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Cite this: J. Mater. Chem., 2011, 21, 15853
HIGHLIGHT
www.rsc.org/materials
Oxocluster-reinforced organic–inorganic hybrid
materials: effect of transition metal oxoclusters
on structural and functional properties†
Silvia Gross*
DOI: 10.1039/c1jm10579e
The focus of this Highlight is on the structural and functional properties which organically
modified transition oxoclusters can provide, once embedded into a polymer matrix, to the
resulting hybrid materials. Some selected case studies are discussed to highlight the role of
these polynuclear inorganic building blocks in determining appealing material properties.
Organic–inorganic hybrid materials are
a wide, manifold and exciting class of
systems which derives from an intimate
combination, often mediated by the
formation of a chemical bond, of inorganic and organic building blocks.1–12
Typically, either inorganic building
blocks (BBs) (clusters, nanoparticles,
fibers, whiskers, lamellae, etc.) are incorporated into a macromolecular polymer
backbone or vice versa, organic molecules
or macromolecules (dyes, biomolecules,
oligomers or polymers) can be embedded
into an inorganic (e.g. silica) matrix.
Materials deriving from a mixture of
inorganic and organic components are
typically classified into two classes:1,2
(i) class I materials, typically referred to
as nanocomposites, and characterised
by weak interactions (van der Waals,
hydrogen bonding, weak electrostatic
interactions) between the two phases, the
organic and the inorganic one;
(ii) class II materials, which are instead
characterised by strong (covalent or ionic)
bonds among the building blocks.
As witnessed by the publication, in the
last 15 years, of over 4000 papers on this
wide class of materials, the filing of more
ISTM-CNR, Dipartimento di Scienze
Chimiche, Universit
a degli Studi di Padova, and
INSTM UdR Padova, Via Marzolo 1, I-35131,
Padova, Italy. E-mail: [email protected]
† Dedicated to the memory of my husband
Professor Klaus M€
uller.
than 500 patents,13 four dedicated textbooks,1,2,7 several thematic sessions at
international conferences and dedicated
specialised conferences, organic–inorganic hybrid materials have gained, in the
last few decades, an increasing interest,
from both the scientific and the technological/industrial point of view, and their
widespread applications have been
already recognized.5a
Nevertheless, they still present unexplored and challenging issues, deriving
from compositional and structural variability. In particular, the most fascinating
aspect of organic–inorganic hybrid
materials, from the point of view of
a chemist, consists in their versatility
deriving from a theoretically infinite
number of combinations of inorganic and
organic building blocks.1–15 Furthermore,
preparation of hybrid materials is not just
a combinatorial and serendipitous task.
In this regard, for the preparative chemists, the most intriguing challenge
concerns the design and focused synthesis
of new building blocks (BBs) and the
tailoring of their surface chemistry to
prepare molecularly homogeneous hybrid
materials characterized by the formation
of covalent bonds between the components, the so-called Class II hybrid
materials.1,2
In fact, as outlined by Kickelbick,15
important chemical issues which have to
be taken into account when dealing with
This journal is ª The Royal Society of Chemistry 2011
this kind of materials are uniformity,
phase continuity, domain size, and
molecular intermixing at the phase
boundaries: all these aspects do in turn
affect the final properties of the resulting
materials.
The design, preparation and suitable
functionalisation of BB and their (in some
cases ordered and hierarchical) assembly
in the final materials can be a time
consuming task. The efforts for more
demanding synthetic routes required for
their preparation are however counterbalanced by the outstanding advantages
this class of materials presents:
(1) overcoming of the structural limits
of conventional materials (polymers,
ceramics, metals, etc.)
(2) fine-tuning of the properties
through variation of:
- composition
- microstructure
- interaction(s) at the interface
(3) design of multifunctionality
(combination of functionalities)
(4) virtually infinite compositional
variability.
The idea behind this ‘‘hybridization’’ is
to overcome the structural limits of
conventional
materials
(polymers,
ceramics, metals, etc.) and to achieve
a noticeable improvement of materials
properties, since generally the resulting
material combines the feature of both
starting systems, i.e. organic and
J. Mater. Chem., 2011, 21, 15853–15861 | 15853
inorganic ones. This idea of combining
inorganic and organic building units into
one material is the underlying principle
also of traditional composites and nanocomposites,2d where this combination is
nevertheless simply addressed by a physical mixing of the two or more components. In this latter case, a generally gross
dispersion of micro- or even macrosized
inorganic fillers into a polymer, mediated
by weak interactions such as van der
Waals interactions or hydrogen bonds, is
achieved. However, the lack of a strong
chemical bond generally leads to poor
mechanical properties, migration and/or
leaching of the guest components within/
from the host matrix, phase agglomeration, and demixing, which are all detrimental for materials properties.
