Sol–gel hybrid materials for aerospace applications: Chemical characterization and comparative investigation of the magnetic properties

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Sol-gel hybrid materials for aerospace applications: Chemical characterization and comparative investigation of the magnetic properties Michelina Catauro, Maria Cristina Mozzati, Flavia Bollino

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S0094-5765(15)00319-7 http://dx.doi.org/10.1016/j.actaastro.2015.08.003 AA5530

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Acta Astronautica

Received date: 10 July 2015 Revised date: 3 August 2015 Accepted date: 5 August 2015 Cite this article as: Michelina Catauro, Maria Cristina Mozzati, Flavia Bollino, Sol-gel hybrid materials for aerospace applications: Chemical characterization and comparative investigation of the magnetic properties, Acta Astronautica, http://dx.doi.org/10.1016/j.actaastro.2015.08.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.


                        

Michelina Catauro1, Maria Cristina Mozzati2 and Flavia Bollino1 1

Department of Industrial and Information Engineering, Second University of Naples, Via Roma

29, 81031 Aversa, Italy 2

Department of Physics, CNISM and INSTM, University of Pavia, Via Bassi 6, 27100 Pavia,

Italy

*Corresponding author: [email protected] Phone +39 0815010360 Fax

+39 0815010204

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ABSTRACT In the material science field, weightless conditions can be successfully used to understand the relationship between manufacturing process, structure and properties of the obtained materials. Aerogels with controlled microstructure could be obtained by sol-gel methods in microgravity environment, simulated using magnetic levitation if they are diamagnetic. In the present work, a sol-gel route was used to synthesize class I, organic-inorganic nanocomposite materials. Two different formulations were prepared: the former consisted in a SiO2 matrix in which different percentages of polyethylene glycol (PEG) were incorporated, the latter was a ZrO2 matrix entrapping different amounts of poly(İ-caprolactone) (PCL). Fourier Transform Infrared Spectroscopy (FT-IR) detected that the organic and the inorganic components in both the formulation interact by means of hydrogen bonds. X-Ray Diffraction (XRD) analysis highlighted the amorphous nature of the synthesized materials and Scanning Electron Microscope (SEM) showed that they have homogeneous morphology and are nanocomposites. Superconducting Quantum Interference Device (SQUID) magnetometry confirmed the expected diamagnetic character of those hybrid systems. The obtained results were compared to those achieved in previous studies regarding the influence of the polymer amount on the magnetic properties of SiO2/PCL and ZiO2/PEG hybrids, in order to understand how the diamagnetic susceptibility is influenced by variation of both the inorganic matrix and organic component.

Keywords: sol-gel method, organic-inorganic hybrid nanocomposites, SQUID magnetometry.

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1. Introduction There is a growing interest towards magnetic gravity compensation in science and technology [1]. Studies in many areas (materials science, fluid physics and combustion, biology and biotechnology) showed that gravity-free environments may be used to improve many industrial processes [2]. In the material science field, in particular, low gravity conditions are required to identify and understand the cause-effect relationship between processing, structure and properties of materials. The investigations include studies of directional solidification, semiconductor and zeolite crystal growth, diffusion in liquid metals, container-less processing of corrosive materials, formation of metal foams, special alloys, composites, special glasses, ceramics and polymers [3, 4]. A synthesis method affected by the gravity force is the sol-gel technique, a method to make ceramic and glassy materials at a relatively low temperature [5]. The increasing attention to this method was due to its versatility which derives from the various special shapes obtained directly from the gel state (e.g. monoliths [6-9], film [10-15], fibers [16-18], and monosized powders), the composition and microstructural control and low processing temperatures which allow to entrap thermolabile molecules (polymers [19-21], drugs [22-26], biomulecules [27], etc) in the inorganic glassy matrix to modify its properties [5, 28]. During a sol-gel synthesis, the transition from a colloidal solution (the ‘sol’) into a solid ‘gel’ phase occurs through hydrolysis and polycondensation reactions of one (or more) metal alkoxide precursor(s). By drying the obtained wet gel, it is possible to prepare xerogels (by exposure to low temperatures) or aerogels (by solvent extraction under supercritical conditions) [5]. The last ones are microstructured, open-pore materials with many properties, such as lightness, high thermal resistance, very low refractive index and sound velocity, and high surface area. For this reason they are used in a wide range of applications, including thermal and acoustical insulation, kinetic energy absorption, electronics, optics, chemistry, biomedicine and aerospace

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industry [29]. Zirconia aerogel prepared via sol-gel route was successfully proposed to prepare thermal barrier coatings (TBCs) on superalloys in aircraft engines [30, 31]. Silica aerogel, because of its optical transparency in the visible spectrum, high surface area, low thermal conductivity, bulk density, refractive index, dielectric constant and sound velocity, was proposed as hypervelocity particle capture medium, thermal insulator and cryogenic fluid containment [29]. The low processing temperature combined with the high sol homogeneity, due to mixing on the molecular scale, makes the sol–gel method an ideal technology to manufacture organic-inorganic hybrid materials by entrapping various organic polymers in a glassy matrix. Those materials are biphasic systems in which the organic and inorganic components are connected on a nanometric scale. The leading idea in their development is to take advantage of the best properties of each component which forms the hybrid, trying to decrease or eliminate its drawbacks, thus achieving a synergic effect which results in the generation of a new material with new properties. Organic-inorganic hybrid coatings, prepared by mixing zirconium tetrapropoxide and methacrilic acid to a siloxane sol, were proposed for corrosion protection of AA7075-TS aluminum alloy under aircraft conditions [32]. Studies curried out using Ground-based facilities for simulation of microgravity [33-39] proved that the final microstructure of the gels obtained via sol-gel route is affected by gravity. In particular, a low gravity induces branched siloxane groups formation, instead of the usual silanol ones. Therefore, microgravity environment allows to obtain rigid gels with a larger pore size, surface area and pore volume than the ones obtained in normal gravity and could be suitable to control final gel microstructure [33]. To simulate long-lasting weightlessness conditions, which are necessary to allow the transition from sol to gel, magnetic levitation technique can be used [40-44]. However, only samples diamagnetic can levitate [40-45], since they receive an upward repulsive force when are appropriately placed at an off-center position of a magnetic vertical

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field which can be strong enough to exceed the force due to the gravity. The analysis of the magnetic properties of sol-gel materials, thus, deserves investigations. In previous works [46, 47], the Authors synthesized organic-inorganic hybrid materials consisting of a SiO2 or ZrO2 matrix in which poly (‫ܭ‬-caprolactone) (PCL) and polyethylene glycol (PEG) were added respectively; the materials emerged as diamagnetic with a diamagnetic susceptibility independent of temperature and which increases with the polymer amount. The present work describes the preparation, by means of a sol-gel method, and the chemicalphysical characterization of two new formulations of organic-inorganic hybrid materials: The former consists of silica (SiO2) as inorganic phase and PEG 400 as organic component, and the latter consists of zirconia (ZrO2) and PCL. Their physicochemical properties were compared with the published SiO2/PCL and ZrO2/PEG hybrids ones in order to study the influence of both phases (organic and inorganic) in the hybrids properties. To allow a homogenous comparison between the hybrid systems, the oxide/polymer ratios used in the previous works have been kept constant in the samples synthesis. In the hybrids preparation diamagnetic substances were used and the polymers were incorporated in different percentages (0, 6, 12, 24, 50 wt%), as a particular attention has been devoted to investigate the magnetic properties of the obtained materials as a function of the polymer amount by means of SQUID magnetometry to verify and quantify their diamagnetic character.

2. Experimental Study 2.1 Sol-gel synthesis Both the formulations of organic-inorganic hybrid materials were synthesized by means of a solgel process. To prepare the SiO2-based formulation, pure tetraethyl orthosilicate (Sigma Aldrich) was used as precursor material and added to a mixture of ethanol, water and nitric acid (൒65.0%, Sigma Aldrich) with molar ratio TEOS:HNO3:EtOH:H2O=1:0.6:6:2. The nitric acid was used to

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catalyze the hydrolytic activity of the precursor. Successively, polyethylene glycol (PEG 400, Sigma Aldrich), previously dissolved in ethanol, was added to the solution under vigorous stirring to obtain a uniform and homogeneous sol. Five hybrid systems were obtained by entrapping different PEG percentages (0, 6, 12, 24, 50 wt%) in the silica matrix (see Table 1). The ZrO2-based hybrids, containing PCL 0, 6, 12, 24, and 50 wt% (see Table1), were synthesized using a Zirconium(IV) propoxide solution (Zr(OC3H7)4 70 wt% in n-propanol, Sigma Aldrich) as precursor of the inorganic matrix and poly(İ-caprolactone) (PCL Mw=65000, Sigma Aldrich) dissolved in chloroform as organic component. Zirconium(IV) propoxide was added

to

a

solution

of

water,

acetylacetone

and

ethanol,

with

molar

ratio

Zr(OC3H7)4:H2O:AcAc=1:1:0.3. Acetylacetone (AcAc) was used in the process as chelating agent to control and decrease the hydrolytic activity of zirconium alkoxide [48, 49]. After gelation, all wet gels were air dried at 45°C for 48h to remove the residual solvents without any polymer degradation. The sol-gel synthesis processes of the SiO2/PEG and ZrO2/PCL hybrids are shown in Figure 1.

2.2 Materials characterization The nature of the interactions between organic and inorganic components in the synthesized hybrid materials was investigated by FT-IR analysis. FT-IR transmittance spectra were recorded in the 400-4000 cm-1 region using a Prestige 21 Shimatzu FT-IR spectrometer, equipped with a DTGS KBr (Deuterated Tryglycine Sulphate with potassium bromide windows) detector, with a resolution of 4 cm-1 (45 scans). T”ƒ•’ƒ”‡– disks were prepared fusing a 1% mixture of pulverized sample in a KBr powder using a hydraulic press (Specac). FTIR spectra were elaborated by IR solution software. The nature of the hybrid materials was investigated by X-ray diffraction (XRD) analysis using a Philips diffractometer. Powder samples were scanned from 2Θ = 5° to 60° using CuKα

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radiation. The microstructure and the surfaces of the synthesized gels were studied by Scanning Electron Microscope (SEM) (Quanta 200, FEI, the Netherlands) equipped with Energy Dispersive X-ray analysis (EDX) (EDAX, USA). A Quantum Design MPMS-XL7 superconducting quantum interference device (SQUID) magnetometer was used for the magnetic characterization of the samples. The field dependence of the magnetization was detected at room temperature in the presence of a magnetic fields ranging between 0 - 3T and 0 - 7T to evaluate the magnetic susceptibility of SiO2/PEG and ZrO2/PCL hybrids respectively. Moreover, to study the temperature dependence of the SiO2/PEG and ZrO2/PCL material magnetic susceptibility in the range of polymers’ life (223-322K), a 3T and 7T magnetic field were applied respectively.

3. Results and Discussion 3.1 Sol-gel synthesis It is known that the gelation process is the result of the hydrolysis of the alkoxide precursors and condensation reactions of the formed oligomers (Eq. 1-3), which proceed through a second order nucleophilic substitution [50]. M(OR)4 + nH2O ՜ M(OR)4-n(OH)n + nROH

(1)

ŁM-OH + RO-SiŁ ՜ ŁM-O- M Ł + ROH

(2)

ŁM-OH + OH-M Ł ՜ ŁM-O-MŁ + H2O

(3)

Where M = Si or Zr and R = C2H5 or C3H7 respectively. After drying, all gels appear as transparent glassy pieces (Figure 1). The ZrO2-bases glasses are of yellow color whereas the SiO2-bases ones are without color.

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3.2 Materials characterization In Figure 2 the infrared spectra of the SiO2 gel (curve a), SiO2/PEG (6, 12, 24, 50 wt%) hybrid gels (curve from b to e) and pure PEG (curve f) were compared. In the SiO2 spectrum (curve a) all typical peaks of the silica sol-gel materials [51, 52] are visible, such as the bands at 1085 (with a shoulder at 1200 cm-1) and 800 cm-1 corresponding to asymmetric and symmetric Si-O stretching motions, the absorption band at 460 cm-1 due to the bending Si-O-Si modes and the two peaks generally observed in alkoxy-derived silica gels, at about 950 cm-1 , assigned to Si-OH bonds, and at 550 cm-1, which is attributed to the 4-fold siloxane residual cyclic structures in the silica network [51-54]. Moreover, a strong sharp peak at 1380 cm-1, due to residual nitrate anions [55], a broad intense band at 3440 cm-1 and the peak at 1640 cm-1, due to –OH stretching and banding vibrations in the hydration water, are visible. The position and the shape of the latter bands suggest the presence of hydrogen-bonded solvent molecules (H2O) and hydrogen-bonded OH groups attached to the Si atoms [56]. In the PEG spectrum (curve f) methylene C–H asymmetric stretching and bending bands are visible at 2870 cm-1 and 1454 cm-1 respectively [57], together with the peaks assigned to terminal C–OH and ethereal C–O stretching at 1250 and 1104 cm-1 respectively and with a little peak at 948 cm-1 attributed to C–C stretching. The SiO2/PEG hybrid materials spectra (from curve b to e) show all typical signals of SiO2 and some polymer picks whose intensity increases with the PEG amount. In particular, the PEG methylene C–H asymmetric stretching and bending bands at 2870 cm-1 and 1454 cm-1 respectively appear already in SiO2/PEG6 spectrum (curve b) whereas the C-C signal (948 cm-1) becomes visible as a shoulder only in SiO2/PEG24 (curve d). Another interesting observation is the change in the shape of the broad band at 3440 cm-1 which suggests the formation of new hydrogen bonds between those –OH groups and the PEG chains (through H-bond acceptors ethereal oxygen atoms or terminal –OH) as described in reaction 4.

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OR H

¨ O ¨

O¨

CH2 CH2 n

¨

H +

Si OR

Si

O

Si

OH

OH

OH H

O

OR

O

Si

ĺH

¨ O ¨

O¨

CH2 CH2 n

OR

¨

OR H

O

Si

(4)

H

H O

O Si

OR

Figure 3 shows the FTIR spectra of ZrO2 gel (curve a) and pure PCL (curve f) compared with those of the ZrO2/PCL hybrid systems (from curve b to e). In the ZrO2 spectrum (curve a) all typical peaks of AcAc-containing zirconia sol-gel materials [56] are present. The broad intense band at 3417 cm-1 is due to –OH vibrations. The position and the shape of that band suggest the presence of hydrogen-bonded solvent molecules (H2O) and hydrogen-bonded OH groups attached to the Zr atom. The bands observed at 1585 and 1377 cm-1 are assigned to C=O vibrations of the AcAc bidentate binding. The bands at 1529 and 1280 cm-1 are attributed to C–C vibrations, the peak at 1425 cm-1 is due to methyl C–H symmetric bending [58], whereas the bands at 1026 and 931 cm-1 are assigned to the C–C–H bending mixed with stretching C–C vibrations of AcAc [56]. The bands placed at 654 and 460 cm-1 are due to Zr–OH and Zr–O–Zr stretching respectively [59], whereas the peak at 422 cm-1 to Zr–OAcAc vibrations [56]. The PCL spectrum [60, 61] (curve f) is dominated by the intense peak at 1730 cm-1 due to C=O stretching. Moreover, the bands at 2930 and 2849 cm-1 correspond to CH2 asymmetric and symmetric stretching respectively, whereas those ones to 1471, 1419, 1398 are due to CH2 bending vibrations and the weak peak at 731 cm-1 to CH2 rocking vibration. The peak at 1294 cm-1 is assigned to the backbone C-C and C-O stretching modes in the crystalline phase. The peak related to ester groups are visible in the region 1250-950 cm-1 and in particular the

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asymmetric and symmetric stretching vibrations of the sp2C-O-C are visible at 1240 and 1180 cm-1 respectively, whereas the vibrations of O-Csp3 appear at 1107, 1045 and 960 cm-1. In the hybrid spectra (curve b to e) the typical bands of both the components, zirconia and polymer, are present and the intensity of the PCL peaks increases in the spectra of the systems containing higher polymer amounts (curve from c to e). Finally, the region of zirconia typical peaks (900-400 cm-1) and in particular the band assigned to Zr–OH stretching at 654 cm-1 changes shape, probably because of the interaction between that hydroxyl groups and the polymer. The concurrent change in the shape of the broad band at 3417 cm-1 suggests, also in those systems, the formation of hydrogen bonds between those –OH groups and the carbonyl groups of the PCL chains (H-bond acceptors) as described in reaction 5.


 









 

൅




















:
:









:
:

՜






 

(5)




The described results are in accordance with the data obtained in previous works where the presence of the H-bounds between the inorganic SiO2 and ZrO2 matrices and the PEG and PCL respectively was detected by means of 13C CPMAS-NMR [20, 62]. Therefore, the obtained materials can be classified as class I hybrids, according to Judenstein P. et. al classification [63]. Those Authors, indeed, proposed a classification of the organicinorganic hybrid materials based on the interactions between the phases. They defined those materials as first class hybrids, if they were characterized by weak bonds (hydrogen bond, Van der Waals forces, etc.) between the two phases, or second class hybrids, if strong bonds (covalent bond or ionic-covalent bond) occurred.

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XRD analysis (Figure 4 and 5) shows that the nature of the synthesized materials does not depend on the polymer amount because all diffractograms, those ones of both inorganic matrices as well as of the hybrid systems, exhibit broad humps, typically observed in completely amorphous materials. Therefore, despite the addition of the polymers (PEG and PCL), the synthesized hybrid systems retain the amorphous structure typical of the SiO2 and ZrO2 inorganic glassy phases. The microstructure of all synthesized materials, inorganics and hybrids, has been studied by SEM (Figure 6) and appreciable differences are not observed in their morphology. In particular, the hybrids are homogeneous and any phase separation is visible even at high magnification, proving that the obtained materials are nanocomposited.

3.3 Magnetic characterization Figure 7 shows the data about the field dependence of the mass magnetization (MvsH) recorded for the SiO2/PEG hybrids. The measurements were carried out at room temperature (with magnetic field ranging between 0 and 3T) to infer the magnetic susceptibility (χ) values of all samples from the slope of the recorded curves. Figure 8 reports the data about the temperature dependence of the magnetization (MvsT) of all SiO2/PEG samples, obtained by applying a 3T magnetic field. The χ-values are stable in the whole investigated temperature range - i.e. the temperature range of life of PEG, above its glass transition (§223K) up to 322 K [64]. The results of these magnetic measurements evidenced the diamagnetic character of the hybrid systems, as a consequence of the well-known diamagnetic character of the end-members of the considered series of samples (SiO2 and PEG). A progressive increase of the absolute χ-value (|χ|) with the increase of the nominal PEG amount is observed. The result can be explained taking into account that the |χ|-value for SiO2 is lower than that one of PEG (as clearly shown in figure 8) and that the diamagnetic susceptibility have an additive character.

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Also the hybrid samples obtained by mixing ZrO2 and PCL with the same oxide/polymer ratios as SiO2/PEG were investigated. Figures 9 and 10 show the results of MvsH measurements performed at room temperature and MvsT measurements carried out by applying a 7T magnetic field in the range 223K-323K, respectively. Also in this case the chosen temperature range is between PCL glassy transition (223K) and its melting point (322 K) [65]. Also for those samples the expected trend of the susceptibility values is observed as function of the polymer amount. The absolute χ-values increase with the increase of PCL content in the samples, and the stability of the susceptibility values in the whole investigated temperature range is verified. The experimental |χ|-values obtained at room temperature from the slope of the MvsH curves for both the studied hybrid systems (SiO2/PEG and ZrO2/PCL) are compared with the values recorded for SiO2/PCL and ZrO2/PEG materials and reported in the previous works [46, 47] (figure 11). It is observed that the |χ|-values of the SiO2-based materials are the highest. It is due to the higher |χ|-value of pure SiO2 with respect to the ZrO2 one. A different behaviour between the SiO2-based hybrids and the ZrO2-based ones is detectable. The |χ|values for the SiO2/PEG samples are generally higher than those recorded for SiO2/PCL systems, notwithstanding the PCL |χ|-value is higher than the PEG one. On the contrary, the experimental |χ|-values of the ZrO2/PCL hybrids are generally higher than those recorded for the corresponding ZrO2/PEG samples, as expected. The different behaviour can be explained after a closer examination of the structure of these materials and after a comparison between the experimental data and the theoretical expected values. The latter were calculated on the basis of the theoretical |χ|- values of the two component of each hybrid system (SiO2 , ZrO2 , PEG and PCL), of their relative amount in the material and considering the additive contribution to the total susceptibility from each component. The results of this comparison are reported in Fig. 12a and 12b for the hybrids containing PEG and PCL,

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respectively. The experimental χ-value achieved for the PEG-free SiO2 sample (-4.55x10-9 m3/kg) does not match with the value reported in literature (-5.23 x10-9 m3/kg [66]). This mismatch can be due to the presence in the sample of residue nitrate ions as consequence of the synthesis procedure. This is proved by the presence in the SiO2-based materials FT-IR spectra (Figure 2) of the band at 1380 cm-1. The theoretical χ-value of NO3- is -3.83x10-9 m3/kg [66]. Therefore, an agreement between the above mentioned experimental and calculated χ-values can be obtained considering the PEG-free SiO2 sample as a mixture of two components (SiO2 and NO3-) composed by about 1/3 of nitrate ions and 2/3 of oxide matrix. The presence of nitrate ions with a lower χ-value than silica matrix one, as the additive character of the diamagnetic susceptibility, causes, thus, a decrease of the experimental χ-value of SiO2 compared to the theoretical one. As the amount of HNO3 used in the synthesis procedure, this hypothesis is conceivable. The theoretical χ-values of the SiO2/PEG samples, reported in figure 12a, were obtained from the calculated χ-value of pure SiO2 and the χ-value of PEG400 experimentally obtained (it is in perfect agreement with the one reported in literature [47]). With the decrease of the SiO2/PEG ratio, the experimental χ-values tend towards the expected ones. This is due to the decrease of the relative NO3- amount in the samples by adding a component with higher χ-value. The same procedure of comparison between experimental and expected data was performed for the ZrO2/PEG hybrids (Figure 12a). Also for this materials the experimental |χ|-value of PEGfree ZrO2 sample (χ=-2.69x10-9 m3/kg) is lower than the theoretical one (χ=-3.47x10-9 m3/kg [66]). In this case the result can be ascribed to the presence in the sample of residual acetylacetone as proved by FTIR analysis (Figure 3). The magnetic susceptibility value of acetylacetone is unavailable. Its effect decreases with decreasing of oxide matrix/PCL ratio because the relative amount of the acetylacetone decrease with the increase of the PCL content in the samples. 13

The comparison between experimental and expected |χ|-values of the two hybrid formulation containing PCL (figure 12b) was achieved following the same procedure used for the two formulation containing PEG. The results show that the experimental values of the SiO2/PCL samples never exceed the expected ones, even for the highest PCL amount. Only for the SiO2/PCL24 sample a perfect agreement is achieved between experimental and expected χvalues. This suggest that the contribution of nitrate ions to susceptibility is higher in SiO2/PCL hybrids than in SiO2/PEG ones, except for SiO2/PCL24 sample for which a negligible effect of the NO3- is recorded. In conclusion, SiO2/PEG and SiO2/PCL showed an analogous behaviour but a more irregular trend is observed in the SiO2/PCL systems. The behaviour of the ZrO2/PCL hybrids, instead, shows the same trend observed for the ZrO2/PEG samples. The comparison between experimental and expected values of both (figure 12b) was performed following the same procedure described above. The mismatch is ascribable to the presence of the acetylacetone and its decrease with decreasing of the oxide/polymer ratio. Nevertheless, ZrO2/PCL materials show a trend slightly more irregular compared to the ZrO2/PEG systems, as well as observed by comparing the two SiO2-based hybrid formulations. In conclusion, SiO2 is the oxide matrix with the highest |χ|-value which, thus, is a suitable component to be used in the synthesis of materials under microgravity conditions by means of magnetic levitation. Moreover, notwithstanding the higher |χ|-value of PCL with respect to PEG, SiO2/PEG hybrids show the best performances (generally higher |χ|-values of the mixed samples for the same oxide/polymer ratio and more regular trend as a function of the polymer amount).

4. Conclusions In this paper, the synthesis and characterization of SiO2/PEG (0, 6, 12, 24, 50 wt%) and ZrO2/PCL (0, 6, 12, 24, 50 wt%) hybrid nanocomposites, obtained via sol-gel, have been

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reported. Their structure and microstructure have been investigated by means of FT-IR, XRD and SEM analyses. The magnetic properties of the synthesized materials were tested by means of SQUID magnetometry and all results are compared with those ones obtained for two hybrid formulation (SiO2/PCL and ZrO2/PEG) and reported elsewhere [46, 47]. The characterization showed that amorphous class I organic-inorganic hybrid nanocomposite materials were obtained. In both the hybrid formulations, the polymer (PEG and PCL) was incorporated into the inorganic networks (SiO2 and ZrO2 respectively) by weak bonds (H-bonds) between the H-bond acceptor groups of the organic polymers and the hydroxyl groups (H-bond donors) of the inorganic matrix. The magnetic characterization confirmed the diamagnetic character of those materials, which display a diamagnetic susceptibility which increases with increasing polymer amount and which is substantially independent of temperature. However, the presence of residual reagents in the synthesized materials (NO3- ions and Acetylacetone in SiO2/PEG and ZrO2/PCL, respectively) causes a mismatch between the theoretical and experimental |χ|-values. The major effect of this phenomenon is that SiO2/PEG hybrids, unexpectedly, achieve |χ|-values higher than those ones recorded for SiO2/PCL, despite |χ|-values of PCL is higher than the one of PEG. Therefore, the measured |χ|-values make the SiO2/PEG system, and in particular SiO2/PEG50 samples, potentially suitable to be synthesized under microgravity conditions by means of magnetic levitation.

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Figure Captions Fig. 1. Flow chart of SiO2/PEG and ZrO2/PCL gels synthesis processes and photograph of representative (a) SiO2/PEG and (b) ZrO2/PCL gels obtained. Fig. 2. FT-IR spectra of (a) SiO2, (b, c, d, e) SiO2/PEG (6, 12, 24 and 50 wt%) gels and (f) pure PEG. Fig. 3. FT-IR spectra of (a) ZrO2, (b, c, d, e) ZrO2/PCL (6, 12, 24 and 50 wt%) gels and (f) pure PCL. Fig. 4. XRD diffractograms of (a) SiO2 and (b) SiO2 + PEG 50 wt% gels. Fig. 5. XRD diffractograms of (a) ZrO2 and (b) ZrO2 + PCL 50 wt% gels. Fig. 6. SEM micrographs of (A) SiO2 and (B) SiO2 + PEG 50 wt% (C) ZrO2 and (D) ZrO2 + PCL 50% wt gels. Fig. 7. Mass magnetization as a function of magnetic field at 300K for SiO2/PEG samples. Fig. 8. Temperature dependence of magnetic susceptibility per unit mass for SiO2/PEG samples. Fig. 9. Mass magnetization as a function of magnetic field at 300K for ZrO2/PCL samples. Fig. 10. Temperature dependence of magnetic susceptibility per unit mass for ZrO2/PCL samples. Fig. 11. Absolute χ-values experimentally obtained for each sample of each investigated hybrid formulations. Fig. 12. Comparison between experimental and calculated χ-values for the samples containing (a) PEG and (b) PCL.

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Label

Inorganic Matrix

Organic Component

SiO2

--

SiO2/PEG6

6 wt% PEG

SiO2/PEG12

SiO2

12 wt% PEG

SiO2/PEG24

24 wt% PEG

SiO2/PEG50

50 wt% PEG

ZrO2

--

ZrO2/PEG6

6 wt% PCL

ZrO2/PEG12

ZrO2

12 wt% PCL

ZrO2/PEG24

24 wt% PCL

ZrO2/PEG50

50 wt% PCL

Table 1 Composition of the synthesized samples

Highlights • • •

Hybrids nanocomposites for aerospace applications were synthesized via sol-gel Both SiO2/PEG and ZrO2/PCL hybrids have a diamagnetic character |Ȥ|-value increases with polymer amount and is independent of temperature

21

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure

Figure