In situ exfoliation of graphite oxide nanosheets in polymer nanocomposites using miniemulsion polymerization

Accepted Manuscript In situ exfoliation of graphite oxide nanosheets in polymer nanocomposites using miniemulsion polymerization Hussein M. Etmimi, Peter E. Mallon PII:

S0032-3861(13)00843-4

DOI:

10.1016/j.polymer.2013.08.060

Reference:

JPOL 16462

To appear in:

Polymer

Received Date: 9 July 2013 Revised Date:

21 August 2013

Accepted Date: 25 August 2013

Please cite this article as: Etmimi HM, Mallon PE, In situ exfoliation of graphite oxide nanosheets in polymer nanocomposites using miniemulsion polymerization, Polymer (2013), doi: 10.1016/ j.polymer.2013.08.060. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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“For table of content use only”

In situ exfoliation of graphite oxide nanosheets in

polymerization

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Hussein M. Etmimi1* and Peter E. Mallon1

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polymer nanocomposites using miniemulsion

Department of Chemistry and Polymer Science, University of Stellenbosch, Private Bag X1,

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Matieland 7602, South Africa

E-mail: [email protected]

Monomer and

Surfactant

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hydrophobe

+

Graphite oxide

Water + Surfactant

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Water

Surfactant solution

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GO nanoplatelets

Polymer

particles

Polymer/graphene nanocomposite latex

Polymerization Miniemulsification by sonication GO nanosheets

Formation of polymer nanocomposite latices based on GO using miniemulsion polymerization.

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Poly (styrene-co-butyl acrylate) nanocomposites containing graphene oxide were synthesized.

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Graphene oxide is prepared from natural graphite and used without further

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modification •

Miniemulsion polymerization was used as a one-step nano-incorporation technique.

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Nanocomposite with exfoliated morphologies were obtained except at high filler

The nanocomposites exhibited improved physical properties compared to pure

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polymers.

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•

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content.

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In situ exfoliation of graphite oxide nanosheets in polymer nanocomposites using miniemulsion polymerization

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Hussein M. Etmimi1 and Peter E. Mallon1*

Department of Chemistry and Polymer Science, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa

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* Corresponding author: Fax: +27218084967, E-mail: [email protected]

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Keywords: Graphene oxide, intercalation, exfoliation, miniemulsion polymerization.

Abstract

Poly (styrene-co-butyl acrylate) (poly(St-co-BA)) nanocomposite latices based on graphene oxide (GO) were synthesized by miniemulsion polymerization. The polymerization procedure

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involved dispersing an aqueous solution of graphite oxide in a monomer phase, followed by emulsification in the presence of a hydrophobe and a surfactant into miniemulsions. The focus was to investigate the suitability of miniemulsion for the synthesis of polymer nanocomposites based on a graphene derivative ((i.e., GO) with exfoliated structure in a one-

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step nano-incorporation technique. Poly(St-co-BA) nanocomposites containing the exfoliated GO nanoplatelets, which have improved mechanical and thermal properties were successfully

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synthesized by the miniemulsion process. The nanostructure of the nanocomposites was investigated by transmission electron microscopy (TEM) and X-ray diffraction (XRD). TEM and XRD indicated that the nanocomposites mainly showed exfoliated morphologies, except at relatively high GO content. TEM also revealed that the nanocomposites latices had the socalled ‘‘armored’’ structure, where the nanosized GO sheets are distributed around the edges of the copolymer particles.

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ACCEPTED MANUSCRIPT 1. Introduction Recent rapid growth in nanoscience has led to the preparation of polymer nanocomposites with enhanced properties compared to those of pure polymers. Currently, one of the most advanced research areas of nanotechnology focuses on the inclusion of nanoparticle fillers into polymers in order to enhance the functional and physical properties of polymers. Some of

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the nanoparticles used to date include nanofibers, silica nanoparticles, carbon nanotubes (CNTs), clays and graphene [1,2]. Perhaps the most studied is the use of clay, due to its ease of modification and availability. However, because of its unique properties, graphene has become the material of choice in the field of nanoscience and nanotechnology in recent years

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[3,4]. Not only do graphene nanosheets provide most of the advantages offered by the nanometer-size fillers but they can be incorporated in both hydrophilic and hydrophobic polymers with some modification [5-8]. Moreover, graphene is naturally available, and thus

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its use is generally economic and cost effective. The low cost of this material, together with its good mechanical, thermal and barrier properties, offer new possibilities for material development of polymer nanocomposites using graphene nanoparticles. The properties of polymer nanocomposites based on graphene strongly depend on how well the graphene nanosheets are dispersed in the final nanocomposites [9]. In fact, the preparation of polymer/graphene nanocomposites (PGNs) based on parent graphene is difficult to achieve.

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Most monomers and polymers can not be easily intercalated between graphene nanosheets in its pristine state. This is mainly because there are no reactive groups on the surface of graphene. Therefore, parent graphene lacks both the space and affinity for polymer molecules (or monomers) to be intercalated into its galleries. Furthermore, the graphene layers are bound

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together by van der Waals forces and π–π stacking interactions, which make the interlayer distance between graphene nanoplatelets very narrow. However, the synthesis of graphene

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oxide (GO) from pristine natural graphite creates many oxygen functionalities on its surface, which could greatly facilitate the interaction and exfoliation of monomer and polymer molecules into graphene galleries [10]. Therefore, GO has been widely used instead of graphene in the synthesis of polymer nanocomposites with intercalated and exfoliated structures [11,12].

GO is generally prepared by the oxidation of natural graphite using a strong oxidizing agent such as potassium permanganate in the presence of concentrated mineral acids (e.g., H2SO4 or HNO3). The oxidation of graphite is currently considered one of the most promising methods for the large scale production of GO nanoplatelets, which can be obtained by exfoliation of graphite oxide sheets. The oxidation leaves many new oxygen-containing groups such as 2

ACCEPTED MANUSCRIPT epoxide, hydroxyl and carboxyl groups on the graphene surface [13,14]. These oxygen groups significantly increase the compatibility between the GO nanoplatelets and polymers. Therefore, GO can be intercalated by various monomers or polymers to prepare different polymer/GO nanocomposites with improved properties [10,11].

