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Loading of PNIPAM Based Microgels with CoFeO Nanoparticles and their Magnetic Response in Bulk and at Surfaces Sebastian Backes, Marcus U. Witt, Eric Roeben, Lucas Kuhrts, Sarah Aleed, Annette M. Schmidt, and Regine von Klitzing J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b03778 • Publication Date (Web): 11 Aug 2015 Downloaded from http://pubs.acs.org on August 18, 2015

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The Journal of Physical Chemistry

Loading of PNIPAM Based Microgels with CoFe2O4 Nanoparticles and their Magnetic Response in Bulk and at Surfaces Sebastian Backes,

Aleed,

y

y

Marcus U. Witt,

y

Annette M. Schmidt,

Eric Roeben,

z

z

Lukas Kuhrts,

and Regine von Klitzing

y

Sarah

,y

Technische Universitat Berlin, Stranski-Laboratorium fur Physikalische und Theoretische Chemie, Strae des 17. Juni 124, 10623 Berlin,Germany, and Department fur Chemie, Institut fur Physikalische Chemie, Universitat zu Koln, Luxemburger Str. 116, 50939 Koln, Germany E-mail: [email protected]

Phone: +49 (0)30 314 23476. Fax: +49 (0)30 314 26602

To whom correspondence should be addressed y Technische Universit at Berlin, Stranski-Laboratorium f ur Physikalische und Theoretische Chemie,

Strae des 17. Juni 124, 10623 Berlin,Germany z Department f ur Chemie, Institut f ur Physikalische Chemie, Universit at zu K oln, Luxemburger Str. 116, 50939 K oln, Germany

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ACS Paragon Plus Environment


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Abstract The present paper addresses the loading of thermoresponsive poly-N-isopropylacrylamide (PNIPAM) based microgel particles with magnetic nanoparticles (MNP: CoFe

2

O @PAA 4

(PAA: polyacrylic acid)) and their response to an external magnetic eld. The MNP uptake is analyzed by transmission electron microscopy (TEM). Obviously, the charge combination of MNP and microgels plays an important role for the MNP uptake, but it does not explain the whole uptake process. The MNP uptake results in changes of size and electrophoretic mobility, which is investigated by dynamic light scattering (DLS) and Zetasizer. The microgels loaded with MNP preserve their thermosensitivity and they show magnetic separability and are considered as magnetic microgels. After adsorption at a surface the magnetic microgels are studied with a scanning force microscope and indentation experiments. The magnetic microgels show an elongation along the magnetic eld parallel to the surface while the height of the microgels (perpendicular to the surface and to the magnetic eld) is compressed. This result is in good agreement with simulations of volume change of ferrogels in a magnetic eld.

1 Introduction Polymeric ferrogels consist of magnetic nanoparticles (MNP), which are embedded in a crosslinked polymeric hydrogel. Since their shape and elastic properties can be controlled dynamically, those materials are of high interest for numerous applications. Their ability to convert magnetic energy into mechanical energy quali es them as soft actuators. Further possibilities of practical use are magnetic robots or applications in biophysics and medicine, e.g. for controlled drug release. In contrast to macroscopic gels, microgels with radii ranging from several hundred nanometers to several micrometers are especially interesting, as they show a faster response to outer stimuli. In addition the microgel particles can be easily processed for coatings with a controlled particle density. Microgels based on the polymer poly-N-isopropylacrylamide 1

