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Trace the polymerization induced by gamma-ray irradiated silica particles
Radiation Physics and Chemistry 125 (2016) 160–164
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Short communication
Trace the polymerization induced by gamma-ray irradiated silica particles Hoik Lee a,n, Jungju Ryu b, Myungwoong Kim c, Seung Soon Im a,d, Ick Soo Kim a, Daewon Sohn b,n a Nano Fusion Technology Research Lab, Division of Frontier Fibers, Institute for Fiber Engineering (IFES), Interdisciplinary Cluster for Cutting Edge Research (ICCER), Shinshu University 3-15-1, Tokida, Ueda, Nagono 386-8567, Japan b Department of Chemistry and Research Institute for Natural Sciences, Hanyang University, Seoul 133-791, Republic of Korea c Department of Chemistry, Inha University, Incheon 22212, Republic of Korea d Department of Organic and Nano Engineering, College of Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea
H I G H L I G H T S
Pre-irradiation of γ-ray enables an effective synthesis of polymer/inorganic nanocomposites toward pre-selected properties. Solution dynamics and electron microscopic studies reveal the formation mechanism of the nanocomposites. Reaction time is a critical parameter to achieve desirable organic/inorganic nanocomposite structure in a given system.
art ic l e i nf o
a b s t r a c t
Article history: Received 8 March 2016 Received in revised form 6 April 2016 Accepted 12 April 2016 Available online 13 April 2016
A γ-ray irradiation to inorganic particles is a promising technique for preparation of organic/inorganic composites as it offers a number of advantages such as an additive-free polymerizations conducted under mild conditions, avoiding undesired damage to organic components in the composites. Herein, we demonstrated a step-wise formation mechanism of organic/inorganic nanocomposite hydrogel in detail. The γ-ray irradiation to silica particles dispersed in water generates peroxide groups on their surface, enabling surface-initiated polymerization of acrylic acid from the inorganic material. As a result, poly (acrylic acid) (PAA) covers the silica particles in the form of a core-shell at the initial stage. Then, PAAcoated silica particles associate with each other by combination of radicals at the end of chains on different particles, leading to micro-gel domains. Finally, the micro-gels are further associated with each other to form a 3D network structure. We investigated this mechanism using dynamic light scattering (DLS) and transmission electron microscopy (TEM). Our result strongly suggests that controlling reaction time is critical to achieve specific and desirable organic/inorganic nanocomposite structure among coreshell particles, micro-gels and 3D network bulk hydrogel. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Nanoparticles Core-shell particles Polymeric composites Nanocomposites γ-Ray pre-irradiation
1. Introduction Nanocomposites have attracted tremendous scientific and technical interests due to their exceptional mechanical, (Afroze et al., 2016; Lee et al., 2013; Ye et al., 2007) electrical, (Chapman and Mulvaney, 2001; Ye et al., 2007) optical, (Chapman and Mulvaney, 2001; Wang et al., 2015; Wilson et al., 2002) and thermal properties (Balayeva and Mamiyev, 2016; Ye et al., 2007; Yoon
n
Corresponding authors. E-mail addresses: [email protected] (H. Lee), [email protected] (D. Sohn). http://dx.doi.org/10.1016/j.radphyschem.2016.04.008 0969-806X/& 2016 Elsevier Ltd. All rights reserved.
et al., 2002). In particular, organic/inorganic nanocomposites have been extensively explored as the platform allows modifying a variety of properties by realizing their potential from bulk to molecular level (Burnside and Giannelis, 1995). A γ-ray irradiation method has been considered as one of promising techniques for an effective synthesis of organic/inorganic nanocomposites, as the process is easily controlled and the system is simultaneously sterilized effectively. Furthermore, since γ-ray initiates polymerization or induces crosslinking in the nanocomposites, the resulting composites do not include any additives such as initiators and crosslinkers or by-products which possibly degrade desired properties in the composites (Chapiro, 1964; Huang et al., 2007).
