pubs.acs.org/Langmuir © 2010 American Chemical Society
Investigation of the Relationship between Hydrogen Bonds and Macroscopic Properties in Hybrid Core-Shell γ-Fe2O3-P(NIPAM-AAS) Microgels J. Rubio-Retama,*,† N. E. Zafeiropoulos,‡ B. Frick,§ T. Seydel,§ and E. Lopez-Cabarcos† †
Departamento Quı´mica Fı´sica II, Facultad de Farmacia, Universidad Complutense, 28040 Madrid, Spain, ‡ Department of Materials Science and Engineering, University of Ioannina, 45110 Greece, and § Institut Laue Langevin, Grenoble, France Received November 24, 2009. Revised Manuscript Received January 20, 2010
We investigate in a hybrid material the interactions existing between magnetic nanoparticles of γ-Fe2O3 and the polymer matrix constituted by core-shell poly(N-isopropylacrylamide-sodium acrylate) microgels. These interactions provoke the shifting of the microgel volume phase transition to higher temperatures when the amount of γ-Fe2O3 increases. The study was performed using different techniques such as incoherent quasi-elastic neutron scattering (IQNS), infrared spectroscopy (FTIR-ATR), and dynamic light scattering (DLS). Below the low critical solution temperature (LCST) of the polymer, the IQNS data confirm that the presence of inorganic nanoparticles affects the PNIPAM chain motions. Thus, in the swollen state both the mean-square displacement of the polymer segments and the diffusive motion of the polymer chains decrease as the iron oxide content increases. The FTIR-ATR study indicates that the reduction of vibrational and diffusional motions of the polymer chains is due to the formation of hydrogen bonds between the amide groups of the polymer matrix and the OH groups of the magnetic nanoparticles. The creation of this hybrid complex would explain the reduction of the swelling capacity with increasing the iron content in the microgels. Furthermore, this interaction could also explain the shift of the polymer LCST to higher temperatures as due to the extra energy required by the system to break the hydrogen bonds prior to the PNIPAM collapse.
Introduction Polymer microgels with LCST constitute an interesting group of materials with applications in nanotechnology due to their fast response to changes in temperature.1 They can undergo a reversible gel-gel volume phase transition between collapsed and swollen states,2 which is the result of the competition between repulsive intermolecular forces and attractive forces.3 The thermal response together with the possibility of incorporating other counterparts such as CdTe quantum dots4,5 and silver or gold nanoparticles6-8 opens the possibility of producing multiresponsive materials for optoelectronics and biotechnology.9 Frequently, the polymer matrix provokes environmental modifications that modify the behavior of the nanoparticles. In other cases, the incorporation of nanoparticles7,8,10 adds new features, like magnetism, giving to the hybrid material potential application as targeted drug delivery systems.11,12 Furthermore, the incorporation of nanoparticles within polymer matrix tends to modify some polymer properties such as swelling capacity, thermal sensitivity, and stability.
In a previous article we presented an alternative route to synthesize a thermosensitive magnetic hybrid material based on core-shell microgels of poly(N-isopropylacrylamide-sodium acrylate) termed P(NIPAM-AAS) covered with γ-Fe2O3 nanoparticles.13 The LCST of PNIPAM shifts to higher temperatures as the iron content increases and disappears completely at iron concentration above 38% (w/w), in agreement with the results obtained by other groups.7,14,15 The aim of this work is to investigate the mechanism underlying the shifting of the P(NIPAMAAS) volume transition temperature due to the presence of γ-Fe2O3 nanoparticles using quasi-elastic neutron scattering (IQNS), infrared spectroscopy (FTIR), and dynamic light scattering (DLS) techniques. The results show that the incorporation of the inorganic nanoparticles in the polymer matrix reduces the diffusion and vibration motions of the polymer chains in the swollen microgels. This behavior is attributed to the formation of hydrogen bonds between the polymer chain and the magnetic nanoparticles, which make them act as cross-linkers that stabilize the polymer network, reducing the swelling and blocking the LCST of the microgels.
