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Novel Pickering Stabilizer Constituted by Graphene Oxide and Carbon Nanotubes for Fabricating Poly(methyl methacrylate) Nanocomposites.

INTRODUCTION

Graphene is a new type of monoatomic layer of carbonaceous nanomaterials with unique two-dimensional (2D) planar structures. The [sp.sup.2]-bonded carbon atoms in graphene constitute the honeycomb lattice structure of the thinnest nanomaterial in the world [1, 2]. Graphene oxide (GO), usually obtained by chemically exfoliating oxidized graphite, is an important derivative of graphene. Due to the unique nanosized configuration and extraordinary physical properties, the application prospect of GO is increasingly extensive [3-6]. Studies have shown that there are many hydroxyl and epoxy groups on the base surface of GO, however, some carbonyl and ionizable carboxyl groups are present at the edges [7-9], Thus, a large number of oxygen-containing groups on the oxidized graphene sheet enable it to be well dispersed in water. As compared with 2D graphene, another one-dimensional (ID) carbon allotropes, carbon nanotubes (CNTs), possessing excellent mechanical, electrical, and thermal properties and large aspect ratio as well [10], have long been recognized as prominent nanoreinforcement for polymer-based composites [11, 12]. However, the dispersion and uncontrollable agglomeration of CNTs in polymer matrix have been an intractable issue [13-17]. In this regard, it holds great promise that GO and CNTs are simultaneously introduced to resin bulk to synergistically improve the functionalities of polymer-based composites [18-20].

It has been reported that GO or functionalized CNTs containing hydrophobic non-oxidized [sp.sup.2]-hybridized carbon domains and hydrophilic oxygen-bearing groups are considered as an amphiphilic species, which is an important structural feature to be used as particle stabilizers for Pickering emulsions [7, 21-23]. Owing to their scientific and technological significance, great efforts have been directed at investigating the oil-in-water (o/w) and water-in-oil (w/o) type Pickering emulsion with polymerizable monomer as oil phase [24-26]. It becomes evident that polymer microspheres or porous polymer composites containing ordered carbon structure formed by GO or CNTs can be conveniently acquired by initiating the polymerization reaction. The influences of covalent functionalization and noncovalent modification on the GO or functionalized CNTs stabilized Pickering emulsion have been studied for several o/w and w/o systems with an emphasis on tuning the amphiphilicity of stabilizer, which is one of the paramount factors governing the morphology and performance of the resulting emulsion and final polymer nanocomposites [27-29].

In this study, GO and CNTs were subjected to a hydrothermal treatment in the presence of 2-ethyl-4-methylimidazole (EMI) in order to prepare a novel carbon nanohybrid Pickering stabilizer. Methyl methacrylate (MMA) monomer was used as oil phase to form o/w type Pickering emulsion. An oil-soluble initiator, azobisisobutyronitrile (AIBN), was employed to achieve the core--shell structured PMMA microspheres enclosed by EMI/GO/CNTs nanohybrids. The mechanical properties and microstructure of the compression molded PMMA nanocomposites were characterized and correlated to the EMI/GO/CNTs stabilizer with respect to level and formulation.

EXPERIMENTAL

Materials

Natural graphite flake (average particle size ~44 [micro]m) was supplied by Qingdao Graphite Factory, China. EMI, MMA, and AIBN were purchased from Shanghai Jingchun Reagents Co., Ltd., China. Carboxylic acid functionalized multiwalled CNTs was supplied from Timesnano, China (purity: > 95%, COOH content: 1.23%, outer diameter: 10-20 nm, inner diameter: <5 nm, length: a few microns). [H.sub.2]S[O.sub.4] (98%), HCl (36%), [H.sub.2][O.sub.2] (36%), and KMn[O.sub.4] were all purchased from Sinopharm Chemical Reagent Co., Ltd, China. All water used in the experiment was deionized water and all chemicals used in the experiment were analytical grade.

