsules [12]. Additionally, the PUF

wall shell cannot protect the epoxy resin core materials

well during the high temperature processing of BMI

owing to its low thermal stability [13]. So high thermal

stable microcapsules containing healing agent are needed

for satisfying the requirement of high temperature proc-

essing and maintaining the outstanding properties of BMI.

Poly(phenylene oxide) (PPO) microcapsules filled with

epoxy resins (PPOMCs) have been synthesized in our

Correspondence to: Li Yuan; e-mail: yuanli@suda.edu.cn

Contract grant sponsor: Priority Academic Program Development of

Jiangsu Higher Education Institutions (PAPD); contract grant sponsor:

National Natural Science Foundation of China; contract grant number:

51273135.

DOI 10.1002/pc.22420

Published online in Wiley Online Library (wileyonlinelibrary.com).

VVC 2013 Society of Plastics Engineers

POLYMER COMPOSITES—-2013

laboratory using 2,6-dimethyl phenol (DMP) and epoxy

resins as materials in the presence of ethylenediamine

(EDA) complexes of Cu(I)(Cu-EDA), their thermal

decomposition temperature (Td) is about 2588C, which is

about 208C higher than that of PUF microcapsules con-

taining epoxy resins [13, 14]. The PPOMCs can withstand

the higher curing temperature and the core materials can

be effectively prevented by the PPO wall shell from

diffusing and consuming during the fabricating of BMI

composites, thus ensuring the realization of self-healing

function. In our previous study, PPOMCs have been

applied to 4,40-bismaleimidodiphenylmethane (BDM)/

O,O0-diallylbisphenol A (BA) system, and the properties

of BDM/BA/PPOMCs have been preliminarily investi-

gated [14]. It has been proved that the PPOMCs can be

controlled by heat and/or force to release the epoxy resin

core materials, the released core materials can polymerize

in the presence of various catalytic groups such as amine

group and hydroxyl group (��OH) to bond the cracks

within the matrix, thus recovering the mechanical property

and increasing the service life. In this study, the influences

of different size and content of PPOMCs on the reactivity

of BDM/BA resin system, the fracture toughness, thermal

property, dielectric property, and self-healing ability of

cured BDM/BA systems were discussed in detail.

EXPERIMENTAL

Materials

BDM was purchased from Xibei Chemical Institute,

China. A(BA) was obtained from Sichuan Jiangyou

Chemical Factory, China. PPO microcapsules filled with

epoxy resins (PPOMCs) were synthesized in our labora-

tory. DMP (analytical grade) used as shell material was

purchased from Rising Chemical, China. Diglycidyl ether

of bisphenol A (DGEBPA, epoxide equivalent weight:

196 g/mol) epoxy resin used as core material was pur-

chased from Wuxi Resin Plant, China. Surfactants sodium

dodecylbenzene sulfonate (SDBS, 99% purity) was pur-

chased from Tianjin Chemical Regents Factory, China.

Chemical pure cuprous chloride (CuCl) was obtained

from Shanghai Guanghua Technology, China. Chemical

pure EDA was obtained from Shanghai Sunheat Chemi-

cals, China. The desired amount of CuCl and EDA were

dissolved in water to prepare the Cu-EDA complex.

Synthesis of Microcapsules

The PPOMCs were synthesized according to the Ref.

14. At room temperature, 30 g DMP and 100 ml deion-

ized water were mixed in a 500 ml three-neck round-

bottom flask equipped with a mechanical stirrer, and the

pH of mixture system was adjusted to about 13 with so-

dium hydroxide aqueous solution. When DMP dissolved

into water, 100 ml of surfactant aqueous solution and a

slow stream of 36 g DGEBPA were added to the solution

to form oil in water (O/W) emulsion under agitation for

20–30 min. Then, the reactor was heated to 30–508C and

the mixture was vigorously stirred at fixed agitation rate

under the oxygen condition, subsequently a small amount

of catalyst Cu-EDA complex was added to the mixture.

After 3–4 h, the reaction was complete. The obtained sus-

pension of PPOMCs was rinsed 2 times with deionized

water and acetone, filtrated, and air-dried for 24 h. The

PPOMCs with different mean diameters were prepared by

selecting different agitation rate (430, 700, and 1000 rpm)

while the other processing parameters are kept constant.

The mean diameter of PPOMCs was obtained on the base

of the PPOMCs size distribution performed by a Malvern

MasterSizer 2000 particle size analyzer.

