Effect of Covalent-Functionalized Graphene Oxide with Polymer and

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Effect of covalent-functionalized graphene oxide with polymer and reactive compatibilization on thermal properties of maleic anhydride grafted polypropylene Na Song, Jingwen Yang, Peng Ding, Shengfu Tang, Yimin Liu, and Liyi Shi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie5031985 • Publication Date (Web): 02 Dec 2014 Downloaded from http://pubs.acs.org on December 8, 2014

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Industrial & Engineering Chemistry Research

Journal Name

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ARTICLE

Effect of covalent-functionalized graphene oxide with polymer and reactive compatibilization on thermal properties of maleic anhydride grafted polypropylene Na Song*‡, Jingwen Yang‡, Peng Ding* , Shengfu Tang, Yimin Liu, Liyi Shi [*] Dr. N. Song, Asso. Prof. P. Ding Research Center of Nanoscience and Nanotechnology, Shanghai University, 99 Shangda Road, Shanghai 200444, PR China E-mail: [email protected] (N. Song), [email protected] (P. Ding)

J.W. Yang, S.F. Tang, Dr. Y.M. Liu, Prof. L.Y. Shi Research Center of Nanoscience and Nanotechnology, Shanghai University, 99 Shangda Road, Shanghai 200444, PR China

1 ACS Paragon Plus Environment


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ABSTRACT: The covalent functionalization of graphene oxide(GO) with bis(3-aminopropyl)-terminated poly(ethylene glycol)(NH2-PEG-NH2) and then the grafting with maleic anhydride grafted polypropylene (MAPP) oligomer matrix using reactive compatibilization were carefully analyzed and verified through detailed investigations. Improvements in the compatibility among the modified GO and matrix, thermal stability, flame property, and crystallization property were achieved through the addition of a small amount of GO grafted MAPP （ PP-g-GO ） . Results of the thermo gravimetric and microscale combustion calorimetry analysises revealed that an increase in Tmax by 51 °C, and the values of the total heat release and peak heat release rate were reduced to 44.4% and 38.9%, respectively, through the addition of 2.0 wt% PP-g-GO relative to pure MAPP. The approach used in this work is an efficient strategy to improve the thermal behavior of polypropylene oligomer with a view to extend their use in advanced technological applications. KEY WORDS: graphene oxide, nanocomposites, covalent bonding, reactive compatibilization, thermal properties INTRODUCTION

matrix

The field of composites has achieved rapid

interactions.2 Thereby, this severely limits the

development in recent years. The unique and

usage of polymer-graphene composites in a wide

outstanding properties of graphene make it the

range of applications. Researchers have focused

best

composites

unfunctionalized

of

choice.1

graphene

is

However, infusible

is

dominated

by

van

der

Waals

their efforts on the creation of solution-processible

and

graphene oxide (GO), one of the most popular

insoluble in polymer matrix. Because the intrinsic

starting materials for functionalized graphene.3

π−π stacking interaction between graphene layers

GO is an effective nanofiller for polymer

easily results in agglomeration, and the interaction

nanocomposites because of its oxygen-containing

between the graphene sheet and the polymer

functional groups that allow the nanodispersion of 2

ACS Paragon Plus Environment

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GO in polar polymer matrixes through simple

GO has been exploited in functionalization and

processes, coupled with the excellent properties

can be used to prepare covalent GO/polymer using

characteristic

carbon-based

both “graft-from” and “graft to” methods.15

nanocomposites.4-7 To date, many work have been

Covalent functionalization of GO with different

reported about the GO-reinforced nanocomposites

surface modifying agents such as alkyl lithium

that using polar polymers as matrix, such as

reagents, isocyanates, diisocyanate compounds

polyurethane (PU) and poly(methyl methacrylate)

and amino compounds is a promising method for

(PMMA),

modification.1, 16 Niyogi et al.17 reported an amide

of

poly(vinyl

all

alcohol)

(PVA),

poly(butylene succinate) (PBS), etc.8, 9

coupling reaction between the carboxyl acid

However, for non-polar polymer, achieving

groups of GO and octadecylamine (ODA). ODA-

enhanced properties of GO/non-polar polymer

modified GO had a solubility of 0.5mg/ml in THF,

nanocomposites seems to be the bottleneck.10-13

and was also soluble in CCl4 and 1, 2-

Therefore, suitable surface modification becomes

dichloroethane.

the key issue to reach homogeneous dispersion

An

effective

approach

to

improve

the

and interface interaction between GO and non-

compatibility between the fillers and non-polar

polar polymer matrixes.

polymer is reactive compatibilization. Reactive

Note that the formation of covalent bonding

compatibilization18-20 is an effective technique to

between GO and the matrix constitutes the

improve

the

strongest type of interfacial interactions; when

morphology control in a variety of incompatible

properly executed, GO could penetrate and

blends. Moreover, new chemical bonds created

become a part of the polymer matrix at molecular

during blend fabrication have much stronger

level.14 GO preparation by controlling the surface

interactions than weaker physical interaction

chemical ingredients is necessary to achieve the

forces like hydrogen bonds and van der Waals

expected results. The essence of highly reactive

forces.21


interfacial

interaction

and


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Based on the above considerations, we propose

polypropylene oligomer(MAPP) is improved but it

an effective approach to prepare high-performance

is still quite low.24 MAPP was used as a

GO/polypropylene(PP) oligomer nanocomposites

compatibilizer that can react with amino groups on

(denoted as PP-g-GO/MAPP) through a two-stage

the surface of the GO-PEG. PP-g-GO can then be

process (see Scheme 1). The first stage involves

acquired. MAPP as a matrix material can also

the synthesis of GO-PEG and the second stage

facilitate suitable solution blending with the

involves

grafted product PP-g-GO.

obtaining

PP-g-GO/MAPP

nanocomposites through solution blending.

