Journal of Non-Crystalline Solids 358 (2012) 1399–1403

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Journal of Non-Crystalline Solids

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One-pot synthesis of novel crown ether polysiloxanes: Promising solidstate ionophores

Mônica F. Belian a,⁎, Dilmo M.S. Leotério b, André Galembeck b,c, Gilberto F. de Sá b, Severino Alves Jr. b

a Departamento de Ciências Moleculares, Universidade Federal Rural de Pernambuco, 52171-900 Recife, Pernambuco, Brazilb Departamento de Química Fundamental, Universidade Federal de Pernambuco, 50670-901 Recife, Pernambuco, Brazilc Centro de Tecnologias Estratégicas do Nordeste, 50670-901, Recife, Pernambuco, Brazil

⁎ Corresponding author. Tel./fax: +55 81 3320 6375.E-mail address: mfbelian@dcm.ufrpe.br (M.F. Belian

0022-3093 © 2012 Elsevier B.V.doi:10.1016/j.jnoncrysol.2012.03.011

Open access under the E

a b s t r a c t

a r t i c l e i n f o

Article history:Received 4 August 2011Received in revised form 13 March 2012Available online 12 April 2012

Keywords:Polysiloxanes;One-pot synthesis;Silacrown ether

The synthesis and characterization of polysiloxane crown ethers (PSCrown—2,5,8,11,14-pentaoxa-1-silacyclotetradecane-polymer) are reported in this work. Tetraethyl orthosilicate (TEOS), tetraethylene glycoland sodium metallic were employed as precursors and the polysiloxanes obtained were characterized byelemental analysis, infrared (FTIR) spectra, thermogravimetric analysis (TG), differential scanning calorimetry(DSC) and nuclear magnetic resonance of 1H, 13C and 29Si. The hybrid surfaces were analyzed using scanningelectronic microscopy (SEM). All hybrids showed a synergistic effect from a thermodynamic viewpoint, sincethe crown ether analogue (15−crown−5) has a low decomposition temperature. The proportional increaseof the silica precursor (TEOS) improved the thermal and mechanical properties.

© 2012 Elsevier B.V. Open access under the Elsevier OA license.

1. Introduction

The sol–gel method for preparing hybrid silica-based polymershas gained scientific importance in the last decades, due to itsextensive possible applications [1–9]. This method is based on thehydrolysis and polycondensation reactions of silicon alkoxides andorganosilanes. These reactions for hybrid matrices production, oftechnological importance, have been extensively explored [10–13].Oxide surfaces modified by organic molecules are now being usedas catalysts, chemical sensors, light filters and phosphors [8–18].In the reaction involving organosilanes and silica, the silanols units(presents in oxide surface) are groups which allow the organic mole-cules introduction (covalently bonded to silica), since are susceptibleto nucleophilic substitution reactions [1–8].

In this context, a macrocycle system has been studied aiming anew proposal of materials which have properties of ion recognition(through the relation between the ion diameter and proposed macro-cycle cavity) and thermodynamic stability of complexation [19–25].“Macrocycle effect" term was first used by Cabbiness and Margerum,who observed an increased stability for the complexes of cyclicligands over those of open-chain ligands [26]. This effect seems tobe related to the chelate effect observed in other complexes. Someinvestigators have assigned a predominantly enthalpic origin to thiseffect, while others argue for a large entropy component [27,28].Among the macrocycle systems studied, crown ethers are significant.Crown ethers, whose molecules exhibit conformational mobility and

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have the structure of a cavity that is lined inside with electron pairsof donor atoms, are capable of specific interactions with many metal-lic ions [29–31]. Synthesized the first time in 1967 by Pedersen [32],crown ethers are characterized by the formation stable and selectivecomplexes with cations.

Taking into account the possible applications of both organosilanesand crown ethers, it is important to produce “crown-silanes” becauseof their wide range of applications, such as ion-selective electrodes(ionophores), heterogeneous catalysts and stationary phases to chro-matography [20–24].