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(PNIPAM) have been among the most studied in the last decades. They are swelling in water and their volume has been shown to depend on temperature, pH, and ionic strength. The mechanical properties can be controlled by the cross-linker density and the comonomer content. Typical E-moduli are between several tens of kPa and several MPa. Embedding nanoparticles can make the microgel sensitive to external elds. Proof of concept studies of hybrid Au NP/ PNIPAM microgels have shown their capability as nanosensors, their reversible switching behavior on thermal and optothermal stimuli. Magnetic microgels represent one kind of multifunctional hybrid materials. Those hybrids combine the properties of their single parts, which are in this case ferro uids and microgels. Research on magnetic microgels aims at the combination of microgel properties, which should be preserved for the hybrids, with the ferro uid's response to magnetic eld. The application of a magnetic eld leads to an alignment of the magnetic moments of the MNP in the direction of the eld. This can happen by two mechanisms. On the one hand, only the magnetic moment can rotate without movement of the particle itself. This inner magnetic mechanism is called Neel mechanism. On the other hand, the whole particle can rotate, which is called Brown mechanism and is dominant for larger particles. An important factor concerning this mechanism is the coupling between the MNP and the polymer matrix, as the MNP can be physically adsorbed within the gel network, or chemically bound to the gel. In the case of strong coupling between MNP and the polymer matrix, a hysteresis has been observed for macroscopic ferrogels in the magnetisation curve, as well as a remanent magnetisation after the eld has been turned o. This ferromagnetic behaviour is a special property of ferrogels, as ferro uids alone are superparamagnetic for suciently small particle sizes. If there is a coupling between MNP and gel, the magnetic torque which is exerted on the MNP is balanced by an elastic counter-torque caused by the deformation of the gel. This means that the magnetisation of the hybrids re ects the elastic properties of the gel. It has been shown that the Young's modulus of macroscopic ferrogels increases in the presence of an external magnetic eld. Other factors in uencing the magnetic response are the 10{14

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concentration of MNP, their distribution in the gel, and their magnetic properties. Several theoretical studies have dealt with the deformation of ferrogels in a magnetic eld. Most ferrogels elongate in eld direction, but depending on their shape and on the concentration of MNP also a contraction is possible for low concentrations and prolate or strongly oblate shapes. The coupling of MNP to the matrix also plays a role, with weak couplings leading to a stretching in eld direction and an overall anisotropic volume loss, whereas strongly coupled hybrids show an isotropic volume loss. A lot of research on magnetic microgels has been done on particles with a core-shell structure. Particles consisting of a MNP core with a polymer shell as well as polymer particles with an MNP shell have been investigated and were shown to combine properties of both MNP and polymer gel. Also clusters of magnetite nanoparticles have been coated with polymer shells of polyacrylic acid (PAA), poly-N-isopropylacrylamide (PNIPAM), and poly-3-acrylamidopropyl trimethylammonium chloride. The resulting magnetic microgels could be magnetically separated with eciencies up to 99.97 %. Another approach is the incorporation of MNP into the meshes of a polymer network. Here, magnetite nanoparticles have been deposited into/onto microgels consisting of copolymers of acetoacetoxyethyl methacrylate and N{vinylcaprolactam P(AAEM{co{VCL). A considerable temperature and pH dependence was preserved for these hybrids. So far, the magnetic microgels were mainly investigated according to their magnetic separability and their magnetisation. In case of thermoresponsive microgels (e.g. PNIPAM, P(AAEM{co{VCL)) it was tested if their thermosensitivity is still preserved after loading with MNP. So far, the microgel composition have not been tuned in order to increase the amount of MNP. Furthermore, nothing is known about the change in microgel shape in an external eld, especially under geometrical con nement, e.g. after adsorption at a surface. The present study on PNIPAM based microgels addresses these problems. The loading of a large number of CoF e O @PAA MNP into a PNIPAM microgel network is aimed. In order to increase the amount of MNP the microgel matrix is tailored 31

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with a comonomer and dierent polymerization starters. The loading of the obtained magnetic microgels is characterized by TEM. Additionally, the hydrodynamic radii (by DLS), the electrophoretic mobility (by Zetasizer), as well as the magnetic separability (by UV{ VIS absorption spectroscopy) are analyzed. Furthermore, the magnetic deformation of the magnetic microgels under geometric con nement at surfaces and the in uence of the MNP loading on elastic properties of the microgels are investigated by means of atomic force microscopy (AFM).

