Synthesis of polymeric nanoparticles containing reduced graphene oxide nanosheets stabilized by poly(ionic liquid) using miniemulsion polymerization

Article abstract of DOI:10.1039/c6sm00269b

Polymeric nanoparticles containing reduced graphene oxide (rGO) nanosheets have been prepared by aqueous miniemulsion radical polymerization of methyl methacrylate (MMA) utilizing poly(ionic liquid) (PIL) as stabilizer to effectively disperse the rGO nanosheets in the monomer phase. The PIL that gave the best results in terms of rGO dispersibility was a block copolymer of the ionic liquid monomer 1-(2-methacryloyloxyethyl)-3-butylimidazolium bis(trifluoromethanesulfonyl)amide ([Mbim][TFSA]) and MMA, the concept being that the MMA units impart solubility in the MMA monomer droplets whereas the IL units act as adsorption sites for rGO. The rGO dispersibility in vinyl monomer was demonstrated to be superior using the above PIL block copolymer compared to the corresponding statistical copolymer or PIL homopolymer. Overall, the approach developed demonstrates how PILs can be employed to conveniently switch (turn ON/OFF) the dispersibility of PIL/rGO via anion exchange reactions, which can be an efficient strategy for synthesis of polymer/rGO nanocomposite materials.

Full text of DOI:10.1039/c6sm00269b

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Synthesis of polymeric nanoparticles containing
reduced graphene oxide nanosheets stabilized by
poly(ionic liquid) using
Cite this:DOI: 10.1039/c6sm00269b
miniemulsion polymerization
Masayoshi Tokuda,ab Mitsuyoshi Yamane,b Stuart C. Thickett,ac Hideto Minami*b
and Per B. Zetterlund*a
Polymeric nanoparticles containing reduced graphene oxide (rGO) nanosheets have been prepared by
aqueous miniemulsion radical polymerization of methyl methacrylate (MMA) utilizing poly(ionic liquid)
(PIL) as stabilizer to effectively disperse the rGO nanosheets in the monomer phase. The PIL that gave
the best results in terms of rGO dispersibility was a block copolymer of the ionic liquid monomer 1-(2-
methacryloyloxyethyl)-3-butylimidazolium bis(trifluoromethanesulfonyl)amide ([Mbim][TFSA]) and MMA,
the concept being that the MMA units impart solubility in the MMA monomer droplets whereas the
IL units act as adsorption sites for rGO. The rGO dispersibility in vinyl monomer was demonstrated to
be superior using the above PIL block copolymer compared to the corresponding statistical copolymer
or PIL homopolymer. Overall, the approach developed demonstrates how PILs can be employed to
conveniently switch (turn ON/OFF) the dispersibility of PIL/rGO via anion exchange reactions, which can
be an efficient strategy for synthesis of polymer/rGO nanocomposite materials.
Received 1st February 2016,
Accepted 21st March 2016
DOI: 10.1039/c6sm00269b
www.rsc.org/softmatter
it has attracted much attention as a functional material in
various fields. However, when graphene is used as filler in
Introduction
Polymer particles have been used in ubiquitous applications polymeric materials to enhance their properties, it is difficult to
such as coatings, adhesives, and films in industrial fields over disperse within both a solid matrix or in a solvent due to strong
several decades.1,2 Recently, the application of polymer particles van der Waals interactions between graphene layers, resulting
has expanded to intelligent materials in sophisticated industry in sheet aggregation. In order to improve the dispersibility of
fields, where polymer particles are used as for example drug graphene, chemical modification of graphene and the use of
transport carriers3–5 (e.g. drug delivery systems) and display surfactants have been reported.19–23
materials in electronic paper.6,7 In order to enable such applications
of polymer in the particle state, novel synthetic routes to polymer organic or halogen anion and exist in the liquid state at room
particles with multifunctional properties are required.
temperature.24–26 They have attracted attention due to their use
Ionic liquids are composed of an organic cation and an
Graphene is an allotrope of carbon consisting of an atomically as environmentally friendly media in various synthetic fields, as
thin two-dimensional ‘‘nanosheet’’ structure.8–10 In 2004, Novoselov well as functional materials such as electrolytes27–29 and CO2
and Geim first reported the preparation and properties of graphene, separation membranes.30–32 Recently, imidazolium-based ionic
in which graphene was prepared by mechanical exfoliation of liquids and poly(ionic liquid)s (PIL) prepared by the polymerization
graphite.8 Thereafter, many researchers have reported new of ionic liquid monomer have received attention as a potential
synthetic methods and further properties of graphene.11–15 As dispersant of graphene due to p–p interactions between imidazolium
graphene possesses various functionalities such as impressive rings and graphene sheets.33–35 Graphene dispersions could be
electrical, optical, thermal, and mechanical properties,16–18 obtained in various solvents using PIL as dispersant because of the
possibility to conveniently control the solubility of PIL by anion
a Centre for Advanced Macromolecular Design (CAMD), School of Chemical
Engineering, The University of New South Wales, Sydney, NSW 2052, Australia.
