Thermoresponsive poly-(N-isopropylmethacrylamide) microgels: Tailoring particle size by interfacial tension control

Polymer 54 (2013) 5499e5510

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Thermoresponsive poly-(N-isopropylmethacrylamide) microgels: Tailoring particle size by interfacial tension control Katja von Nessen a, b, Matthias Karg a, *, Thomas Hellweg b a b

Department of Physical Chemistry I, University of Bayreuth, Universitätsstrasse 30, 95447 Bayreuth, Germany Department of Physical and Biophysical Chemistry, Faculty of Chemistry, Bielefeld University, Universitätsstrasse 25, 33615 Bielefeld, Germany

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 June 2013 Received in revised form 9 August 2013 Accepted 12 August 2013 Available online 20 August 2013

Size control is one of the most important tasks in colloid synthesis. In the present work, the anionic surfactant sodium dodecyl sulfate (SDS) and the cationic surfactant cetyl trimethyl ammonium bromide (CTAB) were used to precisely control the size of poly-N-isopropylmethacrylamide [poly-(NIPMAM)] microgels prepared by precipitation polymerization. For each surfactant the concentration was varied over a broad range below the critical micelle concentration (cmc). The resulting particle size, size distribution, and shape were studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Photon correlation spectroscopy (PCS) was employed to analyze the volume phase transition behavior of the microgels. We show that for both surfactants a decrease in particle size is achieved for increasing surfactant concentrations while the swelling properties of the microgels are nearly unaffected. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Microgel Precipitation polymerization Surfactant

1. Introduction Hydrogels are water-swollen, three-dimensional, physically or chemically cross-linked polymer networks combining liquid-like and solid-like properties. With respect to the gel dimensions, a classification into microgels with dimensions between 10 and 1000 nm in size and macrogels with typical sizes >1000 nm is often used. Of great interest are so-called ”smart” hydrogels which undergo an abrupt and reversible volume change in response to environmental stimuli like temperature [1,2], pH [3e6], ionic strength [7,8], solvent composition [9,10] or electric field [11,12]. During the last decades the development of new hydrogel systems has attracted tremendous interest because these materials open up applications in drug delivery [13e16], photonics [17], catalysis [18,19], tissue engineering [20], sensor design [21,22], and separation technologies [23] due to their excellent properties namely softness, smartness, elasticity, water storage capacity, and biocompatibility. In contrast to macroscopic gels, microgels exhibit significantly faster swelling/de-swelling rates, which is a critical aspect for many applications [24e26]. This effect can be mainly attributed to the much larger surface/volume ratio of microgels compared to their macroscopic counterparts.

* Corresponding author. E-mail address: [email protected] (M. Karg). 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2013.08.027

A well-known and extensively studied thermoresponsive microgel system is based on the monomer N-isopropylacrylamide (NIPAM). Poly-(NIPAM) undergoes a temperature induced volume phase transition (VPT) from a highly swollen to a collapsed state at around 32
C in water. Pelton and Chibante were the first authors to report on the synthesis of thermoresponsive poly-(NIPAM) microgels [27]. The particles, these authors synthesized, had dimensions in the order of 500 nm in diameter in the rather collapsed state as determined by TEM. Based on their synthetic procedure, the common approach for the synthesis of such kind of microgels is the surfactant-free precipitation polymerization in water at temperatures above the lower critical solution temperature (LCST) of the polymerized monomer. Under these conditions, the monomer is still soluble but already rather short oligomers formed in the very beginning of the polymerization will show phase separation. Due to this phase separation, this polymerization strategy leads to spherical polymer objects of several hundred nm dimensions since the system tries to minimize its interfacial area [28]. Much attention was paid to the preparation of microgels with different chemical compositions mainly to alter their volume phase transition behavior. Different functionalities can be easily introduced into the polymer network structure by copolymerization with organic comonomers such as acrylic acid [5,29]. In the last years the polymerization of different monomers and comonomers was used to generate microgels with an even more complex morphology, for example with a coreeshell structure where the

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core is mainly composed of one monomer and the shell constitutes a different composition [30,31]. Despite control over the chemical composition of responsive microgels, a still remaining challenge is the precise control of the final particle size. In case of poly-(NIPAM) microgels, several works can be found in the literature concerning the controlled size variation by using surfactants at different concentrations during the microgel synthesis [32,33]. The addition of a surfactant to the reaction medium leads to a reduction of the interfacial tension during the nucleation phase of the polymerization process. More particles are nucleated at lower interfacial tension and therefore smaller particles can be generated compared to a surfactant-free polymerization. The main processes during the surfactant-assisted precipitation polymerization are illustrated in Fig. 1. Pelton and co-workers reported the synthesis of poly-(NIPAM) latices in the presence of SDS already in 1993 [34]. The authors showed that the radius of the prepared poly-(NIPAM) microspheres decreased exponentially with increasing SDS concentration. More recently, Richtering et al. received the same result for poly-(NIPAM) nanogels [35]. The group of Andersson focussed on the internal microgel structure in dependence of their synthetic protocol [36]. Using temperature dependent 1H NMR spectra and 1H NMR spine lattice (T1) and spinespin (T2) relaxation time measurements it could be shown that poly-(NIPAM) microgels prepared with high surfactant concentrations had a more homogeneous structure than those prepared with low surfactant concentrations. In the present work, we concentrate on the preparation and characterization of microgels based on poly-N-isopropyl methacrylamide (poly-(NIPMAM)). In contrast to poly-(NIPAM), poly-(NIPMAM) shows a volume phase transition at temperatures in the range of 38e44
C [37e39]. This range is very close to the biologically relevant range, which makes poly-(NIPMAM)-based microgels more interesting for potential biological applications such as drug-delivery. All poly-(NIPMAM) microgels were synthesized by precipitation polymerization using two types of surfactants and a broad range of surfactant concentrations. The materials were analyzed by different imaging techniques as well as photon correlation spectroscopy (PCS) to study their volume phase transition behavior in detail. A strong correlation between the particle dimensions and the concentration of surfactant used within the polymerization is found and is described by empirical relations allowing to predict the particle size in future synthesis. 2. Materials and methods 2.1. Chemicals All chemicals are commercially available. The monomer N-isopropylmethacrylamide (NIPMAM, Aldrich, purity 97%) was purified

a

b

c d

Fig. 2. Structures of the used monomer NIPMAM (a), the cross-linker BIS (b) as well as the used surfactants ((c): SDS, (d): CTAB).