H. Lee et al. / Radiation Physics and Chemistry 125 (2016) 160–164
Therefore, γ-ray irradiation has been utilized to tailor the properties of materials to make them more versatile for further applications such as semiconductors, (Xie et al., 1999) membranes, (Kim et al., 2016) food packaging, (Madera-Santana et al., 2016) smart adhesive plasters (Yoshii et al., 1995) and hydrogels (Carenza, 1992). However, direct high-energy γ-ray irradiation to organic materials such as monomers or polymers can induce undesirable chemical reactions such as decomposition or unintended crosslinking in resulting composites. Thus, the resulting chemical/ physical structure and properties are not rationally predicted. To address this challenge, a pre-irradiation method has been proposed and explored where the inorganic materials such as silica fillers directly are treated with γ-ray irradiation which produces peroxide groups on their surfaces (Griscom et al., 1994; Lee et al., 2013). The peroxide groups undergoes homolytic cleavage to result in silyloxy radicals on particle surfaces which allows surface-initiated polymerization (SIP) at mildly heated environment, e.g. 40 °C. Therefore, the pre-irradiation method enables avoiding any damages to organic components in nanocomposites during γ-ray irradiation process. Using this method, we previously reported an additive-free preparation of polymer/inorganic particle nanocomposite hydrogel which exhibited high mechanical strength (Koo et al., 2012; Lee et al., 2013). The results strongly suggested that pre-irradiation method provided an efficient route to tailor physical properties in soft materials, however, details in the mechanism to form the final structure has not been investigated yet. Herein, we systematically studied how the system evolves from γ-ray treated silica nanoparticles, silica/PAA core/shell nanoparticles to microgels by combining solution dynamics and microscopic studies using dynamic light scattering (DLS) and transmission electron microscopy (TEM) techniques. Careful examination of the process as a function of reaction time allows us to gain deep understanding in the formation of 3D network gel structure. Furthermore, current studies highlight the potential of γ-ray to synthesize various types of organic/inorganic nanocomposites from suitable monomer and inorganic particles to achieve desirable properties for target applications.
2. Experimental details
161
the surfaces of silica particles. The resulting samples were stored at 0 °C before the polymerization process. Following previous studies on peroxide formation on the silica surface, (Griscom, 1985; Imai and Hirashima, 1994; Kokatnur and Jelling, 1941) the formation of peroxide groups on the silica particle was confirmed using iodometry which is a common method to verify peroxide groups in various systems (Kim et al., 2009). Potassium iodide and isopropyl alcohol were added to the irradiated silica particle solution. After a reflux for 30 min, the solution gradually turned to yellow color which indicates the formation of I2, confirming the presence of peroxides on the silica particle. Generated peroxide group by γ-ray irradiation is used as an initiator for surface-initiated polymerization. 2.4. Synthesis of PAA/silica nanocomposite 1 mL of Acrylic acid was added to 2 mL of γ-ray irradiated 2 wt% silica particle solution, then dissolved gas was removed by freeze-pump-thaw process for three times. Then, the solution was heated up to 40 °C to initiate the polymerization. The polymerization times was controlled for 30 min, 2 h, 6 h and 12 h, to trace surface-initiated polymerization process. 2.5. Characterizations DLS experiments were performed with a UNIPHASE He-Ne laser operating at 632.8 nm. The detector equipment employed optical fibers coupled to an ALV/SO-SIPD/DUAL detection unit with an EMI PM-28B power supply and ALV/PM-PD pre-amplifier discriminator. An ALV-5000/E/WIN multiple tau correlator with 288 exponentially spaced channels was used to produce correlation functions. The laser beam passed through a cylinder scattering cell which was located in a bath of decaline as a refractive index matching solvent, then passing through a 400 nm pinhole before reaching the detector. The decay rate (Γ) was calculated from the Eq. (1) with correlation functions obtained at nine scattering angles in 30–150°,
Γ = 1/τ
(1)
where τ is an average decay time. The diffusion coefficient (Dt) was calculated using Eq. (2) and Eq. (3).
Γ = Dt q2
(2)
q = (4πn/λ )·sin (θ /2)
(3)
2.1. Materials Tetraethyl orthosilicate (TEOS), ethanol, aqueous ammonia solution (NH4OH) and acrylic acid (AA) were purchased from Sigma Aldrich and were used without further purification. Deionized water (DI water, resistivity of 418.2 MΩ cm) was obtained using Milli-Q system.
where n is a refractive index of a solvent, q is a scattering vector, λ is the wavelength of incident light and θ is a scattering angle. The hydrodynamic radius (Rh) is obtained from Dt using Stokes-Einstein equation (Eq. (4)),
2.2. Silica particle synthesis
Rh = kb T /6πηDt
Silica particles were synthesized using Stöber method (Stöber et al., 1968). A solution of ethanol (150 mL), TEOS (9 mL) and NH4OH (3 mL) in a 250 mL volumetric flask was stirred for 24 h, resulting in spherical silica particles with a radius of 63 nm. The solution was washed thrice with ethanol and subsequently with DI water twice.
where kb is Boltzmann constant, T is temperature and η is solvent viscosity. TEM and STEM images were obtained with JEOL JEM-2100F with operation voltage of 200 kV. Aliquots were taken after desired reaction time, followed by exposing to air with cooling down for quenching. Then a drop of sample was placed on 200 mesh copper Formvar carbon coated TEM grid.
