Thermal and mechanical characterization of poly(methyl methacrylate) nanocomposites filled with TiO2 nanorods

Composites: Part B 43 (2012) 3114–3119

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Thermal and mechanical characterization of poly(methyl methacrylate) nanocomposites filled with TiO2 nanorods Niranjan Patra a,⇑, Marco Salerno a, P. Davide Cozzoli b, Alberto C. Barone a, Luca Ceseracciu a, Francesca Pignatelli a, Riccardo Carzino a, Lara Marini a, Athanassia Athanassiou a,b,c a b c

Istituto Italiano di Techologia, Via Morego 30, 16163 Genova, Italy National Nanotechnology Laboratory, CNR – Istituto di Nanoscienze, Via Arnesano, 73100 Lecce, Italy Center for Biomolecular Nanotechnologies@UniLe, Istituto Italiano di Techologia, Via Barsanti, 73010 Arnesano, Lecce, Italy

a r t i c l e

i n f o

Article history: Received 10 October 2011 Received in revised form 22 February 2012 Accepted 18 April 2012 Available online 25 April 2012 Keywords: A. Nano-structures A. Particle-reinforcement B. Thermomechanical B. Thermal properties

a b s t r a c t Thick films of nanocomposites made of poly(methyl methacrylate) matrix and colloidal anatase TiO2 nanorods fillers were prepared by solvent mixing and solution drop casting. Different concentrations of nanorods were tested in order to examine the influence of the nanoscale fillers on the composites material properties and structure. The thermal properties of the samples were investigated through thermogravimetric analysis, which showed an increase in thermal stability of the nanocomposites on increasing nanorods concentration, for the range of concentrations used. The viscoelastic properties were investigated through dynamic mechanical analysis, which showed an increase in both the storage and loss modulus on increasing nanorods concentration. The in-depth distribution of the TiO2 nanorods in the matrix was evaluated through cross-sectional transmission electron microscopy, which pointed out a uniform dispersion of mesoscale nanorods agglomerates with increasing diameter of 100–200 nm range on increasing nanorods concentration. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Over the last years hybrid nanocomposite materials became of extraordinary interest for the scientific community because of the wide range of properties that can arise from the combination of the peculiar characteristics of the employed nanoparticles (NPs) and polymeric matrices. Indeed, the size-dependent physical and chemical properties of the inorganic NPs, along with the high processability, long-term durability and tunable chemical composition and structure of the organic matrix, may result in materials with unique characteristics that cannot be achieved by the individual components alone [1]. Furthermore, when a high control of the local microstructural arrangement of the NPs in the polymer is obtained, the material properties can be tuned not only inside the range of those of the organic and inorganic constituents, but even novel properties, not fully envisioned from the properties of the single components, may appear [2]. Compared to composites filled with microsized particles [3–5], nanocomposites have showed increased mechanical and rheological properties, reduced gas permeability, enhanced thermal stability, and self-extinguishing fire retardant characteristics [6,7]. For example, a twofold enhancement of the tensile modulus and of ⇑ Corresponding author. Tel.: +39 010 71 781 756; fax: +39 010 72 03 21. E-mail address: [email protected] (N. Patra). 1359-8368/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2012.04.028

the heat distortion temperature, up to 100 °C, can be achieved for nanocomposites with as little as 2 wt.% of inorganic content [8]. Recently, biodegradable polymer based nanocomposites have also been developed [9,10], targeting applications such as cell growth and packaging. Titanium dioxide (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 [4]. Presently investigated applications include photocatalytic systems relying on controlled spatial organization of titania polymorphs [11], and light-responsive coatings with simultaneous antireflective, antibacterial, self-cleaning, and antifogging behavior [12–18]. Such nanocomposites, moreover, present in most cases improved mechanical properties, mainly in terms of elastic modulus [19–22] or creep resistance [23]. The present work investigates the thermo-mechanical properties of nanocomposites of poly(methyl methacrylate) (PMMA) matrix mixed with colloidal anatase TiO2 nanorods (NRs) prepared by solution drop casting. The thermal and mechanical properties of the nanocomposites were characterized by means of thermogravimetric analysis (TGA) and dynamic mechanical–thermal analysis (DMTA), respectively. The obtained results clearly demonstrate that the produced nanocomposites appear to be thermally more stable and exhibit increased elastic modulus compared to the pure polymeric matrix. The structural homogeneity of the

