# A Simple Method for Forming Hybrid Core-Shell  Nanoparticles Suspended in Water

Authors: Jean-Christophe Daigle; Jerome P. Claverie
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

Core-shell hybrid nanoparticles, where the core is an inorganic nanoparticle and the shell an organic polymer, are prepared by a two-step method. Inorganic nanoparticles are first dispersed in water using poly(acrylic acid) (PAA)  prepared by reversible
addition fragmentation chain transfer (RAFT) polymerization as dispersant. Then, the resulting dispersion is engaged in a radical emulsion polymerization process whereby a hydrophobic organic monomer (styrene and butyl acrylate) is polymerized to form the shell of the hybrid nanoparticle. This method is extremely versatile, allowing the preparation of a variety of nanocomposites with metal oxides (alumina, rutile, anatase, barium titanate, zirconia, copper oxide), metals (Mo, Zn), and even inorganic nitrides (Si3N4).


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## Body

## 1. Introduction

The preparation of core-shell nanoparticles  where the core of the
particle is inorganic and the shell is an organic polymer has been the object
of a profusion of recent publications [1, 2].  Indeed, the resulting nanocomposites find wide
applications in the field of coatings and adhesives [3, 4], optics [5],
electronics [6], biomedical [7] and environmental applications [8]. When the
core-shell nanoparticle is suspended in water, the equilibrium arrangement of
the inorganic phase and the organic polymer mainly depends on the interfacial
energies of the mineral/water, polymer/water, and mineral/polymer interfaces.
In most cases, the mineral surface being quite polar, its encapsulation by a
hydrophobic polymer is  not thermodynamically favorable.  Thus, it is not surprising that instead of the
desirable core-shell arrangement, a myriad of other morphologies is
usually observed for these nanocomposites, such as occluded, half-moon, hairy,
strawberry-like, and so on [1, 2].There exist two general routes for the preparation of core-shell
nanocomposites. The first one relies on
the self-assembly of the inorganic nanoparticle with a preformed polymer. For example, layer-by-layer (LbL) deposition
of polyelectrolytes [9] and heterocoagulation of positively charged latex [10]
onto negatively charged inorganic particles can be used to prepare core-shell
type nanocomposites. The second one is
based on the direct formation of the polymer in the presence of the mineral
nanoparticle. Usually, an emulsion
polymerization process is used, because of its robustness, its versatility, and
its ability to yield polymer nanoparticles (latexes) with sizes ranging from 10 nm to 1 micron [11]. Yet, due to the
lack of compatibility between the polymer and the inorganic surface, the
encapsulation succeeds if the inorganic nanoparticle is treated beforehand to
make it more organic-like, for example by adsorption of a  surfactant, resulting in the “hydrophobization”
of the inorganic surface [12, 13]. However,
this strategy is often plagued by a loss of colloidal stability (aggregation of
the inorganic particles). As a means to circumvent this problem, it is possible
to use a miniemulsion polymerization process whereby the surfactant-treated
inorganic particles are dispersed into stable nanometric droplets of an organic
liquid monomer which is later polymerized, to successfully form hybrid
nanocomposites [14–16].  Another route
consists in forcing the inclusion of the inorganic nanoparticle into the
organic shell by tethering it with covalent links. Numerous variants have been used, for example,
by chemically preparing the mineral surface with reacting groups (such as
reactive monomers [17], radical initiators [18], and promoters for controlled
radical polymerization [19]). Although
often successful, this strategy requires a functionalization step which must be
adapted to each nanoparticle (diazonium decomposition on graphite and cobalt [20],
silane [21], and titanate [22] coupling agents on silica and alumina, thiols on
gold [23], etc.). It is clear that there
exists a profusion of methods for the preparation of the core-shell
inorganic-organic hybrid nanoparticles, yet all of those have a restricted
scope, either being limited to a small set of inorganic particles or to a limited
number of polymers and polymer thicknesses (as in the case of LbL
deposition). For the sake of
completion, it is also important to mention that concomitantly to this work [24, 25],
the group of Hawkett [26–28] has recently described a similar approach based on
an interfacial polymerization process used to encapsulate a variety of
inorganic particles.We here report a general method to synthesize hybrid core-shell
nanoparticles. To our surprise, this
method proved to be successful for all types of inorganic compounds we tried,
being metal oxides, metals, or even metal nitrides. The mineral nanoparticles
are first dispersed into water with the aid of a dispersant prepared by
controlled radical polymerization (Scheme1). The shell is then formed upon the addition of
organic monomers via an emulsion polymerization process. The key feature of
this encapsulation method is the use of a living/controlled dispersant which is
reactivated during the polymerization of the shell, thus leading to the
formation of block copolymers. The shell is covalently tethered to the
dispersant, which is adsorbed onto the inorganic nanoparticle surface.Scheme 1
Encapsulation
of an inorganic nanoparticle dispersed in water by PAA (step 1) to form hybrid
core-shell nanoparticles (step 2). The
organic shell is formed of polystyrene (95%) and PAA (5%).In comparison to existing methods of nanoencapsulation, the route we
propose does not require either modification or optimization when one changes
the nature of the inorganic nanoparticles. Importantly, the commercial
inorganic nanoparticles do not need to undergo a specific chemical surface
treatment before use. The preparation of the composite is exclusively performed
in water, thus avoiding the manipulation of organic solvents and bypassing
solvent transfer or solvent exchange steps which are often tedious. Last, this
method is easily scalable, as we were able to target dispersions containing as
much as 30% in weight of composite with no loss of colloidal stability (no
aggregation).


