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Controlling the size and the stability of magnetic clusters formed in Polyacrylamide hydrogels via cobalt-anionic fluoroprobe interactions.

INTRODUCTION

Designing composite materials by using hydrogels is a popular and promising field of research from the viewpoint of both their scientific interest and technological uses. The most applicable way of forming composite materials by using hydrogel is the incorporation of materials such as ceramics, semiconductive and magnetic particles into a hydrogel matrix (1-4).

Hydrogels including magnetic particles so-called magnetic gels can be formed by incorporating the particles in the gel by means of chemical or electrostatic interactions. This process can be carried out during the gelation or via diffusion of the particles into the previously formed gels with or without an external field (5-9). The first magnetic gel synthesized with acrylamide has been reported by Kawaguchi et al. (10). In addition to magnetic gels, magnetic particles embedded in rigid polymeric media have been called as magnetic elastomers (11-13). Magnetic gels and elastomers have been used to control especially the bulk properties, such as deformations (14-16), elastic modulus (17) and quick response (18) to magnetic field, of the gels and/or polymers. Using this gel an intelligent ferrogel was recently developed for controlling the release of drugs by switching the magnetic field (19).

In the literature, fluoroprobes are widely used for monitoring various processes and functions on a microscopic level (20), (21). This technique is based on the interpretation of the change in anisotropy, emission spectra or intensity and viewing the lifetimes of injected fluoroprobes to monitor the change in their microenvironment (22-25). Because of their special features, fluorescence spectroscopy is sensitive and powerful method for analysis of many compounds.

In some recent studies the fluoroprobe, Pyranine (POH), was used as a probe for monitoring, gelation, sol-gel phase transition and the fractal nature of gelling system (26-33). The spectroscopic properties of POH are well known from these works (29), (34-36). It has been shown that a considerable blue-shift, from 515 to 420 nm, occurs in the emission spectra when OH group on POH binds covalently to a vinyl group of the growing polymer chains (29), (30), (33). Upon the initiation of the polymerization the peak appeared around 420 nm increases with time as the 515 nm-peak decreases in intensity. When the polymerization is completed, the peak at 515 nm disappears and only the peak at 420 nm existed. Once POH molecules are bonded to the PAAm strands chemically, they can not be washed out the gel (29), (30), (33). The pyranine interacts electrostatically with positively charged sites of the PAAm strands (29), (30), (33).

POH seems a good candidate for making stable interactions with the metals in the gel. Since it can bind to the polymer strands of the gel chemically over the OH group and interact electrostatically with the metals. Therefore, in this study we used POH molecules as an agent for controlling the size and the stability of the magnetic clusters formed by the [Co.sup.+2] ions in PAAm hydrogels.

EXPERIMENTAL

Materials

The monomer; acrylamide (AAm); the initiator, ammonium persulphate (APS); the activator, tetramethylethyle-nediamine (TEMED); and the multifunctional cross-linkers, methylencbisacrylamide (BIS) were supplied by Merck (Darmstad, Germany). POH and Co(II)aeetate were supplied by Fluka and Aldrich, respectively. All chemicals were used as received.

Synthesis of Hydrogels and PAAm Solutions

PAAm (1) gels were synthesized by using 2 mol/1 AAm (Merck) and 29 mmol/1 BIS (Merck) by dissolving them in 25 [cm.sup.3] of water in which 10 [micro]l of TEMED (tet-ramethylethylenediamine) as an accelerator and 14 mmo1/ 1 APS as an initiator was added. After bubbling nitrogen for 10 minutes, each pregel solution of 3 ml was poured into a 1 X 1 X 4 [cm.sup.3] glass tube; thereafter the glass tube was sealed by a Teflon stopper and put into the heat bath kept at 25[degrees]C. After 24 hours the gels picked out the heat bath. Then, they were dried at 30[degrees]C in an oven. The gels prepared with Co, so-called PAAm-Co gels (2), were prepared for two different concentration of Co(II)acetate, [10.sup.-3] mol/1 (2A), and 0.3 mol/1 (2C). The gels prepared with POH, so-called PAAm-POH gels (3), were prepared with [10.sup.-4] mol/1 concentration of POH. The gels prepared with both Co and POH, so-called PAAm-Co-POH gels (4), were prepared with five different concentration of Co(II)acetate; [10.sup.-3] mol/1 (4A), 0.1 mol/1 (4B), 0.3 mol/1 (4C), 0.5 mol/1 (4D), 0.7 mol/1 (4E) and [10.sup.-4] mol/1 POH for each sample. All the gels (1, 2, 3, and 4) were included the same concentrations of AAm, BIS, APS, and TEMED.

