Methacrylic Acid Based Polymer Networks with a High Content of

Article pubs.acs.org/JPCC

Methacrylic Acid Based Polymer Networks with a High Content of Unfunctionalized Nanosilica: Particle Distribution, Swelling, and Rheological Properties Vesna V. Panic,† Pavle M. Spasojevic,*,†,‡ Tijana S. Radoman,† Enis S. Dzunuzovic,§ Ivanka G. Popovic,§ and Sava J. Velickovic§ †

Innovation Center of the Faculty of Technology and Metallurgy, University of Belgrade, 4 Karnegijeva Street, RS-11000 Belgrade, Serbia ‡ Faculty of Technical Sciences, University of Kragujevac, 65 Svetog Save Street, RS-32000 Cacak, Serbia § Faculty of Technology and Metallurgy, University of Belgrade, 4 Karnegijeva Street, RS-11000 Belgrade, Serbia S Supporting Information *

ABSTRACT: The poor stability and tendency to agglomerate of unfunctionalized nano-SiO2 in the presence of ionic species presents a challenge for preparing poly(methacrylic acid)/nano-SiO2 nanocomposite (NC) hydrogels with desired strength and swelling capability. We proposed a facile and eco-friendly method for the preparation of PMAA/SiO2 NC hydrogels using unfunctionalized silica nanoparticles (NPs) in the form of a suspension. SEM and TEM analyses showed that the NP distribution in the polymer matrix highly depended on the particle concentration. At lower concentrations (up to 13.9 wt %), the NPs were uniformly dispersed as single nanoparticles. With an increase in NP concentration, homogeneously dispersed nanoscale aggregates were formed, while a further increase in the silica concentration led to the formation of homogeneous structures consisting of mutually interacting nanosilica particles coated with PMAA. Swelling experiments confirmed that the silica NPs behaved as adhesive fillers that interacted with PMAA chains, causing the formation of a thin polymer layer strongly adsorbed at the particle interface. The thicknesses of the adsorbed polymer layer, as well as the swelling kinetic parameters, were strongly influenced by nanoparticle size and concentration. Combining nanosilica and PMAA in the form of a soft hydrogel network provided stabilization of the NPs and ensured better mechanical properties of the obtained NC hydrogels compared to pure polymer matrix. The optimal loadings, necessary to ensure the most improved dynamical-mechanical properties, were found in the case of the formation of homogeneously dispersed, nanosized silica aggregates in a PMAA matrix.

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ranging from drug delivery,7,8 tissue engineering,9 agriculture,10 pH-sensitive actuators,11−13 environmental applications,14,15 etc. On a molecular level, highly swellable PMAA hydrogels are elastic cross-linked networks embedded in an excess aqueous solution. The lack of an efficient energy dissipation mechanism and the irregular distribution of cross-linking points in such structures make these hydrogels very brittle.16 Due to poor mechanical properties, methacrylic hydrogels proved to be too weak to withstand the high levels of stress and strain needed for various applications, which threatened to limit their further development. During the past decade, several groups have proposed various strategies to improve the mechanical properties of soft networks including methacrylic based hydrogels.17 The main strategies include the preparation of interpenetrating or semi-interpenetrating polymer net-

INTRODUCTION Hydrogels are polymer materials capable of absorbing large quantities of water without dissolving. Polymer chains are connected by covalent bonds generating a three-dimensional network characterized by a very small cross section of the fibers (100−500 nm), which leads to very large internal surface areas.1 Selection of the appropriate monomer can additionally ensure stimuli-sensitive behavior, nontoxicity, biological inertness, flexibility, the possibility to tailor physicochemical properties, preservation of their shape, etc. Moreover, the ease of preparation and the ability to be prepared at various dimensions have made hydrogels a frontrunner in many fields.2−6 Efforts have been dedicated to the study of hydrogels carrying ionizable groups, such as poly(methacrylic acid) (PMAA) based hydrogels. These types of hydrogels might recognize a stimulus as a signal, judge the magnitude of that signal, and then perform an abrupt change in direct response. PMAA hydrogels, as smart materials capable of responding to environmental pH changes, have ubiquitous applications © 2014 American Chemical Society

Received: February 27, 2014 Revised: November 30, 2014 Published: December 5, 2014 610

dx.doi.org/10.1021/jp5020548 | J. Phys. Chem. C 2015, 119, 610−622

The Journal of Physical Chemistry C

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works18−20 and double-network structure,21 the inclusion of micro- and nanofillers,22−25 increase in the cross-linker concentration,26 copolymerization with hydrophobic monomers or rigid cyclic monomers,27 etc. The incorporation of nanometer-sized particles in the polymer matrix has attracted a considerable interest in the way that these types of structures can imprint new properties into a polymer formulation.28−32 In 2002, Haraguchi and Takehisa33 reported the creation of a novel nanocomposite (NC) hydrogel with a unique organic−inorganic network structure and introduced the concept of incorporation of inorganic particles into polymer matrix to the field of soft hydrogel materials. Among various integrated organic−inorganic nanoparticle systems, the systems based on silica nanoparticles (NPs) occupy an important position in hybrid polymer formulations due to their size-selective synthesis, the ease of organic functionalization, stable chemical properties, and excellent biocompatibility. Furthermore, being a low cost material and generally recognized as safe (GRAS) by the US Food and Drug Administration (FDA), silica NPs are arising as the preferred component in materials with a wide range of potential applications.34,35 However, nanosilica by itself is only stabilized in solution by charge repulsion which, in vivo, for example, is interfered with opsonins causing aggregation and exclusion by the reticuloendothelial system.36 Combining nanosilica and polymer in the form of a soft hydrogel network could provide the stabilization of NPs and ensure better mechanical properties of the obtained NC hydrogels compared to pure polymer matrix. At the same time, the maintenance of significant properties of both components, as the ability to swell, porosity, the availability of polymer and silica active sites, etc., makes NC hydrogels very promising materials. The majority of methacrylic nanocomposite hydrogels with improved mechanical properties were synthesized using nanofillers which establish a strong interaction with the polymer network, such as exfoliated clay, ferric oxide, etc.37,38 Conversely, the replacement of these nanofillers with other inorganic particles such as silica NPs has not provided comparable properties.39 Low or no improvement in the mechanical properties of PMAA composite hydrogels with nanosilica is commonly described as the result of high agglomeration of the silica NPs. In order to overcome these drawbacks, most reported PMAA/SiO2 NC materials have been fabricated by modification of the surface of the NPs. Modification of the nanosilica surface has been generally performed either by grafting macromolecules such as block copolymers or PEG onto its surface or by absorption or reaction with small molecules, such as silane coupling agents, thiols, carboxyl, or phosphonate groups.40−45 In addition, for the specific use of hybrid materials under biological conditions, the membrane mimetic functionalization of silica, based on phospholipids, has been applied.36 All the strategies employed though have limitations. These methods mainly involve a rather sophisticated chemistry which is not off the shelf. Furthermore, the use of toxic solvents and harsh conditions (e.g., long-lasting reactions) can make the material unsuitable for specific applications, e.g., tissue engineering. Modification with macromolecules can also give low-density surface coatings which can adversely affect the application for which the nanoparticle was designed. In this paper, we prepared PMAA/SiO2 nanocomposite hydrogels