2 Experimental section 2.1 Materials

N-Isopropylacrylamide (NIPAM), Dibenzyl ether, poly(acrylic acid) ( M w = 1800 gmol ), Allylamin (AA), 2,2'-Azobis(2-amidinopropane) dihydrochloride (AAPH) were purchased from Sigma-Aldrich. N,N'-Methylenebisacrylamide (BIS), ammonium persulfate (APS) were purchased from Fluka. Fe(acac)3, Co(acac)2, oleyl alcohol were purchased from ABCR. Oleyl amine was purchased from Acros Organics. Oleic acid and sodium hydroxide (0.1 M) were purchased from Fisher Scienti c. For the water puri cation a three stage Millipore Milli-Q Plus 185 puri cation system was used. 1

2.2 Sample Preparation

For the preparation of magnetic microgels (MMG) the pure microgels (MG) and magnetic nanoparticles (MNP) were synthesized separately. Firstly, the MNP have to be soluble in water and secondly, the microgel particles should be adjusted to optimize the loading with MNP.

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2.2.1

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Synthesis of magnetic nanoparticles

Magnetic nanoparticles made of CoF e O @P AA were used. The magnetic nanoparticles were synthesized by thermal decomposition of F e(acac) and Co(acac) according to. For the stabilization of the MNP in water they were covered with a shell of polyacrylic acid (PAA). The particles were transferred in aqueous dispersion via THF, using PAA and NaOH following the protocol of Nelson et al. TEM images show that the diameter of the magnetic core is about 15 nm. The MNP were dispersed in MilliQ water at a pH of 10 to stabilize the MNP. The zeta potential is about -70 mV at pH 8. 2

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2.2.2

Microgel synthesis

To ensure a high loading with negatively charged magnetic nanoparticles, positively charged microgel particles were synthesized. Therefore, the cationic comonomer allylamin (AA) was added during synthesis (MG1 { MG3). Two dierent initiators were used: negatively charged APS or positively charged AAPH. The reference system (MG0) is a pure PNIPAM microgel without comonomer and has an electrophoretic mobility (EM) of -0.45 µmcm/Vs due to the negative starter APS. The microgels were synthezied by a surfactant-free precipitation polymerization. The synthesis was done according to Pelton and Chibante. Monomeric NIPAM, cationic comonomer AA and cross-linker BIS were dissolved in 100 ml MilliQ water. The solution was poured in a glass reactor, heated up to 80 C, and degassed with N for one hour. Then, 1 ml of the initiator (either APS or AAPH) dissolved in Milli{Q water was added. The solution was continuously stirred for 90 min. Afterwards, the mixture was cooled down to room temperature. The microgels were puri ed by dialysis over 10 days against Milli{Q water. Finally, they were freeze-dried for 4 days at 85 C. The chemical composition according to the composition of the preparation mixture of the synthesized microgels is shown in Tab. 1. 44

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Table 1: The concentration in mol

%

is calculated for the total amount of used

polymerization reactant.

Gel c(BIS)[mol%] AA[mol%] Initiator n(Initiator)[µmol] M G0 5 0 APS 100 M G1 5 5 APS 100 M G2 5 5 APS 400 M G3 7 :5 2.5 AAPH 150 2.2.3

Preparation of magnetic microgels

From the microgels three dierent ferrogels, i.e. magnetic microgels were prepared ( M M G1, M M G2, M M G3). From the freeze-dried microgels, stock dispersions were prepared in MilliQ water. The dispersions were shaken for 1 day to let the gels reswell. The MNP were placed in an ultrasonic bath for 10 min before using them for the ferrogel preparation (ph 10). All magnetic microgels were prepared in dispersions consisting of 0 :025 wt% microgel and 0:075 wt% MNP. In order to ensure the colloidal stability of the MNP ( CoF e O @PAA) the pH of the microgel dispersion was adjusted to pH 10 by adding NaOH. Finally, MNP supension and microgel dispersion (both at pH 10) were mixed. The mixture was vortexed at 500 W for 30 min to let the MNP incubate into the gels. To remove free MNP the magnetic microgels were centrifuged at 5000 rpm (2400 g) for 30 min. The supernatant was collected and the precipitant was redispersed in Milli{Q water and vortexed again at 500 W for 30 min. The cleaning process was performed three times until the supernatant was clear. These samples were used for the characterization. 2

