Temperature sensitive poly(N-Isopropyl-Acrylamide) microgel
particles: a light scattering study
Mathias Reufer1,2,Pedro D´ ıaz-Leyva1, Iseult Lynch2and F. Scheffold1
1Department of Physics and Fribourg Center for Nanomaterials, University of Fribourg, CH-1700 Fribourg, Switzerland
2Adolphe Merkle Institute, University of Fribourg, CH-1700 Fribourg, Switzerland
2School of Chemistry and Chemical Biology, University College Dublin, Belfield, Dublin 4, Ireland
Abstract. We present a light scattering study of aqueous suspensions of microgel particles consisting of
poly(N-Isopropyl-Acrylamide) cross-linked gels . The solvent quality for the particles depends on temper-
ature and thus allows tuning of the particle size. The particle synthesis parameters are chosen such that
the resulting high surface charge of the particles prevents aggregation even in the maximally collapsed
state. We present results on static and dynamic light scattering (SLS/DLS) for a highly diluted sample
and for diffuse optical transmission on a more concentrated system. In the maximally collapsed state the
scattering properties are well described by Mie theory for homogenous hard spheres. Upon swelling we find
that a radially inhomogeneous density profile develops.
PACS. 82.70.Dd Colloids – 83.80. Kn Physical gels and microgels – 82.70.Gg Gels and sols
Colloidal particles with adjustable interaction potential
have been of scientific and technological interest in recent
years due to their potential use to control bulk properties
such as viscous flow, optical and also magnetic magnetic
properties [1–7]. Thermo-sensitivemicrogels have been widely
used as model systems [2,8–10]. These materials have also
Send offprint requests to: Frank.Scheffold@unifr.ch
received attention due to their potential applications in
drug delivery or as sensors as a result of their ”respon-
sive” characteristics following changes in their environ-
ment.  One of the most widely studied systems is
poly(N-Isopropyl-Acrylamide) (PNIPAM), a polymer which
has a critical solution temperature of approximately 33◦C.
PNIPAM colloids can be prepared by cross linking PNI-
PAM resulting in microgel particles with tunable softness
Published in "European Physical Journal E:
which should be cited to refer tothis work.
2 Reufer et. al.: Temperature sensitive poly(N-Isopropyl-Acrylamide) microgel particles: a light scattering study
(and swelling degree), these depending on the cross link
density. Another approach is to coat well defined solid
core-particles with PNIPAM, thereby exploiting proper-
ties of both materials. .
Both pure PINPAM and PNIPAM coated particles dis-
play properties due to the tunable network combined with
properties of classical colloids, e.g. crystallization or ag-
gregation. This is very useful to tailor colloidal systems
that can be kept close to the liquid solid transition thus
having the possibility to “temper” these materials  .
Tempering is not possible using most “classical” colloidal
systems that require a change in composition in order to
cross a phase boundary. In this respect temperature sen-
sitive particles are ideal candidates to provide quantita-
tive information about the ergodic-nonergodic transition
in systems with repulsive interactions, both for the glass
transition [13–17] as well as for crystallization .
At temperatures well above 35◦C the PNIPAM particles
are collapsed and behave as solid spheres. The effective
interaction potential can be either attractive due to Van-
der-Waals forces which leads to aggregation and phase
separation , or in the presence of surface charges the
particles can be stabilized in suspension. Upon lowering
the temperature the particles swell by a factor of two to
five in size [2,20]. If the initial particle density is suffi-
ciently high the particle volume increase can drive the
system from a liquid to a solid state. A number of rheo-
logical studies were able to study in detail the apparent
divergence of the viscosity at the transition point and the
emergence of an elastic shear modulus [2,23]. More re-
cently, studies on the internal dynamics and the frequency
and shear rate dependent rheology close to and above the
liquid-solid transition have been reported [24,25].
