ARTICLE pubs.acs.org/Langmuir
UCST-Like Hybrid PAAm-AA/Fe3O4 Microgels. Effect of Fe3O4 Nanoparticles on Morphology, Thermosensitivity and Elasticity Coro Echeverria and Carmen Mijangos* Instituto de Ciencia y Tecnología de Polímeros, CSIC, Juan de la Cierva 3, 28006, Madrid, Spain
bS Supporting Information ABSTRACT: The incorporation of metal oxide nanoparticles into microgels forming hybrid systems gives additional functionalities to the system and widens the field of potential application in biomedicine, biotechnology, and other fields. In particular, there have been very few investigations regarding UCST-like hybrid microgels. In connection with this, we report the preparation of UCST-like hybrid microgels of magnetite nanoparticles (Fe3O4) encapsulated in poly(acrylamide-acrylic acid) microgel matrix via an inverse emulsion polymerization method. The key factor in the preparation of hybrid microgels is the need to divide in two the aqueous phase of the emulsion and feed them separately in order to avoid the aggregation of magnetic nanoparticles prior to polymerization reaction. The morphology, size, and spherical shape of hybrid microgels are determined by scanning electron microscopy. The encapsulation of magnetite nanoparticles within the polymer matrix is confirmed by transmission electron microscopy. Dynamic light scattering is employed to study both the swelling UCST-like behavior and the surface charge of the hybrid microgels. Swelling measurements confirm that the incorporation of magnetite does not affect the thermosensitivity of the system. In order to highlight the rheological behavior that can affect the final potential applications of these hybrid systems, a deep study of the viscoelastic properties is carried out by means of an oscillatory rheometer. The dependence of G0 and G00 of the microgel dispersions with the frequency suggests a gel-like behavior and hence the occurrence of structural organization. In order to understand this structure formation and the influence of the magnetite in the interaction between hybrid microgels, scaling theory was applied. In terms of rheology, the addition of magnetite leads to a change in the interaction between hybrid microgels giving rise to an increase in the elasticity of the system.
’ INTRODUCTION Microgels are intramolecularly cross-linked polymer particles of colloidal size that swell and deswell in a good solvent in response to external stimuli.15 Depending on the composition, they can be sensitive to external conditions such as temperature, pH, magnetic field, light, osmotic strength, and solvent composition,68 with temperature being considerably investigated because it represents an effective stimulus in many applications. It is known that the degree of swelling of gel is controlled by the free energy changes associated to both (i) the mixing of the polymer and water and (ii) the network elasticity as described by Flory-Rhener theory.9,10 Related to that, there exist two general types of responsive microgels: (i) LCST-type (lower critical solution temperature) microgels such as the widely studied poly(N-isopropylacrylamide)1114 and (ii) UCST-type (upper critical solution temperature) microgel such as poly(acrylamideacrylic acid) first reported by Bouillot and Vincent.15 Systems that show LCST-type volume phase transition temperature present a negative change of enthalpy and entropy. As the temperature increases, the entropic contribution to the energy grows leading to the phase separation and, thus, the shrinkage of microgel;16 therefore, LCST-type systems collapse upon heating. On the contrary, UCST-type microgel exhibits a positive swelling r 2011 American Chemical Society
response related to the positive change of the enthalpy and entropy.5,6,15,17,18 The swelling behavior, and hence volume phase transition, is driven by hydrogen bonding that causes the microgel to shrink at temperatures below the UCST and swell at temperatures above the UCST. The interesting features that microgels exhibit can be improved if additional functions are incorporated. For instance, magnetic nanoparticles are able to be coated with biological entities and offer the possibility of manipulation by an external magnetic field gradient.1929 The incorporation of magnetic nanoparticles to polymeric microgel systems gives rise to hybrid systems that combine the features of a polymeric microgel system with the interesting properties presented in magnetic nanoparticles and make them appropriate for applications such as magnetic separation, drug delivery, hyperthermia treatments, and magnetic resonance imaging contrast enhancement.28,3032 Recently, a lot of attention has been paid to developing systems consisting of a thermoresponsive polymer microgel and iron oxide nanoparticles.29,3341 Most of the recent papers Received: February 14, 2011 Revised: May 16, 2011 Published: June 01, 2011 8027