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2.3 Methods 2.3.1

Dynamic light scattering

The hydrodynamic radius was measured with dynamic light scattering (DLS) with a LS Instruments - LS Spectrometer. The light source was a HeNe laser at = 632:8 nm with 21 mW. For the correlation function the LS Spectrometer was used. The scattering inten7



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sity was measured over time for 40 s at dierent angles, ranging from 30 to 130, with a step size of 5. The data were tted by a self written script using the cumulant t procedure.

2.3.2

Electrophoretic Mobility

For the electrophoretic mobility (EM) the Nano-ZS ( = 433 nm, 4 mW) from Malvern is used. For DLS and EM measurements the samples were highly diluted in MilliQ water (c 0:01wt%). 2.3.3

UV-Vis spectroscopy

UV-Vis spectra were collected using a Cary 50 spectrophotometer at a temperature of 20C. The absorbance (A = log II ) is measured over time. The sample was mounted on a neodymium magnet (N d F e B ) to provide the external magnetic eld. 0

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2.3.4

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Transmission electron microscopy

To determine the embedded amount of MNP in the gel, transmission electron microscopy (TEM) was performed on a Cryogen TEM JEOL JEM2100. For the sample preparation TEM copper grids with a carbon lm were used (200 mesh, Science Service, Munich, Germany). The grids were cleaned by a glow discharge for 15 s. 5 µl of the samples were placed on the grids. After one minute the excess liquid was blotted with lter paper. The grids were dried at room temperature and placed inside the TEM with the sample holder (EM21010, JEOL). The TEM was operated at an acceleration voltage of 200 kV and the images were recorded by CMOS camera system (TEMCam-F416, TVIPS). The images were analyzed with ImageJ 1.48v. For the mean loading of the magnetic microgel, the number of MNP in the microgel was averaged for a minimum of ten microgels. 8


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2.3.5

Scanning force microscopy

The magnetic microgels were deposited from aqueous solution on a gold coated silicon wafer by spin coating. All measurements were carried out using an MFP-3D AFM (Asylum Research, Oxford Instruments) with the Variable Field Module 2 (VFM2) extension, which allows the application of a homogenous magnetic eld in lateral direction to the sample. The magnetic eld of the VFM2 is produced by a permanent magnet which is placed between two soft iron pole shoes, on top of which the sample is placed. The ux density can be adjusted by turning the magnet between the pole shoes. Scanning was done with HQ:NSC18/CR-AU BS tips (Mikromash) in intermittent contact mode. Those tips have a typical radius of 8 nm and a chromium-gold coating on their backside. The measurements took place in a drop of water which was put on top of the sample. During one measurement cycle the magnetic ux density was changed from 0 T to a maximum eld of 0 :375 T (which was the maximum value that could be achieved by the VFM2 given the thickness of the sample) and back again to 0 T ( 0:001 T). This procedure was repeated twice, so the magnetic eld was ramped up and down again for a total of three times. The system was left for at least half an hour to equilibrate after the change of the magnetic eld before the measurement took place. Ten individually adsorbed magnetic microgel particles were scanned. Particle analysis was done using the Asylum Research Software. 2.3.6

Elasticity measurements

The Young's modulus E was determined with the same setup as the SFM measurements (MFP-3D AFM with VFM2 extension), but with HQ:CSC38/NO AL tips (cantilever C), which are uncoated and have a full tip cone angle of 40 and a reference spring constant of 0:05 N=m. Before the measurement the cantilever was calibrated on a hard surface to calibrate the cantilever de ection (inverse optical lever sensitivity, InvOLS ) and to determine the exact spring constant. The measurements were done in a water droplet similar to the scanning experiments. Only the approach curves were evaluated. As there was a strong 9



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adhesion after the contact, the retract curves could not be used. Force maps were recorded in the area around a particle, and the force curves which were recorded on the particle were evaluated with the Asylum Research software using the Hertz model to obtain the Young's modulus. For the analysis only the part of the force curve up to 10% indentation was used. Three dierent microgels were recorded for every measurement point and the average was taken.