Preparation of BDM/BA/PPOMCs System

The weight ratio of BDM/BA was fixed as 1:1

throughout the work. The mixture of BDM and BA was

heated to melt at 1308C with stirring, when it melted to

transparent liquid, PPOMCs were added, maintaining the

temperature of 1308C for about 30 min, then the mixture

was poured into a glass mold. After degassing at 1208Cfor 1 h, BDM/BA/PPOMCs systems were cured following

the schedule of 1208C/1 h þ 1508C/2 h þ 1808C/2 h þ2008C/2 h.

Characterization

The structure and morphology of the PPOMCs were

characterized using confocal laser scanning microscope

(LSCM, Leica TCS SP2, Leica Microsystems Gmbh, Ger-

many). As a control, microcapsules were investigated in

the transmission mode at 488 nm line of air-cooled argon

laser. Under these conditions, the wall shell as black area

clearly distinguished from the background. The surface

morphologies of PPOMCs and the fracture surfaces of

BDM/BA/PPOMCs samples were observed using scan-

ning electron microscope (SEM, S-4700), the samples

were coated with a thin layer of gold by sputtering before

SEM observation. The fractured surfaces of the healed

BDM/BA/PPOMCs samples were characterized using the

light microscope (LM, VHX-600 Digital Microscope,

Keyence) for the analysis of the healing behavior.

Differential scanning calorimetry (DSC) measurement

was performed using a TA calorimeter (2910 MDSC, TA)

at a heating rate of 108C/min in a nitrogen atmosphere.

The glass transition temperature (Tg) was determined by

dynamic mechanical analysis (DMA) using a TA Instrument

(DMA 2980). The sample dimension is 35 3 10 3 4 mm3.

A single cantilever clamping geometry was used. DMA

tests were carried out from room temperature to 3008C at a

heating rate of 38C/min and a frequency of 1 Hz.

Gel time of resin systems were performed on a

temperature-controlled hot plate by the standard knife

336 POLYMER COMPOSITES—-2013 DOI 10.1002/pc

method. The gel time required for the resin to stop string-

ing and become quite elastic was taken as the gel time.

Dielectric measurements were performed with Novo-

control Concept 80 broadband dielectric analyzer

(Germany) at a room temperature by the two parallel plate

modes in the frequency range between 102 and 106 Hz.

Samples are about 2 mm in thickness and about 10 mm in

diameter. Before testing, samples were dried at 1008C.Mode-I fracture toughness (KIC) was measured using

an electronic universal testing machine (WDW-200/300,

China), tapered double-cantilever beam (TDCB) samples

were adopted during the experiments. The samples were

pin-loaded and tested to failure at a crosshead speed of

0.5 mm/min. At least three specimens for each system

were tested. In this study, healing efficiency is defined as

the ability to recover KIC [9]. The fractured samples were

fixed using either tape or clamp to ensure that the crack

surfaces can closely contact, then healed at 2208C for

5 h. The healed samples were loaded again to failure.

Average healing efficiencies were reported based on sets

of three samples. The healing efficiency (g) can be calcu-

lated according to Eq. 1.

g ¼ KIChealed

KICvirgin

(1)

RESULTS AND DISCUSSION

Effect of PPOMCs on the Reactivity of BDM/BA ResinSystem

Figure 1 shows SEM and confocal laser scanning mi-

croscopy (LSCM) images of PPOMCs with different

mean diameter. Figure 2 shows the dependence of gel

time of BDM/BA systems without prepolymerization on

the contents and mean diameters of PPOMCs at different

temperature. As described above for the synthesis of

PPOMCs, DMP, CuCl, and EDA were used as materials,

a trace amount of hydroxyl (��OH) and amine groups

exist in PPOMCs, which can be proved by the broad and

strong absorption strong band between 3700 and 3100

cm21 in FTIR spectrum of PPOMCs as shown in Figure 3,

they can accelerate or catalyze the polymerization reaction

of BDM/BA resin systems [15–17], decreasing the gel

time of BDM/BA resin system. The increased content of

PPOMCs implies the increased amount of ��OH and

amine groups, which can cause higher polymerization rate

of BDM/BA and improve the conversion of C¼¼C in

BDM/BA system, then the gel times of all BDM/BA/

PPOMCs decrease with the increase of PPOMCs content.

