Journal of Colloid and Interface Science 455 (2015) 32–38

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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Microspherical hydrogel particles based on silica nanoparticle-webbed polymer networks Makoto Takafuji a,b,⇑, Md. Ashraful Alam a,c, Hiroyuki Goto a, Hirotaka Ihara a,b,⇑ a

Department of Applied Chemistry and Biochemistry, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan Kumamoto Institute for PhotoElectro Organics (Phoenics), 3-11-38 Higashimachi, Kumamoto, Japan c Department of Applied Chemistry and Chemical Engineering, Noakhali Science and Technology University, Sonapur, Noakhali-3814, Bangladesh b

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 11 April 2015 Accepted 19 May 2015 Available online 28 May 2015 Keywords: Spherical hydrogel particle Silica nano particle Water-in-oil suspension Multiple cross-linking

a b s t r a c t Spherical hybrid hydrogel microparticles were facilely fabricated by particulation of an aqueous mixture of hydrophilic copolymer with alkoxysilyl side chains and silica nanoparticles in water/silicone oil suspensions. Inside the aqueous phase, the copolymers were webbed with silica nanoparticles to form polymer network through the silane coupling reactions between reactive side chains of copolymer with the silanol groups on silica nanoparticles. An amino-functionalized silane coupling reagent was used to terminate remaining reactive sites at the surface of the hydrogel particles to avoid aggregation. The size of the spherical hydrogel particles was well controlled by the viscosity of the silicone oil, whereas their properties were controlled by composition. The mechanical strength of the microspherical hybrid hydrogel was increased significantly with increasing the concentration of silica nanoparticles. Ó 2015 Elsevier Inc. All rights reserved.

1. Introduction

⇑ Corresponding authors at: Department of Applied Chemistry and Biochemistry, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan. Fax: +81 96 342 3662. E-mail addresses: [email protected] (M. Takafuji), ihara@kumamoto-u. ac.jp (H. Ihara). http://dx.doi.org/10.1016/j.jcis.2015.05.034 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.

Hydrogels are three-dimensional polymer networks composed of highly hydrophilic cross-linked macromolecular chains that can absorb water and swell to several times their initial volume when placed in aqueous medium [1]. Hydrogels have been of great research interest for the last few decades because of excellent properties like stimuli-responsive behavior, biocompatibility, and

M. Takafuji et al. / Journal of Colloid and Interface Science 455 (2015) 32–38

ability to immobilize reactive functional groups [2–8]. Special attention has been paid for developing novel hydrogels with unique cross-linked structures [9–14]. It has been reported that a cross-linked structure in polymer network hydrogels affects their mechanical and swelling properties. For instance, the slide-ring gel, which is topologically interlocked by figure-of-eight cross-links, shows excellent elasticity and swelling properties because of the ability of the cross-linking points to slide [12]. Nanoclay-containing hydrogels (NC gels) in which polymer chains are cross-linked with the surface of nanoclay particles by electrostatic interactions exhibit remarkable mechanical strength [11]. Swelling/deswelling kinetics as well as elastic properties are remarkably enhanced by the hybrid networks in NC gels through the interactions between the polymer chains and nanoparticles [15]. Recently, we reported hybrid polymer hydrogels containing inorganic nanoparticles as cross-linker [16–18]. These hybrid hydrogels were prepared by mixing an aqueous solution of copolymer with reactive alkoxysilyl side chains with an aqueous suspension of inorganic nanoparticles. During the gel formation process, the cross-linking points are gradually formed through the silane coupling reactions between the alkoxysilyl side chains of the copolymer and silanol groups of silica nanoparticles (SiNPs). Because of the dense silanol groups on their surface, the SiNPs acted as multiple cross-linking points [16]. This gelation system provides various hydrogels with desirable polymer network structure and precisely controlled properties simply by mixing pre-prepared solutions [17]. These attractive features encouraged us to shape the hydrogels into various forms. In this paper, we demonstrate to prepare spherical hybrid hydrogel microparticles consisting of nanoparticle-webbed polymer network in micro-scale. 2. Experimental section