Preparation of EMI/GO/CNT s Stabilizer

GO was synthesized from natural graphite flake according to the modified Hummers method [30]. A certain amount of GO was dispersed in deionized water and sonicated by a Bilon 250Y horn sonicator equipped with a 6 mm diameter tip operating at a frequency of 22 kHz for 30 min in ice-water bath to achieve different initial concentration of GO aqueous dispersion (from 2 to 6 mg [mL.sup.-1]). The pre-calculated amount of CNTs and 100 mL GO aqueous dispersion were agitated by sonication for 2 hr to form the mixture at the GO-to-CNTs mass ratio of 6:1. EMI with the same mass as GO was added to the mixture and stirred for 20 min. EMI/GO/CNTs nanohybrids used as Pickering stabilizer was prepared by hydrothermal reaction at 90[degrees]C for 2 hr with agitation (500 rpm). Using the same conditions, EMI/GO/CNTs nanohybrid stabilizers differing in the GO-to-CNTs mass ratio (from 2:1 to 10:1) were synthesized, in which the total amounts of GO and CNTs were kept constant. The as-synthesized carbon-only stabilizer dispersion was directly employed as the water phase for Pickering emulsion eliminating the tedious drying and ultrasonic processes usually needed for the preparation of common stabilizer.

Preparation of PMMA/GO/CNTs Nanocomposites

The EMI/GO/CNTs dispersion was mixed with a certain amount of MMA monomer containing 1% wt AIBN and sonicated for 10 min to form a Pickering emulsion stabilized by the EMI/GO/CNTs nanohybrids we had prepared. EMI/GO/CNTs encapsulated PMMA microspheres were achieved by free radical polymerization reaction at 80[degrees]C for 6 hr followed by filtration. PMMA/GO/CNTs nanocomposites were fabricated by placing EMI/GO/CNTs encapsulated PMMA microspheres into a mold cavity precoated with mold-releasing agent. The assembly was heated at 200[degrees]C for 0.5 h and then hot-pressed under 10 MPa pressure for 2 hr at 220[degrees]C. PMMA/GO/CNTs nanocomposites with different constitutions were produced in the same way.

Measurements and Characterization

EMI/GO/CNTs nanohybrids were acquired by following the protocol of centrifugation, repeated washes followed by a final vacuum drying at room temperature. Fourier transform infra-red (IR) spectra were collected using a Perkin Elmer Spectrum 100 to analyze the chemical structure of EMI/GO/CNTs nanohybrid stabilizer. The wavelength range was from 450 to 4000 [cm.sup.-1] with a resolution of 4 [cm.sup.-1]. About 1 mg of the sample was mixed with 100 mg of dry KBr and then pressed into a transparent circular flake for IR analysis.

The three-phase contact angle ([theta]) of MMA, water, and the as-synthesized carbon nanohybrid stabilizer was measured according to the compressed disk method [31]. EMI/GO/CNTs nanohybrids were compressed into a 0.5 mm thick film at the pressure of 10 MPa pressure. At 25[degrees]C, MMA was poured into a transparent glass beaker containing the pressed film at the bottom and then an aqueous droplet dyed pink was carefully placed on the film with the help of syringe. The shape of the water drop on the film was photographed by using D3200 digital camera (Nikon, Japan) and the contact angle was directly read with a protractor.

Optical micrograph of the prepared Pickering emulsions on transparent glass slides was obtained by a MP41 microscope (Mshot Co., Ltd., China). Digital photograph of the emulsion was recorded by D3200 camera. The morphologies of PMMA microspheres and the fracture surfaces of PMMA nanocomposites after mechanical testing were observed with SU3500 scanning electron microscope (SEM) after coating with a thin gold membrane to increase the conductance.

The flexural strength of the specimens with the size of 80 mm x 10 mm x 4 mm was tested on the universal testing machine in accordance with GB/T 9341-2000 at the loading speed of 2 mm [min.sup.-1]. The compressive strength of the specimens with the size of 10 mm x 10 mm x 4 mm was measured at the loading speed of 5 mm [min.sup.-1] according to GB/T 1041-2008. Five coupons were measured for each specimen group from which the mean values were reported.