Preparation of Magnetic Hydrogels for Magnetic and Fluorescence Measurements

The magnetic gels were divided into three groups before the measurements. The first group was used without washing, the second and the third groups were washed with pure and high pH (pH = 11) water, respectively. All the gels were dried first and then cut into the pieces with sizes of about 3 X 3 X 2 [mm.sup.3]. Then, all the samples were swelled in sufficient amounts of pure water (controlled swelling) to bring them to certain swelling ratios. 2C was also swelled in [10.sup.-4] mol/1 POH solution ([10.sup.-4] mol/1 POH dissolved in pure water, sample 2Cd) in a similar way.

Fluoresence, Magnetic and FTIR Measurements

The fluorescence spectra of gels were recorded by using a charged-coupled-device (CCD) array camera (Ocean Optics USB2000) (33). The fluorescence measurements were carried out at 90[degrees] position. POH molecules were excited at a wavelength of 400 nm. Magnetic measurements were performed by using SQUID magnetometer (Quantum Design, MPMS XL) at room temperature and under two different external fields, 0.2T and IT. FTIR spectra of gels in powder form were taken by using Perkin-Elmer Spectrum One model FTIR spectrometer. This spectrometer is capable of data collection over a wave-number range of 370-7800 cm . It can be configured to run in single-beam, ratio, or interferogram modes and the best resolution is 0.5 [cm.sup.-1].

RESULTS AND DISCUSSIONS

Magnetic Measurements

Figure 1 represents magnetization of PAAm (1), PAAm-Co (2C), PAAm-POH (3) and PAAm-Co-POH (4C) gels as function of the mass ratio, m/[m.sub.0] (the mass of the gel after swelling/the mass of the dried gel), under the applied magnetic field of IT and 0.2T. The gels were swelled in sufficient amount of water to bring them to certain swelling degrees, and thus the certain mass of the gel. Controlled swelling prevents the diffusion of POH molecules and [Co.sup.+2] ions out of the gel during the swelling processes. As seen from this figure, samples 1 and 3 have diamagnetic behavior which increases slightly with the increasing water content in the gel, so does the mass of the gel. This is due to diamagnetic character of water as discussed in Ref. 37. The gel-composite becomes positively magnetized when ferromagnetic [Co.sup.+2] ions are entrapped in the gels, namely samples 2C and 4C. As seen from Fig. 1, some slight decrease in the magnetization of all gel-composites is observed with increasing water content during the controlled swelling. The characteristic behaviors for the samples do not change with the varying external field.

[FIGURE 1 OMITTED]

It was surprisingly observed that the magnetization of sample 4C was significantly lower than that of 2C although the Co content is the same in each sample. This overall decrease in the magnetization should be due to the interaction between [Co.sup.+2] ions and POH molecules.

The magnetization measurements for washed samples, both for sample 4A and 4C were also carried out after the gels brought to the collapsed state. We observed that the relative change in the magnetization, [DELTA]AM/M = ([M.sub.unwashed] - [M.sub.washed])/[M.sub.unwashed], for 4A and 4C were completely different from each other, ~0% and ~86% for 4A and 4C, respectively.