In this article we discuss the temperature dependent prop-
erties of highly cross-linked microgel particles. The focus
of the present work is on a comprehensive optical charac-
terization of PNIPAM microgel suspensions. We present
results on static and dynamic light scattering (SLS/DLS)
for a highly diluted sample and for diffuse optical trans-
mission on a more concentrated system. A detailed under-
standing of the temperature dependent structural proper-
ties of individual particles is a key ingredient for the study
of the dynamic properties and the phase behavior of dense
suspensions. The parameters of the synthesis were chosen
such that the effective surface charge prevents aggregation
even in the maximally collapsed state. The advantage of
such a charge-stabilized system is that the equilibrium
(or quasi-equilibrium) properties can be studied over the
whole range of temperatures even at high volume fraction.
In the current article we discuss the structural aspects of
our PNIPAM suspension. A detailed study of the high
density phase behaviour and rheological properties will
be presented elsewhere .
The outline of this article is as follows: In section 2 we
describe the particle chemistry and the particle character-
ization by scanning electron microscopy. In Section 3 we
analyze the temperature dependence of the hydrodynamic
radius and in section 4 we discuss results from static light
scattering on a dilute suspension. In section 5 we study
the optical properties (diffuse transmission) of a concen-
Reufer et. al.: Temperature sensitive poly(N-Isopropyl-Acrylamide) microgel particles: a light scattering study3
Fig. 1. Scanning electron microscopy picture of PNIPAM par-
ticles in the collapsed state (Particle radius 260 ± 5 nm.)
trated suspension. Finally, in section 6 the experimental
results are summarized.
2 Sample Preparation
We use free radical cross-linking polymerization of the
monomer N-Isopropyl-Acrylamide (NIPAM) from Acros
Organics (Acros Organics BVBA, Geel, Belgium) with the
(BIS) from Fluka (Fluka Chemie GmbH, Buchs, Switzer-
land). The BIS molecules are essentially two acrylamide
monomer units bridged by a covalent bond.  The poly-
merization reaction is initiated using the ionic salt Potas-
sium Persulfate (KPS, Merck KGaA, Darmstadt, Ger-
many). All chemicals are reagent grade and used with-
out further purification, except NIPAM which is recrystal-
lized from N-Hexane solution. The synthesis is performed
by dissolving a mixture of monomer NIPAM and cross-
linker BIS (at 21.43 mg/ml) in 145 ml deionized and fil-
tered water.1The cross-linking ratio, defined by fbis ≡
[BIS]/([NIPAM] + [BIS]) is 6.7%. Such a relatively high
ratio is expected to produce a rigid gel with approximately
one molecule of cross-linker per 19 molecules of monomer
[20–22]. The mixture (previously degassed for ∼ 30 min-
utes) is heated up to 80◦C under pure nitrogen atmo-
sphere. Then, 72.8 mg of KPS dissolved in 5 ml of degassed
water is added to the mixture to start the polymerization
reaction. The reaction proceeds for at least 4 hours at
constant temperature. Finally the dilute suspension is ex-
tensively dialyzed for several days against deionized water
to eliminate unreacted monomer excess[8,27,29]. The
final step is carried out under normal atmospheric condi-
tions which leads to a finite solvent ionic strength due to
spontaneous dissolution of carbon dioxide in water. Dense
suspensions are prepared using a rotary evaporator. This
procedure increases both the particle density as well as
the ionic strength of the solvent. In turn, at high temper-
atures, we expect to obtain a charge stabilized suspension
with a strongly screened Coulomb interaction potential.
We do not observe any aggregation at high temperatures
which indicates that the suspension is stable.
Figure 1 shows an scanning electron micrograph of our
PNIPAM particles on a solid substrate. The particles are
1The original batch further contained a small quantity of
TiO2 nanoparticles in an initial attempt to create core shell
particles. From transmission electron microscopy we find no
trace of the TiO2 in sample after concentration and purifica-
tion. Therefore the system under study consists exclusively of
PNIPAM particles suspended in water.
