Comparative study of the sol-gel processes starting with different substituted Si-alkoxides

8/18/2019 Comparative study of the sol-gel processes starting with different substituted Si-alkoxides

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Comparative study of the sol–gel processes starting

with different substituted Si-alkoxides

Andrei Jitianu a,*, Alina Britchi b, Calin Deleanu b, Virgil Badescu c,Maria Zaharescu a

a Institute of Physical Chemistry I.G. Murgulescu, 202 Splaiul Independentei, 77208 Bucharest, Romaniab National NMR Laboratory, Institute of Organic Chemistry, C.D. Nenitescu, 202B Splaiul Independentei, 71141 Bucharest, Romania

c Institute of Chemical Research, 202 Splaiul Independentei, 77208 Bucharest, Romania

Received 7 January 2002; received in revised form 11 October 2002

Abstract

In the present work a comparative study of the hydrolysis–polycondensation processes of different Si-substituted

alkoxides, leading to hybrid materials with covalent –Si–C– bonds, was carried out. The following alkoxides were used:

tetraethoxysilane (TEOS), methyltriethoxysilane (MTEOS) and vinyltriethoxysilane (VTEOS). Using gas chromato-

graphy coupled with mass spectrometry (CG-MS), nuclear magnetic resonance ( 29Si-NMR) and infrared spectrometry

(IR), information about the sol–gel process in the mentioned systems were obtained. The differences in the reactivity of

the studied alkoxides are connected with the steric effect of the organic substituents. The reactivity of the alkoxides in

the early stages of the hydrolysis-polycondensation process increased in the order TEOS


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the so-called ORMOSILS (ORganically MOdified

SILicates) or ORMOCERS (ORganically Modi-

fied CERamics) which were first reported by

Schmidt in early 1980s. Schmidt [3] consideredthat the organic groups had two important roles:

as organic modifiers of the inorganic network or as

network formers when the organic group used can

be polymerizable.

It is well known that up to Sanchez [1], many

scientists tried to give a classification of hybrid

materials. He divided conventionally the hybrid

networks in two classes. This classification is based

on the type of bonds established between the or-

ganic and inorganic groups. The class I corre-

sponds to the hybrid materials in which organic

molecules, oligomers or low molecular weight or-

ganic polymers are simply embedded in inorganic

matrices. The class II of hybrid materials corre-

sponds to the type of the network where both or-

ganic and inorganic compounds are bonded

through stronger covalent or iono-covalent

chemical bonds.

So it is well known that the main reactions of

hydrolysis–polycondensation which take place

with this kind of organic modified alkoxides are

the following:

Si (OR)3 + H2O

R R

OHSi(RO)2 + ROH

′ ′

ð1Þ

where R was O–C2H5 and R0 could be O–C2H5 in

the case of TEOS, –CH3 in the case of MTEOS

and –CH@CH2 in the case of VTEOS.

The first reaction (1) is a hydrolysis reaction,while reactions (2) and (3) are condensation reac-

tions. As it can be observed, in the hydrolysis–

polycondensation processes the silicon carbon

bonds are not involved. The silicon–carbon bonds

were found untouched in the final materials. The

presence of the organic groups R0 decreases, gen-

erally, the crosslinking degree.

As can be observed from reactions (1)–(3) the

sol–gel processes are very complex, because in this

kind of synthesis a competition between different

reactions including reesterification, depolymeriza-

tion, and transesterification [4] occurs. It was es-

tablished [5–7] that the identification and the

quantification of the molecular species produced

from reactions (1)–(3) are very important for the

understanding of the initial stages of the hydroly-

sis–polycondensation processes. On the other

hand, it has been proved [5] that the early stages of

the hydrolysis–polycondensation processes are

very important for the future structure and growth

of the polymers and for their final properties [4,6–

9], as well.

Generally the identification of the molecularspecies formed during the early stages of the sol–

gel processes was made by using two important

spectrometric measurements, mainly 29Si-NMR

and/or GC-MS [10]. Involving 29Si-NMR method

(RO) 2 Si OH

R′ R′

(OR) 2SiRO+ (RO) 2 Si O

R′ R′

(OR) 2Si + ROH

ð2Þ

(RO)2 Si OH

R′ R′

(OR)2

SiHO+ (RO)2

Si O

R′ R′

(OR)2

Si + H2

Oð3Þ

264 A. Jitianu et al. / Journal of Non-Crystalline Solids 319 (2003) 263–279


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in the study of the sol–gel route was a very im-

portant step in understanding the mechanisms of

the sol–gel processes.

