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Mechanics versus Thermodynamics Swelling in Multiple-Temperature-Sensitive CoreЦShell Microgels.

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Core–Shell Particles
DOI: 10.1002/anie.200502893
Mechanics versus Thermodynamics: Swelling in
Multiple-Temperature-Sensitive Core–Shell
Ingo Berndt, Crisan Popescu, Franz-Josef Wortmann,
and Walter Richtering*
In the past few years the synthesis of multifunctional nanomaterials has been the focus of various synthetic approaches
in which different chemical functionalities are incorporated at
different positions within the molecule. In this work compounds such as dendrimers, hyperbranched polymers, and
cross-linked micelles have been employed.[1] A second field of
research concerns stimuli-responsive nanomaterials. Here
thermosensitive microgels, often based on poly-N-isopropylacrylamide (PNIPAM), which exhibits a lower critical solution temperature (LCST) of ca. 32 8C in H2O (and ca. 34 8C in
D2O), have been investigated intensively in the past years and
found application in numerous fields.[2] More advanced
polymer architectures can lead to materials with superior
properties, and, for example, core–shell microgels responsive
to two external stimuli (temperature and pH) have been
introduced by Lyon and co-workers.[3]
Multiresponsive core–shell microgels can display unique
behavior since the two domains, core and shell, are mechanically linked. Indeed it has been observed that the degree of
swelling of the core and shell regions is mutally influenced.[4]
However, it is not clear whether such a mechanical connection can lead to qualitatively different properties.
[*] Dipl.-Chem. I. Berndt, Prof. W. Richtering
Institute of Physical Chemistry
RWTH Aachen University
Landoltweg 2, 52074 Aachen (Germany)
Fax: (+ 49) 241-809-2327
Prof. C. Popescu, Prof. F.-J. Wortmann
Deutsches Wollforschungsinstitut (DWI)
RWTH Aachen University
Pauwelstrasse 8, 52056 Aachen (Germany)
[**] Financial support by the Deutsche Forschungsgemeinschaft is
gratefully acknowledged. The authors thank Heiko Huth, Institute of
Physics, University of Rostock, for his support during the DSC
Angew. Chem. Int. Ed. 2006, 45, 1081 –1085
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Herein we report on the thermal behavior of aqueous
(D2O) solutions of core–shell microgels composed of a
temperature-sensitive PNIPAM core and a temperaturesensitive shell of poly-N-isopropylmethacrylamide (PNIPMAM); the latter exhibits an LCST of ca. 44 8C. These
particles combine the two above-mentioned concepts: two
polymers with different temperature sensitivity are combined
in spherical core–shell morphology. The mutual influences
can be controlled by adjusting the core/shell mass ratio, and in
this study a wide range of mass ratios were investigated. In
aqueous solution these particles show a two-step shrinking
behavior corresponding to the LCSTs of core and shell
Figure 1 shows the baseline-corrected and normalized
differential scanning calorimetry (DSC) thermograms of four
PNIPAM core/PNIPMAM shell microgels with core/shell
Table 1: Peak temperatures and calorimetric heats of core–shell microgels and pure core and shell material.
Sample mcore/
Core transition
[kJ mol1]
Shell transition
[kJ mol1]
[a] Pure core. [b] Additional third transition. [c] Pure shell material.
reported for similar systems.[5] This indicates that the thermal
transition of the PNIPAM component in these core–shell
microgels, as measured by DSC, is not altered qualitatively by
the presence of the other component. On the other hand, the
transition enthalpy of the PNIPMAM shell in these core–shell
microgels is always significantly smaller than that obtained for
a pure PNIPMAM microgel with the same cross-linker
content. Apparently the swelling behavior in a shell is
different; this will be discussed further below.
Most important, however, is the case of the core–shell
microgel with the greatest amount of shell component, that is,
with the thickest shell: Here a third peak appears at an
intermediate temperature without any evident relation to the
two polymer components. All three peaks were also observed
upon cooling (Figure 2). All transitions were fully reversible
Figure 1. Normalized DSC thermograms of PNIPAM core/PNIPMAM
shell microgels with core/shell mass ratios of 1:0.23, 1:0.69, 1:1.42,
and 1:2.50 recorded at a heating rate of 2 K min1.
mass ratios of 1:0.23, 1:0.69, 1:1.42, and 1:2.50. All core–shell
samples are based on the same PNIPAM core. The core–shell
microgels exhibit, generally, two peaks, which correspond to
the shrinking processes of the core at low and shell at high
With increased amounts of PNIPMAM in the shell, in
other words, with increased shell thickness, the transition
peak of the core shifts towards higher temperatures from 32.5
to 33.9. The peak temperatures of the shell transition stay
fairly constant at 44.5 8C, except for the case of a very thin
shell where the peak transition is reduced to 41.8 8C. This
observation will be explained further below.
A general kinetic function was employed to deconvolute
the DSC signals and allowed characterizing each peak by its
enthalpy and peak temperature assuming a weighted superposition of each thermal process. Peak temperatures and
calorimetric heats of the thermal transitions of the core–shell
particles as well as of the pure core and shell materials are
listed in Table 1.