*To whom correspondence should be addressed. (1) Daly, E.; Saunders, B. R. Langmuir 2000, 16, 5546. (2) Zhang, Y. Q.; Tanaka, T.; Shibayama, M. Nature 1992, 360, 142. (3) Krazt, K.; Hellweg, T.; Eimer, W. Colloids Surf., A 2000, 170, 137. (4) Agrawal, M.; Rubio-Retama, J.; Zafeiropoulos, N. E.; Gaponik, N.; Gupta, S.; Cimrova, V.; Lesnyak, V.; Lopez-Cabarcos, E.; Tzavalas, S.; Rojas-Reyna, R.; Eychm€uller, A.; Stamm, M. Langmuir 2008, 24, 9820. (5) Zhang, Z.; Zhow, Z.; Yang, B.; Gao, M. J. Phys. Chem. B 2003, 107, 8. (6) Gorelikov, I.; Field, L.; Kumacheva, E. J. Am. Chem. Soc. 2004, 126, 15938. (7) Karg, M.; Lu, Y.; Carbo-Argibay, E.; Pastoriza-Santos, I.; Perez-Juste, J.; Liz-Marzan, L.; Hellweg, T. Langmuir 2009, 25(5), 3163. (8) Wu, W.; Zhou, T.; Zhou, S. Chem. Mater. 2009, 21(13), 2851. (9) Kondo, A.; Kamura, H.; Higashitani, K. Appl. Microbiol. Biotechnol. 1994, 41, 99. (10) Sauzedde, F.; Elaissari, A.; Pichot, C. Colloid Polym. Sci. 1999, 277, 846. (11) Karg, M.; Hellweg, T. Curr. Opin. Colloid Interface Sci. 2009, 14, 438. (12) Fernandez-Barbero, A.; Suarez Ivan, J.; Sierra-Martı´ n, B.; FernandezNieves, A.; de Las Nieves, F J.; Marquez, M.; Rubio-Retama, J.; Lopez-Cabarcos, E. Adv. Colloid Interface Sci. 2009, 147, 88.
Langmuir 2010, 26(10), 7101–7106
Experimental Section Methods. The microgel particles were studied using transmission electron microscopy (TEM) with a JEOL-2000FX microscope operating at 200 kV. Thermogravimetric analysis (TGA) to determine the content of γ-Fe2O3 was performed in a Mettler Toledo TGA/DSC1 equipment. The ATR-FTIR spectra were taken with a Nicolet IR200 FTIR spectrometer equipped with an attenuated total reflection (ATR) setup containing ZnSe crystal (13) Rubio-Retama, J.; Zafeiropoulos, N. E.; Serafinelli, C.; Rojas-Reyna, R.; Voit, B.; Lopez-Cabarcos, E.; Stamm, M. Langmuir 2007, 23, 10280. (14) Bhattacharya, S.; Eckert, F.; Boyko, V.; Pich, A. Small 2007, 3, 650. (15) Pich, A.; Bhattacharya, S.; Lu, Y.; Boyko, V.; Adler, H. J. Langmuir 2004, 20, 10706.
Published on Web 02/09/2010
DOI: 10.1021/la904452c
7101
Article
Rubio-Retama et al.
