Materials Chemistry and Physics 124 (2010) 780–784
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Parylene nanocomposites using modified magnetic nanoparticles Ignacio García a,∗ , A. Ruiz de Luzuriaga a , H. Grande a , L. Jeandupeux b , J. Charmet b , E. Laux b , a ˜ H. Keppner b , D. Mecerreyes a , German Cabanero a b
New Materials Department, CIDETEC - Centre for Electrochemical Technologies, Parque Tecnológico de San Sebastián, Paseo Miramón 196, Donostia-San Sebastián E 20009, Spain HES-SO Arc, Institut des Microtechnologies Appliquées, Eplatures- Grises 17, 2300 La Chaux-de Fonds, Switzerland
a r t i c l e
i n f o
Article history: Received 3 February 2010 Received in revised form 28 June 2010 Accepted 24 July 2010 Keywords: Nanoparticles Nanocomposites Surface modification Parylene
a b s t r a c t Parylene/Fe3 O4 nanocomposites were synthesized and characterized. The nanocomposites were obtained by chemical vapour deposition polymerization of Parylene onto functionalized Fe3 O4 nanoparticles. For this purpose, allyltrichlorosilane was used to modify the surface of 7 nm size Fe3 O4 nanoparticles obtained by the coprecipitation method. The magnetic nanoparticles and obtained nanocomposite were characterized with X-ray diffraction (XRD), infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA) and magnetic measurements (SQUID). The successful incorporation of different amounts of nanoparticles into Parylene was confirmed by FTIR and TGA. Interestingly, increments in saturation magnetization of the nanocomposites were observed ranging from 0 emu/g of neat Parylene to 16.94 emu/g in the case of nanocomposite films that contained 27.5 wt% of nanoparticles. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Poly(p-xylylene)-based polymers, also known as Parylenes, were first developed in 1949 by Szwarc [1]. Thin films of Parylenes are usually obtained by a chemical vapour deposition (CVD) process known as the Gorham process [2]. Parylenes are known for their chemical inertness, low dielectric constant and excellent barrier properties. The polymerization proceeds at room temperature and produces semi-crystalline, transparent conformal and pinhole-free coatings. Over the years, Parylene has established itself as a material of choice used in packaging of circuit boards, semiconductors and hybrid circuits, corrosion resistant coatings and biomedical applications [3]. Recently, Parylene has received considerable scientific attention by its potential application in MEMS devices [4]. There are three forms of Parylene: Parylene N, C and D. Parylene N (di-p-xylylene or di-para-xylylene), the basic member of the series, is a crystalline material. Parylene C and D are monoand dichloro substituted di-p-xylylenes, respectively [5]. Parylene C is the most widely used polymer of the poly(p-xylylene) family because of its biocompatibility and its excellent barrier properties and manufacturing advantages. Metal nanoparticles are receiving a growing interest due to their unique electronic, luminescent and magnetic properties that are not present in the bulk [17–19]. In order to improve the dispersion of nanoparticles inside polymeric material, several surface modifi-
∗ Corresponding author. Tel.: +34 943309022; fax: +34 943309136. E-mail address:
[email protected] (I. García). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.07.060
cation techniques such as grafting from, copolymerization through a covalently linked monomer and grafting to have been reported [20–24]. Nowadays it is well known that polymer nanocomposites have increased the number of application of polymers. This is due to the synergic effect observed in some cases when the polymers and the different kinds of nanoadditives are combined in the form of a nanocomposite [6–12]. Nanocomposites offer nowadays a broad range of technological advantages versus polymers in applications such as barrier properties, flame resistance, cosmetic application, composites reinforcement and anti-bacterial properties. Several authors have shown that transition metals, metal salts, and organometallic complexes, such as those of iron, ruthenium, platinum, palladium, copper, and silver inhibit polymer deposition of Parylene on the substrate [13–16]. For this reason, it is very difficult to obtain nanocomposites based on Parylenes polymers. In this paper, we report the synthesis and characterization of new Parylene nanocomposites with magnetic nanoparticles. In order to avoid the inhibition of Parylene deposition the surface of the iron oxide nanoparticles was modified using allyltrichlorosilane. As a result, we present the first example of multifunctional Parylene C nanocomposite coatings with magnetic properties. 2. Experimental 2.1. Materials Iron(II) chloride tetrahydrate (99%, FeCl2 ·4H2 O) and Iron(III) chloride hexahydrate (99%, FeCl3 ·6H2 O) and ammonium hydroxide solution at 28% in H2 O (99%, NH4 OH), were purchased from Aldrich and used as received. Allyltrichlorosilane (ATCS) was purchased from ABCR and used without any further purification. Dry toluene was purchased by Scharlab and used as received.
