Poly(2-hydroxyethyl methacrylate)-titania hybrids prepared using γ-radiation
Rohan Holmes and Robert Burford
School of Chemical Engineering
The University of New South Wales, Sydney, Australia, 2052
r.holmes@student.unsw.edu.au
ph: 612 9385 5053
fax: 612 9385 5966
One convenient technique used for the production of hybrid materials is to prepare the
inorganic component using sol-gel chemistry. This involves the controlled hydrolysis of a
suitable precursor (titanium tetrachloride, or a titanium alkoxide), and the subsequent
polycondensation reaction of this hydrolysed species to prepare an inorganic network.
Careful control of the reaction conditions (pH, temperature, precursor concentration, and use
of reaction modifiers) allows the inorganic network to be tailored to either the formation of a
three-dimensional network (gel) or a dispersion of nanoparticles in solution (sol).
Titania-polymer hybrids are becoming more widely explored as there are new applications
including high refractive index materials[1] and non-linear optics[2]. Applying the sol-gel
process for titania production is more challenging compared with silica, for which much
scientific literature is available. This is due to the lower electronegativity of titanium and
higher susceptibility to nucleophilic attack, with sol-gel reactions for titania occurring
approximately 100,000 times faster than silica[3]. More complicated coordination chemistry
is therefore employed to slow the sol-gel reactions. Bidentate ligands have been found to be
the most useful in retarding titania sol-gel reactions; they modify the precursor to a form that
is less susceptible to the condensation reaction, by limiting the sites available for this reaction.
Combining sol-gel derived nanoparticles with polymers can be achieved in different ways.
One can simply dissolve an existing polymer and disperse the sol-gel derived nanoparticles.
However, a draw-back of this technique is the internal stresses generated during solvent
removal, which causes cracking of the hybrid. Careful drying conditions are required for the
preparation of continuous crack-free materials. For this reason, in-situ techniques where the
sol-gel reaction and the polymerisation reaction take place concurrently are often employed.
To further minimise problems with solvent removal, a technique where the sol-gel
condensation reaction and the polymerisation occur in the bulk can be applied, and this is
now described.
In biomedical applications and consumer goods, including packaging and carpet underlay,
even trace amounts of monomer need to be rigorously avoided, as this can lead to cytotoxic
effects or sick building syndrome. This aspect can be overlooked by polymer synthetic
chemists but in the opinion of the authors is of considerable importance. For this reason, a
technique that provided high-conversions was required whereas from much of the literature,
it was apparent that thermally decomposing initiators offered poor conversion. Therefore, in
the present preparation of hybrid materials of pHEMA and titania, the polymer component
has been prepared using γ-radiation. This is an important point of difference from other
published work [4, 5] which also describe the preparation pHEMA-titania hybrids, but using
chemical initiators.
We prepare hybrid materials of pHEMA and titania using an in-situ sol-gel reaction and bulk
polymerisation. Hybrids were prepared by mixing hydroxyethyl methacrylate, acetylacetone,
hydrochloric acid, water, and titanium isopropoxide. The 2-propanol leaving group from the
titanium isopropoxide was removed at reduced pressure and the remaining solution added to
glass vials. Samples were irradiated in a MDS Nordion Gammacel 220 excel 60Co γ-source for
20 hours using a dose rate of 0.49kGy/h. The titania loading was varied for the hybrid
materials up to a loading of 15wt.%. titania. The materials were characterised using
thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), transmission
electron microscopy (TEM), measurement of equilibrium swelling, Fourier transform
infrared spectroscopy (FTIR), and solid state nuclear magnetic resonance spectroscopy.
Monomer conversion was also assessed by TGA-FTIR evolved gas analysis of the materials.
