Full text of DOI:10.1557/mrs2001.92

www.mrs.org/publications/bulletin
polymers. In this sense, molecular hybrids
can be considered not only as the combi-
nation of organic polymer and inorganic
materials, but as new materials in their
own right.
New Preparation
In Situ Polymerization Method
Previously, Novak51–53 and Schmidt15
proposed interesting methods for prepar-
ing organic–inorganic polymer hybrids.
They utilized silane coupling agents in the
reactions to introduce organic parts into
the silica matrix. Modification would at-
tach these organic polymers covalently to
the silica network and disperse the poly-
mers in the silica gel homogeneously.
Methods for
Organic–Inorganic
Polymer Hybrids
The inorganic matrix is formed by the
sol-gel reaction, which consists of the hy-
drolysis of the alkyl silicate and the en-
suing condensation reaction, as described
before. This means that the inorganic ma-
trix is constructed by a polycondensation
reaction. Here, as the starting material, an
organic monomer was used instead of the
polymer. In other words, the polymeriza-
tion of the organic monomer was carried
Yoshiki Chujo and Ryo Tamaki
Introduction
Nano-ordered composite materials con-
sisting of organic polymers and inorganic
compounds have been attracting attention
for their use in creating high-performance
or high-functionality polymeric materials.
The term “polymer hybrid” describes blends
of organic and inorganic components with
molecular-level dispersions.1 By combin-
ing these two components, it is possible
to enhance the mechanical strength of or-
ganic polymers with silica particles.2–6 The
high transparency of these materials is
another important property, which makes
them indispensable in the development of
optical waveguides,7–11 optical biosensors,12
nonlinear optical materials,13,14 and contact
lenses.15 Hybrid materials are also poten-
tial candidates for catalysts16–18 and gas-
separation membranes.19–23
subjected to an acid- or base-catalyzed sol-
gel reaction. After drying for several days,
a homogeneous and transparent glass ma-
terial was produced. These colorless, trans-
parent, and homogeneous hybrids were
obtained in a wide range of compositions
that demonstrate the characteristic prop-
erties of these organic polymers.
The high homogeneity of the present
hybrid strongly suggests that the organic
polymer segments and inorganic segment
were blended at the molecular level. This
may be due to the strong hydrogen bonds
between amide carbonyl groups and silanol
groups. As a representative example, Fig-
ure 1 illustrates the interaction between
POZO (1) and silica gel through hydrogen
bonds. The hydrogen bonds in these hy-
brids were confirmed by Fourier trans-
form infrared (FTIR) spectra, in which the
stretching bands due to amide carbonyl
groups were shifted to the lower wave-
number region after the formation of hy-
brids. The molecular dispersion of organic
polymers in a silica matrix has been con-
firmed by the pore size of porous silica
prepared by the pyrolysis of hybrids49 and
also by atomic force microscopy.50
In the hybrid matrix, the organic polymer
segments are dispersed without aggrega-
tion, as described before. Accordingly, the
characteristics of organic polymers as a
group, such as their thermal stability or
glass-transition temperature, can be ex-
pected to differ from those of organic
polymers. In fact, thermally stable hybrid
membranes were prepared without de-
composition of organic parts, even at high
temperature (e.g., 200ꢀC).21 In addition,
the mechanical properties of the polymer
hybrids were highly improved compared
with those of the corresponding organic
Scheme I.
It is generally impossible to disperse silica
gel at the molecular level. However, tetra-
alkoxysilane, a precursor of silica gel in
the so-called sol-gel reaction, can be dis-
solved in organic solvents such as alcohol.
Thus, the sol-gel reaction makes it pos-
sible to incorporate the organic polymer
segments in the network matrix of in-
organic materials.24–37 The elementary re-
actions of the sol-gel procedure are shown
in Scheme I, in which Si-OH species acting
as a Brønsted acid are the key intermedi-
ates.38–42 The polymer hybrids were pre-
pared by means of the sol-gel reaction of
tetra-alkoxysilane in the presence of organic
polymers. Typical examples of organic
polymers incorporated are poly(2-methyl-
2-oxazoline) (POZO) (Structure 1), poly(N-
vinylpyrrolidone) (PVP) (Structure 2), and
poly(N,N-dimethylacrylamide) (PDMAAm)
(Structure 3).43–48 One of these organic poly-
mers was dissolved in ethanol with tetra-
ethoxysilane, and the resulting mixture was
Structures 1, 2, and 3.
