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In this article, the basic principles of core-shell polymers (CSPs) such as definitions, classifications and applications are critically investigated. Introduction of CSPs characterization techniques, such as Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), 1H and 13C Nuclear Magnetic Resonance Spectroscopy (NMR), Small Angle Neutron Scattering (SANS), Nonradioactive Direct Energy Transfer (NRET), Photon Correlation Spectroscopy (PCS) and Dynamic Light Scattering (DLS), are highlighted. In addition, preparation techniques and recent studies of CSPs are briefly discussed. The factors that affect core-shell morphology and properties such as cross-linking radical penetration and diffusion, processing techniques and monomers polarity are considered. Core-shell polymers are structured composite particles consisting of at least two different components, one at the center as a core and surrounding by the second as a shell. Smart properties are considered to be the most desirable characteristics that allow this class of polymer to be used in various applications, particularly in biomedicine as drug delivery systems.
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Cite this: RSC Advances,2013,3,15543
Core–shell polymers: a review
Received 18th March 2013,
Accepted 31st May 2013
DOI: 10.1039/c3ra41296b
www.rsc.org/advances
Ros Azlinawati Ramli, Waham Ashaier Laftah* and Shahrir Hashim
In this article, the basic principles of core–shell polymers (CSPs) such as definitions, classifications and
applications are critically investigated. Introduction of CSPs characterization techniques, such as
Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM),
1
Hand
13
C Nuclear
Magnetic Resonance Spectroscopy (NMR), Small Angle Neutron Scattering (SANS), Nonradioactive Direct
Energy Transfer (NRET), Photon Correlation Spectroscopy (PCS) and Dynamic Light Scattering (DLS), are
highlighted. In addition, preparation techniques and recent studies of CSPs are briefly discussed. The
factors that affect core-shell morphology and properties such as cross-linking radical penetration and
diffusion, processingtechniques and monomers polarityare considered. Core-shell polymers are structured
composite particles consisting of at least two different components, one at the center as a core and
surrounding by the second as a shell. Smart properties are considered to be the most desirable
characteristics that allow this class of polymer to be used in various applications, particularly in biomedicine
as drug delivery systems.
1. Introduction
CSPs are structured composite particles consisting of at least
two different components, one in principle forms the core and
another forms the shell of the particles.
1
In 1961, Hughes and
Brown investigated the physical properties of core–shell
polymer (CSP) and their interesting morphology.
151
This class
of material has attracted much attention because of the
combination of superior properties not possessed by the
individual components. The systems might combine the
characteristics and properties of both shell and core where
the surface properties of the shell are translated to the core,
imparting new functionality to the CSP.
2,3
CSPs have been
used in a number of applications such as impact modifiers,
surface coatings, printing, catalysis, pollution control, sensing,
and drug delivery in biomedical application.
4–7
CSPs are
usually prepared in spherical form, implying a particle
structure with the initially polymerized polymer located at
the centre of the particle, and the later-formed polymers
becoming incorporated into the outer shell layer
8
as shown in
Fig. 1. The core part can be solid, liquid or gas and the shell
material is usually a solid, but its nature depends on the
targeted application.
4
CSPs are prepared in nano and micro
sizes according to preparation methods.
9,10
The approaches that have been used to prepare CSPs rang
between 1 and 3 steps using different techniques such as
dispersion, precipitation and emulsion polymerization.
11–15
Different types of materials have been used to synthesize CSPs,
such as methyl methacrylate (MMA), methacrylic acid (MAA),
styrene (St), divinylbenzene (DVB), acrylic acid (AAc),
N-isopropylacrylamide (NIPAM), ethyleneglycol dimethacrylate
(EGDMA) and N,N9-methylenebisacrylamide (MBA).
16
Advanced techniques such as Transmission Electron
Microscopy (TEM), Scanning Electron Microscopy (SEM),
Photon Correlation Spectroscopy (PCS), Dynamic Light
Scattering, Small angle neutron scattering (SANS), and
Nonradioactive Direct Energy Transfer (NRET) have been used
to characterize CSP and study the core-shell morphology
system. Core-shell monomers and preparation techniques are
summarized in Table 2.
2. Classifications
CSP can be classified depending on state phenomenon into
hydrogels and non-hydrogels. Moreover, CSP hydrogels can be
Department of Polymer Engineering, Faculty of Chemical Engineering, Universiti
Teknologi Malaysia (UTM), 81310 Skudai, Johor, Malaysia.
E-mail: waham1980@yahoo.com.my; Fax: +06-075536165; Tel: +06-0107039350
Fig. 1 Core–shell polymers (CSPs) consisting of central part, which may be a
solid, liquid or a gas, and a covered part, usually a solid.
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divided into NIPAM-based and non-NIPAM-based materials
and non-hydrogel CSPs can be divided into non-aqueous,
organic–inorganic hybrid and single-molecular particles.
Depending on size, spherical CSP can be divided into micro
and nano as shown in Fig. 2. This section of the paper
emphasizes CSP hydrogels and non-hydrogels.
2.1 Core–shell polymer hydrogels
CSP hydrogels are particles with a hydrogel shell surrounding
a non-hydrogel core or particles composed entirely of hydrogel
in both core and shell components.
3
CSP hydrogels have been
produced either to modify the stability and physical properties
of the polymers
17,18
or to impart stimuli-responsive properties
to non-responsive particles.
19
Designing CSP hydrogels with
particular or specific properties requires controlling important
parameters such as size, cross-linking density and the
incorporation of functional groups in both core and shell
compartments during synthesis.
20
CSP hydrogels have
attracted much interest in biomedical applications due to
their properties and core–shell structures.
21,22
CSP hydrogels
made of smart materials have widespread applications,
especially in biomedical areas, as a result of their response
to surrounding environmental changes such as temperature
and pH.
2.1.1 NIPAM based CSP hydrogels. Recently, given current
interest in ‘switchable’ or ‘smart’ materials,
poly(N-isopropylacrylamide (PNIPAM) has been extensively
used as a main component in CSP hydrogels due to its
thermoresponsive properties. PNIPAM display a low critical
solution temperature (LCST) of 32 uC.
23,24
PNIPAM reveals a
drastic decrease in particle size upon heating in aqueous
solution. Its responsive characteristics can be modified by co-
polymerization
25
and core–shell particles can be obtained.
26–28
NIPAM was applied either to core or shell or both core and
shell in CSP production.
29
Senff et al. prepared core–shell latex with a PSt core of 42 nm
hydrodynamic radius and a temperature sensitive PNIPAM
shell using emulsion polymerization. They found that the latex
had rheological properties which could be controlled by
temperature and was stable against flocculation even at
elevated temperatures.
17
Xiao et al. reported the synthesis of
submicron-size monodispersed thermoresponsive core–shell
hydrogel microspheres of 200–400 nm in diameter with
poly(N-isopropylacrylamide-co-styrene) (PNIPAM-co-St) cores
and PNIPAM shells.
30
Different approaches were reported for
preparation of multiresponsive copolymer microgels which
consisted of PNIPAM and acrylic acid. Jones and Lyon
synthesized (P(NIPAM-co-AAc))core/PNIPAM shell and
PNIPAM core/P(NIPAM-co-AAc) shell using two-stage precipita-
tion polymerization.
31
In another approach, copolymer of
NIPAM (core)/AAc (shell) was prepared by one-stage surfactant
free emulsion polymerization (SFEP).
32
2.1.2 Non-NIPAM CSP hydrogel. Non-NIPAM CSP hydrogel
can be divided into two categories based on conventional
monomer and acrylamide derivatives, where both of the
categories were also used to develop ‘‘smart’’ materials. In
the first category, non-NIPAM CSPs are based on conventional
monomer such as hydrophilic AAm, AAc and hydrophobic
MMA, St. The combination of hydrophilic and hydrophobic
monomers is useful in order to produce responsive CSPs. Babu
et al. developed novel core-shell microgels of poly(acrylamide-
co-methyl methacrylate) (P(AAm-co-MMA)) for controlled
release (CR) applications.
15
Xiao et al. developed positively
thermosensitive microgels of poly(acryamide-co-styrene)
(P(AAm-co-St)) core and interpenetrating polymer network
(IPN) shell of poly(acrylamide)/poly(acrylic acid) (PAAm/
PAAc).
14
Ramli et al. synthesized poly(styrene-co-methyl
methacrylate) (P(St-co-MMA)) core and poly(acrylamide-co-
acrylic acid) (P(AAm-co-AAc)) shell microgels for potential
application as drug releasing coatings.
33
In the second category, non-NIPAM core–shell polymers are based
on acrylamide derivatives such as N-n-propylacrylamide (NNPAM),
N-isopropylmethacrylamide (NIPMAM), N-ethylacrylamide and
N-vinylisobutylacrylamide.