33

Japan). Powder samples were prepared by dispersing the polymers in KBr, and compressing the mixture to form disks. 1H NMR spectra of the dried copolymers were obtained by a JNM-LA400 (JEOL Ltd., Japan) instrument at 400 MHz with D2O as solvent and tetramethylsilane as internal standard. 2.3. Preparation of hybrid hydrogel and their microparticulation Nanosilica cross-linked hybrid hydrogels were prepared in test tube. An aqueous solution of pSiHmx was mixed with a certain amount of silica nanoparticle (SiNP) suspension. The reaction mixture was then mixed vigorously for 5–10 s by a vortex mixer and investigated for three weeks to observe hydrogel formation. The gelation property was evaluated by test tube inversion method at 25 °C. Cylindrical-shaped hybrid hydrogels for mechanical strength measurements were prepared in small cylindrical Teflon tubes (inside diameter – 10 mm) with bottom end sealing at 25 °C for one day. The preparation of hybrid hydrogel particles were carried out using w/o suspension method. The mixed aqueous solution of pSiHmx and silica nanoparticles was poured into 100 mL silicone oil and stirred for 6 h. Silane coupling reagents such as 3-aminopropyltrimethoxysilane (APS), and 3-methaclyloy loxypropyltrimethoxysilane were dissolved in silicone oil before pouring the mixture to terminate the unreacted alkoxysilyl groups of pSiHmx at the interface of hydrogel particle. After the desired period, a small volume of suspension was taken out and observed for size measurement by optical microscope. The hydrogel particles in the remaining suspension were collected after keeping the suspension aside for few hours, pouring the supernatant from top, and then wash several times with toluene. Obtained hydrogel microparticles were then dried in vacuum, and then stored. Mechanical strength of hydrogel microparticles were observed before vacuum drying of the particles.

2.1. Materials N-Hydroxyethyl acrylamide (Hm) was kindly supplied from Kohjin Co. Ltd. (Japan) and purified by removing inhibitor using inhibitor remover before use. The initiator, 2,2-azobisisobutyronitrile (AIBN), was purchased from Wako Pure Chemical Industries Ltd. (Japan) and recrystallized from methanol. Silica nanoparticle aqueous suspensions (mean diameter of 14.3 nm) were purchased from Nissan Chemicals Ltd. (Japan). 3-Methacryloxypropyltrimeth oxysilane, (MAPTS) and all other reagents were of analytical grade and were used without further purifications. 2.2. Synthesis of reactive copolymer The reactive copolymer, poly[N-hydroxyethylacrylamide-c o-(3-methacryloxypropyltrimethoxysilane)] (pSiHmx), was synthesized by free radical copolymerization of Hm and MAPTS in methanol at 60 °C. Briefly, required amounts of Hm, MAPTS and AIBN were dissolved in methanol in a three-necked round-bottom flask under stirring. Oxygen was eliminated by bubbling nitrogen through the solution for 30 min. The flask was then placed in an oil bath thermo-stated at 60 °C. The reaction was carried out for 6 h at this temperature. After 6 h, the reaction mixture was removed from the bath and cooled to room temperature. White copolymer product was precipitated out by pouring the reaction mixture drop wise into large amount of acetone. The precipitates were dried in vacuum and again dissolved in methanol for washing and then precipitated again in excess acetone. After repeating these dissolution and precipitation several times, the obtained pure copolymers were dried in vacuum and stored in desiccator for further uses. The homopolymer of N-hydroxyethyl acrylamide (pHm) was also synthesized as a reference. FTIR spectra of pHm and pSiHmx were obtained by a FT-IR 4100 spectrometer (JASCO,