The enthalpy values of the different transitions correlate
very well with the variation in the samples= composition. For
three samples the transition enthalpy of the PNIPAM core
component was found to be 4.8–5.3 kJ mol1 (per mol of the
NIPAM repeating unit), which is in good agreement with data
Figure 2. DSC heating and cooling curve for PNIPAM core/PNIPMAM
shell microgel sample 5 with a core/shell mass ratio of 1:2.50 recorded
at 2 K min1.
and appeared in each heating–cooling cycle. Because of the
above-mentioned peaks shifts, the third thermal transition
cannot be considered to be an overlap of two distinct peaks of
core and shell.
The influence of the composition on the core transition
can be interpreted with a rather simple mechanical model.
The core–shell microgels are considered as spherical particles
with a total radius R. Each particle is composed of a PNIPAM
core of radius r and a PNIPMAM shell of thickness d. We can
assume that the densities of PNIPAM and PNIPMAM are
very similar, so that one can express the core-shell mass ratio
as Equation (1), where a is the relative amount of the
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 1081 –1085
PNIPMAM shell. The radius r of the PNIPAM core is thus
given as Equation (2).
¼ 3 3¼
mshell R r
r ¼ pffiffiffiffiffiffiffiffiffiffiffi
Upon heating from a temperature below the core LCST,
the core shrinks and expells water through the waterpermeable PNIPMAM shell. The heat signal recorded by
DSC is directly proportional to the reduction in the radius of
the core x = r0r, where r0 is the core radius at 25 8C. Core and
shell are connected by an interpenetrating network layer.
Assuming this interaction of core and shell, the temperature
shift may be understood as being caused by an elastic force
developed in the shell, which balances the thermodynamic
force and thus decreases the shrinking force in the core.
The total shrinking force Fcore acting in the core is
temperature dependent and can be expressed by Equation (3), where kC is a material constant of the core. This force
F C ¼ kC T
drives the shrinking process in the core at elevated temperatures and is related to the elastic properties of the PNIPAM
network. Since it is an elastic force one may apply Hook=s law
for the core sphere of radius r0 [Eq. (4)]. Here, SC = 4p(r0x)2
kC T
¼ EC
SC 4pðr0 xÞ2
is the surface on which the force is applied. When a
PNIPMAM shell, which prevents the core from collapsing,
is present, one may describe the same shrinkage by Equation (5).
kCS T 0
SCS 4pðrxÞ2
The subscript CS denotes a material constant for the core–
shell microgel and T’ is the peak temperature shifted due to
the shell. The peak-temperature shift caused by the elastic
force of the shell is now readily inferred as the difference of
the two temperatures above [Eq. (6)], calculated at xp the
core shrinkage at the peak temperature.
DT P ¼ 4pxp
ECS ðr0 xp Þ2 ECS ðrxp Þ2
quently, the temperature of this peak is lowered to 41.7 8C.
For all other samples, the shell resists the collapse of the core,
and the temperature of the shell-shrinking process stays fairly
constant at 44.5 8C.
The sample with the thin shell can be compared with
systems where a thermosensitive polymer is attached to a
rigid particle.[6] In such a case the rigid substrate reduces the
mobility of the chains in the shell, and the transition is shifted
to higher temperatures. In our case the core can be considered
as an “active” substrate and the shrinking core shifts the
transition of the shell material to lower temperatures.
However, the model cannot explain the appearance of a
third peak in case of a very thick shell.
Our group recently established a form factor to analyze
small-angle neutron scattering data (SANS) from such core–
shell microgels.[4d] This model provides the polymer volume
fraction in the core and shell as well as the core size and the
shell thickness. The analysis of SANS data recorded at 39 8C,
where the third peak is found, provides the radial density
profiles shown in Figure 4. The SANS data give additional
information on the swelling behavior of core and shell.
In Figure 3 the experimental and calculated shifts DTP of
the core (PNIPAM) peak temperature are shown, and very
good agreement is observed. The thicker the shell, the
stronger the elastic force and the higher the transition
temperature of the core. The simple elastic model also
accounts for the second peak. For microgels with a very thin
shell, the shell cannot sustain itself against the shrinking force
of the core. When the core collapses, the shell shrinks at lower
temperatures as a result of the confined geometry. ConseAngew. Chem. Int. Ed. 2006, 45, 1081 –1085
Figure 3. Shifts of the core (PNIPAM) peak temperatures as measured
by DSC and calculated from the elastic model compared to data for
the pure core. Thicker shells lead to stronger elastic forces and higher
transition temperatures.
Figure 4. Profiles of the radial polymer volume fraction f(r) of the
PNIPAM core/PNIPMAM shell microgels and of pure PNIPAM core
microgel at 39 8C obtained from SANS.