by recrystallization from n-hexane and dried in vacuum at 25
C. The cross-linking agent N,N‘-methylenebisacrylamide (BIS, purity > 98%, Fluka), the surfactants sodium dodecyl sulfate (SDS, analytical grade, Serva) and cetyl trimethyl ammonium bromide (CTAB, purity  99%, Aldrich) as well as the initiators 2,2‘-azobis(2amidinopropane)dihydrochloride (V50, purity 97%, Aldrich) and potassium peroxodisulfate (KPS, purity 99%, Aldrich) were used as received. Milli-Q water (Millipore, Merck KGaA, Germany) with a resistance of 18.2 MU cm and a total organic content (TOC) < 10 ppb was employed for the synthesis and purification process (centrifugation, ultrafiltration) of the microgels. 2.2. Synthesis of microgels All microgels were synthesized by precipitation polymerization using different surfactants and surfactant concentrations. The structures of the monomer, the cross-linker and the used surfactants as well as their cmc-values are schematically depicted in Fig. 2. All polymerizations were conducted in 250 mL three-necked round bottom flasks equipped with a reflux condenser, a mechanical stirrer (IKA, RW 20) and a nitrogen inlet. For a typical synthesis, the 0.1 M monomer feed was prepared by dissolving NIPMAM (0.01 mol, 1.272 g), BIS (0.53 mmol, 0.082 g) and the desired amount of surfactant in 100 mL of Milli-Q water. The solution was stirred with a rotation speed of 260 rpm and nitrogen was purged through the reaction flask to remove oxygen for 30 min. Afterwards, the temperature was raised to 70
C using an oil bath. After an equilibration time of 30 min, the reaction was started by the rapid addition of the initiator, which was dissolved in 1 mL of Milli-Q water. The clear and coloroless reaction mixture became turbid within the first 10 min after the addition of the initiator. The

Fig. 1. Schematic depiction of the polymerization mechanism. [a] starting point: homogeneous solution of monomer(s), surfactant molecules and initiator molecules, [b] generation of initiator radicals by thermal decomposition of the initiator molecules, [c] propagation e generation of oligoradicals, [d] precursor particles, [e] growing particles, [f] swollen microgels. The surfactant is mainly involved in the steps [b]e[d], but also makes the interfacial tension contribution less important for the growing beads in step [e].

K. von Nessen et al. / Polymer 54 (2013) 5499e5510 Table 1 Summary of polymerization recipes used for the synthesis of the poly-(NIPMAM) microgels. All synthesis were performed with constant agitation of 260 rpm and a constant reaction temperature of 70
C. The nominal cross-linker content was 5 mol%. Surfactant

V (H2O)/ ml

n(NIPMAM)/ mol

n(BIS)/ mmol

n(initiator)/ mmol

n(surfactant)/ mM

SDS CTAB

100 100

0.01 0.01

0.53 0.53

0.025 0.025

0e6.93 0e0.82

2.3. Characterization 2.3.1. Photon correlation spectroscopy (PCS) Photon correlation spectroscopy was performed with an ALV laser goniometer system from ALV-GmbH (Langen, Germany). A diode-pumped solid-state Verdi V2 laser (Coherent Laser Group, Santa Clara, California) with a wavelength of 532 nm and a constant maximum output power of 1 W was used as light source. Timeintensity autocorrelation functions g2 (s) were generated by an ALV-5000/E multiple-s digital correlator. The sample temperature with a precision of 0.01
C was controlled with a Fisherbrand FBC720 thermostat from Fisher Scientific and a toluene index matching bath. Angular dependent PCS measurements were done in a scattering angle range between 30
and 100
at 20
C and at 60
C. Measurements for each angle were repeated three times. All samples were previously filtered to remove dust. Sample concentrations of 0.001 wt% were used for all measurements. All samples were allowed to equilibrate at the used temperatures for at least 20 min. The recorded autocorrelation functions were analyzed by inverse Laplace transformation using the FORTRAN Program CONTIN [40e42]. The calculation leads to a z-averaged relaxation rate G, which is connected to the translation diffusion coefficient DT according to:

G ¼ DT q2

(1)

In this equation, q is the magnitude of the scattering vector, which depends on the wavelength l of the incident light, the refractive index n0 of the continuous medium and the scattering angle q as follows:

q ¼

4pn0

l

  q sin 2

(2)

If purely translational diffusion is observed it is straightforward to calculate the hydrodynamic radius Rh according to the Stokese Einstein equation [43]:

DT ¼

kB T 6phRh

where kBT represents the thermal energy and h the viscosity of the solvent. An important parameter to characterize the swelling capacity of microgels is the swelling ratio a, which can be defined as the ratio between the particle volume in the collapsed (Vcollapsed) and the particle volume in the fully swollen (Vswollen) state.

a ¼

polymerization reaction was allowed to proceed for 6 h. Then, the mixture was cooled down to room temperature. The aqueous microgel dispersions were purified by ultrafiltration and by 3 cycles of centrifugation, decantation and redispersion in Milli-Q water to remove oligomers, unreacted monomers and surfactant molecules. Microgel particles with diameters >100 nm were centrifuged for 20 min at 15,000 rpm, whereas smaller microgel particles with diameters <100 nm were centrifuged for 1 h at 15,000 rpm. After the purification process, the microgels were freeze-dried. Detailed recipes for all synthesized microgels are listed in Table 1.