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Scheme 1. (A) Surface modification of iron oxide magnetic nanoparticles and (B) schematic of the Gorham method that was used for deposition of Parylene C.
2.2. Synthesis of Fe3 O4 nanoparticles Fe3 O4 nanoparticles were synthesized by coprecipitation method of aqueous solution of Fe2+ and Fe3+ . A solution containing 0.496 g of FeCl3 ·6H2 O and 0.200 g of FeCl2 ·4H2 O and 300 mL of pure water was heated to 63 ◦ C. Afterwards, 170 mL of aqueous solution of ammonium hydroxide solutions of 28 wt% NH4 OH was added. The solution was stirred for 15 min, suddenly, a brown powder appeared. The powder was magnetically separated and washed with aqueous solution of ammonium hydroxide of 5 wt% NH4 OH to remove residual ions from the reaction mixture, followed by washing with acetone to remove the water. Then, the obtained Fe3 O4 nanoparticles were dried at 50 ◦ C under vacuum for 1 day. 2.3. Surface modification of Fe3 O4 ATCS, surface modificator, was anchored to surface using a similar procedure reported by several authors [25–27] as shown in Scheme 1A. 40 mL of extra dry toluene, 1 mL of ATCS and 300 mg of Fe3 O4 were mixed at room temperature under nitrogen using magnetic stirring for 24 h. After reaction Fe3 O4 modified with ATCS (Fe3 O4 -ATCS) were subsequently washed 6 times with tetrahydrofuran and then dried under vacuum at 50 ◦ C for 1 day. 2.4. Preparation of Parylene nanocomposites Nanocomposites were prepared by casting of tetrahydrofuran stable solution of 0.5 wt% Fe3 O4 -ATCS onto aluminum substrate using a similar procedure shown by Artukovic et al. [28]. The suspension was sonicated for 1 h and decanted to remove large and heavier bundles from the suspension. Using decanted suspension several quantities of nanoparticles were deposited onto surfaces (3, 6 and 9 depositions of dispersion). After solvent evaporation, Parylene was deposited onto aluminum substrate that bears Fe3 O4 -ATCS particles. Parylene C films were deposited on the nanoparticles coated aluminum substrates according to a LPCVD (low pressure chemical vapour deposition) process also known as the Gorham process [2] (Scheme 1B). The depositions took place, under vacuum, in a COMELEC reactor model 1010. The starting material, dichloroparacyclophane, obtained from Galentis Srl. is first sublimated (130 ◦ C) and then downstream transported through a pyrolysis stage where it is cracked into a monomer at 650 ◦ C. The reactive monomer finally condenses and polymerises at room temperature on the samples in the deposition chamber. The deposition process stops upon complete consumption of the starting material. Under those conditions, 4 g of dichloroparacyclophane are necessary to obtain about 2 m thin Parylene C films on the samples placed in the deposition chamber. During the deposition, the base pressure was kept below 60 mTorr to prevent polymerization in the vapour phase and to avoid contamination and oxidation of the monomers. Films of neat Parylene and Parylene nanocomposites were removed from aluminum substrate and washed with ethanol, water and acetone in order to remove nanoparticles that were not encapsulated by Parylene. 2.5. Characterization X-ray diffraction patters of the iron oxide nanoparticles were collected using the spectrometer X’Pert MPD by ‘Philips’ with Cu K␣ radiation (the wavelength is ˚ = 1.54056 A).
