Elsevier

Polymer

Volume 52, Issue 1, 7 January 2011, Pages 172-179
Polymer

Preparation and characterization of thermosensitive organic–inorganic hybrid microgels with functional Fe3O4 nanoparticles as crosslinker

https://doi.org/10.1016/j.polymer.2010.11.014Get rights and content

Abstract

A methodology is described for the preparation of thermosensitive organic–inorganic hybrid microgels with functional Fe3O4 nanoparticles as the crosslinker and N-isopropylacrylamide (NIPAm) as the monomer. Magnetic Fe3O4 nanoparticles were first prepared via a redox reaction in aqueous solution and then modified with 3-(trimethoxysilyl)propylmethacrylate (TMSPMA) via the silanization. The bonding of multiple TMSPMA monomers on the surface of Fe3O4 nanoparticles renders them as crosslinker. Surfactant-free emulsion polymerization (SFEP) of NIPAm was then carried out with the presence of TMSPMA-modified Fe3O4 nanoparticles at 70 °C in aqueous solution, leading to the formation of thermosensitive PNIPAm-Fe3O4 hybrid microgels crosslinked with Fe3O4 nanoparticles. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), thermogravimetric analysis (TGA), dynamic light scattering (DLS) and physical properties measurement system (PPMS) were then used to characterize the resultant hybrid microgels. The experimental results show that the PNIPAm-Fe3O4 hybrid microgels were spherical in shape with a large size distribution and the Fe3O4 nanoparticles were randomly distributed inside the microgels. The PNIPAm-Fe3O4 hybrid microgels were thermosensitive, exhibiting a reversible swelling and deswelling behavior as a function of temperature. The PNIPAm-Fe3O4 hybrid microgels also show superparamagnetic behavior at room temperature (300 K).

Introduction

Since Pelton and Chibante [1] first reported the preparation and characterization of thermosensitive poly(N-isopropylacrylamide) (PNIPAm) microgels via surfactant-free emulsion polymerization (SFEP) in 1986, microgels have attracted extensive interests due to their potential applications in many diverse fields, specially such as drug delivery, biosensor, templates for nanoparticle synthesis and catalyst [2], [3], [4], [5], [6]. It is thus greatly significant to tailor the properties and functionalities of the microgels. Conventional methods mainly involve the introduction of the second or third monomer during the SFEP of N-isopropylacrylamide (NIPAm), which will lead to the microgels with various functionalities, like response to pH value, UV irradiation of the environments, etc. [7], [8], [9], [10].

In recent decades, a new type of materials, namely organic–inorganic hybrid materials, has emerged [11], [12], [13], [14], [15], [16]. These organic–inorganic hybrid materials possess and combine at the same time the properties of organic components and inorganic components, which render them more superior microstructures and properties. Many researchers have devoted their numerous efforts to the fabrication and characterization of organic–inorganic hybrid materials [3], [4], [6], [11], [12], [13], [14], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. Concerning the research field of microgels, the organic–inorganic hybrid microgels have been reported in recent years [11], [24], [26], [27], [29], [30], [31], [32], [33], [34], [35]. In a recent review, Karg and Hellweg have classified the types of hybrid microgels reported in literature [11]. Three typical classes of organic–inorganic hybrid microgels were discussed. They are: (1) Core-shell microgels with inorganic nanoparticles as cores, of which the gel shells are formed on the surface of inorganic nanoparticles; (2) Microgels filled with inorganic nanoparticles, of which the inorganic nanoparticles are in-situ synthesized inside the microgels; (3) Microgels covered with nanoparticles, of which the microgels are first prepared and the inorganic nanoparticles are coated onto the microgels in the following step. Various inorganic nanoparticles, such as Au, Ag, Fe3O4, SiO2 and TiO2, etc., have been incorporated into the microgels, forming the hybrid systems [17], [19], [20], [22], [23], [28], [29], [30], [31], [32], [33], [34], [35]. Depending on the inorganic nanoparticles used, the resultant hybrid microgels can have various special properties, such as local surface plasmon, magnetic and fluorescence properties, etc. [19], [20], [28], [29], [30], [31], [32], [33], [34], [35], [36] However, most of the hybrid microgels reported in recent years have microgel-shells or microgels prepared with the presence of conventional chemical crosslinker, N,N′-methylene-bisacrylamide (BIS).

