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CHAPTER 3:

NANOCOMPOSITES OF POLY(METHYL METHACRYLATE) AND

BROOKITE TITANIA NANORODS

Niranjan Patra*, Mr., University of Genova and Italian Institute of Technology (IIT) - Genova -

Italy, via Morego 30 - 16163 Genova - Italy, niranjan.patra@iit.it, +39-(0)10-71.781.756, +39-

(0)10-72.03.21,

Alberto Barone, Dr, Italian Institute of Technology (IIT) - Genova - Italy, via Morego 30 - 16163

Genova - Italy, alberto.barone@iit.it, +39-(0)10-71.781.756, +39-(0)10-72.03.21,

Marco Salerno, Dr, Italian Institute of Technology (IIT) - Genova - Italy, via Morego 30 - 16163

Genova - Italy, marco.salerno@iit.it, +39-(0)10-71.781.444, +39-(0)10-72.03.21,

Gianvito Caputo, Dr, National Nanotechnology Laboratory - Lecce - Italy, via per Arnesano 16

(km 5) - 73100 Lecce - Italy, gianvito.caputo@unile.it, +39-(0)832-29.82.70, +39-(0)832-29.82.38,

Athanassia Athanassiou, Dr, National Nanotechnology Laboratory and Center for Biomolecular

Nanotechnologies of IIT - Lecce - Italy, via Barsanti 1 - 73010 Arnesano - Lecce - Italy,

athanassia.athanassiou@iit.it, +39-(0)832-29.57, +39-(0)832-29.82.38,

* Corresponding author

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ABSTRACT

A polymer nanocomposite was produced by using poly(methyl methacrylate) (PMMA) as the

matrix and crystalline Brookite TiO2 nanorods (NRs) as the filler, in a loading range between 5%

and 30% in weight. The colloidal NRs were synthesized through low-temperature hydrolysis of

titanium tetraisopropoxide, and showed a prolate shape with length-to-diameter aspect ratio

around 20. The PMMA/Brookite composites were characterized through atomic force microscopy

(AFM) and transmission electron microscopy (TEM) for the morphology, and through differential

scanning calorimetry (DSC) and nanoindentation for the thermal and mechanical properties,

respectively. It was found that, with respect to the bare PMMA, the glass transition temperature is

increased of about 10°C for all the composites, whereas the reduced modulus and hardness are

substantially increased only for 10 wt% loading. TEM analysis showed evidence of NRs

aggregation on increasing loading.

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N.1. INTRODUCTION

TiO2-based nanostructured materials have emerged in the past decades as a platform on which a

variety of appealing physical-chemical properties coexist with biocompatibility (Chen, 2007).

Presently investigated applications include photocatalytic systems relying on controlled spatial

organization of titania polymorphs (Kawahara, 2002), and light-responsive coatings with

simultaneous antireflective, antibacterial, self-cleaning, and antifogging behavior (Fujishima, 2006).

Most research work has dealt so far with the tetragonal Anatase and Rutile phases, due to the

relative ease with which these polymorphs can be attained (Chen, 2007). However, recent

investigations have highlighted that TiO2 in the orthorhombic Brookite crystal structure can exhibit

superior electrochemical (Koelsch, 2002), catalytic, and photocatalytic (Shibata, 2004)

performances.

In this work, thin film (<1 µm thickness) nanocomposites of Brookite TiO2 nanorods (NRs)

dispersed in polymethyl methacrylate (PMMA) have been prepared by solvent spin coating. The

morphology of the top film surface has been checked by atomic force microscopy (AFM), whereas

the internal NRs distribution of the films has been checked by transmission electron microscopy

(TEM). The functional thermal and mechanical properties of the nanocomposites have been

investigated by differential scanning calorimetry (DSC) and nanoindentation, respectively.

N.2. SAMPLE PREPARATION

Titanium (IV) chloride (TiCl4, 99.999%), titanium (IV) isopropoxide (Ti(OiPr)4, 99.999%), oleic

acid (C17H33CO2H, 90%), oleyl amine (C17H33NH2, 70%), trimethylamine N-oxide dihydrate

((CH3)3NO·2H2O, 98%), and 1-octadecene (C18H36, 90%), as well as all solvents, were purchased

from Aldrich, and were used as received. Water was bidistilled (Millipore Q).

The synthesis was carried out under air-free conditions using a standard Schlenk line setup, as

described in detail elsewhere (Buonsanti, 2008). After the synthesis, extraction/purification

procedures of the nanocrystals were carried out under ambient atmosphere, to remove precursor and

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surfactant/octadecene residuals. The purified nanocrystals were finally dispersed in chloroform,

providing a milky suspension due to the nanocrystals size, which was stable in time. According to

the used procedure, Brookite TiO2 NRs are prepared, which has been confirmed by X-ray

diffraction on solid samples drop-casted from the prepared solutions.

