Influence of acid-base properties of oxides surface on their reactivity towards epoxy compounds

Article abstract of DOI:10.1134/S107036321405003X

Acid-base properties of surface of aluminum, iron, and silicon oxides have been studied by pK spectroscopy. γ-Al₂O₃ and γ-AlO(OH) have been found the most reactive modifiers for epoxy oligomers, due to the presence of active surface groups with pK₁ ≈ 4, capable of chemical interaction with the matrix.

Full text of DOI:10.1134/S107036321405003X

ISSN 1070-3632, Russian Journal of General Chemistry, 2014, Vol. 84, No. 5, pp. 810–815. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © P.A. Sitnikov, A.V. Kuchin, M.A. Ryazanov, A.G. Belykh, I.N. Vaseneva, M.S. Fedoseev, V.V. Tereshatov, 2014, published in
Zhurnal Obshchei Khimii, 2014, Vol. 84, No. 5, pp. 717–722.
Influence of Acid-Base Properties of Oxides Surface
on Their Reactivity Towards Epoxy Compounds
P. A. Sitnikova, A. V. Kuchina, M. A. Ryazanova, A. G. Belykha,
I. N. Vasenevaa, M. S. Fedoseevb, and V. V. Tereshatovb
a Institute of Chemistry, Komi Scientific Center, Ural Branch, Russian Academy of Sciences,
ul. Pervomaiskaya 48, Syktyvkar, 167000 Russia
e-mail: sitnikov-pa@chemi.komisc.ru
b Institute of Technical Chemistry, Ural Branch, Russian Academy of Sciences, Perm, Russia
Received July 22, 2013
Abstract—Acid-base properties of surface of aluminum, iron, and silicon oxides have been studied by рK
spectroscopy. γ-Al2O3 and γ-AlO(OH) have been found the most reactive modifiers for epoxy oligomers, due to
the presence of active surface groups with рK1 ≈ 4, capable of chemical interaction with the matrix.
Keywords: metal oxide, epoxide, polymer, reactive filler, equilibrium constant, homopolymerization
DOI: 10.1134/S107036321405003X
Active fillers can significantly alter utilitarian
properties (mechanical strength, thermal or heat resis-
tance, and electrical conductivity) of polymeric
epoxide-based materials due to formation of interphase
adsorption layer at the polymer–filler boundary [1]. To
date, no unified approach to estimate, predict, and
regulate the interactions at the interphase boundary has
been developed [2].
Metal oxides are widely used as fillers for
thermoplasts and thermosetting plastics. In particular,
in this work we studied well known [3–7] silicon oxide
(aerosil) and various modifications of aluminum oxide
[γ-AlO(OH), γ-Al2O3, α-Al2O3] and iron(III) oxide
[γ-FeO(OH), γ-Fe2O3, α-Fe2O3] in order to elucidate
the effect of acid-basic properties of the oxides surface
on physico-chemical processed taking place upon the
oxides incorporation into epoxy compounds.
Almost any natural or synthetic material can act as
filler for polymer material, including the polymers
themselves, after certain modification of their surface.
Proper selection of the polymer and filler combination
should account for the filler nature, particles shape and
size, its distribution in the matrix as well as possible
interactions with the polymer at the interphase
boundary. Chemical reactivity of the filler surface (that
is in turn determined by its functional composition) is
of particular importance [1, 2].
The most widely used method to investigate the
acid-base (Brønsted–Lowry) properties of metal oxide
materials is potentiometric titration of the suspensions
in aqueous medium. The acid-base properties can be
conveniently represented by рK spectra [8], by plotting
the fraction of the acid-base sites with the
corresponding pK (or their specific concentration q,
mmol/g) as function of pK. рK spectra can be obtained
from potentiometric titration curves.
cHAV0 – cBOHV – [H+](V0 + V)
Gn(рН) = nb(pH) + nH0 = –———————————— + nH0.
m
In the equation above: nH0, amount of hydrogen ions
(mmol/g) adsorbed at the suspension particles at the
starting point of titration pH0; nb(pH), amount of
hydrogen ions (mmol/g) adsorbed at the suspension
particles in the course of titration to the current
pH; V0, suspension volume; сHA, concentration
of monobasic strong acid in the suspension; V,
volume of the added alkali with concentration of сBOH
;
810