Figure 1. Schematic representation of
hydrogen bonds in polymer hybrids.
MRS BULLETIN/MAY 2001
389

New Preparation Methods for Organic–Inorganic Polymer Hybrids
out simultaneously with the sol-gel re-
action. Generally, radical polymerization
was used for the formation of the organic
component. The advantage of this method
might be in the dispersion of the organic
components, that is, the monomer should
be dispersed more easily than the polymer.
Here, N,N-dimethylacrylamide (DMAAm)
was used as a typical example of organic
monomers.54
DMAAm was added as a starting ma-
terial to a sol-gel reaction mixture of
tetramethoxysilane (TMOS) with azobis-
isobutyronitrile (AIBN) as the radical ini-
tiator. The sol-gel reaction was catalyzed
by an acid catalyst to produce a silica gel
matrix. Therefore, it was expected that
organic and inorganic polymerizations
would proceed simultaneously when the
mixture was heated to 60ꢀC. The polymer
hybrids were found to be homogeneous
and transparent. The homogeneity of the
hybrids was assumed to be due to the
strong interaction between the polymer
and silica gel.
It was confirmed that the simultaneous
radical polymerization of DMAAm with
hydrolysis and condensation of TMOS
produced highly homogeneous PDMAAm
and silica gel hybrids. In situ polymeriza-
tion of DMAAm proceeded effectively to
produce PDMAAm with a high molecular
weight. The homogeneity of the hybrids
depended on the amount of the acid cata-
lyst and gelation degree of the silica gel,
indicating the critical role of physical en-
trapment of the polymer in forming silica
gels, as well as the hydrogen bonding
interaction for uniform dispersion of the
polymer. This method seems to offer
greater versatility than prepolymer incor-
poration methods. At the same time, it
does not require modification of organic
monomers to obtain homogeneous poly-
mer hybrids. Thus, the number of proc-
esses required for preparation is reduced.
This method should prove useful for
the synthesis of polymer hybrids starting
from organic polymers without high af-
finity to silica gel, since the aggregation
may be suppressed if the monomer poly-
merizes in the “cage” of the silica matrix.
Based on this idea, homogeneous poly-
styrene and silica gel polymer hybrids
were also prepared.55 A styrene monomer
was introduced into a sol-gel reaction
mixture of TMOS, and the polymerization
was initiated by AIBN, while the sol-gel
reaction of TMOS proceeded to form a sil-
ica gel. The homogeneity was confirmed
quantitatively by measuring the porosity
of charred hybrids using nitrogen poro-
simetry. It was found that polystyrene was
dispersed at the nanometer scale in the
silica matrix.
prepared by applying an in situ polymer-
ization method to a styrene monomer and
divinylbenzene.57 The monomers were
mixed in the sol-gel reaction mixture of
TMOS and subjected to radical polymer-
ization, resulting in transparent glassy
materials. The homogeneity was again
confirmed by nitrogen porosimetry meth-
ods. The organic group was found to be
dispersed at the nanometer scale. The IPN
polymer hybrids obtained were found to
be highly solvent-resistant.
Interpenetrating Polymer
Network (IPN) Hybrids
In in situ polymerization, when a bi-
functional organic monomer is used to-
gether with a monofunctional monomer,
the organic component should be a three-
dimensional cross-linked structure. The
inorganic matrix is a silica gel, which is of
course a cross-linked structure. This mate-
rial is, in fact, an interpenetrating polymer
network (IPN).
Homogeneous IPN polymer hybrids of
PDMAAm gel and silica gel were syn-
thesized by the in situ polymerization
method.56 DMAAm and methylenebis-
acrylamide (MBAAm), as a cross-linking
agent, were added into a methanol solu-
tion of TMOS. Radical polymerization of
DMAAm and sol-gel reaction of TMOS
were carried out simultaneously, resulting
in homogeneous glassy materials. The for-
mation of an IPN structure was confirmed
by solvent extraction. As demonstrated in
Figure 2, while 60% of PDMAAm was ex-
tracted with MeOH from the polymer hy-
brid prepared without MBAAm, the content
of organic segment hardly changed before
and after the extraction when 0.1 equiva-
lent of MBAAm was introduced. These re-
sults indicate that the PDMAAm gel and
silica gel formed an IPN structure with the
addition of MBAAm. The homogeneity of
these IPN polymer hybrids was supported
by nitrogen porosimetry measurements. It
was found that the porous silica obtained
from the polymer hybrids had high sur-
face areas and large pore volumes and
exhibited a sharp peak in pore-size distri-
bution at 1.8 nm. The results indicate a
molecular-level dispersion of the polymer
gels in the IPN polymer hybrids.
