Facile one-pot method of initiator fixation for surface-initiated atom transfer radical polymerization on carbon hard spheres

Article abstract of DOI:10.1002/pola.26728

An efficient and novel one-pot process is developed to immobilize the atom transfer radical polymerization (ATRP) initiators onto the surface of fully pyrolyzed carbon hard spheres (CHSs) via a radical trapping process from the in situ thermal decomposition of bis(bromomethylbenzoyl)peroxide. The CHSs do not require any additional preparative treatment prior to the initiator immobilization. Styrene and methyl methacrylate are polymerized onto initiator-immobilized CHSs by surface-initiated atomic transfer radical polymerization (SI-ATRP). Samples are characterized using Fourier transform infrared, thermogravimetric analysis, scanning electron microscopy, and transmission electron microscopy. These methods of characterization confirmed that all the CHSs are coated with a uniform layer of grafted polymer. This efficient, one-pot immobilization of ATRP-initiators represents an exceptionally simple route for the rapid preparation of various polymer-coated carbon-based nanomaterials using SI-ATRP.

Full text of DOI:10.1002/pola.26728

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Facile One-Pot Method of Initiator Fixation for Surface-Initiated Atom
Transfer Radical Polymerization on Carbon Hard Spheres
1
2
2
3
2
Muhammad Ejaz, Bhanukiran Sunkara, Lakhinder Kamboj, Jibao He, Vijay T. John,
2
1
Noshir S. Pesika, Scott M. Grayson
1
Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States
2
3
Department of Chemical and Biomolecular Engineering, Tulane University, New Orleans, Louisiana 70118, United States
Coordinated Instrument Facility, Tulane University, New Orleans, Louisiana 70118, United States
Correspondence to: S. M. Grayson (E-mail: sgrayson@tulane.edu)
Received 1 February 2013; revised 28 March 2013; published online 22 May 2013
DOI: 10.1002/pola.26728
ABSTRACT: An efficient and novel one-pot process is devel-
These methods of characterization confirmed that all the
oped to immobilize the atom transfer radical polymerization
CHSs are coated with a uniform layer of grafted polymer. This
efficient, one-pot immobilization of ATRP-initiators represents
an exceptionally simple route for the rapid preparation of
various polymer-coated carbon-based nanomaterials using
(
ATRP) initiators onto the surface of fully pyrolyzed carbon
hard spheres (CHSs) via a radical trapping process from the
in situ thermal decomposition of bis(bromomethylbenzoyl)-
peroxide. The CHSs do not require any additional preparative
treatment prior to the initiator immobilization. Styrene and
methyl methacrylate are polymerized onto initiator-immobi-
lized CHSs by surface-initiated atomic transfer radical poly-
merization (SI-ATRP). Samples are characterized using Fourier
transform infrared, thermogravimetric analysis, scanning elec-
tron microscopy, and transmission electron microscopy.
SI-ATRP. V
2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A:
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Polym. Chem. 2013, 51, 3314–3322
KEYWORDS: atom transfer radical polymerization; carbon hard
spheres; nanomaterial; polymer grafts; polystyrene; poly
(methyl methacrylate)
INTRODUCTION Carbon hard spheres (CHSs) represent a ver-
satile alternative to traditional metallic or metallic oxide
nanoparticles because of their robust character, their unique
electronic properties, and their cost-efficient preparation.
Well-defined CHSs can be obtained from a wide range of
readily available carbon-containing precursors such as meth-
method is a relatively “green” process, as the many of pre-
cursors are biomass-based or are byproducts of petroleum
refining and their preparation occurs in pure water at mild
temperature without the presences of any hazardous surfac-
1
3
tants or catalysts. Because of their solid nature, spherodial
geometry, thermal stability, chemical purity, and lightweight
1
2
3
4
5
14,15
ane, glucose, polystyrene (PSt), coal, kerosene, and
properties (lighter than graphite and diamond),
CHSs
6
potato starch using various preparative techniques includ-
show promise as reinforcing agents in light weight compo-
sites or “nano-ball bearings” for lubrication applications.