(Foundation SpeculATR) coupled with a Peltier (Linkam Scientific). Background spectra were taken before sample spectra and subsequently subtracted. Specifically, for each sample containing 20% (w/w) microgels dispersed in water an identical solution without the polymer was used as background. Each sample is the result of 512 spectrum accumulation. The baseline correction was carried out in IGOR pro (version 6.00) by subtracting polynomials in order to make the baseline flat around the peaks of interest in a similar way as it was described by Cremer et al.16 The thermoresponsive character of the γ-Fe2O3/PNIPAM microgels was studied by dynamic light scattering (DLS) using a Malvern Nano-ZS system equipped with a He-Ne laser working at 632.8 nm. The suspension of microgels was diluted to a concentration of 0.02% (w/w) to prevent multiple scattering and to diminish colloidal interactions. The mean diffusion coefficient was derived from the intensity autocorrelation function using cumulant analysis, which was converted into mean particle size via the Stokes-Einstein equation. Other measures carried out with a multiangle dynamic light scattering (Brohaven Bi200SM) were used to corroborate the previous results. Because of the huge difference between DLS and IQNS in the temporal scale resolution (1 10-6, 1 10-3, and 1 10-14-1.10-9 s, respectively), no chain relaxation can be observed in DLS measurements since they are very fast movements within the neutron scale resolution. For that the relaxation obtained in DLS is totally related with the particle diffusion. The PNIPAM molecular dynamics was studied using incoherent elastic (IES) and quasi-elastic (IQNS) neutron scattering. The experiments were performed at the Institut Laue Langevin in Grenoble using the high energy resolution (1 μeV) neutron backscattering spectrometer IN10.17 The outcome in a neutron scattering experiment is the scattering function S(Q,ω) that contains information about the structure and dynamics of the sample (Q = 4π sin θ/λ is the scattering vector with θ and λ being the half of the scattering angle and the incident neutron wavelength, respectively, and ω is the angular frequency). Quasi-elastic spectra were recorded at 290 and 330 K using a sample holder with double-wall hollow-cylinder geometry, which was sealed to avoid D2O evaporation during the measurements. The thickness and concentration of the sample (2% w/w) were selected to yield a transmission of about 85%. Standard ILL procedures and programs were used for corrections (empty cell), normalization, and quasi-elastic peak fit. A more detailed description of the methods has been reported previously.12,13,18 Theory. Dynamics in polymeric systems result from the superposition of faster motions (vibration-like) and slower motions (translational diffusion-like)19,20 that can be separated if they are uncoupled and their time scales are different. The displacement r of a polymer chain segment in water with respect to the center of mass of the microgel displays a diffusive-like motion characterized by a diffusion constant D = K/f 2 DrB K DB r ¼ Dt f Dx2
ð1Þ
where K is the gel elastic modulus and f the friction coefficient between the network and water.11,21 The analysis of the IQNS data was carried out considering the molecular motion formed by the combination of a faster vibrational component with a meansquare displacement Æu2æ1/2 and a translational diffusion motion with diffusion constant D. For Fickian diffusion the dynamic (16) Laura, B.; Zhang, S. Y.; Litosh, V. A.; Chen, X.; Cho, Y.; Cremer, P. S. J. Am. Chem. Soc. 2009, 131, 9304. (17) The IN10 user’s guide can be found at http://www.ill.fr/YellowBook/ IN10/. (18) Rubio-Retama, J.; Frick, B.; Seydel, T.; Stamm, M.; Fernandez Barbero, A.; Lopez-Cabarcos, E. Macromolecules 2008, 41, 4739. (19) Lopez-Cabarcos, E.; Batallan, F.; Frick, B.; Ezquerra, T.; Balta Calleja, F. J. Phys. Rev. B 1994, 50, 13214. (20) Bee, M. Quasielastic Neutron Scattering; Adam Hilger: Bristol, 1988. (21) Tanaka, T.; Hocker, L. O.; Benedek, G. B. J. Chem. Phys. 1973, 59, 5151.
7102 DOI: 10.1021/la904452c
structure factor is a Lorentzian function, with a half-width at halfmaximum DQ2. Thus, the scattering function can be written as " # 1 2 2 1 DQ2 SðQ, ωÞ ¼ exp - Æu æQ δðωÞX 3 π ðDQ2 Þ2 þ ω2
ð2Þ
where δ(ω) is a delta function and X is the convolution product in ω. The mean-square displacement Æu2æ1/2 can be derived from the Q2 dependence of S(Q,0) at fixed temperature, and D is obtained from the fitting of the quasi-elastic component with a Lorentzian function.