(3)

5501

Vcollapsed ¼ Vswollen



Rh R0

3 (4)

2.3.2. Scanning electron microscopy (SEM) The SEM images were recorded on a LEO 1530 Gemini field emission scanning electron microscope (FE-SEM) from Carl Zeiss (Germany) working with an acceleration voltage of 3 kV. The samples were prepared by drying a drop of a dilute aqueous microgel dispersion on a clean silicon wafer at room temperature. The silicon wafers were carefully cleaned by a multi step cleaning process. In the first step, the wafers were treated by sonication sequentially in acetone and methylene chloride for 10 min to remove traces of organic contaminants before further use. Afterwards, the silicon wafers were placed in a solution containing 5 parts of water, 1 part of 30% hydrogen peroxide (H2O2) and 1 part of 27% ammonium hydroxide (NH4OH) and heated to 80
C on a hot plate for 10 min. Finally, the wafers were rinsed several times with Milli-Q water and dried under a stream of nitrogen. All samples were sputtered with a 2 nm platinum layer using a Cressington 208HR sputter coater. Sputtering is necessary to enhance the contrast of the samples and to prevent the accumulation of electrostatic charge at the surface by making the specimen conductive. 2.3.3. Atomic force microscopy (AFM) All AFM images were recorded against air, at room temperature with a MultiMode NanoScope III AFM from Digital Instruments operating in tapping mode. The scan rates were chosen between 0.5 Hz and 1 Hz depending on the scan size. The samples were prepared on 1 cm2 pieces of silicon wafers by drying a dilute drop of a microgel dispersion with a concentration between 0.0006 wt% and 0.03 wt% depending on the particle size at room temperature. 2.3.4. Zeta potential (z) and temperature dependent DLS measurements Zeta potential measurements were performed with a Zetasizer Nano ZS from Malvern Instruments (Herrenberg, Germany) equipped with a 4 mW HeeNe laser (l ¼ 633 nm), a scattering detector positioned at a scattering angle of 173
and a temperaturecontrol jacket for the cuvette. Microgel dispersions with a concentration of 0.01 wt% were used. Electrophoretic mobilities (me) of the particles were measured and converted into z-potentials with the software supplied by the manufacturer using the Smoluchowski equation (z ¼ meh/ε0 ε, where h denotes the viscosity and ε0 ε the permittivity of the suspension). The reported z-potentials were results of the average of 3 successive measurements. All samples were prepared without the addition of salt and had approx. pH 7. Sample concentrations of 0.01 wt% were used for all measurements. All samples were allowed to equilibrate at the used temperatures for at least 10 min. The same instrument was used to analyze the temperature dependent swelling behavior. DLS measurements were done in a temperature range between 30
C and 60
C in 2
C steps with equilibration times of at least 20 min at each temperature. Measurements for each temperature were repeated three times and the autocorrelation functions were analyzed with the method of cumulants.

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Fig. 3. SEM micrographs of poly-(NIPMAM) microgels prepared with SDS (top) and corresponding histograms (bottom) of the particle diameter. For the histograms, the diameters of 300 particles for each sample were measured using the program ImageJ. [a] synthesis with 0.00 mM SDS: 667  23 nm, [b] 0.18 mM SDS: 451  14 nm, [c] 1.04 mM SDS: 315  16 nm, [d] 1.39 mM SDS: 261  13 nm, [e] 2.08 mM SDS: 213  27 nm. The solid lines in the histograms are Gaussian fits.

The results obtained using the Zetasizer Nano ZS and the previously mentioned ALV laser goniometer system were compared and we found very good agreement of the inverse swelling behaviors a1 as a function of temperature (see Fig. S5 in the supporting information). 3. Results and discussion 3.1. Poly-(NIPMAM) microgels prepared with SDS We prepared a series of PNIPMAM microgels with SDS as surfactant at different concentrations below the cmc. SDS is one of the most widely used surfactant in emulsion and precipitation polymerization exhibiting a cmc value of 8.3 mM at 25
C. Due to its negative headgroup charge we performed the polymerizations with the anionic radical initiator KPS leading to slighty negatively charged particles, as will be discussed later in this section. In the following the microgel batches prepared with SDS will be labeled as GEL-x-SDS, where x gives the concentration of SDS in mM used within each polymerization. In order to study the particle size, size distribution and shape, all microgels were characterized with different imaging techniques, namely scanning electron microscopy and atomic force microscopy. Fig. 3 shows representative SEM images for microgels prepared with five different concentrations of SDS. In all cases, spherical particles of low polydispersity were obtained. The SEM images as well as the corresponding histograms of the particle size distribution (bottom) clearly reveal a decrease in particle diameter with increasing SDS concentration (from left to right). The histograms

were obtained by measuring the particle size of 300 particles for each sample using the program ImageJ. For all microgels, the polydispersity is below 10% as can be seen from the narrow half width of the Gaussian fits in the histograms. In addition, the SEM images show areas of hexagonal ordering, which manifests again the low polydispersity. The particle diameter obtained from SEM vary from 667 nm (no SDS) to 78 nm (6.93 mM SDS). The average diameter from the SEM analysis are listed in Table 2. The diameters obtained from SEM correspond to the particle diameter in the nearly fully collapsed state of the microgels due to the high vacuum in the sputter and the electron microscope chamber. Thus larger particle dimensions are expected for aqueous dispersions below the volume phase transition temperature (good solvent conditions). In addition to SEM, we performed AFM measurements on the same microgels. Height profiles obtained in tapping mode are shown in Fig. 4. The AFM images confirm the trend in terms of particle size as obtained from SEM. The analysis of the particle dimension from AFM provides a diameter of 719  23 nm for the poly-(NIPMAM) microgel prepared in the absence of SDS, whereas diameters between 503 and 90 nm were observed for the microgels prepared with different amounts of surfactant. A comparison of both imaging techniques shows that the average particle diameters obtained from AFM were slightly higher than the average particle diameters received from SEM. This can be mainly attributed to the fact that the AFM measurements were performed against air at ambient conditions, while SEM measurements were conducted in a high vacuum chamber. It is noted that the sample preparation procedure

Table 2 Summary of experimental results for poly-(NIPMAM) microgels prepared with SDS: hydrodynamic diameter Dh obtained from angular dependent PCS measurements (at 20
C and at 60
C), volume phase transition temperatures (VPTT) determined from temperature dependent PCS measurements, particle diameter from AFM and SEM measurements and zeta potentials (at 20
C and at 60
C). Sample

D(PCS) 20
C/nm

GEL-0.00-SDS GEL-0.18-SDS GEL-0.35-SDS GEL-0.69-SDS GEL-1.04-SDS GEL-1.39-SDS GEL-1.73-SDS GEL-2.08-SDS GEL-3.47-SDS GEL-5.20-SDS GEL-6.93-SDS

944 776 710 676 616 524 472 414 297 182 111

          

47 39 36 34 31 26 24 21 15 9 6

D(PCS) 60
C/nm 354 307 294 260 237 201 182 159 116 82 49

          