However, despite exhibiting attractive mechanical properties,
CHSs’ incompatibility with most solvents and bulk polymers
results in unstable suspensions, and has prevented their
broad utilization. The surface modification of nanoparticles
enables their physical properties to be tuned, including wett-
ability, solubility, and dispersibility, and therefore is critical
7
8
ing arc-discharge processes, laser ablation, chemical vapor
9
10
deposition, pressure carbonization,
treatment. Many of these methods have been optimized to
yield particles with fine control over the particle diameter
and hydrothermal
1
1
(
typically from tens of nms to single lms) as well as rela-
1
2
tively narrow size distributions. Because of the versatility
demonstrated in the preparation of CHSs, they have potential
for a broad range of applications, including adsorbents, sepa-
ration media, catalyst supports, anode material for lithium
ion batteries, reinforcement in composite materials, and
1
6
to expanding the applications of CHSs. Although adsorbed
surfactants can assist in compatibilizing the CHSs, such sur-
factants can be readily desorbed, resulting in long-term sta-
bility problems leading to aggregation and/or flocculation.
The most robust method for improving the dispersibility of
CHSs is their covalent functionalization with polymer grafts
while the diversity of polymers amenable to grafting offers a
broad range of compatibility. With appropriate polymer
1
2
additives in lubricants. Recent optimization has enabled
the rapid synthesis of narrow polydispersed CHSs on a large
scale via a hydrothermal carbonization (HTC) process. In
addition to providing narrowly disperse CHSs with fine con-
trol over the size (between 100 and 2000 nm), the HTC
V
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grafts, CHSs are expected to yield homogeneous mixtures
with a range of polymers, yielding films or bulk materials
with enhanced physical and electronic properties. In addi-
tion, polymer-grafted CHSs can be optimized for dispersions
with a range of liquids, enabling their use as “nano-ball
of surface reactive sites. Unlike previous conventional immo-
bilization techniques of initiator, our radical-based strategy
does not involve any harsh oxidizing acidic conditions and
can be achieved in a single step via radical fixation.
1
7
A radical fixation route is an ideally suited method for sur-
face immobilization onto CHSs. The CHSs have many open-
ended graphitic flakes and an abundance of defects on the
bearing” additives for lubricants. The hardness and chemi-
cal robustness of the polymer-grafted CHSs, in addition to
their small but uniform size, should yield a high performance
lubricant with enhanced resilience at both high temperatures
4
0
surface, which give them high surface chemical activity. It
is well known fact that carbon materials like carbon black
1
8
and high pressures.
4
1
are strong radical scavengers.
workers
Tsubokawa and co-
4
2,43
reported that radical polymerizations in the
The surface modification of CHSs by covalent attachment of
polymer chains to the substrate surface can be achieved via
presence of carbon black were inhibited and/or retarded,
because of the surface trapping of primary radicals and
growing polymer radicals by carbon black. Although part of
the polymer could be grafted onto the surface, a very low
percentage of polymer mass could be grafted, because of
preferential trapping of low molecular weight primary radi-
cals formed by the decomposition of initiator. However, this
radical trapping behavior can also be used to efficiently func-
tionalize the surface of graphitic materials. For example, Ying
1
9,20
either a “grafting-to” or “grafting-from” approach.
The
“
graft to” approach, generally results in low graft densities
2
1
due to steric complications. In the case of the “grafting
from” approach, also known as surface-initiated polymeriza-
tion (SIP), the polymer is grown from initiator attached to
the substrate and can yield much more densely packed poly-
mer brushes. Among the SIP methods developed, surface-ini-
tiated controlled/“living” radical polymerizations offer both
control over the size of the polymer grafts as well as broad
4
4
et al. reported the derivatization of small-diameter single
walled carbon nanotubes via free radicals generated by the
decomposition of benzoyl peroxide in the presence of alkyl
2
2–24
compatibility with a range of monomers.
Among the dif-
ferent controlled radical polymerization techniques, surface-
initiated atom transfer radical polymerization (SI-ATRP) has
been used most frequently to produce polymer brushes,
because of its chemical versatility: it exhibits compatibility
with a variety of monomers and functional groups, it toler-
ates impurities, and it is relatively insensitive toward resid-
ual traces of oxygen. SI-ATRP enables the efficient synthesis
of well-controlled polymer brushes with narrow molecular
weight distributions on a wide range of surfaces, including:
4
5
iodide. Terrones and coworkers also recently reported the
two-step fixation of ATRP initiators on single-walled carbon
nanotubes. Therefore, we have explored taking advantage of
the radical-trapping ability of the CHS surface to immobilize
the initiator groups, while quenching much of this activity to
minimize the inhibiting effect during subsequent SI-ATRP.