Aiming the produce of solid state ionophores, this paper reportsthe synthesis and characterization of a new polysiloxane crown ether(PSCrown), from tetraethylorthosilicate (TEOS) and tetraethyleneglycol(TEG) in a one-pot synthesis. The characterization of the PSCrown'swas made using elemental analysis, infrared (FTIR) spectra, thermo-gravimetric analysis (TG), differential scanning calorimetry (DSC)and nuclear magnetic resonance of 1H, 13C and 29Si. The hybrid surfaceswere analysed using scanning electronic microscopy (SEM).

2. Experimental methods

Tetraethyl orthosilicate (TEOS, 98%) and tetraethylene glycol(TEG, 97%) were obtained from Aldrich and used without furtherpurification. Elemental analysis (CHN) was performed in a Perkin-Elmer 240 analizer, and the errors were calculated by equation:(TMF−E/E)*100, where: TMF = theoretical molecular formulae andE = experimental data. The infrared spectrum was obtained with aKBr tablet using a Fourrier transform IF66 model spectrophotomerin the 4000−400 cm−1 range, with a spectral resolution of 4 cm−1.Thermogravimetric curves were obtained in a TGA 50/50H Shimadzu

Fig. 1. Synthesis of the PSCrown hybrids (where: TEOS = tetraethylorthosilicate andx=TEOS proportion used).

Table 129Si and 13C NMR chemical shifts of PSCrown hybrids (ppm).

Hybrid TEG:TEOSratio

NMR—ppm (attribution)

29Si 13C

PSCrown 1 1:1 −114.99 (Q2) 58.89 (C–O–Si)−123.27 (Q3) 68.11(C–O–C)−131.42 (Q4) 70.28 (C–O–C)

PSCrown 2 1:1.42 −114.89 (Q2) 59.32 (C–O–Si)−125.26 (Q3) 68.23 (C–O–C)−131.91 (Q4) 70.32 (C–O–C)

PSCrown 3 1:1.84 −115.10 (Q2) 58.77 (C–O–Si)−127.61 (Q3) 68.00 (C–O–C)−133.59 (Q4) 70.23 (C–O–C)

PSCrown 4 1:2.26 −115,07 (Q2) 58.87 (C–O–Si)−124.66 (Q3) 68.03 (C–O–C)−135.73 (Q4) 70.16 (C–O–C)

PSCrown 5 1:2.68 −115.55 (Q2) 58.92 (C–O–Si)−124.27 (Q3) 68.01 (C–O–C)−132.94 (Q4) 70.14 (C–O–C)

PSCrown 6 1:3.10 50.38 (Q1) 58.88 (C–O–Si)−114.99 (Q2) 68.02 (C–O–C)−124.54 (Q3) 70.24 (C–O–C)−133.60 (Q4)

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under N2 atmosphere and a heating rate of 10 °C min−1. The Differ-ential Scanning Calorimetry analyses were performed under N2

atmosphere (50 mL min−1) in a Shimadzu DSC-50WS. The NMRspectra were obtained using VARIAN Unity Plus 300 equipment, ina frequency of 300 MHz to 1H; 75 MHz to 13C and 60 MHz to 29Si, inthe solid state.

The synthesis of the 2,5,8,11,14-pentaoxa-1-silacyclotetradecane-polymer (PSCrown) is shown in Fig. 1.

In a 100 mL two-necked round bottom flask, equipped with adistillation bridge, 1 mmol of TEG and 2 mmol of sodium metallicwere mixed, using 1,4-dioxane as solvent. After 30 min, 1 mmol ofTEOS was added under constant stirring. The mixture was keptfor 3 h at 120 °C to remove the ethanol produced from the reactionsystem. The product was washed with water and dried under vacuumfor 48 h (Yield: 87% based on TEOS). The synthetic procedure issummarized in Fig. 1. Other hybrids were synthesized using thesame methodology, with a variation of the TEOS proportion in 1.00,1.42, 1.84, 2.26, 2.68, 3.10 mmol; resulting in 6 systems, denominatedPSCrown1, 2, 3, 4, 5 and 6 , respectively.

3. Results

Transparent material was obtained only with the 1:1 (PSCrown1)and 1:1.42 ratios (PSCrown2) (TEG:TEOS). The experimental analysisof the higroscopic characteristic was obtained from exposure to airand weighings daily over one week. At the end of this period, thePSCrown materials had acquired a mass of about 35%– 32%.