using a simple method involving only water, monomer, crosslinker, initiator, and silica. We made use of unfunctionalized silica NPs in the form of a suspension. The key properties of the derived materials were investigated as a function of silica content and particle size. Up-to-date experience in the field of nanomaterials shows us that establishing correlations between the structure and properties of a material from one point of view and the parameters of synthesis and material properties, from the other, is essential. At the present time when nanoscale is imperative, we consider that it is necessary to deal at the same time with the structure of methacrylic/silica based nanocomposites on the nanoscale and the effect of nanofiller on their macroscopic properties.

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EXPERIMENTAL METHODS Materials. Methacrylic acid (MAA) (99.5%) was supplied from Merck KGaA, Darmstadt, Germany. The silica suspensions Dispercoll S 3030 (30 wt %, pH 10) and Dispercoll S 4510 (45 wt %, pH 10) with an average diameter of amorphous silica NPs of 9 and 30 nm and a specific surface area of 300 and 100 m2 g−1, respectively, were kindly supplied by Bayer Material Science, Leverkusen, Germany. N,N′-Methylenebis(acrylamide) (MBA) (p.a.) and sodium hydroxide (p.a.) were obtained from Aldrich Chemical Co., Milwaukee, WI. The initiator, 2,2′-azobis-[2-(2-imidazolin-2-yl)propane] dihydrochloride (VA-044) (99.8%) was supplied by Wako Pure Chemical Industries, Ltd., Osaka, Japan. All chemicals were used as received. Synthesis. The synthesis of PMAA/SiO2 NC hydrogels was carried out via free radical polymerization by using the modification of the procedure described in detail previously.46 First, methacrylic acid solutions, prepared with 5.00 g of MAA and given amounts of distilled water (Table 1), were Table 1. Feed Composition and Theoretical Content of Nanosilica in Dry NCH

a

sample

SiOX2/MAA (g/g)

H2O (g)

Dispercoll S (g)

mSiO2,theora (%)

PMAA NCH9-1 NCH9-2 NCH9-5 NCH9-10 NCH9-15 NCH30-1 NCH30-2 NCH30-5 NCH30-10 NCH30-15

0.05 0.10 0.25 0.50 0.75 0.05 0.10 0.25 0.50 0.75

17.63 16.80 165. 0 13.5 9.30 5.13 17.1 16.5 14.8 12.1 9.30

0 0.833 1.667 4.167 8.33 12.5 0.56 1.11 2.78 5.56 8.33

13.9 17.1 25.4 36.0 44.0 13.9 17.1 25.4 36.0 44.0

mSiO2,theor - theoretical content of nanosilica in dry NC hydrogel.

neutralized with the equimolar amount of sodium hydroxide in order to minimize the tendency of SiO2 NPs to coagulate. Afterward, silica suspensions were added very slowly, again, in order to reduce their tendency to gel. The cross-linker (MBA) was subsequently added into the reaction mixtures (0.0358 g) which were then stirred for approximately 20 min at room temperature to ensure homogeneity. Immediately after the addition of the initiator (0.0113 g) under vigorous stirring, the reaction mixtures were poured into plate molds (12 × 12 × 2 mm3) and placed into a drying oven at 80 °C, for 3 h to 611



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Figure 1. FTIR spectra of PMAA/SiO2 NC hydrogels.

The same procedure was utilized to determine the swelling kinetics. The immersed discs were removed from the water at predetermined time intervals, weighed (mt), and put back into the water. The water uptake at time t (SDt) was calculated by using mt instead meq in eq 1. Rheological measurements of the hydrogel formation were performed using a Rheometrics RMS 605 mechanical spectrometer operating in shear mode, parallel plate geometry. After reaching equilibrium in distilled water, the hydrogels were cut into 2 mm thick disks with a diameter of 25 mm. The storage modulus, G′, the loss modulus, G″, and the damping factor, tan δ, were recorded as a function of frequency (varied from 0.1 to 100 rad s−1) at a shear strain of 10% where samples were in the linear viscoelastic range at 20 °C. For each individual disk, the data were averaged over three consecutive runs. Three disks were measured for each type of NC hydrogel.