2.4. Instrumentation FT-IR and 1H NMR spectroscopic measurements were carried out using FT-IR 4100 (JASCO, Co. Ltd., Japan) and JNM-LA400 (JEOL. Co. Ltd., Japan) respectively. 29Si NMR spectroscopic measurements were performed with Varian Unity Inova AS400 (Varian, California, USA). Optical microscopic observations of hybrid hydrogel microparticles were performed by Olympus CX31 (Olympus Co., Japan) and the size distribution analyses were calculated and figured with AZO V250 software. The compression strength measurements of cylindrical hydrogels (10 mm in diameter, 12 mm in height) and microspherical hydrogels were carried out with tensile and compression tester EZ-L (Shimadzu Co. Ltd., Japan) and MCT-211 (Shimadzu Co. Ltd., Japan) respectively. The compressive strength of the hydrogels microparticles was calculated at 40% compression strain and used for comparison. The number average (Mn) and weight average (Mw) molecular weight, and polydispersity indexes (PDI) of the prepared polymers were determined by size exclusion chromatography (SEC) with Shodex Asahipak GF-7M HQ column (Showa Denko K. K., Japan) using dimethylformamide containing 0.01 M lithium chloride as eluent. Polystyrene standards were used as calibration standards. Zeta potential measurements of hydrogel microparticles were carried out in toluene by using Malvern Zetasizer (Nano ZS), UK. 3. Results and discussion 3.1. Preparation of reactive copolymer Water-soluble reactive copolymers (Fig. 1a) were synthesized by free-radical copolymerization of hydrophilic and reactive side

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(a)

(b)

pSiHmx (x = n/m)

Hybrid hydrogel

Fig. 1. Chemical structure of reactive copolymer (pSiHmx) and hybrid hydrogel cross-linked with silica nanoparticles (SiNP).

chain-branched monomers. In this study, Hm was used as a hydrophilic monomer, and MAPTS was used as a monomer with reactive side chains. Copolymers pSiHmx (x = Hm/Si, where Si indicates the reactive MAPTS component) with different compositions were prepared by changing the ratio of Hm and MAPTS in the feed mixture. The homopolymer pHm without reactive side chains was also synthesized as reference. The obtained copolymer pSiHmx and homopolymer pHm were characterized by 1H NMR and FT-IR spectroscopies (Figs. S1 and S2 in Supplementary Material, respectively). The copolymerization ratio, x, of pSiHmx was determined by the integral intensities of the peaks at d = 3.35 and 0.64 ppm. Mn, Mw, and PDI of the pSiHmx were determined by SEC. These results are summarized in Table 1. In this study, five pSiHmx copolymers with different x and Mn/Mw were prepared. 29Si NMR spectra of copolymers pSiHmx showed a sharp single peak at 40.84 ppm (Fig. S3, Supplementary Material) indicating the presence of unhydrolyzed trimethoxysilyl groups on the side chains after polymerization. 3.2. Gelation properties According to our previous report, the gelation time of hybrid hydrogels is strongly affected by composition. For instance, the gelation time shortened as the concentrations of reactive copolymer and SiNPs increased [17]. The molar ratio of co-monomers and polymerization degree in the reactive copolymer also affect gelation time. As shown in Table S1 (Supplementary Material), similar gelation properties were observed in the case of pSiHmx. No gelation was observed in a mixture of pHm and SiNPs with a diameter of 14.3 nm. Copolymer solutions of lower concentration did not form gels, indicating that gelation was occurred through the formation of a SiNP-cross-linked polymer network structure. The molar ratio of Si in the copolymer also influenced gelation properties. As shown in Table 2, the gelation time lengthened as x increased; that is, as the ratio of reactive monomer (MAPTS) in the copolymer lowered. Surprisingly, the copolymer pSiHm93 did not form gel with the same combination as the other copolymers even after three weeks. This can be explained by considering molecular weight. The molecular weight of this copolymer

(2.2  105 Daltons) is quite low compared with that of the other copolymers (Table 2). Therefore, we infer that the chain length of this copolymer was too short to cause the cross-linking with SiNPs to form a three-dimensional hybrid network structure. The effect of size of SiNPs on the gelation properties is presented in Table 3. Gelation time increased when the same amount of larger SiNPs were used as a cross-linker instead of smaller ones. This is because the larger SiNPs had lower specific surface area than the smaller SiNPs, which ultimately provided relatively fewer silanol groups; i.e., reactive sites for cross-linking. The lower the density of reactive sites, the longer is the time required to induce gelation. 3.3. Mechanical properties of hybrid hydrogels The mechanical properties of hydrogels are strongly affected by polymer network structures [19,20]. Haraguchi et al. [11,20] reported that the incorporation of clay sheets in a hydrogel network to form NC gels substantially improved mechanical properties, increasing elongation by about 50 times compared with that of chemically cross-linked hydrogels. These NC gels could withstand high levels of deformation, not only extension and compression but also bending, tearing, twisting, and knotting. Gong et al. [13] reported a general method to obtain very strong hydrogels by forming a double-network (DN) structure using various combinations of hydrophilic polymers which, with an optimized network structure, can sustain compressive pressures as high as several tens of megapascals. Compression stress–strain curves of the SiNP-cross-linked hybrid hydrogels were obtained by a compression tester using a

