Poly(acrylamide) gels with embedded magnetite nanoparticles

Microelectronic Engineering 69 (2003) 324–329 www.elsevier.com / locate / mee

Poly(acrylamide) gels with embedded magnetite nanoparticles a,

a

a

b

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S.G. Starodoubtsev *, E.V. Saenko , A.R. Khokhlov , V.V. Volkov , K.A. Dembo , V.V. Klechkovskaya b , E.V. Shtykova b , I.S. Zanaveskina b a

Physics Department, M.V. Lomonosov Moscow State University, Leninsky Gory, Moscow 119992, Russia b Institute of Crystallography, Russian Academy of Sciences, 59 Leninsky Pr., Moscow 117333, Russia

Abstract Magnetic gels are a new class of soft polymer materials with their properties controlled by magnetic fields. A method was developed for the preparation of magnetite nanoparticles in a matrix of poly(acrylamide) gel. Synthesis of the magnetic spinel iron oxide Fe 3 O 4 in a matrix of poly(acrylamide) gel was performed via coprecipitation of Fe(II) and Fe(III) in alkaline medium. The effects of the cross-link density, the concentration of polymer and salt in the swollen network during preparation on the composition, structure and properties of the magnetic gels were studied by electronography, TEM and small-angle X-ray scattering (SAXS). The crystalline lattice of the particles was determined as magnetite. The average size of the particles, d, calculated from the half-width of the diffraction peaks, is of the order of 10 nm. The obtained data demonstrate a marked difference in size distribution of magnetic particles in gels of different structure.  2003 Elsevier B.V. All rights reserved. Keywords: Magnetic gels; Nanoparticles; Polymer nanocomposites; Superparamagnetism; Iron oxides

1. Introduction Magnetic gels are a new class of soft polymer materials with their properties controlled by magnetic fields. Usually, these materials are swollen polymer networks with incorporated magnetic particles [1–3]. They are capable of deforming in a magnetic field [4–6]. Reorientation of the particles’ magnetic moments in an oscillating magnetic field results in heating of the gel medium (Neel’s relaxation) [7,8]. When the magnetic particles have a size of order 10 nm the gels exhibit superparamagnetic properties [9]. The magneto-elastic properties of the magnetic gels can be used to construct sensors, switches and artificial muscles. Their possible appli-

cations also include various separation, membrane and drug delivery systems. This work is devoted to the synthesis and characterization of the structure and properties of magnetic gels composed of crosslinked poly(acrylamide) (PAAm) and magnetite. Magnetic particles were formed in the gels via coprecipitation of Fe(II) and Fe(III) in alkaline medium. The effects of cross-link density, concentration of polymer and salt in the swollen network on the composition, structure and properties of the magnetic gels were studied by electron diffraction, transmission electron microscopy (TEM) and smallangle X-ray scattering (SAXS).

2. Experimental *Corresponding author. E-mail address: [email protected] (S.G. Starodoubtsev).

PAAm gels were prepared by radical copoly-

0167-9317 / 03 / $ – see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00316-2

S.G. Starodoubtsev et al. / Microelectronic Engineering 69 (2003) 324–329

merization of acrylamide and N,N9-methylenebis(acrylamide) (BAA) in aqueous solution. The copolymerization was performed between glass plates separated by polyethylene gaskets with a thickness of 0.3 mm. The concentration of monomers during gel preparation varied from 6.0 to 50 wt%. The ratio of BAA to AAm was varied from 1:500 to 1:30 mol / mol (Table 1). After washing, the gels were dried. The swelling ratio of the gels was characterized by the ratio F 5 m /m 0 , where m is the mass of the sample in solution, and m 0 is the mass of the dried PAAm network. The dried films were swollen in concentrated aqueous solutions containing FeCl 3 hexahydrate and FeSO 4 heptahydrate in the ratio 2:1 for 24 h. After swelling in salt solutions the films were treated with an excess of a concentrated solution of potassium hydroxide (36 g KOH / 40 ml H 2 O), which resulted in the precipitation of the corresponding oxide, Fe 3 O 4 . Magnetite is obtained according to the reaction 2FeCl 3 1 FeSO 4 1 8KOH → Fe 3 O 4↓ 1 6KCl 1 K 2 SO 4 1 4H 2 O

(1)

The obtained magnetic gels were finally washed with distilled water to neutral pH. The weight fraction of precipitate (in wt%) in the dried magnetic network, Q, was estimated by the gravimetric method. It was shown that the yield of reaction (1) decreases with decreasing concentration of salts and alkali. However, in the region of the very high concentrations used in the study for sample preparation, this dependence becomes insignificant and practically all iron ions are converted into magnetite. The yield of Table 1 Composition of the monomer mixtures during synthesis, the swelling ratios of PAAm gels in water (FH 2 O ) and in a solution of salts (Fsalt ) and the Q values (wt%) of the magnetic gels Sample