The covalent functionalization of GO with NH2-

In this report, hydrophilic bis(3-aminopropyl)

PEG-NH2 and then the grafting with PP oligomer

terminated poly(ethylene glycol)(NH2-PEG-NH2)

through reactive compatibilization were carefully

has been used to covalently functionalize GO

confirmed

(GO-PEG),

cooperative

investigations. The effect of different proportions

reinforcement performance. PEG has a flexible

of covalent functionalized GO on thermal stability,

backbone made of repeating ethylene oxide units.

flame properties, and crystallization properties

The flexibility of the PEG backbone may result in

were investigated in detail. In addition, the

many complicated structural conformations22 that

mechanical properties of PP-g-GO/MAPP/PP

can show better interfacial interaction with PP

blends are also reported.

resulting

in

a

and

analyzed

through

detailed

oligomer. The better interfacial interaction may EXPERIMENTAL SECTION due to the entangle with PP oligomer chains, Raw materials. GO was synthesized from 23

which can enhance the interfacial bonding.

graphite powders (Sinopharm Chemical Reagent Grafting copolymerization offers an effective Co., Ltd., China) using modified Hummer’s way of improving the polarity of PP. Maleic

method(see supporting information).25,

26

MAPP

anhydride is often used to modified PP oligomer; (8–10 wt% the

polarity

of

maleic

anhydride

of

MA),

bis(3-aminopropyl)

grafted terminated

poly(ethylene


glycol)

(NH2-PEG-

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NH2), and N-hydroxysuccinimide (NHS) were

The mixture was heated to 55 °C and refluxed for

purchased from Sigma–Aldrich Co., USA. The

24 h under N2. The mixture was cooled and

molecular weight (Mn) of MAPP is 3900 g/mol.

separated according to molecular weight of 10000

The Mn of NH2-PEG-NH2 is 1500 g/mol.

dialysis bags for 8 h. Deionized water was

EDC•HCl

GL

replaced every half an hour. The mixture was

Biochem, Ltd., China. polypropylene (PP, BJ750)

allowed to stand for 24 h without disturbance. The

are

resultant GO-PEG was obtained by freeze-drying

was

purchased

provided

from

by

Shanghai

SAMSUNG

TOTAL

PETROCHEMICALS Co., Ltd. All other reagents and solvents are analytical grade and used without further purification.

the deposit. Preparation

of

PP-g-GO/MAPP

nanocomposites. PP-g-GO was prepared by

Covalent modification of GO with NH2-PEG-

grafting MAPP on GO-PEG through amidation

NH2. 2.34 g of the resulting GO was suspended in

reaction between the amine groups of GO-PEG

dimethyl formamide (460 mL) and sonicated for

and maleic anhydride groups of MAPP. The

30 min in an ultrasonic bath (SK5210LHC

different proportions of GO-PEG were suspended

KUDOS LHC ultrasonic cleaners, Shanghai

in 100 mL of toluene and disperse uniformly with

KUDOS ultrasonic instrument co., LTD.), the

the aid of sonication for 10 min, then transferred

power used is 53kHz. The GO suspension was

to a three-neck flask and add 10 g of MAPP. The

transferred into a three-neck flask and pre-purged

mixture are reflux at 160 °C (oil bath) under N2

with N2. An aqueous solution (230 mL) containing

for 2 h. The solvent toluene was removed from the

NH2-PEG-NH2 (3.59 g) was added under constant

mixture through a rotary evaporation apparatus

stirring, and then EDC•HCl (0.59 g) and NHS

and dried at 80 °C under vacuum overnight to

(0.61 g) were added into the three-neck flask. The

produce PP-g-GO/MAPP nanocomposites. For

combination of EDC•HCl and NHS can improve

comparison, the prepared GO and 10 g of MAPP

the coupling efficiency of carboxyl and amino.

were refluxed at 160 °C (oil bath) under N2 for 2 h



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to produce GO/MAPP nanocomposites. For the

tests were carried out using an AXIS ULTRFDLD

XRD,

AFM

multifunctional x-ray photoelectron spectroscope

characterizations, a small amount of collected

with an Al Kα radiation source at room

powder was extracted three times with 200 mL of

temperature and under vacuum of 10-7 Pa. Atomic

boiling toluene for 2 h to remove free MAPP. The

force microscopy (AFM) graphs were acquired

powder was dried at 80 °C under vacuum for 2 h

with

to produce PP-g-GO.