Table 2 Effect of Hm/Si composition in pSiHmx on gelation time of the mixture of polymer and SiNP.a Reactive copolymer (5 wt%) SiNP 5 wt% a b c

Table 1 Copolymerization ratio (Hm/Si), molecular weight (Mw) and polydispersity indexes (PDI) of the reactive copolymers. Polymer

Yield (%)

Hm/Si

Mw (105), Daltons

PDI

pSiHm51 pSiHm93 pSiHm100 pSiHm179 pSiHm298 pHm

91.5 82.6 92.0 91.2 92.4 83.0

51 93 100 179 298 –

3.5 2.2 3.7 3.0 3.8 5.8

3.73 3.12 3.62 2.91 3.07 2.87

pHm c

No gel

pSiHm51

pSiHm100

pSiHm179

pSiHm298

pSiHm93b

30 s

3 min

4.5 min

6 min

No gelc

SiNP (14.3 nm, 5 wt%). MW was relatively smaller than other copolymers (see Table 1). No gelation was observed within 3 weeks.

Table 3 Effects of the size of SiNP (14.3 nm, 5 wt%) on gelation time with pSiHm100 (5 wt%) aqueous solution. Size of SiNP

a

SiNP

14.3 nm

16.7 nm

21.9 nm

35.9 nm

78.5 nm

5 wt%

1.5 min

3 min

4 min

60 min

No gela

No gelation was observed within 3 weeks.

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constant compression speed of 30 mm/min at 25 °C. Cylindrical test samples of the hybrid hydrogels were prepared in Teflon tubes with a length of 12 mm and diameter of 10 mm, and the compression tests were carried out after aging each gel for 24 h. Typical compression stress–strain curves of hybrid hydrogels composed of 5 wt% of pSiHm100 with different amounts of SiNPs are shown in Fig. 2. The stress increased slightly at lower compression and then increased exponentially before the gel broke. The compression resistance of the hybrid hydrogels varied with the polymer network structure. If the compression pressure was removed, i.e.,

700

(c)

100 600

80

Stress (kPa)

500

60

(d)

(a) (b)

40

400

20 300

0 0

200

20

40

60

80

100

the gel was unloaded before breaking, the hydrogel readily returned to its original shape back, indicating an elastic nature of the prepared hydrogels. To understand the effect of the composition of the hydrogels on mechanical strength, the stresses at 60% strain of the hybrid hydrogels were compared. As summarized in Fig. 3, the stress was affected by the composition of the hybrid hydrogels, including the concentrations of pSiHmx and SiNPs, x and polymerization degree of pSiHmx. For instance, the strength of the hybrid hydrogels increased with the concentrations of pSiHmx (Fig. 3a) and SiNPs (Fig. 3b). The linear increase of strength with strain indicates that the mechanical strength of the hybrid gels was directly affected by the density of the polymer network structure. In contrast, the compressive strength at break of the hybrid hydrogels was not always high at higher concentrations of pSiHmx and SiNPs. This is probably related to inhomogeneity of the polymer network structure and brittleness of the hydrogels at higher copolymer and SiNP content [17]. The gelation time of the hybrid hydrogels at higher concentrations was too short to form a homogeneous polymer network structure. The strength at 60% strain of the hybrid hydrogels decreased with increasing x in the copolymers (Fig. 3c). Using a polymer with large Mw also improved the strength of the hybrid hydrogels (Fig. 3d). These results might originate from formation of stable hybrid networks through homogeneous cross-linking at these compositions resulting in enhancement of strength.