E-mail: p.zetterlund@unsw.edu.au
b Department of Chemical Science and Engineering, Graduate School of Engineering,
Kobe University, Rokko, Nada, Kobe 657-8501, Japan. E-mail: minamihi@kobe-u.ac.jp
c School of Physical Sciences (Chemistry), The University of Tasmania, Sandy Bay,
TAS 7005, Australia
exchange, as reported by Suh and coworkers.35 In this particular
case, graphene sheets modified by PIL were able to be transferred
between the water phase and the oil phase by anion exchange.
Miniemulsion polymerization is a useful method for preparation
of hybrid nanoparticles composed of organic and inorganic
materials, as well as hollow particles and various other
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structures36–39 including implementation of controlled/living and 1-ethylbromide. 1-Ethylbromide (0.15 mol) was added dropwise
radical polymerization.40 Several groups have reported composite into 1-vinylimidazole (0.13 mol) under vigorous stirring in methanol
polymer particles containing graphene oxide (GO), which is the (10 g). The mixture was poured into a Schlenk flask, degassed using
oxidized form of exfoliated sheets from graphite or graphene, several vacuum/N2 cycles, and then allowed to react in a heated
prepared by miniemulsion polymerization.41–45 We have successfully water bath at 40 1C overnight with constant magnetic stirring
prepared composite nanoparticles ‘‘armoured’’ with GO nanosheets (220 rpm). When the obtained solution was added dropwise
via miniemulsion polymerization, in which GO was used as the sole into diethyl ether (300 mL), a white solid material was obtained,
surfactant.46–52 Such polymer nanocomposite materials containing which was dried under vacuum and confirmed the structure by
GO can be a precursor to composite materials containing reduced 1H NMR. After the obtained [Veim]Br (0.11 mol, 20.5 g) was
graphene oxide (rGO), as the functionality of rGO is similar to that of dissolved in water (25 mL), an aqueous solution (25 mL) of
graphene. However, when reduction of GO to yield rGO is performed Li[TFSA] (0.12 mol, 28.7 g) was added to the [Veim]Br aqueous
in the nanocomposite particle state, it is expected that colloidal solution under vigorous stirring. After phase separation took
stability will be compromised due to the loss of polar functional place, the lower oily layer was collected, washed five times with
groups within the structure of GO (mainly carboxylic acid groups at distilled water and then dried under vacuum. [Veim][TFSA] was
the periphery of the sheets). In previous work, we demonstrated obtained as a pale yellow-colored liquid (yield: 93.0%) (Scheme 1).
the preparation of PIL particles containing rGO by miniemulsion
polymerization using rGO dispersed in an ionic liquid monomer.53
However, with regards to industrial applications, there is a desire to
Preparation of ionic liquid monomer (1-(2-methacryloyloxyethyl)-3-
butylimidazolium bis(trifluoromethanesulfonyl)amide:
[Mbim][TFSA], 517.46 g molꢀ1
)
develop methods applicable to conventional monomers such as
styrenic and methacrylate monomers. In the present work, the
preparation of rGO dispersions stabilized by poly(ionic liquid)
in a conventional vinyl monomer was investigated. Thereafter,
miniemulsion polymerization of the monomer dispersion was
performed to yield composite nanoparticles.
First, synthesis of [Mbim][TFSA] was carried out using MEIZ
and n-butylbromide. MEIZ (5.0 g, 27.7 mmol) and n-butyl-
bromide (4.0 g, 29.2 mmol) were mixed in methanol (10 g),
the mixture was poured into a Schlenk flask, degassed using
several vacuum/N2 cycles, and then allowed to react in a water
bath at 60 1C for 24 h with constant magnetic stirring (220 rpm).