Herein, we describe CHSs with well-defined polymer grafts
by carrying out SI-ATRP from pristine CHSs in only two syn-
thetic steps. In this work, CHSs were produced by the hydro-
thermal process from aqueous sucrose solution. We
developed a one-step method for covalent immobilization of
an ATRP initiator with benzyl bromide moieties on the fully
pyrolyzed CHSs without any prior functionalization. The ini-
tiator-coupled CHSs could be used directly to prepare graft
polymer chains via SI-TARP, including PSt and poly(methyl
methacrylate). This work represents one of the most efficient
methods yet reported for the grafting of ATRP initiators
from carbonaceous materials. Our strategy of fixation of ini-
tiator has three major advantages: it avoids acid treatment,
it involves only one step for initiator immobilization and it
minimizes the inhibiting/retarding effect of CHSs in SI-ATRP.
The synthetic efficiency of this route should broaden the
commercial viability of polymer grafted nanoparticles for a
variety of materials applications.
2
5
26
27
28
29
silicon, clay, gold nanorods, magnetite, silica, poly-
3
0
31
mer films, and cellulose. In addition to preparing well-
defined homopolymer grafts, if the ATRP end groups are
retained at the terminus of the polymer grafts, they can be
reinitiated to yield block copolymer grafts. These combined
advantages of SI-ATRP enable the surfaces of particles to be
readily tailored to exhibit the desired physicochemical prop-
erties for a wide range of applications. Although SI-ATRP
has already been used in an effective way for the surface
modification of nanostructured carbon materials such as car-
bon black nanoparticles, carbon nanotubes, CHSs, and gra-
phene/graphene oxide,
preparation of the surface. For example, recently Gao and co-
3
2
3
3
3
4–38
these involved a multistep
3
9
workers functionalized the pure CHSs surface with ATRP
initiator in four steps to produce polymer grafted CHSs by
SI-ATRP. First, the pristine CHSs were oxidized with concen-
trated nitric acid, producing surface carboxylic acid groups,
which were then reacted with SOCl2 in the second step to
produce acid chloride functionalities, in the third step those
functionalities were treated with an excess of ethylene glycol
to yield surface hydroxyl groups and finally in the fourth
step the initiator functionalized CHSs were prepared by
esterification of the alcohol groups with 2-bromo-2-methyl
propanoyl bromide. The harsh acidic conditions used may
lead to degradation of the CHS core, while the low efficiency
of multistep solid phase reactions results in lower densities
EXPERIMENTAL
Materials
Styrene (99%, Sigma-Aldrich) was purified by passing
through neutral alumina. Methyl methacrylate (99%, MMA)
was purified by passing through basic alumina. Copper (I)
bromide (98%, Sigma-Aldrich) was purified by stirring over
boiling glacial acid (Fisher Scientific) then washing the solid
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and 5 g of anisole in a 25 mL Schlenk flask equipped with a
stir bar. After a 30 min Ar purge, 72 mg (0.50 mmol) of CuBr
was added to above suspension under an Ar flow followed by
addition of 119 mg (0.68 mmol) of PMDETA and 17.00 mg
(
0.096 mmol) of MBB as free initiator. The ratio of the initial
SCHEME 1 Synthetic scheme of bis-(bromomethylbenzoyl)per-
monomer concentration ([M] ) and initiator concentration
oxide BBMBPO. Reagents and conditions: (a) Diethylether/
0
ꢀ
([I] ) ratio was 500. The flask was placed in a thermostatic
oil bath at 95 C and the suspension was stirred for 29 h in
water, 0 C to room temperature, 2 h.
0
ꢀ
an Ar atmosphere. The product was then diluted with THF.