3 Results 3.1 Pre{experiment: Eect of sign of microgel charge on MNP loading

In order to check the eect of sign of microgel charge on MNP loading the negatively charged reference system MG0 without any cationic comonomer AA and the same microgel but with 5% AA were compared. MG0 has an EM of -0.45 µmcm/Vs and MG1 0.91 µmcm/Vs. TEM images of the samples, as shown in Figure 1 show that the uptake was much more successful for the cationic P(NIPAm-co-allylamine) microgel MG1. The PNIPAM sample shows MNP for the most part only on the outside of the microgel.

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Figure 1: Upper row: PNIPAM microgels (sample MG0) with MNP before and during application of a magnetic eld and the respective TEM image; Bottom row: P(NIPAM-coallylamine) with MNP before and during application of a magnetic eld and the respective TEM image. The separation properties of the magnetic-microgels were tested with a permanent magnet. Figure 1 shows the two magnetic-microgel hybrids before and during application of the gradient magnetic eld. The PNIPAM microgel MG0 shows that some degree of separation is achieved; however, the bulk solution still contains a high degree of MNP present. On the other hand, the P(NIPAm-co-allylamine) hybrids show a full separation of solution and hybrid nanoparticles after a couple of minutes in the presence of the magnetic eld. This also suggests that more MNP are located in or at the surface of the microgels, comparatively, allowing a stronger magnetic response. Due to the lack of embedded MNP the reference system (MG0) has not been considered for further investigations. 3.2 Hydrodynamic radius

The hydrodynamic radius was measured with DLS for the microgel (MG) and the magnetic microgel (MMG) at 20 C. The hydrodynamic radii for the samples are shown in Tab. 2. 11



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Table 2:

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Increasing hydrodynamic radii of the gel particles after loading with

MNP.

Microgel rH (25C)[nm] Ferrogel(magnetic microgel) rH (25C)[nm] M G0 249 10 { { M G1 204 9 M M G1 523 40 M G2 240 6 M M G2 527 15 M G3 348 16 M M G3 508 32 Obviously, loading of the microgel particles with MNP leads to an increase in the hydrodynamic radius. 3.3 Electrophoretic Mobility

As a second characteristic measure of the microgels the electrophoretic mobility was measured. The EM changes with swelling and shrinking of the gels. Therefore the EM of the microgel was measured as a function of temperature. The temperature dependent EM before and after loading with MNP is shown Fig. 2(top) and Fig. 2(bottom), respectively. The pure gels (M G1, M G2) show an increasing EM below and a decreasing EM above the VPTT. M G3 shows an overall increase of the positive EM. At room temperature the loading of the positively charged microgel with negatively charged MNP sets the EM to more negative values for the samples. The microgels M G1(M M G1), M G2(M M G2), M G3(M M G3) show a charge reversal from positive to negative EM. For all magnetic microgels the volume phase transition at about 32C is still detectable. For the negatively charged magnetic microgels (MMG1, MMG2, MMG3) the EM becomes more negative above the VPTT. The change of the EM at 20C is shown in Tab. 3. µ 20 MMG Table 3: Electrophoretic mobility

at

C and loading with MNP for the

(average number of NMP per microgel particle).