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Langmuir deal with LCST-type microgels such as PNIPAM, prepared by precipitation polymerization, in which, for the encapsulation of nanoparticles, an intermediate step of surface-functionalization of the magnetic nanoparticles is needed to enhance the biocompatibility with the polymer. However, to our knowledge there is no report concerning UCST-type magnetic hybrid microgels, although the inverse emulsion polymerization technique, used for the preparation of UCST microgels, offers the advantage of avoiding this intermediate step, as reported in the case of the encapsulation of gold nanoparticles.6 In our previous work,5 we developed UCST-type microgels and studied the effect of composition on the morphology, swelling, and rheological properties, as well as the interaction among microgels by means of the scaling theory. This new study represents a step forward, in which the aim is twofold; first, the development of a general and easy method for the preparation of UCST-type hybrid microgels composed of magnetite nanoparticles encapsulated inside microgel matrix and the study of the influence of magnetite on the microgel morphology, structure, and swelling properties. Second, focusing in the rheology, the objective is to determine if the incorporation of magnetite could affect both the viscoelastic properties and the interaction between microgels and hence the rheological properties of microgels dispersion. In connection with this, it is essential to highlight the importance that the rheological characterization has on microgels even if few studies are found regarding such an aspect.4245 In fact, the knowledge of the viscoelastic properties allows anticipation of the behavior of the dispersion under certain conditions of shear directly related to the application mode. This important information would lead us to obtain, by playing with the system composition, the most suitable system able to flow enough and able to overcome the extreme shear conditions that the system could be suffering during the performance. In order to achieve the above objectives, in this work we have prepared magnetic hybrid poly(acrylamide-acrylic acid) thermoresponsive random copolymer microgels and characterized the morphology, structure, and surface charge. The effect of magnetite in the swelling properties as well as in the viscoelastic properties was studied. Finally, in order to understand the effect of magnetite in the interaction and elastic properties of the microgel dispersion, Wu and Morbidelli’s46 scaling theory was applied.
’ EXPERIMENTAL SECTION Materials. Acrylamide (AAm, Aldrich) and acrylic acid (AA, Aldrich) were used as monomer and comonomer, respectively. N,N0 -Methylenebisacrylamide (MBA, Aldrich) and 2,20 -azobis(2-methylpropionamidine)dihydrochloride (AMPA-d, Aldrich) were used as cross-linking agent and initiator of the reaction, respectively. Span 80 (Sorbitan monooleate) (Fluka) and Dodecane (Fluka) were employed as surfactant and organic solvent, respectively. Magnetite (Fe3O4) nanoparticles (MNP) of approximately 10 ( 2 nm size were purchased from NANOGAP Company. The water used in the preparation and characterization of microgel was Millipore Q grade. Method of Preparation. Microgel particles were synthesized by the inverse emulsion (w/o) polymerization method, in which the aqueous phase was dispersed into the oil phase forming water droplets in the organic matrix. The oil phase consisted of the surfactant Span 80 and the organic solvent dodecane. The aqueous phase was composed of AAm and AA monomers, MBA as cross-linker, and water-stabilized magnetite nanoparticles and deionized water. Prior to the polymerization, both phases were purged with N2 for half an hour.