When the same content of PPOMCs was applied, the

smaller PPOMCs have the larger surface areas that can

result in the stronger interface interaction between

PPOMCs and resins, which can increase the polymeriza-

tion reaction rate of BDM/BA resin systems, and then

BDM/BA systems with smaller PPOMCs show shorter gel

time. When the processing temperature is lower (1408C),BDM/BA/PPOMCs system gels slowly [18], the catalytic

effect of PPOMCs on the polymerization reaction of

BDM/BA is more obvious as shown in Figure 2.

Figure 4 shows DSC curves of prepolymerized BDM/

BA systems with different content of PPOMCs with mean

diameter of 40 lm. The curing process of BDM and BA

can be consider as ‘‘ene’’ reaction below 2008C and

FIG. 1. SEM and LSCM images of PPOMCs with different mean diameter.

FIG. 2. The dependence of gel time of BDM/BA systems without pre-

polymerization on the contents and mean diameters of PPOMCs at dif-

ferent temperature.

DOI 10.1002/pc POLYMER COMPOSITES—-2013 337

‘‘Diels-Alder’’ reaction above 2008C [18]. Because BDM/

BA and all BDM/BA/PPOMCs systems have been prepo-

lymized at about 1308C and possibly formed homogene-

ous prepolymers, no obvious melting peaks can be

observed from all DSC curves. For BDM/BA with differ-

ent content of PPOMCs, obvious exothermic peaks above

2008C and weak exothermic peaks below 1758C can be

observed, and as the PPOMCs content increases, the

higher exothermic peak temperatures for BDM/BA system

gradually decrease. The introduction of 10 wt% PPOMCs

can reduce the exothermic peak temperature from 252 to

2288C. Obviously, PPOMCs have catalytic effect on the

polymerization reaction of BDM/BA and can improve the

conversion of C¼¼C in BDM/BA systems when the low

curing temperature is applied.

Fracture Toughness of BDM/BA/PPOMCs Systems

Figure 5 shows the fracture toughness (KIC) of BDM/

BA with different PPOMCs content and size. For the

same-sized PPOMCs, as the content of PPOMCs

increases, the fracture toughness of BDM/BA/PPOMCs

system increases firstly and then decreases. When the con-

tents of PPOMCs with mean diameter of 125, 80, and

40 lm are 5 wt%, the KIC values of BDM/BA/PPOMCs

systems can reach the maximum, they increase by 34%,

47%, and 56%, respectively, as compared to BDM/BA.

The improvement in fracture toughness for BDM/BA/

PPOMCs systems can be explained by the following rea-

sons: Firstly, according to the Ref. 19, PPOMCs can act

as fillers to reduce the stress of resin matrix during the

curing process due to their deformations, and they can

also act as points of the stress concentration under triaxial

stress conditions generating shear yielding or microcrack-

ing in matrix. Secondly, during the microcrack propagat-

ing, crack pinning, or blunting effects of PPOMCs inside

the matrix and the debonding of PPOMCs from matrix

can absorb more energy [20], which can stabilize the

crack and enhance the mechanical property. Thirdly, PPO

can be served as toughener to improve the toughness of

polymer matrix [21–23]; therefore, the PPO composition-

containing PPOMCs embedded in polymer matrix can

toughen BDM/BA matrix. Figure 6 shows SEM morphol-

ogies of the fractured surfaces of BDM/BA and BDM/

BA/PPOMCs. The fractured surfaces of BDM/BA/

PPOMCs (Figure 6b–e) are rougher and more irregular

than that of BDM/BA (Figure 6a), especially near to

region of PPOMCs. Obvious crack pinning or blunting

effects in BDM/BA matrix and the debonding of

PPOMCs from the matrix can be observed as shown in

Figure 6b, c, and d. Good interfacial adhesion between

PPOMCs and the matrix can be implied in Figure 6c and

d for the rough debonded surfaces. Although the addition

of PPOMCs can improve the fracture toughness of BDM/

BA matrix, when the content of PPOMCs increases, the

distance between PPOMCs becomes shorter as shown in

Figure 6e, the interface interaction between PPOMCs and

the matrix become weak, and more flaws occur in the

FIG. 3. FTIR spectrum of PPOMCs.

FIG. 4. DSC curves of BDM/BA systems with different contents of

PPOMCs (40 lm).

FIG. 5. The fracture toughness of BDM/BA with different PPOMCs

content and size.

338 POLYMER COMPOSITES—-2013 DOI 10.1002/pc

matrix, consequently, the fracture toughness of BDM/BA

matrix decreases [24]. When the same PPOMCs content

is used, the smaller PPOMCs have lager total surface

areas, leading to the stronger interface interaction between

PPOMCs and matrix. As a result, relatively obvious

changes in fracture toughness of cured BDM/BA with

smaller size PPOMCs can be observed.