The covalent embedding of different
inorganic building blocks yields instead
more stable materials, endowed with
enhanced mechanical, thermal, and also
functional (e.g. electric, magnetic and
optical) with respect to pure organic
polymers.15
This can be for instance afforded by
using suitable pre-formed inorganic
building blocks. In this framework,
organization of the inorganic and organic
sub-structures of the material is done in
separate steps (the inorganic structures
such as clusters or colloids are already
formed when the organic structures are
built).
The strong chemical interaction
between the organic and the inorganic
part, which relies on the formation of
covalent or ionic bonds, can enhance
remarkably the material properties.
In addition to combining the different
features of the starting materials, new or
enhanced phenomena can emerge as
a result of the molecular intermixing and/
or of the effects at the interface between
organic and inorganic counterparts, and
improvement of structural as well as
functional properties is often observed.
The structural properties are intended
hereafter as properties which are related
to the structure itself of the materials,
such as mechanical (hardness, stiffness,
strength, etc.) and thermal (Tg, decomposition threshold, depolymerisation
temperature, etc.) properties, rheological
behaviour, chemical and photochemical
stability. Structural properties of
organic–inorganic hybrid materials have
been the topic of several contributions
15854 | J. Mater. Chem., 2011, 21, 15853–15861
and of an extensive review on the
mechanical properties by Sanchez and
coworkers.5d
On the other side, functional properties
are those related to one specific functionality of the materials, such as for
instance optical, magnetic, electronic,
electrical properties, piezoelectricity,
biochemical, catalytic activity, etc.1,2,12 As
outlined by Sanchez and Gomez-Romero
in the Introduction of their book, there is
also a quickly expanded area of research
devoted to functional hybrid materials,2
where the emphasis is put ‘‘on chemical,
biochemical, electrochemical activity as
well as on magnetic, electronic, optical or
other physical properties’’.
These enhanced properties can be addressed by the painstaking choice of the
inorganic and organic building blocks
and by a careful tailoring of their mutual
interactions at the interface.1–5,14
Among organic–inorganic hybrid
materials, particularly interesting are
those deriving from the incorporation of
inorganic, metal-based BB into a polymer
matrix.
Actually, incorporation of metalderived species (clusters, polyoxoanions,
complexes, nanoparticles) into an organic
macromolecular backbone is a widely
explored strategy in materials science. In
his review paper, Kickelbick15 has
described the main synthetic approaches
to incorporation into polymer of a wide
variety of inorganic BBs into polymer,
among which:
(1) metals or metal complexes by
coordination interactions
(2) incorporation of unmodified
particles
(3) in situ growth of inorganic particles
in a polymer matrix
(4) surface modification of clusters and
oxoclusters with polymerizable groups
(5) surface modification of clusters and
oxoclusters with initiating groups.
An effective synthetic approach to this
particular type of materials is the socalled ‘‘building block (BB) approach’’
which is extensively described by Kickelbick in his book on hybrid materials1
and in a review article15a as well as by
several other authors to which interested
readers are referred for more detailed
description.1–12,14–16 This bottom up
approach relies on the design and preparation of well-defined molecular or
nanosized structures, with tailored
morphology and chemistry, i.e. defined
size, shape and surface functionalisation.
The surface functionalisation of the BB
plays a primary chemical role, since it
does not only control the compatibility
and solubility of the BB during material
preparation, but also enables its chemical
reaction with other BBs, for instance
organic molecules or monomers, to give
molecularly
homogeneous
hybrid
materials.
As far as the inorganic building blocks
are concerned, typical examples have
been reviewed in the wide available
literature.3,5,15
Beside classical metal clusters such as
those based on M–M interaction,17 which
are however not further discussed in this
paper, interesting polynuclear BBs are
those based on metal/semimetal–oxygen
bonds. Among them, wide interest has
been devoted (i) to silica derivatives and
spherosilicates such as polyhedral silsesquioxanes (POSS),18 (ii) and also to functionalised polyoxometalates (POM),19
which however will not be further discussed in this contribution due to the
broadness of the topics and space
limitations.
