Linearly thermoresponsive core-shell microgels: Towards a new class of nanoactuators

Article abstract of DOI:10.1016/j.polymer.2012.10.001

In this study we present novel core-shell microgels, with a shell made of poly(N-n-propylacrylamide) (pNNPAM) and a core consisting of poly(N-iso-propylmethacrylamide) (pNIPMAM), exhibiting a unique linear temperature response. The effect is produced by the large LCST gap of 23°C between the shell- and the core-forming polymer. We demonstrate that the shell exhibits a temperature induced de-swelling process that is almost independent of the swelling properties of the core. Furthermore the active collapse of the shell forces a collapse of the core (which is known as the so-called "corset-effect"). In a region between 25°C and 41°C the response of the particles is directly proportional to the temperature. Moreover, the core properties were systematically varied, revealing the possibility to linearly change the magnitude of the linear swelling. Hence, these particles are very promising as piezo-like linear nano-actuators.

Full text of DOI:10.1016/j.polymer.2012.10.001

Polymer 53 (2012) 6096e6101
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Linearly thermoresponsive coreeshell microgels: Towards a new class
of nanoactuators
,
b
a,
*
Michael Zeiser a , Ines Freudensprung , Thomas Hellweg
*
a Department of Physical and Biophysical Chemistry, Bielefeld University, Universitätsstr. 25, 33615 Bielefeld, Germany
b Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study we present novel coreeshell microgels, with a shell made of poly(N-n-propylacrylamide)
(pNNPAM) and a core consisting of poly(N-iso-propylmethacrylamide) (pNIPMAM), exhibiting a unique
linear temperature response. The effect is produced by the large LCST gap of 23 ꢀC between the shell- and
the core-forming polymer. We demonstrate that the shell exhibits a temperature induced de-swelling
process that is almost independent of the swelling properties of the core. Furthermore the active
collapse of the shell forces a collapse of the core (which is known as the so-called “corset-effect”). In a
region between 25 ꢀC and 41 ꢀC the response of the particles is directly proportional to the temperature.
Moreover, the core properties were systematically varied, revealing the possibility to linearly change the
magnitude of the linear swelling. Hence, these particles are very promising as piezo-like linear nano-
actuators.
Received 8 August 2012
Received in revised form
28 September 2012
Accepted 2 October 2012
Available online 26 October 2012
Keywords:
Lower critical solution temperature (LCST)
NIPAM NNPAM NIPMAM
Smart colloids
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
to fullfill the requirements of nanoscopic devices. Additionally, the
manufacturing process can be rather complex and therefore cost-
Actuator materials undergo a change of their spatial dimensions
upon application of an external stimulus. Up to now one can find
a great diversity of actuators that fullfill the requirements to react
on electric fields, magnetic fields, pressure or temperature. Besides
commercially established products (bimetal stripes, thermoele-
ments, piezo ceramics) we like to refer to promising artificial
responsive materials, like thin-film multiferroics, which can be
specifically designed to have a strong magnetoelectric coupling [1].
Soft condensed matter provides alternative strategies to realize
responsive materials. For example polymer matrix embedded car-
bonnanotube composites can be synthesized to show a photo-
induced mechanical actuation [2].
In general, the obtained response may be irreversible or
reversible and the actuator may only work under equilibrium or
non-equilibrium conditions. However, actuator-design for nano-
scale applications is still a demanding task since only few systems,
like piezo-materials, are capable to linearly change their size in
a well defined manner. On the way to new actuators one needs to
face challenges like predictability of the response and very often
the macroscopic dimensions of the made materials make it difficult
intensive.
Smart materials are systems that exhibit the characteristics
described above. They comprise a built-in responsivity under
equilibrium conditions and in the case of colloidal hydrogel
particles (microgels) the dimensions are in a range of several
hundred nanometers. Microgels have been intensively studied in
the last 25 years because of their great potential with respect to
applications such as photonic materials, intelligent substrates, or
carriers for catalytic and biotechnological applications [3e5].
Besides their broad applicability, such systems can also serve as
models to study complex phenomena, like glass transitions in
colloids [6]. The properties and applications of microgels were
reviewed in several articles during the last decade [7e11].
