Mammeri, F., Le Bourhis, E., Rozes, L., and Sanchez, C. Journal of Materials Chemistry 15 (35-36), 3787 (2005).
CHAPTER VII
Mechanical Properties of Hybrid Materials.
I. Mechanical properties of organic-inorganic hybrid films (class II)
I.1. Sol-gel silicas modified by hydrophobic –CH3 groups
Fabes et al.1 studied first the mechanical properties of sol–gel silica thin films (thick from 65 to 190 nm) by
microindentation and have first demonstrated the substrate influence on the elastic modulus and hardness
measurements but also the sensitivity of indentation techniques towards the concentration gradient. Recently,
Malzbender et al.2 have carried out an extensive study of the mechanical properties of thin films prepared by
sol–gel routes and deposited on glass substrates. They used two contact probe techniques: nanoindentation and
scratch-test. Studied hybrid materials were formed by co-condensation of tetraethoxysilane (TEOS) and
methyltrimethoxysilane (MTMS) in the presence of silica nanoparticles (up to 70% in volume). Coatings could
be obtained with relatively important thickness (2 to 15 μm). The organic group for these materials is only a
methyl –CH3 group. Without the polymer group, a brittle behaviour was reported. An extensive survey of the
response as a function of coating thickness was made. A drastic increase of hardness as a function of penetration
depth was reported for the thinnest coatings (0.5 μm) while the hardness response was almost a plateau for the 4
μm thick film within the investigated range. Interestingly, as expected, elastic response is affected by the
substrate in the same range of indenter penetration. Under elevated load the thinnest films showed only
delamination and chipping while the thickest films show first radial cracks that were followed by delamination
and chipping. More recently, Etienne-Calas et al.3 reported a study of fracture with the determination of
toughness (considering the effect of residual stresses) on zirconia–methacrylate-based hybrid materials.
I.2. Epoxy-modified silica hybrid materials
Many authors have addressed epoxy–silica hybrid materials.1,3-7 These hybrids were formed from silica
nanoparticles or precursors obtained by hydrolysis–condensation of silicon or transition metal alkoxides and
glycidoxypropyltrimethoxysilane (Glymo) as coupling agent. The epoxy function can be maintained to modify
the hybrid network or polymerized when reactions with amines are carried out, then it will contribute to the
formation of the network. Hardness testing showed variation of modulus and hardness as a function of the
composition. Etienne et al.5 compared the observed response to different mixing rules in a graph showing E =
f(%vol SiO2) for epoxy–silica hybrids. It is to be assumed that all TEOS is converted into silica. The elastic
modulus of silica and GPTMS were taken as 70 GPa and 1.9 GPa respectively while their respective density was
taken as 2.2 g.cm-3 and 1.1 g.cm-3.
I.3. (Meth)acrylate modified silica hybrid materials
More recently, Soloukhin et al.8 addressed materials of type poly((meth)acrylate)–colloidal silica. These
authors used an organosilane (methacryloxypropyltrimethoxysilane) to anchor methacrylate groups at the silica
nanoparticles surface. This colloidal dispersion was introduced in a solution containing organic monomers and a
hybrid sol was formed to produce films. Films of hybrid materials in the range of thickness 15 to 70 μm were
fabricated by photochemical radical polymerisation on polycarbonate substrates. All the resulting films were
transparent. Moreover, nanoindentation tool was shown to yield elastic modulus and hardness data with good
reliability. As expected thickness of the film, filler content and chemical composition are shown to influence the
mechanical response.
Also, Mammeri et al.9 reported a study on the mechanical properties of hybrid thin films based on SiO2–
PMMA materials investigated through nanoindentation tests. This study emphasizes the influence of the hybrid
interface on the mechanical response. This work demonstrated that nanoindentation is an appropriate technique
to characterize such hybrid organic–inorganic thin films once specific procedures of analysis and the use of
appropriate models are determined. Indeed, reproductive indentation modulus and hardness of the hybrid layers
were obtained despite viscoelastic behaviours. This study showed that the mechanical responses of
nanocomposites are not only governed by the composition of the layers but also by the nature and the extent of
the hybrid interface. Several hybrid coatings have been studied, constituted by an inorganic and an organic phase
being either physically mixed or covalently connected. The weak (hydrogen bonds) or strong (covalent bonds)
interactions generated by the hybrid interface lead to nanocomposites which exhibit totally different mechanical
behaviours. Moreover, comparison between hybrid coatings obtained by in situ inorganic polymerization in
Mammeri, F., Le Bourhis, E., Rozes, L., and Sanchez, C. Journal of Materials Chemistry 15 (35-36), 3787 (2005).