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In recent years, many studies have focused on understanding the formation of polymer nanocomposites based on graphene [15-17]. According to the degree of graphene dispersion, the structure of these nanocomposites can be significantly changed. Therefore, the morphology of the obtained nanocomposites can be varied to a large extent. In general, three different morphologies (conventional, intercalated and exfoliated) can be obtained [9]. Each

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type of nanocomposite has different properties compared to pure polymers, which can be mainly controlled by how well the graphene nanosheets are distributed in the polymer matrix. These include properties such as mechanical, thermal and barrier properties [10,11]. The

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preparation of PGNs can be achieved by various synthesis techniques that have become available in the past years [9,11]. These include solution blending or mixing (also called exfoliation-adsorption), [7,18]_ENREF_13 in situ intercalative polymerization [19] and melt mixing (melt intercalative and/or exfoliation process) [20]. In most cases, the graphene nanosheets are exfoliated in a separate exfoliation step prior to the polymerization process. This is generally done by rapidly heating graphite oxide sheets at a very high temperature

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(600–1000 ºC) under N2 atmosphere [21,22].

Recently, great success has been achieved in the preparation of such nanocomposites using in

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situ polymerization of the monomer in the presence of GO nanosheets [19,23,24]. However, reports on the preparation of these composites in emulsion systems are rare. In particular, the use of miniemulsion polymerization for the synthesis of these nanocomposites has not been

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fully investigated. Miniemulsion polymerization is well known to be an effective nanoincorporation polymerization method. In the past, various polymer composites based on filler materials such as clay [25] and CNTs [26] have been successfully prepared using the miniemulsion technique. In miniemulsion, the oil phase, which consists of the monomers and the hydrophobe, is dispersed in the water phase, which contains the surfactant, by a high shear device such as a sonicator [27]. The initial dispersion of the filler nanosheets (a graphene derivative such as GO) within the monomer phase can be achieved by using the high shear device during the miniemulsification process. The use of the high shear device will also lead to the exfoliation of GO nanosheets, which is followed by the in situ polymerization of monomers in the presence of these expanded graphene nanoplatelets. This will lead to the 3

ACCEPTED MANUSCRIPT formation of polymer nanocomposites based on graphene with exfoliated morphology. The advantages of miniemulsion polymerization make it attractive to be used for the synthesis of polymer/filler nanocomposites such as PGNs. A schematic representation of the formation of PGNs by miniemulsion polymerization is shown in Scheme 1.

Surfactant

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Monomer and hydrophobe

+

Surfactant solution

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Graphite oxide Water

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GO nanoplatelets

Water + Surfactant

Polymerization

Polymer

Miniemulsification by sonication

particles

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Polymer/graphene nanocomposite latex

GO nanosheets

Scheme 1: Formation of polymer nanocomposite latices based on GO using miniemulsion polymerization.

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The oxidized graphite nanosheets can be dispersed in water by sonication and then added to a mixture of monomer and a hydrophobe for swelling. Surfactant solution is then added,

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followed by the emulsification process by sonication (i.e., miniemulsification) to obtain the miniemulsion. The sonication step will lead to the expansion of graphite oxide nanosheets to thinner GO nanoplatelets. Upon polymerization, these GO nanoplatelets will be finely distributed within the polymer latex, resulting in the exfoliation of GO in the polymer system. As the polymer chains grow in size, the GO nanoplatelets will be expanded further, resulting in the desired exfoliated morphology.

It was reported in 2010 that GO sheets can act as a surfactant in mixtures of hydrophobic liquids and water due to its amphiphilic character with the unique combination of hydrophilic edges and more hydrophobic basal planes [28,29]. Therefore, GO sheets could be adsorbed at 4

ACCEPTED MANUSCRIPT the oil-water interfaces during the miniemulsion preparation. On the basis of these reports, several authors have focused on the use of GO as the stabilizer using emulsion systems via the so-called “Pickering emulsion polymerization” [30,31]. Recently, Man et al. [32] prepared polystyrene particles armored with nanosized GO by aqueous miniemulsion polymerization of styrene, exploiting the amphiphilic properties of GO in the absence of conventional

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surfactants. The authors used a very similar process described in scheme 1 using GO as the stabilizer without adding any organic surfactant. GO sheets of different sizes were prepared using ultrasonication for different times in combination with separation by centrifugation, allowing investigation of graphene sheets size effects on miniemulsion stability. The best

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results were obtained using the smallest sheets of approximately 20 nm lateral size. The authors showed that this overall approach offers a convenient and attractive synthetic route to

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novel graphene-based polymeric nanostructures.

In a previous article we described the successful use of miniemulsion polymerization for the synthesis of PGNs based on GO with exfoliated structure [33]. However, GO was modified with a surfmer (reactive surfactant), 2-acrylamido-2-methyl-1-propane sulfonic acid prior to use, which widened the gap between the GO nanosheets and facilitated monomer intercalation between its nanogalleries. The current study describes the synthesis of poly(styrene-co-butyl

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acrylate) (poly(St-co-BA)) based on as-prepared GO using miniemulsion polymerization. Unlike in our previous work, the GO nanosheets obtained in this study were used without any further modification step. The emphasis is on determining the suitability of miniemulsion polymerization for the synthesis of polymer nanocomposite latices based on a graphene

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derivative such as GO in a convenient one-step nano-incorporation technique. It will be shown that the exfoliation of graphite oxide nanosheets to GO nanoplatelets within the polymer matrix will be achieved during the miniemulsion process without a prior

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modification or exfoliation step, resulting in the formation of polymer/GO nanocomposites with exfoliated structure. To the best of our knowledge, this is the first report on the exfoliation of as-prepared GO in St and BA monomers using the miniemulsion polymerization technique as a one-step nano-incorporation technique.

