ng C 27 (2007) 172–179www.elsevier.com/locate/msec

Materials Science and Engineeri

Synthesis and characterisation of cellulose/silica hybrids obtained byheteropoly acid catalysed sol–gel process

Sónia Sequeira, Dmitry V. Evtuguin ⁎, Inês Portugal, Ana P. Esculcas

Department of Chemistry/CICECO, University of Aveiro 3810-193 Aveiro, Portugal

Received 20 September 2005; received in revised form 7 November 2005; accepted 18 April 2006Available online 6 June 2006

Abstract

Cellulose/silica hybrids (CSHs) were synthesized by a sol–gel method using eucalyptus bleached kraft pulp as cellulose source and tetraethylorthosilicate (TEOS) as the silica precursor in the presence of heteropoly acids (HPAs) as catalysts. HPAs, and especially tungstophosphoric acidH3PW12O40, showed better catalytic efficiency than conventional mineral acids. Silica was deposited on fibres in the form of a thin film ormesoparticles as revealed by SEM/EDS and AFM analyses. Roughly 40–60% of silica was incorporated into cellulosic material considerablydiminishing its hydrophilicity and improving thermal stability. CSHs were structurally characterised by FTIR, 13C and 29Si solid state NMR. Itwas suggested that proportions of Q2, Q3 and Q4 structures in silica counterpart depended on the synthesis conditions (H2O/TEOS molar ratio andcatalyst concentration among others). A clear relationship between the thermal stability of CSH and the degree of silica crosslinking in hybrids hasbeen observed.© 2006 Elsevier B.V. All rights reserved.

Keywords: Cellulose; Sol–gel synthesis; Heteropoly acid; Organic–inorganic hybrids; TEOS

1. Introduction

Organic–inorganic hybrids (OIHs) are a relatively newtype of composites with interesting mechanical, optical,electrical and thermal properties, which arise from thesynergism between the properties of the starting componentsand depend on the synthesis mode [1]. A significantproportion of recently reported OIHs is synthesised using asol–gel process—a rather flexible and versatile technique. Ina sol–gel process in situ generated inorganic particles areevenly dispersed at the nanometer scale in a polymeric hostmatrix, bounding to the polymer through hydrogen orcovalent bonds thus forming OIHs network. The dispersedinorganic particles govern the properties of hardness,brittleness and transparency, whereas density, free volumeand thermal stability depend on the organic host polymer [2].Typically the starting materials for a sol–gel process arenatural or synthetic polymers, metal alkoxides—M(OR)n(where, M is Si, Ti, Al, etc., and R is CH3, C2H5, C3H7,

⁎ Corresponding author. Tel.: +351 234 370693; fax: +351 234 370084.E-mail address: Dmitry@dq.ua.pt (D.V. Evtuguin).

0928-4931/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.msec.2006.04.007

etc.) and a small amount of acidic or basic catalysts.Hydrolysis (Eq. (1)) and condensation (Eq. (2)) reactionsare basically responsible for polymerisation of the inorganicprecursors as demonstrated below for the tetraethoxysilane(TEOS) under acidic conditions:

ð1Þ

OEt

OEt

OEt

OH Siy

OEt

OEt

OEt Si

OEt

OEt

O Si

n

OH + n EtOH

ð2ÞBesides the primary hydrolysis and polycondensation of

precursors, a sol–gel synthesis involves other important reactionsteps as gelation (accompanied by a sharp increase in viscosity),aging, drying, stabilization (thermal or chemical) and densifi-cation (occurring between 1000 and 1700 °C) [3,4].