After purification of the obtained solution into diethyl ether
(300 mL), a low viscosity yellow liquid was obtained, which was
Experimental
Materials
1
dried under vacuum. The structure was confirmed by H NMR
Methyl methacrylate (MMA, 99%, Aldrich) and ethyl methacrylate
(EMA, 99%, Aldrich) were purified by passing through a column of
activated basic aluminium oxide (Ajax) to remove the inhibitor. 2,20-
Azobis(isobutyronitrile) (AIBN, Aldrich) was purified by recrys-
tallization in methanol. 1-(2-Methacryloyloxyethyl)imidazole
(MEIZ) (495%, The Nippon Synthetic Chemical Industry Co.,
Ltd, Japan), hydrazine hydrate (Sigma Aldrich), 2-cyano-2-propyl
(yield: 65%). Anion exchange process to obtain [Mbim][TFSA]
was carried out in water using [Mbim]Br (5.7 g, 18 mmol) and
Li[TFSA] (4.8 g, 20 mmol) under the same conditions. [Mbim][TFSA]
was obtained as a pale yellow liquid.
Preparation of GO from graphite nanofibers
benzodithioate (CPBD) (97%, Sigma Aldrich), ammonia solution GO was prepared by the oxidation of graphite nanofibers
(28 wt%), graphite nanofibers (Catalytic Materials Ltd, 498%), following a previous report.50 Briefly, 2 g of graphite nanofibers
HCl (Ajax, 32 wt%), H2SO4 (Ajax, 98%), H3PO4 (BDH Chemical, was added to 250 mL of sulfuric acid and phosphoric acid (v/v =
85%), KMnO4 (Ajax), H2O2 (Ajax, 30 wt%), n-butylbromide, 1-ethyl- 9/1), followed by the slow addition of 12 g KMnO4 to the
bromide, 1-vinylimidazole, (Nakalai Tesque Inc., Kyoto, Japan), mixture in an ice bath with stirring (the temperature was kept
lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]) (99.7%, below 10 1C). The flask was placed in an oil bath at 45 1C for
Kanto Chemical Co., Inc., Japan) were used as received. Deionized 17 h with magnetic stirring at 400 rpm to oxidize the graphite
water was used in all experiments.
nanofibers. The mixture was poured onto ice (400 g), treated
with 15 mL of 30 wt% H2O2 aqueous solution and then purified
by centrifugation (8000 rpm, 8 min), followed by several washing
cycles of 3.4 wt% HCl aqueous solution (3 times), acetone
Preparation of ionic liquid monomer (1-vinyl-3-ethylimidazolium
bis(trifluoromethanesulfonyl)amide: [Veim][TFSA], 403.32 g molꢀ1
)
A typical synthesis of imidazolium-based hydrophobic ionic (3 times) and diethyl ether (2 times). The isolated material
liquid monomers was carried out as reported in previous was dried in a vacuum oven (40 1C) and stored in a desiccator
papers.54 Firstly, [Veim]Br was prepared from 1-vinylimidazole until use, and characterized by XPS and FT-IR spectroscopy.
Scheme 1 Structure of ionic liquid monomers.
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Preparation of poly([Veim][TFSA]) (PIL[TFSA]) and reduced
graphene oxide (rGO) composite materials
The reaction mixture was transferred to a round-bottom Schlenk
flask, sealed off with a silicon rubber septum, and purged with
nitrogen for 5 min. Solution RAFT polymerization was carried
out at 60 1C for 18 h. The polymer was isolated by precipitation
in hexane (300 mL) to yield PMMA (macroRAFT agent) as a pink
powder. The monomer conversion was determined to be 74%
via gas chromatography (GC-2014, Shimadzu Corp., Kyoto,
Japan) with helium as the carrier gas, THF as the solvent, and
p-xylene as the internal standard. The number-average molecular
weight (Mn) and molecular weight distribution (MWD) were
analyzed via gel permeation chromatography (GPC) at 40 1C
using two styrene/divinylbenzene gel columns [Tosoh Corp.,
TSK gel GMHHR-H, 7.8 mm i.d. ꢁ 30 cm] with THF as the
eluent, a flow rate of 1.0 mL minꢀ1, a refractive index (RI)
detector (TOSOH RI-8020/21). The columns were calibrated with
polystyrene calibration standards (MW = 1.05 ꢁ 103–5.48 ꢁ
106 g molꢀ1, Mw/Mn = 1.01–1.15). The theoretical molecular
weight (Mn,th) value was calculated using the following equation:
A solution of PIL[TFSA] was prepared by solution polymerization
in acetone. [Veim][TFSA] (5.0 g) and AIBN (0.5 g, 10 wt% based
on monomer) were dissolved in acetone (10 g), transferred to a
round-bottom Schlenk flask, sealed off with a silicon rubber septum,
and purged with nitrogen for 5 min. Solution polymerization was
carried out at 60 1C for 24 h with stirring.