Instead, the focus of this contribution
will be on a relatively recent class of
organically modified metal oxide clusters,1,6,15,20–36 in the following referred as
metal oxoclusters which are polynuclear
complexes characterised by the presence
of O–M–O (M ¼ transition metal, or main
group metal, e.g. Sn, Ba) bonds and have
the general formula MxOy(OR)w(OOR0 )z
(R,R0 ¼ alkyl groups). At variance with
polyoxometalates, they are neutral structures and are generally consisting of an
inorganic core (based on metal–oxygen
polyhedra with some of the oxygen atoms
in m2 or m3 coordination fashions) kept
together by organic molecules, typically
bidentate ligands such as carboxylates. An
example of these oxoclusters is the zirconium tetranuclear one, Zr4O2(OMc)12
(OMc ¼ methacrylate),6,24c whose structure is depicted in Fig. 1.
Among the first reported examples of
these oxoclusters, one of the earliest
structurally characterized examples are
the oxoclusters Ti6O4(OR)8(OOCMe)8
obtained by reaction of Ti(OR)4 (R ¼
alkyl groups) with acetic acid.21
Many of these oxoclusters are functionalised with reactive (generally polymerisable) R0 moieties which enable,
This journal is ª The Royal Society of Chemistry 2011
Fig. 1 Synthesis and structure of the methacrylate functionalised oxocluster Zr4O2(OMc)12.24c
upon free radical copolymerisation with
suitable monomers (typically acrylates,
styrene, etc.), their covalent incorporation into the polymer matrix. The
formation of a strong chemical bond
between the organic and the inorganic
counterparts has several advantages,
since this stable anchoring allows the
achievement of a homogeneous distribution of the guest species in the host matrix
and to overcome problems related to
phase separation, aggregation, and
migration of the oxoclusters in the matrix
or even leaching. An effective approach to
prepare oxocluster-reinforced hybrid
polymers relies on a two-step process,
sketched in Scheme 1, the first involving
the synthesis of a polynuclear inorganic
complex functionalised with a polymerisable unit, the second relying on the copolymerisation with a suitable organic
monomer. This approach allows moreover turning typical linear polymers, such
as polystyrene and poly(methyl methacrylate),
into
highly
crosslinked
networks.1,6,15,20
In the last few years, this approach, i.e.
the embedding of organically functionalised transition metal oxoclusters into
polymers, has been proven to be a very
effective strategy to obtain molecularly
homogeneous organic–inorganic hybrid
materials. Several examples taken from
the current state of the art have been
summarized in Table 1.
The advantages derived from the use of
these oxoclusters have been outlined by
Schubert in some recent reviews,6,20 and
are mostly related to the fact that (i) they
are discrete and structurally well defined
molecular inorganic clusters, (ii) with
a defined composition, and (iii) whose
surface modification can be addressed by
common preparative routes and followed
by standard analytical methods (IR,
NMR).
The described inorganic building
blocks are typically obtained starting
from the corresponding metal alkoxides,
which are chemically modified by suitable
bidentate ligands bearing polymerisable
moieties, giving rise to metal oxoclusters
of well defined structure.6,20
The reaction pattern leading to the
formation of these oxoclusters, whose
mechanism has been thoroughly investigated by Kickelbick et al.,15b is sketched in
Scheme 2.
In principle, these organically functionalised oxoclusters can be also obtained by ligand exchange reactions. In
fact, the carboxylate groups chelating or
bridging the metal atoms are very
dynamic. Several exchange processes
between different coordination sites at the
oxocluster surface and between the oxocluster-bonded carboxylate ligands and
carboxylic acids in solution were observed
by low-temperature NMR spectroscopy
and studied by DFT calculations22 and
also synthetically exploited to obtain new
oxoclusters.24e
However, the method is not limited to
carboxylic acids. For example, Guerrero
et al. reported the synthesis of two Ti
oxoclusters Ti4O(OiPr)8(O3PR)3 and
Ti4O4(OiPr)4(O2PPh2)4 obtained by reaction of Ti(OR)4 with the corresponding
phosphonic or phosphinic acid.23
In the last few years, we and other
groups have extensively explored the
synthesis of different early transition
This journal is ª The Royal Society of Chemistry 2011
metal and main group metal oxoclusters
(with O–M–O moieties) such as those
based on Zr,24,25 Hf,26 Ti,27,28,46 Ti–Zr,27f
Y,29 Ti–Hf,26 Zr–Ti–Hf,26 Ba,30 Ba–Ti,30
Ti-Y,29a,b Nb,31 and Sn.32 Since nineties,
Hubert-Pfalzgraf et al.33,34 have paved the
way to this development by exploring
original routes for the synthesis of
a plethora of mono- and polymetallic (e.g.
Pb–Zr,34a Pb–Ti,34a,b Cu–Y,34c Y,34d lanthanides,34e,f,l La–Zn,34g Ba–Ce,34h Bi–
Ba,34i Zn–Ta,34l Y–Pr,34j Sm–Ti,34l Nb31
and others33) mixed alkoxides and
oxoclusters as potential single-source
precursors for the corresponding mixed
functional oxides (e.g. perovskites).