4Reufer et. al.: Temperature sensitive poly(N-Isopropyl-Acrylamide) microgel particles: a light scattering study
ordered in hexagonal arrays, giving us a qualitative idea
about the rather low polydispersity in the collapsed state.
From the analysis of several dozen particle positions we
obtain an SEM radius of 260 ± 5 nm.
It is well known that even in the maximally collapsed
state PNIPAM microgel particles contain a non-negligible
amount of water molecules. The presence of this bound wa-
ter has to be taken into account for the characterization
of PNIPAM colloidal suspensions. As shown by Lele et al.
 the bound water content of a collapsed PNIPAM gel is
approximately 0.38-0.4 gram per gram of polymer which
corresponds to a bound water volume fraction of approx-
imately ΦWater= 30% if we assume a water density of 1
g/cm3and a bulk density of PNIPAM approximately 1.1
As suggested by Erbe et al.  the bulk refractive index of
PNIPAM for visible light can be estimated by extrapolat-
ing literature dn/dc-values . One finds n = 1.52±0.01.
We note that similar chemical structures such as Poly(N-
lamide, Poly(N-methylacrylamide) all have bulk indizes
for visible light in the range 1.51−1.54 . If we assume
a value of 1.33 for the refractive index of the bound water
we can estimate the refractive index of the particles in the
maximally collapsed state based on the Maxwell-Garnett
mixing rule [35,36]. For the case of small refractive in-
dex variations it can be written as: n ? ΦWater· 1.33 +
ΦPolymer· 1.52 = 1.46 .
Fig. 2. Size characterization by dynamic light scattering
(DLS). Full squares: temperature dependent hydrodynamic ra-
dius RH (error bars denote spread of data points for measure-
ments at three different angles θ = 60,40,20◦) at Φ ∼ 10−5.
Full circles: mean radius of mass distribution R. Open circles:
mean radius R plus diffuse layer 2 σ.
3 Hydrodynamic radius from dynamic light
We determine the hydrodynamic radius of the particles us-
ing standard dynamic light scattering in the single scatter-
ing regime with λ = 532 nm (goniometer system ALV/SP-
125, ALV, Germany). A low volume fraction (Φ ∼ 10−5)
colloidal suspension is filled into a cylindrical glass tube of
10 mm inner diameter. The vial is placed in the center of
a cylindrical vat filled with an index-matching fluid, cis-
trans decahydronaphthalene (decalin), in order to reduce
stray light reflections and scattering from the vial sur-
face. The index-matching fluid is temperature controlled
From the measured time-averaged intensity correlation
Reufer et. al.: Temperature sensitive poly(N-Isopropyl-Acrylamide) microgel particles: a light scattering study5
function, g(2)(q,τ) the translational free diffusion is de-
termined and the particle hydrodynamic radius extracted
by common procedures [37,38]. We restrict the analysis
to relatively low scattering angles (θ = 60,40,20◦). The
experimental data, Figure 2, shows the expected strong
temperature dependence. Since PNIPAM is not soluble in
hot water (T > 33◦) the cross-linked PNIPAM gels col-
lapse at elevated temperatures whereas at room tempera-
ture and below, water is a good solvent leading to a strong
swelling. As a consequence of the relatively high cross-link
density the swelling is less pronounced compared to some
of the previous studies where swelling ratios up to four or
five have been reported [20–22].