Fig. 2. (A) 13C NMR spectrum (75 MHz) of PSCrown system in CDCl3, and

The NRM results of the materials demonstrated a strong similaritywith the PSCrown2; and due to this only its results are shown. The 1HNRM spectrum exhibits one multiplet in the 3−4 ppm region, relatedto the “silcrown” ether protons (CH2−O−), and one band in4.05 ppm related to the OH groups from the silanol groups. The 13CNRM spectrum (Fig. 2A) exhibited signals at 59.32, 68.23 and70.32 ppm, compatible with the silcrown structure. The proposedstructure based on 1H and 13C NMR data was confirmed taking intoaccount the 29Si spectrum (Fig. 2B), in this case of PSCrown2. Thesiloxane polymer proposed structure (Fig. 2B) exhibited silicon unitsof Q2, Q3 e Q4 types. The Q4 silicon unit was chosen assumingsimilarity between Si−O−Si and Si−O−C groups. The 13C and 29SiNMR results for the other hybrids are summarized in Table 1.

The CHN elemental analysis results generally agree with theformula calculated for PSCrown's hybrids; with an error level lowerthan 0.5% (Table 2).

The infrared spectra of PSCrown exhibits the following vibration(stretching) modes: C−O−C (1120 cm−1), Si−O−C (1100 cm−1),Si−O−Si (1070 cm−1) and CH2 (2900 cm−1), suggesting the struc-tures proposed (not shown in this text). The thermogravimetric curveobtained for PSCrown2 (Fig. 3A) exhibits three mass loss steps. Thefirst one (100−120 °C) is related to the release of physisorbed watermolecules. The second and third steps are related to the release of theorganic moiety, as presented in Fig. 3B. All thermogravimetric curves(PSCrown hybrids) are presented in the Supplementary Material.

(B) 29Si NMR spectrum (60 MHz) of PSCrown system in solid state.

Table 2Percentage relationship between signal intensities of Q2, Q3 and Q4 (29Si).

PSCrown 1 2 3 4 5 6

Silicon type %

Q2 25.6 19.5 24.0 18.2 22.4 29.1Q3 52.7 55.7 48.5 50.8 44.9 37.4Q4 21.8 24.8 27.5 31.0 32.7 33.5

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The differential scanning calorimetry (DSC) for all materials pre-sented similar comportment, and, due to this, only the DSC curve ofthe PSCrown2will be shown. The DSC curve, shown in SupplementaryMaterial, exhibits endothermic bands associated with the mass lossesverified in the TG curve. The scanning electronmicroscopies (SEM) forall material are shown in Fig. 4.

4. Discussion

Hybrid systems containing a higher ratio of TEOS showed lowerhigroscopicity, as could be expected, since there was an increase ofsiloxane units (hydrophobic groups), in this case with PSCrowns 5and 6. Depending on the ethyleneglycol used, PSCrown synthesis re-quires a base change, owing to ion adjustment in the cavity formedfrom the precursor (template effect). In all cases, prior deprotonationwas necessary, using sodium, to form a silcrown ether. The metallicsodium was used because the silicrown ether formed is very similarto 15−crown−5, which produces a cavity suitable for this ion.

Analyzing the data of Fig. 2, a higher silica structure can beobserved, with the appearance of Q1 signals in the PSCrown6 hybrid,corroborating with the proposed synthesis. When an analysis ofthe relation of the Q2, Q3 e Q4 de 29Si signals (Table 3) is made, it ispossible to observe an increase in the intensity of the Q4 signal in

Fig. 3. (A) TG curve of PSCrown and (B) main mass

proportion to the increase in the quantity of TEOS. This suggeststhat the length of the polysiloxane chain is longer, in the case ofPSCrown6, could be expected. The signals from Q2 and Q3 did notshow similar behavior.

From an elemental analysis of these results, it is possible toobserve that the increase of the proportion of TEOS in the synthesiscaused a proportional increase in the silicon/carbon. The results,however, showed linearity for all the complexes, with the exceptionof PSCrown1 (Fig. 5). This linear behavior suggests that startingfrom a proportion of 1.42 mmol of TEOS it is possible to have controlover the reaction of the final material produced. This enables arational synthesis with specific properties, such as mechanical andthermal resistance. The proportions between Si/C for systems, whenthe carbons belonging to the ethoxide group (Na+ contra-ion) are nottaken into consideration, were 0.125, 0.125, 0.167, 0.21, 0.25 e 0.29;for the PSCrown1, 2, 3, 4, 5 e 6, systems respectively.

The TG curves of the PSCrown systems demonstrate similarbehavior with the three thermic events relative to loss mass; thefirst refers to the loss of physisorbed water molecules (temperaturearound 75−100°C). The second (loss mass) step showed variabletemperatures around 239 °C (PSCrown1), 242 °C (PSCrown2), 250 °C(PSCrown3), 281 °C (PSCrown4), 285 °C (PSCrown5) and 294 °C(PSCrown6). This behavior is due to the high quantities of TEOSused during the synthesis, causing an increase of the decompositiontemperature of the organically modified silica (improvement ofthe material's thermic properties). The last event observed for thePSCrown systems shows a similar comportment to the second step,the improvement of the thermic properties of which depends on theincrease of the proportion of TEOS used during the synthesis of thePSCrowns. This comportment is expected for the organically modifiedsilica, due to stronger synergism that occurs between organic speciesand the silica matrix. The decomposition study of the materials was

losses observed in thermogravimetry analysis.

Fig. 4. Scanning electron microscopy (SEM) for PSCrown1 (scale bar 5 μm) (A), 2 (scale bar 1 μm) (B), 3(scale bar 10 μm) (C), 4(scale bar 10 μm) (D), 5(scale bar 10 μm) (E) e 6(scale bar 1 μm) (F) hybrids.

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carried out using infrared spectroscopy. The PSCrown2was submittedto thermal treatment at 200 and 400 °C (Supplementary Material).The infrared spectrum for the sample treated at 200 °C exhibits aminor intensity in the C−O−C band. In agreement with the thermo-gravimetric data, the sample treated at 400 °C is constituted of silicawith a minor amount of organic matter, and a corresponding decreasein the intensity of the band associated with the organic matter canbe observed. These results agree with a previous analysis of TG curves.

Fig. 4 presents the typical SEM image of silica after the rapid re-moval of the organic solvent, leading to formation of particle clusters.PSCrowns 5 and 6 show highest particle size when compared withPSCrowns 1, 2, 3 and 4. This is due to the increasing amount ofTEOS used in the synthesis which triggers a greater robustness in

Table 3Elemental analysis data and minimal structure proposed for PSCrown hybrids.

Hybrid %C %H Minimal formulaeproposed

Exp. Theo. Exp. Theo.

PSCrown 1 37.83 37.79 7.12 7.09 Si2C20H44O15Na2PSCrown 2 33.00 32.97 7.17 7.14 Si4C40H88O28Na4PSCrown 3 33.92 33.89 6.88 6.87 Si4C30H67O25Na3PSCrown 4 33.17 33.15 6.36 6.35 Si5C30H69O28Na3PSCrown 5 35.51 35.50 8.03 7.99 Si4C20H48O21Na2PSCrown 6 26.71 26.67 6.30 6.29 Si7C30H73O34Na3

the silica chain, leading to formation of larger clusters. Due to rapidremoval process of the solvent used, the PSCrown hybrids showedthe coalescence phenomenon and superficial irregularities. A signifi-cant homogeneity of the hybrid materials was observed.

Fig. 5. Relationship between TEOS ratio (used in the synthesis) and silicon/carbonration of the obtained materials.

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5. Conclusion

The new synthesized hybrid materials present a versatile alterna-tive for the development of ionophores covalently bonded to a silicanetwork. All hybrids showed a synergistic effect from a thermody-namic viewpoint, since the crown ether analogue (15−crown−5)has a low decomposition temperature. The proportional increase ofthe silica precursor (TEOS) improved the thermal and mechanicalproperties. It also increased the robustness of the inorganic network,due to the increased number of siloxane units in proportion to thecrown “silether” groups. These synthesized systems have the poten-tial for a variety of applications, mainly in the composition of sensorsand catalysts.

Acknowledgements

The authors are grateful to CNPq for fellowships and CNPq andFACEPE for financial support.

Appendix A. Supplementary data

Supplementary data to this article can be found online at doi:10.1016/j.jnoncrysol.2012.03.011.

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