Further examples of metal oxide clusters
based on further transition elements (Fe,
Cr) were obtained from the corresponding metal salts by reaction with unsaturated carboxylic acids or carboxylate
salts, such as [Fe3O(m-OOCR)6L3]X
(OOCR ¼ acrylate or 2-butenate; X ¼
counter-ion) with a triangular M3O
core.35
The chemistry, the tailored synthesis
and modification as well as the structural
issues of these oxoclusters, two of which
are shown in Fig. 2, have been thoroughly
described in some reviews and research
papers.6,20,24-36
These oxoclusters are, as previously
mentioned, suitable BBs for the synthesis
of a wide variety of organic–inorganic
hybrid materials. In particular, different
kinds of organic–inorganic hybrids have
been prepared both as bulk materials and
thin films starting from methacrylatefunctionalised oxoclusters,6,20,36,37 norbornene-2-carboxylate derivatives for
ring-opening metathesis polymerization,38 4-pentynoate derivatives for click
reactions,20,39 2-bromo-isobutyrate derivatives as initiator for atom-transfer
radical polymerization,40 and thiol
carboxylate-substituted metal oxocluster
for thiol–ene polymerisation25 (see
Table 1).
As already outlined in a previous
paper,41 the oxocluster plays in this
framework a two-fold role: from one side,
since its surface is symmetrically functionalised with polymerisable groups, the
oxocluster acts as crosslinking unit and it
increases the interconnection and reticulation of the forming polymer matrix, thus
enhancing the structural properties (in
particular the thermal and mechanical
ones) of the macromolecular backbone.
J. Mater. Chem., 2011, 21, 15853–15861 | 15855
Scheme 1 Two step synthesis of organic–inorganic hybrid materials based on the embedding into
a polymer matrix of organically functionalised inorganic building blocks (S. Gross and M. Bauer,
Adv. Funct. Mater., 2010, 23, 4019–4025, Copyright Wiley-VCH Verlag GmbH & Co. KGaA.
Reproduced with permission).
However, as we will discuss in the
following, the improvement of the thermal
stability has to be mainly traced back to
the presence of an inorganic, oxide-based
filler.42,43 On the other side, since the oxocluster is an inorganic building block,
which can itself be endowed with functional (e.g. optical, magnetic, electric,
photochromic, luminescence, etc.) properties, these properties (if the oxocluster
integrity is retained) can be transferred to
the resulting hybrid material.
In general, the incorporation of an
oxocluster in a polymer matrix through
the formation of stable covalent bonds
induces strong changes in the materials
properties, as extensively outlined in
previous works.20 The changes of some
materials properties include:
linear polymers such as polymethylmethacrylate (PMMA) or polystyrene (PS) are turned into crosslinked
polymers: the polymers are no longer
soluble in organic solvents, but swell
instead, as expected for crosslinked polymers; the crosslinking increases with the
oxocluster amount;
improvement of the thermal stability
with respect to the neat polymers;
improvement of the mechanical
properties (strength, hardness, brittleness,
scratch resistance, etc.);
enhancement of the dielectric properties (e.g. lowering of 3 and tan d);
Table 1 Examples of oxocluster-based hybrid materials
Oxocluster
Monomer
Investigated properties
References
Zr6(OH)4O4(OMc)12
Zr6(OH)4O4(OMc)12
Methylmethacrylate (MMA)
Styrene (Sty)
Thermal, mechanical
Thermal, mechanical
Zr6O4(OH)4(carboxylate)12 (carboxylate ¼ 5norbornene-2-carboxylate or isobutyrate/
methacrylate)
Zr4O2(OMc)12
Norbornene
Thermal, mechanical
42
42–45
and 50
38
Mechanical properties
(flexural strength and
flexural modulus)
37d
Zr4O2(OMc)12
Zr4O2(OMc)12
Bis-[(methacryloyloxy)propoxycarbonylethyl)]
(3-triethoxysilyl)propylamine (1) and (1,3(2)
bismethacryloyloxypropyl)(triethoxysilylpropylaminocarbonyl)butyrate
Methacryloxypropyltrimethoxysilane (MAPTMS)