18 15 15 13 12 10 9 8 6 4 3

VPTT
C

D(AFM)/nm

43.5 45.5 45.0 45.0 45.0 45.5 45.5 45.0 45.0 45.0 46.0

719 503 489 448 410 361 311 269 207 139 90

          

23 22 30 25 24 17 17 24 21 32 20

DSEM)/nm 667 451 421 360 315 261 238 213 140 110 78

          

23 14 13 26 16 13 11 27 19 30 31

z 20
C/mV

z 60
C/mV

6.7 6.3 15.4 16.1 16.8 12.5 9.6 16.2 15.3 12.6 6.6

28.7 31.5 24.2 31.9 30.6 31.1 25.1 29.8 30.4 24.9 22.2

K. von Nessen et al. / Polymer 54 (2013) 5499e5510

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Fig. 4. AFM height profiles (top) and corresponding particle size distribution histograms (bottom) of microgels synthesized with different SDS concentrations. [a] synthesis with 0.00 mM SDS: 719  23 nm, [b] 0.18 mM SDS: 503  22 nm, [c] 1.04 mM SDS: 410  21 nm, [d] 1.39 mM SDS: 361  17 nm, [e] 2.08 mM SDS: 269  24 nm. The solid lines in the histograms are Gaussian fits.

was nearly the same for SEM and AFM investigations, except for the sputtering performed for sample analysis via SEM. Another effect which causes a larger diameter from AFM as compared to SEM is related to the geometry and size of the AFM probe (7e10 nm tip radius). However, the latter effect is rather small compared to the size of the microgel particles. A simulation of the influence of the tip diameter on representative AFM height profiles has shown that the difference in the particle topography is marginal. The ratio of the diameter from AFM and the diameter from SEM are listed in Table 3. The AFM height profiles confirm again the spherical shape of all microgels as well as the low polydispersity. In addition, we performed a cross-section analysis of for the microgels prepared with different amounts of SDS. Fig. S6 in the supporting information shows that not only the lateral dimensions but also the height of the microgels decreases with increasing SDS concentration. The surface charge of the microgel particles was determined by measuring the z-potential at 20 and 60
C. The results are listed in Table 2. It is noted that the interpretation of the z-potential values is rather difficult for these rather large and water swollen microgels and that the z-potentials do not represent the actual particle surface charge. Therefore, we only use these values as an indicator for the sign and magnitude of the surface charge. Due to the anionic radical initiator (KPS) used for all SDS-assisted synthesis, all microgels show a slightly negative z-potential in the swollen state. In addition, the results show that the z-potential of the reference microgel (no SDS) does not differ significantly from the values determined for the microgels prepared with SDS. For the whole series of microgels prepared with SDS, no systematic evolution of

Table 3 Summary of calculated ratios from the diameters obtained by PCS, AFM and SEM measurements. Sample

D(AFM)/ D(PCS, 60
C)

D(SEM)/ D(PCS, 60
C)

D(AFM)/ D(SEM)

D(PCS, 20
C)/ D(PCS, 60
C)

GEL-0.00-SDS GEL-0.18-SDS GEL-0.35-SDS GEL-0.69-SDS GEL-1.04-SDS GEL-1.39-SDS GEL-1.73-SDS GEL-2.08-SDS GEL-3.47-SDS GEL-5.20-SDS GEL-6.93-SDS Average

2.03 1.64 1.66 1.72 1.73 1.80 1.71 1.69 1.78 1.70 1.84 1.75  0.11

1.88 1.47 1.43 1.38 1.33 1.30 1.31 1.34 1.21 1.34 1.59 1.42  0.18

1.08 1.12 1.16 1.24 1.30 1.38 1.31 1.26 1.48 1.26 1.15 1.25  0.12

2.67 2.53 2.41 2.60 2.60 2.61 2.59 2.60 2.56 2.22 2.27 2.51  0.15

the z-potential in dependence on the SDS concentration is observed. This is an indicator that most of the SDS could be successfully removed from the microgel dispersions within the applied purification steps. If significant amounts of SDS would remain adsorbed to the microgel particles an increase of the z-potential with increasing SDS concentration would be expected. At 60
C, the z-potential values increase significantly. On average z ¼ 28 mV was measured for all microgels in the fully collapsed state. In order to study the particle size and size distribution from the ensemble rather than from a limited number of particles, we performed PCS measurements. At first, we studied the angular dependency of the particle diffusion in the swollen state (20
C) and in the fully collapsed state (60
C) to prove whether purely translational diffusion is observed. The results of the angular dependent PCS measurements are shown in Fig. 5. Here the z-average relaxation rate G obtained from CONTIN analysis of the autocorrelation functions is plotted against the square of the magnitude of the scattering vector q2. Representative autocorrelation functions obtained from PCS are shown in the supplementary information (Fig. S1). All correlation functions show a monomodal decay and could be analyzed using the method of inverse Laplace transformation with the program CONTIN. Representative relaxation rate distributions as obtained from the CONTIN analysis are also shown in the supplementary information (Fig. S3). As can be seen in Fig. 5 a linear behavior of G vs. q2 is observed for all microgels. Within the experimental error, the intercept for all linear fits is zero. Hence, purely translational diffusion is monitored for all particles. The StokeseEinstein equation (3) can be applied to calculate the hydrodynamic diameter Dh ¼ 2  Rh. The values of Dh determined from angular dependent PCS measurements at 20
C where the microgels are in the swollen state and at 60
C where the microgels are in the fully collapsed state are listed in Table 2. From the results of the angular dependent PCS measurements shown in Fig. 5 it can already be seen that the diffusion coefficients DT (the slope of the linear fits according to eq. (1)) increase with increasing SDS concentration used in the synthesis. This is related to a decrease in particle size with increasing SDS concentration - a trend which is directly visible from the hydrodynamic diameter listed in Table 2. The largest hydrodynamic diameter (944  47 nm at 20
C and 354  18 nm at 60
C) were determined for the microgel prepared in the absence of SDS. In comparison the respective values for the smallest amount of SDS used within this study (0.18 mM) are 776  39 nm for 20
C and 307  15 nm for 60
C. Hence, the addition of 0.18 mM SDS to the precipitation polymerization leads already to a reduction of the particle size in the order of 18%. For the highest amount of SDS used within this

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K. von Nessen et al. / Polymer 54 (2013) 5499e5510