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The procedure of the reaction was as follows: first, the organic phase was charged into the three-necked round-bottom flask and then the addition of the aqueous phase was done. This aqueous phase was divided in two parts, part I containing AAm, AA, and MBA and half of the water content and part II, magnetite nanoparticles with the other half water content. This division was done in order to avoid the aggregation of the magnetite nanoparticles prior to the mixing with the organic phase. The key factor of this procedure was the simultaneous and controlled addition of these two aqueous phases into the organic phase at 1.5 mL/ min by means of a peristaltic pump while the system was stirred at 475 rpm. Once the inverse emulsion was formed, the flask was heated up to 50 °C. The polymerization was thermally initiated by adding 2,20 -azobis-2-methylpropionamidine-dihydrochloride (AMPA-d) initiator water solution. Immediately, after adding the initiator, the mixture became turbid and the polymerization reaction was allowed to continue under N2 atmosphere for 3 h at a constant stirring of 475 rpm After 3 h, the reaction was cooled down to room temperature while stirring and nitrogen flow was kept. Finally, the prepared systems were purified by removal of organic phase and precipitation of the aqueous phase in ethanol with subsequent washing by centrifugation. All samples were redispersed in deionized water and placed in Spectra/Por dialysis membranes supplied by Spectrum Laboratories (molecular weight cutoff = 3500) for 1 week to remove any unreacted materials. The water was replaced twice per day. Characterization of the Hybrids PAAm-AA Microgels. To visualize the morphology and particle size in the dried state of the microgel particles, environmental scanning electron microscopy XL30 ESEM (Philips) was used. Purified samples redispersed in deionized water were prepared for imaging. A drop of 104 wt % of microgel dispersion was deposited on a glass wafer, dried, and sputter-coated with gold to minimize charging at fixed conditions. Transmission electron microscopy, TEM, at 300 kV and Hitachi SU 8000 HRSEM used in the TE (electron transmission) detector bright field mode were used to observe the iron oxide nanoparticles within the polymer matrix. Samples were dispersed in acetone and then deposited on TEM grids. X-ray diffraction (XRD) pattern of the dried powder samples was performed in a Bruker Advanced D8 diffractometer by using Cu Ka radiation (l = 1.5418 A). TGA measurements were carried out with nitrogen in the temperature range 20900 °C, with a heating rate of 10 °C/min. The equipment used was a TA TGA Q 500. Dynamic light scattering (Malvern Nanosizer Nano S) was used both to determine the swelling behavior with temperature (25 to 40 °C) of the microgel dispersions at a constant pH of 5.5 and to study the surface charge of microgel dispersions as a function of pH, at a constant ionic strength of 0.01 M. The pH of the samples was adjusted by adding HCl. Rheological Behavior. Rheological studies of 1, 2, and 5 wt % concentration microgel dispersions of each sample were carried out using the AR-G2 TA Instruments stress controlled oscillatory rheometer. The geometry used was 60 mm parallel plate. Frequency sweep tests were carried out in the linear viscoelastic regime at 20 °C. Also, strain sweep tests at a constant, nondestructive 0.5 Hz frequency were done. In order to carry out fractal analysis, Wu and Morbidelli’s theory46 was employed, based in our previous work.5
’ RESULTS AND DISCUSSION The studied samples PAAm2%AA2%MBA microgels with, 0, 5, 10, and 15 wt % of magnetite are identified as MG0, MG5MNP, MG10MNP, and MG15MNP, respectively. A constant 2%AA and 2%MBA was chosen, based on our previous work,5 since this concentration gives rise to a collapse temperature close to the physiological temperature. In Table 1 are summarized the samples and conditions studied. 8028
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Table 1. Recipe Used for the Preparation of Hybrid Microgelsa aqueous phase monomer sample
a
AAm mol
organic phase
comonomer AA mol
AA wt % M
3
cross-linker MBA mol
mdod/mspan = 2.25%
MBA wt % M
D mL
Span 80 g
4
Fe3O4
wt % M
MG0
PAAm-2%AA-2%MBA
0.056
1.11 10
2
5.19 10
2
30
0.5056
MG5MNP
PAAm-co-2%AA-2%MBA-5%Fe3O4
0.056
1.11 103
2
5.19 104
2
30
0.5056
5
MG10 MNP PAAm-co-2%AA-2%MBA-10%Fe3O4
0.056
1.11 103
2
5.19 104
2
30
0.5056
10
MG15 MNP PAAm-co-2%AA-2%MBA-15%Fe3O4
0.056
1.11 103
2
5.19 104
2
30
0.5056
15
0
1 wt % of Initiator AMPA-d is added in all samples.
Table 2. Microgel Size Obtained from SEM Micrographs sample MG0
Figure 1. SEM micrographs corresponding to samples (A) MG0, (B) MG5MNP, (C) MG10MNP, and (D) MG15MNP.