## 2. Results and Discussion

### 2.1. RAFT Poly(acrylic acid) as Dispersant

The chosen dispersant for this work is a poly(acrylic acid) (PAA)
homopolymer prepared by reversible addition chain transfer (RAFT)
polymerization, using trithiobenzyl carbonate (TBC) as RAFT agent (Scheme1). Based on the pioneering work of Rizzardo on
RAFT [29], we have shown in the past that this RAFT agent is suitable to
control the polymerization of acrylic acid (AA), yielding PAAs which have a low
polydispersity [30, 31]. Using the analytical methods detailed in [31],
it was found that PAA used here has a degree of polymerization of 86 (43 units
in average on each side of the central trithiocarbonate unit), a polydispersity
index of 1.15, and approxemately 73% of the chains contain a central
trithiocarbonate unit, making them a suitable candidate for reinitiating the
polymerization.PAA prepared by RAFT is an excellent dispersant of metal oxide
nanoparticles such as kaolin, calcium carbonate (calcite), and titanium dioxide
[32, 33]. The adsorption is believed to be driven by electrostatics, and in the
case of calcite [33], the adsorption of a RAFT polymer was shown to be
irreversible and total (no chain in water until saturation is reached). Last, we [34] and others [35, 36] have shown
that PAA prepared by RAFT can be extended with other monomers to form
amphiphilic block copolymers, making PAA prepared by RAFT an ideal choice for
this study.


### 2.2. Dispersion of the Inorganic Nanoparticles

To evaluate the generality of this encapsulation method, a selection of
10 different nanoparticles was
selected for this work, being metal, metal oxides and nitrides, hydrophilic (e.g.,Al2O3), or hydrophobic (e.g., Si3N4).
Silica [37], gold [23], and carbon black [38] nanoparticles were not selected
because very efficient and elegant methods have already been developed for
those nanoparticles. Before embarking upon a description of the encapsulation
process, a few observations relative to the nature of the surface of the
nanoparticles can be made. Coated-TiO2 nanoparticles are coated by a
thin layer of alumina and are commonly used in the pigment industry, in
contrast to anatase and rutile which are often used for their semiconductor and
photocatalytic properties. No effort has been made to treat the commercial
nanoparticles used in this study before use; therefore, the metallic
nanoparticles (Mo and Zn) may be passivated by a thin layer of oxide.The adsorption isotherms of PAA
onto commercial inorganic nanoparticles (Figure1 and Table 1)
indicate that the adsorption of PAA is, as expected, strongly dependent upon
the mineral surface. The isotherms were built by separating the nanoparticles
from the continuous phase by centrifugation, and analyzing
the supernatant for the amount of PAA free, using the method described in [39]. A precise adsorption isotherm could not be built
for anatase and Al2O3 because the serum is always turbid
due to residual amounts of small particles in suspension. The amount of
PAA adsorbed onto Zn, Mo, and Si3N4 nanoparticles was
found to be below measurable levels. This observation may be explained by the
absence of electrostatic interactions in between the negatively charged PAA and
the neutral surfaces of Si3N4, Zn. and Mo (assuming no
passivation for the two latter cases). In the case of BaTiO3 and ZrO2,
all the PAA is adsorbed until the surface is saturated. On the contrary, for
other inorganic particles (CuO, rutile, coated TiO2), the adsorption
isotherm corresponds to a typical Langmuir isotherm, showing a partitioning
between free and bound PAA. The affinity of PAA for the mineral surface can be
quantified by the slope of the adsorption isotherm at low concentrations (Figure 1 and Table 1) and ranks as follows:
ZrO2>BaTiO3>rutile>CuO> coated-TiO2>Zn∼Mo∼Si3N4. In
our experiments, pH was not controlled by a buffer, and measured pH was
comprised between 4 and 7. Influence of
pH and resulting ζ potential
on the behavior of a dispersant are well documented in literature [40]. At higher pH values, not only the adsorption
of PAA on TiO2 nanoparticles becomes unfavorable [28], but also
significant hydrolysis of the chain transfer agent occurs [31].Table 1
Characteristics of the inorganic
nanoparticles with a polystyrene shell.
InorganicS.C.(%)[a]Composition (wt %)Dp[d] (nm)σ[e] (nm)shell[f] (nm)Slope[g]core[b]shell[b]core[c]shell[c]Anatase9.154464852851218nd[h]Rutile10.73862722893271311TiO2-coated15.757437228839111ZrO213.556446931915131074CuO14.9336770308116141.5BaTiO316.65841613993191965Al2O316.4495166341082615ndMo14.3564473271081822≪1Zn27.9534764361823038≪1Si3N411.1613966341553222≪1[a] Solid content of the nanocomposite
in water, as determined by gravimetric analysis. [b] Determined by TGA analysis. [c] Measured from the volume fractions
obtained on electron micrographs. [d]
Particle diameter measured on electron micrographs. [e] Half width of the particle diameter
distribution. [f] Shell thickness measured on electron micrographs. [g] Slope
of the adsorption isotherm (no unit). [h]
not determined.Figure 1
Adsorption isotherm of PAA on
several inorganic nanoparticles ([inorganic] = 200 g/L in water), determined
using the method described in  [39]. The
curves are the best fit of the experimental points with the function f(x)=Kx/(1+ax), which corresponds to a theoretical Langmuir isotherm curve.The plateau levels in the adsorption isotherm correspond to the
saturation of the surface by PAA. These levels are evidently related to the
surface areas of the nanoparticles, and therefore their sizes. The commercial
nanoparticles used here come with various size distributions, and, accordingly,
the position of the saturation plateau varies greatly. ForZrO2 and BaTiO3 nanoparticles which exhibit a strong binding isotherm, the amount of PAA used
for dispersing them prior encapsulation was equal to 1.3 the saturation amount
(100% adsorbed and 30% free). For the
other nanoparticles, an arbitrary amount of 10 g/kg of dispersion was used,
since, whatever the amount used, PAA is partitioned between free and adsorbed. With
this quantity of PAA, the nanoparticles remain dispersed in water for several
hours, which is sufficient to initiate the second step of the synthesis
(encapsulation by an hydrophobic polymer). Lower amounts often result in the
rapid formation of aggregates, as shown by dynamic light scattering, which
ultimately does not lead to the formation of core-shell hybrid
nanoparticles. Importantly,
notwithstanding the position of the saturation plateau, free (i.e., not
adsorbed) PAA is present in the dispersion, either because an amount greater
than saturation is used or because the adsorption isotherm shows a partitioning
between free and adsorbed PAA. This point will be further discussed in the
section on the mechanism of encapsulation.