Fig. 5. Results from angular dependent PCS measurements at 20
C (left) for the swollen state and at 60
C (right) for the collapsed state for selected poly-(NIPMAM) microgels prepared with 0.00 mM, 0.18 mM, 1.04 mM, 1.39 mM and 2.08 mM SDS measured in a scattering angle range between 30
and 100
. The solid lines represent linear fits. The error bars are in the range of the symbol size.

study, the reduction in particle diameter is in the order of 88%. This illustrates the broad range of particle sizes which can be reached by the addition of SDS to the polymerization batch. Since all surfactant concentrations were below the cmc, the particle formation occurred via a homogeneous nucleation mechanism. At this concentration no micelles were present and the particle formation takes place by precipitation in aqueous phase. A schematic representation of this mechanism is shown in Fig. 1. If surfactant molecules are present in the reaction medium during the precipitation polymerization, they will stabilize the precursor particles against aggregation in the early stage of the polymerization. This will allow all nucleated precursor particles to continue growing without strong aggregation and therefore the final particle number is increased while the particle size is decreased for relatively high surfactant concentrations. In the absence of surfactant or in the regime of low surfactant concentrations, the growing particles have a higher tendency to aggregate and hence larger particles are formed and the final number of polymer colloids is decreased. Moreover, also a lower nucleation can be assumed. The ratios between the diameter obtained from SEM, AFM and PCS at 20 and 60
C are given in Table 3. For all microgels prepared with SDS nearly constant ratios D(AFM)/D(PCS, 60
C) and D(SEM)/ D(PCS, 60
C) were obtained. On average the diameter from AFM are 1.75  0.11 larger than the hydrodynamic diameter at 60
C and for the diameter from SEM an average ratio of 1.42  0.18 was determined. It is noted that the ratios for the microgels prepared in the absence of SDS deviate the strongest from these average values and larger ratios D(AFM)/D(PCS, 60
C) and D(SEM)/D(PCS, 60
C) are found. This might indicate a slightly different network structure of this batch of microgels compared to the ones prepared with SDS. We will discuss this difference later on in this section. As previously discussed the diameter from AFM are slightly larger than the ones determined from SEM and an average ratio D(AFM)/D(SEM) of 1.25  0.12 was calculated. In addition the ratios between the hydrodynamic diameter at 20 and 60
C were calculated. These values represent the swelling capacity of the microgels, i.e. the potential increase in particle size due to the uptake of water by the microgel at 20
C. Here, a clear tendency and not a rather constant value is observed. The swelling capacity decreases slightly with increasing amounts of SDS used in the synthesis. To further investigate this effect and to characterize the temperature dependent swelling behavior of the microgels prepared with different amounts of SDS, temperature dependent PCS measurements were performed. Fig. 6 shows the evolution of the hydrodynamic diameter in a temperature range from 30 to 60
C in steps of 2
C (left).

As can be seen in Fig. 6 (left) all microgels show the typical, expected volume phase transition behavior of poly-(NIPMAM). All microgels exhibit a VPPT between 43 and 46
C with the lowest VPTT determined for the microgel prepared without SDS and the highest VPTT for the microgel batch synthesized with the largest amount of SDS. The swelling curves nicely confirm the trend of the particle diameter as observed by SEM and AFM. At each temperature of this PCS data series the diameter of the microgels decrease with increasing amount of SDS. Interestingly, the diameter of all microgels prepared by SDS-assisted precipitation polymerization reach a constant value at temperatures larger than 48
C. In other words these microgels reach the fully collapsed state at around 48
C and no further shrinkage is observed with increasing temperature. In contrast, the microgel prepared in the absence of SDS shows a slight decrease in size for increasing temperatures in a range higher than 48
C. This effect is even more obvious from the deswelling ratios shown in Fig. 6 (right) as calculated according to eq. (4). This indicates that the network structure of the microgel synthesized without SDS is different to the network structure of the SDS series. Since the only difference in all synthesis of this series is the amount of SDS, the only explanation for this different swelling behavior is the cross-link efficiency and cross-linker distribution. For poly-(NIPAM) based microgels chemically cross-linked with N,N‘-methylenebisacrylamide (BIS) prepared via surfactant-free precipitation polymerization it is well known that network structure is rather inhomogeneous with a more cross-linked core and a less cross-linked shell [44,45]. This inhomogeneity is caused by the different reaction kinetics of NIPAM and BIS. Andersson et al. have shown that the internal network structure of poly-(NIPAM) microgels becomes more homogeneous if a surfactant is employed during the synthesis [36]. From our results we see a clear decrease in the swelling capacity with increasing SDS concentrations and a very pronounced difference to the reference microgel (no SDS). Thus, already small amounts of SDS significantly influence the network structure in case of poly-(NIPMAM) microgels. The actual determination of the cross-linker density and the cross-linker distribution inside such microgel particles is a rather difficult task. Small angle neutron scattering would in principle be appropriate to resolve the internal network structure, but scattering profiles for microgels in the typical size range are dominated by the particle form factor in the lower q-range [46,47]. Therefore, we can only assume that the homogeneity and the cross-linker distribution vary in dependence of the amount of surfactant. The decrease of the swelling capacity with increasing SDS concentration points to an increasing cross-link efficiency with increasing SDS concentration, e.g. decreasing particle size. For poly-(NIPAM) microgels several

K. von Nessen et al. / Polymer 54 (2013) 5499e5510

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Fig. 6. Hydrodynamic diameter Dh and inverse swelling ratio a1 as a function of temperature for selected poly-(NIPMAM) microgels synthesized with 0.00 mM, 0.18 mM, 1.04 mM, 1.39 mM and 2.08 mM SDS. The temperature dependent PCS measurements were performed in a temperature range between 30
C and 60
C in steps of 2
C using a Zetasizer Nano ZS.