The wt % of magnetite in the microgels determined through TGA experiments and the XRD analysis of the studied samples are summarized in the Supporting Information. Morphology and Structure of Hybrid Microgel. The morphology of magnetic hybrid microgels was determined by means of SEM and TEM. Figure 1 represents the SEM micrograph corresponding to the reference microgel sample, MG0, and hybrid microgel systems, MG5MNP, MG10MNP, and MG15MNP. From these images, it is observed that the reference sample and samples containing 5% and 10% MNP show spherical shape. However, for the sample containing 15% of MNP, the shape of
diameter (nm) 400430
MG5MNP
250260
MG10MNP MG15MNP
140160 90125
the microgel is not completely spherical, it appears to be slightly deformed and tends to aggregate. Furthermore, from these images it can be deduced that the increase of MNP provokes a decrease in the particle size of microgel. Table 2 summarized the microgel particle size. These results lead us to consider the possibility that MNP acts as cross-linker giving rise to the decrease of particle size. This behavior was also observed by Pich et al.4749 and Richtering et al.32 for thermoresponsive hybrid microgels systems composed of Fe3O4 and ZnO. Figure 2 shows TEM images corresponding to the hybrid microgel samples. Figure 2a,c represents an overall view of MG5MNP (PAAmAA-5%Fe3O4) and MG15MNP, (PAAm-AA-15%Fe3O4), respectively. In both cases, the images showed magnetic nanoparticles (dark spots) embedded in the microgel matrix, with no noticeable free magnetite nanoparticles. Figure 2b,d shows higher magnification of previous images for both samples in which the encapsulation of the magnetite in the microgel matrix is confirmed. For a better observation of the magnetite in the microgel matrix, in Figure 3 micrographs of SEM (with transmission detector) corresponding to the sample MG5MNP (A) and MG15MNP (B) are shown. In both cases, the magnetite nanoparticle encapsulation is clearly observed, and moreover, it also observed that the magnetic nanoparticles are not agglomerated inside the microgel. In order to obtain more information about the hybrid microgel surface charge and thus to confirm the encapsulation of the MNP, measurements of electrophoretic mobility were carried out. Figure 4 represents the electrophoretic mobility measured for the reference microgel sample, MG0, and hybrid magnetic microgel samples, MG5MNP, MG10MNP, and MG15MNP, as a function of pH, in the range pH 28. Starting with the reference sample, MG0, a weak negative charge, probably due to the presence of acrylic acid groups in the surface, is observed. Although we have not presented results concerning magnetite dispersions, it is worth mentioning that, as has been reported in the literature, for magnetite nanoparticles the electrophoretic mobility is positive in acidic solutions and becomes more negative with increasing pH; furthermore, the isoelectric point of magnetite nanoparticles usually is found in the range of pH 67.4951 8029
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Figure 2. TEM micrographs corresponding to the sample MG5MNP (a) and (b) and sample MG15MNP (c) and (d).
Figure 3. SEM micrograph (used with TE detector) corresponding to the samples (A) MG5MNP and (B) MG15MNP.
Figure 4. Electrophoretic mobility of MG0 (black), MG5NPM (red), MG10NPM (green), and MG15NPM (blue) at different pH values at 20 °C.
Regarding the hybrid magnetic microgels, MG5MNP, MG10MNP, and MG15MNP, the three samples present similar results, showing a negative charge in the entire pH range measured. Moreover, this behavior is similar compared to that of the reference sample, MG0. Considering that, as said above, in acid pH magnetite nanoparticles should present positive charge and taking into account that hybrid microgels show negative charge, this result led us to think that the magnetite nanoparticles are
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Figure 5. Evolution of the relative swelling, (d/d0)3 with temperature of MG0 (black), MG5MNP (red), MG10MNP (green), and MG15MNP (blue).