The PSt-grafted CHS particles were isolated from free PSt by
repeated dispersal in THF by bath sonication, centrifugation,
and decanting the free PSt solution. Trace amounts of CuBr
were removed by additional washes of MeOH/THF, aqueous
NH Cl/THF and H O/THF solvents. Finally, the CHSs-PS par-
residue with ethanol and diethyl ether, and finally drying
0
00 000
overnight under vacuum. N,N,N ,N ,N -pentamethyldiethyle-
0
0
netriamime (PMDETA, 99%, Sigma-Aldrich) 4,4 -dinonyl-2,2 -
dipyridal (DNDP, 97%, Sigma-Aldrich), 4-methylbenzyl bro-
mide (MBB, 97%, Aldrich), sodium peroxide (97%, Sigma-
Aldrich), 4-bromomethylbenzoyl bromide (BMBB, 96%,
Sigma-Aldrich), sucrose (ACS reagent Sigma-Aldrich), and all
other reagents were used as obtained from commercial sour-
ces without additional purification. The CHSs were produced
4
2
ticles were vacuum dried overnight. The SI-ATRP of styrene
on CHSs was also carried out in absence of the free initiator
by following the procedure described above but omitting the
addition of MBB. The [M] /[I] was 18,000.
1
1
0
0
by the hydrothermal process,
whereby 0.5 M aqueous
solution of sucrose was hydrothermally treated in pressure
cylinders at 190 C for 6 h. The resultant suspension was
ꢀ
Surface-Initiated Atomic Transfer Radical
Polymerization of MMA
centrifuged, washed repeatedly with water and ethanol and
ꢀ
15.00 mg of ATRP initiator functionalized CHSs were dis-
persed in mixture of 7.5 g (75.0 mmol) of MMA monomer
and 7.5 g of anisole in 25 mL Schlenk flask equipped with a
stir bar and purged with Ar for 30 min. 21.4 mg (0.15 mmol)
of CuBr was added to above mixture under an Ar flow fol-
lowed by addition of 122.2 mg (0.30 mmol) of DNDP and
oven dried at 50 C. The samples were then carbonized at
ꢀ
1
000 C in a tube furnace under N atmosphere for up to 24 h.
2
Synthesis of Bis(bromomethylbenzoyl)peroxide
The initiator precursor bis(bromomethylbenzoyl)peroxide
46,47
(BBMBPO) was synthesized as follows (Scheme 1)
: 5.00 g
27.7 mg (0.15 mmol) of MBB as free initiator. The [M] /[I]0
0
(17.98 mmol) of BMBB in 40 mL of diethyl ether was added
was 490. The flask was placed in a thermostated oil bath at
dropwise to 8.24% aqueous solution of 1.4 g (17.98 mmol) of
ꢀ
ꢀ
95 C and the mixture was stirred for 2 h under an Ar. The
sodium peroxide while stirring at 0 C Upon completion of the
mixture was then diluted with THF. The CHS-PMMA particles
were isolated from free PMMA by repeated dispersal in THF
by bath sonication and centrifugation as described above. The
trace amounts of CuBr were removed with solvent washes as
described above. The CHSs-PMMA particles were finally vac-
uum dried overnight. The SI-ATRP of MMA on CHSs was also
done in absence of the free initiator as described above but
addition of BMBB, the reaction mixture allowed to warm to
room temperature and stir for an additional 2 h. The white pre-
cipitate was filtered, washed with water and diethyl ether and
dried under vacuum for about 2 h. The crude product was
recrystallized from chloroform to form white solid in 51% yield.
1
H NMR (CDCl , d, ppm) 4.5 (s, 4H, CH Br), 7.5 (s, 8H, ArH),
3
2
8.0 (8H, ArH).
without addition of MBB. The [M] /[I] was 19,000.
0 0
Immobilization of ATRP Initiator onto CHSs
by Radical Coupling
Instrumentation
1
H NMR (400 MHz) spectrum was obtained on a Varian Mer-
cury spectrometer (Palo. Alto, CA) using the TMS peak for
calibration. Size exclusion chromatography was carried out
on Waters model 1500 series pump (Milford, MA) with
three-column series from Polymer Laboratories, consisting of
10 milligram of CHSs were dispersed in 50 mL of toluene
and sonicated for 20 min. 0.75 g (1.75 mmol) of BBMBPO
was then added to above mixture. After purging with Ar for
ꢀ
30 min, the reaction was initiated by heating to 105 C and
2
stirred vigorously under Ar for 6 h. Afterwards, the modified
CHSs were collected by repeated dispersal into solvent by
sonication, and then centrifugation. The solvent series used
was first toluene, then dichloromethane, then tetrahydrofu-
ran (THF), and then methanol. The cleaned CHSs were then
dried under vacuum overnight.