(4)
2.3. Surface modification of silica particles by γ-ray irradiation An aqueous solution of silica particle (2 wt%) in a narrownecked ampoule (2 cm in diameter and 5 cm in length) was subjected to 60Co γ-rays at a dose of 10 kGy h 1 (at KAERI, Jeongeup, Korea) at ambient conditions for 2 h to create peroxide groups on
3. Results and discussion The gel formation process can be explained with four stages: (1) initiation on silica nanoparticle surface, (2) propagation of AA
162
H. Lee et al. / Radiation Physics and Chemistry 125 (2016) 160–164
Fig. 1. Schematic illustration of proposed mechanism for the formation of 3D network gel structure in silica/PAA nanocomposite by surface-initiated polymerization: initiation, propagation, and mutual combination of growing chains of several particles.
polymerization from surface resulting in core/shell nanoparticle, (3) microgel formation by an association of several nanoparticles and (4) complete gelation by further associations between microgel domains. This mechanism is graphically depicted in Fig. 1. In details, silica nanoparticle aqueous dispersion was exposed to 60Co γ-ray irradiation to form peroxide groups on particle surfaces, subsequently the peroxide was decomposed into silyloxy radical on silica by mild heating (40 °C), leading to an effective surface initiation in the presence of acrylic acid monomers in a solution and thereby forming thin PAA layer over the silica surface and leading to core-shell structure consisting of silica particles as a core and PAA as a shell. The Rh distribution of as-prepared silica particle is compared to that after SIP of AA from silica particle in Fig. 2(a). The average Rh of bare silica and PAA/silica nanoparticles in distributions were 637 8 nm and 817 18 nm, respectively, confirming the particles are covered by shell layer of PAA as a result of SIP. We confirmed that the Stokes-Einstein relationship is valid to gain Rh value in this system by observing scattering angle dependence (q2) of decay rate (Γ). Fig. 2(b) shows that decay rate values measured from a solution of PAA/silica nanoparticles follows the relationship of Γ ¼Dtq2, indicating scattered signals represent a diffusive mode of dispersed particles. The slope of fitted line was calculated to be
3069.7 nm2/ms, which is converted to 79 nm in Eq. (4), confirming that DLS measurements are suitable to confirm the polymerization of AA at this stage. The inset in Fig. 2(a) is TEM image of PAA/silica nanoparticles; the size is in good agreement with what was measured in DLS. Therefore, upon polymerization for 30 min, PAA was grown from the surface, resulting in 20 nm thick PAA/silica shell/core nanoparticle dispersion. It is worthwhile to note that thin soft polymeric layer was observed in TEM as shown in Fig. 3 (a), suggesting that AA might undergo the self-polymerization or polymerization initiated by hydroxyl radical which is also produced in homolytic cleavage of peroxide group. We further examine the polymerization for longer than 30 min using the same methods. After 6 h, the average Rh was increased to 212 795 nm from 81 718 nm, indicating the formation of large aggregates. Upon the formation of core/shell type particles by SIP, the active chain ends on one particle undergoes mutual termination by radical combination with active chain ends on another particle surface, resulting in association of several particles. Also we note that standard deviation in Rh largely increases at this stage. This is likely due to the nature of recombination in free radical polymerization leading to a broad molecular weight distribution; the aggregates can react with another much larger or much resulting in broad size distribution when the reaction
Fig. 2. (a) The distributions in hydrodynamic radius of silica particles (black) and after polymerization of acrylic acid on silica particles by SIP (red), obtained with DLS measurements. The inset shows TEM image of PAA/silica particles. (b) Plot of scattering angle dependent decay rate for PAA/silica particle solution in Fig. 2(a).