PMMA and hybrid layers obtained with preformed silica nanoparticles emphasized the correlation between the
morphology of the nanocomposites and the mechanical responses.
Another interesting and complete study using microhardness was also performed by Perrin et al.10 on acrylic
polymer–titania-based hybrid materials. Hardness can be related to wear resistance as we have already discussed
in chapter 6. So far, microhardness testing can also be used as a relevant tool to investigate the morphology–
property relationship of acrylic polymer–titania-based thick films.10 The variations in hardness as a function of
time or as a function of temperature allow monitoring of the physical aging of the material (slow ordering of the
molecular chains in a more compact molecular packing) and/or the ending of sol–gel process. That is why a
careful control of the thermal history of the material is necessary to obtain meaningful hardness measurements.
Based on data obtained with a number of commercial amorphous polymers, a unique linear relationship
between Vickers hardness measured at room temperature and at Tg was found. This expression appeared to be
valid only for polymers containing single bonds in the main chain. For the acrylic resin (Tg = 25 °C), this
equation yielded a hardness value of 65 MPa whereas the measured value was found to be 52 MPa.
Obviously, the authors clearly demonstrated the linear increase of hybrids hardness H with the titania
amount (up to 10 vol%), which can be described by a simple additive law: Rice’s model, developed for
conventional composites,11
H = HTiO2φTiO2 + Hpolymer(1 - φTiO2), where φTiO2 is the volume fraction of titania.
The straight extrapolation yields a hardness around 575 MPa, far below the measured hardness of pure sol–
gel titania (1120 ± 40 MPa). This increase in hardness results from titania–polymer interactions. Indeed, titania is
dispersed in the acrylic matrix at a nanometre level, limiting the filler–filler interactions. If the volume fraction
of titania would exceed the three-dimensional percolation threshold (> 15 vol% for a disordered system), an
infinite aggregate of titania would be formed, able to support the stress and limiting the role played by the acrylic
matrix. That would result in a discrepancy between the measured and calculated (from Rice’s model) hardness
values. The hybrid material containing 10.7 vol% of titania was found to be twice as hard as the acrylic matrix
(111 MPa vs 52 MPa respectively).
The hardness of sol–gel-derived titania can be compared to that of titania phases found in the literature.
Usually Mohs hardnesses of rutile, anatase and brookite range between 5.5 and 6.5. Since both the scratching
hardness M (Mohs scale) and the static indentation hardness H (Vickers indentation for example) are determined
by the plastic properties of the material, some attempts were made to correlate Mohs and Vickers hardnesses.12
H = 310.6 exp (0.461 M), where H is the Vickers hardness in MPa and M is the Mohs hardness.
The sol–gel-derived titanium oxo–polymers hardness is lower than that of crystalline titania due to the
softening effect of organic ligands. Up to 10.7 vol% of titania, the hardness H is found to be related to the
density of the hybrids13
lnH = 5.04ρ - 1.54, where H is in MPa and ρ in g.cm-3.
Hardness H was reported to decrease with temperature following the same exponential law as the one found
for semiamorphous polymers14
H = H0 exp(-βT),
where H0 is the hardness at 0 K and β is the coefficient of thermal softening (two values of β were determined, in
the glassy state and in the rubber state).
The Tg values measured by hardness H and DSC show small differences for hybrids containing PMMA and
PET. These small differences of about 10 °C should result from the scale of the measurements. Indeed the
hardness measurement can be considered to be quasi-static if one compares it to the 10 °C.min-1 heating rate
used for the DSC scans.15 For the hybrid materials, b was found to decrease and Tg to increase when increasing
titania content, with a more pronounced effect in the rubbery state. Nevertheless, for all hybrids, the Tg profiles
obtained by DSC were broad and hardly perceptible. Consequently, a precise determination of the glass
transition of such nanocomposites from DSC experiments is not expected preventing careful confrontation.
Finally, a continuous decrease in hardness with loading time clearly revealed the viscoelastic nature of the
hybrids. Depending on the titania content, H decreased by as much as 62–78% over a holding period of one day.
Mammeri, F., Le Bourhis, E., Rozes, L., and Sanchez, C. Journal of Materials Chemistry 15 (35-36), 3787 (2005).
Over the same period, the associated strain due to creep increased from 62 to 110%. For all hybrid materials, the
following typical power law function with time was obtained:
H = H0t-k where H0 is the hardness value after 1 min and k is the creep constant.