Transparent orange hybrid materials were prepared (Figure 1a). It was found that careful
control of the sol-gel reaction was required to prevent the formation of large aggregates that
leads to hazy or opaque products (Figure 1b). Acetylacetone, a bidentate ligand, was found
to be essential in for good control of the sol-gel reactions. However, too much acetylacetone
led to retardation of the organic polymerisation reaction and so determining the optimum
concentration was of key importance.
a
10mm
b
10mm
Figure 1 Prepared hybrids (a) good control of the sol-gel reaction (15wt.%
titania); (b) poor control of the sol-gel reaction.
Residual volatiles in the hybrids were measured using TGA-FTIR evolved gas analysis. For
samples we had prepared using chemical initiators, it was found that diagnostic monomer
peaks were observed in the spectra, which indicated that full monomer conversion was not
achieved. For the hybrids prepared using γ-radiation, there were no diagnostic monomer
peaks observed in the spectra. This was strong evidence that good conversion was achieved.
Titania loadings were determined using thermogravimetric analysis (Figure 2). Unfilled
polymer, and hybrids with titania contents of 3, 6, 9, 12, and 15 wt.% were prepared. It was
determined that the thermal stability was higher for the hybrids than the unfilled polymer,
with the T50 temperature being 40°C higher for the 15wt.% sample compared to the unfilled
polymer. All hybrids were determined to have a T50 value approximately 30°C higher than
the unfilled polymer, and above 450°C, there is a region where the rate of mass loss is far
lower than for the rest of the hybrid. This is due to the formation of a char barrier at the
surface of the residue that impedes pyrolysis. The magnitude of this region on the TGA trace
scales with titania loading.
Figure 2 Thermogravimetric analysis traces for hybrid materials.
Although pHEMA exists as a network polymer, a small glass transition, Tg, can be measured.
For the unfilled sample a Tg of approximately 104°C was measured (Figure 3a). This was
found to be higher than the glass transition temperature that was measured for unfilled
pHEMA prepared using thermally decomposing initiators (95°C). This indicates that either
some residual monomer is still present, plasticising the polymer, or that the polymer prepared
using γ-radiation has a higher cross-link density. As titania content was increased, the
magnitude of the glass transition temperature decreased. This is observed in other hybrid
systems, where the added inorganic component limits segmental motion, leading to a
reduction in the magnitude of Tg. For the 15wt.% hybrid, the glass transition temperature is
almost completely suppressed, as indicated by an almost flat trace (Figure 3b)
Figure 3 DSC traces (a) unfilled polymer prepared using γ-radiation;
(b) 15wt.% hybrid material.
Transmission electron microscopy of thin (100nm) sections prepared via cryoultramicrotomy was used to investigate the dispersion of titania in the polymer. Overall, the
titania was found to be well dispersed in the polymer which was determined form a number
of TEMs which Figure 4a is representative. At higher magnification (figure 4b) it appears
that the titania is actually present in the form of a number of aggregates. However these are
small (<20nm) and do not interfere with the optical transparency of the hybrid. The
formation of aggregates may have a cross-linking effect on the polymer through the
formation of nano-IPN structures which limit chain mobility through entanglements of the
polymer within the aggregate.
a
100nm
b
50nm
Figure 4 TEM images of the hybrid materials (12wt.% titania)
(a) lower magnification; (b) higher magnification.
At equilibrium the unfilled pHEMA increased by approximately 60% compared with the
initial polymer weight. In comparison, the hybrids swelled by only 25%, but there was no
correlation between titania loading and weight gain. This reduction in the swelling indicates
that the chain mobility is affected in a way similar to increasing the cross-link density of the
polymer. By comparing the swelling data and DSC data, it is apparent that the addition of
titania leads to a reduction of chain mobility in the hybrids. This may be simply due to the
presence of titania disrupting the polymer matrix. However, the presence of aggregates leads
us to believe that the formation of entanglements between polymer chains and the titania is
responsible for the reduction in swelling observed.
The structure of these materials, particularly the interaction between the titania and the
polymer, has been further characterised using solid state nuclear magnetic resonance, FTIR,
and Raman spectroscopy.
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