INFLUENCE OF ACID-BASE PROPERTIES OF OXIDES SURFACE
(a)
811
nb
pH
pK
(b)
Fig. 1. Adsorption of hydrogen ions at Al2O3 surface as
function of рН. (Diamonds) 1st direct titration; (squares)
2nd direct titration; (triangles) 1st back titration; and
(circles) 2nd back titration.
m, mass of the solid phase of the suspension in the
sample.
Acid-base properties of oxide materials can be
interpreted taking advantage either of the 2рK or the
4рK model [9].
pK
(c)
The 2рK model is based on the assumption that the
surface charge appears via ionization of amphoteric
groups –S–OH, [Eqs. (1) and (2)] (hereafter S stands
for the sample surface).
SOH][H+]
[–SOH2+]
–S–OH2+ ⇄ –S–OH + H+, K1 = ————— ,
(1)
[–SO–][H+]
–S–OH ⇄ –S–O– + H+, K2 = –————– .
[–SOH]
(2)
pK
Fig. 2. рK spectra of γ-AlO(ОН) (a), γ-Al2O3 (b), and
α-Al2O3 (c).
The equilibrium constants K1 and K2, being
independent of the background electrolyte concentra-
tion, characterize the acid-base sites strength.
–S–OH + Na+ ⇄ –S–O–Na+ + H+,
In the acidic medium, the surface charge is predo-
minantly determined by equilibrium (1), whereas in the al-
kaline media the equilibrium (2) is far more important.
[SO–Na+][H+]
K4 = —————— .
[SOH][Na+]
(4)
The 4pK model accounts for further reactions
involving the background electrolyte, in particular,
partial neutralization of surface charge with its ions.
For example, ion pairs formation involving NaCl is
described by Eqs. (3) and (4).
The high K3 and K4 values point at specific adsorp-
tion of cations, whereas low values of the constants
reflect specific adsorption of anions [8].
Typical plots of nb(pH) recalculated from direct and
back titration curves are shown in Fig. 1 taking γ-
Al2O3 as a representative example. As the results of
sequential titrations were identical, the studied
processes were reversible.
–S–OH2+Cl– ⇄ –S–OH + H+ + Сl–,
[SOH][H+][Cl–]
K3 = –—————— ,
(3)
[SOH2+Cl–]
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812
SITNIKOV et al.
(a)
pK
Fig. 4. рK spectrum of SiO2 (aerosil).
pK
(b)
sites corresponding to рK1 equilibrium (the 4рK
model) were not found at the surface.
Figure 4 shows рK spectrum of nanosized silicon
oxide. Analysis of acid-base properties of SiO2 usually
involves the assumption that its surface charge changes
due to dissociation of surface silanol groups [Eqs. (5)
and (6)].
–Si–OH2+ ⇄ –Si–OH + H+ (рK1 ~ 6),
–Si–OH ⇄ –Si–O– + H+ (рK2 ~ 10).
(5)
(6)
Ions of background electrolytes are known to form
complexes with hydroxyl groups at silicon oxide
surface. Likely, at least one layer of water molecules
separate these weakly absorbed ions and the surface
atoms of oxygen and silicon; this means that ion pairs
or outer sphere complexes are formed [10]. Therefore,
protolytic properties of silicon oxide additional
reaction (7) should be considered (formation of outer
sphere complexes with electrolyte cations).
pK
Fig. 3. рK spectra of FeO(OH) (a) and Fe2O3 (b).
Figure 2 shows рK spectra of various modifications
of aluminum oxide. Following the 4рK model [9], the
peaks in the spectra could be assigned to the following
equilibriums:
–Al–OH2+ ⇄ –Al–OH + H+ (рK1 ~ 4),
–Si–OH + Na+ ⇄ –Si–O–Na+ + H+ (рK4 ~ 9–10).
(7)
–Al–OH ⇄ –Al–O– + H+ (рK2 ~ 10–12),
–Al–OH2+Cl– ⇄ –Al–OH + H+ + Сl– (рK3 ~ 7–8),
Differential scanning calorimetry DSC was applied
to study the physico-chemical processes occurring
upon addition of the powder oxides to phenyl glycidyl
ether. The DSC traces of representative samples
containing Al2O3 are shown in Fig. 5, and the
corresponding interaction parameters are collected in
the table.
–Al–OH + Na+ ⇄ –Al–O–Na+ + H+ (рK4 ~ 8–10).
The рK1 value significantly decreased in the γ-
AlO(ОН)–γ-Al2O3–α-Al2O3 series, being absent in the
case of corundum. The equilibrium state corresponding
to рK2 was practically independent of alumina modi-
fication type.
From Fig. 5 it is to be seen that in the cases of γ-
AlO(ОН) and γ-Al2O3 the DSC curves contained
exothermal peak assigned to chemical interaction of
epoxy groups with the surface acid-base sites of the
oxides, whereas such peak was absent in the case of α-
Al2O3. Similarly to the latter case, DSC curves did not
reveal any chemical interaction of the epoxide with the
filler surface in the cases of iron and silicon oxides.
Figure 3 displays рK spectra of iron hydroxide
derivatives prepared via sol-gel route, FeO(OH)
(Fig. 3a) and Fe2O3 (Fig. 3b). As seen from the plot,
with increasing of annealing temperature water was
eliminated to yield the surface acid-base sites, their
behavior being described with the following equation:
–Fe–OH ↔ –Fe–O– + H+ (рK2 ~ 11). The acid-base
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 5 2014