authors have shown that the swelling capacity decreases with increasing cross-linker content [6]. Since the ratio of NIPMAM to BIS was constant in all of the presented synthesis (5 mol-%), the incorporated amount of BIS can be assumed to be constant for all microgels. However, there are two possibilities for BIS to be incorporated into the polymer network: 1) If only one of the two vinyl groups reacts, BIS is simply incorporated as a monomer unit within a linear poly-(NIPMAM) chain; 2) If both vinyl groups react, BIS acts as a cross-linker molecule and a knot is formed in the growing network. It seems that the probability that both vinyl groups are polymerized and the BIS monomer actually serves as cross-linker increases for decreasing particle sizes of our poly-(NIPMAM) microgels. This would mean that the volume ratio of a lowly crosslinked shell is decreased for increasing SDS concentrations used during the polymerization. This effect is also observable from the cross-sections obtained from AFM height profiles, which we used to calculate the ratio between the height of the particles at the center position and the lateral particle dimensions. We found an increasing height/diameter ratio for increasing SDS concentrations, which is another indicator for a more homogeneous and more cross-linked network structure. These ratios are listed in tab. S1 in the supporting information. As mentioned before, a proof of this assumption is difficult to conduct experimentally and was not focus of the present study. The results obtained from AFM, SEM and PCS reveal a strong dependence of the particle size on the amount of SDS used during

the polymerization. In Fig. 7 (left) the determined diameter from these different techniques are plotted against the SDS concentration. Within the experimental error all diameters show a clear linear decrease with increasing SDS concentration in this semilogarithmic representation. Since the role of the SDS in the synthesis is the reduction of interfacial tension, it is useful to plot the final microgel surface area A0 as a function of c(SDS). This representation is shown on the right of Fig. 7 for results obtained from PCS at 20 and 60
C. The dependence of the particle diameter as well as the microgel surface area A0 on the concentration of SDS could be nicely described using the following empirical equation:

  log D=nm orA0 =nm2 ¼ A þ B  cðsur factantÞ=ðmmol=LÞ

(5)

This simple equation allows us to predict the achievable microgel diameter depending on the SDS concentration. For example, the hydrodynamic diameter in the fully collapsed state (60
C) depends on the SDS concentration as follows:

logðDh ðcollapsedÞ=nmÞ ¼ 2:48  0:12  cðSDSÞ=ðmmol=LÞ (6) This can now be used for synthesizing poly-(NIPMAM) microgels of tailored dimensions and it is useful to rewrite eq. (6) so that the SDS concentration can be calculated for any desired particle diameter:

Fig. 7. Hydrodynamic diameter Dh (left) and microgel surface area A0 (right) obtained from PCS, AFM and SEM measurements as a function of surfactant concentration in semilogarithmic representations. The solid lines are fits according to eq. (5).

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K. von Nessen et al. / Polymer 54 (2013) 5499e5510

Fig. 8. SEM micrographs of poly-(NIPMAM) microgels synthesized with different amounts of CTAB (top) and corresponding size distribution histograms (bottom). For the histograms, the diameters of 300 particles for each sample were measured using the program ImageJ. [a] synthesis with 0.00 mM CTAB: 463  18 nm, [b] 0.15 mM CTAB: 310  15 nm, [c] 0.27 mM CTAB: 218  8 nm, [d] 0.41 mM CTAB: 151  8 nm, [e] 0.55 mM CTAB: 135  12 nm. The solid lines in the histograms are Gaussian fits.

logðDh ðcollapsedÞÞ  2:48 cðSDSÞ=ðmmol=LÞ ¼  0:12

(7)

3.2. Poly-(NIPMAM) microgels prepared with CTAB CTAB has a much lower cmc (1 mM at 25
C) compared to SDS, which is mainly related to the longer alkylchain of CTAB. It is hence expected that CTAB is much more effective in lowering the interfacial tension. Due to the positive headgroup charge of CTAB, we performed all polymerizations of the CTAB series with the cationic radical initiator V50. We varied the CTAB concentration in a broad range below its cmc and performed all polymerizations with the same monomer feed concentration, nominal cross-linker content (5 mol-%) and initiator concentration as done for SDS. The microgels will be denoted as GEL-y-CTAB, where y gives the concentration of CTAB in mM used in the synthesis. Fig. 8 shows SEM micrographs of selected poly-(NIPMAM) microgels prepared with different amounts of CTAB. All microgels have a spherical shape and show narrow particle size distributions with polydispersities below 10%, which is also illustrated by the histograms of the measured particle diameter. In addition, the microgels show hexagonal ordering on the substrates (silicon wafer) which is a consequence of the narrow particle size distribution. A clear reduction of the obtained particle size is observed for increasing amounts of CTAB used within the precipitation polymerization. The Gaussian fits of the particle sizes yielded an

average diameter of 463  18 nm for the microgel prepared in the absence of CTAB. This is significantly smaller compared to the diameter from SEM determined for the poly-(NIPMAM) microgel prepared with KPS in the absence of SDS. However, these values cannot be directly compared due to the different surface charge of the two microgel systems and consequently their different attraction to the substrate used for SEM (silicon wafer). The use of 0.15 mM CTAB within the polymerization leads to a significantly smaller particle diameter of 310  15 nm compared to the GEL-0.00-CTAB sample. This is a reduction in the order of 33%. For the maximum amount of CTAB used within this work (0.82 mM), an average diameter of 110  16 nm was determined, which is a reduction in size in the order of 76%. Hence, CTAB can be very efficiently used to tune the particle size over a broad range. In addition to SEM, AFM height profiles were measured and analyzed in terms of the particle size. The results are shown in Fig. 9. The analysis of the AFM height images provides average particle diameters, which are slightly larger than the values obtained from SEM but the same trend of decreasing particle size with increasing CTAB concentration can be seen. The difference in size from AFM compared to SEM has already been discussed in the previous section. The results from SEM and AFM are summarized in Table 4. The ratios between the diameters from AFM and SEM are presented in Table 5. On average the determined diameter from AFM are by a factor of 1.33  0.18 larger than the diameter from SEM. This value is slightly larger than compared to the microgels synthesized with SDS. Within the error these values are

Fig. 9. AFM height profiles (top) and corresponding particle size distribution histograms (bottom) of microgels synthesized with different CTAB concentrations. [a] synthesis with 0.00 mM CTAB: 540  21 nm, [b] 0.15 mM CTAB: 440  23 nm, [c] 0.27 mM CTAB: 337  16 nm, [d] 0.41 mM CTAB: 239  16 nm, [e] 0.55 mM CTAB: 185  22 nm. The solid lines in the histograms are Gaussian fits.