The gas chromatography coupled with massspectrometry (GC-MS) was less involved in the

clearing up of the sol–gel process, because this

method required volatile and stable compounds in

the conditions of the investigation (temperatures

about 200 C). The silicon alkoxides fulfill these

conditions [10–16].

The first study in which both GC-MS and 29Si-

NMR spectrometry were used for the comparative

identification of the molecular species was made by

Klemperer et al. [12,17]. These studies gave rele-

vant information about the early stages of the

hydrolysis–polycondensation and about molecular

species formed in these steps. The first studies on29Si-NMR appeared in 1980s [17–20], when the

hydrolysis–polycondensation processes of tetra-

methoxysilane (TMOS) and tetraethoxysilane

(TEOS) were considered. Some of the most com-

plete studies carried out using 29Si-NMR spect-

rometry were reported by Assink et al. [20–26].

The authors tried to describe the kinetic behavior

of the hydrolysis–polycondensation processes of

the unmodified and of the organically modified Si-

alkoxides. In the last decade [5], the NMR spect-rometry was developed and new experiments like29Si– 29Si INEPT DQF COSY were involved for

identifying new oligomers. The sequence 29Si– 29Si

INEPT DQF COSY consists of three main parts.

Thus in the first part a refocused INEPT sequence

is used for achieving the sensitivity enhancement.

The second part consists of COSY sequence that

allow observation of Si–Si correlation. The last

part is DQF sequence removes the signals of the

isolated Si atoms [27]. 29 Si-NMR spectrometry was

also involved in the ab initio calculation [7,28,29]

of the chemical shifts for 1 H-NMR and 29 Si-NMR.

The use of the 17O-NMR was very important in

the elucidation of the sol–gel mechanism [30].

During the last years 29

Si-NMR spectrometry wasused for studying different substituted Si-alkoxides

such as methyltriethoxysilane [26,31–33], dimeth-

yldietoxysilane [30], ethyltrimethoxysilane [34],

methyltrimethoxysilane [35], and phenyltrietoxisi-

lane [26].

In our previous works, the hydrolysis-polycon-

densation of some organically modified Si-alkox-

ides [15,16], the thermal stability [17] and ageing

effect [18] of SiO2-based inorganic–organic hybrid

materials were studied.

This paper presents the results of a comparative

study where GC-MS, 29Si-NMR and IR spect-

rometry, were used in the investigation of the early

stages of the hydrolysis–polycondensation pro-

cesses of organically modified Si-alkoxides, like

methyltriethoxysilane (MTEOS) and vinyltrieth-

oxysilane (VTEOS) comparatively with a non-

substituted alkoxide like tetraethoxysilane

(TEOS).

2. Experimental

2.1. Sample preparation

Solutions were prepared using the following

alkoxides as Si-precursors: tetraethoxysilane

(TEOS) methyltriethoxysilane (MTEOS) and vi-

nyltriethoxysilane (VTEOS). The molar ratio

and the preparation conditions are presented in

Table 1.

First, the mixture containing ethanol, water and

hydrochloric acid was prepared and the alkoxides

were added dropwise afterwards under stirring.

Table 1

Chemical composition and the reaction conditions for the studied mixtures

Sample Alkoxide Molar ratio pH Stirring time

(min)

Concentrationa

of Cr(acac)3(mol/l)

EtOH/alkoxide H2O/alkoxide

1 TEOS 1.75 1 3.5 20 5 103

2 MTEOS 1.75 1 3.5 20 5 103

3 VTEOS 1.75 1 3.5 20 5 103

a Only in the sample used for 29Si-NMR studies.