### 2.3. Encapsulation of the Inorganic  Nanoparticles by a Polymer

Once the inorganic nanoparticles are dispersed in water, an organic
monomer (styrene) is added  (step 2, Scheme1), in the presence of a
nonoxidizing water-soluble initiator (4,4′-azobis-4-cyanovaleric acid) and a
small amount of surfactant (sodium dodecylsulfate, SDS) which is necessary to
prevent the final core-shell nanoparticles from aggregating. Oxidizing
initiators are known to potentially decompose the trithiocarbonate [41]. The
amount of initiator added is kept below 20% the amount of trithiocarbonate
units present in the PAA, so as to limit the number of homopolystyrene chains
and to favor the formation of triblock copolymers PS-PAA-PS [33]. However, no
attempt was made to check whether all chains were blocked because the amount of
polystyrene (PS, 3.6 g) formed during this step is greatly superior to the
amount of PAA introduced as dispersant (100 mg). The fraction of polymer
soluble in THF for experiment 3 (TiO2-PS) had a broad molecular
weight distribution as measured by GPC (Mn=24,500g/mol, PDI =
2.2), which indicates that control on MWD is not very effective under these
conditions. Numerous reasons can be
evoked to explain the loss of control:
low efficiency of reinitiating the PAA block with styrene due to a
significant mismatch of polarity [42], partial decomposition of the RAFT agent [31],
trithiocarbonate RAFT agent not very efficient at controlling the
polymerization of styrene [29]. This
work does not aim at using controlled radical polymerization techniques for the
sake of controlling the MWD but it takes advantage of the RAFT technique to
reinitiate a dormant oligomer (PAA dispersant) which has a high chain-end
reactivity. Importantly, the formation of core-shell structures is unsuccessful
if one uses a conventional (dead) PAA, pointing out that block copolymers must
be formed during this experiment, probably in conjunction with an unquantified
amount of PS homopolymer chains.


### 2.4. Characterization of the Core-Shell Nanoparticles

The characteristics of the resulting nanoparticles are consigned in Table1. The solid content of the
dispersion is typically around 10 wt%, but can be as high as 25 wt%, which is
an indication of the colloidal stability of this system. The global composition
of the nanoparticles has been assessed by TGA analysis: in average the
composite is constituted by 50 wt% of inorganic matter and 50 wt% of polymer.
This composition corresponds precisely to the ratio of the weight of mineral
nanoparticles to organic monomer prior polymerization, thus indicating that the
conversion of monomer to polymer is high.The morphology of the nanocomposite
has also been assessed by TEM (Figure2). The inorganic nanoparticle is
encapsulated by a shell of polymer with a thickness comprised between 10 and 40 nm (Table 1), with no aggregates and no partially encapsulated inorganic
nanoparticles. The particle size
distribution (Table 1) was calculated from the surface areas obtained in
a digital treatment of binarized pictures. The thickness of the shell was
directly measured on the micrographs (average of more than 200 measurements,
but several measurements per particle). From the thickness of the shell and the
diameter of the nanoparticle, it is possible to assess the volume fractions of
the core and shell assuming a spherical shape, which can in turn be used to
calculate the corresponding weight fractions, knowing the respective density of
the inorganic particle and of the polymer (density=1). In certain cases, the
composition of the nanocomposite is identical to the one measured by TGA
(anatase, BaTiO3, Zn), but in other cases, the relative amount of
shell is lower than expected. For the latter case, this discrepancy can be
attributed to the presence of polymer nanoparticles devoid of inorganic core,
which can clearly be identified in some of the micrographs (x in Figure 2(b), 2(e), and 2(j)). Once the
polymerization is triggered, the PAA is either adsorbed at the particle surface
or free in water, since an amount greater than saturation is used. Upon
extension by styrene, the copolymer becomes amphiphilic and behaves as an in situ
surfactant which is able to nucleate a new crop of “empty” particles.Transmission electron micrograph of the
inorganic particles encapsulated in a polystyrene shell. (a)BaTiO3, bar = 220 nm; (b) Si3N4, bar = 120 nm;  (c) anatase, bar = 200 nm;  (d) TiO2 coated, bar = 120 nm;
(e)  rutile, bar = 120 nm; (f) ZrO2,
bar = 120 nm; (g) Al2O3, bar = 170 nm; (h) Zn, bar = 220 m ; (i) Mo, bar = 300 nm; (j)  CuO, bar
= 120 nm.
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)The encapsulation method is quite general and is also efficient to form
polymeric shells of lower Tg, for example, shells constituted of a copolymer of
n-butyl acrylate and styrene (50:50 w:w, calculated Tg = 7C°). In this case, the electron micrographs (Figure 3) also clearly indicate the formation of core-shell nanoparticles but the
shell is not spherical, because in the dry state (for TEM analysis) the polymer
can spread on the surface of the grid.
In certain cases, a continuous film is formed due to the coalescence of
several particles together (Figure 3(b) top left). It is interesting to note that the inorganic
nanoparticles in the film are not touching each other, which is a legacy of the
fact that they were encapsulated in a core-shell structure before merging
together into a continuous film.Transmission electron micrograph of the
inorganic particles encapsulated in a poly(styrene-co-butyl acrylate)
shell. (a) Rutile, bar = 380 nm; (b)TiO2 coated, bar = 490 nm; (c)  anatase, bar
= 200 nm.
(a)(b)(c)