Methacryloxypropyltrimethoxysilane (MAPTMS)
Thermal
Dielectric
Zr4O2(OMc)12
Methylmethacrylate (PMMA)
49
37a–c
and 55
37a,i,j
Zr4O2(OMc)12
Zr6O4(OH)48(OOCCH2CHCH2)12$(n-PrOH)]2$ 4
(CH2CHCH2COOH)
Zr12O8(OH)8(MP)24$4(MPA), (MP ¼ HS–(CH2)2–C
(O)O; MPA ¼ HS–(CH2)2–C(O)OH
Zr12O8(OH)8(MP)24$4(MPA), (MP ¼ HS–(CH2)2–C
(O)O; MPA ¼ HS–(CH2)2–C(O)OH
Ti6O4(OPr)8(endo,exo-OOC–Norb)8
Ethyleneglycol diacrylate (EGDA)
Vinyltriethoxysilane
Mechanical, thermal
37k
48
Methacrylic acid (McOH)
Stability of the cluster
25a
Allyl pentaerythritol and trimethylolpropanetris(3-mercaptopropanoate)
Norbornene
Thermal, mechanical
25b
38
Hf4O2(OMc)12
Ti16O16(OEt)24(OC2H4OMc)8
Ti16O16(OEt)24(OEMA)8
Methacryloxypropyltrimethoxysilane (MAPTMS)
CD540 and HEMA
Acrylates
Ti16O16(OEt)32x(OPhCH]CH2)x
Ti oxocluster (structure not provided)
Hydroxystyrene, acetoxystyrene
Hydrogenated acrylonitrile-butadiene rubber
(HNBR)—Therban
Methylmethacrylate (PMMA)
Thermal properties,
crosslinking
Dielectric
Thermal, mechanical
Mechanical,
photochromicity,
optical transparency
Thermal, mechanical
Mechanical, dielectric
Mn12O12{CH2C(CH3)COO}16(H2O)4
15856 | J. Mater. Chem., 2011, 21, 15853–15861
Mechanical, thermal,
barrier
Magnetic
56
47b
28d
28f
37g
and 53
This journal is ª The Royal Society of Chemistry 2011
Scheme 2 Proposed mechanism for oxocluster formation (M ¼ metal, FG ¼ functional group).15b
improved chemical and photochemical stability.
The modification or even improvement
of these properties has to be traced back,
as we will see in the following, not only to
the polymerisation conditions and oxocluster proportion, but also to the
structure and chemical nature of the oxoclusters acting as multifunctional crosslinkers as well as inorganic (nano)-fillers.
Functional effects induced by inherent
properties (e.g. magnetic, optical, etc.) of
the oxocluster or its incorporation in the
polymer chains have also to be taken into
account.
In the following we will describe some
exemplarily case studies, already presented in previous works, in which the
improvement of either functional or
structural properties induced by the
embedding of the oxoclusters is discussed.
Due to space limitation, not the whole
literature on oxocluster-reinforced polymer materials was reviewed, but rather
only some cases, which can help to
understand the role of the oxocluster in
determining/enhancing materials properties, are taken into account.
Improvement of structural
properties
In general, upon embedding of an oxocluster into a polymer matrix, an
improvement of both thermal and
mechanical properties is observed. The
increase of the depolymerisation temperature, of the onset temperatures of
thermal decomposition, of hardness, of
craze initiation stress, etc. were observed
in many studies.37–51
By the step polymerisation method implemented
by
Schubert
and
coworkers,38,44 the Zr6O4(OH)4(OMc)12
(OMc ¼ methacrylate) (Zr6) oxocluster
was copolymerised with polystyrene (PS)
in different amounts (0.24–0.87 mol%
oxocluster proportion). The dynamic
mechanical response of the hybrid was
shown to be typical of thermoplastic
materials. Storage moduli (E0) at room
temperature were slightly higher than that
of neat PS, whereas the crosslinking with
different proportions of the oxocluster did
not modify the linear thermal expansion
coefficient a in the glassy state. The analysis of the stress–strain curves revealed an
increased brittleness with increasing oxocluster proportion although the elastic
properties were still ruled by the PS
matrix. Whereas the changes in the tensile
moduli with increasing crosslinking (i.e.
increasing amount of oxocluster) were not
pronounced, the tensile strength was
significantly improved and exhibited
a distinct positive correlation with
increasing network density. It should be
also underlined that the hybrid network
density correlates well with the oxocluster
proportion.