A. Jitianu et al. / Journal of Non-Crystalline Solids 319 (2003) 263–279 265


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The stirring was maintained 20 min after the last

drop was added. After the preparation, half of the

samples were stored in closed glass vials for GC-

MS and IR measurements at different times.Chromium acetylacetonate (Cr(acac)3) was added

to the other half of samples, so that its final con-

centration in each sample was 5 103 M. Therole of the Cr(acac)3 was that of a spin relaxation

agent for the NMR spectrometry. Many studies

[20,25,26] have shown that the Cr(acac)3 has no

effect on the rates of the sol–gel processes. After

Cr(acac)3 addition the solution were immediately

transferred to a close NMR tube.

2.2. Sample characterization

A double focus 70-Se VG Analytical Mass

Spectrometer coupled with a gas chromatograph

was used under the experimental conditions pre-

sented elsewhere [13,14]. The areas of the mole-

cular species detected by GC were automatically

integrated by the PKD special program, which the

computer of GC-MS was equipped, or by the data

system display of the GC-MS equipment.

The IR spectra were recorded on spectrometer.

The IR measurements on liquid samples were

carried out using KRS-5 standard cells with 0.001mm length from Carl Zeiss Jena. The mixtures

prepared were injected in these type of cells in

order to recorded their IR spectra. A spectral

ranges from 4000 to 2800 cm1 and 1800 to 200

cm1 respectively, with a resolution of 2 cm1 were

investigated.

The 29Si-NMR spectra were measured, at 79.5

MHz, with a Bruker Avance DRX-400 spec-

trometer equipped with an inverse detection mul-

tinuclear probe. A pulse length of 4.6 ls, an

acquisition time of 1 s and a number of scansvarying between 128 and 1024 scans were used, the

line broadening being typically 4 Hz.

The reaction mixture solutions were stored into

a 5 mm NMR tube. The external lock solvent was

DMSO-d 6 and it was introduced in a capillary,

inside the NMR tubes. The 29Si-NMR spectra

were proton broad band decoupled, recorded at

room temperature and they were electronically

referenced with respect to an external tetrameth-

ylsilane standard (TMS) at 0 ppm.

3. Results

In Fig. 1 the chromatograms of the studied re-

action mixture at the start are presented. In order

to obtain a good homogenization of the studied

samples they were mixed for 20 min before being

subjected to different measurements and this mo-

ment was called the start.

From these chromatograms, the number of

molecular species at the start of the reactions was

identified. It can be pointed out that in the case

of the organic modified alkoxides, comparatively

with TEOS, a higher number of molecular species

separated by gas chromatography was found.

The higher number of molecular species ob-

served by gas chromatography in the case of the

substituted Si-alkoxides could be attributed to the

increasing of hydrolysis–polycondensation rate.

This fact was also supported by the identification

of molecular species with higher weight immedi-

ately after finishing the preparation of the reaction

mixture. The compounds separated by GC were

identified by recording their mass spectra and

comparing them with NBS Library spectra and

with our previous works [8,12,13,15,16]. In the

case of unknown species, the strategy for their

identification was to use the chromatographic andfragmentation criteria.

The mass spectra used for the identification in

all mixtures of the monomers and monohydroxy-

lated monomers are presented in Fig. 2 (TEOS –

Fig. 2(a) and (b), MTEOS – Fig. 2(c) and (d),

VTEOS – Fig. 2(e) and (f)). The main studies on

identification of silicon molecular species by mass

spectrometry in the early stages of the sol–gel

processes were realized by Klemperer et al. [12,17],

Wheeler et al. [10,11] and Badescu et al. [36,37].

As it can be observed from Fig. 2, in all cases,there are not very significant differences between

monomer and monohydroxylated monomer. The

monomers and the monohydroxylated monomers,

showed in Fig. 2, presented only a difference of 28

mass units between them. The molecular species

identified by mass spectrometry in all mixtures,

like monomer and monohydroxylated monomers,

are presented in Fig. 2.

These differences which are observed in all mass

spectra might be assigned to the exchange of an



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ethoxy group (–O–C2H5) with a hydroxy group

(–OH). This trend was observed not only in the

case of TEOS, but for all substituted alkoxides and

it confirms that the Si–C bonds from Si–CH3 andSi–CH@CH2 were not submitted to hydrolysis– polycondensation processes. In this process of

hydrolysis, only ethoxy groups were influenced.

The detailed mass spectra studies concerning mo-

lecular species formed in the sol–gel reactions were

presented elsewhere [36,37].