### 2.5.  Encapsulation Mechanism

Why is this encapsulation method so general? Contrarily to what was observed in the case
ofCaCO3 [32], the adsorption of PAA onto the surface of inorganic
nanoparticles is not always irreversible, but ranges from very strong to weak,
as shown by the very diverse adsorption isotherms (Figure 1). We envisage
below two limiting cases, depending on the adsorption behavior of PAA on the
particle surface. Before presenting
these two cases, it is important to stress that the adsorption isotherm
corresponds to the partitioning between the free and adsorbed PAA which itself
can be viewed as the ratio of the adsorption to desorption rate constants. In a
first approximation, the adsorption rate constant does not depend on the
inorganic surface (but only on the diffusion coefficient of the PAA).
Therefore, the partition factor is inversely proportional to desorption rate
constant, hence, proportional to the time the PAA is staying at the
surface.When the polymer is strongly adsorbed, there is no exchange (or a very
slow exchange) between the adsorbed and the free polymer. Once the polymerization
is triggered, the monomer has no other choice than inserting on the adsorbed
polymer and on the free polymer. The growing adsorbed polymer, becoming more
and more hydrophobic, will constitute the shell, whereas the free polymer
eventually becomes amphiphilic and can either generate a crop of new particles
(nucleation event in emulsion polymerization) or adsorb onto the surface
offered by the inorganic nanoparticles now partially “hydrophobized.”
Statistically, this latter event is favored because the polymerization is
performed in the presence of a large amount of inorganic nanoparticles (20 wt%), representing a huge surface area. Thus, most of the polymer serves to the
formation of a shell, but there is a risk that a few polymer-only particles are
created.When there is a rapid exchange between the adsorbed and free polymer
(weak adsorption), each time a monomer unit is inserted, the copolymer
PS-PAA-PS becomes more hydrophobic and the exchange time scale between the
adsorbed and free states becomes slower.
At one point, the exchange becomes so slow (no more solubility of the
polymer in water) that the situation becomes identical to the case of a strong
adsorption: the copolymer is frozen at the inorganic surface.It could be argued that this method would be most successful if the
amount of dispersant would always be below saturation, so as to avoid the
presence of free PAA in the aqueous phase.
However, this would only be possible for those particles showing a
strong irreversible isotherm (BaTiO3,ZrO2, in this
study), which would limit the versatility of the method. Since the precise determination of the
adsorption isotherm can be quite tedious (e.g., in the case of CaCO3,
the CO2 partial pressure must be controlled [31]), it is preferable
to use a method which is efficient notwithstanding the ratio of PAA versus
saturation amount, as long as the resulting dispersion exhibits sufficient
colloidal stability to engage the encapsulation step. In this study, we found that PAA
concentrations of 6–14 g/L are optimal for inorganic nanoparticle
concentrations of 200 g/L. However, the PAA concentration will have to be
adjusted by trial and error when the ionic strength of the aqueous phase is
changed, and if the molecular weight and nature of the dispersant are changed.In this study, SDS surfactant was added to the emulsion recipe to provide
a better stabilization of the latex particles. The SDS concentration (1.2 g/L)
was kept well below cmc (2.4 g/L at 25C°) in order to minimize
secondary nucleation. Indeed, no
secondary nucleation occurs for anatase, BaTiO3, and Zn, seemingly
indicating that secondary nucleation occurs only when PAA is weakly adsorbed,
and it is not due to the presence of SDS.
The presence of a small amount of particles generated by secondary
nucleation has also been reported by Nguyen et al. [28], who have been able to
encapsulate several pigments using a macroRAFT agent based on a random
amphiphilic copolymer containing butyl acrylate and acrylic acid. In their work, no surfactant was used, reinforcing the idea that
secondary nucleation is not due to the presence of surfactant.


## 2.1. RAFT Poly(acrylic acid) as Dispersant

The chosen dispersant for this work is a poly(acrylic acid) (PAA)
homopolymer prepared by reversible addition chain transfer (RAFT)
polymerization, using trithiobenzyl carbonate (TBC) as RAFT agent (Scheme1). Based on the pioneering work of Rizzardo on
RAFT [29], we have shown in the past that this RAFT agent is suitable to
control the polymerization of acrylic acid (AA), yielding PAAs which have a low
polydispersity [30, 31]. Using the analytical methods detailed in [31],
it was found that PAA used here has a degree of polymerization of 86 (43 units
in average on each side of the central trithiocarbonate unit), a polydispersity
index of 1.15, and approxemately 73% of the chains contain a central
trithiocarbonate unit, making them a suitable candidate for reinitiating the
polymerization.PAA prepared by RAFT is an excellent dispersant of metal oxide
nanoparticles such as kaolin, calcium carbonate (calcite), and titanium dioxide
[32, 33]. The adsorption is believed to be driven by electrostatics, and in the
case of calcite [33], the adsorption of a RAFT polymer was shown to be
irreversible and total (no chain in water until saturation is reached). Last, we [34] and others [35, 36] have shown
that PAA prepared by RAFT can be extended with other monomers to form
amphiphilic block copolymers, making PAA prepared by RAFT an ideal choice for
this study.