Microscope

Raman,

FTIR,

XPS,

and

a

NanoScope

IIIa

Controller.

scanning

Scanning

Probe electron

microscopy (SEM) graphs were acquired with a

Preparation of PP-g-GO/MAPP/PP blends. The PP-g-GO/MAPP/PP blends were prepared in

JSM-6700F

emission

scanning

a XSS-300 Torque rheometer at 180 °C for 10 min

microscope. Transmission electron microscopy

with a rotor speed of 60 rpm. Prior to melt mixing,

(TEM) graphs were acquired with a 200CX

PP-g-GO/MAPP and PP were dried in an oven at

transmission

80 °C for 12 h to remove excess moisture. The

Thermogravimetric analysis (TGA) was carried

content of PP-g-GO/MAPP is kept at 0, 0.5, 1.0,

out under nitrogen atmosphere with a DTG-60H

1.5, 2.0, and 3.0 wt %.

thermal analyzer at a heating rate of 10 °C /min.

electron

electron

microscope.

Differential scanning calorimetry (DSC) was

Characterization methods. X-ray diffraction a

carried out under nitrogen atmosphere with a TA

D/MAX2200/PC X-ray diffractometer with Cu–

Q500 HiRes differential scanning calorimeter at a

Kα radiation (λ = 0.154 nm). Raman spectra were

heating rate of 10 °C /min. Heat release rate

obtained using an INVIA confocal micro Raman

curves were recorded on a GOVMARK MCC-2

spectrometer. Fourier transform infrared (FTIR)

microscale combustion calorimeter (MCC) at a

spectra were recorded on an AVATAR370 Fourier

heating rate of 1°C/s. And for TGA, DSC and

infrared spectrometer using potassium bromide

MCC, the tests have been done at least three times

pellets. X-ray photoelectron spectroscopy (XPS)

and the quality of weighing at least accurate to one

(XRD)

patterns

were

collected

with


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over one thousand the amount of the sample to

thereby

ensure the reproducibility. Tensile tests were

performance. The mechanism was proposed as

conducted according to ASTM D 638 using a

shown in Scheme 1.The covalent functionalization

Zwick/Roell Z020 universal testing machine. The

of GO with MAPP chains utilizes a two-step

tests were performed at an extension speed of 100

strategy. First, GO-PEG was prepared by grafting

mm/min. The specimens are the dumbbell with the

NH2-PEG-NH2 onto the GO surface. MAPP was

size of 90 mm ×5 mm (the breadth of cabined

then reacted with GO-PEG through the formation

section) ×2 mm, and the length of cabined section

of amide groups, yielding PP-g-GO.

maximizing

the

outstanding

GO

is 30 mm. All data were the average of five

Evidence of successful grafting of MAPP onto

independent measurements, and the relative errors

the GO surface can be provided by FTIR spectra

committed on each datum were reported as well.

(Figure 1). In the FTIR spectrum of GO, FTIR

Lzod impact tests were conducted according to

absorption peaks at 3418, 1732, 1622, 1415, 1227,

GB/T1843-1996 using a Zwick/Roell HIT5.5P

and 1054 cm−1 are assigned to O-H stretching

pendulum impact testing machine. Notched blends

mode, C=O in carboxylic acid and carbonyl

specimens were used during the experiment. The

groups, C=C skeleton vibrations of unoxidized

dimensions of the specimen used were 80 mm ×

graphitic domains, =C-H in-plane stretching,

10 mm × 4 mm. All data were the average of five

epoxy C-O stretching, and alkoxy C-O stretching,

independent measurements, and the relative errors

respectively.27,

committed on each datum were reported as well.

1109 cm−1 are the symbols of C-H and C-O bond

28

The peaks at 2889 and

in NH2-PEG-NH2,29 respectively. Broad peaks at RESULTS AND DISCUSSION 3407 cm−1 correspond to N-H in-plane stretching Synthesis and characterization of GO, GOof

the

amine

groups.30

The

covalent

PEG, and PP-g-GO. Covalent bonds confer functionalization of NH2-PEG-NH2 onto GO is strong

interfacial

interaction

and

excellent demonstrated by the new peaks at 2867 cm-1 and

compatibility between GO and MAPP matrix,



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1109cm-1, which correspond to the C-H and C-O

Å by the covalent functionalizing NH2-PEG-NH2

bond in NH2-PEG-NH2.

on the surface of GO. Simultaneously, the

The MAPP spectrum shows peaks at 1776 and

characteristic sharp peaks (19.5° and 23.6°; Figure

1715 cm-1, belonging to the C=O stretching

S1) of NH2-PEG-NH2 show almost no deviation in

frequencies of MA and carboxylic acid at self-

GO-PEG.33 After further grafting of MAPP onto

hydrogen-bonded carboxyl groups,31 respectively.

the GO-PEG surface, the diffraction peak assigned

After MAPP grafting onto GO-PEG, the two

to the interlayer spacing of GO is severely

peaks show reduction compared with 2.0 wt% PP-

weakened, causing disorder in the remaining stack

g-GO and MAPP in intensity and a new broad

of PP-g-GO. A new broad peak at around 17.5° to

peak at 3433 cm-1 corresponding to the peak at

25° appears in the XRD pattern of PP-g-GO. The

3429 cm-1 in GO-PEG appears, confirming the

broad peak differs from MAPP peaks with 2θ

acquisition of PP-g-GO.