Recently, the complexity of the synthesized systems increased
because multi-compartment particles exhibit the possibility of
having a specific particle surface, whereas the interior can be
individually modified to incorporate additional desired function-
alities [12,13].
In this contribution we report a novel design strategy of ther-
moresponsive coreeshell colloids which leads to a responsivity
where the size is linearly coupled to the applied temperature. We
think this material is of particular interest because its unique
responsive behavior might be advantageous regarding new actu-
ator designs and responsive coatings.
* Corresponding authors.
E-mail addresses: michael.zeiser@uni-bielefeld.de (M. Zeiser), thomas.hellweg@
uni-bielefeld.de (T. Hellweg).
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2012.10.001

M. Zeiser et al. / Polymer 53 (2012) 6096e6101
6097
Thermoresponsivity in the context of polymers in aqueous
2. Experimental
solutions is a well known phenomenon. The most intensely studied
thermoresponsive polymer is poly(N-iso-propylacrylamide) (pNI-
PAM), which exhibits a lower critical solution temperature (LCST)
at ca. 32 ꢀC [14,15]. Recently, we have shown that in microgels this
phase transition temperature can be taylored in a linear way using
copolymerization of N-alkylacrylamide derivatives [16]. In the
present study a coreeshell system, consisting of two different
temperature-responsive polymers was prepared and characterized.
Such kind of systems can be accessed using copolymerized
microgels of NIPAM with AAc (acrylic acid) as core particles with
a crosslinked pNIPAM shell as was shown by Jones et al. [17,18].
However, copolymerization with non- or less thermoresponsive
materials significantly alters the overall thermoresponsive prop-
erties. In the case of AAc, the phase transition of the NIPAM based
system is broadened due to the incorporated charges. Additionally,
one observes a second phase transition step in the region of the
LCST of AAc at moderate pH values (pH w 8) [19,20]. A structurally
related system was introduced by Richtering et al. [21]. Within their
study they have chosen pNIPAM and its derivative pNIPMAM
(poly(N-iso-propylmethacrylamide)) where the magnitude of the
phase transition is conserved but shifted. They mainly focused on
systems where the polymer in the shell exhibits a larger LCST
(pNIPMAM: 44 ꢀC [22]) compared to the LCST of the pNIPAM cores
[23e29]. The inverse system was less intensely studied despite the
fact that this system shows the so-called “corset-effect”. This
terminology indicates the strong interplay between thermody-
namic and mechanical properties of the shell with the core material
which is also part of the discussion in the mentioned studies above
[17,18,25e28].
2.1. Shell monomer synthesis
N-n-propylacrylamide was synthesized following a classical
SchotteneBaumann reaction described by Hirano et al., using
acryloyl chloride (97%, Aldrich, USA), triethylamine (99%, Grüssing,
Germany), propylamine (99%, Fluka, USA) and methylene chloride
(p. A.) as solvent [30].
2.2. Synthesis of coreeshell microgels
A seeded growth two-step protocol was followed to build up the
coreeshell system [17]. First of all, pNIPMAM core-particles were
synthesized (0.1 mol/L, in water, 400 rpm, 4 h at 70 ꢀC) and purified
by 5 successive ultracentrifugation and re-dispersion steps. The
corresponding feeding ratios of monomer (N-iso-propylmethacry-
lamide, 97% purity, Aldrich, USA), crosslinker monomer (N,N’-
methylenebisacrylamide, 99%, Sigma Aldrich, USA), initiator APS
(ammonium peroxodisulfate, 98%, SigmaeAldrich, USA) and
surfactant (sodium dodecyl sulfate, analytical grade, Serva,
Germany) are listed in Table 1. Every water-based synthesis was
conducted in Milli-Q water (Millipore, Merck KGaA, Germany) with
a resistance of 18 MOhm cm and a TOC-value of <10 ppb. Then
150 mL of a 0.15 wt.% of the corresponding seed-particle dispersion
were equilibrated at synthesis conditions, as described for the core-
particle synthesis, and subsequently coated with NNPAM (0.70 g)
and BIS (0.0175 g) using SDS (0.048 g) and APS (0.093 g), which
resulted in a shell with a nominal crosslinker content of 1.8 mol%.