2. Experimental 2.1. Materials Styrene (St) (99%, Aldrich) and n-butyl acrylate (BA) (99%, Aldrich) were purified by washing with aqueous 0.3 M KOH, followed by distillation at 40 ºC under reduced pressure. Sodium dodecylbenzene sulfonate (SDBS) (99%, Fluka) and hexadecane (HD) (99%, Sigma5

ACCEPTED MANUSCRIPT Aldrich) were used as received. 2,2’-Azobis(isobutyronitrile) (AIBN) (98%) was obtained from Aldrich and purified by recrystallization from methanol. Potassium permanganate (KMnO4) (99%), sodium nitrate (NaNO3) (99%) and hydrogen peroxide (H2O2) (30%) were obtained from Sigma-Aldrich and used as received. Sulfuric acid (H2SO4) (98.08%, Merck) was also used as received. Natural graphite (99.5%) was obtained from Graphit Kropfmühl

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AG (Hauzenberg, Germany) and used without any further purification. Distilled deionized (DDI) water was obtained from a Millipore Milli-Q water purification system. Graphite oxide was prepared by treating natural graphite powder with potassium permanganate in the

2.2. Preparation of graphite oxide from natural graphite

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presence of sulfuric acid, following the method of Hummers and Offeman [34].

A mixture of 10 g of powdered flake graphite and 5 g of sodium nitrate was stirred into 230

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mL of 98% sulfuric acid. The ingredients were mixed in a 1.5 L jar that was cooled to 0 ºC in an ice bath, as a safety measure. While maintaining vigorous agitation, 30 g of potassium permanganate was added to the suspension. The rate of addition was carefully controlled to prevent the temperature of the suspension from exceeding 20 ºC. The ice bath was then removed and the temperature of the suspension brought to 35 ºC, where it was maintained for 30 min. As the reaction progressed, the mixture gradually thickened. After 15 min, the

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mixture became pasty, with a brownish gray color. After 30 min, 460 mL of water was slowly stirred into the paste, causing a violent reaction and an increase in temperature to 98 ºC. The diluted suspension was maintained at this temperature for 15 min. The suspension was then further diluted with ∼420 mL of warm water and 3% hydrogen peroxide to reduce the residual

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permanganate and manganese dioxide to colorless soluble manganese sulfate. Upon treatment with the peroxide, the suspension turned bright yellow. The suspension was filtered and a yellow-brown filter cake was obtained. The filtering was conducted while the suspension was

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still warm to avoid precipitation of the slightly soluble salt of mellitic acid formed as a side reaction. The final solid containing the GO was obtained by centrifugation.

2.3. Miniemulsion copolymerization of St and BA in the presence of graphite oxide The following miniemulsion polymerization procedure for the synthesis of poly(St-coBA)/GO nanocomposite latices was carried out. The graphite oxide was dispersed in DDI water by sonication using a Vibracell VCX 750 ultrasonicator (Sonics & Materials Inc.) for 10 min. The sonicator was set at 80% amplitude and a pulse rate of 2.0 sec (energy ~70 kJ). St and BA monomers, HD and AIBN were stirred for 30 min and then added to the graphite oxide solution. Surfactant solution (2% SDBS relative to monomer) was added and the 6

ACCEPTED MANUSCRIPT mixture was sonicated for 15 min to obtain the miniemulsion latex. The sonication was done in an ice bath to prevent AIBN initiation during this process,. A three-neck round-bottomed flask containing the resultant miniemulsion was immersed in an oil bath at room temperature. The content of the flask was purged with nitrogen for 15 min before increasing the temperature to 75 °C to start the polymerization. The reaction was carried out for 6 h under a

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nitrogen atmosphere, after which it was cooled to room temperature to stop the polymerization. A similar procedure was followed for the synthesis of a poly(St-co-BA) reference without graphite oxide. The various formulations used for the synthesis of poly(St-

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co-BA)/GO nanocomposites and the poly(St-co-BA) reference are tabulated in Table 1.

(g)

(g)

DDI water

(g)

(g)

P(St-co-BA)

-

2.71

2.31

0.102

0.077

50.08

P(St-co-BA)/GO-1

0.05

2.71

2.30

0.101

0.066

50.10

P(St-co-BA)/GO-2

0.10

2.71

2.30

0.103

0.067

50.60

P(St-co-BA)/GO-3

0.15

2.70

2.30

0.100

0.070

50.50

P(St-co-BA)/GO-4

0.20

2.73

2.36

0.107

0.075

50.40

P(St-co-BA)/GO-5

0.25

2.72

2.31

0.105

0.066

50.10

P(St-co-BA)/GO-6

0.30

2.70

2.31

0.105

0.071

50.40

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oxide (g)

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Table 1: Formulations used in the miniemulsion polymerizations of poly(S-co-BA) and poly(S-co-BA)/GO nanocomposite latices Nanocomposite Graphite St BA SDBS (g)/10 g HD DDI water

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2.4. Characterization and analytical techniques Various analytical techniques were used to characterize the graphite oxide samples and the

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poly(St-co-BA)/GO nanocomposites. Nanocomposite samples were obtained from the latices by precipitation. The latex (3 mL) was treated with concentrated hydrochloric acid, the precipitate was washed several times, first with methanol and then with DDI water, and finally dried at 40 °C under reduced pressure. The analytical instrumentation and procedures are now described:

2.4.1. Size exclusion chromatography (SEC) SEC was carried out using a Waters 610 Fluid Unit, Waters 410 Differential Refractometer at 30 °C, Waters 717plus Autosampler and Waters 600E System Controller (run by Millenium 32 V3.05 software). Tetrahydrofuran (THF) (HPLC grade), sparged with IR grade helium, 7

ACCEPTED MANUSCRIPT was used as an eluent at a flow rate of 1 mL/min. Two PLgel 5-µm Mixed-C columns and a PLgel 5-µm guard pre-column were used. The column oven was kept at 35 °C and the injection volume was 100 µL. The system was calibrated with narrow polystyrene (PS) standards (5 mg/mL THF), ranging from 2 500 to 898 000 g mol-1. The nanocomposite samples were dissolved in THF (5 mg/mL) over a period of 24 h and then filtered through a

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0.45 µm nylon filter. All molar mass data are reported as equivalent to the linear PS standards.