PIL[TFSA]/rGO was prepared utilizing an anion exchange process
and chemical reduction of as-prepared PIL[Br]/GO composite
materials as outlined in a previous report.35 GO powder (10 mg)
was added to the PIL[TFSA]/acetone solution (0.11 g, 35 wt%).
After ultrasonication for 10 min, a GO dispersion in PIL[TFSA]/
acetone solution was obtained. Subsequently LiBr (0.2 g) was
added to this dispersion with stirring for 24 h at room temperature
to obtain PIL[Br]/GO composite, resulting in a brown precipitate.
The precipitate was washed with acetone (3 times), and then dried
at room temperature under vacuum, and subsequently analyzed by
XPS. The obtained PIL[Br]/GO powder (5 mg) was added to water
(10 g), and ultrasonicated for 1 h to prepare a PIL[Br]/GO aqueous
dispersion. The chemical reduction of PIL[Br]/GO occurred via the
addition of 10 mL of hydrazine hydrate and 20 mL of ammonium
solution (28 wt%), followed by heating and stirring at 95 1C for
30 min. The resulting dispersion was washed with water via
centrifugation (10 000 rpm, 20 min, 3 times), and dried at room
temperature under vacuum, and then analyzed with XPS. In
order to prepare PIL[TFSA]/rGO, Li[TFSA] (30 mg) was added to
the PIL[Br]/rGO aqueous dispersion, and reacted for 24 h at room
temperature. After washing with water (3 times) and drying in a
vacuum oven (40 1C), a black material was obtained, which was
analysed by XPS.
½Monomerꢂ
Mn;th ¼ MRAFT agent
þ
ꢁ Mmonomer ꢁ Conversion
½RAFTꢂ
where MRAFT agent denotes the molecular weight of the RAFT
agent or the macroRAFT agent. In this system, Mn and Mw/Mn of
the obtained PMMA are 17 000 g molꢀ1 and 1.13, respectively
(Mn,th = 15 000 g molꢀ1).
The obtained PMMA macroRAFT agent (0.2 g, 12 mmol),
[Mbim][TFSA] (0.5 g, 0.97 mmol) and AIBN (0.79 mg, 4.8 mmol)
were dissolved in acetone (3.0 g). The solution was transferred
to a round-bottom Schlenk flask, sealed off with a silicon
rubber septum, and purged with nitrogen for 5 min. Solution
polymerization was carried out at 60 1C for 18 h. The monomer
conversion was determined to be 73% via 1H NMR based on the
vinyl H of the monomer (5.1 and 6.1 ppm). Moreover, the
polymer was isolated by precipitation in ethanol (300 mL),
and Mn of the obtained block copolymer was calculated as
35 516 g molꢀ1 (poly(MMA165-b-[Mbim][TFSA]69)) based on
1H NMR in acetone-d6 by comparing peak integral of imidazolium
part (9.0 ppm) in [Mbim][TFSA] to methoxy groups (3.7 ppm) in
macroRAFT agent.
Preparation of poly([Veim][TFSA]-stat-MMA) statistical
copolymer by solution polymerization
[Veim][TFSA] (5.0 g, 12.4 mmol), MMA (0.53 g, 5.3 mmol) and
AIBN (0.55 g, 10 wt% based on monomer) were dissolved in
acetone (10 g), transferred to a round-bottom Schlenk flask,
sealed off with a silicon rubber septum, and purged with
nitrogen for 5 min. Solution polymerization was carried out
at 60 1C for 24 h. The product was precipitated in water/ethanol
(w/w = 50/50, 300 mL) to remove the residual monomer and
subsequently dried under vacuum at room temperature.