Results and Discussion Synthesis of Magnetic Microgels. Core-shell P(NIPAMAAS) microgels were synthesized by surfactant free radical polymerization in aqueous solution. Subsequently, they were covered with γ-Fe2O3 nanoparticles obtained by precipitation of an FeII/FeIII solution. Depending on the concentration of FeII/ FeIII in the solution, different contents of iron were obtained in the microgels. A detailed description of the microgels preparation can be found as Supporting Information. An interesting characteristic of this synthesis is that the increment of the iron concentration leads to a hierarchical precipitation of the magnetic nanoparticles, from the outer shell toward the inner part of the microgels, as is illustrated in Figure 1. This indicates that γ-Fe2O3 nanoparticles saturate first the carboxylic groups of the shell and later advance inward the microgel. This effect would explain the gradual reduction of the swelling capacity of the microgels (Figure 3 in the Supporting Information) as well as their thermosensitivity when the iron oxide concentration increases. On the other hand, in the collapsed state the size of the core-shell P(NIPAM-AAS) microgels with 28% (w/w) of γ-Fe2O3 was larger than the other microgels (see Figure 4 in the Supporting Information), suggesting that the iron nanoparticles within the network hinder the complete collapse of the polymer. Moreover, the incorporation of γ-Fe2O3 nanoparticles increases the LCST temperature from around 307 K in pure core-shell P(NIPAM-AAS) microgels up to 313 K in hybrid microgels with 28% (w/w) of γ-Fe2O3. This phenomenon might be associated with the dynamics of the polymer chains in the PNIPAM network and how the polymer motions are affected by the presence of the iron nanoparticles. Other factors such as the outward diffusion of water from the inner parts of the microgel or the reduction in pore size with increasing iron content could also contribute to the shift of the transition temperature. Another interesting result observed after the iron oxide incorporation is the increment of polymer degradation temperature (see TGA experiments in Figure 1 of the Supporting Information). The thermal degradation curves of core-shell P(NIPAMAAS) microgels show one weight loss stage similar to those obtained for other acrylic-based polymers.22 The temperature for the maximum weight loss rate, Tp, is 567 K for P(NIPAMAAS) microgels, increasing with the content of γ-Fe2O3 nanoparticles within the microgel. A small amount of nanoparticles is effective to shift the Tp toward higher values, which means they enhance the thermal stability of the microgels. Similar behavior has been reported by Lopez et al.23 for PVA nanocomposites prepared with CoFe2O4 nanoparticles. This finding reported by two research groups in two different polymers might be attributed to the capacity of iron-based nanoparticles to suppress the (22) Razga, J.; Petranek, J. Eur. Polym. J. 1975, 11, 805. (23) Lopez, D.; Cendoya., I.; Torres, F.; Tejada, J.; Mijangos, C. J. Appl. Polym. Sci. 2001, 82, 3215.
Langmuir 2010, 26(10), 7101–7106
Rubio-Retama et al.
Article
Figure 1. TEM images of PNIPAM microgels with (a) 18%, (b) 28%, (c) 38%, and (d) 60% (w/w) of γ-Fe2O3.
thermo-oxidative degradation of the polymer.24 The degradation of the core-shell P(NIPAM-AAS) microgels starts with the backbone scission, which takes place by intermolecular and intramolecular transfer reactions giving a mixture of monomer and oligomers25 as degradation products.26 The reduction of molecular mobility, induced by the presence of nanoparticles, could partially suppress the chain transfer reactions, thus improving the thermal stability of the polymer matrix. The previous results lead us to think that γ-Fe2O3 nanoparticles could modify the polymer dynamics of the hybrid microgels. For that, diffusional motions of polymer chain segments has been investigated using inelastic neutron scattering since these motions are prominent in the quasi-elastic scattering region. However, it is necessary to keep in mind that the segment relaxation observed by IQNS is going to constitute some mean value between “unperturbed” polymer segments and “perturbed ones by the presence of nanoparticles” since with IQNS we cannot differentiate between both states. Furthermore, by dispersing the microgels in D2O, it is possible to obtain enough contrast between the polymer network and the solvent. The elastic neutron-scattering function S(Q,ω=0) normalized to its value at 5 K, S(Q,0)5 K, was measured as a function of the temperature with the backscattering spectrometer IN10 (see Figure 2). To measure S(Q,0), we have set the monochromator and the analyzer to the same energy in such a way that only those neutrons that change their energy by an amount smaller than the energy resolution of the instrument are detected (the so-called “fixed elastic window” method). Polymer dynamics obtained by incoherent quasi-elastic neutron scattering (IQNS) are inferred from the scattering coming from the hydrogen atoms of the polymer network. Taking into account that the P(NIPAMAAS) microgels were synthesized with 5% (w/w) of sodium acrylate and 95% (w/w) NIPAM monomer and that the number of hydrogen per Nipam monomer is 10 and only 3 per acrylate monomer, this lead to the AAS contribution is less than 2% with respect to the total signal. For that the contribution of the ASS part to the signal is much lower than the PNIPAM part. In addition, we have considered that the contribution of the Fe2O3 nanoparticles to the incoherent signal could be considered (24) Dzunuzovic, E.; Vodnik, V.; Jeremic, K.; Nedeljkovic, J. J. Matter Lett. 2009, 63, 908. (25) Cameron, G. G.; Meyer, J. M.; McWalter, I. T. Macromolecules 1978, 11, 669. (26) McNeill, I. C.; Zulfiqar, M.; Kousar, T. Polym. Degrad. Stab. 1990, 28, 131.