2.4.2. Nuclear magnetic resonance (NMR) spectroscopy

H NMR spectroscopy was performed at 20 ºC using a Varian VXR-Unity 300 MHz

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instrument. Nanocomposite samples (30 mg) were dissolved in 2 mL of deuterated chloroform (CDCl3) by stirring overnight. All chemical shifts are reported in ppm downfield

2.4.3. Monomer conversion

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from tetramethylsilane, which was used as an internal standard (δ = 0 ppm).

The monomer conversion in all experiments was determined gravimetrically. Samples were taken from the reaction vessel over time in order to determine the final monomer conversion.

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Monomer conversion was calculated and plotted vs. time for all experiments.

2.4.4. Transmission electron microscopy (TEM) TEM was used to directly visualize the morphology of the poly(St-co-BA)/GO nanocomposites at the nanometer level. Bright-field TEM images were recorded using a LEO

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912 Omega TEM instrument (Zeiss, Germany), at an accelerating voltage of 120 kV. Prior to analysis, miniemulsion samples were diluted with DDI water (0.05%) and placed on 300mesh copper grids, which were then transferred to the TEM apparatus. The average particle

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size of the synthesized latices was determined using computer software, ImageJ (NIH, USA).

A portion of the poly(St-co-BA)/GO miniemulsion latex was dried, then embedded in an epoxy resin, and cured at 60 ºC for 24 h. The embedded samples were then ultra-microtomed with a diamond knife on a Reichert Ultracut S ultramicrotome, at room temperature. This resulted in sections with a nominal thickness of approximately 100 nm. The sections were collected on a water surface and transferred to 300-mesh copper grids at room temperature, which were then transferred to the TEM apparatus. TEM was also used to observe the graphene nanosheets after modification (i.e., oxidation process). Graphite oxide (0.1 g) was

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ACCEPTED MANUSCRIPT dispersed in DDI water (50 g) by sonication. The Graphite oxide samples were diluted with DDI water (0.05%) and placed on 300-mesh grids for analysis.

2.4.5. Fourier-transform infrared (FT-IR) spectroscopy FT-IR spectra were obtained using a Nexus 470 FT-IR spectrophotometer (Thermo Nicolet, by using an attenuated total reflectance unit at a resolution of 4 cm-1.

2.4.6. Differential scanning calorimetry (DSC)

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USA), and recorded by averaging 32 scans. All spectra were acquired from 450 to 4000 cm-1

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DSC was used for the measurement of temperatures and heat flows associated with the phase transitions of the polymer in the nanocomposites. Measurements were carried out on a Q 100 DSC instrument (TA Instruments, USA). The analysis was done by heating samples of less

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than 10 mg from –40 ºC to 250 °C at a heating rate of 10 ºC/min, which were then cooled from 250 °C to –40 ºC, followed by a second heating step. All measurements were conducted under a nitrogen atmosphere, and at a purge gas flow rate of 50 mL/min. The DSC curves were obtained from the second heat cycle.

2.4.7. Scanning electron microscopy (SEM)

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SEM was used to observe the nanostructure of natural graphite flakes before and after the oxidation step. Imaging of the samples was accomplished using a field emission gun SEM instrument (FEI Nova NanoSEM) equipped with an Oxford X-Max EDS detector (University of Cape Town). The graphene samples were carefully mounted on the top of the sample

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holder with double-sided carbon tape. The samples were then coated with a thin layer of gold in order to make the sample surface electrically conducting. Images were recorded between

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500× and 10000× magnification, at 7 kV voltage, with a working distance of ~13 mm.

2.4.8. X-ray diffraction (XRD) XRD patterns were obtained using a X'Pert PRO multi-purpose diffractometer (PANalytical B.V., The Netherlands) equipped with a Cu K (alpha) sealed tube X-ray source (wavelength 1.514 Å). X'Celerator in Bragg-Brentano mode was used as the detector for all analyses.

2.4.9. Thermogravimetric analysis (TGA) TGA measures the weight loss of a material due to the presence of volatile groups as a function of temperature. The loss in weight is attributed to the thermal degradation of functional species as the temperature increases. TGA measurements were carried out on a 9

ACCEPTED MANUSCRIPT Q500 thermogravimetric analyzer (TA Instruments, USA). Sample sizes of less than 15 mg were used for all analyses. Analyses were carried out from ambient temperature to 600 ºC, at a heating rate of 20 °C/min, under a nitrogen atmosphere (nitrogen purged at a flow rate of 50 mL/min).

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3. Results and discussion 3.1. Characterization of graphite oxide

The chemical changes occurring upon the treatment of natural graphite with potassium permanganate in the presence of sulfuric acid were detected by FT-IR spectroscopy. The FT-

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IR spectrum for the natural graphite and its oxidized form are shown in Figure S1 in the supporting information. Similar results recorded for the FT-IR spectra of graphite oxide are reported in literature [35,36]. XRD was also used to determine the average interlayer distance

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(commonly known as the d-spacing) of stacked graphene before and after the oxidation. The average d-spacing of natural graphite and graphite oxide was calculated using the Bragg law and data are tabulated in Table 2. An increase in the d-spacing between oxidized graphene nanosheets in graphite oxide was observed. Further analysis of the obtained graphite oxide was carried out using techniques such as TGA and DSC, which showed good agreement with other reports [36-39]. The TGA and DSC curves of the natural graphite and its oxidized form

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are shown in Figure S2 and S3 in the supporting information, respectively. Table 2: Average interlayer distances (d-spacing) of natural graphite and graphite oxide XRD data Graphite Graphite oxide 2θ (º)

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d-spacing (nm)

26.4

10.5

0.34

0.84

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It is well known that the morphology and nanostructure of PGNs can be varied to a large extent, depending on the degree of graphene distribution in the final nanocomposites. Therefore, the nanostructure of graphite oxide was also studied by SEM and TEM analysis. Figure 1 shows SEM images of the natural graphite flakes at different magnifications. These graphene flakes within pristine graphite have dimensions in the order of 5–10 µm in length and 500 nm in thickness. However, after oxidation, the size of the graphene particles in graphite oxide is reduced considerably due to the effect of the oxidation process. The size of oxidized graphene sheets in graphite oxide was reduced to nanometer scale, as seen in Figure 2 [40]. This is in good agreement with other reports, which showed that the oxidation results in an increase in the d-spacing of graphite oxide relative to natural graphite, which can be 10