The EDS (energy dispersive spectra) analyzes were also performed for the unwashed and the washed samples and the results are given in Table 1 where the properties of 4C washed with pure and with high pH water are completely different. After washing 4C with pure water Co-clusters were destroyed and almost all of the [Co.sup.+2] ions washed out the gel (the percentage amount of [Co.sup.+2] ions in the composite was 38% and less than 0.5% before and after washing with pure water, respectively) resulting in a decrease in the magnetization of the composite material as mentioned above. But washing with high pH water did not affect considerably the Co content of the composite. In the washed sample (35%) it was nearly the same as the amount in the unwashed sample (38%). Although the amount of [Co.sup.+2] ions did not change considerably upon washing with high pH water, an abrupt decrease in the magnetization was observed. This decrease might be due to the interaction between huge amount of [OH.sup.-] ions and [Co.sup.+2] ions. For 4A we could not detect the [Co.sup.+2] ions by SEM probably because of the detection limits of the device. But, from the magnetization results it was observed that washing with pure and high pH water did not affect the magnetization of 4A, which indicates that neither pure water nor the high pH water could destroy the Co-clusters in the gels including low Co concentration.
TABLE 1. Average cluster sizes and the average percentage of
Co in the clusters obtained from EDS analyzes of the composite
materials.

Sample Id    Co         POH      Average cluster  Average Co
           (mol/1)    (mol/1)    size ([micro]m)  amount (%)

  2C         0.3         0             <1             17
  2C (a)     0.3         0             NC              -
  4B         0.1    [10.sup.-4]        <0.5           23
  4B (a)     0.1    [10.sup.-4]        NC              -
  4B (b)     0.1    [10.sup.-4]         7             36
  4C         0.3    [10.sup.-4]      NM (c)           38
  4C (a)     0.3    [10.sup.-4]        NC            <0.5
  4C (b)     0.3    [10.sup.-4]        NM             35
  4D         0.5    [10.sup.-4]         3             29
  4E         0.7    [10.sup.-4]         3             24

NM. not measurable; NC. No cluster is observed.

(a) Washed with pure water.
(b) Washed with high pH water (pH - 11).
(c) Since the structure is rod-like, the cluster size is immeasurable.


In addition to magnetic measurements, fluorescence experiments and FTIR measurements were performed on these gels to elucidate the interaction mechanism between [Co.sup.+2] ions and POH molecules. And, SEM images were taken to estimate the size of the Co-clusters.

Fluorescence Measurements

The fluorescence spectra of the gels in collapsed state -the unwashed gels, the gels washed with excess pure water and with excess high pH water (pH =11)- are presented in Fig. 2. A weak fluorescence intensity is observable for the unwashed gels including high Co concentration (4C). However, the intensity becomes considerable for the gel including low Co concentration (4A) (Fig. 2a).

[FIGURE 2 OMITTED]

When sample 4C was washed with pure water, the fluorescence could be observed due to the decreased quenching effect of the [Co.sup.+2] ions because most of the [Co.sup.+2] ions were washed out the gel upon washing (dashed line in Fig. 2b). But washing with high pH water did not change the fluorescence intensity of 4C. Because none of the [Co.sup.+2] ions could be washed out the gel by washing with high pH water as mentioned before, the fluorescence of POH molecules must have still been quenched by the [Co.sup.+2] ions (dashed line in Fig. 2c). However, the fluorescence was not changed considerably for sample 4A except for some relative decrease in the 480 nm side upon washing with both pure and high pH water. This decrease is due to the diffusion of free POH molecules out of the gel upon washing which results in decreasing probability of excimer structure formed by the free POH molecules (29).

Here, it should be noted that the observed peak around 430 nm corresponds, as proved in a recent work (29), to the fluorescence peak of the POH molecules which are chemically bonded to the PAAm strands over OH groups and interacting electrostatically over [SO.sub.3.sup.-] groups, the other peak around 480 nm belongs to the excimer forms of POH molecules. The fluorescence properties of the free, bonded and excimer forms of POH molecules were discussed in detail in Refs. (29), (30), (33). From the fluorescence spectra of 4A and 4C, it can be concluded that both 4A and 4C include bonded POH molecules but the amount of the bonded POH in 4A is higher than the amount in 4C.