values of approximately 14.0°, 16.8°, 18.5°, 21.4°

To

further

confirm

the

covalent

(Figure S1). All MAPP peaks correspond to the crystalline structure of a-PP.34

functionalization of GO with MAPP chains, the XRD patterns of GO, GO-PEG, and 2.0 wt% PP-

Raman spectroscopy is widely used in the

g-GO are plotted in Figure 2. A sharp peak caused

characterization of carbon materials. Conjugated

by a high degree of order is observed in the GO

and carbon–carbon double bonds result in high

whose diffraction peak is located at 9.1°.32 The

Raman intensities. Figure 3 displays the Raman

peak in GO originates from the interlayer (0 0 1)

spectra of GO, GO-PEG, and 2.0 wt% PP-g-GO.

spacing, which corresponds to an interlayer

The Raman spectrum of GO demonstrates a D at

1355 cm-1,

band

PEG exhibits a diffraction peak that down-shifts to

hybridized carbons and assigned to the breathing

6.1°. The GO-PEG peak indicates that the

mode of κ-point phonons of A1g symmetry. The D

interlayer spacing of GO-PEG is increased to 14.5

band indicates the presence of disordered graphite.


corresponding

to

sp3-

spacing of about 9.7 Å. Compared with GO, GO-

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The G band at 1606 cm-1 corresponds to sp2-

increase in I(D/G) leads to excessive defects in the

hybridized carbons and assigned to the ﬁrst-order

graphitic lattice.36 However, the defects can be

band

partly caused by the formation of covalent bonds

demonstrates the ordered state of GO platelets.35,

between GO and NH2-PEG-NH2 chains.38 The

36

The intensity of the D band is lower than that of

I(D/G) value of 2.0 wt% PP-g-GO changes from

the G band in GO, revealing the presence of less

0.98 (for GO-PEG) to 1.04 because of the

defects and oxygen-containing functional groups

decrease in the average size of the in-plane sp2

in the as-prepared GO.35 In the Raman spectra of

domain (corresponding to the increase of the

the prepared GO-PEG, the intensity of D band

defects) upon thermal reduction during MAPP

(1352 cm−1) is significantly increased compared

grafting onto GO-PEG and solution blending.10

with that of the G band (1597 cm−1), showing

For GO-PEG and 2.0 wt% PP-g-GO, the G-band

similar intensities. The 2.0 wt% PP-g-GO shows

is downshifted by approximately 9.0 and 12 cm−1

higher intensity D band (1352 cm-1) and G band

relative to the same bands in GO. Ni et al.

(1594 cm−1) compared with those of GO. The

reported that the G band of graphene is

intensity ratio of D and G bands (I(D/G))

downshifted when tensile strain (stretching) is

associated with the disordered and ordered crystal

applied because of the elongation of the carbon–

structures of carbon is opposite the average size of

carbon bonds.39 Similarly, functionalization of GO

the sp2 regions.37 The I(D/G) value (calculated by

with NH2-PEG-NH2 and grafting of MAPP onto

the D peak and G peak at highest height in Figure

GO result in the mutual enlargement of PEG and

3) of GO-PEG is increased from 0.72 (for GO) to

MAPP chains. The tensile stress applied to GO is

0.98. According to reports, a large increase in

brought about by the strong interaction between

I(D/G) is caused by the covalent functionalization

chains. The tensile stress weakens the bonds and

implemented on the functional groups within the

lowers vibrational frequency. Some studies have

internal domain of graphene platelets. The large

reported that

scattering

of

E2g

phonons.

The

G


I(D/G) variation is inversely


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Page 10 of 32

correlated with crystallite size (La), which can be

The C1s core-level spectrum of GO-PEG (Figure

calculated using the formula for La (nm) = (2.4 ×

4d) also exhibits C–C(H) (C=C), C–O–C, C–OH,

10−10) × λ4laser (ID/IG)-1 (λlaser is the laser excitation

and C=O types of carbon. The absorbance band

wavelength).40,

Using the formula to calculate

intensity of oxygenated C in epoxy, carbonyl

for La is an indirect way to estimate the defect

groups, and C–OH decreases sharply because of

density of GO defects according to crystallite size.

the removal of most oxygen functional groups in

Based on the data in Figure 3, three crystal sizes

GO. The N atom in GO-PEG has not been

are obtained, namely, 53.5, 39.3, and 37.1 nm.

detected by the C1s core-level spectra of GO-PEG

41

The XPS spectra of GO-PEG show the

because the C–OH (285.6 eV) and C–N peaks

appearance in N1s and an increase in C1s peaks

(285.9 eV) are almost superposed.44 The C/O ratio

compared with that of GO (Figure 4). The atomic

of 2.0 wt% PP-g-GO is estimated to be 3.01:1

ratios of carbon and oxygen (C/O) in GO and GO-

(shown in Figure 4e). To prove that the GO

PEG

respectively,

surface is reduced during MAPP grafting onto

covalent

GO-PEG and solution blending with MAPP, a

functionalization of GO by NH2-PEG-NH2. The

contrast experiment was conducted in this study.