173S. Sequeira et al. / Materials Science and Engineering C 27 (2007) 172–179

The structure and properties of OIHs obtained by sol–gelmethod depend on the synthesis conditions (such as H2O/TEOSratio, presence of organic solvents, pH, catalyst origin and itsamount, temperature, etc.) [5,6]. Particularly, acid catalysedsyntheses normally involve strong mineral acids which maydamage/fragile the acid-labile host polymer matrix during agingand drying steps. Additionally, mineral acids (especially volatileand thermally unstable) can create troubles regarding theequipment corrosion during the material production and limitthe final material application due to environmental concerns. Inthis context the application of strong non-volatile and thermallystable solid acids in sol–gel syntheses, such as heteropoly acids(HPAs), deserves attention.

Cellulose and cellulose derivatives are promising rawmaterials for the synthesis of OIHs [7]. Being the mostnaturally abundant renewable polymer, cellulose possessesseveral unique properties required in different technical areasand biomedicine [8]. Silica derivatives of cellulose are ofespecial interest since these substantially improve the thermalstability of the parent polymer matrix and improve itslipophilic behaviour and the affinity towards specific substrates[8,9]. The incorporation of silica into cellulose or cellulosederivatives has been accomplished with a wide range ofsilylation reagents, the most important being trimethylsilylchloride [9]. However these reactions usually require anorganic solvent such as pyridine, xylene or DMF, and hightemperatures (80–160 °C). Alternatively, silica incorporationby milder sol–gel processes has been reported for cellulosederivatives such as hydroxypropyl cellulose [10,11] andcellulose acetate [12,13]. A sol–gel method has also beenused for deposition of hydrophobic polysiloxane coatings onwood [14] and for the modification of sulphite pulp with watersoluble silicon-containing compounds [15] to increase thehydrophobicity of the final hybrid materials.

To our knowledge practically no information is available onOIHs for composite materials derived from unmodifiedcellulosic fibres (pulp) and silica. The pulp industry producesannually more than 200 million tons of bleached/unbleachedpulp worldwide. Although the major part of this pulp is used forpapermaking, it is progressively applied for the production ofmodified fibres, films, pharmaceutical coatings, food additives,etc. [8]. Cellulose/silica hybrid materials with expectedattractive mechanical, thermal and sorption properties mightbe considered as a potentially interesting application of pulpfibres alternative to papermaking.

In this work are presented results on the synthesis andcharacterisation of cellulose/silica hybrid materials prepared bysol–gel process from Eucalyptus globulus kraft pulp and TEOSusing HPA as the catalyst.

2. Experimental

2.1. Materials

E. globulus industrial bleached kraft pulp (Portucel, Cacia)was used as a cellulose source. This contained about 82% ofcellulose, about 17% of 4-O-methylglucuronoxylan and 0.3%

of ashes. The pulp intrinsic viscosity was 1030 cm3/g (ISO5351-1:1981).

Tetraethoxysilane (TEOS) was a commercial product sup-plied by Sigma-Aldrich Chem. Comp., France (98%, purity).Heteropoly acids (H3PMo12O40, H4SiW12O40, H4SiMo12O40,H3PW12O40) and mineral acids (HCl, HNO3, H3PO4, H2SO4)were p.a. grade products (Sigma-Aldrich Chem. Comp.,France). Heteropolyacids were oven-dried at 170 °C in orderto eliminate the crystalline water. Ethanol was p.a. grade productsupplied by Riedel-de-Häen Chem. Comp., Netherlands.

2.2. Synthesis of cellulose/silica hybrids

Prior to use, the air-dried pulp was disintegrated (swollen) inwater (1 g of oven dried pulp/100 ml of water) and then washedwith ethanol to remove the excess of water. The treated pulpwas filtered off and contained a small amount of residualsolvent. This pre-activation step is necessary to improve theaccessibility of cellulose. A mixture (70 ml) with pre-determined proportion of TEOS, distilled water, ethanol andcatalyst was added to the pre-activated pulp. The reactionproceeded over 24 h at room temperature (∼20 °C) withconstant suspension stirring (200 rpm). The final hybridmaterial was removed by filtration and dried first at 40 °C(24 h) and then at 105 °C (24 h). The percentage of silicaincorporation was determined by weighting the hybrid materialand/or by weighting the residue after calcination at 525 °C(3 h). Usually each synthesis was repeated in triplicate toaverage the yield of hybrid material.