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
The swelling properties of the shell material can be
discussed in terms of the volume change when the temperature is reduced from 50 to 39 8C. The form factor model
described in reference [4d] enables one to calculate the shell
volume at different temperatures, taking into account the
decay of the density profile at the particle surface. We find
that the volume change in the PNIPMAM shell when the
temperature is decreased from 50 to 39 8C is much smaller
than the volume change in a pure PNIPMAM microgel. If we
take the volume change and enthalpy of the pure PNIPMAM
microgel as a reference, we can compare the ratio of
experimental enthalpy values (i.e., DH per repeating unit in
the shell normalized by DH per repeating unit in the pure
microgel) with the ratio of volume changes in the shell and the
pure microgel, respectively. These data are plotted in Figure 5
Figure 5. Plot of the normalized enthalpy of the transition of the shell
component DHPNIPMAM in the core–shell microgels against the normalized volume change of shell component DVshell. The experimental data
of the pure PNIPMAM microgel were used for normalization.
and clearly show that the low enthalpy values found for the
shell transition are correlated with the lower swelling ratio.
Apparently the PNIPMAM shell cannot swell as much as a
pure PNIPMAM microgel can and consequently the heat flow
is reduced.
The analysis of the SANS data also provides important
information on the dimensions of the core in the core–shell
microgel. Compared to the particles in the pure PNIPAM
microgel (bold line in Figure 4), the core dimensions in the
core–shell microgels with mass ratios up to 1:1.42 are only
slightly increased. In agreement with the above model, the
core expands more, the stronger the elastic force from the
shell is, in other words, the thicker the shell is. The sample
with the thickest shell has the largest, most expanded core.
The DSC analysis clearly shows that the third peak at
intermediate temperature must be caused by additional
molecular interactions. In principle these could come from
the interpenetrating network at the core–shell interface.
However, Lyon et al. demonstrated that the thickness of the
interpenetrating network is rather limited[7] and furthermore
should be identical for all samples. Thus the third peak should
be observed with all samples if it were caused simply by the
interfacial region. The experimental finding that the third
peak in the DSC is observed only with the sample where the
thick shell leads to a significant stretching of the core at
intermediate temperature leads us to suggest that the additional thermal transition is caused by the breaking of hydrogen bonds between mechanically stretched chain segments. In
other words, the conformations of the chains in the microgel
network near the core–shell interface are altered when the
elastic force in the shell overcompensates the thermodynamic
force during the PNIPAM transition. These altered chain
conformations lead to the formation of different hydrogen
bonds, the breaking of which leads to the addition thermal
In conclusion, thermal characteristics of PNIPAM core/
PNIPMAM shell microgels in D2O have been investigated by
DSC. DSC thermograms reveal two transitions assigned to
the thermal transitions of core and shell material. With
increased shell thickness a shift of the core transition towards
higher temperatures is observed, which is interpreted in terms
of an elastic model. In contrast, the collapse of the core can
shift the transition of a thin shell to lower temperatures. An
additional thermal transition is found in case of a very thick
shell, suggesting that the balance between the thermodynamic
forces developed in the core material upon heating and the
elastic force resisting a volume change of the shell leads to
stretching of chain segments which allows additional hydrogen bonds to be formed. Thus an additional “chemomechanical” process is generated. Apparently the competition between thermodynamic and elastic forces gives rise to
qualitatively different behaviors thus demonstrating the
unique properties of cross-linked, multiresponsive core–
shell nanomaterials.
Experimental Section
Thermal measurements were carried out with a Perkin-Elmer Pyris 1
DSC. Stainless-steel pans with silicon rubber gaskets were used as
sample holders, and a pan with 30 mg of D2O was used as a reference.
The sample environment was rinsed with a nitrogen flow during
measurement. Approximately 30 mg of a 6-wt % microgel dispersion
in D2O was used per measurement. D2O was used as the solvent so
that the data could be compared with results from small-angle
neutron-scattering experiments. The study was carried out over a
temperature range from 25 to 55 8C with a constant heating rate of
2 K min1. For all samples at least two heating runs and one cooling
run were performed; all transitions were fully reversible and
appeared in each heating–cooling cycle. Several heating–cooling
rates (0.5, 1, 2, 5, 10, and 20 K min1) were employed for sample 3, and
only rates 10 K min1 had an influence on the transitions. The
errors in calculating the enthalpies were determined from the peaks in
repeated cycles and were < 5 %, in most cases < 1 %. The core–shell
samples were all based on the same PNIPAM core with a molar crosslinker content of 1.4 %. The pure PNIPMAM microgel and the
PNIPMAM shell have a cross-linker content of 5 mol %. Synthesis,
removal of soluble polymer chains by ultracentrifugation, and sample
preparation are described elsewhere.[4c,d]
Received: August 15, 2005
Published online: January 10, 2006
Keywords: core–shell particles · gels · polymers ·
thermochemistry · thermodynamics
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 1081 –1085
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Angew. Chem. Int. Ed. 2006, 45, 1081 –1085
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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thermodynamics, coreцshell, temperature, mechanics, versus, swelling, microgels, multiple, sensitive
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