complete the reaction. The derived hydrogel sheets were taken out from the molds, sliced to disks (10 mm in diameter and ca. 2 mm thick), and immersed in distilled water. The water was changed in the next 7 days in order to remove the sol fraction of polymer, unreacted monomer, and unbound nanoparticles. The washed-out NC hydrogels were dried to a constant weight at 40 °C and then stored in a vacuum desiccator before use. The obtained NC hydrogels were denoted as NCHX-Y, where X represents the diameter of SiO2 nanoparticles and Y the SiO2 wt % in the initial reaction mixture. Methods. The Fourier transform infrared spectroscopy (FTIR) measurements were done on a Bomem MB100 FTIR Spectrometric Analyzer using KBr pellets. The morphology of NC hydrogels (surface and cross section) was studied by using a Tescan MIRA 3 XMU fieldemission gun scanning electron microscope (FEG-SEM) operating at 20 kV. Prior to scanning, all the samples were swollen to equilibrium in distilled water, freeze-dried, and coated with Au−Pd alloy using a POLARON SC502 sputter coater. Samples for transmission electron microscopy (TEM) were prepared by embedding a small piece of the hydrogel in epoxy resin (Struers resin, Struers GmbH, The Netherlands). Ultrathin sections (80−120 nm) were cut with a Leica EM UC6 ultramicrotome (Leica, Vienna, Austria). They were floated off on a water surface. After picking them up, the sections were scanned on a Philips CM200 STEM microscope with a LaB6 electron gun and an Olympus SIS Megaview II camera by applying an acceleration voltage of 80 kV. The actual SiO2 content in the synthesized NC hydrogels as well as the thermal stability were determined by using a TA Instruments Q500 thermogravimetric analyzer. The samples (5−10 mg) were heated from 20 to 1000 °C at a heating rate of 10 °C min−1 under a nitrogen atmosphere (flow rate 10 cm3 min−1). To evaluate the swelling behavior of NC hydrogels, the same weights of dried hydrogel discs (0.02 g) were immersed in distilled water or 0.5 M NaCl aqueous solution at room temperature for a day to reach equilibrium. Then, the swollen samples were removed from water and the excess of water was carefully removed by filter paper. The equilibrium swelling degree was calculated as SDeq =

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RESULTS AND DISCUSSION PMAA/silica NC hydrogels were synthesized using unfunctionalized silica NPs of 9 and 30 nm in diameter. Silica was introduced in the form of a suspension. Setting the appropriate reaction parameters described in the Experimental Methods prevented NP coagulation and provided compact and uniform materials. The obtained NC hydrogels were characterized by FTIR, SEM, swelling experiments, TG, and DM analysis to investigate the effect of NP size and concentration on hydrogel morphology and properties. FTIR spectroscopy was used to investigate the structure of the synthesized nanocomposite hydrogels and to confirm the presence of both the PMAA and the SiO2 components. The resulting spectra are shown in Figure 1 together with the spectra of pure PMAA hydrogel and Dispercoll S. The intensities of the bands are in relative units and are omitted because the spectra have been translated vertically to avoid overlapping and to ensure clarity. The FTIR transmittance spectra of SiO2 showed characteristic bands at 1095, 797, and 468 cm−1 corresponding to the stretching, bending, and out-ofplane SiO bonds, respectively.47 The neutralization of PMAA hydrogel was confirmed with a strong band at 1541 cm−1 characteristic for carboxylate anion.48 The band at 1652 cm−1 corresponds to the CO stretching vibration of the carboxylic group, while the band at 1204 cm−1 corresponds to the carboxyl (CO) stretching vibration. Other bands presented in the PMAA spectrum are an intensive band around 3450 cm−1, a double band in the region 2930−3000 cm−1, and a sharp band at 1396 cm−1, corresponding to the stretching of hydroxyl (COH) groups, methylene (CH) groups, and

meq − m0 m0

(1)

where m0 is the mass of the dry disc, meq is the mass at the equilibrium state, and SDeq is the equilibrium swelling degree. 612



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structure with large voids presented in Figure 3a showed the macroscopic structure of pure PMAA hydrogel. This structure was retained in all of the NC hydrogels, regardless of the size and amount of SiO2 nanoparticles (corresponding micrographs are shown in Figure S1, Supporting Information). We studied the state of dispersion of nanoparticles in the PMAA matrix in more detail, as it was expected to have a significant effect on the properties of the synthesized NC hydrogels (Figure 4). It was obvious that the increase in nanosilica content significantly changed the morphology of the NC hydrogels at the nanoscale. In the case of samples NCH9-1 and NCH9-5, the silica nanoparticles were uniformly distributed in the PMAA matrix. In the first case, the NPs were embedded in PMAA without any aggregation, and in the second case, small aggregates, composed of few nanoparticles, could be seen. The morphology of NCH9-2 was similar to the morphology of NCH9-5. The aggregate sizes and shapes in NCH9-2 were similar to the aggregates in NCH9-5 but less densely packed. The insertion of larger NPs (Figure 4b) led to the formation of structures with uniformly dispersed aggregates even at the lowest nanosilica content. The size of the formed aggregates was between 50 and 200 nm (Figure S2, Supporting Information). As seen in Figure 4, the structures of NCH9-15 and NCH3015 consisted of closely packed nanosilica particles, probably connected to each other into larger domains. Bearing in mind the investigated properties of these two materials, high transparency, the absence of nanosilica leaching (see Table 2), and swelling at so high silica content (see Figure 7), it could be assumed that the NPs and/or their nanoscale aggregates were gathered or coated with PMAA. In order to clarify the inner morphology of NCH9-15 and NCH30-15 samples, TEM analysis was included in the research. The recorded TEM micrographs are presented in Figure 5. The TEM micrographs showed the absence of a high level of agglomerization and phase separation. Domains with higher and lower silica concentrations could be observed. In the case of NCH30-15 (larger NPs), the noticed domains were mostly formed of silica nanoparticles, closely packed but spaced apart by a thin polymer coating layer (see Figure S3, Supporting Information). On the other hand, the domains in NCH9-15 contained small aggregates of nanosilica uniformly distributed in the PMAA matrix. This could be attributed to the fact that smaller NPs have a larger surface area and, therefore, a higher tendency for agglomeration.50 The illustrated morphologies are in accordance with the previously conducted investigation of optical transparency.