2.3. Cellulose/silica hybrids characterization

29Si solid-state Magic Angle Spinning Nuclear MagneticResonance (29Si MAS NMR) spectra were recorded on a BrukerAvance 400 spectrometer. Samples were packed into azirconium rotor sealed with Kel-F™ caps and spun at 5 kHz.Acquisition parameters were as follows: 90 ° pulse width 4 μs,contact time 8 ms, pulse delay 60 s.

13C solid-state Cross Polarization-Magic Angle SpinningNuclear Magnetic Resonance (13C CP-MAS NMR) spectrawere registered on a Bruker Avance 400 spectrometer. Sampleswere packed into a zirconia's rotor sealed with Kel-F™ capsand spun at 7 kHz. Acquisition parameters were as follows: 90°pulse width, contact time 1 ms, and the delay between pulses of2 s.

Thermogravimetric analysis (TGA) was carried out on aShimadzu TGA-50 instrument under N2 atmosphere in therange 25–700 °C, with a heating rate of 5 °C/min, using about8 mg of sample placed in a platinum cap.

Differential Scanning Calorimetry (DSC) analyses wereperformed on a Shimadzu DSC-50 instrument under N2

atmosphere, in the range of 25–500 °C with a heating rate of5 °C/min. Samples (about 8 mg) were placed in aluminium non-sealed caps.

Fourier Transform Infrared Spectra (FTIR) were recorded ona Mattson FT-IR spectrometer Model 7300 using KBr pellets at4 cm−1 resolution and acquiring 128 scans per set.

Fig. 1. Pulp weight increment (%) after reaction with TEOS in the presence ofmineral acids (ca. 0.3 mol/l) and heteropoly acids (ca. 3.0×10−4 mol/l).Reaction conditions: 24 h, 20 °C, TEOS/EtOH/H2O solution (20:45:5 v/v/v).

174 S. Sequeira et al. / Materials Science and Engineering C 27 (2007) 172–179

Scanning Electron Microscopy (SEM) images wereobtained on a FEG-SEM Hitachi S4100 microscope coupledwith Energy Dispersive Spectrometer (EDS) and operating at25 kV, using carbon coated samples.

Atomic Force Microscopy (AFM) images were acquired on aSPM MultiMode microscope (Digital Instruments) in TappingMode on air using commercial Si cantilevers TAP 300 with aresonance frequency of about 300 kHz and in Contact Modeusing commercial Si cantilevers FMR (Nanosensors) with aresonance frequency of about 80 kHz.

Water retention value (WRV) of pulp and hybrids wasdetermined by centrifugation method according to the proce-dure proposed by Jayme [16].

3. Results and discussion

3.1. Synthesis of cellulose/silica hybrids

A series of cellulose/silica hybrids (CSHs) was preparedunder different experimental conditions as summarized in Table1. Besides pulp and silica precursor (TEOS) the reactionmixture contained water, an organic solvent (EtOH) and acidiccatalyst. The H2O/TEOS molar ratio, the type and the amount ofcatalyst were the main variables. The ratio between reactionmixture components (H2O/TEOS/EtOH) and pulp has beenmaintained constant in all experiments (about 70) and was highenough to avoid reagent deficiency and the diffusion problemsin bulk. All syntheses were carried out at room temperature(+20–23 °C) since at temperatures +30 °C or higher thespontaneous gel formation has been observed. Hybrids of seriesA (Table 1) were obtained with different acidic catalystsmaintaining other reaction conditions the same. The importanceof H2O/TEOS molar ratio and the amounts of catalyst wereverified synthesising the hybrids of series B and C, respectively.To evaluate the catalyst efficiency (hybrids of series A) thepreparation procedure was slightly different from that describedin the experimental part for the hybrids of series B and C.Namely, prior to drying, all hybrid materials of series A werethoroughly washed with 50 ml of ethanol to remove the excessof TEOS. This procedure assures that only bounded to cellulosesilica remains in the CSH and, consequently, the weighincrement of the final hybrid relatively to the initial pulp is aquantitative index for the silica incorporation thus reflecting theeffect of each catalyst used.