H. Lee et al. / Radiation Physics and Chemistry 125 (2016) 160–164
163
Fig. 3. Structural transformations monitored by TEM in various time, (a) 0.5 h (b) 6 h and (c) 12 h. (d) Tracing the size of particles in solution for various polymerization time using DLS.
further proceeds. This termination leads to further covalent association between particles and the formation of large nanoparticle aggregates as observed in TEM analysis (Fig. 3b). It is worthy to note that the abrupt increase of polymerization solution viscosity was observed after this stage though still the solution is not fully gelated yet, strongly indicating thickening effect which typically occurs when the micro- and nano-domains are chemically or physically associated with others to form microgel type structure (Kim et al., 2004, 2003; Yekta et al., 1995). Upon further polymerization for more 6 h (12 h), the viscosity greatly increased and fully gelated solid was observed, which could not be characterized with DLS as the signal was not on diffusive mode anymore, indicating gel formation. Therefore, the sample was characterized with scanning transmission electron microscope (STEM) shown in Fig. 3c. The dark field image shows all silica particles were embedded in polymer matrix. These observations strongly suggest that the SIP eventually induces an interparticular association by combination of active chain ends, resulting in 3D network structure and hence an organic/inorganic hybrid hydrogel as presented in previous report (Kim et al., 2009; Lee et al., 2013).
4. Conclusions We demonstrated our detailed investigation on the step-wise mechanism of surface-initiated polymerization of organic/inorganic hybrid nanocomposites via γ-ray pre-irradiation method. The polymerization of AA was initiated by radicals on the surface of silica particles, resulting in core-shell type silica/PAA nanoparticles at the first stage. At the second stage, two PAA coated silica particles can be associated by radical combination of two different growing grafted chains on different silica particles. Then, the particles begin to aggregate and form several domains similar to the micro-gel structure. Finally, the polymer chains on each domain begin to extend to adjacent domains resulting in the formation of a 3D network of bulk hydrogel. The result highlights that controlling reaction time is critical to achieve specific and desirable organic/inorganic nanocomposite structure among core-shell particle, micro-gel and 3D network bulk hydrogel. Easily and effectively achieved various organic/inorganic nanocomposite structures by this process could be a valuable finding for exploitation at industrial scale in cosmetics, medical applications such as tissue engineering and drug delivery. Our findings further emphasize the versatility of this method to synthesize various nanocomposite structures from nano to bulk materials.
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Acknowledgments This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (2015M2B2A9032029).
References Afroze, J.D., Abden, M.J., Alam, M.S., Bahadur, N.M., Gafur, M.A., 2016. Development of functionalized carbon nanotube reinforced hydroxyapatite magnetic nanocomposites. Mater. Lett. 169, 24–27. Balayeva, N.O., Mamiyev, Z.Q., 2016. Synthesis and studies of CdS and ZnS-PE/NBR modified thermoplastic elastomeric copolymer nanocomposite films. Mater. Lett. 162, 121–125. Burnside, S.D., Giannelis, E.P., 1995. Synthesis and properties of new poly(dimethylsiloxane) nanocomposites. Chem. Mater. 7, 1597–1600. Carenza, M., 1992. Recent achievements in the use of radiation polymerization and grafting for biomedical applications. Int. J. Radiat. Appl. Instrum. Part C. Radiat. Phys. Chem. 39, 485–493. Chapiro, A., 1964. Radiation chemistry of polymers. Radiat. Res. Suppl. 4, 179–191. Chapman, R., Mulvaney, P., 2001. Electro-optical shifts in silver nanoparticle films. Chem. Phys. Lett. 349, 358–362. Griscom, D.L., 1985. Defect structure of glasses: some outstanding questions in regard to vitreous silica. J. Non-Cryst. Solids 73, 51–77. Griscom, D.L., Hosono, H., Imai, H., Hirashima, H., 1994. Proceedings of the first PAC RIM meeting on glass and optical materials intrinsic- and extrinsic-defect formation in silica glasses by radiation. J. Non-Cryst. Solids 179, 202–213. Huang, T., Xu, H.G., Jiao, K.X., Zhu, L.P., Brown, H.R., Wang, H.L., 2007. A novel hydrogel with high mechanical strength: a macromolecular microsphere composite hydrogel. Adv. Mater. 19, 1622–1626. Imai, H., Hirashima, H., 1994. Intrinsic-and extrinsic-defect formation in silica glasses by radiation. J. non-Cryst. Solids 179, 202–213. Kim, J., Koo, J., Shirahase, T., Takahara, A., Sohn, D., 2009. Preparation of organic/ inorganic hybrid gel after & gamma-ray radiation. Chem. Lett. 38, 1112–1113. Kim, M., Choi, Y.-W., Sim, J.-H., Choo, J., Sohn, D., 2004. End chain length effect of