After synthesis the same purification steps were followed as
described before.
In this study we present novel responsive coreeshell micro-
gels based on N-iso-propylmethacrylamide (NIPMAM) as core
material and N-n-propylacrylamide (NNPAM) (LCST ¼ 21 ꢀC)
forming the shell (see Fig. 1) [22,30]. Thus, the LCST-gap between
2.3. Determination of temperature dependent swelling curves
both materials was significantly increased (
D
¼ 23 ꢀC) compared
Angular dependent photon correlation spectroscopy (PCS)
measurements were performed using a light scattering goniometer
system equipped with a multiple
to the known systems in literature. It is remarkable, that the
combination of both relevant factors, namely the strong interplay
between the low-LCST shell and the high-LCST core, as well as the
large LCST-gap results in a linear swelling behavior. In addition,
we present the synthesis of coreeshell systems containing cores
that exhibit varying swelling properties. The shell properties
were kept constant which allowed us to systematically study the
influence of core-properties on the overall coreeshell
characteristics.
s digital correlator (ALV-5000/E,
ALV-GmbH, Langen, Germany) combined with an Argon-Ion LASER
l
q
¼ 514.5 nm (Spectra Physics Stabilite 2017, Newport Corp., USA).
The swelling curves were measured at a fixed scattering angle of
¼ 60ꢀ using a solid-state LASER (TOPTICA Photonics AG, Germany)
at a wavelength of
l
¼ 661.8 nm and a fast correlator (ALV-6010,
ALV-GmbH, Langen, Germany) with a thermostated bath (Haake
Phoenix II, Thermo Scientific). All measurements were taken with
samples at a concentration of 7.5$10ꢁ4 wt.% in cylindrical Quartz
cuvettes (Hellma GmbH & Co. KG, Germany) with an outer diameter
of 10 mm. The samples were brought to equilibrium at constant
thermal conditions for 15 min prior to the measurement. The
visualized datapoints are average results of at least 5 individual
measurements.
m
n
co
HN
HN
O
O
NH
n
N
O
H
Table 1
Overview of the feeding ratios used for the core particle synthesis. The abbreviations
NIPMAM, BIS, SDS and APS stand for the monomer N-iso-propylmethacrylamide, the
crosslinker monomer N,N’-methylenebisacrylamide, the surfactant sodium dodecyl
sulfate and the initiator ammonium peroxodisulfate.
x
y
co
O
O
NH
Samplea
c(NIPMAM)core
m(BIS)core
c(SDS)core
c(APS)
y
C (2 mol%)
C (5 mol%)
0.0819 M
0.0819 M
0.0819 M
0.0819 M
0.0819 M
0.0819 M
0.0819 M
0.0379 g
0.0947 g
0.1422 g
0.1894 g
0.2370 g
0.2841 g
0.3788 g
0.0011 M
0.0011 M
0.0011 M
0.0011 M
0.0011 M
0.0011 M
0.0011 M
0.0027 M
0.0027 M
0.0027 M
0.0027 M
0.0027 M
0.0027 M
0.0027 M
N
O
H
C (7.5 mol%)
C (10 mol%)
C (12.5 mol%)
C (15 mol%)
C (20 mol%)
Fig. 1. Visualization of the studied coreeshell systems. The core consists of pNIP-
MAM (LCST z 44 ꢀC in water) with varying nominal crosslinker contents between
2 mol% and 20 mol%, while the shell polymer is pNNPAM (z21 ꢀC) with a constant
crosslinker content of 1.8 mol%.
a
The sample name indicates the nominal crosslinker concentration in mol%.

6098
M. Zeiser et al. / Polymer 53 (2012) 6096e6101
Photon correlation spectroscopy is well suited to determine the
based polymer) in the microgel. More details about the influence of
crosslinker content on the phase transition of the microgels can be
found in the supplementary material (Fig. S1).
hydrodynamic properties of microgels [31]. Since our systems show
monomodal particle size distributions, we have treated our data
with a third order cumulant analysis which was first introduced by
D. E. Koppel. In this method the normalized field autocorrelation
function is expanded in a MacLaurin series thus giving us, amongst
others, the first cumulant which corresponds to the relaxation rate
In the second synthesis step differently cross-linked pNIPMAM
microgels were used as seeds for a seeded growth precipitation
polymerization to grow a pNNPAM shell around the cores [17].