RESULTS AND DISCUSSION

IR Spectra Analysis

IR spectra of GO, CNTs, and EMI/GO/CNTs nanohybrids are shown in Fig. la and b. In Fig. la, the spectrum of GO exhibits the characteristic absorption bands at 3390 [cm.sup.-1] (O-H), 2980 [cm.sup.-1] (C[H.sub.2]), 1730 [cm.sup.-1] (C=O), 1630 [cm.sup.-1] (C=C), 1390 [cm.sup.-1] (C-OH), 1230 [cm.sup.-1] (C-O--C), and 1040 [cm.sup.-1] (C--O). The spectrum of CNTs displays three absorption bands at 3420 [cm.sup.-1] (O-H), 1630 [cm.sup.-1] (C=C), and 1390 [cm.sup.-1] (C--OH). In the context of the spectrum of EMI/ GO/CNTs nanohybrids, the features of GO and CNTs are observed together. The marked absorption bands at 3720 [cm.sup.-1] is assigned to free hydroxyls, which is sharply different from those of GO and CNTs. In Fig. 1b, the IR spectra of three specimens in the range of 1000-1800 [cm.sup.-1] are shown to better evaluate EMI/GO/CNTs nanohybrids prepared by the hydrothermal reaction. The peak at 1730 [cm.sup.-1] is attributed to the stretching vibration of C=O linkage of GO. The absorption bands at 1570 [cm.sup.-1] and at 1395 [cm.sup.-1] corresponding to N--H and C=N bonds are indicative of EMI involving in the hydrothermal reaction. It is also shown in Fig. 1b that the hydrothermal reaction results in a frequency shift of C--O bond from 1040 [cm.sup.-1] for GO specimen to 1066 [cm.sup.-1] for EMI/GO/CNTs nanohybrids. This shift to higher frequency is due to the specific interaction occurring in the studied system, which drive EMI, GO, and CNTs to assembly into an amphiphilic carbon nanohybrids. The specific interaction is marked by the greatly decreased intensity and obvious blueshift in the hydroxyls absorption and by the appearance of a weak band at 1114 [cm.sup.-1]. The amphiphilicity of this novel carbon-only Pickering stabilizer can be tuned by hybridizing with hydrophobic CNTs in the presence of EMI during hydrothermal reaction.

Contact Angle Analysis

It has been reported that the type and stability of Pickering emulsions are highly dependent on the wettability of stabilizer captured by the oil-water interface [32]. The stabilizer is preferably wetted by the relatively thermodynamically affinitive liquid, which tend to be continuous phase of the resulting emulsion. As demonstrated in Fig. 2, the three-phase contact angle ([theta]) of MMA, water, and the as-synthesized carbon nanohybrid stabilizer (GO/CNTs = 6:1) is less than 90[degrees], which implies the formation of o/w type emulsions stabilized by EMI/ GO/CNTs nanohybrids. Driven by the supermolecular hydrophobic and [pi]-[pi] interactions between [sp.sup.2] bonded carbon domains pertaining to CNTs and GO, CNTs are adsorbed and surrounded by hydrophilic GO and incorporated into the novel amphiphilic carbon-only stabilizer. In addition, being a five-membered heterocyclic compound composed of acidic and basic nitrogen atoms at nonadjacent positions [33], EMI rings are proposed to mediate between the GO and CNTs allowing the formation of amphiphilic carbon nanohybrid stabilizer through intermolecular hydrogen bonds. The evidence for the three-phase contact angle corresponds well to the IR spectra analysis.

Pickering Emulsion Stabilized by EMI/GO/CNTs Nanohybrids

Pickering emulsion stabilized by solid particles and the ordinary emulsion stabilized organic surfactants qualitatively share the common regular pattern in many respect, but show distinct differences in the applications where particle stabilizers such as GO and CNTs are welcome to endow the final nanocomposites with novel functionalities. However, highly hydrophilic and electrically insulating GO is not amenable to form stable Pickering emulsion and to be used as reinforcement for polymer composites, which motivates numerous researches on modification of GO. In this study, the optical micrograph and photograph of o/w Pickering emulsion stabilized by EMI/GO/CNTs nanohybrids are demonstrated in Fig. 3, which is prepared on the condition that the oil/water volume ratio is 1, the GO/CNTs ratio is 6, and the initial concentration of GO is 2 mg [mL.sup.-1]. The good stability of Pickering emulsion is confirmed in Fig. 3, which was allowed to stand for 8 hr before taking the photograph. It is indicated that there is sufficient time for the following free radical polymerization reaction to approach completion. EMI/GO/ CNTs nanohybrids reside at the interface of MMA droplet and water phase and stabilize the emulsion by reducing the total interfacial energy of the system. The oil/water interface acting as 3D template drives EMI/GO/CNTs nanohybrids to encapsulate MMA droplet without external manipulation. As indicated by optical microscopy in Fig. 3, the emulsion droplets present a relatively homogeneous sizes distribution.