hydrophobically end-capped poly (ethylene oxide) s on their self-assemblies in solution. J. Phys. Chem. B 108, 8269–8277. Kim, M., Sim, J.-H., Sohn, D., 2003. Adsorption of hydrophobically end-capped poly (ethylene oxide) on poly (tetrafluoroethylene) latex particles: light scattering study. Macromolecules 36, 9986–9993. Kim, Y.-S., Seo, K.-S., Choi, S.-H., 2016. Polymeric nanocomposite proton exchange membranes prepared by radiation-induced polymerization for direct methanol fuel cell. Radiat. Phys. Chem. 118, 35–41. Kokatnur, V., Jelling, M., 1941. Iodometric determination of peroxygen in organic compounds. J. Am. Chem. Soc. 63, 1432–1433. Koo, J., Kim, J., Lee, H., Chung, H., Lee, Y., Yi, W., Sohn, D., 2012. Formation and characterization of poly(acrylic acid) on silica particles irradiated by γ-ray radiation. Macromol. Res. 20, 138–142. Lee, H., Ryu, J., Kim, D., Joo, Y., Lee, S.U., Sohn, D., 2013. Preparation of an imogolite/ poly(acrylic acid) hybrid gel. J. Colloid Interface Sci. 406, 165–171. Madera-Santana, T.J., Meléndrez, R., González-García, G., Quintana-Owen, P., Pillai, S.D., 2016. Effect of gamma irradiation on physicochemical properties of commercial poly(lactic acid) clamshell for food packaging. Radiat. Phys. Chem. 123, 6–13. Stöber, W., Fink, A., Bohn, E., 1968. Controlled growth of monodisperse silica spheres in the micron size range. J. Colloid Interface Sci. 26, 62–69. Wang, Y., Chang, H., Jia, L., Zhu, T., Xu, Z., Zhou, T., Li, H., Li, Z., Xu, J., 2015. Development of a visible-light-sensitized THA-based lanthanide nanocomposite for cell imaging. Mater. Lett. 161, 644–647. Wilson, O., Wilson, G.J., Mulvaney, P., 2002. Laser writing in polarized silver nanorod films. Adv. Mater. 14, 1000–1004. Xie, Y., Qiao, Z., Chen, M., Liu, X., Qian, Y., 1999. γ-Irradiation Route to Semiconductor/Polymer Nanocable Fabrication. Adv. Mater. 11, 1512–1515. Ye, Y., Chen, H., Wu, J., Ye, L., 2007. High impact strength epoxy nanocomposites with natural nanotubes. Polymer 48, 6426–6433. Yekta, A., Xu, B., Duhamel, J., Adiwidjaja, H., Winnik, M.A., 1995. Fluorescence studies of associating polymers in water: determination of the chain end aggregation number and a model for the association process. Macromolecules 28, 956–966. Yoon, P.J., Fornes, T.D., Paul, D.R., 2002. Thermal expansion behavior of nylon 6 nanocomposites. Polymer 43, 6727–6741. Yoshii, F., Makuuchi, K., Darwis, D., Iriawan, T., Razzak, M.T., Rosiak, J.M., 1995. Heat resistance poly(vinyl alcohol) hydrogel. Radiat. Phys. Chem. 46, 169–174.
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Short communication
Trace the polymerization induced by gamma-ray irradiated silica particles Hoik Lee a,n, Jungju Ryu b, Myungwoong Kim c, Seung Soon Im a,d, Ick Soo Kim a, Daewon Sohn b,n a Nano Fusion Technology Research Lab, Division of Frontier Fibers, Institute for Fiber Engineering (IFES), Interdisciplinary Cluster for Cutting Edge Research (ICCER), Shinshu University 3-15-1, Tokida, Ueda, Nagono 386-8567, Japan b Department of Chemistry and Research Institute for Natural Sciences, Hanyang University, Seoul 133-791, Republic of Korea c Department of Chemistry, Inha University, Incheon 22212, Republic of Korea d Department of Organic and Nano Engineering, College of Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea
H I G H L I G H T S
Pre-irradiation of γ-ray enables an effective synthesis of polymer/inorganic nanocomposites toward pre-selected properties. Solution dynamics and electron microscopic studies reveal the formation mechanism of the nanocomposites. Reaction time is a critical parameter to achieve desirable organic/inorganic nanocomposite structure in a given system.