Miniemulsion polymerization with PIL copolymer and rGO
Poly([Veim][TFSA]-stat-MMA) (4.5 mg) and hexadecane (50 mg)
were dissolved in EMA (1.0 g) at 60 1C, followed by addition of
rGO (1.0 mg). After ultrasonication at 50% amplitude for 1 min,
rGO dispersed in EMA was obtained. After addition of AIBN
(40 mg), the dispersion was added to a 1 wt% Tween 80 aqueous
solution (15 g), followed by ultrasonication for 5 min. The obtained
emulsion was subsequently transferred to a round-bottom Schlenk
flask, sealed with a silicon rubber septum, and purged with
nitrogen for 5 min. Miniemulsion polymerization was carried
out at 60 1C for 24 h at 240 rpm stirring.
Preparation of poly([Mbim][TFSA]-stat-MMA) statistical
copolymer by solution polymerization
This polymer was prepared using [Mbim][TFSA] (6.42 g, 5.3 mmol),
MMA (0.53 g, 5.3 mmol) and AIBN (0.55 g, 10 wt% based on
monomer) following the same procedure as above, except that
precipitation was conducted in ethanol (300 mL).
Preparation of poly(MMA-b-[Mbim][TFSA]) block copolymer
by solution reversible addition-fragmentation chain transfer
(RAFT) polymerization
In the case of using poly(MMA-b-[Mbim][TFSA]), the block
copolymer (5.0 mg), hexadecane (40 mg) and AIBN (40 mg)
were dissolved in MMA (1.0 g) at room temperature, followed
MMA (2.7 g, 28 mmol), CPBD (30 mg, 140 mmol) as RAFT agent, by addition of rGO (1.0 mg). After ultrasonication at 50%
and AIBN (9.0 mg, 55 mmol) were dissolved in THF (6.0 g). amplitude for 20 min, rGO dispersed in MMA was obtained.
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Miniemulsion polymerization was carried out under the same
conditions as above.
Characterization
TEM images were obtained using a JEOL1400 transmission
electron microscope at an accelerating voltage of 100 kV. The
specimens were prepared by casting a drop of diluted aqueous
polymerized miniemulsion onto a Formvar-coated copper grid
followed by drying at room temperature. X-Ray photoelectron
spectra (XPS) were recorded using a Kratos Axis ULTRA XPS
using monochromatic Al X-rays (1486.6 eV) at 225 W (15 kV,
15 mA). Survey scans were carried out over 1360–0 eV binding
energy range with 1 eV steps and a dwell time of 100 ms; high
resolution scans were run with 0.2 eV steps and a dwell time
of 250 ms. The chemical structures of ionic liquid monomer
were confirmed by 1H NMR. The 1H NMR measurements
were carried out with a Bruker Avance 500 MHz spectrometer
at room temperature in D2O.
Fig. 2 Visual appearances (left) of PIL[Br]/GO dispersion in water before
(a) and after (b) chemical reduction. XPS wide scan spectra (right panel) of
PIL[Br]/GO before (a) and after (b) chemical reduction.
PIL[Br]/GO was readily dispersed in water, which enabled aqueous
phase chemical reduction of GO to be performed using ammonium
solution and hydrazine monohydrate at 90 1C. Fig. 2 shows the XPS
spectra of PIL[Br]/GO before and after reduction. The color of the
dispersion changed from dark brown to black, a visual cue that the
chemical reduction of GO had proceeded.
Typically, the XPS peak corresponding to atomic oxygen
should decrease when rGO is prepared from reduction of GO.
However, N and Br signals (present in PIL[Br]) decreased instead
of the O signal (present in GO), consistent with desorption of
PIL[Br] from the GO surface (Fig. 2b). It is proposed that
reduction of GO by hydrazine and ammonium aqueous solution
proceeds simultaneously with anion exchange of PIL[Br]. Hydroxide
(OH) ion and hydrogen carbonate (HCO3) ion derived from CO2
are present in the aqueous phase, and these ions may partake in
anion exchange reaction with PIL[Br], thus replacing Br. Anion
exchange of PIL[Br] would render the polymer more hydrophilic,
thus making it more prone to dissolution in water relative to
adsorption at the GO.