The creep rate significantly decreases when increasing the filler content (k decreases by as much as 23%
with 2.3 vol% of titania). For higher titania contents, k seems to decrease almost linearly with the volume
fraction of titania. According to the author, the time-dependent process would only be changed when the matrix
characteristic is modified. Thus, the change in creep rate observed with titania–acrylic-based hybrids is another
evidence of the strong interactions between the organic and inorganic components.
I.4. Conclusion
Nanoindentation is an efficient tool allowing for the characterization of the mechanical properties of a large
variety of hybrid thin films. The hybrid materials and their mechanical parameters are gathered in the following
table:
• Polymers like PDMS, (meth)acrylates, epoxy resin, polyimide, polycaprolactone, etc. have been studied.
• The nature of the metal oxide phase could be varied from precursors obtained from sol–gel process
(hydrolysis–condensation of alkoxides) to colloidal silica, oxo-clusters such as POSS or mesoporous silica.16
Elastomer-based hybrid materials start to be characterized by nanoindentation.9,17,18 Regarding nondestructive methods, it is important to know their limits: their interpretations require the knowledge of
density19or to have porous materials20or are in development to be more accurate.21 So far, as stated in the
beginning of the thin film section, other techniques are being used even though they are less popular. Coupling
some of these techniques22 should be one of the next challenges since discrepancies are being observed.4 Only
crossing complementary information will yield an accurate description of the relationships between mechanical
data and microstructure of the hybrid materials.
Table: Indentation investigation of organic-inorganic hybrid thin films.
Hybrid materials
Inorganic materials
Methyltrimethoxysilane/colloidal silica
and TEOS
Techniques
Results
References
Nanoindentation
(home-designed)
Malzbende et al.2
Etienne-Calas et al.3
Methyltrimethoxysilane and/or tetramethyl
tetra(trimethoxysilylethyl)
cyclotetrasiloxane and/or
octa(methoxysilylethyl) octasilsesquioxane
Continuous stiffness,
Nanoindentation
Measurements of E and H but also residual
stresses of films and fracture toughness of
the interface coating-substrate
Determination of E and H, ISE, substrate
effect
Depth-sensing,
Nanoindentation
Elastic modulus measurements in good
agreement with DMTA measurements
Li et al.17
AFM,
DMTA,
Tensile testing,
Continuous stiffness,
Nanoindentation
Nanoindentation
(home-designed)
Nanoindentation
Concentration gradient, study of mechanical
properties as a function of wt% Ti, substrate
effect on E and H
Perrin et al.10
Xiong et al.23,24
Nguyen et al.25
Soloukhin et al.8
Rubbery materials
PDMS-vinyl cross-linked to
Si[OSi(Me)2H]4 by hydrosilylation
Thermoplastics
Acrylic resin (butyl acrylate, MMA,
methacrylic acid or MPTMS, butyl
acetate)/Ti(OBun)4, acac or acetic acid
(Meth)acrylate matrix/colloidal
silica/MPTMS
MPTMS/methacrylic acid/Zr(OiPr)4
Yim et al.22
Lyu et al.
MPTMS (prehydrolyzed or not)/HEMA
(as monomer or preformed polymer)
Nanoindentation,
Taber abrasion,
Residual stresses
Vickers hardness,
Erosion testing
Hydroxy or triethoxysilane poly(ecaprolactone) (PCL–OH or PCL–Si)
Scratch resistance
tests
Determination of E, H as a function of wt%
Si
Measurements of E and H but also residual
stresses of films and fracture toughness of
the interface coating-substrate
Influence of O–I composition and class I/II
hybrid interface. Specific procedure to
consider viscoelastic behaviour
Influence of the O–I ratio but also the nature
of the inorganic filler and the morphology
on the mechanical properties
Characterization of films/substrate (PMMA)
adhesion
Nanoindentation,
DMTA
Determination of E and H as a function of
the wt% and type of POSS
Huang et al.28
Nanoindentation,
Study of compromise between increase of E
Douce et al.7
PMMA/MPTES/TEOS or colloidal silica
Thermosets
Polyamic acid with terminal
amine/epoxymonofunctionalized POSS
(other groups are aliphatic or cyclohexyl)
Glymo and colloidal silica (modified by
Etienne-Calas et al.3
Mammeri et al.9
Habsuda et al.26
Messori et al.27
Mammeri, F., Le Bourhis, E., Rozes, L., and Sanchez, C. Journal of Materials Chemistry 15 (35-36), 3787 (2005).