We kept the shell properties constant via fixed synthesis protocols
as well as fixed monomer feeds. The synthesis was performed at
elevated temperatures to avoid shell-monomer diffusion inside the
collapsed cores [17,18,21,25e28]. Due to the described procedure, the
corresponding shell polymer covers preferentially the core surface
and thereby an interpenetration of shell material into the core is
reduced and only a small interpenetration layer is obtained [25,26].
As a next step we also studied the influence of temperature on
the hydrodynamic coreeshell radius. The behavior of the coree
shell system is significantly changed in contrast to the unper-
turbed core system. In Fig. 3a a sketch of the temperature induced
transitions of the coreeshell microgels is shown. This totally
reversible swelling/deswelling process of the new particles can be
subdivided into three distinct regions of different behavior. As an
example the corresponding plot of the hydrodynamic radius for
a coreeshell microgel with 10 mol% cross-linker in the core is
shown and discussed (Fig. 3b).
G
[32]. All temperature dependent measurements were carried out
at a fixed scattering angle of 60ꢀ. This is appropriate because
angular-dependent measurements revealed the purely diffusional
nature of the observed relaxation at this angular position. The
hydrodynamic radius is related to the relaxation rate via the
StokeseEinstein equation:
kBT
Rh
¼
(1)
G
6
ph
q2
with q ¼ 4
p
n/
l
sin(
q
/2) being the magnitude of the scattering
vector, the thermal energy kBT, the viscosity
index of the solvent n.
h and the refractive
2.4. Electron microscopy of dried microgels
In region I, we observe a nonlinear decrease of the particle size
with increasing temperatures up to 25 ꢀC. This behavior indicates
the occurrence of a typical collapse of the shell forming pNNPAM,
which exhibits its transition around 21 ꢀC. Above this transition
temperature, water becomes a bad solvent for the shell material
which initiates the shrinking process. At temperatures larger than
25 ꢀC the regular phase transition of the shell material is finished.
Region II points to the temperature window where the hydro-
dynamic radius of the microgel is linearly changing with temper-
ature. In detail, this linear relation is found in a temperature range
Concentrated particle suspensions were previously spin-coated
on Si-wafers (1000 rpm), dried and sputtered with a 1.3 nm thick
Pt-layer (Sputter Coater 208HR, Cressington, UK). The measure-
ments were carried out using a FE-SEM instrument (Gemini 1530,
Zeiss, Germany) with an acceleration voltage of 3 kV (Inlens-
detector).
3. Results and discussion
The first step on the route to coreeshell microgels was the
synthesis of pNIPMAM based cores. A series of seven core-particles
with different amounts of crosslinker (N,N0-methylenebisacrylamide;
BIS) was synthesized via a precipitationpolymerization technique. The
hydrodynamic radii of these core-particles were studied in a temper-
ature range between 10 ꢀC and 60 ꢀC using photon correlation spec-
troscopy (see Fig. 2). The observed behavior is similar to the results
found for pNIPAM particles [33]. Increasing the amount of incorpo-
rated crosslinker leads to a diminishing swelling capability of the
microgels. Additionally, a broadening of the volume phase transition is
observed. Both effects are a result of an increasing network rigidityand
larger fraction of non-responsive polymer material (pure crosslinker
between 25 ꢀC and 41 ꢀC ( T z 15ꢀ C). It is the intermediate
D
temperature range between the two LCST values of the core and
shell material, respectively.
The synthesis is performed at elevated temperatures (80 ꢀC)
thus the shell material covers fully collapsed seeds. Because of the
presence of the shell-“corset”, the core material cannot retain its
original shape at temperatures lower than its own phase transition
temperature. At each point in the swelling curve the core is
mechanically stressed due to the presence of the shell and vice
versa. Thus the shell actively compresses the core, which retains its
shape simply due to its network elasticity and its osmotic modulus.