INFLUENCE OF ACID-BASE PROPERTIES OF OXIDES SURFACE
As the constants K1 and K2 reflect the strength of
813
Т, °С
acid-base sites, the presence of pK1 line at about 4 in
the рK spectrum of γ-AlO(ОН) and γ-Al2O3 confirmed
the higher reactivity of those oxides as compared with
other studied objects.
2
That was further confirmed by IR spectroscopy
data. In particular, heating of mixture of phenyl
glycidyl ether with γ-AlO(OH) (90°C, 1 h) led to
weakening of the absorption bands assigned to
vibrations of epoxy groups (750–950 cm–1) (Fig. 6),
thus confirming consumption of those groups in the
course of phenyl glycidyl ether polymerization at the
surface of alumina.
–0.6
Heat flow, W/g
Fig. 5. DSC traces of the model mixtures based on phenyl
glycidyl ether containing α-Al2O3 (1), γ-AlO(ОН) (2), and
γ-Al2O3 (3).
Appearance of the absorption band at 1650–
1600 cm–1, assigned to stretching of С=О conjugated
with oxygen atom, suggested the following scheme of
the epoxide ring opening (in the scheme, Al is
aluminum atom at the filler surface).
glycidyl ether to yield polymeric ether, the alumina
active sites acting as polymerization initiators.
O
O_ + H2C
H2
C
H
O
Al
CH
R
Al
OH + H2C
O
C
O_
O
H2
C
H2
C
+ 2H+
H2C
O
CH
R
Al
O
C
O
Al
O
O
Heating of the mixture of phenyl glycidyl ether
with γ-Al2O3 led to significant weakening of the epoxy
group absorption bands as well. Moreover, the bands
assigned to vibrations of aliphatic C–O–C bonds
appeared at 1150–1060 cm–1 (Fig. 7). Those spectral
changes pointed at homopolymerization of phenyl
Besides the discussed interactions, IR spectra
revealed appearance of the surface hydroxyl groups,
similar to those present in γ-AlO(OH) (weak band at
1640–1630 cm–1) (Fig. 7b). The hydroxyl groups
appeared due to hydrolysis reaction [7].
100
90
80
70
60
50
40
30
20
10
0
ν, cm–1
Fig. 6. IR spectrum of the phenyl glycidyl ether–γ-AlO(OH) mixture: before heating (1) and after annealing at 90°С during 1 h (2).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 5 2014

814
SITNIKOV et al.
Thermal parameters and heat effect of interaction in the
model mixtures of phenyl glycidyl ether with γ-AlO(ОН)
and γ-Al2O3
phenyl glycidyl ether initiated by the active surface
groups of the filler. Less reactive iron oxides, silicon
oxide, and corundum did not participate in such
interactions. The latter oxides could modify the
polymer matrix properties due to formation of hydro-
gen bonds between epoxy groups and the surface acid-
base sites of the oxides.
Oxide
component
Т
start, °C
Tmax, °С
Tend, °C
Q, J/g
γ-AlO(ОН)
80
76
105
105
125
125
8.6
6.2
γ-Al2O3
EXPERIMENTAL
–Al–O–Al– + H2O ⇄ 2–Al–OH.
Aluminum oxide was prepared by precipitation
from saturated solution of aluminum nitrate (analytical
pure grade) with aqueous ammonia (23 wt %,
chemical pure grade); the precipitant was added till pH
of 7.5 was reached. Various modifications of alumina
were prepared by annealing of the synthesized
aluminum hydroxide: at 300°C [γ-AlO(OH)], 650°C
(γ-Al2O3), or 1200°C (α-Al2O3) during 1 h, after
heating at 60 deg/h [11].
Heating of phenyl glycidyl ether mixed with other
studied oxides did not lead to any changes of IR spectra.
To conclude, we demonstrated that of the studied
oxides only γ-Al2O3 and γ-AlO(OH) contained the
acid-base sites with behavior described by the
–Al–OH2+ ↔ –Al–OH + H+ equilibrium and рK1 of
about 4 (the 4pK model). Containing the reactive
surface groups, those modifications acted as reactive
fillers for epoxy polymers. In their mixtures with
phenyl glycidyl ether, chemical interaction of epoxy
rings with the surface hydroxyl groups of the oxide
was observed as well as homopolymerization of
Derivatives of iron hydroxide were prepared via the
sol-gel route from iron(III) chloride [12]. Silicon oxide
was prepared by decomposition of silicon chloride
(chemical pure grade) vapor with water vapor at 1000°С.
(a)
(b)
1250
750
1250
750
ν, cm–1
ν, cm–1
Fig. 7. Selected regions of IR spectra of the phenyl glycidyl ether–γ-Al2O3 mixture before heating (a) and after annealing at 90°С
during 1 h (b).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 5 2014

INFLUENCE OF ACID-BASE PROPERTIES OF OXIDES SURFACE
The oxides acid-base properties were studied by рK
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ACKNOWLEDGMENTS
This work was financially supported by basic
research program of Ural Branch, Russian Academy of
Science (projects nos. 12-33-004-ARKTIKA and 12-I-
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