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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
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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.
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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
Assigned Chemical shifts d (ppm)
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
Assigned Chemical shifts d (ppm)
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.
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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
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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
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