Langmuir 2010, 26(10), 7101–7106
Figure 2. Logarithm of the elastic scattering function S(Q,0) integrated over all angles and normalized at 5 K (where all motions are frozen) as a function of the temperature for the heating (rate 1 K/min) of PNIPAM-co-acrylic with different percentage of γ-Fe2O3.
as negligible since the incoherent scattering cross sections are: 80.2 barn for H, 0.4 barn for Fe, and 0.0008 barn for O. (Further information about polymer dynamics and IQNS is provided in refs 17-20.) The elastic scans provide an overview about the temperature dependence of the molecular dynamics. In Figure 2, three regions can be distinguished: (i) below 277 K the scattered intensity decreases with increasing temperature in agreement with the harmonic behavior; (ii) between 277 and 307 K the intensity sharply decreases with the melting of D2O; (iii) above 307 K the intensity increases up to a nearly constant value for temperatures higher than 313 K. This behavior can be interpreted as follows: At low temperature the D2O is frozen and there are only a few vibration motions, and most of the scattered intensity passes through the energy window of the analyzer. Above the melting point of heavy water (276.82 K) the motion of the molecules explains the broadening of the elastic line signal, and only the central part of it passes through the energy window of the analyzer, with the consequent drop of the intensity. Between 307 and 313 K the microgels collapse due to the volume phase transition, and the elastic intensity exhibits a steep increase, indicating that some motions in the sample are hindered which narrows the elastic line. DOI: 10.1021/la904452c
7103
Article
Rubio-Retama et al. Table 1. Charge of the Microgels as a Function of the Iron Oxide Content microgel charge (C/g) iron oxide loaded (%)
Figure 3. Apparent mean-square displacement of the polymer network as a function of temperature for PNIPAM microgels (green) and P(NIPAM-AAS) microgels with different amounts of γ-Fe2O3.