Thermodynamic Properties of BDM/BA/PPOMCs

Figure 7 shows the glass transition temperature (Tg) ofBDM/BA/PPOMCs systems obtained from DMA testing.

The addition of PPOMCs can increase Tg value of cured

BDM/BA. When the contents of PPOMCs with mean

diameters of 125, 80, and 40 lm are 2 wt%, the maxi-

mum Tg values for BDM/BA/PPOMCs can be obtained,

and they are 25, 15, and 138C higher than that of BDM/

BA. The improved Tg value is the fact that the ��OH and

amine groups in PPOMCs can catalyze the polymerization

reaction of BDM/BA, enhancing the cross-linking density

of matrix and the conversion of C¼¼C in BDM/BA.

When PPOMCs content increases, more epoxy resins,

hydroxyl groups, and amine groups can be introduced to

BDM/BA resin system. The increased epoxy resins reduce

the Tg of BDM/BA systems for their lower molecular

weight, and the increased hydroxyl and amine groups can

improve the polymerization reaction rate of BDM/BA res-

ins and lead to more nonuniform structure formed in the

matrix, lowering the Tg of BDM/BA matrix [25]. The

negative effect of PPOMCs on the Tg value of BDM/BA

increases with the content of PPOMCs, and then the Tgvalues of BDM/BA cannot constantly increase as shown

in Figure 7. In this study, BDM/BA systems with 2 wt%

PPOMCs have the maximum Tg values despite of the

PPOMCs size.

When the same PPOMCs content was applied to BDM/

BA resins, the smaller PPOMCs have larger total surface

area to volume ratio as compared to the larger PPOMCs,

the interface interaction between smaller PPOMCs and res-

ins becomes stronger, the polymerization rate of BDM/BA

increases as indicated by the shorter gel time (Figure 2)

and more nonuniform structures can be formed in the ma-

trix. So the Tg of BDM/BA with smaller PPOMCs is

slightly lower than that of BDM/BA with larger PPOMCs.

Effect of PPOMCs on the Dielectric Properties of BDM/BA Systems

Figure 8 shows the dependence of dielectric property

of BDM/BA/PPOMCs on the mean diameter and content

FIG. 6. SEM morphologies of fractured surface of cured BDM/BA and BDM/BA/PPOMCs systems.

FIG. 7. Tg values of BDM/BA/PPOMCs systems.

DOI 10.1002/pc POLYMER COMPOSITES—-2013 339

of PPOMCs from 102 to 106 Hz. The dielectric constant

and dielectric loss (Tan d) of BDM/BA are 4.11–4.34 and

0.0036–0.0145, respectively. As the content of PPOMCs

increases, the dielectric constant of BDM/BA gradually

decreases. This phenomenon can be explained by the fol-

lowing facts: Firstly, due to the trace amount of ��OH

and amine groups in PPOMCs, PPOMCs can catalyze the

polymerization reaction of BDM/BA and improve the

crosslinking density of the matrix. Secondly, the wall

shell material PPO of PPOMCs can decrease the dielectric

constant of BDM/BA for its lower dielectric constant

(2.45) [26]. The addition of PPOMCs can also decrease

the dielectric loss of BDM/BA owing the increased cross-

linking density of the matrix and the lower dielectric loss

(tan d ¼ 0.0007) of PPO. But PPOMCs contain unreacted

epoxy resins and possible impurity, which can increase

the dielectric loss. As a result, the dielectric loss of

BDM/BA cannot decrease proportionally to the content of

PPOMCs. When the same contents of PPOMCs are used,

it seems that smaller PPOMCs can lead to lower dielectric

property of BDM/BA because of the larger surface areas

and more evenly dispersed PPOMCs in BDM/BA system

[27]. It can also be found from Figure 8 that the dielectric

loss of BDM/BA in the range of 102–106 Hz gradually

becomes steady with the content of PPOMCs, especially

in 104–106 Hz, the reason is the steady dielectric property

of PPO over a wide frequency range.