C/O ratio in NH2-PEG-NH2 affects the C/O ratio

Controlled GO-PEG(cGO-PEG) was processed

of GO-PEG after functionalization. In Figure 4c,

under the same condition used in solution

two distinct deconvolutions of the C1s peaks at

blending (without MAPP). The (C/O) ratio was

binding energies of about 284.8 and 286.9 eV for

estimated to be 2.38:1 (shown in Figure 4e),

GO emerge, which can be assigned to GO C–C(H)

which is higher than that of GO and GO-PEG, but

(C=C) and epoxide groups (C–O–C). The other

lower than that of 2.0 wt% PP-g-GO, suggesting

two peaks located at binding energies of 285.6 and

that the process reduced part of the GO.

are

2.35:1

suggesting

the

and

2.16:1, successful

287.5 can be assigned to the C–OH and C=O

The AFM measurements of GO platelets and 2.0

(carbonyl C) functional groups,42, 43 respectively.

wt% PP-g-GO are shown in Figure 5. Using AFM


Page 11 of 32

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measurement, both surface and height profiles of

surface is relatively rough with many obvious

GO platelets can be accurately and easily

protuberances

on

the

obtained. The GO area is small, which may be due

protuberances

are

formed

to ultrasonic treatment (Figure 5). GO platelets are

entanglement of neighboring polymer chains

especially smaller in the AFM image of GO-PEG

anchored on the PP-g-GO.46

GO

surface.

by

the

The mutual

(shown in Figure 5b), possibly due to ultrasonic

The morphology of the GO was confirmed by

treatment before the reaction, wherein GO reacted

TEM in Figure 6f. The monolayer or multilayer

with NH2-PEG-NH2. The distribution of points on

GO platelets are rippled and resemble crumpled

GO can be interpreted as the formation of covalent

silk veil waves.16,

bonds between GO and NH2-PEG-NH2 chains

PEG (Figure 6g) is different from that of GO,

mentioned in the Raman discussion. The results

where dark blocks from NH2-PEG-NH2 are clearly

obtained from the AFM and Raman analyses are

visible

complementary. The height profile diagram of the

functionalization of GO by oxygen-containing

AFM image indicates that the thickness of a

functional groups and their direct use as reactive

platelet is around 0.8 nm, corresponding to the

sites can successfully functionalize NH2-PEG-

typical thickness of the single- layer GO platelet

NH2 on the surface of GO platelets.49

(–0.8 nm).45 The height of GO-PEG is around

on

the

Compatibility

47, 48

GO

The morphology of GO-

platelet.

between

GO

Hence,

and

the

MAPP

1.72 nm, which is higher than that of GO because

matrix after reactive compatibilization. As

of the NH2-PEG-NH2 chains grafted onto the GO.

expected, the morphology is effectively controlled

The height of PP-g-GO (2.86 nm) is apparently

by the assumptive reactive compatibilization. The

higher than that of prepared GO because of the

compatibilizing effect of MAPP can be confirmed

introduction of functional groups on PP-g-GO.

by SEM observations (Figure 6a, b, c and d). The

The surface of GO is mostly smooth with only few

bright regions reflect the high conductivity of

wrinkles and folds. However, the PP-g-GO

graphene platelets.50 Holes or cavities can be



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Page 12 of 32

observed in the SEM image of 2.0 wt%

MAPP matrix. Some parts of the disk were yellow

GO/MAPP (Figure 6a). However, the dispersion

but transparent, whereas other parts were black

of 2.0 wt% PP-g-GO/MAPP (Figure 6c) is

because of the partial reduction of GO (shown in

represented by bright regions that are well

the middle of Figure 6e). Thus, GO has poor

distributed in the MAPP matrix (dark region)

compatibility with the MAPP matrix because the

without obvious congregation, explaining the

agglomerates can be seen by the naked eye. For

direct functionalization of GO with NH2-PEG-

the 2.0 wt% PP-g-GO/MAPP, the presence of a

NH2. Compatibilization of MAPP can improve the

black uniform disk (shown in the right of the

dispersion.

higher-magnification

Figure 6e) indicates that the 2.0 wt% PP-g-

(Figure 6b and d) images were used as further

GO/MAPP is uniformly dispersed in MAPP. The

proof to illustrate the reduced agglomeration of

uniform dispersion is attributed to increased

the 2.0 wt% PP-g-GO, which is attributed to the

compatibilization. From the results of the above

strong interfacial interaction between PP-g-GO

analysis, functionalized GO could compatibly

and MAPP.36,

However, the interfaces of 2.0

react with MAPP matrix during solution blending,

wt% PP-g-GO/MAPP still can be recognized at

thus improving the dispersion of GO in the MAPP

higher magnification.

matrix.

For

51

clarity,

The compatibilizing effect of functionalized GO

From the above microscopic and macroscopic

in PP-g-GO/MAPP nanocomposites was proven

observations,

by the photographs (Figure 6e) of fabricated disks

compatibilizing mechanism of MAPP in the PP-g-

obtained by hot pressing (results shown in inset).