Table 1Experimental conditions used for the preparation of cellulose/silica hybrids

Hybrid H2O/TEOS(mol/mol)

EtOH/TEOS(mol/mol)

Catalysts Catalystsconcentration(mol/l)

A 3.2 8.7 HNO3, H2SO4,H3PO4, HCl,PW12, SiW12,SiMo12, PMo12

Mineral acids: 0.3Heteropolyacids:3.0×10−4

B 4.4 8.3 PW12 3.0×10−4

C 4.4 8.3 PW12 3.0×10−4, 4.2×10−4

and 6.0×10−4

3.1.1. Catalyst effectDifferent mineral acids (HCl, HNO3, H3PO4 and H2SO4),

conventional catalysts in sol–gel syntheses, and severalheteropoly acids (HPAs) were used in the preparation ofCSHs. HPAs belong to the inorganic metal–oxygen clusteranions containing highly symmetrical core assemblies of MOx

units (M=V, Mo, W) forming a quasi-spherical structure [7]. Aseries of polyanions of Keggin type constituted of twelve MO6

octahedra surrounding the central coordination heteroatom ofgeneral formula Hx[XM12O40]

x− (X=Si, P; M=Mo, W) wasemployed in this study. HPAs possess a very low negativecharge on the surface bridging oxygens (low basicity) and aretherefore strong Brønsted acids, even stronger than mineralacids [7,17]. For example, the acidity of concentrated aqueoustungstophosphoric acid H3PW12O40 (PW12 for short) is higherby about 1.5 units of the Hammet acidity function (H0) thanH2SO4 [17]. HPAs are soluble in polar organic solvents thoughtheir dissociation is often stepwise. HPAs are thermally stableup to 350–400 °C and, unlike most of the strong mineral acids,have more affinity to silica than to the cellulosic material. Thesefeatures are important regarding the eventual structural stabilityof the final CSH. To our knowledge the efficiency of sol–gelacidic catalysis with HPA has not yet been assessed.

The results obtained for the silica incorporation into the pulpusing different catalysts are presented in Fig. 1. Theconcentration of HPAs was about 103 times lower thanconventional mineral acids and corresponded to about 30–50times less catalyst weight load. Nitric acid was the mosteffective catalyst among the tested conventional acids thoughthe former is less strong than, for example, hydrochloric orsulphuric acids. This means that the catalytic effect of mineralacids cannot be explained only by their protolytic properties.Since in the acid-catalysed sol–gel synthesis the polyconden-sation (Eq. (2)) is the rate limiting step of the silica networkformation [5] it is reasonable to propose that exactly thisreaction step is better promoted by nitric acid than with othermineral acids applied.

Among HPAs the best catalytic activity was observed withPW12, which was even better than with nitric acid (Fig. 1).Overall, the efficiency of TEOS hydrolysis/condensation

Water content

52

43

0

20

40

60

7% (v/v) H2O 10% (v/v) H2O

% S

iO2

(w/w

)

Fig. 2. Influence of water content on pulp weight increment (%). Reactionconditions: 24 h, 20 °C, TEOS/EtOH/H2O solution 20:45:5 (v/v/v) (H2O/TEOS=3.2 mol/mol) and 20:43:7 cm3 (H2O/TEOS=4.4 mol/mol), PW12

concentration of 3.0×10−4 mol/l.