The analysis of the evolution of the molecular

species obtained during the sol–gel processes

identified by GC-MS was performed by using the

PKD special program. Due to this program, each

molecular species separated by GC was quantified

from the area under each peak, appearing in

chromatograms. The early stages of sol–gel pro-

cesses was studied by GC-MS up to 96 h. In order

to carry out this study the molecular species

formed at different periods from the start of the

sol–gel process were identified.

In Figs. 3–5 the evolution of molecular species

versus time up to 96 h for each studied reaction

mixture are presented.

As can be observed from Fig. 3, in the case of

the mixture prepared with TEOS, in the first steps,

up to 24 h, the predominant molecular specieswere the monomer, monohydroxylated monomer

and dimers. It was established that, after 96 h, the

dimers and trimers are predominant molecular

species, but a higher quantity of monomer still

exists.

From Fig. 4, the evolution of the molecular

species in the reaction mixture prepared with

MTEOS can be observed. It can be pointed out

that even from the start of the hydrolysis–poly-

condensation process, small quantities of mono-

mer and monohydroxylated monomer appeared.After 96 h, the predominant molecular species

found in the reaction mixture were tetramers and

pentamers. Another important observation is that

after 96 h, only traces of the monomer appear. The

presence of this molecular species could be ex-

plained by the reversible equilibrium of reactions

(1)–(3) which occur in the sol–gel system.

In Fig. 5 the evolution of the molecular species

formed up to 96 h in the reaction mixture con-

taining VTEOS is presented. In the first step,

Fig. 1. Chromatograms of the studied mixtures: (a) TEOS,

(b) MTEOS and (c) VTEOS, at start of the sol–gel reaction.



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besides the monomer, which is not in a very high

quantity, an important amount of dimers was

identified. Most of species formed after 96 h from

the start of the sol–gel reaction were dimers andtrimers. Although in this mixture an important

quantity of tetramers is presented, these species are

not predominant. The monomer still exist, but in

small quantity.

The GC-MS results are in good agreement withthe absolute values obtained from 29Si-NMR data.

Fig. 2. Mass spectra of monomers and monohydroxylated monomers respectively, for all reaction mixture: (a) and (b) for TEOS;

(c) and (d) for MTEOS; (e) and (f) for VTEOS.



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In Tables 2–4 the values of the 29Si-NMR chemical

shifts for the molecular species formed in all

studied reaction mixtures are presented. In Figs.

6–8 the 29

Si-NMR spectra of each alkoxides usedand for each system studied are also presented.

As it can be observed from Tables 2–4 and from

Figs. 6–8, pure alkoxides were presented as stan-

dards. From these spectra it can be observed that

the values of 29 Si-NMR chemical shifts (d) for each

pure alkoxide are very different. This influence on

the d values was assigned to the inductive +I effect[6,38]. Hook [6] and Boilot et al. [38] considered

that the influence of the inductive effect +I on the d

values is higher than that of other parameters. As

0 20 40 60 80 100

0

50000

100000

150000

200000

250000

300000

350000

400000

450000 Monomers

Monohydroxylated mo nomers

Dimers

Trimers

Tetramers

Pentamers

[ a .

u .

]

Time / h

Fig. 3. Variation of molecular species versus time, in the reaction mixture prepared with TEOS, up to 96 h.

0 20 40 60 80 100

0

50000

100000

150000

200000

250000

300000

350000

400000

450000

Monomers

Monohydroxylated monomers

Dimers

Trimers

Tetramers

Pentamers

Hexamers

Heptamers

Octamers [ a

. u

. ]

Time / h

Fig. 4. Variation of molecular species versus time, in the reaction mixture prepared with MTEOS, up to 96 h.



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a general rule, it was observed that with increasing

of the net positive charge on the silicon (Siþ) in-

volved an upfield of 29Si-NMR chemical shifts.