CH3 bending vibrations, respectively. From the FTIR spectra of NC hydrogels, it could be observed that the CO stretching vibration of carboxylic anions in PMAA tended to shift to higher wavenumbers, suggesting the formation of hydrogen bonds between the COO− groups of methacrylic acid and the OH groups at the silica surface. In addition, with increasing silica content in the hydrogels, there was an increase in intensity of the SiOSi stretching vibration band at 1115 cm−1 and out-of-plane SiO bond band at 468 cm−1, as marked in Figure 1. Optical transparency of the PMAA/SiO2 NC hydrogels with a relatively high content of nanosilica of 13.9−44.0 wt % (calculated to the dry state, Table 1) was observed. This interesting but also unique characteristic of this kind of polymeric materials is presented in Figure 2.

Figure 2. Optical transparency of PMAA/SiO2 NC hydrogels and unfilled PMAA hydrogel.

The transparency of NC hydrogels with a nanosilica content up to 25.4 wt % (NCH9-1, -2, and -5 and NCH30-1, -2, and -5) was not much affected, when compared to pure PMAA hydrogel. The mentioned NC hydrogels were completely transparent because they have phase domains smaller than the wavelengths of visible light,49 and for that reason, transparency was retained even though the content of silica was increased. Further increase in the silica content (up to 44 wt %) led to the manifestation of turbidity, probably duo to the formation of larger-sized silica aggregates that scatter visible light. The aforementioned presumptions were further investigated by SEM analysis through examination of the interior morphology of the synthesized NC hydrogels. Cross-sectional FEG-SEM micrographs of lyophilized samples, previously swollen to equilibrium, are presented in Figures 3 and 4. The interconnecting, three-dimensional porous

Figure 3. FEG-SEM micrographs of PMAA hydrogel. 613



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Figure 4. FEG-SEM micrographs of NC hydrogels.

stages were noticed. In the first stage (below 250 °C), weight loss occurred due to the evaporation of physically bonded water molecules and intra- and intermolecular anhydride formation, followed by the elimination of water between pairs of carboxyl groups.51 It could be seen from Figure 6b that the DTG curves of neat PMAA and NCH30 samples had several overlapping peaks in the first decomposition region. At the same time, samples NCH9-5 and NCH9-15 had only one peak in this region, at 110 and 140 °C, while sample NCH9-10 had two peaks at 125 and 195 °C. This indicated that the presence of smaller NPs influenced PMAA anhydride formation or the diffusion of formed water through the sample, which would be the subject of further investigations. The second weight loss stage, starting from about 450 °C, was attributed to intensive decomposition where fragmentation of the poly(methacrylic anhydride) backbone chain was dominant. However, no significant increment in the onset decomposition temperature for all the NC hydrogels implied the absence of strong chemical bonding between the NPs and the polymer network.52 The third weight loss stage, observed in the PMAA hydrogel, NCH9-5, and NCH30-5, started at about 800, 600, and 650 °C, respectively. It was attributed to the decomposition of hydrocarbons and aromatic compounds produced by the aromatization reaction associated with the previous PMAA decomposition stage. The residue of PMAA hydrogel thermal decomposition (10.5%) consists of free carbon and highmolecular-weight hydrocarbons.53 As FEG-SEM and TEM analyses showed, in the case of NC hydrogels with a silica content higher than 24.5 wt %, NPs were uniformly arranged in the whole PMAA matrix, practically without large domains of bulk polymer. The formed structures prevented the formation of stabile hydrocarbons and aromatic compounds in the temperature range from 550 to 800 °C. This most likely happened because polymer domains around the nanoparticles acted differently compared to the bulk PMAA in pure hydrogel and decomposed directly to free carbon and hydrocarbon structures. The fact that NCH9-5 and NCH30-5 also have the third decomposition stage, less pronounced and shifted to lower temperatures compared to pure PMAA hydrogel, further confirms the previous claim.

Table 2. Thermogravimetric Data and Theoretical and TG Amounts of SiO2 in the NC Hydrogels sample

T5 (°C)

T10 (°C)

PMAA

102

135

sample

T5 (°C)

T10 (°C)

NCH9-1 NCH9-2 NCH9-5 NCH9-10 NCH9-15 NCH30-1 NCH30-2 NCH30-5 NCH30-10 NCH30-15

107 104 100 108 111 104 111 117 141 189

139 135 131 143 145 141 147 157 182 224

T40 (°C)

RPMAA (%)

T40 (°C)

459 mSiO2,theor (%)

10.5 mSiO2,TGA (%)

474 475 483 488 508 469 474 473 492 515

13.9 17.1 25.4 36.0 44.0 13.9 17.1 25.4 36.0 44.0

11.4 14.8 22.6 32.7 41.9 11.7 15.0 24.7 33.2 42.8

Figure 5. TEM micrographs of NCH9-15 (left) and NCH30-15 (right).

Thermal stability plays an important role in determining both the technical applications and processing conditions of polymeric nanocomposites and is usually investigated by TGA. The decomposition (TG) curves of representative samples are shown in Figure 6a and the corresponding DTG curves in Figure 6b. The weight loss of pure PMAA hydrogel and NC hydrogels with silica loading up to 24.5 wt % (NCH9-5 and NCH30-5, showed as representatives) occurred in three stages, while, in the case of NC hydrogels with higher nanofiller content, two 614



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Figure 6. (a) TG and (b) corresponding DTG curves of PMAA/SiO2 NC hydrogels with 9 nm (left) and 30 nm (right) SiO2 nanoparticles.

nanocomposites with relatively high NP content (50 wt %).56 Popovic and Katsikas have found that NPs had a catalytic effect on certain thermal decomposition stages, as evidenced by the TGA of some polymethacrylates.57 Considering all, we could assume that the incorporated silica nanoparticles affected the NC hydrogel thermal properties in two opposite ways. (1) First, due to the generated interactions between the NPs and the PMAA matrix, the thermal stability of all the NC hydrogels was increased compared to pure PMAA. The higher the silica content and the smaller the particle radius, the more interactions were established and the thermal stability increased. (2) On the other hand, there was a catalytic effect of the silica NPs on the PMAA thermal degradation and that lowered the NC hydrogel thermal stability. Consequently, the higher silica content and smaller particle radius provided a stronger catalytic effect and the thermal stability decreased. Thus, the incorporation of smaller NPs enhanced both phenomena more than the incorporation of larger ones. The overall effect of the inclusion of smaller silica NPs on the thermal stability of NC hydrogels was then less positive than in the case of larger NPs, due to the higher neutralization of these two phenomena. Therefore, the thermal stability of the synthesized materials decreased in the following order: NCH30 > NCH9 > pure PMAA hydrogel. Considering the application of hydrogels as a special type of polymeric materials, their imperative characteristic is swelling behavior. As the successful incorporation of unmodified nanosilica in the PMAA matrix has been demonstrated in the previous section, special attention was given to the influence of the size and amount of nanoparticles on the swelling of the derived NC hydrogels.