art ic l e i nf o
a b s t r a c t
Article history: Received 8 March 2016 Received in revised form 6 April 2016 Accepted 12 April 2016 Available online 13 April 2016
A γ-ray irradiation to inorganic particles is a promising technique for preparation of organic/inorganic composites as it offers a number of advantages such as an additive-free polymerizations conducted under mild conditions, avoiding undesired damage to organic components in the composites. Herein, we demonstrated a step-wise formation mechanism of organic/inorganic nanocomposite hydrogel in detail. The γ-ray irradiation to silica particles dispersed in water generates peroxide groups on their surface, enabling surface-initiated polymerization of acrylic acid from the inorganic material. As a result, poly (acrylic acid) (PAA) covers the silica particles in the form of a core-shell at the initial stage. Then, PAAcoated silica particles associate with each other by combination of radicals at the end of chains on different particles, leading to micro-gel domains. Finally, the micro-gels are further associated with each other to form a 3D network structure. We investigated this mechanism using dynamic light scattering (DLS) and transmission electron microscopy (TEM). Our result strongly suggests that controlling reaction time is critical to achieve specific and desirable organic/inorganic nanocomposite structure among coreshell particles, micro-gels and 3D network bulk hydrogel. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Nanoparticles Core-shell particles Polymeric composites Nanocomposites γ-Ray pre-irradiation
1. Introduction Nanocomposites have attracted tremendous scientific and technical interests due to their exceptional mechanical, (Afroze et al., 2016; Lee et al., 2013; Ye et al., 2007) electrical, (Chapman and Mulvaney, 2001; Ye et al., 2007) optical, (Chapman and Mulvaney, 2001; Wang et al., 2015; Wilson et al., 2002) and thermal properties (Balayeva and Mamiyev, 2016; Ye et al., 2007; Yoon
n
Corresponding authors. E-mail addresses: [email protected] (H. Lee), [email protected] (D. Sohn). http://dx.doi.org/10.1016/j.radphyschem.2016.04.008 0969-806X/& 2016 Elsevier Ltd. All rights reserved.
et al., 2002). In particular, organic/inorganic nanocomposites have been extensively explored as the platform allows modifying a variety of properties by realizing their potential from bulk to molecular level (Burnside and Giannelis, 1995). A γ-ray irradiation method has been considered as one of promising techniques for an effective synthesis of organic/inorganic nanocomposites, as the process is easily controlled and the system is simultaneously sterilized effectively. Furthermore, since γ-ray initiates polymerization or induces crosslinking in the nanocomposites, the resulting composites do not include any additives such as initiators and crosslinkers or by-products which possibly degrade desired properties in the composites (Chapiro, 1964; Huang et al., 2007).
H. Lee et al. / Radiation Physics and Chemistry 125 (2016) 160–164
Therefore, γ-ray irradiation has been utilized to tailor the properties of materials to make them more versatile for further applications such as semiconductors, (Xie et al., 1999) membranes, (Kim et al., 2016) food packaging, (Madera-Santana et al., 2016) smart adhesive plasters (Yoshii et al., 1995) and hydrogels (Carenza, 1992). However, direct high-energy γ-ray irradiation to organic materials such as monomers or polymers can induce undesirable chemical reactions such as decomposition or unintended crosslinking in resulting composites. Thus, the resulting chemical/ physical structure and properties are not rationally predicted. To address this challenge, a pre-irradiation method has been proposed and explored where the inorganic materials such as silica fillers directly are treated with γ-ray irradiation which produces peroxide groups on their surfaces (Griscom et al., 1994; Lee et al., 2013). The peroxide groups undergoes homolytic cleavage to result in silyloxy radicals on particle surfaces which allows surface-initiated polymerization (SIP) at mildly heated environment, e.g. 40 °C. Therefore, the pre-irradiation method enables avoiding any damages to organic components in nanocomposites during γ-ray irradiation process. Using this method, we previously reported an additive-free preparation of polymer/inorganic particle nanocomposite hydrogel which exhibited high mechanical strength (Koo et al., 2012; Lee et al., 2013). The results strongly suggested that pre-irradiation method provided an efficient route to tailor physical properties in soft materials, however, details in the mechanism to form the final structure has not been investigated yet. Herein, we systematically studied how the system evolves from γ-ray treated silica nanoparticles, silica/PAA core/shell nanoparticles to microgels by combining solution dynamics and microscopic studies using dynamic light scattering (DLS) and transmission electron microscopy (TEM) techniques. Careful examination of the process as a function of reaction time allows us to gain deep understanding in the formation of 3D network gel structure. Furthermore, current studies highlight the potential of γ-ray to synthesize various types of organic/inorganic nanocomposites from suitable monomer and inorganic particles to achieve desirable properties for target applications.