FTIR Measurements and SEM Images

FTIR spectra of 2C, 3, 4A, and 4C are given in Fig. 3. The peak at 493 [cm.sup.-1] (corresponding to the metal-oxygen vibrations and the metal-oxygen-hydrogen bending vibrations (38), (39)) observed only for 4C clearly indicates binding of [Co.sup.+2] ions to the POH molecules over OH groups and form Co-clusters via POH bridges. The peaks at 1555 and 1410 [cm.sup.-1] are assigned to the antisymmetric stretching [v.sub.asym](COO) and symmetric stretching [v.sub.asym](COO) vibrations of the free acetate ions, respectively (40). The other peaks in each spectrum are belonging to a typical PAAm (41).

[FIGURE 3 OMITTED]

SEM images of the samples 2C, 4C, and the samples 2Cw (2C washed with water), 4Cw (4C washed with water), 4Cb (4C washed with high pH water) in their collapsed states are presented in Figs. 4 and 5, respectively. The Co-clusters formed in 2C, 4C, and 4Cb arc clearly visible (see arrows). In 2C, they are formed probably due to the ponderamotive forces (37) between the [Co.sup.+2] ions. On the other hand, in sample 4C, they should be formed via both the electrostatic interactions between the empty electronic states of [Co.sup.+2] ions and the free electrons of [SO.sub.3.sup.-] groups, and covalent binding of POH molecules to [Co.sup.+2] ions over OH groups on POH molecules as supported by the magnetic, fluorescence and FTIR measurements. SEM-EDS analysis carried out on these clusters also indicated that these regions composed of [Co.sup.+2] ions together with carbon and oxygen atoms. The EDS analysis for these samples was summarized in Table 1. The EDS analysis showed that the cluster like structures formed in 4Cw do not include [Co.sup.+2] ions, this cluster probably formed due to the heterogenic nature of the PAAm systems. Comparison of Figs. 4b with 5b states that the rod-like structure formed in 4C was destroyed completely and the [Co.sup.+2] ions leaked out the gel upon washing. We observed that when 4C washed with high pH water the rod-like structure observed in 4C was destroyed again but the Co-clusters did not leak out the gel (Fig. 5c). This may be due to neutralization of [Co.sup.+2] ions with [OH.sup.-] ions which may result in aggregation. Surprisingly, for a certain Co concentration, 0.3 mol/1, the globule-like structure (Fig. 4a) of the Co-clusters transforms to a rod-like structure (Fig. 4b) for the samples prepared with the POH molecules. For sample 2C, where most of the Co-clusters washed out the gel upon washing (see Table 1), the polymer structure becomes more porous due to probably washing out of small clusters, which are not connected to the infinite network as seen from Fig. 5a.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

The samples with five different [Co.sup.+2] concentrations were prepared to examine the size dependence of Co-clus-ters on Co concentrations. The average size of these clusters seems changes with the composition of Co and POH contents as seen in Fig. 6. When Co concentration is increased, average size of the clusters increases up to 3 [micro]m and then saturates (see Table 1): for [10.sup.-3] mol/1 Co including gel the average size of the clusters is 0, for 0.1 mol/1 it is less than 0.5 [micro]m, for 0.3 mol/1 it is indefinable because the structure completely changes from globule like to rod-like as seen in Fig. 4b, for 0.5 mol/1 and 0.7 mol/1 it is about 3[micro]m. Here it should be noted that for [10.sup.-3] mol/1 we could not detect [Co.sup.+2] ions from the EDS analysis of SEM images which may be due to insufficiency of the Co content for detection by SEM. As a result, it may be concluded that the average cluster size increases roughly with increasing Co concentration. However, more detailed experiments may be needed for a strict quantitative analysis.