Upon increasing temperature, the pNIPMAM solubility successively
a
b
Fig. 2. Swelling behavior of microgels. a, Hydrodynamic radius Rh as a function of temperature T for different pNIPMAM core systems containing nominally different crosslinker
contents. b, Normalized radius for the same systems as in a, with the fully collapsed state as reference condition.

M. Zeiser et al. / Polymer 53 (2012) 6096e6101
6099
a
b
Fig. 3. Linear temperature response. a, Schematic illustration of a coreeshell microgel which undergoes three regions of different swelling behavior (completely reversible
process). b, Corresponding classification of previously mentioned regions in an exemplary Rh(T)-diagram of a coreeshell 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.
decreases while the active compression of the shell is still present.
As a result, the microgel shrinks with increasing temperature. Since
this change in size is linear, we propose that this effect can be
related to the polymeresolvent interaction parameter, which is in
a first approximation inversely proportional to the temperature for
trend is reasonable because the mechanical properties of the core
should have a significant influence on the coreeshell particle
properties. Theoretical models like the FloryeRehner theory
underline this trend as well, since the thermodynamic phase
behavior of a neutral gel can be described by two terms for the
osmotic pressure inside the gel, namely mixing and elastic contri-
butions [34]. In Fig. 4b we demonstrate that the slope k, which
captures the radial swelling capability, is directly proportional to
the core crosslinker content as well. Thus from a quantitative point
of view we determined the key parameter, namely the core cross-
linker content, that can be utilized to directly tune the responsive
behavior of the coreeshell particles. To our knowledge such
a response in combination with an easily accessible tuning
parameter has not been found before for any thermoresponsive
system. This behavior could e.g. be exploited to produce materials
serving as linear actuators on a nm scale. The desired magnitude of
temperatures below the phase transition (c
f Tꢁ1) [34,35].
Finally region III corresponds to an active collapse of the core
inside the coreeshell microgel, because we reach the transition
temperature of the core material. However, due to the “corset-effect”
the compression of the core is already in an advanced stage and only
a small change in terms of particle size is observed in this region.
We verified the linear swelling behavior for all synthesized
coreeshell microgels. Taking a closer look at the normalized radius
of the particles in Fig. 4a it is obvious that the magnitude of the
slope of the linear fit increases with decreasing crosslinker
concentration in the core. From a qualitative point of view this
0
b
2.2
2.0
1.8
1.6
1.4
1.2
1.0
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1
CS(2mol%)
a
k(c(BIScore))
CS(5mol%)
CS(7.5mol%)
CS(10mol%)
CS(12.5mol%)
CS(15mol%)
CS(20mol%)
Linear Fit
-0.005
-0.010
-0.015
-0.020
-0.025
-0.030
k
25
30
decreasing
core-crosslinker
35
40
0
5
10
(BIS)core
15
mol%
20
10
20
30
40
/ °C
50
60
c
/
T
Fig. 4. Influence of the core-crosslinker. a, Temperature dependence of the normalized radius with the fully collapsed condition as a reference (Inset: magnified view on region II
where the solid lines indicate linear regressions). b, Corresponding slopes k within region II for different core-crosslinker contents (y-error: as obtained from the fit, x-error: ꢂ1 mol%).
The solid line again indicates a linear regression.

6100
M. Zeiser et al. / Polymer 53 (2012) 6096e6101
different between both systems. In other words it can be stated that
there is also a significant influence of the core crosslinker content
on the swelling capability of the microgels. This trend is consistent
for both types of particles, we conclude that the core determines
the swelling behavior of the coreeshell system.
40
30
20
10
0
(
c
(BIScore
exp-fit C
(BIScore
exp-fit CS
)
) C
max
(c
)) CS
max
Similar results can be found for dried mixtures of different coree
shell particles which were studied using SEM (Fig. 6b). This method
captures surface dependent properties and therefore the particles
tend to have enhanced contrast the more crosslinker the cores
contain. Upon drying, soft spheres (microgels with low crosslinker
contents) exhibit a flat “pancake”-like structure where we obtain
only low contrast and brightness levels relative to the substrate,
whereas harder spheres retain their spherical shape having high
contrast and brightness [36,37]. We were able to confirm these
correlations by reference measurements of the individual particles
on different substrates (see supplementary material Fig. S3). This
already indicates the same trend as was shown for the light scat-
tering results, where the coreeshell system was significantly influ-
enced by the core properties. Furthermore the drying patterns of the
particles deviate significantly from each other. However, SEM only
shows the differences in qualitative way.