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nanocomposites, which is crucial for the enhancement of the physical properties of any composite material [24], is also investigated by transmission electron microscopy on cross sections of the samples. 2. Experimental

Table 1 Characteristic values of PMMA. Characteristics of PMMA

Values

Mol. wt (Mw) by GPC Total impurities Refractive index Transition temperature Density

120,000 62.0% N20/D1.49 Tg 105 °C 1.188 g/mL at 25 °C

2.1. Materials All chemicals were of the highest purity available and were used as received. Titanium tetraisopropoxide (TTIP, 97%), titanium tetrachloride (TiCl4, 99.999%), trimethylamine N-oxide dihydrate (TMAO, 98%), oleic acid (OLAC, 90%), 1-octadecene (ODE, 90%), oleyl amine (OLAM, 70%), and PMMA of Mw 120,000 were purchased from Sigma–Aldrich (Milan, Italy). The detailed characteristics properties of PMMA are given in Table 1. All solvents used were of analytical grade and were also purchased from Sigma–Aldrich. Used water was bi-distilled (Millipore Q). 2.2. Synthesis of colloidal TiO2 nanorods OLAC capped anatase TiO2 NRs were synthesized by a slightly modified literature protocol using a standard Schlenk line setup [25,26]. Prolate NRs with mean equatorial diameter of 3–4 nm and mean length of 25–30 nm were obtained by low temperature TMAO-catalyzed hydrolysis of TTIP [25]. In a typical synthesis, 15 mmol of TTIP dissolved in 70 g of degassed OLAC was reacted with 5 mL of an aqueous 2 M TMAO solution at 100 °C for 72 h. The TiO2 NRs were separated from their growing mixture upon 2-propanol addition, and were subsequently subjected to repeated cycles of redissolution in chloroform and precipitation with acetone to wash out surfactant residuals. Finally, optically clear NR stock solutions in chloroform were prepared to be used as the filler for the nanocomposites. As a result of the process, the NRs OLAC capping was specifically constituted by oleate anions, bonded to Ti atoms on the surface of TiO2 (R-COOA) in a bidented configuration. 2.3. Preparation of the nanocomposites Composite solutions of PMMA and TiO2 NRs were prepared by blending the chloroform solution of the polymer (200 mg/mL) with the chloroform solution containing the NRs (16 mg/mL). Various volume amounts of TiO2 NRs solution were added to a fixed volume of PMMA solution. The viscosity of the final solution was kept constant for all concentrations by adding the necessary chloroform volume. The nanocomposite samples are here mentioned as PMMAXNRs, with ‘X’ values of 2, 4, and 8, respectively, representing the weight percent concentration of the NRs in the polymer. For the DMA analysis, thick PMMA and nanocomposite films were prepared by drop casting the 2, 4, and 8 wt.% PMMA–NRs solutions onto clean Teflon sheets to prevent the sticking of the film onto the substrate. All the samples were dried in a vacuum oven at 90 °C for 15 h, in order to remove the residual chloroform. The thickness of the films was 0.10 mm, as measured by a digital micrometer (Mitutoyo, USA). 2.4. Thermal analysis TGA measurements were carried out on a Q500 TA apparatus (TA Instruments, New Castle, USA), working in N2 atmosphere between 30 °C and 500 °C, with a heating rate of 10 °C/min and a flow rate of 60 mL/min. The same samples prepared for the DMA measurements were also used for the TGA measurements. The amount of solid sample was approximately 8 mg in weight. From the TGA