GO/MAPP blend after GO functionalization

For pure MAPP, when the sun passes the MAPP, a

through NH2-PEG-NH2 is explained below. The

bright yellow transparent disk is visualized

surfaces of unfunctionalized GO possess oxygen-

(shown in the left of Figure 6e). For 2.0 wt%

containing functional groups, whereas the MAPP

GO/MAPP, the GO did not evenly disperse in the

surface does not contain a large number of polar


and

analysis

results,

the

Page 13 of 32

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functional groups. Therefore, the prepared GO is

calorimetry, MCC also known as pyrolysis

not adequately dispersed in MAPP, and some GO

combustion flow calorimetry (PCFC), which is a

particles still aggregate. Functionalizing GO with

convenient and fast method for laboratory

NH2-PEG-NH2 and reactive compatibilization

evaluation of the flame properties. It is based on a

with MAPP can form the product PP-g-GO, which

TGA-like degradation of the polymer in nitrogen,

can exhibit intermolecular interactions with

followed by combustion of the gases produced in

MAPP matrix through its grafted PP chains.

air.55 In the process of MCC measurement, two

Therefore, when the proportion is appropriate, the

heat release results are detected, namely, heat

dispersion of GO in MAPP can be improved.

release rate (HRR) and total heat release (THR).

Consequently, the compatibility between the GO

HRR is an important parameter because it

and the MAPP matrix is enhanced, facilitating the

represents

uniform dispersion of GO in the MAPP matrix.

parameter, which is used to predict the flame

the molecular level

flammability

PP-g-GO/MAPP

resistance and behavior of a substance.56, 57 HRR

nanocomposites. We report the effect of adding

can be used to predict the combustion behavior of

functionalized GO to MAPP. GO can be reduced

a material in a real fire. THR, which determines

at 160 °C during solution blending.52 GO has a

how big a fire could be, is another important

highly extended structure with superior gas barrier

parameter for fire hazard evaluation. Once the

ability.53 Therefore, its outstanding flame property

ignition takes place, THR steadily increases with

is believed to be due to its ability to form a

burning time and get a steady state before the

continuous, protective char layer that acts as a

flameout happens. Thus, an efficient filler for

thermal insulator and a mass transport barrier.54

flame properties should be able to reduce THR

MCC

Performance

of

test

for

effectively when it is incorporated into a

properties

of

polymer.58 The effect of different proportions of

polymer materials through oxygen consumption

PP-g-GO on HRR curves is shown in Figure 7.

is

an

investigating

effective and the

combustion

rapid



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Page 14 of 32

The peak heat release rate (PHRR) value of

The THR and PHRR values (Table S1) of PP-g-

MAPP increases up to 811.6 W/g. Upon the

GO/MAPP nanocomposites and the change of

incorporation of 2.0 wt% GO, the PHRR value of

component contents are shown in Figure S2. The

MAPP decreases to 718.5 W/g with reduction of

THR and PHRR values of all PP-g-GO/MAPP

about 11.5%, demonstrating that GO can be an

nanocomposite components decreases before the

effect additive for flame property of GO/MAPP

addition of 2.0 wt% PP-g-GO, and then increase

nanocomposites.

not

with the addition of PP-g-GO. By comparing the

uniformly disperse in MAPP, thus affecting the

values of PHRR and THR for MAPP (Table S1),

flame properties of nanocomposites (shown in

we observed that the proportion of PP-g-GO is

Figure. 6e). Unlike MAPP, NH2-PEG-NH2 has

less than that of 2.0 wt%. PP-g-GO improves the

inherent flame property (with PHRR value of

flame properties of PP-g-GO/MAPP gradually.

527.9 W/g, in Figure 7), so the introduction of

This may because GO cannot be burned to release

NH2-PEG-NH2-functionalized GO into the MAPP

heat, and the PP-g-GO as nanofillers for MAPP

matrix can not only compatibly react with MAPP,

can effectively decrease the THR value of the

but can also remarkably reduce flammability of

nanocomposites. However, when the amount of

PP-g-GO/MAPP nanocomposites. When 2.0 wt%

PP-g-GO is increased to above 2.0 wt%, the

of PP-g-GO is added to MAPP, the PHRR value

improvement gradually decreases, suggesting that

sharply decreases to 495.9 W/g (about 38.9%

when

reduction), demonstrating that the excellent

2.0 wt%, the uniform dispersion of PP-g-GO in

dispersion of PP-g-GO particles and the interfacial

the MAPP matrix is negatively altered. Therefore,

interaction between PP-g-GO and the matrix

PP-g-GO could not form adequate continuous

confer

network structures to reduce heat and mass

the

However,

retardancy

GO

does

properties

of

PP-g-

the

proportion

of

PP-g-GO

exceeds

transfer between the gas and condensed phases,

GO/MAPP to achieve the expected result.

and protect the underlying MAPP from heat.59 To


Page 15 of 32

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achieve such behavior with GO, as with all

the chains on PP-g-GO.61 When the proportion is

nanocomposites, a good nanofiller dispersion

higher than 2.0 wt%, the improvement of Tmax and

within the matrix must be achieved.