[FIGURE 6 OMITTED]

Magnetization measurements, fluorescence data, FTIR spectra and SEM images reveal the following results; (i) the fluorescence intensity of the POH molecules are quenched considerably in the collapsed state of the gel including especially high Co content (0.3mol/1). (ii) When the gels prepared with [Co.sup.+2] ions and POH molecules together, both the bonded and the free (unbounded) POH molecules exist together in the gel as seen from the fluorescence data where both short wavelength (around 435 nm) and high wavelength peaks (around 480 nm) appeared together as discussed in (29). (iii) The Co-POH clusters in sample 4A are very stable and can not be dispersed in pure and high pH water because the fluorescence intensity and the magnetization did not change considerably after washing process. The stability of these clusters is probably due to that most of the POH molecules belong to these clusters are bonded to the PAAm strands over OH groups. This point can be confirmed by the fluorescence spectra where only the 435 nm-peak corresponding to the bonded POH molecules is observed (29), (30), (33). At the same time, the existence of the 480 nm-peak in addition to 435 nm-peak is an indication that these POH molecules are close to each other, thus, they form excimer corresponding to 480 nm-peak. This special confirmation (both the POH molecules are close to each other and bonded to the PAAm strands from one ends as depicted in Scheme 1) results probably in strong electrostatic interactions between [Co.sup.+2] ions and the POH molecules in geometrically restricted regions. Therefore, the clusters formed in this way become more stable against the pure and high pH water, (iv) In sample 4C, the POH molecules are probably enclosed by [Co.sup.+2] ions due to high Co concentrations. This prevents most of the POH molecules to bind PAAm strands but results in binding of POH molecules to the [Co.sup.+2] ions over OH groups as supported by the FTIR spectrum of 4C. The Co-clusters including unbounded POH molecules which are bound to [Co.sup.+2] ions instead of PAAm strands are unstable against washing with pure water. The clusters are dispersed probably due to water and washed out the gel upon washing with excess water. Therefore, the magnetization decreases abruptly while the fluorescence intensity increases considerably upon washing, where quenching effect decreases due to most of the [Co.sup.+2] ions leaked out the gel. (v) The possible cluster structures formed in the sample 4B-4E and 4A, and the effect of the washing on these clusters can be modeled as represented in Scheme 1. (vi) SEM images give a rough estimation for average size of the clusters whereas it is not possible to interpret the interactions. However, fluorescence, magnetic and FTIR measurements reveal the structure depicted in Scheme 1.

[ILLUSTRATION OMITTED]

CONCLUSION

The results of this work showed that the clusters formed between [Co.sup.+2] ions and bonded POH molecules via ionic interactions as represented in Scheme 1 are very stable at low Co concentration ([10.sup.-3] mol/l). These stable clusters formed between [Co.sup.+2] ions and POH molecules can be obtained only during the polymerization. This is due to the fact that POH molecules bind chemically to the PAAm strands over OH groups during the polymerization. Consequently, this results in a considerable decrease in the magnetization of the gel; the restricted geometry due to specific interaction between Co, POH and polymer matrix inhibits the alignment of dipole moments of [Co.sup.+2] ions involve in this clusters. When the POH molecules were diffused into the PAAm-Co gel after the preparation, no change in the magnetization was observed and they behave like flocculants and decrease the average size of the Co-clusters dispersing them via attractive electrostatic interactions. Surprisingly, for a certain Co concentration, 0.3 M, the globule-like structure of the Co-clusters transforms to a rod-like structure, which results again in a considerable decrease in the magnetization, for the samples prepared with the POH molecules. The mechanism underlying the formation of the rod-like structure is not known at present and is open for the future researches in this field.

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Ali Gelir, (1) Esra Alveroglu, (1) Mustafa Tulun, (1) Yasar Yilmaz, (1) Huseyin Sozeri (2)

(1) Department of Physics, Istanbul Technical University, Maslak 34469, Istanbul, Turkey

(2) TUBITAK-UME, National Metrology Institute, PO Box 54 TR-41470, Gebze/Kocaeli, Turkey

Correspondence to: H. Sozeri; e-mail: huseyin.sozeri@ume.tubitak.gov.tr

DOI 10.1002/pen.21597

Published online in Wiley InterScience (www.interscience.wiley.com).

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Author:Gelir, Ali; Alveroglu, Esra; Tulun, Mustafa; Yilmaz, Yasar; Sozeri, Huseyin
Publication:Polymer Engineering and Science
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Date:Apr 1, 2010
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