PLgel 5 lm Mixed C (300 3 7.5 mm ) and PLgel 5 lm 500
Å (300 3 7.5 mm ) columns. The system was fitted with a
2
Model 2487 differential refractometer detector. THF was
used as mobile phase (1 mL/min flow rate). The calculated
molecular weight was based on calibration using linear PSt
standards. Data was collected and processed using Precision
Acquire software. The Fourier transform infrared (FTIR)
spectroscopy was performed using a NEXUS 670 FT-IR SEP.
Analyte was mixed with KBr and ground into a fine powder
by mortar and pestle and compacted into a pellet by apply-
ing pressure. Thermogravimetric analysis (TGA) data was
acquired using TA Instruments TGA 2950 Thermogravimetric
Surface-Initiated Atomic Transfer Radical Polymerization
of Styrene
The initiator functionalized CHSs were used to initiate the
ATRP, and a representative example (with styrene monomer)
is provided below. 10.00 mg of initiator-functionalized CHSs
were dispersed in mixture of 5 g (48.01 m mol) of styrene
ꢀ
Analyzer with heating rate of 10 C per minute under N2
3
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atmosphere. Data was processed using TA Instruments Uni-
versal Analysis software. Transmission electron (TEM) mi-
croscopy data was collected with a JEOL 2010 operated at
200 kV. Field emission scanning electron microscopy (SEM)
data was obtained using Hitachi S-4800, operated at 20 kV.
The SEM samples were prepared by putting the fine solid
powder of particles onto carbon conducting tape.
RESULTS AND DISCUSSION
Synthesis of CHSs
Spherical, porous CHSs were prepared by a hydrothermal
1
1
method.
The measured Brunauer–Emmett–Teller surface
2
area was around 320 m /g and average pore size measured
from the high resolution TEM (HRTEM) image was of about
0.4–1.5 nm.
Immobilization of Bis(bromomethylbenzoyl) Peroxide
BBMBPO) on CHSs
FIGURE 1 FTIR spectra of (below) pristine CHSs and, (above)
(
CHSs-BBMBPO.
To carry out SI-ATRP on CHSs, the ATRP initiator must first
be immobilized on surface of CHSs. Generally the ATRP initia-
tors are immobilized on carbon materials in multiple steps. In
rapid route for the surface attachment of ATRP initiators and
is particularly useful for large-scale syntheses.
4
0,47
this work, we explored free radical addition of BBMBPO
as a 1 step initiator fixation process to the CHS surface
Scheme 2). It has been reported that graphitic surfaces have
The surface attachment of initiator could be confirmed by
infrared spectroscopy and TGA. In the FTIR spectra of CHSs,
there are few characteristic signals, as expected for a fully
pyrolyzed graphitic surface [Fig. 1(a)]. However after the
reaction with BBMBPO, the spectrum [Fig. 1(b)] of modified
CHSs exhibits characteristic peaks, including a carbonyl band
(
4
8
ꢀ
a reproducible free radical acceptor ability. At 105 C,
BBMBPO decomposes into free radicals that can form covalent
bonds directly with the conjugated graphitic surface, as well
as defect sites. Similar reactivity has been noted for the radi-
4
4,45,49
cal surface grafting of multi-walled nanotubes.
As a
2
1
21
located at 1730 cm and an ester CAO band at 1270 cm
.
result of this reaction, benzyl bromide functionalities are cova-
lently attached onto the surfaces of CHSs that can act as ini-
Finally, the presence of aromatic rings is suggested by the
2
1
5
0
very weak signal located at 735 cm , representing the reso-
nance of aryl hydrogen atoms.
tiators for SI-ATRP of styrene and MMA
(Scheme 2).
Unreacted initiator was washed from the particles by
repeated suspension, centrifugation, and decanting of the su-
pernatant. The dispersibility of particles after treating with
coupling agent was improved significantly in organic solvents
such as toluene or anisole. This method represents a simple,
The grafting of the initiator could be further confirmed by
the addition of pyrolyzable weight onto the CHSs during TGA
studies. The thermograms of the pristine CHSs and initiator
immobilized CHSs are shown in Figure 2. The pristine CHSs
SCHEME 2 Stepwise SI-ATRP process for grafting polymer
brushes from CHSs.
FIGURE 2 TGA curves of (a) pristine CHSs, (b) CHSs-Br.