glymo or dimethyloctylchlorosilane)
Scratch-test
Glymo/Si, Ti, Zr alkoxides
Knoop
microindentation,
3-points dynamic
bending
Nanoindentation,
Residual stresses
Glymo/Si alkoxide
Glymo/colloidal silica or in situ sol–gel
generated silica
Epoxysilane/bisepoxide spacer/colloidal
silica/various dyes
Polyester resin/TEOS
Other techniques
Mesostructured silica (Pm3n with CTAB
or Im3m with block copolymer)
Heteropolysiloxanes
HEMA/Zr or Al alkoxides
Nanoindentation
(home designed),
3-points dynamic
bending
Vickers hardness,
Scratch-test
Nanoindentation
Ellipsometry by
adsorption–
desorption cycle of
water
Microacoustic
technique
4-points bending
testing,
Brillouin
spectroscopy
and deterioration of scratch-resistance as a
function of wt% of fillers
Determination of E as a function of the
curing time. Discrepancy between bending
and indentation (surface is stronger then
bulk)
Residual stresses % hardness. Better
mechanical performances when Glymo is
used with TEOS
Enanoindentation < Ebending. Influence of the Si
content on E and H. Use of mixing rules E =
f(vol% Si)
Innocenzi et al.4,29
Robertson et al.6
Etienne et al.5
Influence of silica content on the abrasion
resistance
Prehydrolyzation of the precursor to avoid
evaporation of Si compounds and preserve
the mechanical performances. Five loading–
unloading cycles for visco-elastic creep to
be minimized
Mennig et al.30
Analysis of the film thickness evolution to
determine E.20 For porous materials only!
Boissière et al.20
Frings et al.31
Dabadie et al.32
Determination of elastic constants, E, G, B,
ν if the density is known. Actual drawback
for hybrid materials: γ between transverse
and longitudinal modes are unknown (0.507
for PMMA)
Di Maggio et al.21
II. Relation between nanostructure and mechanical properties
O–I hybrid materials combine both the advantages of organic polymers (flexibility, lightweight, good
impact resistance and process ability, etc.) and inorganic materials (high mechanical strength, good chemical
resistance, thermal stability, etc.). The properties of hybrid networks strongly depend on the degree of phase
dispersion that can be reached and consequently on many chemical parameters relative to their elaboration (O–I
ratio, molecular weight of macromonomers, number of anchoring groups, reactivity of cross-linking alkoxide
reagents, processing, solvent, etc.). Even slight changes in the elaboration parameters may lead to a wide variety
of hybrid O–I networks (as seen previously). Consequently, the knowledge of the nanostructure is required to get
further understanding of their viscoelastic behaviour. Generally, mechanical properties of polymers reinforced at
the nanoscale level are determined by the complex relation between the nature and the size of the filler, the
hybrid interface and the nature of the interactions between the organic and inorganic components. The crucial
role of nanostructure and microstructure is illustrated in this part.
II.1. Functionalized PDMS or PTMO as organic network formers
Wilkes et al.33 and Mark et al.34 simultaneously developed in the 80s a new kind of composite material
incorporating polymeric materials with metal oxo–polymers obtained by the sol–gel process. Indeed,
condensation (by sol–gel reactions) between silanol or alkoxysilyl-terminated PDMS with TEOS leads to hybrid
materials with improved and tunable mechanical properties. Mechanical and rheological properties of these in
situ filled elastomers strongly depend on the choice of the synthesis conditions. The elastic modulus and stiffness
of these materials increase with the TEOS content, obviously because of an increased number of cross-linking
points. The importance of these cross-links was clearly demonstrated by Wilkes et al.35 when varying the number
of functional triethoxysilyl groups in the synthesis of PTMO–TEOS hybrids. As this parameter increases, the
hybrid network becomes more and more cross-linked and thus stiffer. Moreover, the increase of the observed
elastic modulus is correlated with the decrease of the swelling properties. Mechanical properties of PTMO–
TEOS are strongly improved as compared to those of hybrids made from PDMS.36
In sol–gel processes, the promotion of hydrolysis or condensation reactions of alkoxy silane is governed by
the choice of acidic or basic catalysts which directly influence the microstructure and the structure (and then the
properties) of PDMS–TEOS hybrids. For instance, Huang et al.33 have reported that the flexibility and ultimate
Mammeri, F., Le Bourhis, E., Rozes, L., and Sanchez, C. Journal of Materials Chemistry 15 (35-36), 3787 (2005).
elongation of HCl-catalyzed PDMS–TEOS elastomers increased with increasing acid content. PDMS–TEOS
hybrids (Mw = 1700 g.mol-1) could be rubbery even when the inorganic component was in excess (70 wt%).37 By
increasing the acid concentration, these materials exhibit properties comparable to those of organic rubbers. In
such acidic conditions the resulting material consists of a matrix of medium chain length and small silica
particles, with a porous structure, which allows the production of the free volume required for chain motion.