traces, differential thermogravimetric (DTG) plots were calculated as the first derivative of the TGA curve. 2.5. Mechanical analysis All the measurements were performed on a Q800 dynamic mechanical–thermal analyser (TA Instruments, New Castle, USA). The thickness and width of the films was 0.10 mm and 6.40 mm, respectively. The span length of the films was always kept constant (12 mm). The tests were carried out in uniaxial tensile mode, applying a sinusoidal deformation with a frequency of 1 Hz and an amplitude of 5 lm. The temperature was ramped from 30 °C to 150 °C at a rate of 3 °C/min. This rate was maintained throughout all the test runs, so that there was a minimum temperature lag between the sample and the furnace environment. Isothermal creep tests were also carried out in tensile mode at constant temperature, with an initial static load of 0.001 N at 80 °C and then at constant stress of 0.2 MPa for 10 min. 2.6. TEM analysis The samples for TEM imaging were prepared by transversally cutting the nanocomposites thick films by means of a EM UC6 ultramicrotome (Leica Mikrosystems, Wetzlar, Germany). A clean diamond knife with cutting edge of 45 reaction generating a radical was used to obtain sample cross-sections of 70 nm thickness at ambient temperature of 25 °C, which subsequently were placed on a 150-mesh carbon coated copper grid. The TEM imaging was performed with a JEM 1011 instrument (JEOL, Japan), operated at an accelerating voltage of 100 kV. 3. Results and discussion 3.1. Thermal behavior In Fig. 1 the results of the thermal analysis under inert N2 atmosphere for both bare PMMA and PMMA–NRs nanocomposites are reported. In case of pure PMMA film (black curve) first the removal of adsorbed water occurs, up to 110 °C, which is hardly visible in figure due to the compressed vertical scale. From Tonset  110 °C up to Tend1  213 °C a first mass loss step is observed. This step is assigned to removal of residual solvent from the casted solution, originally trapped inside the film during its drying out. In support of the origin of the first mass loss step, an independent measurement performed on PMMA powder is also reported as a reference (gray curve). In this case the step at lowest temperature range does not appear, as the sample does not contain any solvent. At higher temperatures the PMMA degradation occurs, which can be separated in two major mass loss steps. Previous observations of other authors [27,28] assigned PMMA degradation to scission of weak links, whose presence is attributed to the polymerization technique, and to random chain scission. The scission of weak links should also depend on the film thickness, as a consequence of the change in the diffusion time of the volatile radicals out of the polymer film [27,28]. The first degradation step observed here

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occurs from Tend1 up to Tend2  323 °C, and can be attributed mainly to PMMA depolymerization to form MMA monomer. Any reaction generating a radical (including b-scission) is expected to degrade a large number of polymer chains by chain-transfer process with monomer formation. The second step, from Tend2 upto Tend3  455 °C, is attributed to random scissions within the polymer chain and monomer volatilization. A similar three-step process is also observed for the nanocomposite samples, colored1 curves in Fig. 1. Again, the first mass loss step is assigned to residual solvent removal, and the second and third steps are the actual PMMA degradation reactions. As expected, the final amount of residual mass observed at 500 °C is approximately proportional to the TiO2 NRs concentration, as shown on the right side of Fig. 1 after the horizontal axis break, where an expanded vertical scale is used. Actually, due to the NR surfactant the residual mass is approximately 61% of the NR concentration, as 39% is estimated to be the mass of the OLAC capping molecules removed during the process. The DTG thermograms, Fig. 2, give more evident indication of the change in both position and intensity of the maxima of mass loss rate for the different steps, DTGmax i (with i = 1 to 3). In the nanocomposites the position of the first two maxima shifts towards higher temperatures, by 25 °C for the first step and 10 °C for the second step, whereas the position of the third maximum remains approximately the same. The observed shifts towards higher temperatures on increasing NRs concentration (see also Fig. 2 inset) are qualitatively consistent with the increase in glass transition temperature presented in a previous work of our group [14]. The slight shift of the third maximum toward lower temperature (3 °C only) can be explained by the presence of OLAC surfactants, which deteriorate above 380 °C. On the other hand, the peak intensity (i.e. the maximum rate) is decreased with respect to PMMA for the first two mass loss steps, whereas it is increased for the third step. This reversed behavior of the third step with respect to the previous two is expected, since at very high temperatures the complete thermal degradation of the organic content should eventually occur. The shift towards higher temperatures of the peak position and the decrease in peak intensity of the first two mass loss steps describe an increased thermal stability in the nanocomposites, which can be partly justified by the decreasing polymer content in the samples with increasing NRs concentration. However, this is not the only reason for the observed effect, since it can be noticed that the changes do not scale with the NRs concentration. We assume that the higher thermal stability of the nanocomposites with respect to bare PMMA at temperatures below 325 °C are due to the presence of nanofillers, which might limit the motion of polymer segments, limiting the interaction of their end groups with the produced free radicals. In this way chain transfer reactions are suppressed and depolymerization is limited. Since oxide TiO2 particles are highly thermally stable, we believe that they act as a barrier to heat, preventing the nanocomposites to degrade soon, resulting in an increase in thermal stability of the nanocomposites.