33–35
These monomers were specifically
used to polymerize temperature-sensitive microgels. In aqueous
solution, the thermal response of these microgels manifests as a
drastic decrease of their volume. In order to allow control of the phase
transition over a broad range, copolymerization is suitable.
34
Copolymer microgels based on NNPAM and NIPMAM were prepared
by Wedel et al. and Zeiser et al. using precipitation polymerization
technique. Both studies showed that the resulting core–shell
microgels have linear thermoresponsive characteristics in a region
between 22 uCto45uC.
34,36
Fig. 3 shows the structure of monomer
structures of acrylamide derivatives that are normally used to prepare
non-NIPAM CSP hydrogel.
2.2 Non-hydrogel core–shell polymer
Non-hydrogel CSPs can be divided into non-aqueous, organic–
inorganic hybrid and single/unimolecular particles. Non-
aqueous CSPs are used in paints and coating applications as
pigments and binder. The core might be solid polymer particle
or rubber and the shell is made of hard polymer.
7
However,
most of the methods that explain non-aqueous CSP produc-
tion are described in the patent literature.
37–41
Inorganic–organic hybrid materials possess huge potential
in the synthesis of new functional materials
42
for light-
emitting and quantum-dot devices, photonics, photodetectors,
solar cells, biomedical and sensor applications.
43–50
Hybrid
CSPs are usually prepared in nano size particles and also
known as hybrid nanoparticles (NPs). The core of this
particular class of core–shell nanoparticles is made of a
polymer, such as polystyrene, poly(ethylene oxide), polyur-
ethane, poly(vinyl benzyl chloride), poly(vinyl pyrrolidone),
dextrose, surfactant, and different copolymers, such as
acrylonitrile butadiene styrene, poly(styrene acrylic acid), and
poly(styrene methyl methacrylate). In addition, shells were
prepared from different materials, such as metals, metal
oxides, metal chalcogenides and silica.
51
Methods of synthe-
sizing core–shell NPs typically involved physicochemical or
chemical processes. In physicochemical processes, an organic
or inorganic substance is precipitated at the core surface
during solvent evaporation or adsorption by means of
electrostatic and chemical or biochemical interactions. In
chemical processes, the polymerization is performed directly
in the presence of inorganic particles.
52
The most convenient
method to prepare core–shell NPs is encapsulation of
inorganic core into a polymeric shell via grafting the polymers
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on the surface of the particles.
42
Silica is the most used
inorganic material for core–shell NPs production. This is due
to the ability of silica to prevent core aggregation and outsider
environmental attacks and the wide practical uses of silica in
separation, biotechnology, medicine, chemosensors and coat-
ing. The silica shells provide an additional benefit of
facilitating biocompatibility and biofunctionalization. In
addition, core materials in organic–inorganic core/shell NPs
can be a dye, fluorescent conjugated polymer and co- or
terpolymer, etc.
53
Single-molecular particles, also known as unimolecular
micelles, are non-crosslinked CSPs consisting of amphiphilic
block or graft polymers in which hydrophobic and hydrophilic
segments are covalently connected with the dendritic or
hyperbranched core. Single-molecular particles demonstrate
a micellar behavior as a single molecule
54–57
and are usually
prepared by a multi-step process, such as combination of
alkyne polycyclotrimerization and alkyne-azide ‘‘click’’ chem-
istry,
54
tandem coordination, ring-opening and hyperbranched
polymerization,
58
self-condensation and ATRP.
55,56,59
Single-
molecular particles have attracted great attention from
scientists due to their potential in nanotechnology and
biomedical applications, such as a signal-molecular template,
live-cell imaging, drug carrier and drug release.
55,57,59–61
3. Preparation methods of core–shell
polymers
CSPs are typically prepared by a series of consecutive
emulsion, dispersion or precipitation polymerization
sequences with different monomer type. Usually, CSP particles
are prepared by multi-step procedures using seed particles as a
core material, where the second or third stage monomer is
polymerized in the seed latex particles. These seed particles
may be prepared in a separate step, or form in situ during the
polymerization.
3,23,62
These techniques however have the
significant limitations of being both expensive and time-
consuming, due to the multi step procedures. One-stage
reaction is a facile method to prepare polymer particles with
core–shell morphology.
63
Dispersion polymerization is a technique which produces
polymer particles in the range of 1–15 mm. The formed
polymers are insoluble in continuous phase (e.g. ethanol,
methanol, water).
64
Dispersion polymerization is considered a
class of precipitation polymerization. However, larger particles
and irregular shape of polymer particles were produced in
precipitation polymerization.
65,66
Moreover, emulsion poly-
merization is the main process for the preparation of
commercial emulsion, which involves a monomer that has
limited solubility in water.
67
The particle diameter is typically
within the range of 1–10 mm.
68
Fig. 4 illustrates common
methods to prepare CSPs described by Li and Stover.
5
Two-
stage emulsion polymerization (I) was the first general method
developed to prepare CSP. CSP particles were also prepared by
emulsion polymerization using reactive surfactants (II). The
formation of core–shell particles by step-wise hetero-coagula-
tion of smaller cationic polymer particles onto larger anionic
polymer particles, followed by heat treatment has also been
reported (III). Block copolymers can also be used to produce
core–shell type polymer nanospheres via block copolymeriza-
tion (IV).
3.1 Emulsion polymerization
Emulsion polymerization synthesized process is commonly
used to produce water based resins with a variety of
physicochemical and colloidal properties. The reaction system
is characterized by emulsified monomer droplets (ca. 1–10 mm
in diameter, 10
12
–10
14
dm
23
) dispersed in a continuous
aqueous phase with the assistance of an oil-in-water surfactant
at the very beginning of polymerization. Monomer swollen
micelles (ca. 5–10 nm in diameter, 10
19
–10
21
dm
23
in number)
may also exist in the reaction system, provided that the
concentration of surfactant in the aqueous phase is above its
critical micelle concentration (CMC). Most of the monomer
molecules dwell in giant monomer reservoirs (i.e. monomer
droplets). The polymerization is initiated by the addition of
initiator.
69
The emulsion polymerization technique is a
commercially and technologically important reaction system.
Currently, emulsion polymerization is the beginning of a
worldwide industry. This technique continues to grow through
its versatile reaction and its ability to tailor the properties of
the emulsion polymer produced.
68
Emulsion polymerization can be carried out using contin-
uous, batch and semi-batch process.
68
Batch processes are of
limited versatility for producing emulsion and are mainly use
in academic research with simple reaction formulations.
Accordingly, novel polymerizations were developed to replace
the batch process, such as semi-batch continuous process
70
and pre-emulsified semi-continuous seeded emulsion poly-
merization method.
71
Semi-batch is a versatile process and
mainly use in industry. The process is widely used for all
emulsion polymerization due to the highly exothermic nature
of free radical polymerization and limited heat removal
capacity in big scale reactors.
69
Moreover, semi-batch process
offers stringent quality control to produce emulsion products
with controlled particle morphology and polymer composition
and allows influence on the properties and applications of the
emulsion products.
69
Any properties, such as composition,
polymer architecture, particle size distribution, particle mor-
phology and molecular weight distribution, can be tailored by
this process.
72,73
The most significant difference between batch and semi-
batch emulsion polymerization is the polymerization ingre-
dients, such as surfactant, monomer, water and initiator, can
be fed to the process reaction system during the polymeriza-
tion as depicted schematically in Fig. 5. Consequently, the
residence time of particle nuclei distribution in semi-bath
emulsion polymerization process is broader. These features
make semi-batch emulsion polymerization kinetics and
mechanisms more complicated in contrast with batch
process.
69
Semi-batch process allows two types of feed stream, M (neat
monomer) feed and E (emulsion) feed
74,75
as shown in Table 1.
In M feed process, the reactor is initially charged with the
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monomer, water, emulsifier and initiator and polymerization
is carried out by feeding the remaining monomer continu-
ously. However, in E feed process, all of the monomers used
are pre-emulsified.
72
The beneficial effects of adding monomer and other
ingredients continuously are:
(1) Good temperature control with extra cooling of poly-
merization process.
(2) Easy to control polymerization rate by keeping process
‘starved’.
(3) Flexible control of molecular weight.
(4) Good polymer composition control.