In Situ Hydrolysis Method
When poly(vinyl alcohol) (PVA) was
used as an organic polymer in the sol-gel
reaction, the aggregation of the polymer
took place due to the interaction among
hydroxyl groups in the organic compo-
nent, which caused phase separation. As a
result, heterogeneous and milky materials
were obtained. Generally, PVA can be pre-
pared by hydrolysis of poly(vinyl acetate).
Here, acid-catalyzed in situ hydrolysis of
poly(vinyl acetate) was performed simul-
taneously with the acid-catalyzed sol-gel
reaction.58 It should be noted that ester
groups of poly(vinyl acetate) can act as
hydrogen bond acceptors to interact with
silanol groups.
In the presence of poly(vinyl acetate), the
acid-catalyzed sol-gel reaction of TMOS was
carried out. The degree of hydrolysis of
poly(vinyl acetate) was estimated by FTIR
or 13C CP/MAS (coupled plasma/magic
angle spinning) nuclear magnetic reso-
nance (NMR) and was found to be con-
trolled (0–85%) by the amount of acid
catalyst. As a result, a homogeneous and
transparent polymer hybrid of PVA and
silica gel was effectively obtained by this
procedure.
Homogeneous IPN polymer hybrids of
polystyrene gel and silica gel were also
ꢀ–ꢀ Interaction
The interactions utilized to integrate or-
ganic and inorganic phases are generally
classified into two groups: covalent bond-
ing and hydrogen bonding interactions.
On the other hand, ꢁ–ꢁ interaction is
known as one of the types of attractive
non-covalent bonding interactions that
play a critical role in, for example, the sta-
bilization of the double-helical structure of
DNAand complexation in host–guest sys-
tems.59 This ꢁ–ꢁ interaction can be used in
the preparation of homogeneous polymer
hybrids. The principle of this method is an
interaction between the organic and in-
organic components by ꢁ–ꢁ interaction.
For this interaction, phenyltrimethoxysi-
lane (PhTMOS) was used as a starting ma-
terial for the sol-gel reaction, by which
phenyl groups can be incorporated into
the inorganic matrix. As an organic com-
ponent, polystyrene was used. Thus, poly-
Figure 2. Loss of organic-component
PDMAAm-silica interpenetrating
polymer network (IPN) hybrids by
MeOH extraction. PDMAAm (Structure 3)
is poly(N,N-dimethylacrylamide),
MBAAm is methylenebisacrylamide,
and DMAAm is N,N-dimethylacrylamide.
390
MRS BULLETIN/MAY 2001

New Preparation Methods for Organic–Inorganic Polymer Hybrids
styrene and silica gel polymer hybrids
were prepared utilizing a sol-gel reaction
of PhTMOS.60 When an alkyl- (or aryl-)
substituted alkoxysilane is used as a start-
ing material, the alkyl group is introduced
into the silica gel, since the Si–C bond
does not undergo hydrolysis. Phenyl groups
would be introduced into the silica gel by
the sol-gel reaction of PhTMOS. As shown
in Scheme II, the alkoxysilane was added
to a tetrahydrofuran (THF) solution of
polystyrene, followed by the addition of
an acid catalyst. For comparison, other
alkoxysilanes such as TMOS, methyltri-
methoxysilane (MTMOS), and isobutyl-
trimethoxysilane (iBuTMOS) were also
used as starting materials. The mass ratio
of alkoxysilane to polystyrene was 0.1 or
1.0. The homogeneity of the obtained
polymer hybrids was evaluated optically.
Scheme II.