As we mentioned earlier, the NC hydrogels were fabricated using organically unmodified silica nanoparticles. Although this method is very convenient from the processing point of view, it has some potential threats, such as the leaching of silica NPs from the polymer network. To explore the possible leaching of SiO2 during the washing out of hydrogels, the TG method was used to calculate the actual silica content in the NC hydrogels after washing out.54 It is easy to show that the content of SiO2 in the NC hydrogels can be calculated from mSiO2,TGA =

100RPMAA − R c RPMAA − 1

(2)

where RPMAA is the residue (wt %) at 1000 °C of the starting PMAA hydrogel, mSiO2,TGA is the weight percentage of SiO2 in the NC hydrogels, and Rc is the residue at 1000 °C (wt %) of NC hydrogels. The data obtained from the TG curves of PMAA and NC hydrogels are listed in Table 2, including the SiO2 content calculated by using eq 2 and the temperatures at which 5% (T5), 10% (T10), and 40% (T40) of decomposition occurs. It could be seen that the amounts of SiO2 determined by using the data acquired by TGA were similar to the expected values. Thus, it could be concluded that the leaching of silica nanoparticles did not occur at a noteworthy level. The incorporation of nanosilica, in the whole investigated range, shifted T5, T10, and T40 to higher values, indicating the higher thermal stability of all the NC hydrogels compared to pure PMAA hydrogel. Surprisingly, this shift was much more pronounced in the case of bigger NPs. In most cases, the presence of smaller NPs, having a larger surface area, provides nanocomposites with more resistance to decomposition at high temperatures.55 Although unusual, the obtained reverse trend has already been reported for other methacrylate based 615



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Figure 7. Equilibrium swelling degree of PMAA/SiO2 NC hydrogels in distilled water (left) and 0.5 M NaCl aqueous solution (right); solid symbols calculated to mass of dry NC hydrogels, and open symbols calculated to mass of MAA.

latter was also verified by recalculating the equilibrium swelling data of the polymeric network. The decrease in the calculated SDeq values with increasing silica content comes from the contribution from the immobilized polymer chains constrained by the nanofiller surface and signifies higher polymer/ nanoparticle interactions and the restriction of movement of polymer chains between adjacent cross-linking points. Bearing in mind the above presented results, the role of silica nanoparticles could be regarded as analogous cross-linking points, the increasing number of which decreased the swelling ratio of the NC hydrogels. Since the same masses of silica NPs of different sizes were used in the corresponding NC hydrogels from the two series, they contain a different number of particles; the smaller the size of the NPs, the larger number of particles is included. Thus, when comparing the swelling of samples with the same content of silica, fewer physical crosslinks (i.e., fewer NPs used in the NC hydrogel preparation) were found to be the reason why the larger diameter of the NPs led to higher values of SDeq. An analytical expression for the equilibrium swelling degree of a filled elastomer (SDeq), derived by Lequeux and coworkers,60 is given as a function of the equilibrium swelling degree of the pure polymer matrix (SDeq,PMAA) and the volume fraction of particles in the initial state (φSiO2):

To deal with the behavior of polymeric nanocomposites, many authors have included the perturbed region in the modeling of their properties.58,59 If the polymer in the vicinity of the NPs exhibits distinct properties from those of the bulk matrix, due to polymer/ nanoparticle interactions, it is expected that the NC swells less than the unfilled matrix. On the contrary, however, if there is no adhesion at all between the polymer matrix and the NPs, NPs do not restrict the swelling of the network and the filled hydrogel can have an even higher equilibrium swelling degree due to the formation of cavities filled with solvent around the particles.60 Measurement of the equilibrium swelling degree (SDeq) was, therefore, used as a test to study the presence and the density of interactions generated between the PMAA network and the nanoparticles in the synthesized NC hydrogels. Figure 7 (left) shows that the equilibrium swelling degrees of both types of nanocomposite hydrogels in water were decreased compared to pure PMAA hydrogel. Bearing in mind that SiO2 does not swell, the effect of inclusion of SiO2 on the swelling behavior of the derived materials was estimated through the reduced values of the equilibrium swelling degree calculated for the polymeric matrix (excluding the NP weight) (SDeq,MAA). In this case, a steep decrease in the SDeq,MAA after insertion of the SiO2 nanoparticles was followed by a gradual decrease in its values with a further increase in the amount of silica. This could be explained by the rather complex effect of nanosilica. On the one hand, polymer/nanoparticle interactions increase the cross-linking density and hinder swelling, while, on the other hand, embedded silica nanoparticles increase the osmotic pressure and enhance swelling because of counterions from the partial dissociation of silanol groups.23 Obviously, as the SDeq of NC hydrogels were lower compared to the SDeq of neat PMAA hydrogel, the first effect was stronger. If there were no second effect, the SDeq decrease with increasing SiO2 concentration would be even more pronounced. In order to only assess the impact of interactions between PMAA and nanosilica, the equilibrium swelling degree of NC hydrogels was also measured in high ionic strength solution (0.5 M NaCl) where the electrostatic interactions were screened. It is clearly displayed in Figure 7 (right) that the swelling of NC hydrogels monotonously decreased in 0.5 M NaCl with increasing amount of nanosilica, revealing the formation of additional physical cross-links.23,61 As the amount of polymer was kept constant in all of the NC hydrogels, the