Catalyst concentration, mol/l

46

52

49

40

44

48

52

56

3 • 10-4 4.2 • 10-4 6 • 10-4

% S

iO2

(w/w

)

Fig. 3. Influence of catalyst loading on pulp weight increment (%) Reactionconditions: 24 h, 20 °C, TEOS/EtOH/H2O solution 20:43:7 (v/v/v) (H2O/TEOS=4.4 mol/mol).

175S. Sequeira et al. / Materials Science and Engineering C 27 (2007) 172–179

reactions and the interaction of siloxane pre-polymer withcellulose in the presence of HPAs increased in the followingorder: H3PW12O40 (PW12)>H4SiW12O40 (SiW12)>H4SiMo12O40 (SiMo12) >H3PMo12O40 (PMo12). This is almost inagreement with reported acidic strength in acetone (pK1 valuesin parentheses): PW12 (1.6)>SiW12 (2.0)≈PMo12 (2.0)>SiMo12 (2.1) [17]. The unexpected higher catalytic activity ofSiMo12 than that of PMo12 could be tentatively explained by thesecondary structure of heteropolyanions formed in the presenceof EtOH and TEOS, which seems to influence their final acid/basic properties.

Since the highest percentage of incorporated silica (31.3%(w/w)) into a cellulosic material was observed in the presence ofPW12, the optimisation of CSHs synthesis was carried out usingPW12 as catalyst.

3.1.2. Effect of H2O/TEOS ratioThe molar proportion of water and TEOS is a crucial factor

for the synthesis of siloxane pre-polymers. The water contentinfluences the rate of TEOS hydrolysis, which in turn affects theseries of hydrolysis/condensation reactions characteristic for asol–gel process. Theoretically, to convert completely siliconalkoxide to SiO2, two moles of water are needed per each moleof precursor [18]. In practice, this molar H2O/TEOS ratio is notsufficient probably due to the formation of non-cyclicintermediate species and should be maintained higher than 2.5[19]. Thus, an excess of water is required to prepare cellulose/silica hybrids.

Results presented in Fig. 2 show that the increase of watercontent from 3.2 to 4.4 mol H2O/mol TEOS (7% and 10% (w/w), respectively) favours the silica incorporation into cellulosicmaterial. This may be related to increase of the molecularweight of siloxane polymers due to the promotion ofalkoxysilane condensation reactions with increase of H2Omolar proportion in the reaction system [20]. The siloxane pre-polymers of higher molecular weight eventually interact easierwith pulp than those with smaller molecular weight. 13C CP-MAS NMR spectra of CSH obtained with molar ratios H2O/TEOS 3.2 and 4.4 and the same PW12 concentration (series A

and B, Table 1) did not show the characteristic signals at 16.9and 58.5 ppm assigned to methyl and methylene carbons,respectively, in ethoxyl moieties of the precursor. This factindicates complete hydrolysis of TEOS during the CSHsynthesis.

H2O/TEOS molar ratios higher than 4.4 promotes fastnetwork growth leading to gelation, i.e. the formation of infinitepolymeric network. This spontaneous process increases drasti-cally the SiO2 content in hybrid material, which is not bound topulp fibres.

3.1.3. Catalyst loadThe amount of catalyst in the reaction system H2O/TEOS/

EtOH influences significantly both hydrolysis (Eq. (1)) andcondensation reactions (Eq. (2)). Increasing the catalyst toalkoxysilane ratio normally favours the hydrolysis rate ratherthan condensation reactions [5]. As a consequence the averagelength of siloxane polymer chains decreases [20]. This factobviously is related to the decrease of bound silica when thecatalyst amount was increased (Fig. 3). Probably, higher catalystconcentration led to smaller size of silica domains on the fibresurface negatively affecting the hybrid yield.