Due to the issues encountered with the above synthetic
route, PIL[TFSA]/rGO was instead synthesized via an alternative
process whereby rGO was first prepared via reduction of an
aqueous GO dispersion,53 followed by synthesis of PIL[TFSA]/
rGO utilizing a two-step anion exchange process (Fig. 3). As the
first step, rGO was dispersed in PIL[TFSA] acetone solution
by ultrasonication for 10 min (Fig. 3a), demonstrating that
PIL[TFSA] performed well as stabilizer of rGO (in the absence of
PIL[TFSA], rGO agglomerated (coagulation) instantaneously in
pure acetone after 10 min ultrasonication). PIL[TFSA]/rGO has
Results and discussion
Preparation of PIL[TFSA]/rGO composite
In its pure form, rGO cannot be effectively dispersed in hydro-
phobic monomers. In order to increase the level of compatibility,
rGO was modified with hydrophobic PIL. The hydrophobicity/
hydrophilicity of PIL can be tuned by changing the anion;
PIL[TFSA] is hydrophobic whereas PIL[Br] is hydrophilic. The
lateral dimensions of the GO sheets employed in this work were
approximately 100 nm (DLS). GO was ultrasonicated in a PIL[TFSA]
acetone solution according to a previous procedure,35 yielding a
stable brown dispersion (Fig. 1a). The reduction of GO with
hydrazine is generally performed in water, which thus requires
that PIL/GO can be dispersed in water. Anion exchange treatment
of PIL[TFSA]/GO with LiBr resulted in formation of the more
hydrophilic PIL[Br]/GO, which can be dispersed in water. As shown
in Fig. 1, a brown material precipitated after addition of LiBr to the
acetone dispersion. XPS analysis of the precipitate (Fig. 1, right),
revealed two strong signals corresponding to N and Br, consistent
with the formation of PIL[Br]/GO. The compositions calculated
from XPS on the basis of Wang’s report55 were 20.4 wt% (GO) and
79.6 wt% (PIL[Br]), which is close to the initial stoichiometry.
Fig. 3 Visual appearances (a–d) of synthesis process of PIL[TFSA]/rGO
composite materials utilizing anion exchange: (a) PIL[TFSA]/rGO dispersion
prepared by ultrasonication in acetone, (b) after addition of LiBr to
PIL[TFSA]/rGO dispersion in acetone, (c) aqueous dispersion of PIL[Br]/
rGO, and (d) after addition of Li[TFSA] to PIL[Br]/rGO aqueous dispersion.
Fig. 1 Visual appearances (left) of GO dispersion in PIL[TFSA] acetone
solution before (a) and after (b) addition of LiBr. XPS wide scan spectra
(right panel) of GO (a) and PIL[Br]/GO (b).
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Table 1 Dispersibility and solubility testsa of PIL[TFSA]/rGO and PIL[TFSA],
respectively, in various monomers
PIL[TFSA]/rGO
PIL[TFSA]
Styrene
MMA
EMA
ꢁ
n
n
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
n
n
ꢁ
ꢁ
ꢁ
ꢁ
LMA
BzMA
PhEMA
t-BA
a
ꢁ: not dispersible or insoluble, n: swelling.
10 min (Fig. 5a). The rGO remained dispersed in EMA when the
dispersion was added to Tween 80 aqueous solution (Fig. 5b),
with no observable rGO precipitate from the deep blue mini-
emulsion that was obtained after ultrasonication (Fig. 5c). After
miniemulsion polymerization using AIBN at 60 1C for 24 h, the
color of the miniemulsion remained grayish but significant rGO
precipitation was observed (Fig. 5d). As shown in Fig. 5e and f,
the diameter of the obtained particles was approximately 50 nm.
TEM imaging revealed enhanced contrast around the perimeter
of the particles, possibly indicating some rGO had precipitated
from inside particles (Fig. 5e). For comparison, a control
experiment was conducted without copolymer, in which case
rGO instantaneously precipitated in EMA, and the contrast around
the perimeter of the obtained particles after polymerization was
even more clearly observed (Fig. 5g). These results indicated that
Fig. 4 XPS wide scan spectra of rGO (a), PIL[Br]/rGO (b), and PIL[TFSA]/
rGO (c).
now been prepared – however, in order to isolate this composite
material as a solid, two anion exchange steps were performed.
First, after addition of LiBr to the dispersion, phase separation
occurred (Fig. 3b) due to the insolubility of the formed PIL[Br]
in acetone. Interestingly, the precipitate obtained after washing
with acetone could be dispersed instantaneously in water with-
out ultrasonication (Fig. 3c). Subsequently, when Li[TFSA] was
added to the aqueous dispersion, a black precipitate was
obtained due to anion exchange from the bromide (hydrophilic)
to [TFSA] (hydrophobic) anion (Fig. 3d). This precipitate (after
washing with water) could be dispersed in acetone without
ultrasonication, demonstrating how the dispersibility of PIL/
rGO can be readily switched utilizing anion exchange.