encapsulated within the matrix and not in the surface of the microgel. Swelling Behavior. Dynamic light scattering was used to examine the effect of the incorporation of magnetite on the thermosensitivity of the obtained microgel. Figure 5 showed the evolution of the relative swelling, that is, the average volume of swollen particles at a specific temperature over the average volume of the particles in the collapsed state, with temperature for samples MG0, MG5MNP, MG10MNP, and MG15MNP. The main conclusion that can be deduced from this graph is that the thermosensitivity is not eliminated with the incorporation of MNP, and therefore, all hybrid microgel samples show a UCST-like behavior. As previously reported for PAAm-AA/ AuNp microgels, this transition can be attributed to hydrogenbonding driven forces between PAAm and PAA.6,18 At temperatures below the UCST, hydrogen-bonding forces dominate and maintain the particles in a collapsed state; however, as the temperature increases, these bonds weaken and a hydrophilic front is set up within the microgel giving rise to the swelling of microgel. From the figure, it is also extracted that MNP affected the swelling behavior of the microgel, decreasing its capacity to swell to almost the half, with the incorporation of just 5% of MNP. This trend is also observed for the samples containing 10% and 15% of MNP, with the swelling of MG15MNP being extremely reduced. These results suggest that MNP could interact with the polymer chains acting as cross-linker and therefore decrease the ability to swell, as was also determined by SEM micrographs. In order to determine if MNP has any effect on the UCST or collapse temperature of the hybrid microgels, Figure 6 represents the evolution of the normalized size with temperature for all the systems studied. Taking the value of the transition temperature at half of the swelling (0.5), it is observed that the transition temperature occurs over a very narrow range of temperatures, at 35, 34.5, 33.5, and 35.5 °C for samples MG0, MG5MNP, MG10MNP, and MG15MNP, respectively. As seen, hybrid systems containing 5% and 10% of MNP undergo a slight shift toward lower temperatures compared to the reference sample, but the sample containing 15% MNP does not follow this trend and presents a transition temperature equal to that of the reference sample within the experimental error. The fact that there 8030
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Figure 6. Evolution of the normalized diameter with temperature of samples MG0 (black), MG5MNP (red), MG10MNP (green), and MG15MNP (blue).
is not a higher shift of the collapse temperature with the concentration of MNP can be explained by taking into consideration two competing effects: on one hand, the effect of interactions that occur between the polymer matrix and MNP giving rise to a decrease of the transition temperature, as it happens for the samples containing 5% and 10% MNP and, on the other hand, the effect related to interactions occurring among the MNPs themselves, that provoke a shift toward higher temperatures, explained by the fact that additional energy should be needed to overcome the interparticle forces inside the microgel network.48 Rheology of Hybrid Microgels. As stated in the Introduction, in order to obtain relevant information that could help to determine the most suitable system for further applications, it is very important to have in-depth knowledge of the characteristic rheology of the studied hybrid microgel systems. The results obtained in relation to such an aspect would determine, as a first approach, the more appropriate concentration of dispersion and content of magnetics in the system. As reported in our previous work5 and as has been demonstrated by other authors,52,53 microgel dispersions possess a gellike behavior due to interactions occurring between microgel particles that provoke reversible clustering formation. Once the encapsulation of the magnetite in the microgel matrix was demonstrated, now, the question is whether the hybrid microgels also show a gel-like behavior and if the incorporation of magnetite nanoparticles would affect the viscoelastic properties of the system. In Figure 7, the frequency sweeps of the reference microgel sample, MG0, and microgel loaded with Fe3O4 nanoparticles, MG5MNP, MG10MNP, and MG15MNP, are plotted. As previously reported,5 the reference microgel sample, MG0, presents a gel-like behavior characterized by an elastic modulus higher than the viscous modulus in the entire frequency range and a constant elastic modulus at zero frequency. This same behavior is observed for magnetite loaded microgels, MG5MNP-MG15MNP, in which G0 is higher than G00 and a constant elastic modulus is observed at zero frequency. Therefore, the addition of magnetite nanoparticles does not eliminate the gel-like behavior of the system. Figure 8AD represents the strain sweep tests carried out for the reference microgel sample and for microgel loaded with 5, 10, and 15 wt % of Fe3O4 at three different concentrations of
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Figure 7. Elastic modulus (filled symbol) and viscous modulus (empty symbol) as a function of frequency for 1 wt % microgel dispersion of the sample MG0 (diamond), MG5MNP (square), MG10MNP (circle). and MG15MNP (triangle).