More explicitly, the d values are shifted to more

negative values when the positive charge on the

silicon atom increases.In the case of TEOS, the ethoxy groups were

classified like moderate electron acceptors. From

this reason, as it can be observed, it could be ex-

plained the fact that 29Si-NMR chemical shift in

the case of TEOS (d ¼ 81:138 ppm) appeared at

the most negative value. In the case of MTEOS,

the methyl groups are classical weak electron do-

nors. As a result of the presence of this organic

groups in MTEOS, the positive charge on the sil-

icon decreases and the values of the 29Si-NMR

chemical shifts in this case was orientated todownfield. In the case of MTEOS, the 29Si-NMR

chemical shift (d ¼ 42:701 ppm) presents thelowest absolute values from all studied alkoxides.

In the case of the VTEOS, the silicon atoms are

bonded directly to a sp2 carbon atom. The vinyl

Table 229Si-NMR chemical shifts of reaction mixture obtained with TEOS

Assigned Chemical shifts d (ppm)

Alkoxide Different time after sol–gel reaction was generated

Start 30 min 24 h 48 h 96 h

Si–(OH)4 – )72.355 )72.392 – – –

Si–(OEt)(OH)3 – )74.342 )74.378 )74.482 )74.454 –

Si–(OEt)2(OH)2 – )76.515 )76.549 )76.607 )76.602 )76.600

Si–(OEt)3(OH) – )78.980 )79.001 )79.027 )79.021 )79.024

Si–(OEt)4(TEOS) )81.138 )81.905 )81.912 )81.909 )81.911 )81.924

Si–O–Si–(OEt)(OH)2 – – – )83.982 )83.981 – Si–O–Si–(OEt)2(OH) – – – )86.219 )86.220 )86.220(Si–O)2 –Si–(OEt)2 – – – )88.723 )88.724 )88.729Si–O–Si–(OEt)3 – – )89.016 – – – (Si–O)2 –Si–(OEt)(OH) – – – – )94.034 )93.800

0 20 40 60 80 100

0

100000

200000

300000

400000

500000

Monomers

Monohydroxylated Monomers

Dimers

Trimers

Tetramers

Pentamers

Hexamers

[ a

. u

. ]

Time / h

Fig. 5. Variation of molecular species versus time, in the reaction mixture prepared with VTEOS, up to 96 h.



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group was generally considered like a weak elec-

tron-acceptor group. Under this circumstances,

the 29Si-NMR chemical shift (d ¼ 58:289 ppm)for VTEOS was found to be between the 29Si-

NMR chemical shift for TEOS (d ¼ 81:138 ppm)and the 29Si-NMR chemical shift for MTEOS

(d ¼ 42:701 ppm). It can be concluded, in good

agreement with other studies [6,38], that the 29

Si-NMR chemical shift could be predicted based on

the observation of inductive +I effects. Engelhardt

and Michel [39] explained this easiness to predict

the 29Si-NMR chemical shift taking into account

the inductive +I effects by the linear correlation,

between the net charges of the silicon and the

relative screening constant in the )40 and )120

ppm chemical shift range, when the paramagnetic

shield of the 29Si nucleus in the tetra-coordinated

silicon compound is calculated. Using this con-

cept, it could be explained the 29Si-NMR chemical

shifts for the products resulted from hydrolysis–

polycondensation reactions.

As it can be observed from Fig. 6 and Table 2,

in the case of TEOS at start, in the 29Si-NMR

spectra, besides monomer, all hydroxylated mono-

mers appeared. These hydroxylated monomers

appeared toward more positive values. This factcould be explained by an exchange of the ethoxy

groups with hydroxyl groups, which determines

the decreasing of the positive charge on the silicon,

the OH groups having a higher ionic character

than the –OC2H5 groups.

In the early stages of hydrolysis–polyconden-

sation, the molecular species with higher degree of

condensation were identified step by step by 29Si-

NMR spectrometry. Up to 96 h, using the 29Si-

NMR spectra, the trimers as molecular species

Table 329Si-NMR chemical shifts of reaction mixture obtained with MTEOS


Alkoxide Different time after sol–gel reaction was generatedStart 30 min 24 h 48 h 96 h

CH3 –Si–(OEt)(OH)2 – )40.106 )40.132 – – –

CH3 –Si–(OEt)2(OH) – )41.435 )41.445 )41.473 – –

CH3 –Si–(OEt)3(MTEOS) )42.701 )42.941 )42.949 – – –

Si–O–Si–(CH3)(OH)2 – – – – – )48.833Si–O–Si–(CH3)(OEt)(OH) – )49.347 )49.353 )49.386 )49.375 )49.310Unknown species – )45.505 )49.513 )49.536 )49.536 )49.537