Consequently, the PDMS chains can curl and uncurl in the presence of external stress and the materials exhibit a
rubbery elasticity.
There have been important developments in the area of elastomeric polymers. Among these, we shall
mention network chains of controlled stiffness, fluorosiloxane elastomers, new thermoplastic elastomers, etc.
Interesting elastomeric composites include those with in situ generated ceramic-like particles, clay-like layered
fillers, polyhedral oligomeric silsesquioxanes (POSS). On the other hand, new characterization techniques are
being developed for elastomers, and there have been new developments in elasticity theory and in elastomer
processing.38
The introduction of transition metal oxides such as titania,39 or zirconia40 in hybrid materials like
ethoxysilylterminated PDMS–TEOS leads to an improvement in the mechanical properties of the hybrid
elastomers. It is especially true in the case of PTMO–silica hybrids for which the ultimate strength and elastic
modulus increase in the presence of titania-based component.41 Indeed, titanium alkoxides, with higher reactivity
than that of silicon alkoxides allow a better completion of hydrolysis–condensation reactions41,42 and thus a
higher degree of cross-linking.
II.2. (Meth)acrylate matrixes
Inspired by the results of Wilkes and Mark, Coltrain et al. first prepared organic polymer–silica hybrids by
in situ polymerization of TEOS or TMOS in the presence of both trialkoxysilane functionalized and
unfunctionalized poly((meth)acrylates).43 They showed that trialkoxysilane functions would delay phase
separation and a steady increase in Tg with increasing silicate content was observed.
Among numerous studies on PMMA- and PHEMA-based hybrid materials, we shall mention that of
Habsuda et al.26 who have investigated methacryloxypropyltrimethoxysilane (prehydrolyzed –pMPTMS or not –
MPTMS) and 2-hydroxyethylmethacrylate (as monomer HEMA or as polymer PHEMA)-based hybrid materials.
A large range of nanocomposites were synthesized without chemical bonding between HEMA and MPTMS
prior to radical polymerization. The composites were brittle (from 9 to 26 wt% of silica) and showed yellow to
brown colours. The aspect of the coatings strongly depends on whether MPTMS was introduced in HEMA or
PHEMA: those with MPTMS appeared to be solid samples with smooth surfaces while those with pMPTMS
were brittle and cracked. The glass transition temperatures of all the hybrid composites were found to be higher
than those of the neat polymers (PHEMA, PMPTMS or pMPTMS where P is for organic polymerization and p
for inorganic polycondensation). The intensity and width of the Tg peaks were observed to depend on the
composition, the degree of mixing and the morphology. Therefore, the peaks associated with the Tg were broader
and weaker in the case of pMPTMS-based hybrids than for MPTMS-based hybrids of the same composition. So
far, pMPTMS-based nanocomposites were denser than unhydrolyzed systems due to a better packing of the
components. Hardness of these systems was measured and correlated to the morphologies developed in the
nanocomposites. Only PHEMA-based hybrids yielded a reliable study of the Vickers hardness, which was found
to be only slightly sensitive to the silica content, drawing the conclusion that morphology is the most important
parameter determining the hardness.
Previous studies on poly(silicic acid)–HEMA-based hybrid materials containing up to 10 wt% silica have
been reported.44 However, the mechanical properties and the overall appearance of the PSA–HEMA composites
were negatively affected when the inorganic content was increased. Indeed, no covalent bond was formed
between PSA and HEMA. Their absence proved to be of great importance in regards of the mechanical
performances of the material during erosion testing. Indeed, better properties were obtained after addition of a
coupling agent (MPTMS) that leads to Si–C links.
However, the use of trialkoxysilanes as coupling agents is mainly limited to the preparation of silica-based
hybrid materials. Up to now, there have been very few studies reporting the preparation of organic polymer–
titania hybrid materials by this method although titania has unique mechanical, thermal, optical and electronic
properties.45 For example, Wilkes et al. successfully prepared high-refractive index O–I hybrid materials by
using Ti(OiPr)4 inorganic precursor to react with triethoxysilane-capped poly(arylene ether ketone) and
poly(arylene ether sulfone).46
Mammeri, F., Le Bourhis, E., Rozes, L., and Sanchez, C. Journal of Materials Chemistry 15 (35-36), 3787 (2005).