dispersion. The concentrations of the microgel dispersion chosen were 1, 2, and 5 wt %. From the graphs, a general and similar behavior can be observed in all the studied samples. First, as the applied strain increases the elasticity and viscosity modulus remains constant up to a value where the linearity is broken and where both moduli are dependent on the applied deformation. This suggested more complex structure formation in microgel dispersions that break when a certain deformation is applied.5 Second, elastic modulus is higher than viscous modulus for all samples and concentrations of microgel dispersions. Third, the viscous modulus G00 undergoes a slight increase at high deformation in the nonlinear range associated with a weak-strain hardening of the colloidal gel. This fact could be attributed to a loss of energy due to the deformation applied to the system that provokes interactions in the microgel.52 Finally, from these graphs it can be deduced that, as the concentration of the sample increases, both elastic and viscous moduli increase in all cases. This behavior could be explained by considering that a higher amount of microgel particles in the dispersion will hinder the flow itself giving rise to higher elasticity. Besides the important information obtained from the study of the influence of the strain in microgel dispersions, the main objective of this experiment is to obtain the relevant parameters elastic modulus plateau, G00 , and the critical deformation, γ0, for all the samples as indicated in the figure, which are necessary for the characterization of the systems and further scaling model application, as will be explained later on. Regarding the effect of magnetite nanoparticle concentration, if we plot the evolution of the elastic modulus plateau with the Fe3O4 content (Figure 9A) it can be seen that the incorporation of 5% Fe3O4 increases the elastic modulus by almost an order of magnitude with respect to the reference sample, and therefore a reinforcement of the system is achieved. However, the addition of 10% Fe3O4 does not increase the elastic modulus, and furthermore, the addition of 15% Fe3O4 to the microgel provokes a weakening in the elastic response. Now, if we consider the effect of Fe3O4 content on the limit of linearity or critical deformation (Figure 9B), we observe that the critical deformation increases up to a value of magnetite concentration between 10% and 15% Fe3O4, in which the critical deformation started to decrease. This 8031
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Figure 8. Evolution of the elastic modulus (full symbol) and viscous modulus (empty symbol) for 1 wt % (square), 2 wt % (circle), and 5 wt % (triangle) concentration of microgel dispersion, of samples (A) MG0; (B) MG5MNP; (C) MG10MNP; (D) MG15MNP.
Figure 9. (A) Elastic modulus plateau evolution and (B) critical deformation evolution with the Fe3O4 content for 1 wt % (square), 2 wt % (circle), and 5 wt % (triangle) concentration of microgel dispersions.
value could be considered as an optimal or critical concentration of magnetite, in which above this value the additional magnetite does not improve the elasticity of the sample. Considering the results obtained from the viscoelastic characterization, it could be assumed that the incorporation of the magnetite does not prevent structure or clustering formation as
observed from strain sweep tests. Even so, the question is whether the incorporation of the magnetite could affect the interaction between microgels due to their magnetic properties. In order to elucidate this question and, therefore, to relate the colloidal gel structure to the rheological properties, a fractal analysis, helped by the Wu and Morbidelli model scaling theory 8032
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Figure 10. (A) Elastic modulus plateau evolution with concentration of dispersion and (B) critical deformation as a function of concentration of dispersion, for the samples: MG (diamond), MG5MNP (square), MG10MNP (circle), and MG15MNP (triangle).
Table 3. Summary of the Results Obtained by Applying Wu and Morbidelli’s Scaling Theory Wu and Morbidelli sample
A
Df
B
R
regime
MG0: PAAm-AA
1.97
1.40
2.40
0.95
Weak
MG5MNP: PAAm-AA þ 5% Fe3O4
2.27
0.82
1.62
0.35
Transition(Strong)
MG10 MNP: PAAm-AA þ 10% Fe3O4 MG15 MNP: PAAm-AA þ 15% Fe3O4
2.04 1.43
0.50 0.29
1.70 1.24
0.50 0.53
Transition Transition
(Supporting Information), was carried out on the hybrid microgels. To apply the Wu and Morbidelli theory, the plateau values of G0 and γ0 already determined from the strain sweep experiments (Figure 8) were collected and thus plotted as a function of microgel dispersion concentration in double-logarithmic form in Figure 10A and B, respectively. From figure 10A, it can be extracted that the elastic modulus plateau increases with the concentration of the dispersion for all the samples. Concerning the critical deformation or limit of linearity represented in Figure 10B, it is observed that for the reference microgel sample, MG0, the limit of linearity increases with the concentration of the dispersion. This means that, with the increase of microgel particle concentration, more deformation needs to be applied in order to provoke the breakage of clustering. On the contrary, hybrid magnetic microgels, MG5MNPMG15MNP, showed an opposite behavior. As the concentration of the dispersion increased, the limit of linearity decreased slightly; thus, less deformation is needed to provoke the clustering breakage. These graphs indeed show that both G0 and γ0 exhibits a power law or a scaling relationship with concentration for all the samples and can be fitted to the following equations: G00 jA γ0 jB with A and B being the values obtained from the slope. By applying these slope values in the Wu and Morbidelli equations (eqs 12 in Supporting Information), it is possible to determine the fractal dimension, microscopic elastic parameter (R), and therefore the link regime of the systems. The obtained results are summarized in Table 3.