Si–O–Si–(CH3)(OEt)2 – )50.774 )50.782 )50.815 )50.804 )50.816Unknown species – – – )50.948 )50.943 )50.957

(Si–O)2 –Si–(CH3)(OH) – – – )57.526 )57.526 )57.601(Si–O)2-Si-(CH3)(OEt) – – – )58.929 )58.926 )58.933

Table 429Si-NMR chemical shifts of reaction mixture obtained with VTEOS


Alkoxide Different time after sol–gel reaction was generated

Start 30 min 24 h 48 h 96 h

CH2@CH–Si–(OEt)(OH) 2 – )55.163 )55.182 )55.215 – –

CH2@CH–Si–(OEt)2(OH) – )56.716 )56.728 )56.743 )56.740 )56.752

CH2@CH–Si–(OEt)3(VTEOS) )58.289 )58.493 )58.502 )58.528 )58.525 )58.538

Si–O–Si–(CH@CH2)(OH)2 – – – – )62.915 )62.917Si–O–Si–(CH@CH2)(OEt)(OH) – )64.370 )64.379 )64.404 )64.401 )64.419Unknown species – )64.517 )64.525 )64.534 )64.531 )64.541

Si–O–Si–(CH@

CH2)(OEt)2 – )

66.011 )

66.022 )

66.034 )

66.030 )

66.042(Si–O)2 –Si–(CH@CH2)(OH) – – – )72.432 – – (Si–O)2 –Si–(CH@CH2)(OEt) – – – )73.907 73.903 )73.916



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with a high degree of condensation were identified

and in the same time, the tetra- and tri-hydroxy-

lated monomers disappeared. These compounds of

condensation presented the 29Si-NMR chemical

shift at values more negatives than those for the

alkoxides. This could be explained by the inductive

+I effects of the –OSi groups.

In the case of TEOS, 29Si-NMR data are in

good agreement with the GC-MS data and the

signal for TEOS still exists even after 96 h.

For MTEOS, as it could be observed in Table 3

and in Fig. 7, the 29Si-NMR spectra present at

start not only the monomer and hydroxylated

monomers, but also the presence of dimers. It can

Fig. 6. 29Si-NMR spectra of the reaction mixture prepared with TEOS: (a) TEOS; (b) start, (c) after 30 min, (d) after 24 h, (e) after

48 h, (f) after 96 h from start.

Fig. 7. 29

Si-NMR spectra of the reaction mixture prepared with MTEOS: (a) MTEOS; (b) start, (c) after 30 min, (d) after 24 h, (e) after48 h, (f) after 96 h from start.



11/17

be also observed that the specific peak for the

alkoxide has a lower intensity than the specific

peak for the monohydroxylated monomer. All

these observations show that in the first step, the

hydrolysis–polycondensation reaction of the mix-ture prepared with MTEOS was much faster than

in the case of reaction mixture prepared with

TEOS. This conclusion was supported by all the29Si-NMR spectra recorded at different moments

of time. At 24 h after the sol–gel reaction was

generated, it can be observed that the character-

istic peak for the monomer disappeared and after

48 h, the signal for monohydroxylated monomer

was not evidenced. In the case of 29Si-NMR

spectrum recorded at 96 h after the sol–gel reac-

tion was generated, only the characteristic peaksfor the molecular species with higher degree of

condensation (up to trimers) can be observed.

These data are in a good agreement with GC-MS

data, and from this it was concluded that the rate

of hydrolysis–polycondensation of the reaction

mixture prepared with MTEOS is higher than for

reaction mixture prepared with TEOS.

The behavior of the reaction mixture prepared

with VTEOS was found from the 29Si-NMR data

ranging between MTEOS and TEOS as it results

from GC-MS data, as well. From 29Si-NMR data

presented in Table 4 and in Fig. 8, it can be ob-

served the presence of dimers in the first steps after

the sol–gel reaction was generated, besides the

monomer and the hydroxylated monomers, wasidentified. As in the case of MTEOS, the intensity

of the peak of the monohydroxylated monomer

was found to be higher than the specific peak of

the monomer. Comparatively with the reaction

mixture prepared with TEOS, in the case of the

reaction mixture prepared with VTEOS, the pres-

ence of dimers was identified in the first steps.