K. von Nessen et al. / Polymer 54 (2013) 5499e5510

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Table 4 Summary of experimental results for poly-(NIPMAM) microgels prepared with CTAB: Hydrodynamic diameter Dh obtained from angular dependent PCS measurements (at 20
C and at 60
C), volume phase transition temperatures (VPTT) determined from temperature dependent PCS measurements, particle diameter from AFM and SEM measurements and zeta potentials (at 20
C and at 60
C). Sample

D(PCS) 20
C/nm

GEL-0.00-CTAB GEL-0.15-CTAB GEL-0.27-CTAB GEL-0.41-CTAB GEL-0.55-CTAB GEL-0.69-CTAB GEL-0.82-CTAB

1010 578 443 338 293 212 194

      

51 29 22 17 15 11 10

D(PCS) 60
C/nm 383 223 171 131 114 94 84

      

19 11 9 7 6 5 4

VPTT
C

D(AFM)/nm

43.5 45.0 45.5 45.5 46.0 46.0 46.0

540 440 337 239 185 150 125

comparable. Moreover, we performed a cross-section analysis on the basis of the AFM profiles. The results for selected microgels are shown in the supporting information (Fig. S6, right). Once more, we observe a decrease in the lateral particle dimensions as well as the particle height for increasing CTAB concentration used during the polymerization. Due to the cationic character of the V50 radical initiator, the microgels prepared within the CTAB series are expected to show a positive surface charge. This was confirmed by measuring the z-potentials at 20
C and at 60
C. As expected, positive z-potentials were obtained for all microgels prepared with V50. The fact that the measured z-potential of the reference microgel (no CTAB) is in the range of the measured zpotential for the poly-(NIPMAM) microgels prepared with CTAB, indicates that the purification processes were sufficient to remove CTAB from the reaction batches. The z-potentials at 20
C vary between 5.2 mV and 9.5 mV, whereas at 60
C a variation between 24.4 mV and 29.1 mV is observed. The hydrodynamic dimensions and the temperature dependent swelling behavior of the poly-(NIPMAM) microgels prepared with CTAB were investigated by PCS. First, we performed angular dependent PCS measurements at 20
C, where the microgels are expected to be swollen and at 60
C, where the particles should be fully collapsed (see Fig. 10). A clear linear dependence of the relaxation rate G on the square of the magnitude of the momentum transfer q2 can be seen for all samples of this series. Representative autocorrelation functions measured by PCS as well as relaxation rate distributions as result from the performed CONTIN analysis are shown in the supplementary material (Figs. S2 and S4). Again monomodal decays were observed for all microgels and the CONTIN analysis yielded narrow distribution functions of G. In addition, Fig. 10 shows that purely translational diffusion is monitored. The StokeseEinstein equation (eq. (3)) was applied to calculate the hydrodynamic diameter Dh of the microgel colloids. The values of Dh determined at 20 and 60
C from angular dependent PCS measurements are listed in Table 4. For all microgels the particle size decreases with increasing surfactant amount. Hence, the PCS measurements confirm the results obtained from SEM and AFM. Due to the difference in size determined from these different techniques, we calculated their

      

21 23 16 16 22 18 11

D(SEM)/nm 463 310 218 151 135 124 110

      

18 15 8 8 12 17 16

z 20
C/mV

z 60
C/mV

6.2 5.2 9.5 8.2 8.9 8.9 6.6

24.4 25.0 29.1 27.5 26.7 26.5 24.6

ratios as listed in Table 5. On average the diameter determined from AFM are by a factor of 1.70  0.23 and for SEM by a factor of 1.28  0.12 larger compared to the hydrodynamic diameter in the fully collapsed state. Compared to the results for SDS-assisted polymerizations as given in Table 3, these values are in very good agreement within the experimental error. In addition we calculated the ratio between the diameter from PCS at 20 and 60
C as a measure for the swelling capacity of the particles. These values, listed in Table 5, show the same tendency than the microgels prepared with SDS. The swelling capacity decreases with increasing CTAB concentration. For small amounts of CTAB (0.15e0.55 mM), the swelling capacity is slightly lower compared to the microgel prepared in the absence of CTAB. In contrast, the swelling behavior of microgels synthesized with high amounts of CTAB (0.69e 0.82 mM) is more affected, which is obvious from the small values of the swelling capacity. This indicates a more homogeneous network structure of the microgel particles, which might be related to the cationic radical inititiator (V50), which was used in this CTAB series. The thermoresponsive properties of the microgels were examined by temperature dependent PCS measurements. Fig. 11 (left) shows the evolution of the hydrodynamic diameter Dh. The typical volume phase transition behavior of poly-(NIPMAM) is observed for all samples with VPTTs between 43.5 and 46.0
C. As already observed for the SDS series, the VPTT slightly increases with increasing amount of CTAB used in the synthesis. The swelling capacity of these microgels is presented in Fig. 11 (right) in terms of the deswelling ratio a1 as a function of temperature. These values were calculated according to eq. (4). It can be seen that the swelling capacity decreases with increasing amounts of CTAB. Thus, we conclude that the microgel network structure is influenced by the presence of CTAB leading to a more homogeneous structure for large amounts of CTAB and potentially a higher crosslink efficiency. These effects have also been observed for the microgel series prepared with SDS as discussed in the previous section. Again, this trend is also observable from the cross-sections obtained from AFM height profiles. The height/diameter ratio increases with increasing CTAB concentrations. These ratios are listed in Table S2 in the supporting information.

Table 5 Summary of calculated ratios from the diameters obtained by PCS, AFM and SEM measurements. Sample

D(AFM)/D(PCS, 60
C)

D(SEM)/D(PCS, 60
C)

D(AFM)/D(SEM)

D(PCS,20
C)/D(PCS, 60
C)

GEL-0.00-CTAB GEL-0.15-CTAB GEL-0.27-CTAB GEL-0.41-CTAB GEL-0.55-CTAB GEL-0.69-CTAB GEL-0.82-CTAB Average

1.41 1.97 1.92 1.82 1.62 1.60 1.49 1.70  0.23

1.21 1.39 1.27 1.15 1.18 1.32 1.31 1.28  0.12

1.17 1.42 1.55 1.58 1.37 1.21 1.14 1.33  0.18

2.64 2.59 2.59 2.58 2.57 2.26 2.31 2.51  0.17

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K. von Nessen et al. / Polymer 54 (2013) 5499e5510

Fig. 10. Results of angular dependent PCS measurements at 20
C (left) for the swollen state and at 60
C (right) for the collapsed state for selected poly-(NIPMAM) microgels prepared with 0.00 mM CTAB, 0.15 mM CTAB, 0.27 mM CTAB, 0.41 mM CTAB, 0.55 mM CTAB at a scattering angle range between 30
and 100
. The solid lines are linear fits. The error bars are in the range of the symbol size.