⎛ SD1/3 eq,PMAA − 1 SDeq (φSiO ) = SDeq,PMAA ⎜⎜1 − 2 SD1/3 ⎝ eq,PMAA ⎞3 3(1 − ν) ⎟ × ′ ) ⎟⎠ 2(1 − 2ν) + (1 + ν)(φcp /φSiO 2

(3)

where ν is the Poisson coefficient (taken equal to 0.5), φcp the volume fraction at random close packing (taken equal to 0.64), and φ′SiO2 the corrected volume fraction of SiO2. The volume fraction of SiO2 must be corrected because of the existence of a polymer coating layer which is strongly adsorbed at the particle interface, and it has different swelling behavior compared to the bulk polymer and has a thickness t (nm) 3 ⎛ t⎞ ′ = φSiO ⎜1 + ⎟ φSiO 2 2 r0 ⎠ ⎝

(4)

where r0 is the radius of neat particles (nm).60 616



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Figure 8. Superposition of experimentally obtained SDeq values in (a) water and (b) 0.5 M NaCl aqueous solution (■) and master curves derived from the Lequeux model for different thicknesses of the adsorbed polymer layer (lines).

Table 3. Thickness of the PMAA Layer Adsorbed on Silica Nanoparticles According to the Lequeux Model, Kinetic, and Dynamical-Mechanical Parameters

a

SiO2/MAA

tH2O (nm)

tNaCl (nm)

V H2 O (cm3 g−1)

VNaCl (cm3 g−1)

k × 102 (min−1)

n

teq (h)

R2

Gp′ (Pa)

Gp″ (Pa)

Gc′ a (Pa)

Gred′ (kPa)

PMAA NCH9-1 NCH9-2 NCH9-5 NCH9-10 NCH9-15 NCH30-1 NCH30-2 NCH30-5 NCH30-10 NCH30-15

7.5 5.8 3.4 2.7 2.0 13.1 9.4 5.5 1.6 0.8

4.8 3.1 2.8 2.4 2.1 12.2 9.5 8.2 5.0 3.4

6.8 4.1 1.7 1.2 0.8 2.1 1.2 0.6 0.1 0.06

2.9 1.4 1.2 1.0 0.8 1.9 1.3 1.0 0.5 0.3

0.20 0.73 1.52 6.94 4.61 5.49 0.25 0.47 1.56 1.91 5.91

1.33 0.95 0.90 0.82 0.80 0.70 1.07 1.05 1.10 1.08 1.09

5 4.5 5 4 3.5 1 6 6 5.5 4 2

0.980 0.982 0.998 0.983 0.989 0.988 0.996 0.993 0.996 0.991 0.981

692.7 2830 4300 6462 1980 836 2662 3191 3578 1421 769

73.3 63.5 240 278 101 42.3 283 357 776 277 43.1

693 701 710 740 804 885 701 710 740 804 885

6.18 21.9 31.9 45.5 11.5 4.45 22.5 26.5 28.0 10.5 5.31

The Guth−Gold model66 predictions for the elastic modulus.

could be attributed to the fact that at higher SiO 2 concentrations silica−silica interactions became dominant over PMAA−silica interactions; i.e., the NC properties were mostly controlled by the formation of a complex filler structure rather than by the adsorption of polymer chains on the surface of the nanoparticles.62 In the case of NC hydrogels with smaller NPs and lower NP concentration (NCH9-1, NCH9-2, and NCH9-5), there was a clear difference between tH2O and tNaCl, while in the case of samples with higher NP concentrations (NCH9-10 and NCH9-15), the values of tH2O and tNaCl became very similar. As the thickness of the formed polymer layer reflects the strength of the PMAA−silica interactions, it was

Master curves derived from the presented model together with the experimental data obtained in water and 0.5 M NaCl aqueous solution are presented in Figure 8. According to the applied model, it is clear that in the case of all the investigated NC hydrogels there was an adsorbed polymer layer around the nanoparticles that affected the swelling of the nanocomposites, because otherwise there would be good agreement with the master curve for t = 0. The size and the amount of incorporated nanofiller had a strong effect on the thickness of the formed polymer layer, as presented in Table 3. The obtained results show that, in the case of both sizes of SiO2 nanoparticles, an increase in particle concentration led to a decrease in the thickness of the formed layer. This 617



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Figure 9. Frequency dependencies of (a) the storage modulus and (b) the damping factor of PMAA/SiO2 NC hydrogels swollen to equilibrium.

where n is a characteristic exponent describing the mode of the penetrant transport mechanism, coefficient k is a characteristic rate constant that relies on the hydrogel structure, t is the swelling time, and the other symbols were previously described. The applied equation was applicable from the beginning to 70−85% of the swelling process, with very good correlation (Table 3). The calculated kinetic parameters together with the time required to reach equilibrium swelling (teq) are listed in Table 3. The addition of silica nanoparticles notably increased the rate of swelling and decreased the time required to reach equilibrium. Analyzing values of parameter n, it could be concluded that the water transport mode through the hydrogel was affected by the incorporation of nanosilica and also by the size of the inserted particles. The swelling kinetics of pure PMAA hydrogel was controlled by the rate of the network expanding (i.e., relaxation of the elastic polymer network). NC hydrogels with 30 nm sized silica had n values close to 1, indicative for case II water transport also controlled by the rate of relaxation of the polymer network but time independent. The inclusion of 9 nm sized NPs changed the type of water penetration to non-Fickian, revealing swelling kinetics controlled by two processes, the relaxation of polymer chains and diffusion.63 It was also notable that, in the case of NC hydrogels with smaller NPs, the parameter n was much more sensitive to the change of the silica concentration. Reducing its value from 0.95 to 0.70 with increasing SiO2 content, it demonstrated the increasing influence of the rate of water diffusion on the overall swelling rate, which is in accordance with the predicted role of NPs as additional cross-links. Unlike polymer microcomposites, where the dynamicmechanical behavior is mainly determined by the characteristics of the filler and matrix and their respective concentrations, the