In a related study on polymerisation of
styrene with the Zr6 oxocluster, it was
Fig. 2 Two examples of methacrylate (left) and thiolate (right) functionalised oxoclusters: the
barium–titanium Ti10Ba2(m3-O)8(m2-OH)5(m2-OMc)20(OPrOMe)2 (ref. 30) and the Zr or Hf dodecanuclear M12O6(OH) 6(MP) 30$3(MPA) (M ¼ Zr, Hf, MPA ¼ mercaptopropionate).25a
This journal is ª The Royal Society of Chemistry 2011
shown that both the tensile moduli at
room temperature and the strain hardening moduli, as determined in compression tests at large deformations, increased
linearly with the oxocluster proportion.44
In one case, also the flexural strength
was determined in hybrid materials
produced by crosslinking of Zr4 oxocluster with different monomers.37d
Further studies have been carried out,
which are devoted to the investigation of
the effect of polymerisation methodology,
oxocluster nature and content on
mechanical properties.25b,37k As far as the
thermal properties are concerned, it has
been generally observed as the thermal
stability of PMMA or PS based hybrids is
increased upon embedding of even small
proportion of the oxocluster.37,38,42–47 In
the beginning, it has been argued that
these effects were determined by the
enhanced crosslinking induced by incorporation of the symmetrically functionalised oxocluster. Nevertheless, as
previously mentioned and as highlighted
by a systematic work by Kogler and
Schubert45 on the copolymerisation of
differently substituted zirconium oxoclusters (functionalised with both polymerisable and non-polymerisable ligands)
with styrene, the origin of the thermal
stability enhancements has mainly to be
traced back to the filler effect of the
inorganic clusters rather than in their
participation in network formation. In
fact, despite the higher crosslinking of the
materials obtained from the oxocluster
with polymerisable moieties, the two
materials present comparable onset
temperatures of thermal decomposition
and glass transition temperatures. These
findings highlight as the enhanced
thermal stability is mainly to be ascribed
to the filler effect of the oxoclusters rather
than to their participation in network
formation. Crosslinking provides extra
benefits, such as insolubility of the hybrid
materials.
Similar trends in the mechanical and
thermal properties were observed also in
Ti oxocluster based hybrid materials.
Sanchez et al.46 reported on different kinds
of hybrids obtained by copolymerisation
of CD540 (ethoxylated bisphenol A
dimethacrylate) and 2-hydroxyethyl
methacrylate
(HEMA)
with
the
organically modified titanium-oxoclusters
Ti16O16(OEt)24(OC2H4Mc)8
(Ti16),
whereas further Ti oxocluster-based
J. Mater. Chem., 2011, 21, 15853–15861 | 15857
hybrid materials have been reported by
the same authors in further publications.47
Among other investigations, dynamic
mechanical analysis (DMA) showed the
incorporation of Ti oxoclusters in the
matrix leads to a significant alteration of
the hardness as shown by nano-indentation measurements. Thermogravimetric
analysis also indicated a significant
enhancement of the thermal stability
compared to the neat matrix, which in this
case was ascribed by the authors, contrary
to what reported by Kogler and Schubert,45 to the antioxidant effect of the
titanium oxoclusters. In a further work,47c
the effectiveness of the oxoclusters in
enhancing the glass transition temperature of the resulting hybrid was clearly
shown, and a linear relationship between
the concentration of titanium oxocluster
and the glass transition temperature was
outlined. In this work the authors ascribed
the positive effects on the mechanical
reinforcement at the rubbery state of the
hybrid nanocomposite to the presence of
the oxocluster, being an inorganic object
with high volume and molar mass covalently bonded to the polymer medium.
The oxocluster, being rigidly anchored to
the polymer matrix and due to the high
functionality leading to a higher crosslink
density, decreases, however, the molecular
mobility
of
the
macromolecular
backbone.
Also the embedding of zirconium
oxoclusters in silica-derived hybrids led to
an improvement of the mechanical properties of the resulting materials, as evidenced by Di Maggio and coworkers in
the case of the Zr12 oxocluster copolymerised with vinyl trimethoxysilane.48
Instead, slight improvements of the
thermal stability were pointed out in the
materials obtained by copolymerisation of
the Zr4O2(OMc)12 (Zr4) oxocluster with
methacryloxypropyltrimetoxysilane.49
Finally, it was pointed out44,50 as the
final thermal and mechanical properties
of the hybrid material are not only
affected (i) by the oxocluster nature and
(ii) amount, and (iii) by the ratio of
functional and non-functional capping
ligands, but also, at a remarkable extent,
(iv) by the polymerisation conditions.
Although the effect of the nature of the
oxocluster on the observed variations in
materials properties is verified, it is not
easy to rationalise. It has been argued that
it may be related to the polymerization
15858 | J. Mater. Chem., 2011, 21, 15853–15861
kinetics, to oxocluster aggregation
(although it occurs at the nanoscale, and
no remarkable effects on the mechanical
properties are expected) or to a modification of the polymer structure induced by
the presence of the inorganic unit.43 Di
Maggio et al. suggested that the
oxocluster could be also involved in the
initiation
of
the
polymerisation
reactions.51
In conclusion, Schubert et al.44 ascribed
the changes observed in the thermomechanical properties to the concurrence
of different factors, the most important
ones being:
- the oxoclusters act as multifunctional
crosslinkers
- the oxoclusters also act as inorganic
(nano-)fillers, and
- the oxoclusters aggregate to some
(minor) extent within the organic matrix.