ACCEPTED MANUSCRIPT attributed to the intercalation of oxygen-based groups between its layers. Hence, the interlayer distance of graphite can be increased from 3.35 Å of the original graphite to 6–10 Å, depending on the interlamellar water content and the extent of the intercalation process [41].

b

20 µm

4 µm

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a

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Figure 1: SEM images of natural graphite: a) at low magnification and b) at high magnification.

b

600nm

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a

300nm

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Figure 2: SEM images of graphite oxide: a) at low magnification and b) at higher magnification. As evident from XRD analysis, (see Table 2), the interlayer spacing between oxidized

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graphene nanoplatelets had increased considerably in the graphite oxide sample compared to natural graphite. The natural graphite resulted in an intense d002 diffraction peak, indicating that the graphene layers are arranged in an ordered structure with 0.34 nm spacing. On the other hand, graphite oxide exhibited only one peak at a lower 2θ of 10.5°, with an interlayer distance of 0.84 nm. The GO nanosheets in graphite oxide were also observed by TEM (dispersed in water) at the nanometer level (see Figure 3). The TEM images clearly show that the thick graphite oxide sheets consist of thinner GO nanosheets, with sizes in the nanometer level (<100 nm). This is in agreement with SEM images in Figure 2._ENREF_37

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b

a

500nm

100nm

3.2. Characterization of the nanocomposite latices 3.2.1. Monomer conversion and latex stability

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Figure 3: TEM images showing a) thinner nanosheets of graphite oxide and b) a different area of the same sample of graphite oxide at higher magnification.

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Figure 4 shows the monomer conversion of the miniemulsion polymerization of St and BA in the presence of different quantities of GO. For comparison, monomer conversion of St and BA monomers in the absence of GO is also shown. AIBN was used as the initiator and the polymerization was carried out at 75 ºC. All latices prepared were stable and polymerization reactions proceeded with high final monomer conversions (80–95%). It should be noted here that at 4 wt% GO loading the conversion reached 80% after 100 min of the reaction, which

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was then decreased to ~75% probably due to some coagulation. The evolution of monomer conversion with time clearly shows that the polymerization was relatively fast and there was no inhibition period. This is in agreement with result found in the literature in the use of miniemulsion polymerization ofstyrene using GO as the surfactant [32]. Moreover, it is noted

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that all reactions had similar polymerization rates and there was no significant effect of GO loading on the rate of polymerization. These findings are similar to those of other researchers, who investigated the effect of other filler content (i.e., clay) on the monomer conversion in

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the preparation of polymer/clay nanocomposites. Moraes et al. [42] investigated the synthesis of poly(St-co-BA) via miniemulsion polymerization and observed no significant difference in conversion with an increase in clay loading.

Figure 5 shows digital photographs of the latices that were prepared using different GO contents (1 and 5 wt%). A photograph of the poly(St-co-BA) latex that was made without GO (Figure 5a) is also shown for comparison. The color of the latices varied from white for the latex made with no GO to light blue for the latex made with 1 wt% GO. The latex made with higher GO loading (5 wt%) exhibited a dark gray color (see Figure 5c). The images were

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ACCEPTED MANUSCRIPT taken ~5 months after the latices were prepared. The images clearly show that all latices were stable, even after 5 months. 100

60

40

0.0 wt% GO 2.0 wt% GO 4.0 wt% GO 6.0 wt% GO

20

0 0

50

100

150

200

250

300

350

Time (min)

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Conversion (%)

80

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c

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a

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Figure 4: Monomer conversion of the miniemulsion polymerization of St and BA in the presence of different quantities of GO content (0, 2, 4 and 6 wt%).

Figure 5: Digital photographs showing poly(St-co-BA) miniemulsion latices: a) latex without GO, b) latex containing 1 wt% GO and c) latex containing 5 wt% GO. 3.2.2. FT-IR analysis of nanocomposites

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Although FT-IR spectroscopy is the most widely used technique for the characterization of polymers, the use of this technique for the analysis of polymer nanocomposites can be very

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difficult [43]. In the case of polymer nanocomposites made with GO, the analysis is complicated due to the presence of the GO particles in the nanocomposite. Table 3 shows the main absorption bands that can be seen in the FT-IR spectra of GO, pure poly(St-co-BA) and poly(St-co-BA)/GO nanocomposites. The actual FT-IR spectra of pure poly(St-co-BA) and poly(St-co-BA)/GO nanocomposites (containing 1 and 5 wt% GO) are shown in Figure S4 in the supporting information.

Evidence of the formation of poly(St-co-BA)/GO nanocomposites was obtained by comparing the main absorption bands in the FT-IR spectra of pure poly(St-co-BA) with those of poly(Stco-BA)/GO nanocomposites. The vibration bands at 3000–3600, 1715 and 1047 cm–1 are 13

ACCEPTED MANUSCRIPT associated with –OH, C=O and C–O of GO, respectively [35]. The adsorption bands at 1727 cm-1 and the bands in the range 3090–3026 cm–1 are associated with C=O and hydrogen atoms attached to aromatic groups (Ar–H) of poly(St-co-BA), respectively [44]. It can be seen that all the components of the nanocomposite materials are present in the final product. All the expected bands of GO and poly(St-co-BA) are seen in the FT-IR spectrum of the poly(St-

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co-BA)/GO nanocomposites, confirming the formation of poly(St-co-BA) nanocomposites containing GO.