Hybrid solutions with Brookite TiO2 NRs dispersed in a liquid polymer phase were prepared by

adding solutions of NRs to PMMA, with different relative NRs-to-PMMA mass concentration

(shortly termed ‘loading’ in the following) of 5, 10, 20 and 30 wt %. The nanocomposite thin

films for AFM and nanoindentation measurements were prepared by spin-coating the NRs-PMMA

solutions onto properly cleaned glass substrates at 1000 rpm for 60 s, (using a Sawatec SM-180-BT

spinner, Germany). The thickness of the films was in all cases 0.7 µm, as measured by a

profilometer (XP-2, AMBIOS Technology, USA), which is sufficiently high to provide reliable

nanoindentation measurements.

N.3. SAMPLE CHARACTERIZATION METHODS

The top surface of the nanocomposites films was investigated with a MFP-3D AFM (Asylum

Research, USA), working in ambient air in Tapping mode with gold coated silicon probes (NSG10,

NT-MDT, Russia) with a resonant frequency of ~250 kHz.

Low-resolution TEM images were recorded with a Jeol JSM 1011 microscope operated at an

accelerating voltage of 100 kV. The samples for this analysis were prepared by spin-coating 1:10

diluted nanocomposite solution onto carbon-coated Cu grids, which allowed to obtain 200 nm

thick films, appropriate for TEM analysis. Since the goal in this analysis is to check the distribution

of NRs in the nanocomposite solution, no solution with 0% NRs loading was considered. On the

contrary, the starting solution with NRs only, without polymer matrix, was used.

DSC was carried out on a Pyris Diamond SII (Perkin-Elmer, USA), heating from 50 to 200oC

with a rate of 10oC/min in nitrogen atmosphere (flow rate 20 mL/min). The sample weights were of

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7-8 mg in all the measurements. The instrument was calibrated using Indium and Zinc as the

standard materials.

Nanoindentation experiments were performed by using a NanoTest instrument (Micro Materials

Ltd., UK) equipped with a Berkovich pyramidal indenter, with nominal radius of curvature of the

tip ~50 nm. The equipment was calibrated by iterative indentation cycles into fused silica (with

loads ranging from 0.5 to 200 mN). The indentations were performed in a cabinet with constant

temperature of 23°C, in mechanical and electrical low-noise conditions, under load-control. The

maximum load range (0.12-0.18 mN) was chosen such that the indentation size effect and the tip

effect at low loads were minimized, and the influence of the glass substrate at higher loads was

negligible. For each load, indentations were repeated ten times on different regions of the film

surface, with a 0.01 mN/s loading and unloading rate and a 60 s dwell period at peak load.

N.4. RESULTS AND DISCUSSION

After optical microscopy inspection at low magnification, higher magnification analysis of the film

top surface was performed by means of AFM. For each NRs-PMMA loading , both 10 μm and

3 μm scan size images were taken, in several regions of the films. In Fig. N.1a) and b)

representative AFM images at the higher magnification are shown.

The nanocomposites films with increasing NRs loading exhibit increasing surface roughness.

The red regions show areas where the height was above the maximum value in the color bar by the

side, taken as the height distribution mean plus two standard deviations. In Fig. N.1c) the

summarizing plot of root mean square (RMS) roughness versus loading shows a linear increase,

in the considered range. This result is qualitatively similar to the previously studied case of

Anatase TiO2 NRs (Patra, 2009). However, in the present case the roughness measured at the

highest loading =30% is between one and two orders of magnitude higher (15 nm instead of

2 nm for 3 m scan, and 100 nm instead of 3 nm for 10 m scan, respectively). In particular,

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the significantly higher increase for the 10 m scan images shows that the roughness is better

sampled in this case on such a lateral size scale. Both this large lateral scale of the roughness texture

and the high RMS values are hints of NRs aggregates forming close to the film surface.

Imaging nanocomposite thin films with TEM allowed us to better assess the possible occurrence

of NRs aggregates under the surface. The results are shown in Fig. N.2.

Fig. N.2 shows the TEM micrographs of a) bare TiO2 Brookite nanocrystals and b) PMMA-TiO2

NRs nanocomposites. In Fig. N.2a) it appears clearly that the NRs have rather high elongation.