However, the interplay of these issues
and their individual contribution to the
macroscopic properties makes a general
conclusion concerning the variation of the
materials properties not trivial.
Improvement or endowment of
functional properties
The functional properties of the final oxocluster-reinforced hybrid material can
be either related to some inherent property of the oxocluster itself, which is
transferred to the material, or to some
functional effect which is determined by
the presence of the oxocluster into the
polymer backbone, for instance due to
chain dynamics modulation or due to the
formation of voids in the chains packing.
In the following, selected examples on the
possibility to endow the final hybrid
material with functional properties are
discussed.
The first case is concerned with one
functional property of the oxocluster
which is tout court transferred to the
resulting hybrid material, as in the case
of a polymer matrix embedding
magnetic oxoclusters described by
Schubert et al.37g Mn12 clusters of
general composition Mn12O12(OOCR)16
are in fact well known and prototypal
examples of magnetic molecular clusters.
The superparamagnetic properties and
the magnetic bistability reported by
Sessoli et al.52 in these Mn oxoclusters,
due to the slow relaxation of the
magnetization, have disclosed the
possibility to use them for storing
information at a single molecular level.
Moreover, their superparamagnetic-like
behaviour in a perfect mono-dispersed
particle size distribution would allow to
use them as model systems to test theoretical predictions such as the quantum
tunnelling of magnetization. In the
quoted work by Schubert et al.,37g the
authors embedded the Mn oxocluster in
polyacrylate matrix and then investigated, inter alia, the magnetic properties
of the resulting materials which were
proven to be the same as the isolated
oxocluster. In particular, ac magnetic
susceptibility measurements show superparamagnetic behaviour above about
8 K, the precise temperature depending
on the sample and oxocluster contents.
Below this temperature, the relaxation
time needed for the moments to follow
the alternating ac magnetic field was
found to follow the Arrhenius law with
energy barriers that increase as the
oxocluster separation increases. This is
an important point showing the oxocluster amount can be used to tune the
functional properties of the materials. In
conclusion, the study showed that the
polymerization of the magnetic clusters
in the presence of organic monomers
allows the preparation of magnetic
materials that can be processed like
typical organic polymers but retain the
properties of the embedded molecular
magnets. A very similar study was also
carried out by Willem et al., leading to
similar results.53
A similar effect (i.e. transferring of oxocluster properties to the materials)
would be expected in the case of optical
properties, but to the best of our knowledge, they have not yet been investigated
in oxocluster-based hybrid materials.
Actually, since the amount of oxocluster
embedded in the polymer network is
generally low (1–3 at%), no relevant
change is expected in the variation of
optical properties such as, for example,
transparency and refractive index.
Instead, among optical properties
induced by the nature of the oxocluster, it
should be mentioned the photochromicity
observed by Sanchez and coworkers in
hybrid materials produced by the
embedding of the Ti16 oxocluster into
polymethymethacrylates.47b The resulting
hybrid materials become dark blue upon
UV-Visible irradiation, and this effect
This journal is ª The Royal Society of Chemistry 2011
was ascribed to the absorption created by
the intervalence band associated with the
photogeneration of localized titanium(III)
polarons. Indeed the presence of mixedvalence Ti(III)–Ti(IV) entities were shown
through UV-Visible and EPR measurements.54 This photochromic behaviour is
reversible in the presence of oxygen which
yields the back oxidation of the Ti(III)
centers into Ti(IV).
The third example deals instead with
functional properties of the material
which are not properties of the oxocluster
but are instead induced by the presence of
the oxocluster. In particular, some of our
recent papers37a–c,h,55,56 described the
interesting dielectric properties of polymethylmethacrylate-based hybrid materials embedding zirconium or hafnium
oxoclusters in different monomer : oxocluster molar ratios (rc). The investigation of the electrical properties of the
hybrids was carried out by broadband
dielectric spectroscopy. In general, we
observed, in the materials prepared by
embedding of the oxocluster, a lowering of
the dielectric constant and of tan d. In this
case, the improved dielectric properties
with respect to the bare polymer were
ascribed to modulation of chain dynamics
induced by the symmetrically functionalised oxocluster and to the creation of voids
in the polymer matrix induced by the
presence of the oxocluster itself. In
the case of polymethylmethacrylate
embedding the Zr4 oxocluster,37a–c
the impedance spectroscopy qualitative
investigations allowed us to conclude that:
(i) the electrical properties of the hybrid
materials are strongly affected by the rc
ratio,
(ii) at high rc value (low amount of
oxocluster), the dielectric properties of
the material are close to that determined
in a pristine PMMA system,
(iii) the inorganic oxoclusters, being
covalently anchored to the matrix, do not
migrate in the materials.