Morphological Characteristics of EMI/GO/CNTs Encapsulated PMMA Microspheres

SEM images at low and high magnification of EMI/GO/ CNTs encapsulated PMMA microspheres originated from Pickering emulsion shown in Fig. 3 are revealed in Fig. 4a and b, respectively. The loosely aggregated PMMA particles are composed of microspheres with the diameter of a few microns, which are approximately consistent with those of the emulsion droplet given in optical micrograph in Fig. 3. The free radical polymerization of MMA gives rise to the core-shell structured PMMA microsphere adhering to each other, which is characterized by the coarse surface constituted by GO and CNTs. From the mechanistic point of view, the reaction process for this Pickering emulsion is proposed to essentially be suspension polymerization based on the used oil-soluble initiator AIBN. EMI/ GO/CNTs nanohybrids act as Pickering stabilizer at the stage of emulsion formation and remain at the MMA/water interface. Subsequently, GO and CNTs are frozen on the surface of PMMA core by the following polymerization reaction. It is reasonable to expect that the resulting carbon-only nanohybrid shell with the combined features of chemically reactive GO and hydrophobic conjugated CNTs can synergistically confer novel functionalities to PMMA nanocomposites.

Mechanical Properties of PMMA/GO/CNTs Nanocomposites

In this study, flexural and compressive strengths of PMMA nanocomposites were tested to evaluate the effects of the amount and formulation of nanohybrid stabilizer, respectively. In Fig. 5, flexural and compressive strengths of PMMA/GO/ CNTs nanocomposites as a function of initial GO concentration with the ratio of GO/CNTs constant (6:1) are shown. The flexural and compressive strengths show monotonously decreasing trends with increased initial GO concentration. The flexural strength of the nanocomposites is 73.1 MPa at an initial GO concentration of 2 mg [mL.sup.-1]. In comparison with flexural strength, the effect of the amount of nanohybrid stabilizer on the compressive strength is more pronounced. PMMA nanocomposites prepared using initial GO concentration of 2 mg [mL.sup.-1] exhibits compressive strength of 218.4 MPa. However, an initial GO concentration of 6 mg [mL.sup.-1] significantly reduces the compressive strength of the nanocomposites to 178.5 MPa.

The roles of stabilizer formulation in flexural and compressive strengths of PMMA nanocomposites while holding stabilizer amount constant are shown in Fig. 6. The effect of varying stabilizer composition is remarkably different from what is observed in stabilizer level study. PMMA nanocomposites fabricated via Pickering emulsion stabilized by the carbon nanohybrids at a GO/ CNTs ratio of 6:1 presents the peak values for flexural strength of 73.1 MPa and for compressive strength of 218.4 MPa, respectively. It is well established that the size of the droplets and the total interfacial area of oil/water system usually show a basically opposite trend in a typical emulsion and the increased EMI/GO/ CNTs stabilizer level is responsible for the reduction in the droplet size of Pickering emulsion [32]. At the processing parameters adopted in compression molding operation in this study, the nonviscous carbon nanohybrid shell is disadvantageous for the melt PMMA core to fuse into monolithic composites, which can deteriorate by increasing stabilizer level.

As seen in Fig. 6, the optimized mechanical properties are achieved for PMMA nanocomposites produced by using the stabilizer containing the GO-to-CNTs mass ratio of 6:1. During hydrothermal reaction, excessive EMI cannot be the component constituting the carbon nanohybrid stabilizer and is removed by the repeated washes. However, excessive hydrophobic CNTs collect into aggregates. As for excessive GO, the repulsive effect induced by ionizing oxidized carbon atoms adversely affect the adhesion of PMMA microsphere to each other. Thus, excessive GO or CNTs leads to the decreased mechanical properties of the final nanocomposites by overcoating PMMA microspheres. It is unneglectable that the difference in average molecular weight of PMMA chains caused by the size of monomer droplets has inevitable effect on the mechanical properties of the PMMA nanocomposites.