Table 2 Monomer/materials that form CSP, preparation methods of CSP and their classification
Materials
Preparation method Classification Ref.Core Shell
St St, MPS Emulsion NPs
83
2VP NIPAM Dispersion Microgel
132
NIPAM, AAc NIPAM Precipitation Microgel
4,115
NIPAM, AAc/AFA NIPAM, APMA Precipitation Nanogel
9
NIPMAM, AAc/AFA NIPMAM, APMA
NIPAM AAc Emulsion Nanogel
32,79
NIPMAM NIPAM Precipitation Microgel
10
St, CdS/ZnS MMA Miniemulsion Nanoscale
133
TBA St Emulsion Microgel
134
MMA St Emulsion Microparticle
135
(PBA/MMA/MAA) PSt/AN, PBA/MMA Emulsion Microparticle
7
PI AAc/AAm Self-assembly Nanogel
136
NIPMAM NNPAM Dispersion Microgel
36
MMA AAm Emulsion Microgel
15
AAM, St AAm, AAc Emulsion Microgel
14,90
MMA St Emulsion Microparticle
137
NIPAM CMCS Emulsion Microgel
138
NIPAM NIPAM, APBA Precipitation Microgel
139
Silica PSt Emulsion NPs
52
NIPAM 4VP Dispersion Nanogels
11
NIPAM NIPAM/NIPAM, BMA Precipitation Nanogels
89
St PFA Emulsion Microparticle
1
PDVB, St VAc Emulsion Microparticle
80
BMA, PheMMA NIPAM, AnMA Precipitation NPs
62
AN NIPAM Emulsion Nanogel
82
NIPAM 2MBA Emulsion Nano/Microgel
81
SAM Silica Emulsion NPs
53
DVB CMS Precipitation Microgels
5
NIPAM NIPMAM Emulsion Microgels
25,119
NIPAM SA Emulsion Microgels
110
St Py Emulsion Nanoparticle
63
NIPAM, St NIPAM Emulsion Microgel
30
St NIPAM Emulsion Microgel
24
NIPAM PEI/Chitosan Dispersion Microgel
122
BMA, An/Phe PBBT, An/Phe Emulsion NPs
107
St NIPAM Photoemulsion Microgel
92
MMA St Dispersion Microparticles
140
NIPMAM NIPAM, AAc Precipitation Microgel
141
NIPAM St Emulsion Microgel
126
BA BA, St Emulsion Nanogels
91
AL CH L-b-L NPs
125
St, MMA AAm, AAc Emulsion Microgel
33
CMONS Silica Spray-drying NPs
44
ClO
4
,BF
4
,PF
6
PEDOT Electro-polymerization NPs
46
DTH-Fe
3
O
4
Silica Silanol hydrolysis NPs
47
PF PEG Polycyclotrimerization, ‘‘click’’-chemistry
54
NIPAM, HFMA PEGMEMA Emulsion Microgels
142
Table 1 Two types of feed stream in semi-batch process
Process Initial charge Continuous feeding
M feed water, initiator, emulsifier and part of monomer remaining monomer
E feed initiator and part of pre emulsified monomers remaining pre emulsified monomer
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In semi-batch processes, composition drift can be reduced
by feeding the monomer mixture into the reactor at the same
rate.
76
Semi-batch technique can also be used to manufacture
emulsions with non-uniform size and large composition
drift.
72
There is no other polymerization technique which is
more versatile than the semi-batch emulsion polymerization
processes. Lin et al. prepared thermoresponsive CSPs of
P(NIPAM-co-AAc) or poly(N-isopropylacrylamide-co-sodium
acrylate) (P(NIPAM-co-SA)) copolymer using batch process
surfactant-free emulsion copolymerization (SFEP). The poly-
merization was carried out for 2 h at 70 uC and a stirring rate
of 200 rpm. During the polymerization, the pH values of the
NIPAM/AAc and NIPAM/SA systems were measured to be 3–4
and 6–6.2, respectively. The obtained latex particles were milky
white with an average diameter of 200 to 500 nm.
77
Dupin
et al. polymerized 2-vinylpyridine (2VP) in the presence of
cationic surfactant, DVB cross-linker, and a hydrophilic
monomer, such as monomethoxy-capped poly(ethylene glycol)
methacrylate (PEGMA), using emulsion polymerization pro-
cess. The one-shot batch synthesis was carried out at 60 uC for
24 h. pH-Responsive microgel particles were obtained with
10% solid content and pH 8. In addition, the mean particle
diameter of samples ranged from 370 to 970 nm with narrow
size distribution.
78
Zhang et al. prepared and characterized a
series of poly(NIPAM-co-AAc) nanogels with temperature and
pH sensitivity by surfactant-free emulsion polymerization
(SFEP) via semi-batch and batch process. For semi-batch
method, the monomer of NIPAM, MBA, and AAc were
dissolved in deionized water and added drop wise to
Fig. 4 The common methods to prepare CSPs. Reproduced from ref. 5 by
permission of American Chemical Society.
Fig. 2 Classification of core–shell polymers (CSPs) depending on different point
of view.
Fig. 3 Monomer structures of acrylamide derivatives.
Fig. 5 Flowchart for a typical semi-batch emulsion polymerization process.
Reproduced from ref. 69 by permission of Elsevier.
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ammonium persulfate (APS) solution at intervals of 2 h and
reaction temperature of 70 uC. Otherwise, for conventional
batch method, the same amounts of NIPAM, MBA, and AAc
were dissolved in deionized water and heated to 70 uC. After 30
min, APS solution was injected to initiate the reaction. The
objective of this work was to prepare the ultimate pH and
temperature sensitive nanogels.
79
Pusch and van Herk
synthesized core–shell particles with a polydivinylbenzene
(PDVB), St and vinyl acetate (VAc) shell via two-stage emulsion
polymerization. A core with a layer of polystyrene (PSt)
polymerized onto the PDVB seed latex. Then, VAc was
polymerized onto PDVB/PSt seed lattices. The obtained
transparent lattices with a core–shell structure are especially
interesting because this material is very promising for
biocompatible optical devices, due to its unique properties.
80
Serrano-Medina prepared nano/microgels of
poly(N-isopropylacrylamide-co-ethylene glycol methyl ether
methacrylate-2-methacryloyloxybenzoic acid) (P(NIPAM-co-
PEGMEMA-co-2MBA)) by one-stage surfactant free emulsion
polymerization (SFEP). The core consisted of NIPAM, while
poly(ethylene glycol) methyl ether methacrylate (PEGMEMA)
formed the shell. The purpose of this study was to produce
hydrolysable nanogels for drug release and nanogel elimina-
tion from the body.
81
Khan synthesized copolymer microgels
which consised of temperature sensitive cores and pH
sensitive shells by one-stage SFEP method. The microgels
were obtained from NIPAM and AAc. Using this approach, it
was possible to prepare core–shell microgels as stable colloids
with 50–60 nm size. The high sensitivity of these P(NIPAM-co-
AAc) microgels to small changes in pH and temperature
suggest that they could be useful in drug delivery applications
where small changes in pH or temperature may exist or may be
imposed.
32
Sahiner synthesized thermo-responsive core–shell
nanogels of poly(acrylonitrile-co-N-isopropylacrylamide (P(AN-
co-NIPAM)) using one-stage emulsion polymerization techni-
que. Different sizes and morphologies such as core–shell and
connected beads were obtained by varying the feed ratios of
monomers and cross-linkers. The resulting particles are highly
monodisperse with a size range of 50–150 nm. Fig. 6 depicts
the proposed reaction scheme for the simultaneous polymer-
ization and cross-linking of AN and NIPAM monomers in an
oil-in-water microemulsion system.
82
Ni et al. synthesized hybrid nanoparticles (NPs) with a
polystyrene core and a hybrid copolymer shell in a two step
process: emulsion polymerization of styrene and subsequent
copolymerization of styrene with c-methacryloxypropyltri-
methoxysilane (MPS). The effects of operating conditions on
the copolymer microstructure were investigated. The outcome
of this study is that stable lattices with uniform copolymer
compositions can be achieved through the semi-continuous
addition of MPS to the reactor. Fig. 7 shows schematic
formation of the core–shell NPs by semi-batch emulsion
polymerization.
83
3.2 Dispersion polymerization
This technique allows synthesis of micro particles in the range
of 1–15 microns.
64
Most of the ingredients in this process,
including surfactant, initiators and monomers, are soluble in
continuous organic phase and which form polymers that are
insoluble in continuous phase.
64,65
Li et al. reported the
preparation of narrowly distributed nanogels by two-stages
dispersion polymerization. At first, the core particles com-
posed of PNIPAM were synthesized and then the core particles
were used as nuclei in the following stage for subsequent shell
addition of poly(4-vinylpyridine) (P4VP).
11
A similar method
was reported by Laus et al. and Okubo et al. for the preparation
of CSP hydrogel particles in micron size.
84–86
Fig. 6 A schematic representation of the copolymerization and cross-linking
reaction mechanism of AN with NIPAM in sodium dodecyl sulfate (SDS) micelles.
Reproduced from ref. 82 by permission of Elsevier.
Fig. 7 Schematic representation of the formation of the core–shell NPs by semi-batch emulsion polymerization. Reproduced from ref. 83 by permission of American
Chemical Society.
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3.3 Precipitation polymerization
Initiation and polymerization occurs in homogeneous solution
and the formed polymers are soluble in the polymer phase.