Transparency of the polymer hybrids was
attained only when silica gel particles em-
bedded within polystyrene were smaller
than the wavelength of light. Transparent
polymer hybrids were obtained for both
mass ratios when PhTMOS was used as a
starting material for the sol-gel reaction. In
contrast, polymer hybrids became translu-
cent or turbid when TMOS, MTMOS, or
iBuTMOS was used. Mesityltrimethoxy-
silane (MesTMOS) gave homogeneous
polymer hybrids when the mass ratio was
0.1. However, the homogeneity deterio-
rated as the mass ratio was increased to
1.0. The result might be attributed to
methyl groups on the phenyl ring of
MesTMOS, which are suspected to inter-
rupt the ꢁ–ꢁ interaction. When phene-
thyltrimethoxysilane (phenethylTMOS)
was used as the starting material, homo-
geneous polymer hybrids were obtained
for both mass contents. These results
demonstrate that a phenyl ring is neces-
sary for the homogeneous dispersion of
polystyrene and silica gel.
By using this idea, poly(diallyl phtha-
late) or polycarbonate prepared from
bisphenol-A could be utilized as an or-
ganic component.61 In all cases, very trans-
parent and homogeneous polymer hybrid
materials were obtained by the sol-gel
reaction of PhTMOS or phenethylTMOS.
As observed in the case of polystyrene,
TMOS, MTMOS, and iBuTMOS gave only
inhomogeneous polymer hybrids. These
results indicate that ꢁ–ꢁ interaction is quite
effective for the synthesis of homoge-
neous polymer hybrids of silica gel and
organic polymers having aromatic groups.
Cubic phenylsilsesquioxane also pro-
duced a homogeneous hybrid material with
polystyrene through ꢁ–ꢁ interaction.62 The
polymer hybrid obtained showed high
solubility in common organic solvents and
good processability.
Scheme III.
diate properties between plastics and
glasses (ceramics). In addition, the compo-
sition of the hybrids can be widely varied.
In other words, the hybrids can be used to
modify the organic polymer materials or
to modify the inorganic glassy materials.
Thus, the exploration of new preparative
methods for organic–inorganic polymer
hybrids should be one of the most im-
portant technologies for the creation of
high-performance or high-functionality
polymeric materials.
Ionic Interaction
Ionic interaction between anion and
cation is one of the simplest interactions.
For example, polystyrene sulfonic acid
was used as an organic component. As an
inorganic component, 3-aminopropyltri-
ethoxysilane, a popular silane coupling
agent, was used in order to produce an
ionic interaction with the sulfonic acid
groups in the organic component. Thus, in
the presence of polystyrene sulfonic acid,
the sol-gel reaction of a mixture of TMOS
and 3-aminopropyltriethoxysilane was car-
ried out, as illustrated in Scheme III.63 The
resulting hybrid materials were found to
be very homogeneous and transparent.
This method via ionic interaction might be
applied to other organic polymers having
sulfonic acid or carboxylic acid groups.
Poly(acrylic acid) is one such example.
References
1. Y. Chujo and T. Saegusa, Adv. Polym. Sci. 100
(1992) p. 11.
2. G.S. Sur and J.E. Mark, Eur. Polym. J. 21
(1985) p. 1051.
3. J.E. Mark and G.S. Sur, Polym. Bull. 14 (1985)
p. 325.
4. S.J. Clarson and J.E. Mark, Polym. Commun.
28 (1987) p. 249.
5. C.-C. Sun and J.E. Mark, Polymer 30 (1989)
p. 104.
Conclusions
The molecular hybrids of organic poly-
mers and silica gel are expected to show
many possibilities as new composite ma-
terials. These hybrids may show interme-
6. J.E. Mark, CHEMTECH (April 1989) p. 230.
7. C.J. Wung, Y. Pang, P.N. Prassad, and F.E.
Karasz, Polymer 32 (1991) p. 605.
MRS BULLETIN/MAY 2001
391

New Preparation Methods for Organic–Inorganic Polymer Hybrids
8. S. Motakef, T. Suratwala, R.L. Roncone, J.M.
Boulton, G. Teowee, G.F. Neilson, and D.R.
Uhlmann, J. Non-Cryst. Solids 178 (1994) p. 31.