2. Experimental details
161
the surfaces of silica particles. The resulting samples were stored at 0 °C before the polymerization process. Following previous studies on peroxide formation on the silica surface, (Griscom, 1985; Imai and Hirashima, 1994; Kokatnur and Jelling, 1941) the formation of peroxide groups on the silica particle was confirmed using iodometry which is a common method to verify peroxide groups in various systems (Kim et al., 2009). Potassium iodide and isopropyl alcohol were added to the irradiated silica particle solution. After a reflux for 30 min, the solution gradually turned to yellow color which indicates the formation of I2, confirming the presence of peroxides on the silica particle. Generated peroxide group by γ-ray irradiation is used as an initiator for surface-initiated polymerization. 2.4. Synthesis of PAA/silica nanocomposite 1 mL of Acrylic acid was added to 2 mL of γ-ray irradiated 2 wt% silica particle solution, then dissolved gas was removed by freeze-pump-thaw process for three times. Then, the solution was heated up to 40 °C to initiate the polymerization. The polymerization times was controlled for 30 min, 2 h, 6 h and 12 h, to trace surface-initiated polymerization process. 2.5. Characterizations DLS experiments were performed with a UNIPHASE He-Ne laser operating at 632.8 nm. The detector equipment employed optical fibers coupled to an ALV/SO-SIPD/DUAL detection unit with an EMI PM-28B power supply and ALV/PM-PD pre-amplifier discriminator. An ALV-5000/E/WIN multiple tau correlator with 288 exponentially spaced channels was used to produce correlation functions. The laser beam passed through a cylinder scattering cell which was located in a bath of decaline as a refractive index matching solvent, then passing through a 400 nm pinhole before reaching the detector. The decay rate (Γ) was calculated from the Eq. (1) with correlation functions obtained at nine scattering angles in 30–150°,
Γ = 1/τ
(1)
where τ is an average decay time. The diffusion coefficient (Dt) was calculated using Eq. (2) and Eq. (3).
Γ = Dt q2
(2)
q = (4πn/λ )·sin (θ /2)
(3)
2.1. Materials Tetraethyl orthosilicate (TEOS), ethanol, aqueous ammonia solution (NH4OH) and acrylic acid (AA) were purchased from Sigma Aldrich and were used without further purification. Deionized water (DI water, resistivity of 418.2 MΩ cm) was obtained using Milli-Q system.
where n is a refractive index of a solvent, q is a scattering vector, λ is the wavelength of incident light and θ is a scattering angle. The hydrodynamic radius (Rh) is obtained from Dt using Stokes-Einstein equation (Eq. (4)),
2.2. Silica particle synthesis
Rh = kb T /6πηDt
Silica particles were synthesized using Stöber method (Stöber et al., 1968). A solution of ethanol (150 mL), TEOS (9 mL) and NH4OH (3 mL) in a 250 mL volumetric flask was stirred for 24 h, resulting in spherical silica particles with a radius of 63 nm. The solution was washed thrice with ethanol and subsequently with DI water twice.
where kb is Boltzmann constant, T is temperature and η is solvent viscosity. TEM and STEM images were obtained with JEOL JEM-2100F with operation voltage of 200 kV. Aliquots were taken after desired reaction time, followed by exposing to air with cooling down for quenching. Then a drop of sample was placed on 200 mesh copper Formvar carbon coated TEM grid.
(4)
2.3. Surface modification of silica particles by γ-ray irradiation An aqueous solution of silica particle (2 wt%) in a narrownecked ampoule (2 cm in diameter and 5 cm in length) was subjected to 60Co γ-rays at a dose of 10 kGy h 1 (at KAERI, Jeongeup, Korea) at ambient conditions for 2 h to create peroxide groups on
3. Results and discussion The gel formation process can be explained with four stages: (1) initiation on silica nanoparticle surface, (2) propagation of AA
162
H. Lee et al. / Radiation Physics and Chemistry 125 (2016) 160–164
Fig. 1. Schematic illustration of proposed mechanism for the formation of 3D network gel structure in silica/PAA nanocomposite by surface-initiated polymerization: initiation, propagation, and mutual combination of growing chains of several particles.