The resulting particles have a high polydispersity (PDI) with
irregular shape.
65,66
Preparation of microgels or hydrogels
based on poly(N-isopropylacrylamide) (PNIPAM) by precipita-
tion polymerization has been widely conducted for medical
applications.
87,88
Lyon’s group in Georgia studied thermoresponsive CSP
hydrogel particles, which are commonly synthesized by two-
stage precipitation polymerization. Core particles composed of
cross-linked PNIPAM or P(NIPAM-co-AAc) were synthesized
first and then used as nuclei for subsequent polymerization of
P(NIPAM-co-AAc) and PNIPAM, respectively. This approach has
the advantage of the thermally triggered collapse of the
growing polymer chains to control the nucleation and growth
phases.
31,89
3.4 Other techniques to prepare core–shell polymers
CSPs have been prepared using other techniques such as the
three-step synthesis approach which was used to prepare
thermoresponsive CSP by Xiao et al. In the first steps, core–
shell seeds with (P(AAM-co-St)) cores and PAAM or P(AAM-co-
BMA) shell layers were prepared by emulsifier-free emulsion
polymerization. In this second step, PAAM shells were
fabricated on the microsphere seeds by free radical polymer-
ization. Then, the seeds were immersed in an aqueous AAc
solution containing KPS and MBA as initiator and cross-linker,
respectively. The three-step approach is illustrated in
Fig. 8.
14,90
A single-molecular particle of hyperbranched conjugated
polyelectrolyte (HCPE) was synthesized by Pu et al. using a
combination of alkyne polycyclotrimerization and alkyne–
azide ‘‘click’’ chemistry. In this study, polyfluorene (PF) were
used as the core substance, covered by linear poly(ethylene
glycol) (PEG) as the shell.
54
The three-step technique was used
by Cai and Liu to synthesize a novel single-molecular/
unimolecular nanoparticle, multi hyperbranched poly[2-((2-
bromopropionyl)oxy)ethyl acrylate)-g-poly(N-isopropylacrylamide]
(HPBPEA-g-PNIPAM), via atom transfer radical polymerization
(ATRP). In the first step, HPBPEA was synthesized by self-
condensing vinyl polymerization (SCVP) based on 2-((2-bromo-
propionyl)oxy)ethyl acrylate) (BPEA). The second step included
the synthesis of a hydrophobic core with HPBPEA molecules,
which was initiated using ethylene glycol dimethacrylate
(EGDMA). Finally, HPBPEA grafted PNIPAM was synthesized
using ATRP. Fig. 9 shows a schematic illustration of the
synthesis routes of single-molecular nanoparticles multi-
HPBPEA-g-PNIPAM.
56
Mu et al. prepared a monodisperse and multilayer core–shell
(MMLCS) via surface cross-linking emulsion polymerization.
1-Nonylphenyloxy-2-deca(oxyethylene)-3-allyloxypropane
ammonium sulphate (SE-10N) was used as polymerizable
surfactant and sodium alkylated diphenyl ether disulfonate
(DSB) as anionic surfactant. Fig. 10 shows the preparation of
multilayer core–shell (MMLCS) emulsion via surface cross-
linking emulsion polymerization. The PBA core was synthe-
sized by seed polymerization using the PBA seed at 75 ¡2uC
for 3.5 h. The inner shell was obtained during the second stage
of polymerization and the outer shell was formed during the
third stage of polymerization.
91
Thermosensitive PSt–PNIPAM core–shell particles were
synthesized using photoemulsion polymerization technique.
The synthesis was carried out in three steps as illustrated in
Fig. 11. In the first step, a PSt core with 5 mol% NIPAM was
prepared by emulsion polymerization, which was covered by a
thin layer of photoinitiated 2-[P(2-hydroxy-2-methylpropiophe-
none)]-ethyleneglycol methacrylate (HMEM) in the second
step. Finally, the shell of a cross-linked PNIPAM was formed by
photoemulsion polymerization. This new synthesis strategy
may produce a thermosensitive shell of PNIPAM networks with
more homogeneous cross-linking density.
12,92
Kim et al. fabricated monodisperse core–shell microgels
based PNIPAM by capillary microfluidic technique. The pre-
microgel drops were generated by a capillary-based micro-
fluidic device, which were then polymerized in situ with redox
reaction as shown in Fig. 12. This technique allows incorpora-
tion of additional materials into the core–shell gels and
precisely controls the particle size in the range of 10–1000 mm
without the need to sacrifice the monodispersity of the
sample. The versatility of this approach can be used to
develop novel biomaterials for applications in drug delivery,
artificial muscles, and cancer therapy.
93
Fig. 8 Schematic illustration of the preparation procedure of thermoresponsive
CSP using three-stage approach. Reproduced from ref. 14 and 90 by permission
of Wiley and Elsevier.
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4. Development of latex particle
morphology of CSP
Development of the latex particle morphology of CSP is
affected by many variables such as cross-linking, radical
penetration and diffusion, batch and semi-batch processing
and polarity of monomers.
4.1 Effect of cross-linking
A cross-linker is a multifunctional monomer normally used in
free-radical polymerization to construct network structure.
94
The concentration of cross-linker is mainly dependent on
polymerization technique, with lower cross-linker content
needed in emulsion polymerization than in bulk and solution
polymerization. Furthermore, semi-batch process uses a lower
concentration of cross-linkers than batch process. In emulsion
Fig. 9 Schematic illustration of the synthesis route of single-molecular CSP multi-HPBPEA-g-PNIPAM. Reproduced from ref. 56 by permission of Wiley.
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polymerization, cross-linker is used to control the particle
morphologies and enhance the mechanical properties of the
product.
95
Cross-linking of the seed polymer creates elastic
forces which compete with the interfacial forces to control the
particle structure, which is very sensitive to the level of cross-
linking.
96,97
Fig. 10 Schematic illustration of preparation of MMLCS emulsion and nanostructured polymer film. Reproduced from ref. 91 by permission of Elsevier.
Fig. 11 Schematic representation of the preparation of PSt–NIPAM core-shell particles by photoemulsion polymerization. Reproduced from ref. 12 and 92 by
permission of Elsevier and Wiley.
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Durant et al. have predicted the effect of cross-linked seed
latex particles on equilibrium particle morphology of two
component particles, which is considered to be occluded
morphology (OCC), inverse core–shell (ICS) and core–shell (CS)
structures. The effects of seed latex cross-linking on the
resultant equilibrium morphology of composite particles can
be computed from the basic mechanics of amorphous
polymers, as applied to rubberlike elasticity. According to
Durant and Sundberg, CS structure can be transformed into
OCC structure due to a change in the free energy as illustrated
in Fig. 13. If the ultimate morphology is to be ICS, the original
cross-linked seed polymer must be deformed from its original
shape of a sphere into an outer shell.
96,97
Sheu et al. prepared core-shell latices by seeded emulsion
polymerization of styrene (St) into polystyrene (PSt) latices
with varying amounts of DVB cross-linker. The resulting latex
particle structures were investigated with SEM. The results of
this study indicated that the shell structure is changed due to
an increase in the degree of cross-linking in the seed latex, as
shown in Fig. 14. PSt formed a homogeneous shell on un-
cross-linked PSt seed. The morphology of the shell changed to
a snowman structure when PSt seed was cross-linked with
around 0.2% of DVB. At 6% of DVB the shape of the shell
changed into a raspberry structure.
80,98,99
4.2 Radical penetration and diffusion
The reaction temperature and method of monomer feeding
during polymerization has an effect on diffusion and reaction
of both polymer radicals and monomers within the latex
particle. Monomers easily penetrates to the latex particle in a
manner that likely results in a uniform concentration profile
across the particle, even with slow monomer feeding and
glassy seed polymers. On the other hand, polymer radicals may
be restricted to the periphery of the particles when the radical
flux is high enough and the monomer feed is slow enough for
glassy seed polymers, but probably not for low T
g
seed
polymers.
100
Ivarsson et al. and Jo
¨nsson studied the influence of the
relative difference between apparent glass transition tempera-
ture, T
g
, and reaction temperature within particle on the
ability of oligomeric radicals to penetrate the particle. Both
studies found the degree to which polymer radicals penetrated
the particles can be controlled by manipulating the glassy
nature of the polymer host and its T
g
during reaction.
101–103
Fig. 15 shows the possible particle morphologies produced
from differing penetration depths of radical polymers.
100,104
4.3 Batch and semi-batch processing
Batch process provides a spectrum of monomer concentration
from 80 wt% down to a fraction of a percent at the end of the
conversion. By contrast, semi-batch process offers constant
monomer concentration during polymerization process.
Consequently, the diffusivity of polymeric species remains
constant and, most importantly, the diffusivity of the radicals.