Self-Healing of BDM/BA/PPOMCs Composites

Figure 9 shows the dependence of the healed KIC val-

ues of BDM/BA systems on the content and diameter of

PPOMCs. Figure 10 shows the healing efficiency (g) of

healed BDM/BA/PPOMCs systems. For BDM/BA without

PPOMCs, due to the incomplete reaction of C¼¼C, the

possible healing may be realized when the two crack

surfaces can be brought together in close contact. The

healed KIC and g values of BDM/BA systems increase

with the content of PPOMCs. Because increasing the con-

tent of PPOMCs can introduce more epoxy resin healing

agent into the matrix, the crack surfaces of fractured sam-

ples can adhere well on the condition of sufficient epoxy

resins. In this study, when the same content of PPOMCs

is used, varying the size of PPOMCs does not signifi-

cantly change the healed KIC value of BDM/BA, but gvalues of BDM/BA show an increasing trend with the

increase of PPOMCs size. From Figures 9 and 10, it can

be found that using clamp to fix samples can obtain larger

FIG. 8. The dependence of dielectric property of BDM/BA/PPOMCs on the mean diameter and content of PPOMCs. [Color figure can be viewed in

the online issue, which is available at wileyonlinelibrary.com.]

340 POLYMER COMPOSITES—-2013 DOI 10.1002/pc

KIC and g values than using tape, the reason is the fact

that the crack surfaces can be clamped tightly, thus

increasing the contact areas between crack surfaces. In

this study, 10 wt% PPOMCs can recover 57–79% of the

original KIC of BDM/BA/PPOMCs systems when the

fractured samples are fixed using clamp.

Figure 11 shows LM images of the fractured surfaces

of healed BDM/BA/PPOMCs systems. Obvious healed

areas can be observed from Figure 11a–c. The realization

of healing function in BDM/BA matrix is mainly attrib-

uted to the fact that at high temperature, the released core

material epoxy resins from PPOMCs can polymerize in

the presence of ��OH and amine groups in PPO wall

shell and the incomplete reaction of BDM/BA, the poly-

merized epoxy resins can bond the crack surfaces to-

gether. In the region of PPOMCs, the polymerized epoxy

resins can be observed. For the healed BDM/BA/PPOMCs

systems, two modes of the release of core material epoxy

resins can be suggested. One mode is that during the

loading, the cracks rupture PPOMCs to release the core

FIG. 9. The dependence of the healed fracture toughness(KIC) of

BDM/BA systems on the content and diameter of PPOMCs.FIG. 10. The healing efficiency (g) of BDM/BA/PPOMCs systems.

FIG. 11. LM images of the fractured surfaces of healed BDM/BA/PPOMCs systems.

DOI 10.1002/pc POLYMER COMPOSITES—-2013 341

materials into the crack plane, it seems that this mode is

difficult happened because of the high tear resistance and

elongation of PPO and no fractured PPOMCs in the ma-

trix indicated by Figure 6; the another one is that during

the heating process, the thermal expansion stress of core

materials and the melted PPO allow the PPOMCs to

release the core materials into the crack plane.

CONCLUSION

Novel high performance BDM/BA/PPOMCs systems

were prepared using low temperature processability in this

work. The addition of PPOMCs can enhance the reactivity

of BDM/BA owing to the ��OH and amine groups in

PPOMCs. When the same content of PPOMCs is used, the

smaller PPOMCs may have significant influence on the

reactivity of BDM/BA owing to their larger surface area.

The appropriate content of PPOMCs can improve the frac-

ture toughness of BDM/BA because of the crack pinning

or blunting effects of PPOMCs, PPO toughener, the

debonding of PPOMCs from matrix and good interfacial

adhesion between PPOMCs and the matrix. BDM/BA sys-

tems with 5 wt% PPOMCs have the maximum KIC value,

which increases by 34–56% as compared to BDM/BA.

Because of the catalytic effect of PPOMCs on polymeriza-

tion reaction of BDM/BA, the addition of PPOMCs can

improve the crosslinking density of matrix, and BDM/BA/

PPOMCs systems with 2 wt% PPOMCs show 25–138Chigher Tg than BDM/BA. BDM/BA/PPOMCs systems

have lower dielectric constant and dielectric loss owing to

the increased crosslinked density of resins matrix and the

introduction of low dielectric property of PPO. Because

the core material epoxy resins of PPOMCs can be released

into crack surfaces and polymerize in the presence of

��OH and amine groups under controlled temperature, the

crack surfaces of the matrix can be bonded together by the

polymerized epoxy resins. The self-healing efficiency of

BDM/BA can be influenced by the contact areas between

crack surfaces, the size, and the content of PPOMCs.

When the fractured samples are fixed using clamp, BDM/

BA with 10 wt% PPOMCs can have the self-healing effi-

ciency of 57–79% after heat treatment at 2208C for 5 h.

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