expected that tH2O > tNaCl because the electrostatic interactions were screened in high ionic strength solution. This trend was obeyed by NCH9-1, NCH9-2, and NCH9-5, indicating a significant fraction of the PMAA−silica interactions in the overall interactions when silica NPs were uniformly disperse in the polymer matrix (as single NPs or nanosized aggregates), i.e., when the NPs were away from each other. In the case of closely packed silica NPs, the PMAA−silica interactions represented only a small fraction of the overall interactions and were not affected by screening, so tH2O ≈ tNaCl. The same mass of smaller NPs contained 37 times more NPs than the same mass of the larger ones and around all of them there was a polymer coating layer of the average thickness t. Consequently, the level of generated interactions between silica and PMAA in the synthesized NC hydrogels could be estimated by the total specific volume of the formed polymer layer around all of the NPs (V). As the engaged NPs were spherical, and the model used assumes the same interactions in all directions, the total specific volume of the formed polymer layer around the NPs could be calculated as the volume of the polymer sphere shell reduced to the NP mass (see Table 3). It became clear that the NCH-9 series had higher values of V than the NCH-30 one, indicating that when PMAA and smaller NPs interacted more polymer was adsorbed on the NP surface. The obtained result was in accordance with the lower SDeq values for the NCH-9 series. The swelling kinetics were investigated by using the wellknown semiempirical equation (eq 5) mt = kt n meq

(5) 618



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aggregates of NPs, themselves nanosized and homogeneously dispersed in PMAA, reinforced the polymer matrix more strongly through the generation of PMAA−silica and silica− silica interactions. Further increase in the silica content (NCH9-10, NCH9-15) weakened the PMAA−silica interactions. This was confirmed by the lower values of the thickness of the adsorbed polymer layer around the NPs and the increased amount of silica−silica interactions. The latter was due to the small distance between the NPs as TEM analysis indicated. The existence of more and larger aggregates led to viscoelastic moduli lower than that for pure PMAA, which could be attributed to the domination of polymer−polymer and silica−silica interactions and the fact that the formed aggregates could not act as a physical network and, consequently, be mechanically effective.66 The fine distribution of NPs and good adhesion with the polymer matrix at the said lower nanosilica concentrations68,65 were further confirmed by the recorded decrease in the storage modulus with increasing particle size. In order to provide an additional proof in favor of the existence of SiO2/PMAA interaction, we compared the experimentally obtained values of the storage modulus to the theoretical prediction of the well-known Guth−Gold model, given as

key factors that affect the dynamical-mechanical performances of nanocomposites are found to be the nanoparticle aspect ratio and distribution inside the polymer matrix and the interactions generated between the polymer chains and NPs.49,64 In general, the addition of silica NPs to a polymer network can have a different effect on the viscoelastic modulus as compared to that of neat polymer. The viscoelastic modulus usually tends to increase with increasing nanoparticle volume fraction. Sometimes the effect is linear, but it can also be nonlinear65 and even occur in a limited range of nanoparticle content where G′ increases until a certain nanoparticle concentration is introduced in the NCs and afterward it decreases.66 In Figure 9, it can be seen that all the nanocomposite hydrogels, as well as PMAA hydrogel, had a frequency independent response of G′ within the frequency range from 0.1 to 100 rad s−1 that corresponds to the rubber plateau. Hydrogels showing this behavior could be classified as “perfect gels”, and values of Gp′ ∼ 104 listed in Table 3 (corresponding to ω = 1 Hz) confirmed that the derived NC hydrogels showed soft rubbery-like properties.67 Furthermore, the low values of the damping factor (tan δ < 1) (Figure 9b) confirmed the dominance of elastic solid behavior. For all the employed samples, tan δ slightly decreased with the increase in frequency in the investigated frequency range and was of the same order of magnitude. In the case of NC hydrogels with smaller SiO2 particles, the values of the damping factor were lower compared to that of the neat hydrogel, while, for NC hydrogels with larger SiO2 particles, the tan δ values were similar to that of neat hydrogel. The obtained results indicated that the incorporation of smaller SiO2 nanoparticles in the PMAA matrix enhanced its elasticity for all the examined SiO2 loadings. Compared to pure PMAA hydrogel, the addition of silica NPs led to an increase in the viscoelastic moduli of all NC hydrogels. It could be seen that, with increasing silica content, the values of both moduli, G′ and G″, were changed. They increased notably up to 5 wt % SiO2 and afterward decreased with a further increase in the nanosilica concentration. In order to investigate the NP concentration that corresponded to the maximal G′ values, three additional samples for both series were synthesized with SiO2/MAA weight ratios of 0.15, 0.20, and 0.35 (NCH-3, NCH-4, and NCH-7) (Table T1, Supporting Information). The obtained Gp′ values of 5252, 5935, and 5150 Pa for NCH9-3, NCH9-4, and NCH9-7 and 4760, 4851, and 4220 Pa for NCH30-3, NCH30-4, and NCH30-7, respectively, confirmed that the NCH-5 samples had the maximal Gp′ for both series. On the basis of all the presented results, we think that in the case of nanocomposites with moderated interactions, such as in the presented case, where hydrogen bonds and physical interactions are the strongest interactions generated between PMAA and silica, the distribution of NPs and their interaction with polymer have a major impact on all the properties, including the dynamic-mechanical ones. However, our results also indicate that the interactions between the NPs themselves are of high importance. That is why, in the case of sample NCH9-1 with a uniform distribution of single NPs, we obtained higher G′ values compared to PMAA but lesser values than in the case of NCH9-2 and NCH9-5. In the case of NCH9-1, silica NPs were “far away” from each other and the increase in G′ was only a consequence of PMAA−silica interactions. On the other hand, in the case of NCH9-2 and NCH9-5, small

Gc′ = Gp′ (PMAA)(1 + 2.5φSiO + 14.1φSiO 2) 2

2

(6)