4 Static light scattering
We determine the particle form factor over a temperature
range 15◦C ≤ T ≤ 40◦C using a home-built 3D Light Scat-
tering setup . We use a solid state laser TUI Optics DL
100R (TUI Optics GmbH, Munich, Germany) λ = 680.4
nm, two avalanche photomultipliers Perkin Elmer SPCM-
AQR-13-FC (Perkin Elmer Inc, Fremont CA, U.S.A.) and
a digital multi-tau correlator (Correlator.com, Bridgewa-
ter NJ, U.S.A.). The sample is contained in a cylindrical
optical glass tube of 5 mm inner diameter, placed in the
center of a vat filled with Cis/Trans decalin to reduce stray
The q dependence of the scattering intensity I(q) ∝ P(q)
of a dilute sample (Φ ≈ 4 × 10−5) is displayed in Fig-
ure 3 for different temperatures. For uncorrelated spher-
ical particles I(q) is given by the scattering form factor
Table 1. Hydrodynamic radius RH and static light scatter-
ing parameter from a best fit to a RGD-form factor assuming
a gradually decreasing density profile with a mean radius R
convoluted with a Gaussian of width σ
P(q) = k2
dΩ, where k0 = 2πns/λ. In the high
temperature limit the experimental data can be described
by a Mie calculation for a homogenous sphere of diameter
242 nm, refractive index n = 1.46 and a polydispersity of
The SLS-particle size in the maximally collapsed state
R = 242nm is found in rather good agreement with the
hydrodynamic radius RH? 260nm. The result is also con-
sistent with the SEM analysis although, if under vacuum
conditions all the water is released, we would actually ex-
pect to find a radius of about 215 nm. There are a number
of possible reasons for this slight difference. For example
we cannot exclude the possibility that some residual wa-
ter remains bound to the polymer or that the particles are
slightly deformed leading to a flattened shape . Small
errors in the alignment of the electron microscope or the
6Reufer et. al.: Temperature sensitive poly(N-Isopropyl-Acrylamide) microgel particles: a light scattering study
Fig. 3. Temperature dependent Form Factor P(q) measured under dilute conditions (Φ ≈ 4 × 10−5). Dash-dotted lines: Mie
calculations for homogenous spheres, polydispersity 9%: (RMie = 242 nm, n=1.46, T = 40◦C), (RMie = 275 nm, n=1.42,
T = 30◦C) and (RMie = 350 nm, n=1.37, T = 10◦C). Solid lines: RGD form factor for a mass distribution with mean radius R
convoluted with a Gaussian with standard deviation σ, polydispersity 9.5 ± 0.5%.
calibration could also explain this difference. More exper-
iments would be needed to provide a conclusive answer,
which is beyond the scope of this work.
Lowering the temperature the first minimum is ex-
pected to shift to lower q values due to the increasing
particle size [42,22]. Such a variation is indeed observed.
A quantitative description of the data however requires
modeling of the density profile in the particle.
Stieger and coworkers  suggested to convolution of
the density profile for a particle of radius R with a Gaus-
sian of width σ, implying an effective steric radius of order
RH≈ R +2σ. An alternative approach suggested by Ma-
son and Lin is the ”uniform core linear shell” . Both
models provide an excellent fit to the data for temper-
atures of T = 25◦C and below where the particles are
swollen and the effective refractive index is sufficiently
low for the Rayleigh-Gans-Debye (RGD) approximation
(|n/nsolvent− 1| ? 1) to be valid. The ”uniform core lin-
ear shell” model however results in an outer cutoff radius
substantially smaller than the hydrodynamic radius (data
not shown). For this reason we have chosen to apply the
Gaussian approximation. We think that the latter model
captures better the slow and gradual decrease of the par-
ticle density in the particle corona. One should keep in
mind however, as pointed out by Mason and Lin, that the
missing cutoff makes this model unphysical at very large
Reufer et. al.: Temperature sensitive poly(N-Isopropyl-Acrylamide) microgel particles: a light scattering study7
Both the ”uniform core linear shell” and the Gaussian
approximation were originally applied to neutron scatter-
ing data. In the RGD-limit light and small angle neutron
scattering on a two component system are sensitive to
the same physical properties, namely density fluctuations.
Light scattering probes fluctuations of the refractive index
(n/nsolvent− 1) whereas neutron scattering probes fluctu-
ations in the nuclear scattering length density. Both quan-
tities are proportional local polymer density. Therefore, up
to a pre-factor, the model by Stieger and coworkers 
can be applied to both light and neutron scattering data.