After the first anion exchange process (a) to (b) in Fig. 3, the
atomic percentages of N and Br increased compared to the original
rGO. These peaks correspond to PIL[Br], which accounts for
66 wt% of the total weight from calculation of the fractional
composition based on the XPS data. After the second anion
exchange process, the signal corresponding to Br disappeared,
coupled with the appearance of a signal from F and S (present
in the TFSA counterion). These results indicate that anion
exchange proceeded efficiently and the obtained material was
indeed PIL[TFSA]/rGO (Fig. 4).
Miniemulsion polymerization with PIL/rGO
Prior to miniemulsion polymerization, the dispersibility of
PIL[TFSA]/rGO in various monomers was investigated. Although
rGO precipitates instantaneously in common vinyl monomers,
PIL[TFSA]/rGO is expected to be dispersible in some monomers
similar to the acetone system described above. However, the
dispersibility was poor in most monomers, although swelling
was observed for MMA and EMA (Table 1).
To improve the solubility of PIL[TFSA] i.e. poly([Veim][TFSA])
in monomer, a statistical copolymer of [Veim][TFSA] and MMA
Fig. 5 Visual appearances of the synthesis process of polymer particles
containing rGO by miniemulsion polymerization: (a) rGO dispersion in
was prepared, and its solubility in various monomers was EMA/copolymer; (b) rGO dispersed monomer in Tween 80 aq. before and
(c) after ultrasonication; (d) obtained emulsion after miniemulsion poly-
subsequently examined. Poly([Veim][TFSA]-stat-MMA) (mol/mol =
70/30 assuming 100% monomer conversion) was soluble in EMA
at 60 1C up to 0.45 wt%. rGO was subsequently dispersed in EMA/
merization; (e) TEM photograph of obtained particles; (f) number-based
DLS distributions of diameter of obtained particles prepared by miniemulsion
polymerization of rGO-dispersed EMA; (g) TEM photograph of obtained
copolymer (0.45 wt% copolymer in EMA) by ultrasonication,
particles by miniemulsion polymerization of EMA/rGO droplets in Tween 80
resulting in a dispersion that remained stable for approximately aqueous solution.
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some rGO is expected to be located inside the particles in the with some minimal rGO precipitation at the bottom of the
presence of poly([Veim][TFSA]-stat-MMA). Nevertheless, most rGO emulsion (Fig. 6c). When miniemulsion polymerization was
appeared to have precipitated and was located at the bottom of the carried out using AIBN at 60 1C for 24 h, the miniemulsion
emulsion, demonstrating that the rGO stability in monomer using after polymerization was stable and the obtained particles were
this statistical copolymer was insufficient.
spherical based on TEM imaging (Fig. 6d and e). Quantitative
The concept of using poly([Veim][TFSA]-stat-MMA) to enhance analysis by gravimetry revealed that 30 wt% of the rGO had
the stability of rGO in monomer is that the MMA units impart precipitated before polymerization, and the amount of precipitated
solubility in the monomer whereas the IL units act as adsorption rGO remained constant throughout the polymerization. When
sites for rGO. From this perspective, a statistical copolymer removal of rGO was performed by centrifugation (10 000 rpm,
chain structure may have been far from optimal – a block 2 min) prior to miniemulsion polymerization, no rGO precipitate
copolymer with distinct IL and MMA segments would be was observed after polymerization, resulting in successful
expected to be more efficient.
formation of similarly sized polymeric nanoparticles (Fig. 6f
Block copolymer synthesis with MMA/IL monomer was there- and g). This result suggests that rGO precipitation mainly
fore conducted using RAFT polymerization with a methacrylate occurred during the ultrasonication step as opposed to during
ionic liquid monomer ([Mbim][TFSA]; [Mbim][TFSA] : [MMA] = the actual polymerization, with 70 wt% of the initial rGO being
70: 30 mol/mol) instead of [Veim][TFSA] (because [Veim][TFSA] is located inside the obtained particles.