The intensity trend is similar for the three samples studied, but there is a decrease in intensity with increasing γ-Fe2O3 content and the effect is more remarkable above 313 K. The average spatial scale of the vibrational motion of the scattering particles (mostly hydrogen), the apparent mean-square displacement Æu2æ1/2, can be obtained analyzing the Q dependence of S(Q,0) at fixed temperature ln
SðQ, 0Þ 1 ¼ - ðÆu2 ðTÞæ -Æu2 ð5 KÞæÞQ2 SðQ, 0Þ5 K 3
ð3Þ
Assuming that at 5 K there are no motions (Æu2(5 K)æ g 0), we can infer Æu2(T)æ from the dependence of the intensity with Q2. To get rid of the water coherent scattering that could contaminate the detectors placed at Q higher than 1.5 A˚-1 only the first three detectors were used to obtain the mean square displacement (see Figure 5, Supporting Information). The results obtained are presented in Figure 3. Here we can see that in the frozen state the value of the squared amplitude is around 0.7 A˚2, independently of the microgel composition. However, when D2O melts, Æu2(T)æ increases abruptly until reaching an approximately constant value that depends on the composition of the microgels. Thus, the highest γ-Fe2O3 content corresponds with the smallest Æu2(T)æ value. Above the volume transition temperature the behavior is different. At 307 K begins the collapse of the microgels, and the process involves the approaching of neighbor polymer chains and the release of the swollen water. The result of this process is a compact polymer structure whose vibrational motions are restricted and the value of Æu2æ resembles that of the polymer matrix in the frozen state (see Figure 3). However, the value of the mean-square displacement varies depending on the composition of the polymer matrix. Thus, for the P(NIPAMAAS) the steep decrease is not so remarkable as in the neat PNIPAM microgels, probably due to the presence of COOgroups that locally hinder the collapse by electrostatic repulsions (the charge of these microgels is 23.6 C/g much higher than 4.7 C/ g for pure PNIPAM). In such scenario we find two different polymer arrangements: one formed by PNIPAM monomers completely collapsed and the other formed by swollen poly(acrylate) entanglements with higher vibrational level.2 7104 DOI: 10.1021/la904452c
23.6 0.0
16.5 2.5
4.1 5
0.23 10
In the case of the microgels with γ-Fe2O3, the COO- groups are partially neutralized (the charge of these microgels is only 0.23 C/g), and the variation in the vibrational amplitude is not as large as for pure PNIPAM. We have observed that the amount of iron oxide nanoparticles required to reduce the charge of the P(NIPAM-AAS) microgels to a similar level existing in pure PNIPAM microgels is around 5% (w/w), as is shown in Table 1. These results seem to indicate that around 5% of the iron oxide nanoparticles interact with the AAS portion, and further increment in nanoparticles loading would interact with PNIPAM. This interpretation is supported by the fact that thermal and dynamical properties begin to change at nanoparticles concentration higher than 5%. All these results suggest that at low iron oxide loading (∼10%) the nanoparticles are preferentially distributed on the outer part of the microgels (neutralization of the superficial charge) collapsing the microgel’s AAS outer shell due to the strong interaction existing between the nanoparticles and the polymer chains. This effect is concomitant with a hydrodynamic diameter reduction (Supporting Information), indicating a relationship between collapse and nanoparticles presence. This could be the reason why at nanoparticles loading higher than 5% they introduce a physical barrier that hinders the polymer chain vibrations. It can be seen in Figure 3 that the mean-square displacement of the polymer network below the LCST is reduced when the amount of magnetic nanoparticles is increased, indicating that the barrier is proportional to the amount of nanoparticles. This effect is attributed to the strong interactions between nanoparticles and polymer chains that act as cross-linking points, increasing the rigidity of the polymer matrix. Furthermore, as was shown in Figure 3, the iron content affects the collapsing capacity of the microgels, which is almost blocked when the percentage of γ-Fe2O3 is close to 30% and completely disappears for 38% iron content.12 To further clarify this point, we have used IR spectroscopy to determine the interactions that take place between the nanoparticles and the polymer matrix. This technique is highly sensitive to the local environment of the molecules, specially the vibration of the amide group. Figure 4 shows the IR spectra of the P(NIPAM-AAS) microgels with different amounts of γ-Fe2O3 at 290 and 327 K measured in H2O. At 290 K, the core-shell P(NIPAM-AAS) microgels spectrum presents two bands: the amide II at 1560 cm-1 (contribution of N-H ∼60% bending and C-N ∼40% stretching vibration) and the amide I at 1625 cm-1 (containing contributions from the CdO stretching vibration ∼80% with a minor contribution from the C-N stretching vibration). The position of the amide I at 1625 cm-1 indicates that most of the carbonyl bonds are hydrogen bonded with water as it was previously described by Maeda et al.27 and Cremer et al.10 This scenario changes completely when the microgels collapse; the amide I and amide II bands split into two, appearing a new amide II band at around 1560 cm -1 and a new amide I band at 1650 cm-1. These new bands inform about the formation of intra- and intermolecular CdO 3 3 3 H-N hydrogen bonds during the PNIPAM matrix collapse.28 (27) Yamauchi, H.; Maeda, Y. J. Phys. Chem. B 2007, 111, 12964. (28) Maeda, Y.; Higuchi, T.; Ikeda, I. Langmuir 2000, 16, 7507.