AAm (wt%)

BAA /AAm

FH 2 O

Fsalt

Q

1 2 3 4 5 6

6 6 6 15 30 50

1 / 30 1 / 100 1 / 500 1 / 100 1 / 100 1 / 100

14.7 21.3 36.0 9.7 6.0 4.4

18.2 24.6 39.8 12.3 7.5 5.3

73.4 79.4 86.6 61.0 53.8 46.5

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reaction (1) in the gel phase also depends on the time of treatment of the gel films containing iron salts by alkali. Analyses of cuts of the films after the reaction showed that, for the obtained PAAm films, 30 min treatment with KOH is enough to complete reaction (1) in the whole volume of the gel. At significantly smaller times the central part of the gel films remains untreated due to the rather slow diffusion of KOH into the gel. In parallel with reaction (1), hydrolysis of PAAm occurs. Hydrolysis is accompanied by the appearance of ionized carboxylic groups on the PAAm chains and an increase of the swelling ratio of the gels in water. For instance, an increase in the treatment time with alkali from 30 min to 24 h leads to an increase in F of gel 2 from 12 to 38. Electron diffraction measurements were performed using an electron diffraction camera EMP-102 (U 5 75 kV). TEM images were obtained using a JEM100C microscope (U 5 100 kV). Small-angle X-ray scattering measurements were performed using the diffractometer ‘‘AMUR-K’’ at wavelength l 5 0.1542 nm. Kratky-type (infinitely long slit) geometry was used with a sample-to-detector distance of 673 mm and an entrance slit width of 0.2 mm to cover the range of momentum transfer 0.12 , q , 5.5 nm 21 , where q 5 4p sin u /l and 2u is the scattering angle. The scattering intensity from a dilute system of particles is proportional to the spherically averaged scattering from a single particle [10]: I(q) 5 kA2 (q)l V `

sin(qr) E r kr(r) 3 r(r)l ]] dr qr 2

5 4p

v

(2)

r 50

where A(q) is the scattering amplitude with q 5 4p sin u /l, 2u is the scattering angle, r is the vector which determines an observation point corresponding to the scattering center, l is the wavelength, and k l V indicates averaging over all spatial angles in reciprocal space. The spatial model of particle structure can be represented by the scattering density, which is described by the corresponding function r (r). The program GNOM [11,12] was used to estimate the distance distribution functions H(S) using the formula

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S2 H(S) 5 ]]2 2p

`

sin(qS) E q I(q)]]] dq qS 2

(3)

q50

Taking into account the polydispersity of the samples studied the program GNOM was also applied to evaluate the volume distribution function for the specimens. The size distributions of the scattering particles in the systems were computed using a spherical approximation of the particles. The indirect transform program GNOM was used to solve the integral equation and to determine the distribution profiles: R max

I(q) 5

E P(R)m (R)i (qR) dR 2

0

(4)

Fig. 1. Dependences of the ratio m mag /m PAAm (1) and f (g / g) (2) on the ratio m sol /m PAAm .

R min

where P(R) is the size distribution function, R is the radius of the sphere, R min and R max are the minimum and maximum radii, respectively, i 0 (x) 5 h[sin(x) 2 x cos(x)] /x 3 j 2 is the sphere form factor and m(R) 5 (4p / 3)R 3 Dr, where Dr is the difference between the scattering length density of the particles and that of the other part of the system. The value of R min was kept zero; that of R max was selected for each individual data set by successive runs with different values of this parameter. The magnetic properties of the gels were characterized by the value of the intrinsic force, f (g / g), that acted on 1 g of precipitate contained in the sample positioned in a magnetic field of constant configuration. Discs of the same diameter were cut from the gel films and placed in the field of a permanent magnet. The magnet-to-gel distance, L, was set constant; the thickness of the film was much smaller than L. The values of f were measured using balances.

tration of entanglements between the polymer chains of PAAm in a concentrated solution of the monomer. In a concentrated solution of mixed salts the swelling ratio of the gels is higher than in water. The latter result can be explained by the higher density of the salt solution in comparison with water. Fig. 1 shows the dependence of the weight of the precipitate obtained in the gel phase, m mag /m PAAm , on the amount of salt solution, m sol /m PAAm , absorbed by 1 g of dried PAAm before reaction with KOH. It can be seen that the amount of precipitate in the gels with different network structures is directly proportional to the amount of absorbed solution. It can be assumed that (i) the presence of the polymer matrix has a small effect on the yield of reaction (1) in the gels and (ii) the solubility of Fe(II) and Fe(III) salts

3. Results and discussion The parameters of the gels obtained in the study are listed in Table 1. An increase in the cross-linked density of the network and, especially, an increase in the AAm concentration at synthesis, decrease the swelling ratio of the gels. The latter effect is explained by a significant increase of the concen-

Fig. 2. Electron diffraction patterns obtained from sample 2.