For reference microgel sample, MG0, a value of R = 0.95 is observed, so that it presents a weak-link regime. In terms of Wu and Morbidelli, this result corresponds to an elasticity derived from the interaction between microgel or intrafloc interactions. With the incorporation of 5% MNP, the value of R is changed from 0.95 to 0.35, from a weak link regime to a regime that is found to be closer to a strong link. That means that the elasticity of the MG5MNP system comes from the interaction between aggregates (interflocs) with a slight contribution of the interaction between microgel or intraflocs. As the concentration of Fe3O4 increases to 10% and 15%, the value of R obtained is 0.50 and 0.53, respectively. These samples do not follow the expected trend showing a displacement to a transition regime, which means that the interaction between microgel (intra) has increased, contributing to the overall elasticity of the system. The results obtained can be explained considering that, depending on the amount of iron oxide nanoparticles, they could act simply as inorganic filler or as magnetic nanoparticles. In the case of MG5MNP, nanoparticles probably act simply as inorganic filler reinforcing the polymeric matrix, as seen before from viscoelastic studies, causing an increase of the interaction between aggregate. However, in the case of MG10MNP and MG15MNP, nanoparticles not only cause the reinforcement of the polymer matrix, but also provoke an increase in the interactions occurring between individual microgels, behavior that could be attributed to the magnetic force occurring between MNP. Concerning the fractal dimension (Df), as observed in Table 3, as the amount of MNP increases the fractal dimension decreases from a value of 2.4 for sample of reference to 1.61.7 for MG5MNP 8033
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Langmuir and MG10MNP samples. That corresponds to a change from a more spherical to a more planar system. These results agreed in some manner with the change of regime observed with the increase of magnetite.
’ CONCLUSIONS Hybrid microgels composed of poly(AAm-AA) and magnetite nanoparticles have been synthesized and characterized. The incorporation of magnetite nanoparticles into the microgel matrix was performed by the addition of nanoparticles into the inverse emulsion polymerization reaction. TEM images and surface charge analysis confirmed that the encapsulation of magnetite into the microgel matrix is achieved in an efficient manner. Concerning the swelling behavior and thermosensitivity of the microgels, the incorporation of MNP does not affect this property, although there is a shift in the collapse temperature compared to that of the reference microgel probably due to interaction between magnetite nanoparticles. Regarding the rheology and fractal analysis, two main conclusions are extracted: first, that the elastic modulus of the microgel dispersion is increased with the addition of magnetite up to a critical value where the matrix is not further reinforced. This value is comprehended between 10 and 15 wt % of magnetite. Second, by means of scaling theory it can also be observed that the incorporation of magnetite increases the interaction between microgel particles and hence the elasticity. However, the fact that the higher the hybrid microgel concentration, the lower the critical deformation, means that less deformation is needed to break the structure or agglomerates formed. Hence, hybrid microgels become more interesting than raw microgel system, due to its characteristic rheology, for applications in which they need to be used as carriers through fluids. Furthermore, the knowledge of rheological behavior allows us to obtain the most favorable system, by playing with concentration and magnetite nanoparticle content, for processing and further application. Therefore, this report illustrates a direct method for the preparation of multistimuli UCST-like magnetic hybrid microgels with potential interest for biomedical applications such as controlled drug delivery systems and hyperthermia treatments, now in progress. ’ ASSOCIATED CONTENT
bS
Supporting Information. Description of DLS, TGA, XRD analysis and scaling theory. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail address:
[email protected].
’ ACKNOWLEDGMENT Financial support from Ph.D. fellowship of the University of Perugia and CICYT (MAT2008-1073) is acknowledged. The authors acknowledge Prof. Asua for providing DLS facility, Margarita Hernandez for AFM measurements, and David Gomez for SEM images.
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