Further quantitative results were obtained after simulation of the experimental data with an equivalent circuit
composed of a resistance in parallel with
a constant-phase element. For both rc ¼
100 and rc ¼ 207 specimens, very high
resistances were measured, thus indicating
that both these networks are characterized
by excellent dielectric properties.
Taken together, these results indicate
that oxocluster-based hybrids are
interesting materials to be used as insulator gate film in microelectronic devices
such as field effect transistors. The effect
of the amount of the oxocluster can be
rationalized by considering that, with the
increasing content of oxocluster, also the
interphase proportion increases, which in
turn lead to a modulation of the chain
dynamics.
A related example of improvement of
functional properties, and ascribable to
the presence of the oxocluster, is the fairly
good barrier properties against corrosion
evidenced for a polyacrylate matrix
embedding the Zr4 oxocluster.37j In this
regard, electrochemical impedance spectroscopy (EIS) was performed in order to
evaluate if the coatings actually protect
the metallic substrate from corrosion.
Although some improvements in the
process are required, the hybrid coatings
appear promising as barrier against
corrosion and they generally behave
better than pure PMMA, when deposited
on different aluminium alloy substrates.
Although the water uptake of hybrids is
greater than that of pure PMMA, the
impedance modulus |Z| measured for the
sample coated by hybrids is greater especially at long time.
to covalently disperse a electroluminescent
molecular cluster into a compact, waterproof, transparent, structurally stable and
dielectric matrix for the production of
high performing alternating current
powder
electroluminescent
lamps
(ACPEL).55
To tackle this stimulating enterprise,
the bewitching role of chemistry, and in
particular of inorganic chemistry,
encompasses not only the synthesis of
new inorganic BBs (for instance based on
rare earth metals) with appealing functional properties such as luminescence or
magnetic properties, or chemical activity,
but also the skilful choice and design of
their surface functional groups to match
them with suitable organic counterparts
providing further functionalities. In this
regard, K€
ogerler et al.58 have recently and
successfully explored the heterometal
expansion of oxoclusters, which enables
to introduce into the cluster structure
magnetic ions (e.g. Ni, Mn, Fe). This
strategy discloses new (synthetic)
perspectives for the introduction of
further (functional) heterometal(s) in the
oxocluster core and the further development of novel heterometallic based hybrid
materials
with
novel
functional
properties.
Conclusions
In conclusion, hybrid materials design
and synthesis is a multifaceted and stimulating research field which entails
a profound chemical knowledge of
composition/structure–property relationships as well as interdisciplinary efforts to
harness all their potentialities for structural and/or functional applications.
The new frontier which can be envisioned in this fascinating chemistry playground is the search for synergic activity
and multifunctionality, both deriving from
a combination of different BBs each
providing different functions or properties. This means for instance to obtain
a hybrid dielectric thin layer for microelectronics applications, which is however
endowed with transparency and with
outstanding
thermomechanical
and
chemical stability required in operative
conditions. But combination of functions
means also to merge for instance the
magnetic properties of a recently developed57 thiophene-functionalised copper
dimer with the electric properties of the
conductive polymer polythiophene,12 or
This journal is ª The Royal Society of Chemistry 2011
Acknowledgements
The University of Padova, the National
Research Council (CNR) of Italy, and
the Italian Consortium INSTM (Italy)
are acknowledged for providing money
and equipments. The author would like
to warmly thank Prof. U. Schubert
(Technische Universit€
at Wien, Vienna,
Austria) for his scientific and human
teachings, as well as all the cooperation
partners at European and Italian
Universities and colleagues at the ISTMCNR and at the University of Padova
who gave their valuable contribution to
the quoted works. Last but not least, I
would like to express my sincere gratitude and appreciation to some of my
Diploma, Bachelor and Master students
at the University of Padova (F. Faccini,
F. Graziola, F. Maratini, M. M. Montolli, S. Mascotto, and A. Zattin) who
have fruitfully contributed, in these
years, to the work in the field of transition metal oxoclusters and organic–
inorganic hybrid materials.
J. Mater. Chem., 2011, 21, 15853–15861 | 15859
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