FTIR spectra were recorded with a FTIR spectrometer (IFS66v, Bruker). The nanoparticles were pressed together with KBr to form pellets, which were analyzed via the signal averaging of 32 scans at a resolution of 2 cm−1 . Blind measurements of neat KBr were also obtained and subtracted from the final spectra. TGA was performed with a TA Instrument TGA Q500 under a nitrogen atmosphere at a heating rate of 10 ◦ C/min from room temperature to 800 ◦ C. Atomic force microscopy images of the samples were obtained in a tapping mode at room temperature using a scanning probe microscope (Molecular Imaging’s PicoScan) equipped with a nanosensors tips/cantilever having a resonance frequency of approximately 330 KHz and a spring constant of about 42 N/m with a tip nominal radius lower than 7 nm. Magnetic properties of samples were performed using “Superconducting Quantum Interference Device” (SQUID) Quantum Design Magnetic Property Measurement System (MPMS) 5S. MPMS allows to study magnetization (M) and susceptibility of small quantity of samples. The samples were measured at 300 K and a magnetic field between 50,000 Oe and −50,000 Oe were applied.
3. Results and discussion 3.1. Synthesis of Fe3 O4 nanoparticles First, Fe3 O4 nanoparticles were synthesized by coprecipitation of Fe2+ and Fe3+ using ammonium hydroxide solution. After isolation, they were characterized by several analytical techniques to study the composition and size. Fig. 1 shows the XRD pattern of the Fe3 O4 nanoparticles where it shows characteristic peaks of magnetite nanoparticles. The XRD pattern of Fe3 O4 is similar to bibliographic results of magnetite nanoparticles. The experimental d-spacing from X-ray patterns are 2.95, 2.52, 2.09, 1.61 and 1.47 at the characteristic peaks of 2-, 30.27, 35.59, 43.13, 57.22 and 62.87, respectively, which is similar to another author data where d-space for Fe3 O4 are 2.97, 2.53, 2.10, 1.62 and 1.48. According to the Scherrer equation, the average diameter of the iron oxide nanoparticles was calculated as 8.3 nm from the width of the diffraction peak. Scherrer equation supplies information about relative sizes of crystals from several samples but in order to know absolute value, a reference sample is necessary. Several authors [26,27,29–33] have shown that AFM allows to determinate the size of organic and inorganic nanoparticles. Representative AFM images and corresponding height profile are shown in Fig. 2. Samples were prepared through the solvent evaporation of very diluted dispersions of Fe3 O4 nanoparticles on silicon wafers. In the height profiles, differences between the apparent particle diameter and height can be observed. This fact can be attributed to the tip geometry smearing effect. For this reason, the real size
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Fig. 3. SQUID magnetization of unmodified magnetic nanoparticles at 300 K. Fig. 1. XRD patters of Fe3 O4 as-synthesized nanoparticles.
of the Fe3 O4 was measured with the estimated height rather than lateral lengths from the height images. Height profile showed 7 nm size for Fe3 O4 nanoparticles. Fig. 3 shows the magnetization curve of unmodified magnetic nanoparticles. The specific saturation magnetization (Ms) value for Fe3 O4 is found to be 67.7 emu/g. The decrease in saturation magnetization compared to bulk magnetite is due to the small size of the particles. Zero coercitivity on magnetization curves indicate superparamagnetic behaviour. 3.2. Surface modification of Fe3 O4 nanoparticles After isolation, Fe3 O4 nanoparticles were covered by allyltrichlorosilane using hydroxyl groups on magnetite surface and chlorosilane group of the modificator.
FTIR analysis confirmed that ACTS was anchored on Fe3 O4 nanoparticles. As shown in Fig. 4, two new peaks were observed in the Fe3 O4 nanoparticles covered ATCS. One of them was attributed to asymmetric stretch vibration of Si–O–Si (1080 cm−1 ). Fe3 O4 and Fe3 O4 -ATCS nanoparticles were analyzed by TGA in order to determinate the amount of ATCS cleaved on nanoparticles (Fig. 5). The weight loss of the Fe3 O4 nanoparticles is 3.53% for the whole temperature range because of the removal of adsorbed physical and chemical water. The amount of ATCS coated on the surface of the nanoparticles was calculated on the basis of the TGA data to be 4.39% with respect to the unmodified Fe3 O4 . As shown in Fig. 6, the saturation magnetization value for Fe3 O4 -ATCS was found to be 57.5 emu/g. The value of saturation magnetization is not directly correlated with Ms of Fe3 O4 because the amount of ATCS in the sample is not considered in the calculation. 3.3. Synthesis and characterization of Parylene/Fe3 O4 nanocomposites Finally, Parylene deposition was investigated onto the Fe3 O4 ATCS nanoparticles deposited on aluminum substrates by drop casting. After deposition, thin transparent homogeneous brownish films were obtained. The nanocomposite films were removed,
Fig. 2. AFM images of Fe3 O4 nanoparticles. Height (left) and phase (right) and corresponding height profile (center).