Tonset is weakened. The consequence brought

The result of TGA analysis is shown in Figure

about by entangled chains in PP-g-GO is higher

8, with detailed data summarized in Table 1. Tonset

than 2.0 wt% proportion, which decreases the

is defined as the temperature where 5 wt% mass

distribution of PP-g-GO. The increase of Tmax and

loss occurs. Tmax (estimated from the differential

Tonset of PP-g-GO/MAPP is not as obvious as that

analysis of TGA curve) is defined as the

of less than 2.0 wt% proportion. An increase in

temperature where maximum weight loss rate

Tmax by 51 °C could be observed with only 2.0

takes place.

wt% GO (shown in Table 1), which is higher than

MAPP shows Tonset of 253 °C and Tmax of

the addition of 2.0% graphene reported by Song.62

407 °C (Table 1). Tonset and Tmax are determined

The increase in Tmax and Tonset clearly indicates

by oxygen-containing functional groups in the

that thermal stability is improved by adding PP-g-

maleic anhydride (MA) and degradation of MAPP

GO.63

backbones.60 As shown in Figure 8 and Table 1,

nanocomposites, the Tonest of the nanocomposite

when the additive proportion of PP-g-GO is higher

increases by 17 °C (270 °C for 0.5% PP-g-

than 0.5 wt%, much higher Tmax of 437, 457, 458,

GO/MAPP, 253 °C for MAPP). Tmax is decreased

458, and 415 °C are observed. Tonset is slightly

by 14 °C (393 °C for 0.5% PP-g-GO/MAPP,

higher than the corresponding MAPP, except for

407 °C for MAPP) because when a very small

1.0 wt% PP-g-GO/MAPP, whose Tonset is slightly

amount of PP-g-GO (for 0.5wt%) is added to the

lower than that of MAPP. The increase in Tmax is

MAPP, the GO in the PP-g-GO relative to MA in

attributed to the entanglement between chains in

MAPP is less likely to degrade below 300 °C, but

PP-g-GO. The slight increase of Tonset is caused by

is easily degraded above 300 °C. We therefore

the close molecular weight between MAPP and all

concluded that well-dispersed GO could serve as

For


0.5

wt%

PP-g-GO/MAPP


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Page 16 of 32

physical barriers that mitigate the transport of

components of PP-g-GO/MAPP is 147 °C, except

decomposed volatile products outward of the

in the 0.5% proportion of PP-g-GO.

MAPP matrix during pyrolysis, and enhance the

In terms of crystallization, the addition of PP-g-

carbonization of the polymeric matrix.64

GO can accelerate the crystallization, so the trends

The crystallization behavior of MAPP is shown

of Tm1 (caused by smaller or imperfect motion of

in Figure 9a with the detailed data of Tc in Table

MAPP) and PP-g-GO/MAPP are weakly elevated.

1. Compared with MAPP, Tc of PP-g-GO/MAPP

However, the Tm1 of 0.5% of PP-g-GO/MAPP is

shows a decreasing trend after the first increase. Tc

still 146 °C, which can be attributed to the weak

fluctuates between 112 and 117 °C, which is

enhancing effect of 0.5 wt% PP-g-GO in Tm1.

attributed to the increased and then reduced

The tensile strength and notched Lzod impact

dispersion of PP-g-GO between PP-g-GO and

strength of the neat PP and PP-g-GO/MAPP/PP

MAPP.65 However, the Tc of the composites are

blends are shown in Figure 10. It can be seen that

both higher than that of MAPP, possibly because

after adding the PP-g-GO/MAPP to PP, the tensile

of the addition of PP-g-GO that acted as a

and impact strengths have been changed with the

nucleating agent for promoting the crystallization

amount of PP-g-GO/MAPP. And the tensile and

of MAPP to some degree.65 The melting behaviors

impact strengths at first increase with the increase

are shown in Figure 9b. The melting points of

of the PP-g-GO/MAPP until maximum are

MAPP and PP-g-GO/MAPP are almost equal (Tm2

reached, but with the further increase in PP-g-

of pure MAPP is about 156 °C in Table 1). The

GO/MAPP content, the tensile and impact

presence of double melting peaks can be related to

strengths start to decrease. The highest tensile

the occurrence of smaller or imperfect a-PP,66

strength value of PP-g-GO/MAPP/PP blends is

consistent with the results of the XRD test. As

31.02 MPa when 1.0 wt % of PP-g-GO/MAPP is

shown in Table 1, the first melting temperature

added, which increases by 6.97% as compared to

(Tm1) of MAPP is 146 °C, whereas the Tm1 of all

that of pure PP. The highest impact strength value 16


Page 17 of 32

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of PP-g-GO/MAPP/PP blends is 14.80 kJ/m2

nucleation ability. Notably, the flame properties of

when 1.5 wt % of PP-g-GO/MAPP is added,

PP-g-GO/MAPP nanocomposites was better. The

which increases by 12.46% as compared to that of

THR and the PHRR values sharply declined to

pure PP. The improved mechanical properties

26.8 kJ/g (about 44.4% reduction) and 495.9 W/g

should be attributed to the large interfacial area

(about 38.9% reduction) relative to those of pure

and the strong interaction between the PP and PP-

MAPP. The results demonstrate the effects of the

g-GO/MAPP.67

covalent bonds at the PP-g-GO/MAPP interfaces,

CONCLUSIONS

which

In our research, we studied functionalized GO

interactions, simultaneous improvement of GO

as a filler for MAPP to fabricate PP-g-GO/MAPP

dispersibility, and enhanced performances of the

nanocomposites. In this study, we found that NH2-

PP-g-GO/MAPP

PEG-NH2 has a better flame property compared

GO/MAPP nanocomposites can also be master

with MAPP. Furthermore, NH2-PEG-NH2 as a

batches to apply into the polyolefin to study the

polymer

GO

mechanical properties without agglomeration. The

oxygen-containing

work presented herein broadens the application of

functional groups on the surface of GO. Once

polyolefin and will be of interest to communities

functionalized, GO could be used in the MAPP

in graphene-based nanocomposites.