Recently, Xiong et al.24 have prepared O–I hybrid materials based on methacryloxypropyltrimethoxysilanecapped acrylic resin and titania by mixing acrylic resins with titania obtained from Ti(OBun)4 hydrolysis with
different water content and pH. Nanoindentation results showed an enhancement of the hardness H and the
elastic modulus E when increasing the titania content. In comparison with the pure organic polymer, H increased
by 350% (from 0.030 to 0.135 GPa) while E was improved by 58% (from 2.80 to 4.48 GPa) when only 5 wt% of
titania was added. These results suggested important crosslinking between the two phases, through residual OH
and OR groups from titania and TMOS-capped polymer chains, forming Si–O–Ti covalent bonds. However, the
increased inelastic modulus saturated as the content of titania was further increased to 20 wt% because of the
limited number of Si–OH groups and the aggregation of titania (demonstrated by SAXS measurements).
Consequently, the trends were the following: E0% < E20% < E5%.
The effects of the amount of water and pH on the sol–gel process as well as the effects of the ratio of mixed
solvents on the mechanical properties were also demonstrated (for the nanocomposites containing 5 wt% titania).
Both the elastic modulus and hardness were observed to decrease when a larger amount of water was used.
Indeed, low water content favours the formation of small-size titania oxo–polymers having open structures and
extended reactive surfaces that are prone to interact with the organic polymer. Stronger interactions and better
miscibility between the two phases result in better mechanical properties.
Another important parameter influencing the mechanical properties of hybrid materials is the pH values
during the sol–gel process. Hybrid films prepared under acid-catalyzed conditions exhibit excellent hardness and
elastic modulus (0.135 GPa and 4.48 GPa with H2O/Ti = 4) due to low phase separation and the open structure
of titania in the polymer matrix (revealed by SAXS).47 However, the sample prepared under neutral conditions
shows a chain-like structure of titania with smaller size. Moreover, the reaction between the reactive groups of
titania and the polymer chain is very slow, leading to a decrease in cross-linking and mechanical properties
(0.113 GPa and 4.04 GPa). Base catalysis dramatically increases phase separation yielding the lowest values of E
and H (0.104 GPa and 3.55 GPa).
Solvents used for the materials synthesis were also found to influence the mechanical properties. Indeed, an
increase of the butyl acetate–ethanol ratio caused a significant increase in phase separation. Titania networks
were more compact with larger sizes, resulting in weaker interfacial interaction with the polymer and thus in a
decrease of the elastic modulus E (3.12 GPa) and hardness H (0.075 GPa), even under acidic conditions.
Physical mixtures of organic polymers and preformed inorganic particles may lead to a separation in
discrete phases, resulting in poor mechanical properties, especially if particle aggregation occurs. These
drawbacks are avoided using functional polymers that interact with the surface of the particles or modify the
surface of the particles.48 Another alternative is the encapsulation of small inorganic particles by a polymer layer
(so-called core–shell particles), which can be carried out applying an emulsion polymerization process.
Polymerization occurs primarily at the surface of unmodified particles due to the absorption of the monomer on
the surface and is followed by the polymerization in the adsorbed layer.49 On this basis, Vitry et al.50 have
realized the copolymerization reaction of 3-methacryloxypropyltrimethoxysilane (MPTMS) with styrene and nbutyl acrylate monomers through emulsion polymerization. The so-produced hybrid co- and ter-polymers
P(BuA–co-MPTMS) and P(Sty–co-BuA–co-MPTMS) latexes were cast into films that displayed good optical
transparency and homogeneity up to 20 wt% of MPTMS after coalescence of the particles. The materials were
not soluble in good solvents for PBuA and P(BuA–co-Sty), indicating that the polymer chains were
homogeneously crosslinked throughout the film. Copolymer samples (PBuA and P(Sty–co-BuA)) display typical
behaviour of amorphous thermoplastic polymers. However, the films obtained from the hybrid latexes showed
improved dynamical mechanical properties indicating the formation of a reinforcing organo–mineral network in
the nanocomposite. The dynamic storage modulus of the hybrids increased when increasing the silane content,
while the tanδ peak shifted to higher temperatures, broadened and decreased in intensity, demonstrating the
formation of a continuous rigid network due to the presence of MPTMS in the sample. The main relaxation peak
shifted towards higher temperatures (for example from -33 °C to -15 °C by adding 10 wt% of MPTMS in
PBuA). At the same time, the dynamic modulus in the rubbery plateau G’ increased drastically (2 orders of
magnitude up to around 10 MPa with 20 wt% of MPTMS). Similar trends were observed with the P(Sty–coBuA) copolymer-based hybrids. With 10 wt% (20 wt%) of MPTMS, a shift of 17 °C (40 °C) of the maximum of
tanδ was observed. The observed increase in G’ was again around two decades, which means that all the hybrids
behave like cross-linked polymers.