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magnetite, Fe 3 O 4 . The characteristic size of the crystallites was estimated from the full width at half-maximum intensity of the peak using the Scherrer formula:

l L 5 ]]] bs cos u

Fig. 3. TEM image obtained from sample 2.

in water containing the gels is similar to that in aqueous solution. The latter assumption is supported by the fact that the dependence of f on the ratio m sol /m PAAm for the gels shown in Fig. 1 is rather weak. However, there is a tendency for a decrease of the magnetic force with increase of the polymer content of the gel. The crystalline structure of the precipitate in the gels was determined by electron diffraction. A typical electron diffraction pattern obtained from the magnetic samples is shown in Fig. 2. The presence of continuous concentric rings in the electron diffraction pattern demonstrates the random polycrystalline structure of the precipitate in the gel. The positioning of the rings corresponds to the crystalline structure of

(5)

where bs is the full width at half-maximum intensity of the peak (in radians) observed at a mean scattering angle of 2u. The characteristic size of the crystallites was ca. 10 nm. Fig. 3 shows the TEM image obtained from sample 2. The large dark round areas in the image have an average size of ca. 80–120 nm. Electron diffraction from these objects gives a picture analogous to that shown in Fig. 2. Hence, they have a polycrystalline structure and consist of randomly distributed magnetite domains of size ca. 10 nm. From the values of the density of magnetite (5.2 g / cm 3 ) and PAAm (1.33 g / cm 3 ) it is easy to calculate the volume fraction of magnetite, vm , in the dried magnetic composites. Such calculations give vm 5 0.5 for sample 2. The fraction of area occupied by the images of polycrystalline domains in the TEM picture is smaller than 0.05. No other types of crystal, but magnetite, can be detected by electron diffraction measurements. It can be assumed that there are a large number of much smaller magnetite crystals in the PAAm matrix together with large polycrystalline domains. These crystals are not detected by TEM due to their small size.

Fig. 4. SAXS profiles I(q) obtained from gel samples 1–3 (a) and 4 and 5 (b). The numbers on the curves correspond to the samples.

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Fig. 5. Size distribution functions, P(R), calculated from SAXS data obtained from gel samples 1–3 (a) and 4 and 5 (b). The numbers on the curves correspond to the samples.

Fig. 6. Distance distribution functions, H(S), calculated from SAXS data obtained from gel samples 1–3 (a) and 4 and 5 (b). The numbers on the curves correspond to the samples.

Fig. 4 shows the SAXS profiles obtained from magnetic gels 1–5 swollen in water. A monotonous decrease in intensity is observed for all gels in this study. The size distribution functions, P(R), and distance distribution functions, H(S), calculated for the magnetic gels are shown in Figs. 5 and 6, respectively. All size distribution functions have a maximum at about R55 nm (diameter 10 nm). This value is very similar to the value obtained by electron diffraction measurements. The strong background scattering makes accurate estimation of the anisometry of the particles impossible, but the obtained profiles of the distance distribution functions (Fig. 6) correspond to the rather elongated particles formed in the systems.

Acknowledgements This work has been supported by International Association (INTAS 00-243).

References [1] Gy. Torok, V. Lebedev, L. Cser, Gy. Kali, M. Zrinyi, Physica B 297 (2001) 40–44. [2] D. Szabo, I. Czako-Nagy, M. Zrinyi, A. Vertes, J. Colloid Interface Sci. 221 (2000) 166–172. [3] D. Szabo, G. Szeghy, M. Zrinyi, Macromolecules 31 (1998) 6541–6548. [4] P. Xulu, G. Filipcsei, M. Zrinyi, Macromolecules 33 (2000) 1716–1719.

S.G. Starodoubtsev et al. / Microelectronic Engineering 69 (2003) 324–329 [5] M. Zrinyi, J. Feher, G. Filipcsei, Macromolecules 33 (2000) 5751–5753. [6] M. Zrinyi, L. Barsi, A. Buki, J. Chem. Phys. 104 (1996) 8750–8756. [7] M. Babincova, D. Leszczynska, P. Sourivong, P. Cicmanec, P. Babinec, J. Magn. Magn. Mater. 225 (2001) 109–112. [8] Gy. Torok, V. Lebedev, L. Cser, M. Zrinyi, Physica B 276–278 (2000) 396–397.

329

[9] D. Leslie-Pelecky, R. Rieke, Chem. Mater. 8 (1996) 1770– 1783. [10] L. Feigin, D. Svergun, Structure Analysis by Small-angle X-ray and Neutron Scattering, Plenum Press, New York, 1987. [11] D. Svergun, A. Semenyuk, L. Feigin, Acta Crystallogr. A 24 (1988) 244–250. [12] D. Svergun, J. Appl. Crystallogr. 25 (1992) 495–503.