matrix

ASSOCIATED CONTENT

modifier

functionalization

through

can through

reactive

be

used

in

compatibilization.

include

strengthening

of

nanocomposites.

interfacial

The

PP-g-

Compared with pure MAPP, the composites had Supporting Information better compatibility, as well as excellent flame XRD patterns for MAPP and NH2-PEG-NH2. property, thermal stability, and nucleation ability. The data of peak heat release rates and total heat In terms of proportion, we found that 2.0 wt% release values for compatibilized PP-g-GO/MAPP proportion yielded high compatibility, as well as nanocomposites, MAPP, NH2-PEG-NH2, as well better flame property, thermal stability, and as uncompatibilized GO/MAPP. This material is



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Page 18 of 32

available free of charge via the Internet at

Funding Sources

http://pubs.acs.org.

Our study was financially supported by the National Natural Science Foundation of China (51303101), the Natural Science Foundation of Shanghai (09ZR1411600), and the Postdoctoral Science Foundation of China (20110490709). The authors declare no competing financial interest.

AUTHOR INFORMATION Corresponding Author

ACKNOWLEDGMENTS

* Tel.: +86 21 66134726, Fax: +86 21 66134726. E-mail:

The authors would like to thank Prof. Yuliang Chu and Prof. Weijun Yu for their help in obtaining SEM and TEM measurements.

[email protected](N. Song), [email protected] (P. Ding).

Author Contributions ‡These authors contributed equally


Page 19 of 32

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Scheme 1. Mechanism of PP-g-GO/MAPP synthesis. (For interpretation clearly for the molecular models of the PP-g-GO/MAPP, The orange platelet represents the GO phases, the magenta lines represent MAPP.)



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Figure 1. FTIR spectra of GO, NH2-PEG-NH2, GO-PEG, MAPP, and 2.0 wt% PP-g-GO.

Figure 2. XRD patterns of GO, GO-PEG, and 2.0 wt% PP-g-GO.


Page 20 of 32

Page 21 of 32

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Figure 3. Raman spectra of GO, GO-PEG, and 2.0 wt% of PP-g-GO (excitation wavelength: 633 nm).

Figure 4. XPS spectra and C1s XPS spectra of GO (a and c, respectively), GO-PEG (b and d, respectively), and corresponding carbon-to-oxygen ratio of XPS spectra (e).



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Figur5. AFM images of GO platelet and 2.0 wt% PP-g-GO (a and b, respectively).


Page 22 of 32

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Figure 6. (a) SEM images of 2.0 wt% GO/MAPP at magnification of × 1, 000 and (b) ×10000. (c) 2.0 wt% PP-g-GO/MAPP at magnifications of (d) × 1, 000 and (e) × 10000. Photographs of MAPP(left in e), 2.0 wt% GO/MAPP (middle in e), and 2.0 wt% PP-g-GO/MAPP (right in e). The inset in e is the template of hot pressed MAPP. The dimensions of the platelets are 10 mm (diameter) and 2 mm (thickness). (f) TEM images of GO and (g) GO-PEG.



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Page 24 of 32

Figure 7. HRR curves for compatibilized PP-g-GO/MAPP nanocomposites, NH2-PEG-NH2, and uncompatibilized GO/MAPP nanocomposite without direct functionalization of GO with NH2-PEG-NH2.

Figure 8. TG curves of compatibilized PP-g-GO/MAPP nanocomposites and MAPP.


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Figure 9. (a) Melting curves and (b) crystallization curves of compatibilized PP-g-GO/MAPP nanocomposites and MAPP.



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Page 26 of 32

Table 1. Thermal property of MAPP and its nanocomposites.

Nanocomposites (composition in wt%) MAPP 0.5%PP-g-GO/MAPP 1.0%PP-g-GO/MAPP 1.5%PP-g-GO/MAPP 2.0%PP-g-GO/MAPP 3.0%PP-g-GO/MAPP 5.0%PP-g-GO/MAPP

Tm1 (℃) 146 146 147 147 147 147 147

Tm2 (℃)

Tc (℃)

Tonest (℃)

Tmax (℃)

156 156 156 156 156 156 156

111 112 116 116 117 114 114

253 270 247 255 259 255 255

407 393 437 457 458 458 415

Tonset and Tmax are defined as the temperatures where 5 wt% mass loss occurs and the temperature where the maximum weight loss rate takes place. Tm and Tc represent the melting temperature and the crystallization temperature, wherein Tm1 is the first melting temperature and Tm2 is the second melting temperature.

Figure 10. (a) Tensile strength and (b) Lzod impact strength of PP and PP-g-GO/MAPP/PP blends.


Page 27 of 32

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43, 6515-6530.

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Effect of Covalent-Functionalized Graphene Oxide with Polymer and

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