Langmuir 2010, 26(10), 7101–7106
Rubio-Retama et al.
Article
Figure 4. FTIR absorption spectra of the amide I and amide II bands for core-shell P(NIPAM-AAS) microgels (blue) and core-shell P(NIPAM-AAS) microgels with different amounts of γ-Fe2O3 at 290 K (left) and 327 K (right) in H2O.
The γ-Fe2O3 precipitation onto these microgels provokes important changes in the FTIR spectra (Figure 4, left panel). At 290 K, the FTIR spectrum of core-shell P(NIPAM-AAS) microgels with 10% (w/w) of γ-Fe2O3 content resembles those obtained for P(NIPAM-AAS) microgels at 327 K. Additionally, a new band arises in the spectrum at 1635 cm-1, which becomes stronger when the amount of iron precipitated in the microgels increases. We attribute this band to the interaction between the CdO groups of the polymer and the OH groups of the γ-Fe2O3 nanoparticles surface. The CdO of amide would act as a hydrogen-bond acceptor while the γ-Fe2O3 nanoparticles would act as hydrogen-bond donor (CdO 3 3 3 H-O-Fe2O3). The presence of CdO 3 3 3 H-N hydrogen bonds at 290 K and the new band at 1635 cm-1 could indicate that γ-Fe2O3 nanoparticles interact with the polymer chains and bring them closer favoring the interaction, through hydrogen bonds, between neighbor amide groups giving as result an FTIR spectrum at 290 K similar to the P(NIPAM-AAS) microgels at 320 K. The OH groups on the γ-Fe2O3 nanoparticles can contribute to the formation of multiple hydrogen bonds with the polymer matrix making the nanoparticles behave as cross-linking agents, reducing the swelling capacity of the polymer matrix. Assuming a 1:1 conversion of the CdO species, the integrated area of amide I band measured with different percentage of γ-Fe2O3 nanoparticles gives the ratio of the molar absorptivity of, component I at 1625 cm-1 (CdO 3 3 3 H2O), component II at 1650 cm-1 (Cd O 3 3 3 H-N), and component III at 1635 cm-1 (CdO 3 3 3 HO-Fe2O3), relative to the total contribution of all these bands. From this analysis (see Supporting Information, Figure 6) one can observe that the fraction of CdO groups that take part in hydrogen bond with water at 290 K is conspicuously reduced as result of the γ-Fe2O3 incorporation. This reduction is concomitant with an increment in the percentage CdO groups which interact with H-N groups and with γ-Fe2O3. The FTIR information seems to indicate that the presence of the γ-Fe2O3 nanoparticles creates strong interactions between the inorganic material and the polymer, which modify the ratio of CdO groups able to interact through hydrogen bond with water molecules, altering the polymer LCST. If this assumption is correct, and the polymer forms a complex with the magnetic nanoparticles, the diffusion of the polymer chain should be affected. The diffusive motions of the polymer network at 290 and Langmuir 2010, 26(10), 7101–7106
Figure 5. S(Q,ω) function clearly shows the different dynamics of P(NIPAM-AAS) networks in the swollen (290 K) and collapsed (330 K) state with different amount of γ-Fe2O3 nanoparticles. The dotted line corresponds to the resolution function.