In the existence of soft seeds, radicals can diffuse easily
throughout the particles. However, in the existence of glassy
seeds (T
g
higher than reaction temperature), radicals may not
penetrate inside the particles. Thus, there is a second stage of
in which the polymer mainly forms in the outer region of the
particle and directly manipulates the final polymer morphol-
ogy.
104
Fig. 12 Drop formation of pre-microgel drops in a capillary microfluidic device.
Reproduced from ref. 93 by permission of Wiley.
Fig. 13 Free energy pathway for transforming a core–shell particle into an
occluded structure (OCC). Reproduced from ref. 96 by permission of American
Chemical Society.
Fig. 14 Schematic drawings of core–shell structures of PDVB/PSt latices with
increasing cross-linking of the core latex. Reproduced from ref. 80 by permission
of American Chemical Society.
Fig. 15 Possible particle morphologies produced from differing radical pene-
tration depth. Reproduced from ref. 100 and 104 by permission of Elsevier and
Taylor & Francis.
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4.4 Effects of highly polar monomers
Monomers of carboxylic acid such as methacrylic, acrylic,
maleic, itaconic and fumaric acids, have been used in multi
and single step emulsion polymerization approaches. Acrylic
acid can be identified unevenly incorporated inside the
polymer particles, near or on the particle surface. However,
methacrylic acid (MAA) participates more evenly into particle
phase during reaction and is incorporated more uniformly
inside the copolymer. Based on the previous reports, research
on the effect of polar monomers was focusing on methacrylic
acid (MAA).
104
Karlsson et al. reported the results of adding MAA into
styrene seeds latex and second-stage polymers of isoprene–
styrene. The result showed that adding 10% MAA to the
second-stage monomers resulting in the poly(isoprene-co-
styrene) moving to the outside of the particle, forming a
‘sandwich’ structure. When the acid was added to the seed
polymer instead of the second-stage, the structures resembled
hemispheres.
105
Fukuhara and Sundberg reported the results
of adding MAA varing from 0–10% into poly(butyl acrylate-co-
styrene) (P(BA-co-St)) seeds lattices. Second-stage polymers
were either PMMA or less polar poly (butyl methacrylate)
(PBMA). In the case of the poly(methyl methacrylate) (PMMA),
the structure changed from a well defined core–shell (at 0%
MAA) to an apparent homogeneous particle at 10% MAA. In
contrast, using PBMA led to changing the particle morphology
from hemispherical (at 0% MAA) to an inverted core–shell at
10% MAA.
104,106
5. Characterizations
CSP properties mainly depend on the internal structure of the
particles, therefore it is necessary to find the right method and
technique that are able to deliver information on the
distribution of the polymers which form the core and the
shell of the particles.
107
5.1 TEM and SEM
SEM and TEM are usually used to investigate the polymer
morphology as these techniques can provide valuable detail on
the internal structure of core–shell particles.
107
SEM and TEM
were used by Li and Stover to study the particle morphology of
cross-linked core–shell polymer microspheres in terms of
diameter and homogeneities. The findings indicated that both
the core particles and the final core–shell particles have
spherical shape and smooth surface as shown in Fig. 16(A) and
16(B).
5,108
Kirsch et al. investigated the particle morphology of poly(n-
butyl acrylate)/poly(methyl methacrylate) (PBA/PMMA) compo-
site latex particles using TEM and different staining techni-
ques. Preferential staining with ruthenium tetraoxide (RuO
4
)
and a Pt-shadowing technique was used to distinguish
between soft and hard phases. Fig. 17(A) shows that the PBA
rich phase appears darker than a PMMA phase which is not
stainable with RuO
4
and Fig. 17(B) indicated that PBA particles
are almost completely covered by the PMMA phase and no
shadow is detectable.
109
Lin et al. applied the staining technique with uranyl acetate
(UAc) to observe the distribution of the anionic (carboxylic
acid) functional group in the PNIPAM-based copolymers
microgels. Acrylic acid distribution was imaged in TEM
photograph as shown in Fig. 18. The anionic sites were
selectively stained with UAc acetate to appear darker in these
images.
110
5.2
1
H and
13
C nuclear magnetic resonance spectroscopy
(NMR)
Solid-state NMR
1
H spin-diffusion method (TEM) is used to
clarify the complex structures in the current latex system. This
characterization allows quantitative determination of the
extent of coverage of the core by the shell polymer and the
interphase thickness. Mellinger et al. applied spin-diffusion
techniques, exploiting demagnetization and remagnetization
effects in three-component system consisting of a PBA/PSt/
PAAc latex film containing residual water, as shown in Fig. 19
which showsa three-dimensional representation of the remag-
netization process.
111
Ishida et al. examined the structure of core–shell type
polymer particles composed of PBA and PMMA using a magic
angle spinning (MAS) system of solid-state
13
C NMR as shown
in Fig. 20. They found that the polymer particles of PBA/PMMA
(BM) do not have a typical core–shell type phase-separated
structure, but rather an incompletely phase-separated struc-
Fig. 16 (A) SEM image of poly(DVB-55) microspheres (PP112-02) with porous
shells prepared in a toluene : acetonitrile(40 : 60) mixture. (B) TEM micrograph
of PP112-02 microspheres having dense poly(DVB-55) cores and porous
poly(DVB-55) shells. Reproduced from ref. 5 by permission of American
Chemical Society.
Fig. 17 TEM pictures of samples with different soft to hard phase ratio
contrasted by (a) preferential staining with RuO4 and, (b) Pt-shadowing
technique. Reproduced from ref. 109 by permission of Wiley.
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ture in which small PMMA domains are dispersed in the
continuous PBA phase.
112
5.3 Small angle neutron scattering (SANS)
This technique can provide more detailed information of
internal structure of the entire CSP, such as the relationship
between temperature and the hydrodynamic radius depending
on scattering intensity. In addition, scattering data is used to
organize the density profile and the distance between the
center and the outer of the particles. SANS data obtained with
a core–shell microgel are shown in Fig. 21.
25
SANS machine
allows us to cover a large q-interval (qis the magnitude of
scattering vector) ranging down to q-values usually probed in
light scattering experiments. Therefore, the machine is well
suited to characterize colloidal particles with rather large
diameters and crystals formed by these particles.
24
Wai et al.
reported the SANS study of polymer particle morphology
formed by polymerization of trideuteromethyl (methyl-d
3
)
methacrylate in the presence of a well characterized core of
hydrogenated (50/50) P(St-co-MMA) seed. The resulting poly-
mers possess a core–shell structure as determined by SANS
where the radius of the core is approximately 50 nm and the
shell radius 14 ¡6 nm.
113
Fig. 18 Observation of ultrathin cross sections of copolymer particles stained with a 1% uranyl acetate (UAc) solution. Reproduced from ref. 110 by permission of
Wiley.
Fig. 19 Three-dimensional representation of the remagnetization process.
Reproduced from ref. 111 by permission of Wiley.
Fig. 20 Shows high-resolution solid-state
13
C NMR spectra of the core–shell
polymer particles BM, PBA homopolymer and PMMA homopolymer, which were
measured at 30 uC by cross polarization (CP)/MAS or dipolar decoupling (DD)/
MAS spectroscopy. Reproduced from ref. 112 by permission of Elsevier.
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5.4 Nonradioactive direct energy transfer (NRET)
NRET has been used in a wide variety of polymeric systems in
order to gain dynamic and morphological information at the
nanometer scale. Fluorescence techniques, especially energy
transfer studies, have been widely used in static and dynamic
studies of both natural and synthetic polymers. NRET is a
process by which energy is transferred between a photoexcited
chromophore and a ground-state chromophore via a dipole-
induced dipole interaction.
114
There is a limitation in the
quantitative interpretation of the data due to the overall extent
of the energy transfer which is still small, even when there is
substantial mixing; nevertheless, trends are apparent.
8
Marion
et al. studied the internal structure of core–shell latex particles
using the NRET technique. Analysis of donor fluorescence
decay was carried out by choosing a donor concentration
profile within the particles. Using a simple model for the
concentration profile of the shell polymer in the latex particles,
a quantification of the diffuse interface between the core and
the shell of the particles was possible. Good fits of the
theoretical fluorescence decay curve to the experimental
decays were found. Fig. 22 shows the comparison of the
donor concentration profiles obtained for two different
compositions of latex, CS3 (full line) and CS8 (dashed line)
from the best fit of eqn (1) (i.e., eqn (1) is the theoretical
fluorescence decay curve which should fit the experimental
data to the fluorescence decay data).
107
(1)
where I
D
is a quantity proportional to the donor concentration,
C
D
is the donor concentration, C
A
is the accepter concentra-
tion, ris the distance from the center of the composite
particle, tis the donor fluorescence lifetime, tis time and Ris
the particle radius. Gan and Lyon developed the synthetic
techniques that used in the creation of core–shell particles to
spatially localize NRET fluorophores within the microgel core
and shell, such that thermo-induced changes in the core–shell
interface are directly probed by energy transfer across that
interface.