100

3.4. Preparation of hybrid hydrogel particles and their properties 0

0

20

40

60

80

100

Strain (%)

Stress at 60% compression (kPa)

Fig. 2. Compression stress–strain curves of the hybrid hydrogels containing 5 wt% pHmSi100 and (a) 15, (b) 10, (c) 7.5, and (d) 5 wt% SiNP. The inset is the same graph at lower compression stresses from 0 to 100 kPa.

120

Hydrogel microparticles as a unique class of polymeric materials have received immense interest, especially because of their exceptional properties arising from the combination of their colloidal nature with internal network structure [21]. These three-dimensional microscale networks are highly functional

120

(a)

[pSiHm100]

100

: 2.5 wt% : 5.0 wt% : 7.5 wt%

80 60

60

40

40

20

20

0

5

10

15

0 0

120

2.5

5

7.5

Concentration of pSiHm100 (wt%)

Concentration of SiNP (wt%)

Stress at 60% compression (kPa)

: 2.5 wt% : 5.0 wt% : 10 wt% : 15 wt%

80

0

(b)

[SiNP] 100

120

(c)

100

[pSiHm100] / [SiNP] : 2.5 wt% / 5.0 wt% : 5.0 wt% / 5.0 wt% : 5.0 wt% / 10 wt% : 5.0 wt% / 15 wt%

80 60

100

60 40

20

20

0

100

200

300

Composition of copolymer (Hm/Si)

(d)

: pSiHm100 (Mw = 3.7 x 10 5) : pSiHm97 (Mw = 2.2 x 10 5)

80

40

0

[pSiHmx] = 7.5 wt%

0

0

5

10

15

Concentration of SiNP (wt%)

Fig. 3. Compression strength at 60% strain of hybrid hydrogels; (a) compressive strength versus SiNP content, (b) compressive strength versus copolymer concentrations, (c) compressive strength versus copolymer ratios, and (d) compressive strength of hydrogels of different molecular weight versus SiNP content.

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materials with tremendous potential for a wide variety of applications such as optics [22,23], colloidal crystals [24], sensors [25], and release experiments in materials science and biomedical research [26–29]. Several methods including suspension polymerization and suspension cross-linking [30,31] have been proposed to tailor the bead size, morphology and encapsulation efficiency of hydrogel microparticles for different applications [32–34]. However, it is still difficult to fabricate uniform beads with controlled shape and size for specific applications. Various approaches have been explored to address these challenges [35–38]. For example, polymer beads that efficiently encapsulate bioactive materials have been prepared by phase separation [35], and in situ polymerization [36]. Microparticles with improved shape have been fabricated by combining a phase-separation process with in situ polymerization of the polymeric particles [37]. To further advance this direction of research, we fabricated hybrid hydrogel microparticles with controlled shape and size using a simple, efficient, and facile method. Spherical hybrid hydrogel microparticles were prepared by dropping an aqueous mixture of pre-prepared reactive copolymer and SiNPs into silicone oil with or without APS at room temperature, and suspending the mixture for a certain incubation period under mechanical agitation. To control the size of the microparticles, the roles of different SiNP contents in hydrogel compositions, and viscosities of silicone oil were investigated. Optical microscope images of the obtained microparticles are shown in Fig. 4.

a

Individual spherical microparticles formed when APS was used as a terminating agent in the suspension media, whereas the particles aggregated without the use of APS (Fig. 4a). Microparticles obtained without the use of APS were irregularly shaped with a wide range of diameters ranging from several tens to several hundreds of micrometers. Most of the hydrogel particles aggregated together in the suspension media. This is probably caused by the formation of cross-linked structures at the interface between hydrogel particles through reaction of unreacted side chains of pSiHmx and remaining silanol groups of the SiNPs. To avoid such aggregation, we used the silane-coupling reagent APS to terminate the remaining reactive sites of the SiNPs in pSiHmx. As a result, transparent spherical hydrogel particles that were well dispersed in the suspension media were obtained (Fig. 4b–e). The hydrogel particles were extracted from silicone oil to evaluate their surface charge. f-potential measurements confirmed that the surface of the obtained hydrogel particles was positively charged (+18.7 mV). We used silicone oil with three different viscosities, 50, 100, and 300 centistokes (CS), as suspension media. The size of the resultant particles was smaller when silicone oil of higher viscosity was used, and comparatively bigger using silicone oil of lower viscosity. For example, the average diameters of the obtained particles were 1236, 292 and 55 lm when the viscosities of the silicone oil were 50, 100, and 300 CS, respectively, for aqueous mixtures with the same composition [pSiHm100] = 5 wt%, [SiNP] = 5 wt% (Fig. 4c–e). The sizes of the microparticles were further