3.2. Mechanical behavior DMTA analysis results are usually expressed through the dynamic modulus components, i.e. the storage modulus (E0 ), which describes the elastic response to the deformation, and the loss modulus (E00 ), which is related to the viscous response. DMTA analysis was carried out to measure the temperature dependence of 1 For interpretation of color in Figs. 1–4 and 6, the reader is referred to the web version of this article.

storage modulus (E0 ) and loss modulus (E00 ) of bare PMMA and TiO2 nanocomposites, as shown in Fig. 3. Fig. 3a and b show the variation with the temperature of the storage and loss moduli of the nanocomposites measured at a frequency of 1 Hz, for different filler concentrations. It can be seen that all the curves of the nanocomposites (colored lines) lie above the curve of the bare PMMA (black line). At a temperature of 30 °C, the storage modulus and the loss modulus increments range between 30% to 45%, and 15% to 40%, respectively, whereas in both cases the highest values are recorded for the lowest concentration (2 wt.%). These increments in moduli fall within the same order of magnitude of similar nanocomposite materials [29]. The significant improvement in both storage and loss modulus is due to the PMMA chains becoming stiffer due to the incorporation of TiO2 particles on the PMMA, restricting the chain movement. In all the nanocomposites, the 2 wt.% NRs shows the highest improvement in properties. This is also due to the fact that on increasing the NRs concentration the aggregates size is also increasing because of the mutual interaction of the nanoparticles trying to hinder themselves inside the polymer matrix. Actually, upon increasing the NRs concentration the amount of OLAC surfactant used to prevent NRs aggregation is also increasing, which is concurring in decreasing the Tg of the materials [30]. Actually, both storage and loss modulus decrease drastically at around 100 °C, which means the materials undergoes in the Tg region. Furthermore, it is clear from Fig. 3b that the height of the loss modulus peak decreases and the curve broadens with increasing TiO2 NRs content close to the glass transition region. A more detailed insight in the observed differences between different NRs concentrations can be given after considering the TEM analysis (see Section 3.3). In Fig. 3b, the peaks of loss modulus can be associated to the Tg of the materials. A little change in the peak position as a function of the particles concentration is observed for 4 and 8 wt.%, which may be due to the increase in crosslinking density. Basically, interactions between the particles and the matrix are mainly physical, while the interface does not confer mobility to the chains in the studied concentrations [31,32]. Creep compliance values are shown in Fig. 4. An improvement, i.e. lower creep compliance, of all the nanocomposites with respect to the bare PMMA is observed. In this case the improvement is roughly proportional to the TiO2 NRs concentration, which suggests that it is related mainly to the higher viscosity induced by the fillers. The reduction in compliance ranges from a factor 0.8 for 2 wt.% concentration to 0.4 for 8 wt.% in agreement with other studies made both in similar stress conditions [29,31] and in higher stress or longer time conditions [23]. 3.3. Transmission electron microscopy TEM analysis of samples cross-sections can provide important insight concerning the quality of the NRs dispersion in the PMMA matrix. Fig. 5a shows TEM images of bare TiO2 NRs and Fig. 5b–d showing cross-sections of PMMA–NRs nanocomposites samples containing different NRs concentrations. The images reported in Fig. 5b–d are representative of the morphology of the nanocomposites cross-sections. The efficiency of the TiO2 NRs in modifying the properties of the PMMA is primarily determined by the degree of its dispersion. The dark lines or spots in the micrographs are TiO2 NRs aggregates. Aggregates of TiO2 NRs appear already at the lowest NRs concentration of 2 wt.%. Nevertheless, the aggregates are uniformly distributed for all the filler concentrations, resulting in rather homogeneous nanocomposite samples at the considered length scale. The analysis of the aggregate domains is reported in Fig. 6. Both the surface density of the aggregates (‘grains’) and their mean diameter are plot (in blue and red, respectively). Single aggregate

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Fig. 1. Thermogravimetric analysis of PMMA and PMMA–TiO2 nanocomposites in N2 atmosphere.