13C CP-MAS NMR spectra of CSHs (not shown) revealed,surprisingly, a notable amount of incompletely hydrolysedethoxy moieties (characteristic resonances at 16.9 and58.5 ppm) when the catalyst concentration was increasedtwice (from 3×10−4 to 6×10−4 mol/l). This indicates thepresence of a less branched silica network. It may be proposedthat under increased catalyst load the extent of hydrolysisremains incomplete due to the reversibility of the hydrolysisreaction (Eq. (1)), i.e. the excess of PW12 moved partially thereaction equilibrium to the left (formation of ethoxysilanemoieties).

3.2. Cellulose/silica hybrids characterization

3.2.1. Image analysisThe silica deposition on cellulosic fibres was monitored by

SEM coupled with EDS. As an example, SEM images of hybridseriesA, obtained with PW12 as catalyst, are presented in Fig. 4.

Fig. 4. SEM images of hybrid material series A obtained with PW12 as catalyst.

Fig. 5. AFM image (tapping mode) of hybrid material series A.

50

60

70

40090014001900

wavenumber, cm-1

Tra

nsm

itanc

e, %

1640

800

1080

45095

0

BP

BP/SiO2

1220

Fig. 6. FTIR spectra of bleached pulp (BP) and hybrid material series A(BP/Silica). PW12 was used as catalyst.

176 S. Sequeira et al. / Materials Science and Engineering C 27 (2007) 172–179

It was verified that silica was deposited predominantly in theform of a thin film or as isolated or grouped mesoparticles (1–4 μm in diameter). Simultaneously, the relatively large-scaleparticles of 10–15 μm were detected that were mostly localizedat the intersection of neighbouring fibres thus bounding themand reinforcing the composite structure (Fig. 4). Large-scaleparticles were more abundant in hybrids obtained with highermolar ratio H2O/TEOS and with lower concentration of catalystfor the same molar ratio of water and silica precursor.

The size and the shape of domains in the silica film on pulpfibres were assessed by AFM. Fig. 5 shows the fibre region ofhybrid series A covered by a silica film. The AFM analysisrevealed that the silica film comprised conjugated round-shapedomains of 0.05–0.3 μm. This film was discontinuous andinterrupted by uncovered regions of the fibre's surface.

3.2.2. Structural analysis by FTIR and NMRFTIR spectra of hybrids were very similar and clearly

different from the spectrum of starting material (pulp). As anexample, Fig. 6 shows a spectrum of bleached kraft pulp andhybrid A. A broad band centred at ∼3400 cm−1 attributed toO–H stretching of silanol groups and water is not shown in thespectra. The deformation band at 1640 cm−1 confirms thepresence of bound water. This band is much less pronounced in

the spectrum of hybrid material than in spectrum of pulp. Infact, the Water Retention Value (WRV) of initial pulp (94%)was much higher than that of hybrid materials (20–24%)indicating clearly the enhancing of hydrophobic character ofCSHs. Typical bands of silica at the low wave number rangeassociated with rocking, bending (or symmetric stretching) andasymmetric stretching of SiO2 inter-tetrahedral oxygen atoms[21], are expected at 450, 800 and 1080 cm−1. The bands at 450and 800 cm−1 were detected in the hybrid material. The thirdband expected at 1080 cm−1 is overlapped with a broad bandbetween 1000 and 1150 cm−1 attributed to O–H bending ofprimary and secondary alcohol groups of cellulose [9]. FTIRspectra confirmed the presence of cyclic silica (SiO)6 structuresas revealed from characteristic band at 1220 cm−1. The bandcorresponding to the Si–O cellulose vibration that could

-40 -50 -70 -80 -90 -100 -110 -120 -130 -140 -150 ppm

Q2

Q3Q4

Q2

Q3

Q4

Q2

Q3

Q4

-60

-70 -80 -90 -100 -110 -120 -130 -140 ppm-60

-70 -80 -90 -100 -110 -120 -130 -140 ppm-60

MAS RMN 29Si

Hybrid A

MAS RMN 29Si

Hybrid B

MAS RMN 29Si

Hybrid C

Fig. 7. 29Si MAS NMR spectra of hybrid series A (a) and B (b) obtained with3×10−4 mol/l concentration of PW12 and hybrid series C (c) obtained with6×10−4 mol/l concentration of PW12. Specific reaction conditions are presentedin Table 1.