330 K were measured by IQNS at the spectrometer IN10. At 290 K, the quasi-elastic signal was measured only at the two detectors placed at Q < 0.86 A˚-1 since for the rest of Q values the motions are beyond the accessible dynamic window of IN10. On the contrary, at 330 K the intensity was measured over the seven detectors of the IN10 spectrometer. In Figure 5 we show the quasi-elastic component, for Q = 0.5 A˚-1, at the two selected temperatures for samples with different content of iron nanoparticles. Figure 5 shows that depending on the state of the microgels, swollen or collapsed, the polymer chains present different dynamics. In the swollen state (290 K) the S(Q,ω) function is wider than in the collapsed state, which indicates that at low temperatures the system behaves like soft matter becoming harder above the LCST (collapsed state). Additionally, we can observe that when the amount of iron increases, the difference between S(Q,ω) in the swollen and collapsed states is reduced, indicating a hardening of these microgels at low temperature. The fitting of the quasi-elastic data was made using eq 2, and the result of the fitting is shown as a continuous line in Figure 5. The fitting was improved by introducing a flat background, coming from the water dynamics,18 whose value was determined from the data of the tails of the curve. From the fitting of S(Q,ω) we have obtained the half-width at the half-maximum Γ(Q), which is represented versus Q2 in the Figure 6. As can be seen in Figure 6 the dependence is linear, Γ = DQ2, and from the slope we have calculated the apparent diffusion coefficient, D, of the polymer chain with respect to the center of mass of the magnetic microgel. Table 2 summarizes the polymer diffusion coefficients obtained at 290 and 330 K. DOI: 10.1021/la904452c
7105
Article
Rubio-Retama et al. Table 2. Diffusion Coefficient of the Samples with Different Content of Iron at 290 and 330 K sample
neat P(NIPAM-AAS) microgels P(NIPAM-AAS) microgels with 10% of γ-Fe2O3 P(NIPAM-AAS) microgels with 28% of γ-Fe2O3
D at 290 K -11
D at 330 K
m /s (2.4 ( 0.6) 10 (1.6 ( 0.2) 10-11 m2/s -11 (0.4 ( 0.1) 10 m2/s 2
(4.5 ( 0.9) 10-13 m2/s (3.8 ( 1) 10-13 m2/s (3.6 ( 0.9) 10-13 m2/s
The variation of D for the same sample follows an opposite tendency to most solids and liquids for which D increases when temperature is raised. In addition, the incorporation of γ-Fe2O3 nanoparticles reduces D at both the swollen and collapsed states, and the decrease is higher at higher iron content. The reduction of D as the iron content increases support the hypothesis of formation of strong interactions between the polymer network and the γ-Fe2O3 nanoparticles. It seems the nanoparticles interact with the polymer behaving as cross-linkers reducing their swelling capacity and hindering the LCST of the microgels.
Figure 6. Half-width at half-maximum of the Lorentzian quasielastic component measured as a function of Q2 for the microgels with different iron content in the swollen (290 K) and collapsed (330 K) states.
These values are in good agreement with previously published values obtained from the network diffusion in macroscopic P(NIPAM)29 gels or microgels.30 Thus, for example, Shibayama and co-workers published a D value of 4.5 10-11 m2/s for PNIPAM gels with a cross-linking degree of 1.2%, while Karg and co-workers computed D values close to 4 10-11 m2/s for P(NIPAM) microgels with a different cross-linking degree. (29) Shibayamna, M.; Takata, S.; Norisuye, T. Physica A 1998, 249, 245. (30) Hellweg, T.; Kratz, K.; Pouget, S.; Eimer, W. Colloids Surf., A 2002, 202, 223.
7106 DOI: 10.1021/la904452c
Conclusions In this article we have studied the physicochemical interaction that exists between γ-Fe2O3 nanoparticles and PNIPAM polymer chains, which form a hybrid complex stabilized by hydrogen bonds. This interaction, mainly hydrogen bonds, could reduce the mean-square displacement of the polymer segments from an average value of 5.6 A˚2 in pure P(NIPAM-AAS) microgels down to a value of 3.6 A˚2 in the P(NIPAM-AAS) microgels. In addition, the diffusion coefficient of the polymer segments with respect to center of mass of the microgel became smaller when the iron oxide content was increased, and this result is associated with the increment of the LCST and the polymer degradation temperature. In summary, we have attempted to explain the way in which short scale interactions such as hydrogen bonds influence macroscopic properties like swelling degree or LCST shift. Acknowledgment. This work was supported by the Ministry of Science and Technology (MAT2009-14234) and the BSCHUCM program for research groups (GR58/08). Partial support of COST Action D43 is also acknowledged. Supporting Information Available: Experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2010, 26(10), 7101–7106