Gel µ[µmcm/Vs] Ferrogel µ[µmcm/Vs] µ[µmcm/Vs] M G0 0:45 { { { M G1 +1:17 M M G1 1:06 2:23 M G2 +0:91 M M G2 1:20 2:11 M G3 +2:57 M M G3 1:02 3:59 12


loading { 133 16 275 36 243 29

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3.4 Loading of microgels with MNP (TEM)

The amount of adsorbed MNP and the distribution in the gels was investigated by TEM. Fig. 3 shows an exemplary TEM image of M M G2. Small MNP in the range of 15 nm and larger microgel particles with a broader density pro le can be seen. Obviously, the microgel particles consist of a dense core and a rather uy shell, where most of the MNP adsorb. The second information that can be obtained from the TEM image is the spacing between the MNP. It shows that the MNP are well separated and homogeneously distributed over the gel pro le. A tendency for a hexagonal packing of the magnetic microgel is observed. Almost no single MNP exist between the magnetic microgels. From the TEM images the loading with MNP can be determined. The number of MNP per microgel particle is presented in Tab. 3. Obviously, no sytematic relation between EM and loading exists. 3.5 Magnetic separability (Absorption measurement)

A rst experiment to investigate the magnetic properties of the magnetic microgel is to pull them through the solution in an external magnetic eld. Therefore, absorption measurements with a UV-Vis spectrometer were performed. The samples were measured under an external magnetic eld. The decreasing absorbance over time in the external magnetic eld is shown in Fig. 4. 3.6 Adsorbed microgels: Microgels under geometrical con nement

Since MG2 (MMG2) shows the highest loading with MNP this microgel was used for further investigation at the surface. A comparison of individually adsorbed magnetic microgels (M M G2) and pure microgels (M G2) without MNP is shown in Fig. 5. The images were taken by SFM in air. For the magnetic microgels, the MNP in the less dense shell of the 13



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microgel are clearly visible around the gel particles. In contrast, the pure gels have a smooth spherical shape. The deformation of the magnetic microgel particles M M G2, swollen in water, under the in uence of a lateral magnetic eld, was observed using scanning force microscopy. The magnetic eld was increased step-like from 0 T to 0 :375 T three times and an image of ten particles was recorded after every change of the eld. Fig. 6 shows the relative change in volume, height and width (width: parallel to the eld direction and parallel to the slow scan direction). Each value with the magnetic eld is normalized with respect to the cross section which was taken in the absence of the magnetic eld. Both volume and height of the magnetic microgel decrease by up to 6% in the magnetic eld, whereas the width increases by up to 7%. This means that the microgels are elongated in the direction of the magnetic eld, which leads to a attening and overall slightly smaller volume of the gels . The elastic properties of the swollen magnetic microgels and their dependence on an external magnetic eld were measured by force mapping of the area around a single particle. As has been shown before, the highest Young's modulus is found at the center of a microgel, whereas the modulus decreases as one gets closer to the edge of the gel. This is attributed to the fact that less dense materials are generally softer, and as the cross-linker density is lower in the outer regions of the gel, the Young's modulus decreases towards the outer periphery (core{shell structure). This is shown exemplary in Fig. 7, which depicts the cross section and elastic properties of an exemplary magnetic microgel (without external eld). The obtained values for the Young's moduli are summarized in Tab. 4. The values are the averaged Young's moduli in the center of three dierent particles. They were measured for a pure microgel (M G2), as well as magnetic microgels ( M M G2) with the external magnetic eld being ramped up to 0:375 T and down again twice. Two results can be obtained here. First, the pure microgel has a higher Young's modulus than the magnetic microgel. Second, there is no measurable in uence of an external magnetic eld on the Young's modulus of 21,22

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the magnetic microgels. M G3 and M M G3 show a similar trend, with values of 1453 kPa for M G3, and only 623 kPa (at 0 T) and 767 kPa (at 0 :375 T) for M M G3. Table 4: Averaged Young's moduli in the center of magnetic microgel particles

2 M M G2 Pure gel 0T 0.375 T 0T 0.375 T 0T 531 55 kPa 448 64 kPa 421 72 kPa 426 21 kPa 426 26 kPa 442 13 kPa

in

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Figure 2: The electrophoretic mobility of microgels (MG, top) and magnetic microgels (MMG, bottom) in dependence of temperature. 16


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Figure 3: Transmission electron microscopy image of the M M G2. The mean value for the embedded MNP is n = 275 36.