Another promising route for the reinforcement of acrylic matrixes is the use of nano-building blocks
(NBBs). Titanium and zirconium oxo-clusters are examples of such NBBs. Schubert et al.51 have shown
improvement of mechanical properties by copolymerization of such clusters with a methacrylate matrix. When
Mammeri, F., Le Bourhis, E., Rozes, L., and Sanchez, C. Journal of Materials Chemistry 15 (35-36), 3787 (2005).
the polymerization of the methacrylate monomer was performed in presence of 10 wt% of Zr4O2(OMc)12
(methacrylate-substituted oxozirconium cluster), the flexural strength was almost the same (47 MPa) but the
flexural modulus was increased from 1000 MPa to 1900 MPa. Moreover, contrary to the neat polymer, the
hybrid materials did not lose their flexibility upon water storage. Hybrid materials synthesized by polymerizing
MMA with 0.3 mol% of Ti6O4(OEt)8(OMe)8 (methacrylate-substituted oxotitanium cluster) also exhibited
improved mechanical properties as compared to the neat PMMA.52 Indeed, DMTA data show that the glass
transition temperature increases drastically with very small changes in composition (from 97 °C to 127 °C). The
storage modulus was also higher for the Ti-doped hybrid samples. An effective reinforcement of methacrylate
thermosets or thermoplastic polystyrene by an oxo–alcoxo titanium cluster has been recently reported.53,54 The
organic surface of the Ti16O16(OEt)32 cluster can be selectively modified by transalcoholysis reactions leading to
new functional clusters Ti16O16(OEt)32-x(OR)x, where R can be a non-reactive alkoxy group such as a propoxy or
phenoxy, or a reactive one such as a methacrylate or styrenic group, and x is the number of modified alkoxy
ranging from 4 to 16 in function of the acidic power of the alcohol.55 The Ti16O16(OEt)32 cluster was modified by
methacrylate functions by reaction with HEMA and copolymerized with a methacrylate matrix. The clusters
which lead eight methacrylate groups act as effective nano-fillers covalently linked and dispersed in the organic
thermoset matrix since the thermo-mechanical properties of the resulting nanocomposites are strongly enhanced
above the glass transition temperature with respect to the neat methacrylate matrix (an increase of glass
transition temperature and of the storage modulus with increasing modified cluster content was observed).
II.3. Polyimides as organic network formers
Polyimides constitute a very interesting group of strong, thermally and chemically resistant polymers. They
are often used instead of glass and metals, such as steel, in many demanding industrial applications. In particular,
because of their thermal stability, they can be used in circuit boards, insulation, fibres for protective clothing,
adhesives, etc.
Inorganic fillers (silica or titania) have been added to polyimides to control and modify their characteristics.
Homogeneous distributions of added nanoparticles were achieved by sol–gel process.56 Morikawa et al.57 and
Nandi et al.54 have first investigated the synthesis of oxide–polyimidebased hybrids from carboxylic or
ethoxysilyl functionalized polyimide polymers hydrolyzed and condensed with Si or Ti alkoxides. The rigidity of
the polyimide backbone (relative to high Tg) slows down the mobility of the formed metal oxo-clusters,
preventing the agglomeration of large particles and consequently, highly dispersed metal oxo–polymers were
generated by hydrolysis–condensation of alkoxides.
Transparent and flexible free-standing films were obtained with both functionalized polyimides but in the
case of carboxylic-functionalized polymers, a loss in strength was reported for the highest silica filler
concentrations (> 35 wt%). While in the case of ethoxysilyl-functionalized polymers, elevated toughness was
reported for a silica concentration up to 70 wt%. The dynamical mechanical studies of the hybrids indicate that
the motion of the polyimide chains is constrained in the hybrid matrix, especially when the polyimide
macromonomers containing a large amount of ethoxysilyl groups were used. The tensile strength of the hybrid
films showed no evolution when silica content was increased, whereas the tensile modulus was reported to
increase, except when the polyimide matrix did not contain any ethoxysilyl anchoring groups.