These observations indicated a higher reactivity in

the early stage of the reaction mixture obtained

with VTEOS comparatively with the mixture pre-

pared with TEOS. In the 29

Si-NMR spectra re-corded after 96 h from the start of the sol–gel

reaction, the specific signals for monomer and

monohydroxylated monomer were found. These

observations showed that the reaction mixture

prepared with VTEOS had a lower rate of hy-

drolysis–polycondensation than in the case of the

reaction generated with MTEOS. In both cases of

the reaction mixtures prepared with organically

modified alkoxides, it was observed that, taking

into account the inductive +I effect of the organic

Fig. 8. 29 Si-NMR spectra of the reaction mixture prepared with VTEOS: (a) VTEOS; (b) start, (c) after 30 min, (d) after 24 h, (e) after

48 h, (f) after 96 h from start.



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groups, the 29Si-NMR chemical shifts of the mo-

lecular species resulting from reactions could be

predicted.

The IR spectroscopy involved observation of the evolution of the entire system. In Figs. 9–11 the

IR spectra of the TEOS, MTEOS and VTEOS

respectively, are presented. In each figure, the IR

spectra of the alkoxide and the IR spectra for the

reaction mixtures at start and after 96 h are given.

As it can be observed from Fig. 9, in the case of IR

spectra of TEOS, all specific absorption bands

characteristic for TEOS, were found [40]. In the

case of the reaction mixture prepared with TEOS

all characteristic bands reported by Bertoluzza

et al. [40] were identified. The characteristic peaks

for formation of Si–O–Si bonds: m as Si–O–Si at

1115 and 1090 cm1, m Si–O(H) at 975 cm1, m s Si–

O–Si at 800 cm1

, and d Si–O–Si at 470 cm1

,respectively were found in the IR spectra. Beside

these characteristic peaks for the inorganic net-

work, the presence of the ethoxy groups was found

in all spectra by identification of the absorption

bands: m as CH3 and m as CH2 at 2990 cm1 and 2940

cm1, respectively, and other specific peaks in the

range 1500–1270 cm1. Between the IR spectra of

TEOS and the IR spectra of reaction mixture

containing TEOS, at start and after 96 h, respec-

tively, no significant differences could be noticed.

Fig. 9. IR spectra of the reaction mixture obtained from TEOS: (a) TEOS; (b) start, (c) after 96 h from start.



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This fact could be explained by the presence of the

important quantity of the unhydrolyzed monomer

in the solution mixture, as was shown using the

other methods of investigation. Only the effect of

dilution with ethanol was pointed out by thepresence of the absorption bands at 890 cm1,

m OH at 3350 cm1 and d HOH at 1650 cm1.

The IR spectra of MTEOS, showed beside the

characteristic bands of vibration for the Si–O–Si

bonds, the absorption bands due to the presence of

the Si–C bonds: ds CH3 at 1275 cm1, r Si–C at 830

cm1, and m s Si–C at 670 cm1 [41–43]. In this case,

between the IR spectrum of the alkoxides and the

IR spectra of reaction mixture obtained from

MTEOS at start and after 96 h, significant differ-

ences appeared in the ranges between 1500 and

1270 cm1 and between 1000 and 600 cm1, as it

can be observed in Fig. 10. In this regions the

characteristic bands of the ethoxy groups appeared

and the modification of the bands in this regionindicated the exchange of surrounding of silicon

atoms. These modifications were assigned to the

formation of the other molecular species besides

the monomer. This fact confirmed that the reac-

tion mixture prepared with MTEOS was more

reactive comparatively with the mixture prepared

with TEOS.

In the case of the IR spectrum of VTEOS pre-

sented in Fig. 11, besides of the absorption bands

characteristic for Si–O–Si and Si–C bonds the

Fig. 10. IR spectra of the reaction mixture obtained from MTEOS: (a) MTEOS; (b) start, (c) after 96 h from start.