Table 3: FT-IR data of GO, poly(St-co-BA) and poly(St-co-BA)/GO nanocomposites Poly(S-co-BA) (cm–1) Poly(S-co-BA)/GO Functional group GO (cm–1)

3000–3600

-

C=O

1715

-

C–O

1047

>C–C<

-

C–H

-

C=O

-

Ar–H

-

–CH2–

-

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–OH

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nanocomposite (cm–1) 3000–3600 1728 1061

1022, 753

1022, 754

2920, 2847

2926, 2851

1727

1732

3090, 3061, 3026

3069, 3030

1492, 1448

1496, 1447

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-

3.2.3. Chemical composition of poly(St-BA) nanocomposites The polymerization of St and BA was carried out in the presence of GO. Poly(St-co-BA)

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reference without GO was also prepared under similar conditions to those employed for the synthesis of the nanocomposites. The composition of the poly(St-co-BA) and poly(St-co-

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BA)/GO nanocomposites was analyzed using 1H NMR spectroscopy [45]. The amounts of St and BA in the nanocomposites are tabulated in Table 4. Table 4: Amounts of St and BA in the nanocomposites as calculated from 1H NMR Nanocomposite GO content (wt%) St (wt%) BA (wt%) P(St-co-BA)

0.0

54.0

46.0

P(St-co-BA)/GO-1

1.0

53.0

47.0

P(St-co-BA)/GO-3

3.0

53.3

46.7

P(St-co-BA)/GO-5

5.0

54.0

46.0

14

ACCEPTED MANUSCRIPT The composition of all nanocomposites is very close to the feed composition of St and BA (54.1 wt% St and 45.9 wt% BA) that were added to the initial formulation. The 1H NMR spectra of pure poly(St-co-BA) and poly(St-co-BA) in the nanocomposites (synthesized at

3.2.4. Effect of GO loading on molecular weight of poly(St-co-BA)

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different GO loadings) are shown in Figure S5 in the supporting information.

Table 5 tabulates the molecular weights (weight average molecular weight, M¯ w and number average molecular weight, M¯ n ) and dispersity (Ð) of the poly(St-co-BA) reference and poly(St-co-BA)/GO nanocomposites prepared using different quantities of GO. All the

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synthesized polymers have relatively high molecular weights. This is common for polymers prepared by miniemulsion polymerization with very small particles and can be attributed to the compartmentalization effect [46,47]. When the particle size is very small, the molecular

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weight increases due to segregation of propagating radicals in different particles leading to reduced termination rates. Furthermore, no significant effect of the GO concentration on the molar mass of the polymer in the nanocomposites was observed. As the GO loading increased, the nanocomposites had similar molecular weights. Increasing the GO content also had no effect on the Ð values, which remained effectively constant with different GO loadings. The similarities between the polymers synthesized in the presence of different GO

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loadings is important since this implies that any changes in the nanocomposite properties are due to the GO content and not fundamentally due to differences in the polymer matrix.

¯ n , M¯ w and Ð of poly(St-co-BA) nanocomposites prepared using different Table 5: M

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quantities of GO (0, 2, 4 and 6 wt%)

GO content

¯n M

M¯ w

(wt%)

(g/mol)

(g/mol)

P(St-co-BA)

0

7.10 × 105

1.53 × 106

2.16

P(St-co-BA)/GO-2

2

7.22 × 105

1.63 × 106

2.26

P(St-co-BA)/GO-4

4

6.10 × 105

1.41 × 106

2.32

P(St-co-BA)/GO-6

6

7.65 × 105

1.62 × 106

2.12

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Nanocomposite

Ð

3.3. Nanocomposite morphology and properties Various analytical techniques were used to characterize the obtained nanocomposites: XRD, TEM, TGA and DSC. Analyses were carried out for nanocomposites containing 0, 2, 4 and 6 wt% of GO. 15

ACCEPTED MANUSCRIPT 3.3.1. XRD analysis Figure 6 shows XRD patterns of poly(St-co-BA) nanocomposites prepared with different quantities of GO. XRD proved that the final structure of the nanocomposites is influenced by the GO filler content in the nanocomposites. The structure of the nanocomposites changed significantly when the amount of GO in the nanocomposite increased. Therefore,

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nanocomposites made with different GO loadings have different XRD patterns depending on the quantity of GO incorporated into the sample. However, the nanostructure showed generally an exfoliated morphology, as revealed by the XRD pattern in Figure 6, except that created with GO content of 6 wt%. This is evident from the fact that no diffraction peak was observed for nanocomposites containing <6 wt% GO and the appearance of a broad peak at

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about 2θ = 10° for the nanocomposites containing 6 wt% GO. The broad peak at 2θ = 20° observed in the XRD scattering pattern corresponds to poly(St-co-BA) (amorphous halo) in

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the nanocomposites [48]. 400

0 wt% GO 2 wt% GO 4 wt% GO 6 wt% GO

350

Intensity (a.u.)

300 250 200 150 100

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50 0

10

20

30 ο

2θ ( )

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Figure 6: XRD patterns of neat poly(St-co-BA) (0 wt% GO) and poly(St-co-BA)/GO nanocomposites prepared with different quantities of GO (2, 4 and 6 wt%).

As seen in Figure 6, there is no diffraction peak for the nanocomposites containing 2 and 4

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wt% of GO, which indicated that these nanocomposites had mainly exfoliated structure. This suggests that the GO nanosheets have been separated by the polymer chains in these nanocomposites, leading to a fully exfoliated structure. However, in the case of nanocomposites with higher GO content (6 wt%), a more defined peak appears at 2θ = 10º, corresponding to an intercalated structure with more graphene order. This could be attributed to the fact that when the GO content is high (i.e., 6 wt%), the GO nanosheets tend to recombine in stacks of GO, leading to more ordered material (less broad XRD peaks). This is due to presence of oxygen-containing groups on GO nanosheets, which could interact with each other by hydrogen–hydrogen bonding in the presence of water molecules [13]. It should

16

ACCEPTED MANUSCRIPT be noticed here that the interlayer distance of GO in this nanocomposite measured about 0.88 nm, which is still greater than that for pure GO.