an unconjugated monomer and thus difficult to polymerize
In order to confirm the existence of rGO inside the particles,
using RAFT). First of all, in order to investigate whether the the final emulsion was treated with acetone to dissolve PMMA
change in IL monomer type had any influence on the rGO following the same procedure previously reported.53 As shown
stability, the statistical copolymer poly([Mbim][TFSA]-stat-MMA) in Fig. 7a, the solution after acetone treatment exhibited the
was also prepared (same molar composition as poly([Veim][TFSA]- Tyndall effect, unlike acetone solutions of PMMA and PMMA-b-
stat-MMA)). Poly([Mbim][TFSA]-stat-MMA) exhibited better poly([Mbim][TFSA]) in the absence of rGO. This would be due to
solubility in monomer (soluble in MMA, and swollen by EMA) dispersed rGO stabilized by p–p interactions between rGO and
than poly([Veim][TFSA]-stat-MMA), but rGO/MMA in the presence PIL units of block copolymer in acetone after dissolution of the
of either copolymer remained stable for less than 10 min. The PMMA particles. The particle size distribution (DLS; Fig. 7b)
block copolymer PMMA-b-poly([Mbim][TFSA]) was soluble in before acetone treatment comprised nanometer-sized particles
EMA as well as MMA at room temperature, suggesting that the (Dn = 160 nm), consistent with the TEM imaging (Fig. 6g). After
monomer solubility was remarkably improved by changing the acetone treatment, significantly smaller nanoparticles (Dn = 27 nm)
structure from statistical copolymer to block copolymer. More- were detected, consistent with single rGO sheets. The size
over, a dispersion of rGO in MMA in the presence of the block reduction of rGO (compared to that of the initial GO) was due
copolymer (0.5 wt%; by ultrasonication) remained stable for at to further ultrasonication during the synthesis of rGO as well
least 12 h (Fig. 6a), i.e. a significant improvement compared as during the preparation of rGO dispersed in monomer.
to the systems using either statistical copolymer, despite the Based on a particle diameter of 160 nm and assuming circular
same PIL content in the block copolymer (70 mol%) as that disc-like rGO sheets of diameter 27 nm and considering
of the statistical copolymers (70 mol%). As shown in Fig. 6b, 70 wt% of the initially loaded rGO being located within the
rGO dispersed in monomer remained stable after addition polymer particles,53 one arrives at an average of 1.4 rGO sheets
of the rGO dispersion to a 1 wt% Tween 80 aqueous solution. per particle, i.e. one rGO sheet in every particles. From these
After ultrasonication, a milky-gray stable emulsion was obtained results, it has been demonstrated that PMMA composite
particles containing rGO using PMMA-b-poly([Mbim][TFSA])
were successfully prepared.
Fig. 6 Synthetic route to polymer particles containing rGO by miniemulsion
polymerization: (a) rGO dispersion in MMA/block copolymer, (b) rGO-
dispersed MMA in Tween 80 aqueous solution before and (c) after ultra-
sonication; (d) emulsion obtained after miniemulsion polymerization;
(e) TEM photograph of obtained particles; emulsion (f) and obtained
particles (g) after polymerization of emulsion which was removed free
rGO by centrifugation.
Fig. 7 (a) Observation of Tyndall phenomenon using a laser pointer
for (left) emulsion after acetone treatment and (right) acetone solution
dissolving PMMA and PMMA-b-poly([Mbim][TFSA]); and (b) number-
based DLS distributions of diameter of PMMA particles containing rGO
prepared by miniemulsion polymerization (line) and after acetone treatment
(dashed line).
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Conclusions
A novel approach towards synthesis of polymeric nanoparticles
containing rGO nanosheets (lateral dimensions of approximately
27 nm) within the particle interior has been developed. The
approach is based on aqueous miniemulsion radical polymerization
using the nonionic surfactant Tween 80. The challenge of effectively
dispersing the rGO nanosheets in the monomer phase (EMA or
MMA) was overcome by use of poly(ionic liquid) (PIL) as stabilizer.
The most effective stabilization was achieved using a block
copolymer of the ionic liquid monomer [Mbim][TFSA] and MMA,
the concept being that the MMA units impart solubility in the MMA
monomer droplets whereas the IL units act as adsorption sites
for rGO. The rGO dispersibility in vinyl monomer was demon-
strated to be far superior using the above PIL block copolymer
compared to the corresponding statistical copolymer or PIL
homopolymer. Overall, this work has demonstrated the pre-
paration of composite polymeric nanoparticles containing rGO
via the miniemulsion technique, with a view towards expanding
the range of applicable monomers that can be used to prepare
such nanocomposite materials.
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Acknowledgements
This work was partially supported by a Grant-in-Aid for Scientific
Research (Grant No. 26288103) from the Japan Society for the
Promotion of Science (JSPS) and by a Research Fellowship of JSPS
for Young Scientists (given to M. T.).
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