## 2.2. Dispersion of the Inorganic Nanoparticles

To evaluate the generality of this encapsulation method, a selection of
10 different nanoparticles was
selected for this work, being metal, metal oxides and nitrides, hydrophilic (e.g.,Al2O3), or hydrophobic (e.g., Si3N4).
Silica [37], gold [23], and carbon black [38] nanoparticles were not selected
because very efficient and elegant methods have already been developed for
those nanoparticles. Before embarking upon a description of the encapsulation
process, a few observations relative to the nature of the surface of the
nanoparticles can be made. Coated-TiO2 nanoparticles are coated by a
thin layer of alumina and are commonly used in the pigment industry, in
contrast to anatase and rutile which are often used for their semiconductor and
photocatalytic properties. No effort has been made to treat the commercial
nanoparticles used in this study before use; therefore, the metallic
nanoparticles (Mo and Zn) may be passivated by a thin layer of oxide.The adsorption isotherms of PAA
onto commercial inorganic nanoparticles (Figure1 and Table 1)
indicate that the adsorption of PAA is, as expected, strongly dependent upon
the mineral surface. The isotherms were built by separating the nanoparticles
from the continuous phase by centrifugation, and analyzing
the supernatant for the amount of PAA free, using the method described in [39]. A precise adsorption isotherm could not be built
for anatase and Al2O3 because the serum is always turbid
due to residual amounts of small particles in suspension. The amount of
PAA adsorbed onto Zn, Mo, and Si3N4 nanoparticles was
found to be below measurable levels. This observation may be explained by the
absence of electrostatic interactions in between the negatively charged PAA and
the neutral surfaces of Si3N4, Zn. and Mo (assuming no
passivation for the two latter cases). In the case of BaTiO3 and ZrO2,
all the PAA is adsorbed until the surface is saturated. On the contrary, for
other inorganic particles (CuO, rutile, coated TiO2), the adsorption
isotherm corresponds to a typical Langmuir isotherm, showing a partitioning
between free and bound PAA. The affinity of PAA for the mineral surface can be
quantified by the slope of the adsorption isotherm at low concentrations (Figure 1 and Table 1) and ranks as follows:
ZrO2>BaTiO3>rutile>CuO> coated-TiO2>Zn∼Mo∼Si3N4. In
our experiments, pH was not controlled by a buffer, and measured pH was
comprised between 4 and 7. Influence of
pH and resulting ζ potential
on the behavior of a dispersant are well documented in literature [40]. At higher pH values, not only the adsorption
of PAA on TiO2 nanoparticles becomes unfavorable [28], but also
significant hydrolysis of the chain transfer agent occurs [31].Table 1
Characteristics of the inorganic
nanoparticles with a polystyrene shell.
InorganicS.C.(%)[a]Composition (wt %)Dp[d] (nm)σ[e] (nm)shell[f] (nm)Slope[g]core[b]shell[b]core[c]shell[c]Anatase9.154464852851218nd[h]Rutile10.73862722893271311TiO2-coated15.757437228839111ZrO213.556446931915131074CuO14.9336770308116141.5BaTiO316.65841613993191965Al2O316.4495166341082615ndMo14.3564473271081822≪1Zn27.9534764361823038≪1Si3N411.1613966341553222≪1[a] Solid content of the nanocomposite
in water, as determined by gravimetric analysis. [b] Determined by TGA analysis. [c] Measured from the volume fractions
obtained on electron micrographs. [d]
Particle diameter measured on electron micrographs. [e] Half width of the particle diameter
distribution. [f] Shell thickness measured on electron micrographs. [g] Slope
of the adsorption isotherm (no unit). [h]
not determined.Figure 1
Adsorption isotherm of PAA on
several inorganic nanoparticles ([inorganic] = 200 g/L in water), determined
using the method described in  [39]. The
curves are the best fit of the experimental points with the function f(x)=Kx/(1+ax), which corresponds to a theoretical Langmuir isotherm curve.The plateau levels in the adsorption isotherm correspond to the
saturation of the surface by PAA. These levels are evidently related to the
surface areas of the nanoparticles, and therefore their sizes. The commercial
nanoparticles used here come with various size distributions, and, accordingly,
the position of the saturation plateau varies greatly. ForZrO2 and BaTiO3 nanoparticles which exhibit a strong binding isotherm, the amount of PAA used
for dispersing them prior encapsulation was equal to 1.3 the saturation amount
(100% adsorbed and 30% free). For the
other nanoparticles, an arbitrary amount of 10 g/kg of dispersion was used,
since, whatever the amount used, PAA is partitioned between free and adsorbed. With
this quantity of PAA, the nanoparticles remain dispersed in water for several
hours, which is sufficient to initiate the second step of the synthesis
(encapsulation by an hydrophobic polymer). Lower amounts often result in the
rapid formation of aggregates, as shown by dynamic light scattering, which
ultimately does not lead to the formation of core-shell hybrid
nanoparticles. Importantly,
notwithstanding the position of the saturation plateau, free (i.e., not
adsorbed) PAA is present in the dispersion, either because an amount greater
than saturation is used or because the adsorption isotherm shows a partitioning
between free and adsorbed PAA. This point will be further discussed in the
section on the mechanism of encapsulation.


## 2.3. Encapsulation of the Inorganic  Nanoparticles by a Polymer

Once the inorganic nanoparticles are dispersed in water, an organic
monomer (styrene) is added  (step 2, Scheme1), in the presence of a
nonoxidizing water-soluble initiator (4,4′-azobis-4-cyanovaleric acid) and a
small amount of surfactant (sodium dodecylsulfate, SDS) which is necessary to
prevent the final core-shell nanoparticles from aggregating. Oxidizing
initiators are known to potentially decompose the trithiocarbonate [41]. The
amount of initiator added is kept below 20% the amount of trithiocarbonate
units present in the PAA, so as to limit the number of homopolystyrene chains
and to favor the formation of triblock copolymers PS-PAA-PS [33]. However, no
attempt was made to check whether all chains were blocked because the amount of
polystyrene (PS, 3.6 g) formed during this step is greatly superior to the
amount of PAA introduced as dispersant (100 mg). The fraction of polymer
soluble in THF for experiment 3 (TiO2-PS) had a broad molecular
weight distribution as measured by GPC (Mn=24,500g/mol, PDI =
2.2), which indicates that control on MWD is not very effective under these
conditions. Numerous reasons can be
evoked to explain the loss of control:
low efficiency of reinitiating the PAA block with styrene due to a
significant mismatch of polarity [42], partial decomposition of the RAFT agent [31],
trithiocarbonate RAFT agent not very efficient at controlling the
polymerization of styrene [29]. This
work does not aim at using controlled radical polymerization techniques for the
sake of controlling the MWD but it takes advantage of the RAFT technique to
reinitiate a dormant oligomer (PAA dispersant) which has a high chain-end
reactivity. Importantly, the formation of core-shell structures is unsuccessful
if one uses a conventional (dead) PAA, pointing out that block copolymers must
be formed during this experiment, probably in conjunction with an unquantified
amount of PS homopolymer chains.