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presence of vinyl groups connected to silicon at-

oms was identified, since the following character-

istic vibrations are present: m (@CH2)/(@CH) at

3070 cm1, m C@C at 1600 cm1, bs (@CH2)/b

(@CH) at 1410 cm1 and c (@CH2) at 1015 cm1

[43]. For the reaction mixture prepared with this

alkoxide at start and after 96 h, no significant

differences were evidenced. Besides the bandsspecific for the presence of ethanol (880 cm1) and

water (m OH at 3340 cm1 and d HOH at 1655

cm1), a change in the range between 1000–600

cm1, with respect to the IR spectra of the sol–gel

reaction mixture at the start was observed. Like in

the case of MTEOS, these modifications explained

the evolution of ethoxy groups, the exchange of

them with hydroxy groups and, in the same

time, the modification of Si–O–Si bonds especially

by the increasing of the intensity of the band m sSi–O–Si at 790 cm1. This behavior of VTEOS

classified the reactivity of the reaction mixture

prepared with this alkoxide between TEOS and

MTEOS like other method of investigation dis-

cussed above.

Another possibility to study the processes of

hydrolysis–polycondensation for all investigatedalkoxides, was to observe the modification of r

CH3 vibration bands. This band appeared as a

sharp shoulder at 1170 cm1 in the case of all IR

spectra for all pure alkoxides. It is very easy to

observe in the IR spectra (Figs. 9–11) that after the

start of the hydrolysis–polycondensation processes

for all reaction mixtures studied, the intensity of

this band decreases. After 96 h, in all studied cases

this band become a very small shoulder.

Fig. 11. IR spectra of the reaction mixture obtained from VTEOS: (a) VTEOS; (b) start, (c) after 96 h from start.



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4. Discussion

Taking into account the evolution of molecular

species presented in Figs. 3–5, some comments canbe drawn concerning the reactivity of the studied

reaction mixtures. It can be observed that MTEOS

presents the highest reactivity because, in this case,

after 96 h, molecular species with highest degree of

polycondensation were identified, like octamers,

the monomer being only in traces. On the other

hand, the reaction mixture prepared with TEOS

has the lowest reactivity. This fact was supported

by the presence in the system of molecular species

with lower degree of polycondensation, up to

pentamers after 96 h. The presence of an impor-

tant quantity of monomers was also identified. The

reactivity of VTEOS ranges between these two

limits. The average reactivity of VTEOS was ex-

plained by the presence in the samples measured

after 96 h – not as predominant species – of the

high polycondensed molecular species up to

hexamers, as well. The GC-MS is a very useful

method but only for studying the trend of the re-

activity in the sol–gel processes at the early stages.

From the 29Si-NMR data it was observed

the same trend of reactivity TEOS


16/17

observed, the relative rates of hydrolysis–poly-

condensation could be predicted taking into ac-

count the inductive +I and steric effects.

5. Conclusions

The processes of hydrolysis–polycondensation

in acid catalysis of organically modified alkoxides

like MTEOS and VTEOS, was studied compara-

tively with the unsubstituted alkoxide, as TEOS.

The processes were studied using the GC-MS,29Si–NMR, and IR spectrometry. Using these

methods of investigation, the relative reactivity of

the alkoxides was identified.

The GC-MS was able to separate the molecularspecies formed and to identify them based on their

molecular weight. From the MS, besides the

identification of the weight of molecular species

formed, it was showed that the Si–C bond was

not destroyed during hydrolysis–polycondensation

processes. This observation was confirmed by IR

spectrometry. The 29Si-NMR spectrometry evi-

denced the type of molecular species formed and

their structure. The modification of the structure

of all molecular species and the bonds formed or

destroyed during the sol–gel processes were evi-denced using the IR spectroscopy.

Taking into account the inductive effect, it

could be predicted by 29Si-chemical shifts of the

organic substituted alkoxides. At the same time,

knowing the inductive and steric effects, the rela-

tive rate of the hydrolysis and condensation pro-

cesses could be predicted.

All the method of investigation were comple-

mentary and all of them indicated the same trend

of increasing of the hydrolysis–polycondensation

rate in the following order: TEOS


17/17

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[43] L.D. Ou, A.B. Seddon, J. Non-Cryst. Solids 210 (1997) 187.

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Documents

Comparative study of the sol-gel processes starting with different substituted Si-alkoxides