114
In another contribution, they used NRET to
analyse cross-linker heterogeneity in core–shell nanoparticles.
To interrogate the composition of these various network
densities in the region of the core–shell interface, NRET was
used via covalently localizing the donor and acceptor in the
core and shell, respectively.
62
5.5 Photon correlation spectroscopy (PCS)
PCS is a powerful method for investigation of the shape and
motion diffusion of polymer colloids.
8
Temperature-pro-
grammed photon correlation spectroscopy (TP-PCS) was used
to determine the mean particle sizes and particle size
distributions. This technique has been applied extensively to
the characterization of such materials due to the potential for
in situ size characterization of soft materials that cannot be
reliably sized by electron microscopy, due to deformation and/
or dehydration under vacuum.
89
PCS has been used to
characterize the volume phase transition (VPT) behaviour
and shell thickness of CSP as a function of pH and
temperature.
4,115
Fig. 23a and b compare the VPT behavior
of lightly cross-linked (1 mol% MBA, panel a) and highly cross-
linked (10 mol% MBA, panel b) P(NIPAM-co-AAc) core
microgels in conditions both above and below the AAc pK
a
.
4
5.6 Dynamic light scattering (DLS)
DLS is the method used most to characterize microgels in
dilute solution,
116
for structural analysis of microgel parti-
cles
12
and to get information about the size of nanoparticles.
DLS allows determination of the hydrodynamic radius of a
particle based on the relationship between the time dependent
fluctuations in the scattered light and the rate of diffusion of a
particle in solvent. In DLS experiments, all the measurements
were made at 90uangle. Fig. 24 shows that the hydrodynamic
diameter of the core–shell particles was found to be 104 nm
with DLS measurements with a very narrow PDI.
82
Fig. 21 SANS scattering data taken at 25, 39, 50 uC. Reproduced from ref. 25 by
permission of American Chemical Society.
Fig. 22 Comparison of the donor concentration profiles obtained for CS3 and
CS8 from the best fit of eqn (1) to the fluorescence decay data. Reproduced
from ref. 107 by permission of American Chemical Society.
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6. Properties
Combinations of two polymers or materials are normally used
to enhance one or both of the components and to achieve
better properties. The core–shell method is one of the
techniques that has been used to combine two different
materials to reach superior properties, which are difficult to
achieved by blending them. Using the core–shell method to
prepare CSP hydrogels was used to enhance the physicochem-
ical properties such as swelling and stimuli-responsive.
3
Swelling behaviors, such as extent of swelling and quick
response upon changes of external stimuli, are interesting
properties of microgels.
117,118
The characteristics of swelling
behaviors depend on the cross-linking density and thickness
of the particle shells.
119
In the domain of responsive hydrogels, the most commonly
studied are those composed of thermoresponsive polymers,
which undergo dramatic changes in network swelling as a
function of temperature. The most widely studied class of
thermoresponsive microgels are those composed of NIPAM. At
temperatures above y31 uC, PNIPAM undergoes an endother-
mic, entropically driven, phase transition from a random coil
to a collapsed globule.
3
One particularly useful feature of hydrogel materials is the
high porosity of the polymeric network.
3
Pores may possibly
formed in hydrogels by phase separation during polymeriza-
tion, or exist within the polymer network. The factors that
control the volume fraction of the pores, pore sizes and pore
interconnections are the cross-link density and composition of
network polymeric chains.
120
Furthermore, a uniform micro-
gel particle size is essential for drug delivery systems (DDS).
121
Moreover, drug release kinetics can be dominated by mono-
dispersed microgel particles, which facilitate the formulation
of intelligent DDS.
30
CSPs for paints and coatings are
concerned with degree of opacity (obliteration, cannot be
easily penetrated). Typically, titanium dioxide (TiO
2
) is used to
increase opacity of emulsion paint. Khan et al. studied the
composition of shell which could provide optimum opacity.
They found that CSP with a shell containing St/AN in a ratio of
60 : 40 shows similar performance with 15% reduction of
TiO
2
. This approach may be used to reduce the quantity of
TiO
2
in paint, thus reducing the cost of the paint.
7
Organic–inorganic hybrid nanoparticles, in general, have
dual properties from both the inorganic and organic materials.
The inorganic material, especially a metal oxide coating on an
organic material, is beneficial in several respects such as
increased strength of the overall material, resistance to
oxidation, thermal and colloidal stability, and abrasion
resistance. At the same time, these particles also show
polymeric properties such as excellent optical properties,
flexibility, and toughness, and in addition they can improve
the brittleness of the inorganic particles.
51
The micellar structure of single-molecular particles is static
rather than dynamic due to the connection of hydrophobic
and hydrophilic segments with dendritic or hyperbranched
core, which offers structurally stable monodisperse macro-
molecules.
55
Single-molecular particles do not disassemble
upon dilution and are stable to environmental changes due to
their covalent nature, thereby providing excellent in vivo
stability. Furthermore, the highly branched structure of the
copolymers provides many end groups for further functiona-
lization.
61
Fig. 24 Particle size distribution of 0.5% cross-linked (P(AN-co-NIPAM)).
Reproduced from ref. 82 by permission of Elsevier.
Fig. 23 Temperature-induced VPT behavior of P(NIPAM-co-AAc) core particles
above (#, pH 6.5) and below (N, pH 3.5) the pK
a
(4.25) of acrylic acid as
monitored by photon correlation spectroscopy: (a) 1 mol% MBA; (b) 10 mol%
MBA. Reproduced from ref. 4 by permission of American Chemical Society.
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7. Recent study on core–shell polymers
Core–shell polymers have attracted enormous research inter-
est, both from the point of view of fundamental science and
for prospective applications. In addition, the unique proper-
ties of CSP attracted scientists to study and developed new
microgel systems and re-investigated older systems using
advanced techniques and methods. In this section, previous
studies are reviewed. Multiresponsive CSPs were prepared by
Leung and his coworkers, with which they developed a new
method to prepare temperature sensitive core surrounded by
pH sensitive shell. The core–shell gels were prepared by graft
copolymerizations of NIPAM with either poly(ethyleneimine)
(PEI) or chitosan in aqueous media. The unique core–shell
nanostructures of PNIPAM/PEI or PNIPAM/chitosan exhibited
remarkable responses to pH and temperature.
122
Yu et al.
prepared monodisperse CS microspheres composed of a
PNIPAM core and a biocompatible poly(2-hydroxyethyl metha-
crylate) (PHEMA) shell by microfluidic emulsification, free-
radical polymerization and ATRP as shown in Fig. 25. The
thermo-responsive swelling/shrinking of the PNIPAM core and
opening/closing behaviour of the PHEMA shells is highly
attractive for developing DDS.
123
Lee et al. demonstrated that core–shell poly(styrene/pyrrole)
P(St/Py) particles were successfully prepared by using Fe
3+
-
catalyzed oxidative polymerization with emulsifier-free emul-
sion polymerization in aqueous medium. The resulting P(St/
Py) particles showed excellent electrical conductivity (2.21 S
cm
21
) due to the core–shell morphology. This method can be
effectively utilized to prepare structured functional polymeric
materials with various morphologies and inner structures for
bio- or chemical-sensor applications and electrical devices.
Fig. 26(a) shows a schematic for the formation of core–shell
P(St/Py) particles and (b) the detailed reaction mechanism of
pyrrole monomers via Fe
3+
-catalyzed oxidative polymeriza-
tion.
63
Chi et al. employed poly(ethylene glycol) ethyl ether
methacrylate (PEGEEMA) as a core and poly(ethylene glycol)
methyl ether methacrylate (PEGMEMA) and poly(acrylic acid)
as a shell to produce thermoresponsive core–shell microgels.
The resultant core–shell particles have a very narrow size
distribution and can self-assemble into colloidal crystals. This
particle will not only have thermal responsive properties like
PNIPAM polymer, but also have biocompatibility.
29
Zhang et al. reported a facile method to create a ‘‘living’’ in
situ gelling system for controlled formation of hydrogels from
a hyperbranched polymer (BAP) with disulfide-linked core–
shell structures. In another contribution, they developed a
general approach to controlled formation of microgels/
nanogels for producing hydrogel particles with customizable
structures and properties from a disulfide-linked core–shell
BAP. An inverse emulsion technique is used to obtain micro-
or nanodroplets of a disulfide-linked core–shell BAP. The
approach could produce fine-tunable micro/nanodrug carriers,
having broad implications in diagnostics and therapeutic
delivery systems. The general approach was summarized in
Fig. 27.
124
Haidar et al. studied a controlled-release protein delivery
system consisting of core–shell hybrid nanoparticles (NPs).