In the RGD approximation the scattering form factor is
given by P(q) ∝ [A(q)]2with
A(q) = 3exp[−(σq)2/2][sin(qR)−qRcos(qR)]/(qR)3(1)
It is straightforwardto account for polydispersity using
standard procedures  (polydispersity values for best fit
to the data are 9.5 ± 0.5%). We obtain an excellent fit to
the data for temperatures up to T = 30◦C. T = 30◦C is
however a borderline case. Here, we obtain a perfect RGD
fit with σ ≈ 0, while a Mie calculation with the same
parameters displays differences. This inconsistency shows
that the RGD-condition(|n/nsolvent− 1| ? 1) is not fully
met. As a consequence, in this transition regime, the thick-
ness of the diffuse corona σ is underestimated.
Figure 2 summarizes the results of our analysis. For tem-
peratures T = 25◦C and below the results from static
(open circles) and dynamic light scattering (solid squares)
match almost perfectly. However in the transition regime
at T = 30◦C we observe a noticeable difference.
It is worthwhile to add that our study represents one of the
first attempts to quantitatively model the full from factor
(including the first minimum) of PNIPAM particles us-
ing static light scattering. Our results clearly show that
such data can be of similar or even better quality than
neutron scattering data but with significantly less sample
preparation requirements and at a fraction of the cost.
This should prove very useful for future studies of these
very interesting systems in particular for particle radii of
about 200nm and above. A detailed understanding of the
rheological properties in the solid state for example will
require quantitative input concerning the internal polymer
5 Diffusing Transmission
The optical transmission through a cell of thickness L
is determined by the scattering cross section of the in-
dividual particles, the particle number density and inter-
particle correlations characterized by the structure factor
S(q). For dilute non-interacting systems (S(q) ≡ 1) the
optical properties can be characterized by the scattering
mean free path .
which is found to be inversely proportional to the total
scattering cross section of a particle σsc. Initially this leads
to an exponential decay of the unscattered intensity trans-
mitted through a slab of thickness L: T0 = exp(−L/l).
8Reufer et. al.: Temperature sensitive poly(N-Isopropyl-Acrylamide) microgel particles: a light scattering study
With increasing density multiple scattering contributions
become important until in the case of a highly turbid sys-
tem light propagates diffusively and the total transmission
coefficient can be written as 
T ∝ l∗,L ? l∗
The typical length characterizing the diffusion process
is the transport mean free path l∗which can be written
quite generally in terms of the mean square scattering
? ? denotes the angular average over all scattering angles,
weighted by the scattering probability.
Positional correlations between particles affect the optical
density of concentrated colloidal suspensions because they
change the angular distribution of scattered light emerging
from each scattering event and therefore change the value
of l∗. For correlated systems we replace P(q) by the full
scattering function P(q)S(q), thus we obtain
Therefore the general expression for l∗in a correlated
suspensions of spherical particles reads [45,46]:
We now want compare these predictions to the scatter-
ing properties of our thermosenstive PNIPAM particles.
A dense suspension is prepared using a rotary evaporator
from the initial as-synthesized stock solution. From drying
and weighing we find a mass density of φ ≈ 14.6% w/w.
Under the assumption that the dry sample is absolutely
water free this would imply a volume fraction in the col-
lapsed state of Φ ≈ 19 %. We perform a diffuse transmis-
sion experiment with a frequency-doubled Nd:YV04solid
state laser Coherent “Verdi” (Coherent Inc., Santa Clara
CA, U.S.A.) operating at λ = 532 nm. The laser beam
is slightly expanded to illuminate the sample with a spot
size diameter of approximately 5 mm. The sample is kept
in a regular glass cell with inner dimensions 10 × 2 mm
(Hellma, Germany). The scattered light leaving the sam-
ple is collected by a single-mode optical fiber placed in
transmission geometry. The transport mean free path l∗
at each temperature is obtained by measuring the ratio of
the intensity of the transmitted light through the sample
compared to the value obtained for a reference sample of
known value l∗[47,48].
Figure 4 shows the corresponding increase of the trans-
port mean free path upon decreasing the temperature. The
optical transport mean free path increases from l∗=53 μm
in the high temperature limit (T = 40◦) to l∗= 289μm at
T = 10◦.