polymerization from surface resulting in core/shell nanoparticle, (3) microgel formation by an association of several nanoparticles and (4) complete gelation by further associations between microgel domains. This mechanism is graphically depicted in Fig. 1. In details, silica nanoparticle aqueous dispersion was exposed to 60Co γ-ray irradiation to form peroxide groups on particle surfaces, subsequently the peroxide was decomposed into silyloxy radical on silica by mild heating (40 °C), leading to an effective surface initiation in the presence of acrylic acid monomers in a solution and thereby forming thin PAA layer over the silica surface and leading to core-shell structure consisting of silica particles as a core and PAA as a shell. The Rh distribution of as-prepared silica particle is compared to that after SIP of AA from silica particle in Fig. 2(a). The average Rh of bare silica and PAA/silica nanoparticles in distributions were 637 8 nm and 817 18 nm, respectively, confirming the particles are covered by shell layer of PAA as a result of SIP. We confirmed that the Stokes-Einstein relationship is valid to gain Rh value in this system by observing scattering angle dependence (q2) of decay rate (Γ). Fig. 2(b) shows that decay rate values measured from a solution of PAA/silica nanoparticles follows the relationship of Γ ¼Dtq2, indicating scattered signals represent a diffusive mode of dispersed particles. The slope of fitted line was calculated to be
3069.7 nm2/ms, which is converted to 79 nm in Eq. (4), confirming that DLS measurements are suitable to confirm the polymerization of AA at this stage. The inset in Fig. 2(a) is TEM image of PAA/silica nanoparticles; the size is in good agreement with what was measured in DLS. Therefore, upon polymerization for 30 min, PAA was grown from the surface, resulting in 20 nm thick PAA/silica shell/core nanoparticle dispersion. It is worthwhile to note that thin soft polymeric layer was observed in TEM as shown in Fig. 3 (a), suggesting that AA might undergo the self-polymerization or polymerization initiated by hydroxyl radical which is also produced in homolytic cleavage of peroxide group. We further examine the polymerization for longer than 30 min using the same methods. After 6 h, the average Rh was increased to 212 795 nm from 81 718 nm, indicating the formation of large aggregates. Upon the formation of core/shell type particles by SIP, the active chain ends on one particle undergoes mutual termination by radical combination with active chain ends on another particle surface, resulting in association of several particles. Also we note that standard deviation in Rh largely increases at this stage. This is likely due to the nature of recombination in free radical polymerization leading to a broad molecular weight distribution; the aggregates can react with another much larger or much resulting in broad size distribution when the reaction
Fig. 2. (a) The distributions in hydrodynamic radius of silica particles (black) and after polymerization of acrylic acid on silica particles by SIP (red), obtained with DLS measurements. The inset shows TEM image of PAA/silica particles. (b) Plot of scattering angle dependent decay rate for PAA/silica particle solution in Fig. 2(a).
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Fig. 3. Structural transformations monitored by TEM in various time, (a) 0.5 h (b) 6 h and (c) 12 h. (d) Tracing the size of particles in solution for various polymerization time using DLS.
further proceeds. This termination leads to further covalent association between particles and the formation of large nanoparticle aggregates as observed in TEM analysis (Fig. 3b). It is worthy to note that the abrupt increase of polymerization solution viscosity was observed after this stage though still the solution is not fully gelated yet, strongly indicating thickening effect which typically occurs when the micro- and nano-domains are chemically or physically associated with others to form microgel type structure (Kim et al., 2004, 2003; Yekta et al., 1995). Upon further polymerization for more 6 h (12 h), the viscosity greatly increased and fully gelated solid was observed, which could not be characterized with DLS as the signal was not on diffusive mode anymore, indicating gel formation. Therefore, the sample was characterized with scanning transmission electron microscope (STEM) shown in Fig. 3c. The dark field image shows all silica particles were embedded in polymer matrix. These observations strongly suggest that the SIP eventually induces an interparticular association by combination of active chain ends, resulting in 3D network structure and hence an organic/inorganic hybrid hydrogel as presented in previous report (Kim et al., 2009; Lee et al., 2013).
4. Conclusions We demonstrated our detailed investigation on the step-wise mechanism of surface-initiated polymerization of organic/inorganic hybrid nanocomposites via γ-ray pre-irradiation method. The polymerization of AA was initiated by radicals on the surface of silica particles, resulting in core-shell type silica/PAA nanoparticles at the first stage. At the second stage, two PAA coated silica particles can be associated by radical combination of two different growing grafted chains on different silica particles. Then, the particles begin to aggregate and form several domains similar to the micro-gel structure. Finally, the polymer chains on each domain begin to extend to adjacent domains resulting in the formation of a 3D network of bulk hydrogel. The result highlights that controlling reaction time is critical to achieve specific and desirable organic/inorganic nanocomposite structure among core-shell particle, micro-gel and 3D network bulk hydrogel. Easily and effectively achieved various organic/inorganic nanocomposite structures by this process could be a valuable finding for exploitation at industrial scale in cosmetics, medical applications such as tissue engineering and drug delivery. Our findings further emphasize the versatility of this method to synthesize various nanocomposite structures from nano to bulk materials.
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Acknowledgments This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (2015M2B2A9032029).
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