They formulated the core–shell NPs via L-b-L self assembly of
alginate (AL) and chitosan (CH) on liposomes. Their results
demonstrate that this delivery system features an extended
shelf life and can be loaded immediately prior to administra-
tion, thus preventing any loss of the protein.
125
Ramli et al. developed a new method to prepare hydrogel
CSPs using pre-emulsified semi-batch emulsion polymeriza-
tion. In this approach, pre-emulsified monomer was first
prepared followed by polymerizing monomers in a pro-
grammed manner. The resulting CSP cannot simply be
described as a core–shell type due to the interplay of
thermodynamics, as well as kinetic, parameters during the
polymerization process. Accordingly, a ‘‘raspberry’’-shaped
structure was formed during the transition morphologies.
Fig. 28 shows SEM images (a) and an illustration of
‘‘raspberry’’-shape of the structure of the core particle (b).
33
Zeiser et al. reported a novel design strategy of linearly
thermoresponsive core-shell microgels, with a shell made of
Fig. 25 Schematic illustration of the preparation procedure of the proposed
microsphere with PNIPAM core and PHEMA shell: (a) Microfluidic preparation of
monodisperse emulsion droplets containing NIPAM monomer and poly(vinyl
alcohol) (PVA) polymer, (b) polymerization of PNIPAM core with the emulsion
droplet as synthesis template, and (c) fabrication of PHEMA shell on the PNIPAM
core via ATRP method. Reproduced from ref. 123 by permission of Elsevier.
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poly(N-n-propylacrylamide) (PNNPAM) and a core consisting of
poly(N-isopropylmethacrylamide) (PNIPMAM). In the region
between 25 uC and 41 uC, the response of the particles is
directly proportional to the temperature as shown in Fig. 29.
This material is of particular interest because its unique
responsive behavior might be advantageous for new actuator
designs and responsive coatings.
36
Peng et al. studied a novel approach for preparing hollow
PSt particles by seed emulsion polymerization. The particles
are composed of PNIPAM cores and PSt shells and became
hollow upon dehydration of the PNIPAM microgels. Hollow
Fig. 26 (a) Schematic illustration of the mechanism for the preparation of core-shell poly (St/Py) particles. (b) Detailed reaction mechanism of pyrrole monomers via
Fe
3+
-catalyzed oxidative polymerization. Reproduced from ref. 63 by permission of American Chemical Society.
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particles have shown potential applications in drug delivery,
catalysts, controlled release etc. Fig. 30 shows the preparation
route of the hollow particles.
126
Other contributions related to
hollow CSPs are described by a number of researchers.
127–131
8. Applications
CSP features, such as colloidal stability, three dimensional
networks and dispersed particles which can expel water, are
being actively investigated due to technological applications in
various areas such as industry, medicine, environmental
cleanup,
143
surface coating,
144
drug delivery,
145
optoelectric
switches
146
and others. The ability of CSPs to respond to
external stimuli (temperature, ionic strength, pH, external
stress and solvent nature) reversibly has widened the research
related to this area. Nanoparticles and microgel particles are of
particular interest because of their intrinsic properties, such as
narrow particle size distribution (PSD), small size, microheter-
ogeneous structure and high surface volume.
61,144
CSPs have been used as micro/nanocarriers in drug
delivery.
61,147,148
For example, Lapeyre et al. developed new
multiresponsive core–shell microgels consisting of a PNIPAM
core and P(NIPAM-co-acrylamidophenylboronic acid)
(P(NIPAM-co-APBA)) shell. The core–shell particles in the
collapsed state (T=40uC) have diameters and PDI ranging
from 159 to 215 nm and 0.009 to 0.025, respectively. This work
has developed the concept of a temperature-responsive/
hydrophilic core combined with a glucose-responsive shell.
These CSP microgels, which have a response to glucose at
physiological pH, were applied to deliver insulin. Results have
revealed that the extent of core swelling was regulated by its
own internal stimulus, i.e., temperature, and shell compres-
sion, which is proportional to glucose concentration.
139
Yang et al. developed a new type of single-molecular
particles as drug nanocarriers for targeted cancer chemother-
apy. The core was a hyperbranched aliphatic polyester, Boltorn
H40, the inner hydrophobic layer composed of random
copolymer of poly(malic acid) and poly(e-caprolactone) (PMA-
co-PCL) segments, while the outer hydrophilic shell was
composed of poly(ethylene glycol) (PEG) segments. An antic-
ancer drug, doxorubin (DOX) was conjugated onto the PMA
segments by acid sensitive hydrazone bonds. The studies
indicated that very little DOX was released at pH 7.4 which
meant DOX-conjugated single molecular particles exhibited
excellent stability during blood circulation. Furthermore, once
the micelles were internalized by the cancer cells, the pH-
sensitive hydrazone linkages were cleavable by the intracel-
lular acidic environment, which initially caused a rapid release
of DOX. These findings indicated that the single-molecular
particles exhibited a pH-triggered drug release profile and
potentially excellent in vivo stability, which may provide a very
promising approach for targeted cancer therapy.
61
Photonic crystals (PCs) from CSPs are highly promising for
a variety of nanoscaled optoelectric devices. Up to now,
Fig. 27 (A) Schematic illustration of the core/shell separation process –
dissociation of the shells and cross-linking of the cores – and (B) schematic
depiction of the synthetic approach to controlled formation of (multilayered)
hydrogel particles. Reproduced from ref. 124 by permission of American
Chemical Society.
Fig. 28 (a) SEM images and (b) illustration of the ‘‘raspberry’’-shape of the structure of the core particle. Reproduced from ref. 33 by permission of Elsevier.
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semiconductor quantum dots (QDs) have been incorporated
into the voids of colloidal crystals by in situ growth method.
However, the incorporation of QDs does not protect the QDs
from oxidation and the method does not allow the formation
of high quality QDs, known for low PDI and strong
photoluminescence (PL). Accordingly, Fleischhaker and
Zentel prepared composites PCs from CSP, which incorporated
highly fluorescent CdSe quantum dots (QDs) in the core by
modified miniemulsion polymerization. CSP of different
diameters have been self-assembled to colloidal photonic
crystals (PCs) with photonic stop bands located in the visible
range of the electromagnetic spectrum. The controlled
combination of electronic confinement, originating from the
QDs, and photon confinement, due to the periodic dielectric
structure of the colloidal crystal, as it has been realized in this
work presents a huge platform for the design and construction
of novel optoelectronic devices based on PCs.
133
The use of CSP microgels for controlled uptake and release
of active species has a great potential. Microgel particles are
attractive for controlled uptake and release because they
respond much more rapidly to external stimuli in comparison
to macrogels, and they are highly stable to aggregation, at least
in their swollen state.
132
The thickness of the gel layer offers
control over the size of the final shell formed. The responsive
nature of the gel layer to external triggers, such as tempera-
ture, can be used to control their catalytic activity.
149
Microgel
particles with temperature-responsive PNIPAM cores and pH
responsive poly(4-vinylpyridine) (P4VP) shells prepared by Li
et al. showed that the shell does not significantly perturb the
temperature-induced volume phase transition of the core.
11
Accordingly, Bradley and Vincent improve the CSP properties
by introducing pH-responsive poly(2-vinylpyridine) (P2VP) as a
core and temperature-responsive PNIPAM as a shell. The
uptake and release of an anionic surfactant from the microgels
has been investigated as a function of solution pH and
temperature. The results indicate that the surfactant mainly
interacts with the PVP core. The results indicate that
electrostatic attraction between the anionic surfactant and
the cationically charged core of the microgel particles is the
dominant mechanism for absorption of the surfactant into the
core–shell microgel particles.
132
The use of polymeric systems particularly provides a clear
optimization to develop controlled release (CR)
142,150
formula-
tions to achieve the desired therapeutic results at the target
site. Solubility of drug in the polymer matrix is an important
factor in controlling the delivery. One important step in the
development of CR drug delivery systems is the loading of the
drug into the polymeric matrix. Drug loading within the
polymer matrix depends upon the release, which involves
Fig. 29 Linear temperature response. (a) Schematic illustration of a core-shell microgel which undergoes three regions of different swelling behavior (completely
reversible process). b, corresponding classification of previously mentioned regions in an exemplary R
h
(T)-diagram of a core-shell microgel system with 10 mol% cross-
linked cores. In region I we find a restricted shell collapse, while region II covers the linear swelling behavior. Region III indicates the occurrence of an active core
collapse. Reproduced from ref. 36 by permission of Elsevier.
Fig. 30 Preparation of hollow particles with PNIPAM microgels as the cores.
Reproduced from ref. 126 by permission of Elsevier.
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factors such as rate and swelling degree of the microparticles,
drug/polymer interactions, drug solubility and its concentra-
tion. Babu et al. presented new experimental data on the
development of novel 5-fluorouracil-loaded poly(acrylamide-
co-methylmethacrylate) CSP microgels for CR applications.