The fourth possible class of organic–inorganic hybrid microgels is that the hybrid microgels are in-situ formed via covalent crosslinking with functional inorganic nanoparticles without adding any conventional chemical crosslinker. In other word, the functional inorganic nanoparticles act as chemical crosslinkers. Currently, reports about such fourth class of organic–inorganic hybrid microgels are still rare. Clay nanoparticles were first successfully used as crosslinking agent for the preparation of organic–inorganic hybrid hydrogels [37], [38], [39], [40] and microgels [41] in the absence of conventional chemical crosslinker. However, the crosslinking mechanism of the clay particles is not well-defined and still unclear [37], [38], [39], [40], [41]. It remains unknown whether there are covalent bonds between the clay particles and the network chains. For organic–inorganic hybrid solid nanoparticles, modified clay nanoparticles were successfully encapsulated inside latex particles via covalent bonds [42]. In another report, magneto-responsive polystyrene (PS) gels were fabricated by using functional Fe nanoparticles as crosslinking agent via the surface-initiated atomic transfer radical polymerization (SI-ATRP) without adding any conventional chemical crosslinker [43]. However, the mechanism of crosslinking and network formation was not well-established as well and still under debate for such Fe–PS hybrid gels. The authors argued that the crosslinking points were formed via the interparticle termination of propagating radicals on the Fe nanoparticles [43]. Here, we reported the successful preparation of thermosensitive organic–inorganic PNIPAm-Fe3O4 hybrid microgels with functional Fe3O4 nanoparticles as the crosslinker and NIPAm as the monomer. The PNIPAm chains were covalently crosslinked with surface-functional Fe3O4 nanoparticles. The crosslinking mechanism was well-defined in the present work, which involved the radical crosslinking polymerization of multiple carbon–carbon double bonds grafted on the surface of Fe3O4 nanoparticles and the formation of covalent bonds between Fe3O4 nanoparticles and network chains. No conventional chemical crosslinker was used. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), thermogravimetric analysis (TGA), dynamic light scattering (DLS), and physical properties measurement system (PPMS) were then used to characterize the functional Fe3O4 nanoparticles and resultant hybrid microgels. We demonstrated that the thermosensitive and magnetic PNIPAm-Fe3O4 hybrid microgels were indeed obtained, which were spherical in shape with a relatively large size distribution. The Fe3O4 nanoparticles were randomly distributed inside the hybrid microgels.

Section snippets

Chemical and materials

N-isopropylacrylamide (NIPAm: 99%) and 3-(trimethoxysilyl)propylmethacrylate (TMSPMA: 98%) were purchased from Acros Organics. Ferrous chloride tetrahydrate (FeCl2·4H2O), anhydrous ferric chloride (FeCl3), potassium peroxydisulfate (K2S2O8) (Sinopharm Chemical Reagent Co. LTD) and ammonia solution (25%, Hangzhou Changzheng Chemical Reagent Co. LTD) were used without further treatment. All other reagents were of analytical grade and used as received. Deionized water was used.

Preparation and surface modification of Fe3O4 magnetic nanoparticles

The preparation of Fe

Results and discussion

The synthesis procedure for the thermosensitve PNIPAm-Fe3O4 hybrid microgels consists of the following three steps as shown schematically in Scheme 1: (1) the preparation of magnetic Fe3O4 nanoparticles via redox reaction; (2) the surface modification of magnetic Fe3O4 nanoparticles by using TMSPMA; (3) the synthesis of thermosensitive PNIPAm-Fe3O4 hybrid microgels via SFEP by using TMSPMA-modified Fe3O4 nanoparticles as the crosslinker and NIPAm as the monomer.

Fig. 1 shows the TEM morphologies

Conclusions

The thermosensitive organic–inorganic PNIPAm-Fe3O4 hybrid microgels were successfully prepared via surfactant-free emulsion polymerization (SFEP) by using N-isopropylacrylamide (NIPAm) as the thermosensitive monomer and 3-(trimethoxysilyl)propylmethacrylate (TMSPMA)-modified magnetic Fe3O4 nanoparticles as the crosslinker. The TMSPMA-modified magnetic Fe3O4 nanoparticles were confirmed to behave as crosslinkers, leading to the formation of PNIPAm-Fe3O4 hybrid microgels. The resultant PNIPAm-Fe3O

Acknowledgments

The authors thank the National Natural Science Foundation of China (No. 20604022, 20874087) and 863 project (No. 2009AA04Z125) for financial supports.

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