II.4. Epoxy–polyimide organic network formers
Like polyimides, epoxy networks are thermosets used in the microelectronic and aerospace industry as new
high-performance composite materials with high end-use temperatures. Epoxy-resin-containing polyimide is one
of these systems being investigated for high temperature applications.58 Unmodified epoxy resins have limited
applications because of their inherent brittleness and relatively poor thermal stability, while the linear polyimides
present tuned flexibility and elevated thermal resistance. By combining the advantages of both polymers
polyimide–epoxy composites are expected to yield outstanding performances. The physical mixing is the most
common approach to prepare polyimide-epoxy composites28 despite some drawbacks due to phase separation
after solvent removal. Huang and Lee have reported the preparation of hybrid materials with polyamic acid with
terminal amine groups instead of traditional epoxy hardeners and POSS with epoxide groups allowing
improvement of the thermal mechanical properties (especially the changes in Tg).28,59 The effect of POSS content
on macroscopic properties has been studied using two types of POSS, one bearing aliphatic groups and another
bearing cyclohexyl groups. The nanocomposite networks were predominantly formed by the linkage between the
terminal amine groups of the polyimide molecules and the epoxide groups. Although the cross-link densities in
nanocomposites increased significantly (tanδ peak shifted to significantly higher temperature while it weakened
and broadened significantly and the rubbery storage moduli clearly increased), the compressive modulus,
Mammeri, F., Le Bourhis, E., Rozes, L., and Sanchez, C. Journal of Materials Chemistry 15 (35-36), 3787 (2005).
hardness (measured by nanoindentation) and thermal expansion changed only slightly. For example, elastic
modulus E and hardness H decreased (from 5 to 4 GPa and from 0.5 to 0.35 GPa respectively) when aliphatic
POSS was used instead of cyclohexyl groups. E increased only slightly and H decreased (from 0.5 to 0.4 GPa)
when cyclopentyl–POSS content was increased.
Li et al.60 have also reported that monofunctional POSS–epoxy matrix hybrids exhibited lower Tg than the
neat epoxy resin probably because of incomplete curing (up to 25 wt% of POSS). On the contrary, Lee et al.61
have argued that cyclohexyl or cyclopentyl POSS introduced at a molecular level in epoxy resin of bisphenol A
can significantly delay the physical aging process in the glassy state.
Laine’s group has modified epoxy resins by a series of octa and polyfunctional silsesquioxanes with various
inert groups R and aminophenyl and dimethylsiloxypropylglycidylether groups.62,63 The dynamic mechanical
properties, fracture toughness and thermal stability were studied for different types of R groups, tether structures
between epoxy matrices and POSS cages and the defects in silsesquioxane cages. At the same time, Ni et al.64
have performed the in situ polymerization of DGEBA with octanitrophenyl– and octa-aminophenyl–POSS
(ONP– and OAP–POSS respectively). All mixtures were homogeneous, revealing a good miscibility of POSS
with epoxy precursors and the resulting structures depended on the types of corner groups of POSS. The storage
modulus in the glassy state and in the rubbery state was higher than that of the neat epoxy for ONP– and OAP–
POSS-based hybrids, indicating the reinforcement induced by POSS. Nevertheless, the mechanical properties
were improved when OAP–POSS was used and when a better nanoscale dispersion of POSS cages was obtained
in the material.
II.5. Conclusion
The microstructure of O–I hybrid materials depends not only on the nature of the organic and inorganic
components but also on the synthesis procedure.44 Indeed, Hajji et al.65 have shown that homogeneous and
optical transparent hybrid materials can be formed when bulk free radical polymerization of HEMA was
conducted with the sol–gel polymerization of TEOS precursors. The two components form a bicontinuous
interpenetrated network. Additional studies on 2-hydroxyethylacrylate (HEA)–TEOS systems have revealed a
drastic change in the microstructure when the relative rates of the two polymerization processes are changed.
Particularly, when sol–gel polymerization is faster, a better degree of dispersion can be achieved.
The chemical nature of the organic component can also be of primary importance to obtain adhesion to the
polymeric substrate that has to be protected. Thus, it can influence the microstructure and the morphology
development during the coating process. In this respect, hydroxy-terminated poly(e-caprolactone) (PCL), a
commercially available aliphatic polyester presents some interesting features because of its miscibility with
several polymers. Moreover it can be synthesized in different architectures with a well defined number of
functional end-groups suitable for subsequent reactions.66
Regarding the choice of inorganic components, numerous academic studies report the use of nano-building
blocks (NBBs). The resulting hybrid materials are very interesting model compounds to establish an accurate
relation between the microstructure of the hybrid nanocomposites and their mechanical properties. We have
reviewed Si, Ti, Zr oxocluster-based hybrid materials which exhibit improved mechanical properties as
compared to the neat polymers. We expect further improvements soon in the literature. Generally, the glass
transition temperature (Tg) is efficiently increased upon copolymerization with the associated oxo-cluster
monomer. Romo-Uribe et al.63 have shown, using rheological measurements, that an increasing proportion of
POSS also affects the polymer dynamic and a high rubbery plateau is observed at elevated concentrations of
POSS. The tensile modulus of POSS-reinforced polyurethanes is also greatly increased by introducing such
pendant inorganic POSS.67
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