Higher drug loadings and faster release rates have been
observed by the solvent evaporation technique. Sustained and
prolonged drug release rates have been observed from the in
situ drug-loaded microparticles of this study. The study further
demonstrated that by exploiting the relationship between the
outer shell–inner core combinations, the drug action would
offer important new information to design improved core–
shell microparticles for fine-tuning of drug release, as well as
for a deeper understanding of the drug action mechanisms
through the core–shell structures.
15
CSPs in paints and coating applications are used to
improve the hiding power of coatings and as pigments. The
chief ingredients of shell in this CSP are styrene–acrylonitrile/
butyl acrylate/methyl methacrylate copolymer. These air voids
and non-pigmented emulsions are transparent to light, but
they have different refractive indices (RI; air – 1.0 and binder –
1.58, in general any binder has a RI in this range), causing
opacity in the dried film of the emulsion paint. Incorporation
of such a core–shell polymer in water based paints, together
with well dispersed titanium dioxide, will enhance the opacity
of the paint. The optimum composition of shell in the CSP of
poly(butyl acrylate-co-methyl methacrylate-co-methacrylic acid)
(PBA/MMA/MAA) core and a poly(styrene-co-acrylonitrile) (PSt/
AN), poly(butyl acrylate-co-methyl methacrylate) (PBA/MMA)
shell was investigated by Khan et al. and was applied in
emulsion paint as a paint binder.
7
9. Conclusion
This paper presents a general overview of core–shell polymers
(CSPs). The combination of a core in the center surrounded by
the shell seems to have unique properties of both the raw
materials of the core and the shell. However, applications of
CSPs are expanding in various areas such as impact modifiers,
surface coatings, printing, catalysis, pollution control, sensing,
and drug delivery in biomedical applications. Morphological
analysis of CSPs is the most important data that can provide
detailed information about internal structure which leads to
an estimation of their properties. The environmental
responses of CSPs, such as thermoresponse and pH sensitivity,
are the most important characteristic for drug delivery
applications. TEM, SEM, NMR, SANS, NRET, PCS and DLS
were used to provide important data in terms of the final
structure of CSPs. According to the literature, emulsion
polymerization seems to be the most used technique to
prepare CSPs in different size. NIPAM is the most commonly
used monomer to prepare CSPs, due to its high response to the
environmental change.
Abbreviations
AAm Acrylamide
AAc Acrylic acid
AFA 4-Acrylamidofluorescein
AL Alginate
AN Acrylonitrile
An Anthracene
AnMA 9-Anthryl methacrylate
AnMMA 9-Phenanthryl methyl methacrylate
APBA Acrylamidophenylboronic acid
APMA N-(3-aminopropyl) methacrylamide hydro-
chloride
APS Ammonium persulfate
ATRP Atom transfer radical polymerization
BA Butyl acrylate
BAP Hyperbranched polymer
BF
4
Boron
BIBB 2-Bromoisobutyryl bromide
BM PBA/PMMA
BMA Butyl methacrylate
BPEA 2-((2-bromopropionyl)oxy)ethyl acrylate)
2BMA 2-Butyl methacrylate
CC Cross polarization
CH Chitosan
ClO
4
Chlorine
CMC Critical micelle concentration
CMCS Carboxymethyl chitosan
CMS Chloromethylstyrene
CMONS a-[(4-Methoxyphenyl)methylene]-4-nitro-
benzeneacetonitrile
3CMS 3-Chloromethylstyrene
CR Controlled release
CS Core-shell
CSP Core-shell polymer
CSPs Core-shell polymers
CuBr Cuprous bromide
CuCl Cuprous chloride
DD Dipolar decoupling
DDS Drug delivery systems
DLS Dynamic light scattering
DOX Doxorubin
DSB Diphenyl ether disulfonate
DTH 3;5-di-tert-butyl-2-hydroxybenzaldehyde
DVB Divinylbenzene
EGDMA Ethyleneglycol dimethacrylate
Fe
3
O
4
Ferum Oxide
5-FU 5-Fluorouracil
GMA Glycidyl methacrylate
HCPE Hyperbranched conjugated polyelectrolyte
HFMA 2,2,3,4,4,4-hexafluorobutyl methacrylate
HMEM 2-[P(2-hydroxy-2-methylpropiophenone)]-
ethyleneglycol methacrylate
HMTETA 1,1,4,7,10,10-
Hexamethyltriethylenetetramine
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HPBPEA poly[2-((2-bromopropionyl)oxy)ethyl acry-
late)-g-poly(N-isopropylacrylamide]
ICS Inverse core-shell
IF Inner fluid
IPN Interpenetrating polymer network
KPS Potassium persulfate
L-b-L Layer-by-layer
LCST Lower critical solution temperature
MAA Methacrylic acid
MAS Magic angle spinning
MBA N,N9-Methylenebisacrylamide
2MBA 2-Methacryloyloxybenzoic acid
MF Middle fluid
MPS c-Methacryloxypropyltrimethoxysilane
MMA Methyl methacrylate
MMLCS Multilayer core-shell
NaSS Sodium p-styrene sulfonate
NIPAM N-isopropylacrylamide
NIPMAM N-isopropylmethacrylamide
NMR Nuclear magnetic resonance spectroscopy
NNPAM N-n-propylacrylamide
NPS Hybrid nanoparticles
NRET Nonradioactive direct energy transfer
OCC Occluded
OF Outer fluid
PEDOT Poly(3,4-ethylenedioxythiophene)
PAAm Poly(acrylamide)
PAAc Poly(acrylic acid)
PAN Poly(acrylonitrile)
PBMA Poly(butyl methacrylate)
PBBT Poly(butyl methacrylate-co-butyl acrylate-
co-trifluoroethyl methacrylate)
PBA Poly(butyl acrylate)
PBBT Poly(butyl methacrylate-co-butyl acrylate-
co-trifluoroethylmethyacrylate)
PCL poly(e-caprolactone)
PEI Poly(ethyleneimine)
PF Polyfluorene
PFA Perfluoroalkyl acrylate
PF
6
Phosphorus
PI Polyisoprene
Py Pyrrole
PPy Poly(pyrrole)
PBMA Poly(butyl methacrylate)
PMA poly(malic acid)
PMDETA 1,1,4,7,7-pentamethyldiethylenetriamine
PMMA poly(methyl methacrylate)
PCs Photonic crystals
PCS Photon correlation spectroscopy
PDI Polydispersity
PDVB Polydivinylbenzene
PEG Poly(ethylene glycol)
PEGEEMA Poly(ethylene glycol) ethyl ether methacry-
late
PNIPAM Poly(N-isopropylacrylamide)
PNIPMAM Poly(N-isopropylmethacrylamide)
PNNPAM Poly(N-n-propylacrylamide)
PEGMA Poly(ethylene glycol) methacrylate
PEGMEMA Poly(ethylene glycol) methyl ether metha-
crylate
P2VP Poly(2-vinylpyridine)
P4VP Poly(4-vinylpyridine)
PHEMA Poly(2-hydroxyethyl methacrylate)
Phe Phenanthrene
PL Photoluminescence
PSD Particle size distribution
PSt Polystyrene
PVA Poly(vinyl alcohol)
PVAc Poly(vinyl acetate)
PVP Poly(vinyl pyridine)
QDs Quantum dots
RI Refractive indices
RuO
4
Ruthenium Tetraoxide
SANS Small angle neutron scattering
SEM Scanning electron microscopy
T
g
Glass transition temperature
St Styrene
SA Sodium acrylate
SAM Substituted acetylene monomers
SCVP Self-condensing vinyl polymerization
SDS Sodium dodecyl sulfate
SFEP Surfactant free emulsion polymerization
TFEM Trifluoroethylmethacrylate
TEM Transmission electron microscopy
TFEM Trifluoroethyl methacrylate
TiO
2
Titanium Dioxide
TP Temperature-programmed
TMEDA Tetramethylethylenediamine
t-BHP tert-Butyl hydroxyperoxide
UAc Uranyl acetate
VAc Vinyl acetate
VPT Volume phase transition
2VP 2-vinylpyridine
4VP 4-vinylpyridine
W/O Water in oil
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... 127 Additionally, the polymerization techniques, which are often used for the structures with an organic core or shell, are based on the deployment of various monomers to form a three-dimensional (3D) network structure; a polymeric core can be prepared in advance or the shell can be synthesized using polymerization methods. 128 Chemical precipitation involves the conversion of a solution of ions to a solid particle or an insoluble form or the creation of a supersaturated solution and can be further subdivided into different categories such as direct precipitation, coprecipitation, and homogeneous precipitation. During the precipitation method, the release of ions regulates the nucleation and particle growth, where different factors may affect the properties of the final NPs, namely, the concentration of ions and the pH. ...
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