0,0

0,2

0,4

0,6

0,8

1,0

0 2 400 600

T (°C)

w /

w 0

BP

BP/sílica

sílica

Fig. 8. TGA traces of silica, bleached pulp (BP) and hybrid series A.

177S. Sequeira et al. / Materials Science and Engineering C 27 (2007) 172–179

confirm the covalent bonding between silica and cellulose isalso expected in the range of 1000–1150 cm−1 and was notclearly detected. The efforts in the detection of cellulose linkagewith siloxy moiety using differential 13C CP-MAS NMRspectra of initial pulp and hybrids have also failed. These facts,however, do not signify that these linkages do completely notexist. Probably, the abundance of cellulose-silica covalentbonds is very low (<3–5 mol%).

Additional structural information on the silica counterpart ofhybrids was obtained using solid-state NMR. 29Si MAS NMRspectroscopy provided information about the proportions of Qn

species (Qn represents a Si atom bonded to n other Si atoms viaO-bridges) allowing quantification of crosslinking degree. 29SiMAS NMR spectra (Fig. 7) obtained for hybrids of series A, Band C, respectively, showed two major non-resolved signals at−100 ppm and −110 ppm assigned to Q3 and Q4 resonances,respectively, and a weaker peak at −92 ppm corresponding toQ2 species [22,23]. Q1 species expected at −81 ppm were notdetected which indicates that the silica network is essentiallycomposed of cyclic units (Q3 species) crosslinked by oxygenbridges (Q4 species) [23].

Q2 species indicate the presence of small amounts of linearsegments [23]. The ratio of peak areas Q1/Q2/Q3/Q4 was(0:0:1:1) for hybrid series A (prepared with 7% of water andPW12 concentration of 3×10−4 mol/l) (0:1:7:6) for hybrid B(prepared with 10% of water and PW12 concentration of3×10−4 mol/l) and (0:1:6:5) for hybrid C (prepared withPW12 concentration of 6×10−4 mol/l and with 10% of water).The degree of TEOS conversion in condensation reactions, η,defined as the ratio of effective (fef) and potential (fpot)functionalities of Si substituted by OSi moieties, can becalculated by Eq. (3):

g ¼ feffpot

ð3Þ

where fef ¼P

4n¼1ðxn � nÞ, fpot ¼ 4�P

4n¼1ðxnÞ and xn is the

mole fraction of Qn structures (calculated from correspondingpeak areas). For hybridsA,B andC ηwas found to be 0.88, 0.84and 0.83, respectively. These results confirm the formation of ahighly branched silica network during CSH preparation. Theincrease of water content in the reactionmixture (hybridB) led tothe reduction of η, i.e. affects negatively the degree of networkcrosslinking. A similar effect was observed with the increase ofacid catalyst concentration during CSH synthesis (hybrid C).

3.2.3. Thermal analysisTypical TGA curves for silica, bleached pulp and hybrid

material (series A obtained with PW12 as catalyst) are presentedin Fig. 8. A small weight loss at around 60 °C corresponds towater release. Thermal degradation of organic material in

-1

-0,5

0

0,5

1

1,5

2

2,5

3

0 100 200 300 400

T (°C)

Hea

t flo

w (m

W)

3 . 10-4 mol/l HPA

6 . 10-4 mol/l HPA

Fig. 9. DSC traces of hybrids series B and C. Specific reaction conditions arepresented in Table 1.