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Figure 4: The absorbance in dependence of time during exposure to an external magnetic eld. The absorbance is normalized to A(t = 0). Time "0" marks the time where the external magnetic eld was applied. The magnetic microgel is pulled out of the beam path by the external magnetic eld.

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Figure 5: Comparison of adsorbed unloaded microgel M G2 (top) and microgel particles loaded with magnetic nanoparticles M M G2 (bottom) recorded by SFM in intermittent contact mode against air. 19



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Figure 6: Relative change in volume, height and width parallel to the eld direction of adsorbed magnetic microgels M M G2 after applying a magnetic eld of 0 :375 T. The magnetic eld was ramped up from 0 T to 0 :375 T three times (three cycles).

Figure 7: Cross section and Young's modulus of an exemplary magnetic microgel in the absence of an external eld. 20


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4 Discussion The microgel with the highest positive EM (MG3) could be obtained using a positively charged comonomer (AA) and a positively charged initiator (AAPH). Although the amount of comonomer was lower for MG3 (2.5 mol%) than for MG1 and MG2 (5 mol%) the EM was twice as high (2.57 µmcm/Vs vs. 1.17/0.91 µmcm/Vs). The reason might be the reduced tendency for aggregation between like charged comonomer and initiator. In case of oppositely charged comonomer (AA) and initiator (APS) aggregation was considered as a reason for uncontrolled polymerisation, especially for higher amounts of comonomer. The temperature dependent EM indicates a VPTT at about 32 C. The sign of the microgel charge seems to have an important impact on the loading with MNP. Microgels cannot be loaded with like charged MNP, while oppositely charged MNP lead to a pronounced loading. Comparing the dierent positively charged microgels (MG1(MMG1), MG2(MMG2), MG3(MMG3)) it is obvious that electrostatic interaction is not the only important factor. Since there is no simple relation between EM and loading capacity it is assumed that also the internal structure of the microgels plays an important role for the MNP uptake. A decrease in starter concentration leads to a smaller amount of charged starter in the core. This may explain the dierence in MNP uptake for M G1(M M G1) and M G2(M M G2). With M G3(M M G3) the EM and the structure of the gel are changed to investigate the structual eect. The higher value of the EM for M G3 than for M G1 and M G2 by increasing size is an eect of a more charged particle. With increasing crosslinker concentration the internal structure changes to smaller meshes and therefore to a steric hinderance of the MNP. Loading with MNP has several eects. First, the magnetic microgels show a higher hydrodynamic radius for all three microgels. This indicates that the repulsive force between the charged MNP is stretching the microgel matrix. With a magnetic moment of 3.77*10 Am one can calculate , the ratio between the magnetic dipole-dipole interaction and the thermal 19

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energy at room temperature:

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= 4d mk T

(1)

2

0

3

B

with the vacuum permeability , the magnetic moment m, the particle diameter d, the Boltzman constant kB , and the temperature T . The obtained value of = 0:58 shows that the thermal energy dominates, which supports the observation that the particles are stable at 20 C and do not form aggregates due to magnetic interaction. From the Grahame equation one can calculate the charge density 0

45

= 2 er kB T sinh 2ke T 0

B

(2)

with the vacuum permitivity , the relative permitivity r , the Debye length , the zeta potential , and the elementary charge e. An average charge of Z=14 e per MNP is obtained. The electrostatic repulsion is 6.4 times larger than the magnetic dipole dipole atraction. Second, due to the negative charge of the MNP (-70 mV at pH 10) the positive EM of the microgel decreases and even leads to a charge reversal for M G1(M M G1), M G2(M M G2), and M G3(M M G3). By increasing the temperature beyond the VPTT, the EM becomes more negative for all magnetic microgels. The MMG show the typical behavior for microgels, i.e. the value of the EM increases above the VPTT since the charges are accumulated at the outer shell of the microgels. This non monotonic behaviour is tipical for oppositely charged starter and comonomer. After loading with MNP this eect is not essential anymore and the behaviour becomes monotone. The ability to pull the magnetic microgel out of the dispersion is a clear hint for a net magnetic moment which can be induced by the external magnetic eld. During synthesis the MNP are not oriented and the directions of the dipole moments are isotropically distributed. That means that the magnetic microgel has no net magnetic moment in absence of an external magnetic eld. The fact that magnetic microgel particles can be moved by an external magnetic eld is a clear hint that the MNP can be oriented within the microgel 0