b

300 µm

300 µm

d

c

300 µm

300 µm

f

e

100 µm

100 µm

Fig. 4. Optical microscopic images of the hybrid hydrogel particles prepared in various suspension media. (a) [pSiHm100] = 5 wt%, [SiNP] = 5 wt%, Silicone oil-100 CS without APS, (b) [pSiHm100] = 5 wt%, [SiNP] = 2.5 wt%, Silicone oil-100 CS with APS, (c) [pSiHm100] = 5 wt%, [SiNP] = 5 wt%, Silicone oil-100 CS with APS, (d) [pSiHm100] = 5 wt%, [SiNP] = 5 wt%, Silicone oil-50 CS with APS. (e) [pSiHm100] = 5 wt%, [SiNP] = 5 wt%, Silicone oil-300 CS with APS, (f) [pSiHm100] = 5 wt%, [SiNP] = 5 wt%, Silicone oil-100 CS with MAPTS.

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M. Takafuji et al. / Journal of Colloid and Interface Science 455 (2015) 32–38

40

(a) 30

Stress (MPa)

controlled by the composition of the aqueous mixture of copolymer and SiNPs. For instance, the average sizes of the microparticles were 223 and 292 lm when the content of SiNPs was 2.5 and 5 wt%, respectively, with a fixed copolymer concentration of 5 wt% (Fig. 4b and c). The related histograms for each experiment are illustrated in Fig. 5. The average sizes of the obtained hybrid hydrogel particles were similar to those of polymer microspheres prepared by conventional suspension polymerization [39,40] but were larger than those prepared using a membrane filter. [41] Further tuning of particle size and distribution would be possible by using up-to-date particulation methods such as emulsion and spraying. Other silane coupling reagents with various functional groups such as n-Octadecyltrimethoxysilane (ODS), MAPTS, and Mercapt opropyltrimethoxysilane (MPS) were also used as silane coupling reagents to terminate reactive sites. Using all of these reagents, nearly spherical hydrogel particles were obtained but they were partially coagulated in the suspension media (Fig. 4f). These results indicate that the suspension of aqueous mixtures of pSiHmx and SiNP was stabilized by termination of remaining reactive sites at the interface of hydrogel particles with the silane coupling reagents. The APS-terminated hydrogel particles were particularly stable because of the electrostatic repulsion between them. The compression strength of the dispersed hybrid hydrogel particles was evaluated; typical stress–strain curves of hydrogel microparticles prepared under different experimental conditions are shown in Fig. 6. To compare the strength of different hydrogel microparticles, the average compressive strength at 40% strain of 10 microparticles has been considered, and the values are summarized in Table 4. As can be observed, the hydrogel microparticles possess very high compressive strength in the megapascal range, and the strength increased with increasing the crosslinker content (SiNP) in the composition. For example, the compressive strength

20

(c) 10

0

0

20

40

60

Compression (%) Fig. 6. Typical compression strength–strain curves of the hybrid hydrogel particles. (a) [pSiHm100] = 5 wt%, [SiNP] = 5 wt%, prepared in silicone oil-300 CS, diameter of original microparticle = 55 lm, (b) [pSiHm100] = 5 wt%, [SiNP] = 5 wt%, prepared in silicone oil-100 CS, diameter of original microparticle = 292 lm, (c) [pSiHm100] = 5 wt%, [SiNP] = 2.5 wt%, prepared in silicone oil-100 CS, diameter of original microparticle = 223 lm.