178 S. Sequeira et al. / Materials Science and Engineering C 27 (2007) 172–179

bleached pulp is observed at 305 °C and at 345 °C for the hybridmaterial. This increase in degradation temperature indicates thestrong organic–inorganic phase interactions (probably also thecovalent bonding) greatly influencing the thermal resistance.Very similar results were obtained for hybrids of series B and Calthough the temperature corresponding to the maximumthermal degradation rate was lower. Thus, CSH obtained atH2O/TEOS molar ratio 4.4 (hybrid B) showed a degradationtemperature of 11 °C lower (334 °C) than CSH obtained atmolar ratio 3.2 and at the same catalyst concentration. Similarly,increasing the catalyst concentration from 3×10− 4 to6×10−4 mol/l at H2O/TEOS molar ratio 4.4 (hybrid C) thethermal stability of hybrid decreased (from 334 °C to 316 °C).Therefore, a clear parallel between the silica network structureand the thermal stability of CSH can be drawn. As the degree ofthe silica crosslinking increases the thermal stability of CSHincreases as well. Apparently, the synthesis conditions leadingto the formation of highly branched silica also favours its strongbounding to cellulosic fibre surface. Additionally, the appear-ance of non-reacting ethoxy moieties in hybrid material Caffected its thermal stability.

The results of DSC analyses of hybrid materials corroboratedwith TGA data. Fig. 9 shows DSC traces of hybrids series Cobtained with different concentration of PW12 at H2O/TEOSmolar ratio 4.4. The endothermic peaks in the region 310–340 °C correspond to segmental motions in hybrid materialsunder thermal decomposition. A clear shift of transitiontemperature (Tg) from 335 °C to 310 °C was observed forhybrids when the catalyst concentration was increased from3×10−4 to 6×10−4 mol/l.

4. Conclusions

Organic/inorganic hybrids based on cellulose fibres andsilicon alkoxides were synthesized by a sol–gel method underacidic conditions. HPAs were used as catalyst for the first timeand showed a higher catalytic efficiency than that ofconventional mineral acids (HCl, HNO3, H3PO4, H2SO4).

Tungstophosphoric acid H3PW12O40 was the best catalystamong HPAs used in this study.

The effect of sol–gel synthesis conditions on the yield ofhybrids and the structure of the silica counterpart has beenevaluated. It was suggested that the increase of H2O/TEOSmolar ratio up to 4.4 favours the hybrid yield, whereas catalystconcentration higher than 3×10−4 mol/l affected negatively theCSH yield. The best CSH yield (152%) was reached at H2O/TEOS/EtOH/PW12 molar ratio 4.4/1/8.3/0.00002. It wasverified based on a 29Si NMR study that silica network isessentially composed of cyclic units (Q3 species) crosslinked byoxygen bridges (Q4 species) in the presence of a small amountof linear Q2 fragments. The synthesis conditions affectednotably the structure of silica counterpart of hybrids. Overall,increasing the H2O/TEOS molar ratio more than 3.2 and using acatalyst concentration higher than 3×10−4 mol/l favoured theappearance of linear siloxane fragments and the decreasing ofcrosslinked chains in silica. Incomplete condensation reactionsof TEOS led to weaker bounding of silica to the pulp fibres. Itwas also suggested that if the covalent bonding betweencellulose and silica in hybrids exist, the frequency of thesebonds should be very low (<3 mol%).

Hybrid materials showed considerably higher hydrophobic-ity (about 4 times) and thermal stability when compared to thestarting fibrous material (bleached kraft pulp). SEM/EDS andAFM image analyses revealed that silica is deposited on thefibre surface in the form of a thin film or as mesoparticles (up to1–4 μm in diameter), which were localized at the intersection ofneighbouring fibres thus bounding them and reinforcing thecomposite structure. The density of the final hybrid material canbe changed varying the conditions of it post-treatment after thesynthesis (such as pressing at different temperatures, etc.). Theevaluation of physical properties of CSH materials (strength,thermal conductivity, etc.) is in progress.

Acknowledgments

The authors wish to thank the Regional Research InnovationProgram PRAI-CENTRO (FEDER), Project “New Materialsbased on Cellulose Fibres” for financial support of this work.

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