The dependence of the hydrodynamic diameter in the swollen and collapsed state as well as the diameter obtained from SEM and AFM on the amount of CTAB used in the synthesis is represented in Fig. 12. The particle diameter from the CTAB series depend on the CTAB concentration in an exponential fashion. The fits to the data are fits according to eq. (5). For CTAB the following connection between the hydrodynamic diameter in the fully collapsed state and the CTAB concentration c(CTAB) can be derived:

logðDh ðcollapsedÞ=nmÞ ¼ 2:41  0:62  cðCTABÞ=ðmmol=LÞ

(8)

This can now be used for synthesizing poly-(NIPMAM) microgels of tailored dimensions and it is useful to reform eq. (8) so that the CTAB concentration can be calculated for any desired particle diameter:

logðDh ðcollapsedÞ=nmÞ  2:41 cðCTABÞ=ðmmol=LÞ ¼  0:62

(9)

In the following section the results for SDS and CTAB will be directly compared.

employed in the precipitation polymerization. Fig. 13 compares the hydrodynamic diameter in the swollen state (20
C) and the collapsed state (60
C) as a function of surfactant concentration normalized with the cmc of the respective surfactant at the synthesis conditions (70
C). Since the polymerizations were conducted at 70
C, the cmc values at this temperature are relevant. We used a value of cmc (70
C) ¼ 10.8 mM for SDS [48] and cmc (65
C) ¼ 1.55 mM for CTAB [49]. As Fig. 13 nicely demonstrates the data points for the two temperatures nicely fall together on master curves when the surfactant concentrations are divided by the cmc values. For the collapsed state, as an example, we can now derive the following linear dependence:

logðDh ðcollapsedÞÞ  2:48 cðsurfactantÞ=cmc ¼  1:25

(10)

This means it is now possible to predict the influence of any ionic surfactant on the particle dimensions. However, whether this hypothesis really holds will be investigated in the future, since the use of surfactants other than SDS and CTAB was beyond the scope of the present work.

3.3. Comparison of the poly-(NIPMAM) microgels prepared with SDS and CTAB

4. Conclusions

For both surfactants, SDS and CTAB, we found an exponential dependence between particle size and concentration of surfactant

Thermoresponsive microgels composed of chemically crosslinked poly-(NIPMAM) were prepared by surfactant-assisted

Fig. 11. Hydrodynamic diameter Dh and inverse swelling ratio a1 as a function of temperature for selected poly-(NIPMAM) microgels prepared with 0.00 mM CTAB, 0.15 mM CTAB, 0.27 mM CTAB, 0.41 mM CTAB, 0.55 mM CTAB. The temperature dependent PCS measurements were performed in a temperature range between 30
C and 60
C in steps of 2
C using a Zetasizer Nano ZS.

K. von Nessen et al. / Polymer 54 (2013) 5499e5510

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Fig. 12. Hydrodynamic diameter Dh (left) and microgel surface area A0 (right) obtained from PCS, AFM and SEM measurements as a function of surfactant concentration in semilogarithmic representations. The solid lines are fits according to eq. (5).

precipitation polymerization allowing precise control of the particle size through the concentration of the surfactant. Using CTAB as surfactant and a cationic radical initiator, positively charged microgels were achieved, whereas SDS in combination with an anionic initiator led to negatively charged particles. Analysis using SEM and AFM has shown that spherically shaped polymer colloids of low polydispersity (<10%) were obtained for all batches. Particle dimensions determined from these imaging techniques show that the particle diameter decreases systematically with increasing surfactant concentration for both surfactants used. This trend is also confirmed by PCS measurements, which were used to analyze the hydrodynamic particle diameter of the ensemble and to study the volume phase transition behavior. PCS measurements at 60
C, where the microgels are in the fully swollen state, show that the particle diameter can be varied from 354 nm for synthesis without SDS down to 49 nm for 6.93 mM SDS used within the precipitation polymerization (KPS as initiator). For CTAB as surfactant, the values are 383 nm for synthesis without CTAB and 84 nm for 0.82 mM CTAB used within the polymerization (V50 as initiator). Due to the lower cmc of CTAB, CTAB influences the particle size already at much lower concentrations compared to SDS. When the surfactant concentrations are normalized with the cmc values of the surfactant we obtain a very similar influence of the surfactant concentration on the particle size. The results from temperature dependent PCS measurements reveal the typical volume phase

transition behavior of poly-(NIPMAM) for all microgels prepared in this study. For both surfactants, a slight increase of the volume phase transition temperature as well as a decrease of the swelling capacity with increasing surfactant concentration is observed. This is most likely due to a more homogeneous network structure and cross-link efficiency for microgels prepared with surfactant. The results of this work show that a very precise size control of poly-(NIPMAM) microgels is achieved by the addition of defined amounts of surfactant to the reaction medium. In addition, via the choice of the surfactant and the charge of the ionic radical initiator it is possible to influence the sign of the particle surface charge. This is of relevance for 2D assembly processes on charged substrates such as silicon wafer or glass. Due to the low polydispersity of the microgels nicely ordered structures can already be achieved from simple drop-casting of dilute microgel dispersions on solid substrates. Acknowledgment We would like to thank Dr. Beate Förster and Martina Heider (BIMF, University of Bayreuth) for assistance with the SEM investigations. MK acknowledges financial support from the Verband der Chemischen Industrie through the Fonds der Chemischen Industrie. This project was supported by the DFG through SFB 840 (TP A4). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2013.08.027. References

Fig. 13. Comparison of the hydrodynamic diameter Dh obtained from PCS measurements at 20
C and at 60
C as a function of surfactant concentration divided by the cmc of the respective surfactant at reaction temperature conditions (70
C) for poly(NIPMAM) microgels prepared with SDS and CTAB. The solid lines are linear fits. The cmc values were adopted from Refs. [48,49]. (a)A cmc value for CTAB determined at 65
C was used [49].

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