Fig. 4. Typical creep tests of bare and loaded PMMA at 80 °C.

Fig. 2. Differential thermogravimetric analysis of PMMA and PMMA–TiO2 nanocomposites in N2 atmosphere.

domains at 2 wt.% NRs concentration (Fig. 5b) have mean diameter of 100 nm, and undergo a significant increase in size with increasing NRs concentration from 2 to 4 wt.% (mean domain diameter 170 nm in Fig. 5c), tending to a saturation at 8% concentration (diameter 200 nm in Fig. 5d). The large error bars for the aggregate diameter represent the widespread distributions in the respective aggregate populations. In the same graph, the whole histograms are also shown, in red as the respective (mean) data points. The histogram widths are at the scale with the axis values, and have all single bin width of 15 nm. From the histograms it is clear that the distribution of aggregates diameter changes from monomodal at 2% to bimodal already at 4% and even more at 8%. However, the increase in overall mean diameter is still significant. Concurrently, the density of aggregate domains is decreasing, starting from 3.4 lm2 in Fig. 5b, and changing to 1.4 lm2 in Fig. 5c and 1.3 lm2 in Fig. 5d. Obviously, both grain density

and particles diameter tend to saturation at 8%, and this trend is especially clear for the grain density, which remains constant between 4% and 8% NRs concentration. Associating this analysis with the mechanical characterization results, it can be concluded that the aggregate domains density, rather than the nanoparticles concentration, is the key parameter for the improvement of both elastic and damping properties of the nanocomposites, whereas improvements in the short term creep compliance is a bulk effect closely related to the filler concentration. The uniform mesoscale aggregate distribution resulting from the TEM images is also in agreement with the improvement in thermal stability of nanocomposites observed previously with respect to the pure polymer [33]. Actually, the uniformly distributed inorganic aggregates may prevent quick heat diffusion into the polymer matrix limiting further material degradation [34]. In a previous work it has been demonstrated that the in-depth NRs distribution is not uniform and the NRs rather tend to accumulate close to the film surface when the sample preparation is done by the spin coating technique. Instead, the use of the drop casting technique herein seems to lead to a homogeneous distribution of aggregates of NRs throughout the sample’s depth, probably due to a different dynamic of PMMA solution drying. 4. Conclusions Nanocomposites of PMMA incorporating TiO2 NRs prepared by drop casting were investigated in terms of thermal and dynamic mechanical behavior. Both the thermal stability and the elastic modulus appear to be increased in the nanocomposites, up to

Fig. 3. Typical DMTA curves of (a) storage modulus E0 and (b) loss modulus E0 0 of PMMA–TiO2 nanocomposites with different concentration as a function of the temperature.

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Fig. 5. TEM images of (a) bare TiO2 NRs (magnification 35,000X), and cross-sections of PMMA–NRs nanocomposites with different concentration: (b) 2, (c) 4, and (d) 8 wt.%, (magnification 750).

Fig. 6. Results of the aggregate domains analysis performed on the TEM images from Fig. 5. The plot shows the number of aggregates per unit area and the aggregate mean diameter along with the respective size distributions. The spline lines are just guides to the eye.

8 wt.% NRs concentration compared to the pure polymer. Electron microscopy analysis of the films cross-sections showed occurrences of uniformly distributed NRs aggregates in the polymer matrix, whose size scales approximately with the NRs concentration. The observed improvement of the thermo-mechanical properties of the materials can be ascribed to the good dispersion of these aggregates. In particular, the enhancement in storage (elastic) and loss (viscous) modulus is attributed to chains interlocking, which depends mainly on the aggregates density, i.e. on the distribution, whereas the instantaneous viscosity, measured on short time creep tests, is roughly proportional to the filler concentration. The thermal properties enhancement could arise from the limit in the motion of polymer segments due to the nanofillers. Therefore, the end groups of the polymer chains become less reactive with the free radicals suppressing the depolymerization. References [1] Fragouli D, Resta V, Pompa PP, Laera AM, Caputo G, Tapfer L, et al. Patterned structures of in situ size controlled CdS nanocrystals in a polymer matrix under UV irradiation. Nanotechnology 2009;20(15):155302–11. [2] Villafiorita-Monteleone F, