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Available online at www.sciencedirect.com

Materials Chemistry and Physics 108 (2008) 24–28

Characterization of sol–gel-derived polyhydridosiloxanepre-ceramic polymer

Volodymyr D. Khavryuchenko a, Ireneusz Natkaniec b, Yuriy O. Tarasenko a,Oleksiy V. Khavryuchenko c,∗, Sergei A. Alekseev c, Vladyslav V. Lisnyak c,∗

a Institute for Sorption and Problem of Endoecology, National Academy of Sciences of Ukraine, 13 General Naumov Str., UA-03164 Kiev, Ukraineb Joint Institute for Nuclear Research, 141980 Dubna, Russian Federation

c Chemical Department, Kyiv National Taras Shevchenko University, 64 Volodymyrska Str., UA-01033 Kyiv, Ukraine

Received 25 April 2007; received in revised form 27 August 2007; accepted 29 August 2007

bstract

The polyhydridosiloxane was manufactured by sol–gel method from triethoxysilane. Methanol, hexane and water adsorption/desorption isotherms

f the polyhydridosiloxane were measured at 293 K and textural parameters were determined. Fourier transform infrared and inelastic neutroncattering spectroscopic examination of the polyhydridosiloxane along with the quantum chemical simulation have resulted in the theoreticallyrounded assignment of the spectral bands and evaluation of the theoretical model of the polyhydridosiloxane globule. 2007 Elsevier B.V. All rights reserved.

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eywords: Amorphous materials; Adsorption; Inelastic neutron scattering; Vib

. Introduction

Polyhydridosiloxane (PHS) is hydrophobic unwettablelobular microporous non-swelling polymer of generalormula (HSiO3/2)n, analogous to hydrogen silsesquiox-ne (H-silsesquioxane), hydrosilsesquioxane, polyhydrogenilsesquioxane or hydridosiloxane xerogel materials. The back-one of the PHS is formed by the siloxane network. The surfacef the PHS is covered by hydrophobic Si–H groups. The chem-cal behaviour of the Si–H groups inside pores and at the outerurface differs strictly.

The PHS, used as pre-ceramic polymer, can be thermally con-erted into temperature-resistant and/or non-wetting coatings1], low-k dielectric ceramic microstructures for modern elec-ronics [2], and is an attractive candidate to joining of SiC/SiComposites for fusion energy systems [3].

The thermal treatment of the PHS upon the formation of coat-

ngs or joining materials leads to the elimination of SiH groups,hich occurs non-uniformly, with some remanent groups inside

he pores of the PHS. Fourier transform infrared (FTIR) spec-

∗ Corresponding authors.E-mail addresses: alexk@univ.kiev.ua (O.V. Khavryuchenko),

isnyak@chem.univ.kiev.ua (V.V. Lisnyak).

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254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2007.08.033

spectroscopy

roscopy and especially FTIR-microspectroscopy can be usedor the monitoring of the process completion.

A set of problems is encountered when studying amorphousaterials, e.g. PHS, since most of methods of theoretical exam-

nation are based on translation symmetry operations, which areon-applicable in the case of amorphous and semi-amorphousolids. So, more sophisticated theoretical models should be used.lso, unlike crystalline substances the methods of direct struc-

ural analysis cannot be applied to amorphous matter, so statisticethods, such as vibrational spectroscopy, are commonly used.In order to characterize the spectral properties of the PHS,

he FTIR and inelastic neutron scattering (INS) spectroscopictudy has been performed along with quantum chemical (QC)imulation of the PHS structure and vibrational spectra. Thisakes possible to evaluate a model of thermal degradation of

he PHS, binding the structural changes with vibrational spectraharacteristics.

. Experimental

.1. Sol–gel synthesis of the PHS

Triethoxysilane (HSi(OC2H5)3, Aldrich, 95%, No. 39014-3), deionizedater (Aldrich, ACS reagent, 99.99+%, 27073-3, HPLC grade), 1,4-dioxane

Sigma–Aldrich, 99.9%, 27053-9, HPLC grade) and ammonia water (Aldrich,

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V.D. Khavryuchenko et al. / Materials

CS reagent, 28.0–30.0% as NH3, No. 221228) were used to prepare the PHS.ll solvents and chemicals used in this study were proanalysis quality. The.001 wt.% ammonia water with pH 9.7 was prepared from the Sigma–Aldricheagent by dilution.

Typical sol–gel synthesis of the PHS involves the hydrolysis of triethoxysi-ane in the presence of acid or base catalysts and organic solvents according toeaction:

HSi(OC2H5)3 + 3nH2O = [HSiO3/2]n + 3nC2H5OH

In a typical preparation of the PHS sample, 1,4-dioxane solutions of the tri-thoxysilane (1:1) and 0.001 wt.% ammonia water (3:2) were vigorously mixedt room temperature for 1 h, followed by adding triply deionized water for solydrolysis (20 mol% excess with respect to the stoichiometric amount requiredor hydrolyzing the precursor to its corresponding hydroxide).

Five minutes after the beginning of the dropping, the reaction mixture beganpalization and was further stirred for 3 h to complete the reaction. Fine PHSarticles were produced in the reaction mixture. The product was washed withioxane in the the Soxhlet extractor.

After washing, the product was distilled under reduced pressure in rotaryvaporator to remove the solvent, after which a dry white powder was obtained.he CHN-element analysis results in hydrogen contents H = 1.9%. Therefore, itas confirmed that a product of the formula HSiO3/2 was obtained.

.2. Characterization of the PHS

Characterization of the PHS was accomplished by several techniques suchs pycnometry, acid–base titrometry and pH-measurements, differential ther-al and thermo-gravimetric analysis, temperature-programmed desorption mass

pectrometry, X-ray diffraction (XRD), adsorption/desorption analysis, vibra-ion spectroscopy (FTIR, INS) in complex with computer simulation of structurend theoretical vibrational spectra of the PHS globule.

The bulk density of the PHS were determined by weight method. The trueensity of the PHS amorphous powder was measured by the pycnometry (Accu-yc 1330 Pycnometer, Micromeritics, Norcross, GA).

The pH of the PHS water suspension were determined by means of a HI 8314H-meter. PHS suspensions were prepared at concentrations up to 12.5 wt.%.he preparation technique of the stabilized aqueous dispersions of the PHS

ncludes three stages: stirring of starting components, ultrasonic dispergation andentrifugation. The suspensions were ultrasonicated for 60 min and centrifugatedt 2000 rpm for 15 min.

The acid–base titration of the PHS powder was performed with 1 M NaOHqueous solution and the quantity of H2 evolved was measured volumetrically.

The differential thermal and thermo-gravimetric (DTA–DTG) analysis waserformed on as-obtained PHS powder at a heating rate of 10 K min−1 in a vac-um using a Paulik–Paulik–Erdey Q-1500 D derivatograph (MOM, Budapest,ungary) with a Derill converter, which simultaneously records TG, DTG andTA curves. The water desorption was detected by the temperature-programmedesorption mass spectrometry (TPDMS) method [4] using a MX-7304 A masspectrometer (SELMI, Sumy, Ukraine).

XRD data were collected using a Phillips PW 1800 powder diffractometerith Ni-filtered Cu K� radiation to confirm the amorphous state of the PHSowders.

.2.1. Adsorption and desorption data collection and analysisMethanol, hexane and water adsorption/desorption isotherms were measured

t 293 K with an adsorption apparatus with MacBen–Baker quartz scales. Beforeeasurements, the PHS samples were pre-evacuated at 400 K until constantass.

The textural parameters: adsorption volume at saturation pressure (Vs), thick-ess of adsorbed monolayer from the density of liquid (hd) and from the Kiselevodel (hK), surface area from the Brunaurer–Emmet–Teller (SBET) and from

he Kiselev (SK) models [5] were calculated from methanol and hexane adsorp-

ion/desorption isotherms. The Kiselev model is based on cylindrical pore model6] and the Kelvin equation, expressing the surface area as SK =

∫ asat

ahyst2da/rk,

here da is the differential of adsorption (cm3 g−1) and rk is the core radii fromhe Kelvin equation, while the thickness of adsorbed layer is neglected. Theurface area can be calculated by integration of dS in the limits from the initial

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istry and Physics 108 (2008) 24–28 25

oint of hysteresis loop up to the saturation. Average value of the surface area,valuated from the desorption and adsorption branches of the isotherm, was usedor further calculation, in particular, for determination of the hK in the initialoint of the hysteresis loop. Pore size distributions were calculated using theelvin equation [6,7] corrected on the thickness of the adsorbed layer in the

nitial point of the hysteresis loop.

.2.2. Vibration spectroscopyThe Fourier transform infrared spectra were measured on a Nicolette 320X

TIR instrument. Solid state spectra were recorded in Nujol suspensions usingBr plates. The spectra mean resolution was typically 4 cm−1 and 128 scans for

ach spectrum were collected.The INS spectra for the PHS were measured in the frequency range of

0–10,000 cm−1 using inverted geometry time-of-flight spectrometers (NERA),nstalled in an IBR-2 nuclear pulse reactor at the Joint Institute for Nuclearesearch (JINR) Dubna, Russia [8]. All spectra were recorded at 10 K to reduce

he Debye–Waller factor. The pyrolytic graphite analyser with a resolution ofa. 2–3% �E/E (NERA) has been used. Known weight (ca. 30 g) of finelyround sample was evenly loaded into aluminium foil sachets (5 cm × 16 cmross-section) and mounted onto a centre stick, which was placed in a closed-ycle refrigerator in the instrument. The sample was then left to cool to 10 Knd the spectra were recorded. The INS spectra were stored in the form ofmplitude-weighted density of states (AWDS) versus wave number.

The quantum chemical (QC) simulation of the spatial structure (seeupplementary materials) and vibrational spectra of the PHS in a cluster approachas been performed in order to interpret the experimental data and obtain theodel for the theoretical speculations. The PM3 [9] computations have been per-

ormed in the framework of self-developed computation software for the clusterimulation “QuChem” [10] previously approved in complex investigation ofilica and other amorphous materials [11–14]. The COSPECO software [15],pplied earlier [11,16–18], has been used to simulate the vibrational spectra ofhe PHS. The experimental spectra have been used to solve the inverted vibrationroblem for the QC-simulated clusters. Two types of vibrational spectra haveeen simulated, namely, IR and INS.

. Results and discussion

.1. Physical–chemical characteristics of the PHS

According to the XRD data, the PHS samples obtained aremorphous without a sign of crystallinity. As the result of theeight and pycnometric measurements, the bulk and the trueensity of the PHS are determined as 0.25 and 1.60 g cm−3,espectively.

The total amount of water was found to be <0.7 mass%ccording to mass loss value obtained from the DTA–DTG datand attributed solely to water desorption as confirmed fromhe TPDMS results. Concentration of active hydrogen in theHS determined by titrometry is equal to 450 ml g−1, the wateruspension pH is found to be 6.7.

.2. Adsorption/desorption isotherms

The isotherms of the methanol and hexane adsorp-ion/desorption refers to the fourth-type isotherms with H2-typeysteresis loop according to the IUPAC classification [7] (Fig. 1).uch isotherms are typical for the mesoporous adsorbentsormed as agglomerate of nano-sized primary particles, and they

an be used for the textural parameters calculations [6].

The textural parameters, namely Vs, hd and hK, SBET andK and maxima of pore diameters (Dpor) calculated from theethanol and hexane isotherms are listed in Table 1. Pore radii

26 V.D. Khavryuchenko et al. / Materials Che

Fig. 1. Isotherms of methanol, hexane and water adsorption/desorption on thePHS.

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ig. 2. Pore radii distributions from the methanol and hexane desorptionsotherms.

istributions, calculated from the desorption branches of thesotherms, which correlated with the sizes of the pore entrances6], are shown in Fig. 2.

The values of Vs calculated from the isotherms of methanoldsorption are in a good agreement with that from hexanedsorption, indicating complete filling of the pores at the satura-ion pressure. Other parameters, represented in Table 1, differsor both adsorbates significantly. The hK of hexane is higherhan hd, and the SBET is significantly higher than SK. This indi-ates the presence of small mesopores and/or the micropores,n which the adsorption of hexane cannot be described in termsf the Kelvin equation [6]. The pores of this type are excluded

rom the pore radii distribution, calculated through the Kelvinquation, and the correction factor of hK gives overestimatedore sizes values.

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able 1he textural parameters of the PHS, calculated from the methanol and hexane isother

dsorbate Adsorption volume atsaturation pressure,Vs (cm3 g−1)

BET surface area,SBET (m2 g−1)

Kiselev surface area,SK (m2 g−1)

ethanol 0.441 110 509exane 0.450 885 257

mistry and Physics 108 (2008) 24–28

Oppositely, the SBET is significantly lower than SK, and hKs smaller than hd, according to methanol adsorption isotherm.onsequently, the methanol monolayer is not formed on theHS surface at p/ps = 0.3, corresponding to the initial point of

he hysteresis loop on the isotherm. However, the capillary con-ensation occurs, and the isotherm is appropriate to be used forhe calculation of textural parameters from the Kiselev model5]. Thus, the real values of Ssurf and Dpor are close to theata, obtained from the methanol adsorption applying Kise-ev model (Table 1). The values obtained are typical for the

esoporous adsorbents formed as agglomerate of nano-sizedrimary particles, which size is in the range with the poresize.

In contrast to the isotherms of methanol and hexane adsorp-ion, the isotherm of water adsorption relates to the first-typeccording to the IUPAC classification [7]. The value ofs = 0.013 cm3 g−1 determined from the isotherm of waterdsorption is significantly lower, than that for methanol andexane (see Table 1). Hence, the PHS can be considered asssentially hydrophobic material, and the adsorption of H2O onhe PHS correlates with the presence of silanol groups (Si–OH).nitial part of water adsorption isotherm can be linearized in theoordinates of the BET model, giving the following parame-ers: the BET constant CBET = 120 and the adsorption capacityf the monolayer am = 0.44 mmol g−1. In the case of specificdsorption the am matches the number of specific adsorptionites [6], i.e. the silanol groups. Previously, the values of ambtained from H2O isotherms were applied for the calculationf the silanol groups concentration in the samples of chemicallyodified silicas [19].

.3. Structure of the PHS globule determined by vibrationpectroscopy and computer simulation

The FTIR and INS spectra recorded for the PHS are demon-trated in Figs. 3 and 4.

In order to assign the experimentally observed bands both inhe FTIR and INS spectra of the PHS, the model of PHS chainorming an amorphous globule, has been evaluated (Fig. 5). Itonsists of 28 silica units, including 28 [SiO3H] links and twoerminal OH-groups. The cluster geometry has been completelyptimized by energy and the inverted vibrational problem haseen solved in respect to the experimental IR spectrum (seeupplementary materials).

riginate from the Si–H valent vibrations. The band splits intoigh- and low-frequency components, referring to the outer- andntra-globular Si–H groups stretchings, correspondingly. The

ms

Thickness of adsorbedmonolayer from the densityof the liquid, hd (nm)

Thickness of adsorbedmonolayer from theKiselev model, hK (nm)

Maxima of porediameters, Dpor,(nm)

0.407 0.118 2.920.603 0.844 4.59

V.D. Khavryuchenko et al. / Materials Chemistry and Physics 108 (2008) 24–28 27

Fig. 3. The QC-simulated IR spectrum of the PHS model, fitted along the FTIRexperimental spectrum: (a) 400–1800 cm−1 and (b) 2000–2400 cm−1 regions.Line 1: experimental spectrum; line 2: simulated spectrum. Delta-functions oftfi

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he QC-simulated vibrational modes are represented as thin solid sticks in allgures. Asterisks denote Nujol bands.

ame band in the INS spectrum was observed 100 cm−1 higher,robably due to the low accuracy of the INS evaluation at theigh-frequency region.

The asymmetrical νSi–O vibrations bands of IR spectrum at070 and 1140 cm−1 are strictly resolved due to the reductionf the symmetry of the silicon polyhedra (SiO4) to quasi-C3v iniO3H-link. These vibrations are pure by form, while others are

ixed, thus can be assigned conditionally only.The IR spectrum shows a typical silica broad bands about

00 and 460 cm−1, referring mainly to the mixed δO−Si−O +

ig. 4. The QC-simulated INS spectrum of the PHS model in the formmplitude-weighted density of states (AWDS) versus wave number, fitted alonghe experimental spectrum. Line 1: experimental spectrum; line 2: simulatedpectrum.

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asSi−O + δO−Si−H vibrations. Also the weak sharp bands at 946nd 976 cm−1 are observed, originating from the δSi–O–H vibra-ions of the silanol groups of the Si–H groups, partially oxidizedn air. The strong sharp bands at 830, 865 cm−1 and shouldert 905 cm−1 refers mainly to the δO–Si–H vibrations. This ispproved by the presence of very intensive bands in the INS spec-rum at the same region, since the vibrations involving H-atomave much higher intensity in the INS spectra.

The low frequency band at 40–70 cm−1 of the INS spectrumefers mainly to the χ

asymSi−O vibrations. The bands at the region

f 1200–2000 cm−1 of the INS spectrum originate from the firstvertones and combinations of the δO–Si–H vibrations [20].

. Conclusion

The polyhydridosiloxane was manufactured by sol–gelethod from triethoxysilane. Methanol, hexane and water

dsorption/desorption isotherms of the polyhydridosiloxaneere measured at 293 K and textural parameters were deter-ined in the framework of the BET and the Kiselev model.he values obtained are typical for the mesoporous adsorbents

ormed as agglomerate of nano-sized primary particles, whichize is in the range with the pores size. The silanol group con-entration in the PHS was found to be 0.44 mmol g−1 from theater adsorption isotherm.The IR and INS spectra of the PHS have been measured.

he QC simulation of the structure and vibrational spectra ofHS cluster has allowed the complete assignment of the bands

n experimental spectra. The QC-simulated model of the PHSlobule might be used for the further studies of the PHS oxidationuring storage and thermal treatment, including simulation ofhe products vibrational spectra.

ppendix A. Supplementary data

Supplementary data associated with this article can beound, in the online version, at doi:10.1016/j.matchemphys.007.08.033.

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