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FIGURE 3 SEM images of (a) Pristine CHSs, (b) CHSs-BMBPO and (c) CHSs-PSt-FI. TEM images of (d) Pristine CHSs (e) CHSs-
BBMBPO, and (f) CHSs-PSt-FI.
exhibit a good stability at high temperatures, with only 2%
weight loss at 800 C in an inert atmosphere. However,
negligible compared to the ꢁ500-nm diameter nanoparticles.
These data confirm that the surface immobilization of the
ATRP initiator can be achieved with a substantial amount of
surface functionalities, but without substantially altering the
morphology or integrity of the CHSs substrate.
ꢀ
BBMBPO-immobilized CHSs (CHSs-Br) exhibit a measurable
ꢀ
ꢀ
loss of mass between 300 and 500 C with 157 C onset
ꢀ
temperature point and 405 C inflection temperature point
that represents about 5% of the sample’s total weight. The
initiator concentration and density calculated from this 5%
weight loss difference from the modified CHSs to pristine
CHSs indicate approximately 2.8 initiator moieties per 1000
carbons or 0.233 mmol per g. This concentration is very
near to those reported in similar studies of CHSs (3.4 initia-
tors per 1000 carbons, or 0.268 mmol per g) that required
SI-ATRP of Styrene and MMA from Initiator-Immobilized
CHSs
The SI-ATRP was tested via the polymerization of two com-
mon monomers, namely, styrene (St), and MMA. Both were
ꢀ
polymerized in anisole solvent at 95 C from the initiator-
functionalized CHSs using the CuBr/PMDETA catalytic sys-
tem for 29 h or the CuBr/ DNDP catalytic system for 2 h for
St and MMA, respectively. In both cases the polymerizations
were carried out twice, once in the presence of an analogous,
unimmobilized free initiator, MBB and once without it (but
otherwise identical polymerization conditions). The sacrificial
initiators are typically used in SI-ATRP to control the poly-
merization. The generated soluble polymer can also be used
as an indirect measurement of molecular weight and polydis-
persity index of the surface grafted polymer chains, assum-
ing that the kinetics of surface polymerization is similar to
3
9
multiple steps for initiator fixation. The porous CHSs used
2
in this study have surface area 320 m /g and average pore
sizes of about 0.4–1.5 nm (measured from image of the
HRTEM microscope). From the above data the calculated sur-
2
face initiator density is about 0.47/nm , which includes the
surface of the nanopores within the overall surface area
measurements. FTIR analysis provides further evidence of
surface grafting, as the change in IR spectra upon reaction
with BBMBPO corresponds to a combination of grafted side
wall aromatic rings, and the distorted fragments of the exter-
nal CHSs layers. In addition, the increased tendency of the
modified CHSs to form suspensions in organic solvents such
as toluene, anisole, chloroform and THF further demon-
strates that the CHSs’ surface has been modified to a sub-
stantial extent with organic moieties.
5
1–53
that in solution by fast exchange process.
The free poly-
mer formed in the solution was separated from the grafted
CHSs by centrifugation, and characterized by GPC. A molecu-
lar weight of 81,000 with a PDI around 1.43 was observed
by GPC for the free PSt produced in the SI-ATRP experiment
in the presence of the sacrificial initiator (ꢁ80% conversion
of monomer) while molecular weight of 144,000 with a PDI
around 1.60 was detected by GPC for free PSt produced in
SI-ATRP experiment without any sacrificial initiator (ꢁ20%
conversion of monomer). The latter soluble polymer was
formed by adventitious solution phase initiation. There is
strong possibility of thermal initiation of monomer in solu-
tion. For the PMMA polymerizations, a molecular weight of
46,000 with a PDI around 1.37 was observed for free PMMA
The field emission SEM and TEM studies, on the other hand,
show little change in the size or morphology upon surface
modification. As prepared, the CHSs appear as aggregates
that could be clearly visualized in both the SEM [Fig. 3(a)]
and TEM [Fig. 3(d)]. Upon initiator fixation, there was no
obvious change in structure observed in either the SEM [Fig.
3(b)] or the TEM [Fig. 3(e)]. These results are expected, as
the sub-nanometer thickness of the initiating groups are
3
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PSt-NFI, Fig. 4(a)] and with added free initiator [CHSs-PSt-FI,
Fig. 4(b)] including the aromatic C@C ring stretches (1453
21
and 1493 cm ). For PMMA-grafted CHSs without [Fig. 5(a)]
and with [Fig. 5(b)] free initiator, strong ester carbonyl signals
2
1
were observed at 1730 cm . The presence of dense polymer
grafts on the CHS surfaces could be further confirmed by their
substantially improved dispersibility in good solvents for the
corresponding polymer grafts, such as THF or CHCl3.