This model provides calculation of the storage modulus of incompressible, rubbery matrix systems filled with hard, spherical, weakly interacting particles, taking into account only the storage modulus of the neat polymer matrix (Gp′(PMAA)) and the volume fraction of the filler.69,70 The fact that the experimental moduli were up to 8.7 times higher than the corresponding predictions (Table 3) strongly implied that, besides being a hydrodynamic reinforcement of the network, the nanoparticles certainly acted as additional cross-link points.61 Since NC hydrogels showed a significant decrease in the storage modulus with an increasing degree of swelling, the obtained values of Gp′ were reduced to the dry state for comparison, by using eq 7: ′ = Gp′ SD1/3 Gred eq

(7)

where Gred′ is the reduced storage modulus.71 The values of Gred′ showed the same complex change with increasing nanosilica content as the Gp′ values (Table 3). Assuming that the presence of both the cross-linker (MBA) and the silica nanoparticles contributes to the obtained G′ values, as well as the fact that the MBA/MAA molar ratio was kept constant in all samples, the reinforcement of PMAA/SiO2 NC hydrogels was calculated in the equilibrium swollen and dry state as the ratio of the corresponding values of the storage modulus of the NC hydrogels to the G′ modulus of the pure PMAA hydrogel. As presented in Figure 10, the reinforcement effect was evident in the equilibrium swollen as well as in the dry state, but it was a little more pronounced than in the former case. The presented comparison is very useful, as it clearly presents the optimum nanofiller loading (0.25 SiO2/MAA wt ratio, i.e., 25.4 wt % of SiO2). In the case of NC hydrogels with moderately strong polymer/nanofiller interactions, determina619



Article

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors would like to acknowledge funding from the Ministry of Education, Science and Technological Development of the Republic of Serbia, through Project No. 172062 “Synthesis and characterization of novel functional polymers and polymeric nanocomposites”. The authors also thank Djordje Veljovic who gave a significant contribution in the FEG-SEM analysis and Nancy Dewit, Christiaan Van Roost, and Milan Stamenovic from AGFA Materials and AGFA Graphics for great help regarding the TEM analysis.

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Figure 10. Reinforcement of NCH9 (black symbols) and NCH30 (red symbols) in the equilibrium swollen (solid symbols) and dry (open symbols) states.

ABBREVIATIONS G′, storage modulus (Pa); Gp′, plateau storage modulus (Pa); Gred′, reduced storage modulus (Pa); G″, loss modulus (Pa); k, characteristic coefficient that relies on the hydrogel structure (min−1); m0, mass of dry NC hydrogel disc (g); MBA, N,N′methylenebis(acrylamide); meq, mass of NC hydrogel at equilibrium swollen state (g); mSiO2,TGA, weight percentage of SiO2 in the NC hydrogels (wt %); mt, mass of NC hydrogel at time t (g); n, characteristic exponent describing the mode of the penetrant transport mechanism; NC, nanocomposite; NP, nanoparticle; PMAA, poly(methacrylic acid); r0, radius of neat particles (nm); Rc, residue at 1000 °C of NC hydrogels (wt %); RPMAA, residue at 1000 °C of starting PMAA hydrogel (wt %); SDeq, equilibrium swelling degree; SDeq,MAA, reduced values of equilibrium swelling degree calculated to polymeric matrix (excluding NP weight); SDeq,PMAA, equilibrium swelling degree of the pure polymer matrix; SDt, swelling degree at time t; t, swelling time (h); t, thickness of the formed polymer layer around NPs (nm); T10, temperature at which 10% of decomposition occurs (°C); T40, temperature at which 40% of decomposition occurs (°C); T5, temperature at which 5% of decomposition occurs (°C); tan δ, damping factor; tH2O, thickness of the formed polymer layer around NPs after swelling in water (nm); tNaCl, thickness of the formed polymer layer around NPs after swelling in NaCl aqueous solution (nm); VA-044, 2,2′-azobis-[2-(2-imidazolin-2-yl)propane] dihydrochloride; φcp, volume fraction at the random close packing; φSiO2, volume fraction of SiO2 in the initial state; ′ 2, corrected volume fraction of SiO2; ν, Poisson coefficient; φSiO ω, frequency (rad s−1)

tion of the optimal loading is necessary to ensure improved dynamic-mechanical properties.

■

CONCLUSION A simple synthetic strategy to obtain NC hydrogels made from neutralized methacrylic acid and unfunctionalized silica NPs was described. Although unfunctionalized, silica NPs did not coagulate and settle even at high concentrations (up to 42.8 wt %). Depending on the NP concentration, they were arranged as single nanoparticles, nanoscale aggregates, or as a homogeneous structure, which consisted of mutually interacting nanosilica particles coated with PMAA (SEM analysis). The obtained hydrogels were capable of uptaking high amounts of water (SDeq ranging from 90 to 450 g g−1). Also, they showed improved viscoelastic moduli (up to 8.7 times) and enhanced elasticity compared to the neat hydrogel. These properties were highly dependent on the silica nanoparticle diameter and silica/ MAA weight ratio, indicating the existence of polymer/ nanoparticle interactions. By analyzing data from SEM, DMA, and swelling experiments, it was shown that silica NPs behaved as adhesive fillers that interacted with PMAA chains, causing the formation of a thin polymer coating layer strongly adsorbed at the particle interface. However, in order to give more definite conclusions regarding the polymer/nanosilica interactions, it is necessary to investigate the surface properties, shape, and size of the formed nanosilica aggregates. In the future, it would be interesting to study the influence of the degree of neutralization of MAA on the properties of the NC hydrogels. The influence of particle shape is another parameter worth investigating.

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ASSOCIATED CONTENT

S Supporting Information *

(1) Figure S1: FEG-SEM micrographs of NC hydrogels; bars, 50 μm. (2) Figure S2: FEG-SEM micrographs of NC hydrogels; bars, 200 nm. (3) Figure S3: TEM micrographs of NC hydrogels. (4) Table T1: Swelling and DMA properties of additional samples: NCH9-3, NCH9-4, and NCH9-7. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Methacrylic Acid Based Polymer Networks with a High Content of

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