Table 4 Compressive strength of hydrogel microparticles. Composition

60

pSiHm100 (%)

SiNP (%)

5 5 5

5 5 2.5

Viscosity of suspension media

Average particle size

Strength at 40%

(Centistokes)

(lm)

(MPa)

300 100 100

55 292 223

16.80 14.20 4.97

70

(a)

50

(b)

60 50

%

40

%

(b)

30

40 30

20

30

950

850

650

750

450

550

350

250

45 40

(c)

25

(d)

35

20

30

%

%

50

950

850

650

750

550

350

450

250

0

50

10

0

150

10

150

20

15

25 20

10

15 10

5

Particle size (µm)

255

285

225

195

165

135

75

105

45

0

15

2850

2550

2250

1950

1650

1350

750

1050

150

450

5 0

Particle size (µm)

Fig. 5. Particle size distributions of the hybrid hydrogel particles; (a) [pSiHm100] = 5 wt%, [SiNP] = 5 wt%, Silicone oil-100 CS with APS, (b) [pSiHm100] = 5 wt%, [SiNP] = 2.5 wt%, Silicone oil-100 CS with APS, (c) [pSiHm100] = 5 wt%, [SiNP] = 5 wt%, Silicone oil-50 CS with APS. (d) [pSiHm100] = 5 wt%, [SiNP] = 5 wt%, Silicone oil-300 CS with APS.

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M. Takafuji et al. / Journal of Colloid and Interface Science 455 (2015) 32–38

of hydrogel microparticles containing 2.5 and 5 wt% of SiNPs were 4.97 and 14.20 MPa, respectively, with a fixed copolymer concentration of 5 wt% (Fig. 6b and c). It was expected from the compositional point of view, as the introduction of inorganic component (SiNP) in the hydrogel network favors the improvement of mechanical properties [17,19,20]. However, the size of hydrogel microparticles having same composition (pSiHmx = 5 wt%, SiNP = 5 wt%) did not have significant effect on the strength as shown in Fig. 6a and b. The compressive strength at 40% strain was 16.80 and 14.20 MPa for microparticles with average diameter of 55 lm, and 292 lm, respectively. These results suggest that the mechanical strength of hydrogel microparticles can be easily controlled by their initial composition. The hybrid hydrogel particles broke at around 50% strain, which was comparatively earlier than bulk hydrogels (Fig. 2), which broke at about 80% strain. This is because the deformation of the surface of spherical particles was much larger than that of the cylindrical test pieces of bulk hydrogel. However, the compressive strength of the hydrogel microparticles was considerably higher than that of the bulk hydrogels. For example, the strength of hydrogel at 40% strain with a composition of [pSiHm100] = 5 wt% and [SiNPs] = 5 wt% was 14.20 MPa in the case of microparticles, but only 0.87 kPa for the bulk hydrogel. We think that this improvement in the mechanical properties might be due to more homogeneous distribution of SiNPs throughout the networks in the hydrogel microparticles compared to that in the bulk hydrogels.

4. Conclusions Mechanical properties of silica nanoparticle cross-linked hybrid hydrogel were evaluated in detail. Due to the incorporation of inorganic nanoparticle as cross-linker instead of organic chemical in the polymer network, the compressive strength of the hybrid hydrogels was affected [10,11,17,19] significantly by the concentration of the SiNPs. Furthermore, concentration as well as the monomer ratio in the reactive copolymer also affected the mechanical properties. A facile methodology was developed to fabricate hybrid hydrogel microparticles in water/oil suspensions without any complicated apparatus such as a porous filter [41,42] or microreactor [38]. The size and properties of the hydrogel particles were well controlled by controlling the viscosity of suspension media and composition of the hydrogel particles. We believe that the method used to prepare hybrid hydrogel particles presented in this study will reveal new practical applications. Acknowledgments This study was supported by Industrial Technology Research Grant Program from New Energy and Industrial Technology Development Organization (NEDO) of Japan and Grant-in-Aid for

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Microspherical hydrogel particles based on silica nanoparticle-webbed polymer networks.

Spherical hybrid hydrogel microparticles were facilely fabricated by particulation of an aqueous mixture of hydrophilic copolymer with alkoxysilyl sid...
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