9. S. Motakef, T. Suratwala, R.L. Roncone,
J.M. Boulton, G. Teowee, and D.R. Uhlmann,
J. Non-Cryst. Solids 178 (1994) p. 37.
27. Y. Wei, W. Wang, J.-M. Yeh, D. Yang, and
J.K. Murray, Adv. Mater. 6 (1994) p. 372.
28. Y. Wei, D. Yang, L. Tang, and M.K.
Hutchins, J. Mater. Res. 8 (1993) p. 1143.
29. Y. Wei, J.-M. Yeh, D. Jin, X. Jia, and J. Wang,
Chem. Mater. 7 (1995) p. 969.
Chem. A27 (1990) p. 1603.
45. Y. Chujo, E. Ihara, S. Kure, K. Suzuki, and
T. Saegusa, Makromol. Chem., Macromol. Symp.
42/43 (1991) p. 303.
46. Y. Chujo, Polym. Mater. Encyclopedia 6 (1996)
p. 4793.
47. T. Saegusa and Y. Chujo, Makromol. Chem.,
Macromol. Symp. 51 (1991) p. 1.
10. M. Yoshida and P.N. Prassad, Appl. Opt. 35
(1996) p. 1500.
30. M.W. Ellsworth and B.M. Novak, J. Am.
Chem. Soc. 113 (1991) p. 2756.
11. C. Xu, L. Eldada, C. Wu, R.A. Norwood,
L.W. Shacklette, J.T. Yardley, and Y. Wei, Chem.
Mater. 8 (1996) p. 2701.
12. B.C. Dave, B. Dunn, J.S. Valentine, and J.I.
Zink, Anal. Chem. 66 (1994) p. 1120.
13. C. Claude, B. Garetz, Y. Okamoto, and S.
Tripathy, Mater. Lett. 14 (1992) p. 336.
14. B.L. Davies, M. Samoc, and M. Woodruff,
Chem. Mater. 8 (1996) p. 2586.
15. G. Philipp and H. Schmidt, J. Non-Cryst.
Solids 63 (1984) p. 283.
16. M.T. Reetz, A. Zonta, and J. Simpelkamp,
Angew. Chem., Int. Ed. Engl. 34 (1995) p. 301.
17. M.A. Harmer, W.E. Farneth, and Q. Sun,
J. Am. Chem. Soc. 118 (1996) p. 7708.
18. U. Schubert, New J. Chem. 18 (1994) p. 1049.
19. S.P. Nunes, J. Schultz, and K.V. Peinemann,
J. Mater. Sci. Lett. 15 (1996) p. 1139.
20. M. Smaihi, T. Jermoumi, J. Marignan, and
R.D. Noble, J. Membr. Sci. 116 (1996) p. 211.
21. R. Tamaki, Y. Chujo, T. Yazawa, and K.
Kuraoka, J. Mater. Chem. 9 (1999) p. 1741.
22. J. Marchese, N. Ochoa, and C. Pagliero,
J. Chem. Technol. Biotechnol. 63 (1995) p. 329.
23. C. Guizard and P. Lacan, New J. Chem. 18
(10) (1994) p. 1097.
31. B.K. Coltrain, W.T. Ferrar, C.J.T. Landry,
T.R. Molaire, and N. Zumbulyadis, Chem. Mater.
4 (1992) p. 358.
32. J.J. Fitzgerald, C.J.T. Landry, and J.M.
Pochan, Macromolecules 25 (1992) p. 3715.
33. C.J.T. Landry, B.K. Coltrain, and B.K. Brady,
Polymer 33 (1992) p. 1486.
34. C.J.T. Landry, B.K. Coltrain, J.A. Wesson, N.
Zumbulyadis, and J.L. Lippert, Polymer 33
(1992) p. 1496.
35. C.J.T. Landry, B.K. Coltrain, M.R. Landry,
J.J. Fitzgerald, and V.K. Long, Macromolecules 26
(1993) p. 3702.
36. C.J.T. Landry and B.K. Coltrain, J. Macromol.
Sci., Pure Appl. Chem. A31 (1994) p. 1965.
37. K.G. Sharp, in Hybrid Organic–Inorganic
Composites edited by J.E. Mark, Y.C. Lee, and
P.A. Bianconi (American Chemical Society,
Washington, DC, 1995) p. 163.