0
5
10
(BIS)core
15
mol%
20
25
c
/
Fig. 5. Swelling capability of the microgels. Maximum swelling coefficient for
differently cross-linked cores and the corresponding coreeshell systems. The lines
indicate equally weighted exponential fits.
4. Conclusion
In conclusion this study presents the synthesis of non-NIPAM
based coreeshell microgels. It was found that the combination of
distinct monomers for core and shell leads to a new system class
with extraordinary linear swelling properties. In our study, every
mechanical response can be realized by simply changing the core
crosslinker content.
In the present work the number of different crosslinker contents
was sufficiently large to allow a more detailed analysis of the
relationship between the maximum swelling ratio amax and the
crosslinker content. amax, which is the maximum hydrodynamic
volume normalized to the corresponding fully collapsed state,
decreases with increasing BIS molar ratio (see Fig. 5). This
decreasing maximum swelling capacity can be described by a single
exponential decay and is accompanied by a broadened phase
transition. Furthermore, the observed exponential trend can be also
found for the corresponding coreeshell particles. It is remarkable
that the decay constant is identical within the standard deviation
for the parameter sets of the core particles and the corresponding
coreeshell particles. The computed baseline parameters slightly
deviate from each other. Only the amplitude is significantly
single coreeshell microgel exhibits a region of at least
D
T ¼ 15 ꢀC,
where the hydrodynamic radius was linearly dependent on the
temperature. This is a result of the enhanced “corset-effect” real-
ized due to the use of two polymers with largely different LCST.
Moreover, we demonstrate how differently cross-linked cores
influence the overall coreeshell swelling behavior in bulk solu-
tions. In more detail, it was shown that the magnitude of the linear
response may be tuned, again in a linear manner, over a wide range
by simply changing the crosslinker content of the employed cores.
We like to point out that this system class is a very promising
candidate for two types of applications. Since the transition of the
shell material occurs around room temperature, the introduced
coreeshell microgels exhibit a dense shell at temperatures above
25 ꢀC. This addresses to their usability as model systems, which can
fill the gap between hard-spheres and fuzzy, soft colloids. Addi-
tionally, we think this system class is of immediate relevance with
respect to actuator design in general, where a simple control and
predictability of the responsive behavior is advantageous.
Future studies include small angle neutron scattering with the
goal to determine the temperature dependent shell/core density-
and size-ratio which are essential to verify our current under-
standing for the involved physical processes and to develop a suit-
able theoretical model.
Acknowledgments
MZ likes to thank Christian Hannappel and Matthias Bie-
ligmeyer for fruitful discussions and support during synthesis and
measurements. The authors thank the DFG priority program
SPP1259 “Intelligente Hydrogele” for financial support.
Fig. 6. Shape retention of dried particles. SEM-micrograph of a mixture of three
coreeshell particles with different core-crosslinker contents (2 mol%, 10 mol% and
20 mol% BIS) dried on a Si-wafer and sputtered with a 1.3 nm thick layer of Pt. The
softness of the particles, which can be related to the brightness and contrast, is
decreased for those systems that contain cores with larger amounts of crosslinker
(scale bar: 200 nm). Additionally the morphology is significantly changed which can be
also seen from the magnifications on the right side (no scale bar given). Hence, already
in the dry state the interaction between core and shell is obvious.
Appendix A. Supplementary data
Supplementary data related to this article can be found at
http://dx.doi.org/10.1016/j.polymer.2012.10.001.

M. Zeiser et al. / Polymer 53 (2012) 6096e6101
6101
[21] Berndt I, Richtering W. Doubly temperature sensitive coreeshell microgels.
Macromolecules 2003;36:8780.
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