Since the diameter is 5 nm and the average length is 100 nm, the typical aspect ratio is 20. This

is much higher than the previously investigated Anatase TiO2 NRs (Patra, 2009), which was 8,

(diameter of 3-4 nm, length of 25-30 nm). Probably as a consequence of this high aspect ratio,

aggregates are formed in the nanocomposites (Fig. N.2b)), due to the high probability of the NRs to

meet and stick to each other already from the solution phase, despite the surfactant capping. Even

higher agglomeration may occur in nanocomposites with higher loading (>5%).

DSC measurements allowed us to collect curves of heat flow versus temperature in the

nanocomposite films prepared from solutions with different , which are shown in Fig. N.3a). The

glass transition temperature Tg of the respective nanocomposites is identified by the flex point in

these curves, and has been plot in Fig. N.3b). The change in specific heat capacity at constant

pressure Cp versus the inorganic filler (Cp=0) has also been plot in Fig. N.3b), as a useful control

parameter for the experiment. This parameter should be as constant as possible, and indeed only a

minor decrease occurs for the samples with NRs loading =10%. In Fig. N.3b) it also appears that

the Tg increases suddenly for Φ=5%, and comes to an almost constant value for all the higher

loadings. This behavior is different from the previously observed Anatase NRs (Patra, 2009), for

which a rather linear increase was observed over the whole 5-30% range. It seems that a

threshold effect occurs in Brookite NRs nanocomposites, already at a loading as low as =5%. This

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is probably due to the high aspect ratio of the brookite NRs, which get easily agglomerated for

loading higher than that limit.

Indentation experiments were carried out to determine the hardness (H) and the reduced modulus

(Er) of the nanocomposite materials, whose results are shown in Fig. N.4. For the roughest films

with (see Fig. N.1c)) it was not possible to obtain clear and repeatable measurements,

probably due to the uncertain determination of the contact point in our load-penetration curves. In

Fig. N.4 the statistical deviations of the data-points have been represented with a box diagram,

where the inner square symbol and horizontal line represent the mean and the median, respectively.

The box boundaries are the lower and upper quartile, and the outer vertical bars extend to largest

and smallest values within 1.5 inter-quartile range from box boundaries, (data-points outside this

range are plotted separately as crosses). Considering the mean values, one can see that compared to

TiO2 Anatase NRs as the filler (Patra, 2009) only minor changes in both H and Er are observed in

the present case, on increasing . In particular, for H the apparent increase occurring between Φ=0

and Φ=10% is followed by a consistent drop for Φ=20%. Furthermore, the spreads of the

measurements are comparatively large, which is attributed to the increase in sample inhomogeneity

also measured as film surface roughness.

The significance of the observed differences on Φ was statistically analyzed by ANOVA. For H

only the point at Φ=10% was significantly higher than the others (at a level of significance p<0.05).

Probably for =20% the aggregated NRs have the effect of a weak point rather than a reinforcing

agent, and on pressing the film they push away the polymer and sink in without opposing much

resistance, due to the lack of an extended networking. The differences occurring on are more

marked for Er, for which ANOVA showed that all the nanocomposite data-points (=5, 10, 20%)

are significantly higher (p<0.05) than the value for bare PMMA (=0). Furthermore, again the

difference is stronger for =10%, (p<0.01). Obviously, the mechanical effects of NRs aggregation

occur for higher than the thermal effect of Tg.

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N.5. CONCLUSIONS

Thin films of nanocomposites of PMMA and Brookite nanocrystals were prepared, and the resulting

morphology, thermal and mechanical properties were characterized. It was found that these long

NRs (aspect ratio 20) tend to agglomerate even at a comparatively low loading of 5% wt, which

also reflects in a highly increased surface roughness (at least one order of magnitude higher) if

compared to similar nanocomposites made with Anatase nanocrystals (of aspect ratio 8). The glass

transition temperature increased of approximately the same amount (+10°C) for all the composite

loadings. The hardness increased significantly only at 10% loading, whereas the reduced modulus

was clearly increased at all loadings, with a maximum also at 10%.

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Figue captions

Fig. N.1. High resolution AFM topographical maps (3 μm scan size), showing nanocomposite

films with different NRs loading Φ in the starting solution of a) 5% and b) 20%. c) plots of RMS

roughness extracted from the AFM images.

Fig. N.2. TEM micrographs showing a) bare TiO2 Brookite NRs from the as synthesized colloidal

solution, and b) aggregated NRs in a nanocomposite film spin-coated on a grid from a 1:10 diluted

=5 % solution.

Fig. N.3. DSC thermal traces of nanocomposites prepared from solutions with different Brookite

NRs loading and .

Fig. N.4. Statistical box diagrams of a) hardness H and b) reduced modulus Er, measured at

different Brookite NRs loading and .

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