3.3.2. TEM analysis TEM was used to observe the morphology of the synthesized poly(St-co-BA)/GO

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nanocomposites at the nanometer level. The TEM images in Figure 7 show the poly(St-coBA)/GO latices containing 1 and 2 wt% GO. The images show polymer particles with sizes ranging from 80 to 200 nm, which is in the typical range of a miniemulsion polymerization (50–500 nm) [27]. The particle size distribution is narrow, which is an indication that no

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secondary particle nucleation occurred during the polymerization. The GO nanosheets are seen in the TEM images as dark lines around the polymer particles (see Figure 7). These are

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areas where the GO nanosheets bridged the particles in the so-called linked particle formation.

a

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b

200 nm

200 nm

Figure 7: TEM images of poly(St-co-BA)/GO nanocomposite latices made with a) 1 and b) 2 wt% GO relative to monomer.

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Figure 8 shows latices that were made with higher GO concentration relative to monomer, i.e., 4 and 6 wt% GO. The TEM images show that the latices had relatively broad particle size

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distribution and more small particles started to appear at higher GO concentrations. These small particles are probably caused by secondary nucleation, which will result in the formation of polymer particles with different sizes [49]. It can also be seen that some of the polymer particles are partially deformed due to the film drying that occurred during the TEM analysis. The observed particle deformation most probably occurred during the sample preparation due to the low Tg of the poly(St-co-BA) copolymer. It could also be caused by melting of the copolymer under the electron beam of the TEM instrument [50]. It is well known that polymer particles can undergo radiation beam damage or melting when exposed to the high energy electron beam of TEM [51,52].

17

ACCEPTED MANUSCRIPT a

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b

500 nm

500 nm

Figure 8: TEM images of poly(St-co-BA)/GO nanocomposite latices made with a) 4 and b) 6 wt% GO.

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Figure 8 also shows that the latices prepared with higher GO content exhibited similar morphology to those synthesized with lower GO loadings. The GO nanosheets are seen in the

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TEM images as dark lines attached to the polymer particles. The GO nanosheets tend to form a link between these miniemulsion particles, leading to the formation of linked polymer particles, where the polymer particles tend to link together to house the GO aggregates. This is probably caused by the presence of many oxygen-containing groups on the GO, which results in the formation of GO nanosheets with different properties. Although GO has been identified as an extremely hydrophilic material, recent studies of its chemical structure reveals

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a dual (hydrophilic–hydrophobic) structure of GO [28,53].

The simultaneous presence of hydrophilic oxygen groups on the surface of hydrophobic graphene nanosheets renders GO functionally similar to an amphiphilic molecule. Therefore, GO nanosheets may act as a surfactant, where these nanosheets are distributed around the

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polymer particles forming so-called armored polymer particles. These morphologies could have a significant effect on the overall stability of the synthesized latices. Kim et al. [28]

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showed that GO could act like a surfactant, as measured by its ability to adsorb on interfaces and reduce the surface or interfacial tension. They attributed this to the fact that GO has an amphiphile structure with hydrophilic edges and a more hydrophobic basal plane. Man et al. [32] used a very similar approach for the preparation of polystyrene particles using GO as the sole stabilizer without adding any organic surfactant. To investigate the effect of the size of the graphene sheets on miniemulsion stability, the authors prepared GO sheets of different sizes using ultrasonication for different times in combination with separation by centrifugation. They found that the best results were obtained using the smallest sheets of approximately 20 nm in size.

18

ACCEPTED MANUSCRIPT The morphology of the obtained nanocomposites was also studied by analyzing the microtomed films cast from the nanocomposite latices by TEM. Results indicated that the nanocomposite films had mainly an exfoliated morphology (in agreement with XRD analysis). Figures 9 and 10 show the TEM images of the dried films obtained from the latices that contain 2 and 4 wt% GO relative to monomer, respectively. The dark lines represent the

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GO nanosheets and the polymer matrix appears as relatively bright domains. In most areas the GO is dispersed as thin layers in the polymer matrix, which results in the formation of a highly exfoliated structure. a

1000 nm

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b

200 nm

Figure 9: TEM images of poly(St-co-BA)/GO nanocomposites at 2 wt% GO loadings: a) low magnification image and b) higher magnification image. a

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b

1000 nm

200 nm

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Figure 10: TEM images of poly(St-co-BA)/GO nanocomposites at 4 wt% GO loading: a) low magnification image and b) higher magnification image. However, films made with higher GO content (6 wt% GO) exhibited stacking of GO nanoplatelets, which indicates that the GO nanoplatelets were not completely exfoliated. Therefore, these nanocomposites exhibited mainly intercalated morphology at GO loading of 6 wt% (in agreement with XRD results in Figure 6). The TEM images of poly(St-co-BA)/GO nanocomposites made with 6 wt% GO content are shown in Figure 11. Compared to the images in Figures 9 b and 10 b, the images in Figure 11 b indicate that most of the graphene nanoplatelets in GO are stacked in an orderly manner, exhibiting a more defined intercalated 19

ACCEPTED MANUSCRIPT morphology. This is also in agreement with the results of XRD analysis of these nanocomposites, which showed a less broad peak at 2θ = 10 (see Figure 6, 6 wt% GO).

b

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a

200 nm

SC

1000 nm

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Figure 11: TEM images of poly(St-co-BA)/GO nanocomposites at 6% GO loading: a) low magnification image and b) higher magnification image. 3.3.3. Thermal analysis

The thermal properties of the obtained nanocomposites were investigated by TGA. Figure 12 displays the TGA weight loss curves of poly(St-co-BA) and the poly(St-co-BA)/GO nanocomposites made with different GO concentrations. There is no significant weight loss of the pure poly(St-co-BA) copolymer below 380 ºC and its thermal degradation only occurs in

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one step at ~380 ºC.

100

0 wt% GO 2 wt% GO 4 wt% GO 6 wt% GO

60

EP

Weight (%)

80

40

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20

0 360

380

400

420

440

460

ο

Temperature ( C)

Figure 12: TGA thermograms of poly(St-co-BA) nanocomposites made with different quantities of GO.

and

poly(St-co-BA)/GO

As evident in Figure 12, the thermal stability of poly(St-co-BA) increased noticeably in the presence of GO and the synthesized nanocomposites are more thermally stable relative to the neat copolymer. The onset temperature of degradation for the poly(St-co-BA) in the nanocomposites increased by 10–15 °C in the presence of GO relative to the neat copolymer. This indicates that the incorporation of GO into the polymer chains leads to better thermal 20

ACCEPTED MANUSCRIPT stability of the polymer. This is in agreement with other findings in the literature, where nanocomposites based on graphene exhibited a great improvement in the thermal stability compared to that of neat polymers [54,55]. The improvement in thermal stability of poly(Stco-BA) in the presence of GO can be attributed to the intercalation of polymer chains into the lamellae of GO. The polymer chains are trapped between the graphene nanoplatelets in the

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nancomposites. These GO nanoplatelets may act as an insulator between the heat source and the surface area of polymer where the combustion occurs [56]._ENREF_54 The presence of GO nanoplatelets may also hinder the diffusion of volatile decomposition products within the nanocomposites by promoting char formation. The char formed layer act as a mass transport

SC

barrier that retards the escape of the volatile products generated as the polymer decomposes [48]. The enhancement of the nanocomposites’ thermal stability has also been attributed to the

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movement restriction of the polymer chains inside the graphene nanogalleries [48].

3.3.4. Glass transition temperature of poly(St-co-BA) in the nanocomposites The glass transition temperature (Tg) of poly(St-co-BA) in the nanocomposites was determined by DSC analysis. The Tg of the nanocomposites containing different GO loadings are summarized in Table 6. All nanocomposites exhibited one Tg value, which corresponds to the Tg of the poly(St-co-BA) copolymer. This suggests that all the monomers are incorporated

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in the copolymer and no homopolymerization of St or BA occurred. Furthermore, as the amount of GO was increased, a considerable change in the Tg of the copolymer in the nanocomposites was observed. A shift of ~5.0 ºC in the Tg of poly(St-co-BA) was observed with the addition of only 2 wt% of GO. The increase in the Tg value of poly(St-co-BA) in the

EP

nanocomposites can be attributed to restricted chain mobility of the polymer chains caused by the presence of GO nanosheets.

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Table 6: Tg of poly(St-co-BA) and poly(St-co-BA)/GO nanocomposites prepared with different GO content (0, 2, 4 and 6 wt%). Nanocomposite GO content (wt%) Tg (ºC) P(St-co-BA)

-

28.0

P(St-co-BA)/GO-2

2

32.5

P(St-co-BA)/GO-4

4

32.0

P(St-co-BA)/GO-6

6

32.0

The strong interaction between polymer chains and GO nanoplatelets, which have a high aspect ratio results in a significant decrease in the polymer segments’ mobility near the

21

ACCEPTED MANUSCRIPT polymer–graphene interface, leading to a higher mechanical performance [57,58]. However, as seen in Table 6 the increase in the Tg was not a function of GO loading, and all analyzed films had similar Tg values. This is because the morphology of the nanocomposites changed significantly as the GO content increased, as indicated in the XRD results (see Figure 6). The nanocomposites exhibited only exfoliated morphologies at relatively low GO content. At a

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higher GO content the nanocomposites showed mainly intercalated structures. This change in the nanocomposite’s morphology may counteract the effect of GO loading on the Tg of the copolymer in the nanocomposites.

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Our findings are similar with what is reported in the literature, namely addition of a small quantity of graphene nanoparticles to polymer nanocomposites could lead to a significant improvement in the mechanical properties of polymers. Ramanathan et al.[59] showed that

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the addition of only 2 wt% of graphene to PMMA led to a significant increase of 31% in the mechanical properties (measured as modulus) of the polymer. They attributed this to the strong hydrogen–bonding interaction of oxygen–functionalized graphene and the mechanical interlocking at the wrinkled surface that may restrict segmental mobility of polymer chains near polymer–graphene surfaces. They also found that the same behavior was observed for poly(acrylonitrile) (PAN), where a significant shift in the Tg of the polymer at only 1.0 wt%

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of GO content was obtained. They attributed this to the presence of hydroxyl groups on the GO surface, which promoted a positive shift in the Tg due to favorable non–covalent interactions with the PAN chains. Tg shifts of between 10 and 20 ºC have also been previously

4. Conclusion

EP

reported [60,61].

Poly (styrene-co-butyl acrylate) (poly(St-co-BA)) nanocomposite latices containing exfoliated

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graphene oxide (GO) nanoplatelets were successfully synthesized by miniemulsion polymerization. The nanoscale GO sheets were prepared from natural graphite flakes following a novel procedure, which effectively led to the formation of functional graphene nanoplatelets. The GO nanoplatelets were used without any further modification step. The polymerization proceeded to relatively high monomer conversion and produced stable nanocomposite latices containing the as-prepared GO nanoplatelets. Miniemulsion polymerization was, therefore, suitable for the formation of polymer latices containing the GO nanoplatelets that were exfoliated in situ without a prior exfoliation step. Results showed that miniemulsion is a convenient method for the preparation of polymer nanocomposites based on a graphene derivative (i.e., unmodified GO) in a one-step nano-incorporation technique. 22

ACCEPTED MANUSCRIPT The overall approach offers a new route for the synthesis of exfoliated polymer nanocomposites based on as-prepared GO with improved properties. Analysis of GO revealed that the graphene nanosheets were successfully functionalized with oxygen-containing groups such as hydroxyl and carboxylic groups. FT-IR results confirmed the formation of poly(St-coBA)/GO nanocomposites with various quantities of GO. The degree of GO dispersion was

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determined by XRD and TEM. TEM was used to determine the nanocomposite morphology by directly visualizing the latex particles and their films at the nanometer level. XRD was used to confirm the structure of the nanocomposites, i.e., intercalation and/or exfoliation of GO nanosheets within the polymer matrix. Examination of the nanocomposite films by TEM

SC

indicated the formation of poly(St-co-BA)/GO nanocomposites with exfoliated morphologies. XRD analysis proved the formation of exfoliated nanocomposites except at relatively high GO content. TGA and DSC showed that the poly(St-co-BA) copolymer in the

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nanocomposites exhibited higher thermal and mechanical properties compared to pure copolymer, which was not a function of GO loading.

Acknowledgments

This study was financially supported by the International Center for Macromolecular

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