Microstructure of PMMA Nanocomposites

Figure 7a and b shows SEM micrographs at low magnification and high magnification of fracture surface of compression molded PMMA/GO/CNTs nanocomposites. Marked different from the well-known smooth fracture surface of amorphous glassy PMMA, PMMA/GO/CNTs nanocomposites display rough fracture morphology. In the presence of non-viscous carbon nanohybrid stabilizer, it is difficult for the melted PMMA core being close to each other to completely fuse and coalesce into a monolith integrating GO and CNTs. Amphiphilic carbon nanohybrid shells are wetted by PMMA melt as evidenced by the obscured interface in Fig. 7a. A cellular structure consisting of discrete polymer domains and irregular light colored stripe is characteristic of PMMA/GO/CNTs nanocomposites fabricated in this study. It is inferred that EMI/GO/CNTs nanohybrid stabilizers enclosing MMA droplet are ultimately represented by a carbon nanohybrids network throughout the final nanocomposite. The poor interparticle adhesion of carbon nanohybrids wrapped PMMA microsphere is confirmed in Fig. 7b. The appearance of a few micron-sized holes is attributed to the detachment of intact PMMA microspheres from the densely stacked particles during mechanical testing. Thanks to the fact that the focus of research on polymer nanocomposites based on CNTs and/or GO is more on functionalities than on mechanical behaviors. The objective of mechanical test in this study is to explore the relationship of the as-synthesized EMI/GO/CNTs stabilizer, the characteristics of the resulting Pickering emulsion, and the microstructure of the final nanocomposites. Further investigations on the processing-microstructure-property interplays of polymer composites based on the carbon nanohybrids stabilized Pickering emulsion, especially on electrical and thermal conductivities, are in progress.

CONCLUSIONS

A strategy for elaborating polymer nanocomposites containing GO and CNTs based on Pickering emulsion has been proposed. The novel carbon nanohybrid stabilizer was synthesized by EMI, GO, and CNTs in a hydrothermal reaction. The amphiphilicity of the novel carbon-only nanohybrids was characterized by IR spectra and contact angle measurement, which lead to the stability being imparted by the reduction of o/w interfacial tension and enabled it to be used as Pickering stabilizer. The free radical polymerization of MMA initiated by AIBN gave rise to EMI/GO/CNTs wrapped PMMA microsphere. The flexural and compressive strengths of compression molded PMMA nanocomposites as a function of carbon nanohybrid stabilizer level and formulation were tested to assess the processing-microstructure-property interplays of the nanohybrid composites. It has been found that the mechanical properties of PMMA nanocomposites decrease with the increased amount of carbon nanohybrid stabilizer. The peak values for the mechanical properties are attributed to the optimized stabilizer formulation. As suggested in SEM analysis, it is reasonable to infer that the incorporated carbon nanohybrid stabilizer in polymer nanocomposites based on Pickering emulsion routes play different roles with respect to load transfer and electron transport.

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Baichen Wang (ID), Yongna Liu, Wei Li, Yu Gao

Liaoning Key Laboratory of Advanced Polymer Matrix Composites, Shenyang Aerospace University, Shenyang 110136, China

Correspondence to: B.C. Wang; e-mail: wang_baichen@126.com Contract grant sponsor: National Natural Science Foundation of China; contract grant numbers: 50703024; 51373102.

DOI 10.1002/pen.24807

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

Caption: FIG. 1. IR spectra of EMI/GO/CNTs in the ranges of (a) 4000-450 [cm.sup.-1] and (b) 1800-1000 [cm.sup.-1].

Caption: FIG. 2. Schematic representation of the three-phase contact angle ([theta]) of the as-synthesized EMI/GO/CNTs stabilizer at the MMA-water interface. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. Photograph and optical micrograph of Pickering emulsion. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 4. SEM micrographs at (a) low and (b) high magnification of EMI/ GO/CNTs encapsulated PMMA microspheres.

Caption: FIG. 5. Flexural and compressive strengths of PMMA/CO/CNTs nanocomposites as a function of initial GO concentration at the GO-to-CNTs mass ratio of 6:1. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 6. Flexural and compressive strengths of PMMA/GO/CNTs nanocomposites as a function of the ratio of GO/CNTs with EMI/GO/CNTs stabilizer level constant. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 7. SEM micrographs at (a) low magnification and (b) high magnification of fracture surface of PMMA/GO/CNTs nanocomposites.
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Author:Wang, Baichen; Liu, Yongna; Li, Wei; Gao, Yu
Publication:Polymer Engineering and Science
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Date:Nov 1, 2018
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