A control experiment was carried out in which pristine
CHSs, without immobilized ATRP initiator, were subjected to
identical polymerization conditions as the above graft poly-
merization from CHSs. FTIR studies suggest that no grafted
polymers were formed. The spectra of pristine particles
were almost identical before and after these control
experiments.
To quantify the amount of grafted polymer relative to the
CHS core, TGA was used to characterize the CHSs-polymer
grafted samples. All of initiator moieties and polymers are
FIGURE 4 FTIR spectra of (a) CHSs-PSt-NFI, (b) CHSs-PSt-FI,
ꢀ
assumed to be completely pyrolyzed at 800 C leaving only
and (c) PSt.
the residual CHS cores. Figure 6(a,b) indicate that the grafted
ꢀ
ꢀ
PSt decomposes around 500 C with 310 C onset tempera-
ꢀ
ture point and 480 C inflection temperature point from
produced from sacrificial initiator (ꢁ60% conversion of
monomer) while a molecular weight of 171,000 with a PDI
around 1.57 was detected by GPC for free PMMA produced
in the absence of sacrificial initiator (ꢁ15% conversion of
monomer). The latter soluble polymer was formed by adven-
titious solution phase initiation, though the source of initia-
tion is unclear. The reduced polydispersity and polymer
molecular weights observed with sacrificial initiator implies
that for both monomers the polymerization is more con-
trolled by the addition of sacrificial initiator than without it.
It should be noted here that when there was low concentra-
tion of overall initiators (both immobilized and free), and
both of the PSt grafted CHSs which is comparable to the
decomposition of pure PSt with an onset temperature of 387
C and an inflection temperature point at 465 C [Fig. 6(c)].
After 29 h of polymerization in the presence of free initiator,
it can be concluded that PSt was grown from CHSs-PSt-FI,
representing about 9.1% of the particles overall mass [Fig.
ꢀ
ꢀ
6(a)]. For the PSt-grafted CHSs prepared in the absence of
free initiator, an increased mass of grafting (17.47% PSt by
mass) was grown from CHSs-PSt-NFI, [Fig. 6(b)] during the
same time of polymerization. The above results show that
the amount of grafted PSt on CHSs can be controlled simply
by tuning the amount of sacrificial initiator added without
changing other parameters. Furthermore, Figure 7(a,b)
higher ratios of monomer to initiator, there was little amount
II
of deactivating Cu /Br species generated, which resulted in
2
polymer with higher molecular weight and broad PDI.
When the concentration of overall initiator was increased by
adding free sacrificial initiator, there is more control over po-
lymerization and molecular weight control during ATRP can
5
4
be achieved via the persistent radical effect. From “free ini-
tiator” to “no free initiator” polymerization there is signifi-
cant increase in molecular weight of free polymer. The
polymer grafted particles demonstrated improved colloidal
stability in organic solvents (good solvents for polymer co-
rona) than bare particles which exhibited negligible compati-
bility in all solvents.
The bromobenzyl initiator was selected because of its effi-
cient mode of attachment onto the CHSs, however, a more ef-
ficient initiator will likely yield improved graft densities,
increased molecular weights, and reduced polydispersities.
The polymer grafted CHSs were characterized qualitatively by
FTIR spectroscopic studies to verify the presence of surface-
bound polymer. Characteristic PSt signals were observed in
the FTIR spectra of PSt without added free initiator [CHSs-
FIGURE 5 FTIR spectra of (a) CHSs-PMMA-NFI, (b) CHSs-
PMMA-FI and (c) PMMA.
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fraction) the polymer graft densities on the CHSs can be cal-
culated. The computed values are 0.0021 and 0.0023 chains/
2
nm for CHSs-PSt produced in the presence of free sacrificial
initiator and without it respectively. Similarly graft densities
22
of PMMA were calculated: 0.0068 and 0.0039 chains nm
with and without sacrificial initiator, respectively. One possi-
ble reason for this low density of polymer brushes relative
2
2 55
to PSt brushes on silicon wafers (about 0.40 chains nm
)
and PMMA brushes on gold nanoparticles (about 0.30 chains
2
2 51
nm
)
might be the presence of nanometer sized pores on
the surface of CHSs resulting in narrow inner pore’s surface
areas that are incapable of grafting large polymer chains. It
should be clarified that the above graft density calculations
include the surface area of any pores or cavities that may be
present on the outer surface. Because the values of graft
densities for both PSt and PMMA systems have same order
of magnitude, it is believed that the technique is highly re-
producible, and the incorporation of unbound, “free initiator”
has little effect on the overall graft densities.
FIGURE 6 TGA curves of (a) CHSs-PSt-FI (b) CHSs-PSt-NFI, and
(
c) PSt.
indicates that the grafted PMMA decomposes with an onset
temperature of about 256 C and an inflection point at 445
SEM and TEM studies enable us to investigate the nanoscale
morphologies and the surface structure of polymer grafted
CHSs. Figure 3(b,c) present typical SEM images of CHSs ini-
tiator immobilized and CHSS-PSt-FI, respectively. In both
images, CHSs are with almost same overall shape as pristine
CHSs. In TEM observations, the polymer phase is brighter
than the CHSs phase because more electrons are transmitted
through the polymer regions than through the CHS regions.
A typical TEM image of polymer-grafted CHSs is shown in
Figure 8(b). The CHSs are completely enveloped by a visible
continuous and uniform smooth film of grafted PSt as com-
pared to initiator-grafted CHSs [Fig. 8(a)]. Similarly results
were observed in TEM studies of CHSs-PMMA revealing a
continuous and uniform smooth film of grafted PMMA. In
HRTEM images, the polymer grafts can be more easily distin-
guished from the CHS core [Fig. 9]. In case of the fusion
points of pristine particles, the high and lower contrast dif-
fused continuously without any definite sharp boundary
between them whereas in the case of polymer grafted par-
ticles, there is very clear sharp boundary (interface) between
lower contrast (polymer layer) and carbon particles. From
these images the thickness of the PSt grafted films are calcu-
lated to be 19.4 and 32 nm in case of CHSs-PSt produced in
the presence [Fig. 9(a)] and absence of sacrificial initiator
ꢀ
ꢀ
C which is comparable to decomposition of pure PMMA
ꢀ
[
Fig. 6(c)] with an onset temperature of 312 C and an
ꢀ
inflection temperature point at 420 C. After 2 h of polymer-
ization in the presence of free initiator, CHSs-PMMA-FI, [Fig.
7
(a)] consisted of 16.3% by mass of PMMA whereas for
CHSs-PMMA-NFI, prepared in the absence of free initiator,
5.4% of the overall mass corresponded to PMMA, [Fig.
(b)]. The increase in weight loss from “free initiator” to “no
3
7
free initiator” samples is simply due to the increase in mo-
lecular weight of the polymer brushes on the surface during
polymerization. Like PSt system above results show that the
molecular weight of grafted PMMA chains on CHSs can be
controlled simply by changing the feed ratio of monomer to
overall initiators (free and surface-bound).
Based on the TGA analyses, surface area of CHSs, the
reported density of polymer in bulk and the presumed mo-
lecular weight of grafted polymer, (based on soluble polymer
FIGURE 7 TGA curves of (a) CHSs-PMMA-FI (b) CHSs-PMMA-
FIGURE 8 TEM images of (a) CHSs-BBMBPO, and (b) CHSs-
NFI, (c) PMMA.
PSt-FI.
3
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CHSs both in the presence and in the absence of free initiator.
It is noteworthy, that in addition to the single step initiator
grafting, that the CHSs did not require any acid treatment
prior to the functionalization. SEM and TEM confirmed that
the morphology of the CHSs was unaltered during initiator fix-
ation and SI-ATRP, and the CHSs were homogenously and
evenly covered with a layer of PSt or PMMA (13–47-nm
thick). It is expected that these grafted CHSs could be efficient
fillers for the fabrication of reinforced polymer as the pres-
ence of grafted polymer should increase the polymer-CHSs
interaction, thus ensuring an excellent load transfer.
ACKNOWLEDGMENTS
ME and SMG would like to acknowledge the Tulane Program
for Bridge Research Support and the Gulf of Mexico Research
Initiative (2012-II-798) for support, VTJ acknowledges the
PKSFI grant program through the University of New Orleans
and NSP acknowledges the NSF (CBET-1034175).
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