22


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along the axis of the magnetic eld. Either the MNP are not xed and can rotate freely within the microgel mesh size or they transfer the torque to the microgel structure. The measurement has shown that increasing the concentration of MNP inside the microgel results in a faster movement under an external magnetic eld. The TEM images show that the MNP are isotropically distributed over the gel pro le. For a homogeneous MNP distribution within the microgel particles one would expect a higher concentration of MNP in the center of the microgel particles. Since the TEM image is a 2D projection the observed isotropic distribution of MNP over one microgel particle leads to the assumption that the MNP are rather attached to the outer shell of the microgels. The increasing contrast of the microgel particles towards their center indicates a higher polymer density due to increasing concentration of crosslinker towards the center of the microgel particle. This is already known from literature and is explained by faster reaction kinetics of the cross-linker in comparison to the monomer. The density gradient is also supported by mechanical studies, which show a higher Young's modulus for the center of the microgels than for the outer shell (Ref. and present paper). With increasing crosslinker concentration the mesh size decreases and the MNP are not allowed to penetrate due to sterical hinderance. This also explains why the loading decreases for the M M G3 (highest crosslinker concentration),has the highest positive charge. although MG3 The adsorbed magnetic microgels ( M M G2) elongate along the direction of the magnetic eld, and contract perpendicular to the surface and to the eld. This behavior corresponds to 2D simulations by Weeber et al . The MNP are not chemically bound to the microgel and thus can rotate without strong interaction with the gel. In this case, the application of a magnetic eld might lead to the formation of chain-like arrangements of MNP parallel to the eld. This could explain the observed deformation. Obviously, the arrangement of MNP is not signi cant enough to in uence the Young's moduli of the gels, but only the presence of the MNP already decreases the Young's moduli. This eect might be related to the increase in the microgel size after loading with MNP. This increase is probably caused by repulsion 46

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between the MNP, which makes the meshes inside the microgel grow. Hence, more water can get inside the gel and make it swell more. The fact that there is a charge reversal due to the loading with MNP is a hint that not all charges are compensated and that additional counterions are introduced into the system, that increases the osmotic pressure and thus might be another explanation for the swelling. This swelling could explain why the hybrid magnetic microgels are softer than pure microgels.

5 Conclusion Magnetic microgels made of PNIPAM based microgels and CoF e O @P AA MNP have been successfully prepared and characterized. In case of oppositely charged microgels and MNP a pronounced MNP uptake by the microgels could be detected. Furthermore, no simple relation between the value of the EM of the microgels and the MNP uptake could be found. This implies other factors like the internal structure being important for the loading capacity. The hydrodynamic radius increases after the MNP incubation due to the electrostatic repulsion of the MNP and an increase in osmotic pressure. This is supported by the measurements of the elasticity and the decrease of the Young modulus for the magnetic microgel compared to the pure gel. The magnetic microgels show a magnetic separability but their thermosensitive response is still preserved after loading with MNP. Additionally, a response of the ferrogel to an external magnetic eld has been detected. Magnetic microgels elongate in eld direction and their volume shrinks anisotropically. This eect is however not strong enough to induce a pronounced change in their elastic behavior. The type of magnetic microgel particles presented here are promising candidates for actuating systems. Future work will address the adjustment of the microgels and MNP in order to get stronger magnetic response. 2

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4

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Acknowledgement

The authors thank the DFG for funding via the priority program "Feldgesteuerte PartikelMatrix-Wechselwirkungen: Erzeugung, skalenbergreifende Modellierung und Anwendung magnetischer Hybridmaterialien" (SPP 1681, KL 1165/18). References

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Nanoparticles and Their Magnetic Re - American Chemical Society

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