By lowering its density the cross-linked gel changes
its optical properties. In fact, the swollen gel has a lower
optical contrast compared to the collapsed gel. Further
scattering contrast is lost as particles start to touch and
Reufer et. al.: Temperature sensitive poly(N-Isopropyl-Acrylamide) microgel particles: a light scattering study9
Fig. 4. Transport mean free path l∗as a function of temper-
ature for φ ? 14.6% w/w. Solid line: Mie result, l∗=56 μm,
for homogenous hard spheres with radius R = 242nm at an
effective volume fraction of Φ ≈ 19 %
deform. For perfectly homogeneous particles at an effec-
tive volume fraction of 1 there should be no scattering.
Residual scattering can be linked to density fluctuations
that are due to a) the imperfect space filling of the PNI-
PAM microgel particles b) an inhomogeneous radial den-
sity distribution of the individual particles and c) posi-
tional correlations. In our case positional correlations con-
tribute only weakly to the increase of l∗since the structure
factor peak is located at small values of q compared to the
cut-off value 2 · k0.
At high temperatures the system is fluid, particles are
homogenous and moreover the density is only moderately
high. Under these conditions it should be possible to make
a quantitative prediction of l∗using equation (6) [46,49].
For our analysis we use the radius obtained from static
light scattering, R = 242 nm. Structural correlations are
taken into account using the Perkus-Yevick structure fac-
tor for a monodisperse suspension of hard spheres. For
pure water as a solvent (ns= 1.33) and a particle index
ns= 1.46 we find a value of l∗= 56μm (l∗= 48μm with
S(q) ≡ 1) at a suspension volume fraction of Φ = 19%.
Such an exceptionally good agreement with the experi-
mental result l∗= 53μm must be somewhat fortuitous
given the limited accuracy of our input parameters such
as particle refractive index and the effective volume frac-
Finally we would like to add a comment concerning the
background medium refractive index. Previous studies on
mono- and biomodal suspension of polystyrene spheres
(n? 1.6) up to Φ ? 40% found good agreement between
theory (Eq. 6) and experiment using the refractive index
of pure water [46,50]. For high refractive index particles
(n > 2) at high volume fraction (in air) it is however nec-
essary to take into account an increased effective refractive
as shown by Rivas et al.  and more recently Reufer and
co-workers . Following the latter approach in our case
we obtain nsolvent,eff? 1.346 and thus l∗= 75μm.
6 Summary and Conclusions
In summary we have presented a comprehensive study of
the scattering properties of a temperature tunable col-
loidal system. It would be interesting to further extend
our approach by using additional techniques such as Ul-
tra Small Angle X-Ray or Neutron Scattering to access
information on smaller length scales as done in some of
the previous work on similar systems. The particular sys-
10 Reufer et. al.: Temperature sensitive poly(N-Isopropyl-Acrylamide) microgel particles: a light scattering study
tem presented in this work displays some interesting prop-
erties which make it very suitable to study the phase be-
havior of colloids with repulsive interactions. The particles
are charge stabilized in the maximally collapsed state and
therefore fully stable even at high temperatures. The op-
tical transport mean free path at T = 40◦C furthermore
indicates that at high temperatures the system essentially
behaves as a suspension of hard spheres. This in turn will
allow the system to reversibly cross the phase boundary
from a dense colloidal suspension to an arrested system
upon changing the temperature.
Work supported by the Top-Nano 21 Project, the Swiss
National Science Foundation project 200020-117762 and
Marie Curie network Grant No. MRTN-CT2003-504712.
Authors also thank Christoph Neururer and Daniela
Curdy for SEM facility and synthesis assistance, Peter
Schurtenberger,James Harden, Reinhard Sigel, Nasser Ben
Braham, Veronique Trappe, Joaquim Clara Rahola, Fred-
eric Cardinaux and Pavel Zakharov for discussions.
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