Fig. 4. FTIR spectrum of Fe3 O4 and Fe3 O4 -ATCS.
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cleaned and characterized by FTIR and TGA. Fig. 7 shows FTIR spectra of Fe3 O4 -ATCS, Parylene and Parylene/Fe3 O4 -ATCS nanocomposite. The characteristics bands of Parylene were observed in the nanocomposite. Furthermore, Parylene/Fe3 O4 -
Fig. 8. TGA curves of neat Parylene and nanocomposites that contain several quantities of Fe3 O4 -ATCS.
Fig. 5. TGA curves of Fe3 O4 and Fe3 O4 -ATCS.
Fig. 9. SQUID magnetization of neat Parylene and nanocomposites that contain several quantities of Fe3 O4 -ATCS at 300 K.
Fig. 6. SQUID magnetization of Fe3 O4 -ATCS at 300 K.
Fig. 7. FTIR spectrum of Fe3 O4 -ATCS, Parylene and Parylene with Fe3 O4 -ATCS.
ATCS nanocomposite shows a new broad shoulder at 563 cm−1 in the peak at 606 cm−1 . This shoulder can be attributed to Fe–O bond of iron oxide nanoparticles at 570 cm−1 confirming the incorporation of iron oxide nanoparticles in Parylene. In order to determinate the quantity of nanoparticles inside Parylene, neat Parylene and nanocomposites were analyzed by TGA. As shown in Fig. 8, the weight loss after TGA analysis decreases proportionally to number of drops that were deposited on aluminum substrate. The weight loss of neat Parylene is 68.7 wt% and the weight loss of nanocomposites of 3, 6 and 9 drops are 60.0 wt%, 50.6 wt% and 41.2 wt% respectively. The nanocomposite films with 3, 6 and 9 drops contain 8.7 wt%, 18.1 wt% and 27.5 wt% of magnetic nanoparticles, respectively. Interestingly, the magnetic properties of Parylene/Fe3 O4 nanocomposites were analyzed by SQUID measurements (Fig. 9). From the plot of magnetization versus magnetic field, the magnetic saturation was determined for neat Parylene and the obtained nanocomposites. As expected, bulk Parylene films did not show any magnetic response. In the case of nanocomposites, an increased magnetic response is observed due to the presence of the Fe3 O4 nanoparticles. Increments in saturation magnetization ranged from 0 emu/g of neat Parylene to 16.94 emu/g of nanocomposites that contain the highest percentage of nanoparticles. Since the value of
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Ms is proportional to the quantity of magnetic material per gram, this increment reflects the increased amount of nanoparticles in the different nanocomposites. This results show that the method allowed to prepare Parylene nanocomoposite films with magnetic response depends on the initial amount of nanoparticles. 4. Conclusions A new technique for the preparation of nanocomposites based on magnetic nanoparticles embedded in Parylene has been proposed. This technique can open a new way on the generation of functional Parylene coatings. The surface modification using ATCS avoided the inhibition of Parylene deposition on iron oxide nanoparticles. The magnetic nanoparticles and the obtained nanocomposites were characterized with XRD, FTIR, SQID, AFM and TGA techniques. These techniques demonstrated the successful synthesis of Parylene nanocomposites with tunable magnetic properties. Acknowledgements The present work was supported by the European project MULTIPOL (FP6–NMP4 - STREP 033201) and Ministerio de Educación y Ciencia (project MAT2007-66798-C03-03). The authors gratefully ˜ for the kind experimental assistance. acknowledge V. Munoz
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