## 2.4. Characterization of the Core-Shell Nanoparticles

The characteristics of the resulting nanoparticles are consigned in Table1. The solid content of the
dispersion is typically around 10 wt%, but can be as high as 25 wt%, which is
an indication of the colloidal stability of this system. The global composition
of the nanoparticles has been assessed by TGA analysis: in average the
composite is constituted by 50 wt% of inorganic matter and 50 wt% of polymer.
This composition corresponds precisely to the ratio of the weight of mineral
nanoparticles to organic monomer prior polymerization, thus indicating that the
conversion of monomer to polymer is high.The morphology of the nanocomposite
has also been assessed by TEM (Figure2). The inorganic nanoparticle is
encapsulated by a shell of polymer with a thickness comprised between 10 and 40 nm (Table 1), with no aggregates and no partially encapsulated inorganic
nanoparticles. The particle size
distribution (Table 1) was calculated from the surface areas obtained in
a digital treatment of binarized pictures. The thickness of the shell was
directly measured on the micrographs (average of more than 200 measurements,
but several measurements per particle). From the thickness of the shell and the
diameter of the nanoparticle, it is possible to assess the volume fractions of
the core and shell assuming a spherical shape, which can in turn be used to
calculate the corresponding weight fractions, knowing the respective density of
the inorganic particle and of the polymer (density=1). In certain cases, the
composition of the nanocomposite is identical to the one measured by TGA
(anatase, BaTiO3, Zn), but in other cases, the relative amount of
shell is lower than expected. For the latter case, this discrepancy can be
attributed to the presence of polymer nanoparticles devoid of inorganic core,
which can clearly be identified in some of the micrographs (x in Figure 2(b), 2(e), and 2(j)). Once the
polymerization is triggered, the PAA is either adsorbed at the particle surface
or free in water, since an amount greater than saturation is used. Upon
extension by styrene, the copolymer becomes amphiphilic and behaves as an in situ
surfactant which is able to nucleate a new crop of “empty” particles.Transmission electron micrograph of the
inorganic particles encapsulated in a polystyrene shell. (a)BaTiO3, bar = 220 nm; (b) Si3N4, bar = 120 nm;  (c) anatase, bar = 200 nm;  (d) TiO2 coated, bar = 120 nm;
(e)  rutile, bar = 120 nm; (f) ZrO2,
bar = 120 nm; (g) Al2O3, bar = 170 nm; (h) Zn, bar = 220 m ; (i) Mo, bar = 300 nm; (j)  CuO, bar
= 120 nm.
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)The encapsulation method is quite general and is also efficient to form
polymeric shells of lower Tg, for example, shells constituted of a copolymer of
n-butyl acrylate and styrene (50:50 w:w, calculated Tg = 7C°). In this case, the electron micrographs (Figure 3) also clearly indicate the formation of core-shell nanoparticles but the
shell is not spherical, because in the dry state (for TEM analysis) the polymer
can spread on the surface of the grid.
In certain cases, a continuous film is formed due to the coalescence of
several particles together (Figure 3(b) top left). It is interesting to note that the inorganic
nanoparticles in the film are not touching each other, which is a legacy of the
fact that they were encapsulated in a core-shell structure before merging
together into a continuous film.Transmission electron micrograph of the
inorganic particles encapsulated in a poly(styrene-co-butyl acrylate)
shell. (a) Rutile, bar = 380 nm; (b)TiO2 coated, bar = 490 nm; (c)  anatase, bar
= 200 nm.
(a)(b)(c)


## 2.5.  Encapsulation Mechanism

Why is this encapsulation method so general? Contrarily to what was observed in the case
ofCaCO3 [32], the adsorption of PAA onto the surface of inorganic
nanoparticles is not always irreversible, but ranges from very strong to weak,
as shown by the very diverse adsorption isotherms (Figure 1). We envisage
below two limiting cases, depending on the adsorption behavior of PAA on the
particle surface. Before presenting
these two cases, it is important to stress that the adsorption isotherm
corresponds to the partitioning between the free and adsorbed PAA which itself
can be viewed as the ratio of the adsorption to desorption rate constants. In a
first approximation, the adsorption rate constant does not depend on the
inorganic surface (but only on the diffusion coefficient of the PAA).
Therefore, the partition factor is inversely proportional to desorption rate
constant, hence, proportional to the time the PAA is staying at the
surface.When the polymer is strongly adsorbed, there is no exchange (or a very
slow exchange) between the adsorbed and the free polymer. Once the polymerization
is triggered, the monomer has no other choice than inserting on the adsorbed
polymer and on the free polymer. The growing adsorbed polymer, becoming more
and more hydrophobic, will constitute the shell, whereas the free polymer
eventually becomes amphiphilic and can either generate a crop of new particles
(nucleation event in emulsion polymerization) or adsorb onto the surface
offered by the inorganic nanoparticles now partially “hydrophobized.”
Statistically, this latter event is favored because the polymerization is
performed in the presence of a large amount of inorganic nanoparticles (20 wt%), representing a huge surface area. Thus, most of the polymer serves to the
formation of a shell, but there is a risk that a few polymer-only particles are
created.When there is a rapid exchange between the adsorbed and free polymer
(weak adsorption), each time a monomer unit is inserted, the copolymer
PS-PAA-PS becomes more hydrophobic and the exchange time scale between the
adsorbed and free states becomes slower.
At one point, the exchange becomes so slow (no more solubility of the
polymer in water) that the situation becomes identical to the case of a strong
adsorption: the copolymer is frozen at the inorganic surface.It could be argued that this method would be most successful if the
amount of dispersant would always be below saturation, so as to avoid the
presence of free PAA in the aqueous phase.
However, this would only be possible for those particles showing a
strong irreversible isotherm (BaTiO3,ZrO2, in this
study), which would limit the versatility of the method. Since the precise determination of the
adsorption isotherm can be quite tedious (e.g., in the case of CaCO3,
the CO2 partial pressure must be controlled [31]), it is preferable
to use a method which is efficient notwithstanding the ratio of PAA versus
saturation amount, as long as the resulting dispersion exhibits sufficient
colloidal stability to engage the encapsulation step. In this study, we found that PAA
concentrations of 6–14 g/L are optimal for inorganic nanoparticle
concentrations of 200 g/L. However, the PAA concentration will have to be
adjusted by trial and error when the ionic strength of the aqueous phase is
changed, and if the molecular weight and nature of the dispersant are changed.In this study, SDS surfactant was added to the emulsion recipe to provide
a better stabilization of the latex particles. The SDS concentration (1.2 g/L)
was kept well below cmc (2.4 g/L at 25C°) in order to minimize
secondary nucleation. Indeed, no
secondary nucleation occurs for anatase, BaTiO3, and Zn, seemingly
indicating that secondary nucleation occurs only when PAA is weakly adsorbed,
and it is not due to the presence of SDS.
The presence of a small amount of particles generated by secondary
nucleation has also been reported by Nguyen et al. [28], who have been able to
encapsulate several pigments using a macroRAFT agent based on a random
amphiphilic copolymer containing butyl acrylate and acrylic acid. In their work, no surfactant was used, reinforcing the idea that
secondary nucleation is not due to the presence of surfactant.


## 3. Conclusion

In this preliminary report, a
remarkably simple technique which is convenient for the preparation of a wide
range of inorganic-organic hybrid nanoparticles has been unraveled. The method
is successful for both hydrophilic nanoparticles (e.g., alumina) or hydrophobic
ones (Si3N4, e.g.). This uncomplicated and rapid method
has been evaluated in an aqueous only environment, which is advantageous for
many applications. Yet, in a future work, the possibility to adapt this
strategy to an organic solvent will be assessed, starting with a
living/controlled dispersant in organic medium and extending the chains with a
monomer such as the copolymer does not exhibit any solubility in the organic
solvent. Another avenue which needs to
be explored in the future is the possibility to use other controlled
polymerization techniques. If the mechanism proposed above for the formation of
the hybrid particles is exact, then, there should be no limitations as to the
choice of the controlled mechanism. Finally, although the scope of the radical
emulsion polymerization process is very broad, it often leads to polymers with
ill-defined microstructures which are not easily functionalized. Conceptually, the method described here,
which can be viewed as a method of “grafting from” the dispersant by a reactive
process, could be extended to a variety other polymerization techniques (ROMP,
late transition metal catalysis, ROP, etc.) thus opening the route to an easy
path for the formation of functionalized core-shell nanoparticles.


## 4. Experimental

### 4.1. Materials and Analytical Methods

Nanoparticles ofTiO2 rutile, TiO2 anatase, ZrO2, and CuO were purchased at MTI corporation (2700 Rydin Rd, Unit
D, Richmond, Calif., USA).
The TiO2 nanoparticles, from Dupont de Nemours, were 95% TiO2-coated
by a thin layer of alumina. All other chemicals were obtained at Aldrich
Company. TGA measurements were obtained using the SII Exstan 6000 TG/DTA 6200
of Seiko Instruments Inc. The sample was
heated at 10C°/min with a plateau at 250C° for 10
minutes, and then heated at 10C°/min with a plateau at 450C°
for 15 minutes, then heated to 900C°. The amount of polymer corresponds to the
quantity degraded between 250 and 450C°, and the amount of inorganic
matter to the remaining solid. Below 250C°,
water is evaporated. TEM micrographs were
obtained with a Tecnai 12 Biotwin microscope equipped with a W source (80 and
120 kV) and a 13 Mpixel Morada camera, except for the Si3N4 nanocomposite for which a 1 Mpixel Gatan camera was used. The samples were obtained by depositing a
dilute sample on a gold grid coated with a formvar film. No contrast agent was used. The images were analyzed using imageJ software
developed by the NIH (http://rsb.info.nih.gov/ij/). Low voltage (80 kV) favored
contrast between the core and the shell, but higher voltage (120 kV) favored
resolution at high magnification. The
molecular weight distribution of the polymer generated in experiment 3,
relative to polystyrene standards, was determined on a Waters GPC equipped with
an RI detector and a set of Polymer
Laboratories Mixed C columns. The UV-Vis spectrophotometer was a Cary 50 set at 210 nm.


### 4.2. Synthesis of PAA

PAA synthesis is described in details in  [30–34]. The following quantities were used:
402 mg of trithiobenzylcarbonate (TBC, 1.58 mmol), 10 mL of acrylic acid (0.132 mol), 48 mg of azobisisobutyronitrile (AIBN, 0.29 mmol), and refluxing ethanol
40 mL. After 2 hours, the reaction was stopped by placing the mixture in
contact to air and adding an inhibitor (BHT).
The solution was then concentrated by rotary evaporation until
approximately 15 mL of ethanol were left and the viscous solution was slowly added to 480 mL of
benzene under vigorous stirring. The
precipitate was collected and redissolved in the minimum of ethanol. This cycle
of dissolution precipitation was repeated four times. The polymer was then dried in vacuo.


### 4.3. Typical Synthesis of a Core-Shell Nanoparticles

4 g ofZrO2,
210 mg of PAA, and 20 mL of nanopure water (ρ=18.1MΩ⋅cm) were placed in a 50 mL beaker
immersed in an ice bath. The mixture was
stirred with a magnetic stir bar and sonicated during 6 minutes with a
sonicator (VibraCell 400 W) equipped with a microtip. An off-white dispersion of
ZrO2 in water was obtained.
In a 250 mL round-bottom flask, butyl acrylate (2.0 mL), styrene (2.0 mL),
50 mg of sodium dodecyl sulfate, and 20 mL of the ZrO2 dispersion
were slowly stirred and degassed during 30 minutes by sparging it with
argon. The mixture was heated to 80C°.
A solution containing 43 mg of 4,4′-azobis-4-cyanovaleric acid, 2 droplets of
sodium hydroxide (30%), and 20.2 mL of ultrapure water was added continuously
over 4 hours (5cm3/hour). The
role of the sodium hydroxide was to facilitate the dissolution of the initiator
(carboxylic acid). When the addition was over, the mixture was left at 80C°
for one hour in order to complete the reaction.


### 4.4. Determination of the Adsorption Isotherm [39]

A series of stock solutions of PAA in water
(50–200 mg/L) was
prepared and used to build a calibration curve by measurement of the absorption
at 210 nm.TiO2 (2 g) were
added to 10 mL of water (concentration 200 g/L identical to the one used for
the encapsulation) containing a known amount of PAA. The mixture was sonicated for 3 minutes and
left for 2 hours. The nanoparticles were
separated from the serum by centrifiguation (30 minutes at 4000 rpm). The concentration of PAA in the serum was
then analyzed by UV-VIS.


## 4.1. Materials and Analytical Methods

Nanoparticles ofTiO2 rutile, TiO2 anatase, ZrO2, and CuO were purchased at MTI corporation (2700 Rydin Rd, Unit
D, Richmond, Calif., USA).
The TiO2 nanoparticles, from Dupont de Nemours, were 95% TiO2-coated
by a thin layer of alumina. All other chemicals were obtained at Aldrich
Company. TGA measurements were obtained using the SII Exstan 6000 TG/DTA 6200
of Seiko Instruments Inc. The sample was
heated at 10C°/min with a plateau at 250C° for 10
minutes, and then heated at 10C°/min with a plateau at 450C°
for 15 minutes, then heated to 900C°. The amount of polymer corresponds to the
quantity degraded between 250 and 450C°, and the amount of inorganic
matter to the remaining solid. Below 250C°,
water is evaporated. TEM micrographs were
obtained with a Tecnai 12 Biotwin microscope equipped with a W source (80 and
120 kV) and a 13 Mpixel Morada camera, except for the Si3N4 nanocomposite for which a 1 Mpixel Gatan camera was used. The samples were obtained by depositing a
dilute sample on a gold grid coated with a formvar film. No contrast agent was used. The images were analyzed using imageJ software
developed by the NIH (http://rsb.info.nih.gov/ij/). Low voltage (80 kV) favored
contrast between the core and the shell, but higher voltage (120 kV) favored
resolution at high magnification. The
molecular weight distribution of the polymer generated in experiment 3,
relative to polystyrene standards, was determined on a Waters GPC equipped with
an RI detector and a set of Polymer
Laboratories Mixed C columns. The UV-Vis spectrophotometer was a Cary 50 set at 210 nm.


## 4.2. Synthesis of PAA

PAA synthesis is described in details in  [30–34]. The following quantities were used:
402 mg of trithiobenzylcarbonate (TBC, 1.58 mmol), 10 mL of acrylic acid (0.132 mol), 48 mg of azobisisobutyronitrile (AIBN, 0.29 mmol), and refluxing ethanol
40 mL. After 2 hours, the reaction was stopped by placing the mixture in
contact to air and adding an inhibitor (BHT).
The solution was then concentrated by rotary evaporation until
approximately 15 mL of ethanol were left and the viscous solution was slowly added to 480 mL of
benzene under vigorous stirring. The
precipitate was collected and redissolved in the minimum of ethanol. This cycle
of dissolution precipitation was repeated four times. The polymer was then dried in vacuo.


## 4.3. Typical Synthesis of a Core-Shell Nanoparticles

4 g ofZrO2,
210 mg of PAA, and 20 mL of nanopure water (ρ=18.1MΩ⋅cm) were placed in a 50 mL beaker
immersed in an ice bath. The mixture was
stirred with a magnetic stir bar and sonicated during 6 minutes with a
sonicator (VibraCell 400 W) equipped with a microtip. An off-white dispersion of
ZrO2 in water was obtained.
In a 250 mL round-bottom flask, butyl acrylate (2.0 mL), styrene (2.0 mL),
50 mg of sodium dodecyl sulfate, and 20 mL of the ZrO2 dispersion
were slowly stirred and degassed during 30 minutes by sparging it with
argon. The mixture was heated to 80C°.
A solution containing 43 mg of 4,4′-azobis-4-cyanovaleric acid, 2 droplets of
sodium hydroxide (30%), and 20.2 mL of ultrapure water was added continuously
over 4 hours (5cm3/hour). The
role of the sodium hydroxide was to facilitate the dissolution of the initiator
(carboxylic acid). When the addition was over, the mixture was left at 80C°
for one hour in order to complete the reaction.


## 4.4. Determination of the Adsorption Isotherm [39]

A series of stock solutions of PAA in water
(50–200 mg/L) was
prepared and used to build a calibration curve by measurement of the absorption
at 210 nm.TiO2 (2 g) were
added to 10 mL of water (concentration 200 g/L identical to the one used for
the encapsulation) containing a known amount of PAA. The mixture was sonicated for 3 minutes and
left for 2 hours. The nanoparticles were
separated from the serum by centrifiguation (30 minutes at 4000 rpm). The concentration of PAA in the serum was
then analyzed by UV-VIS.



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*Source: 609184-2008-09-28.xml*