38. H. Schmidt, H. Scholze, and A. Kaiser,
J. Non-Cryst. Solids 63 (1984) p. 1.
39. C.J. Brinker, K.D. Keefer, D.W. Schaefer,
R.A. Assink, B.D. Kay, and C.S. Ashley, J. Non-
Cryst. Solids 63 (1984) p. 45.
48. T. Saegusa and Y. Chujo, Makromol. Chem.,
Macromol. Symp. 64 (1992) p. 1.
49. Y. Chujo, H. Matsuki, S. Kure, T. Saegusa,
and T. Yazawa, J. Chem. Soc., Chem. Commun.
(1994) p. 635.
50. M. Toki, T. Chow, T. Ohnaka, H. Samura,
and T. Saegusa, Polym. Bull. 29 (1992) p. 653.
51. B.M. Novak, Adv. Mater. 5 (1993) p. 422.
52. C.L. Jackson, B.J. Bauer, A.I. Nakatani, and
J.D. Barnes, Chem. Mater. 8 (1996) p. 727.
53. B.M. Novak, Macromolecules 24 (1991)
p. 5481.
54. R. Tamaki, K. Naka, and Y. Chujo, Polym. J.
30 (1998) p. 60.
55. R. Tamaki, K. Naka, and Y. Chujo, Polym.
Bull. 39 (1997) p. 303.
56. R. Tamaki and Y. Chujo, Bull. Chem. Soc. Jpn.
71 (1998) p. 2749.
57. R. Tamaki and Y. Chujo, J. Mater. Chem. 8
(1998) p. 1113.
58. R. Tamaki and Y. Chujo, Appl. Organomet.
Chem. 12 (1998) p. 755.
59. C.A. Hunter and J.K.M. Sanders, J. Am.
Chem. Soc. 112 (1990) p. 5525.
60. R. Tamaki, K. Samura, and Y. Chujo, Chem.
Commun. (1998) p. 1131.
40. F. Orgaz and H. Rawson, J. Non-Cryst. Solids
82 (1986) p. 57.
24. J.L.W. Nell, G.L. Wilkes, and D.K. Mohanty,
J. Appl. Polym. Sci. 40 (1990) p. 1177.
25. H. Schmidt, J. Non-Cryst. Solids 178 (1994)
p. 302.
26. Y. Wei, R. Bakthavatchalam, D. Yang, and
C.K. Whitecar, Polym. Prepr. (Am. Chem. Soc.,
Div. Polym. Chem.) 32 (1991) p. 503.
41. C.J. Brinker and G.W. Scherer, J. Non-Cryst.
Solids 70 (1985) p. 301.
42. C.J. Brinker and G.W. Scherer, Sol-Gel Sci-
ence (Harcourt Brace, Boston, 1990).
43. Y. Chujo, E. Ihara, S. Kure, and T. Saegusa,
Macromolecules 26 (1993) p. 5681.
44. T. Saegusa and Y. Chujo, J. Macromol. Sci.,
61. R. Tamaki, S.-Y. Han, and Y. Chujo, Polym.
Prep. Jpn. 47 (1998) p. 1016.
62. R. Tamaki and Y. Chujo, Polym. Prep. Jpn. 48
(1999) p. 1068.
63. R. Tamaki and Y. Chujo, Chem. Mater. 11
(1999) p. 1719.
ꢀ
VII International Conference on Advanced Materials, ICAM 2001
August 26–30, 2001
Cancun, Q.R., Mexico
Nanostructures, magnetic materials, fracture mechanics,
thin films, semiconductors, biomaterials, computer simulations, sol-gel,
synthesis, steelmaking, polymers, ceramics, surface engineering,
quantum dots, fullerenes, liquid crystals, catalysis,
archaeological materials, tunneling, glasses, cements, superconductivity,
corrosion, material characterization, metals, alloys and phase transitions,
amorphous materials, renewable-energy materials, composites,
http://www.icam2001.unam.mx
recycling materials, and optical properties.
Information: María Luisa Marquina, Departamento de Física, Facultad de Ciencias,
Universidad Nacional Autónoma de México, 04510 México, D.F.
committee@icam2001.unam.mx
Academia Mexicana de Ciencia de Materiales
International Union of Materials Research Societies
www.mrs.org/publications/bulletin
392
MRS BULLETIN/MAY 2001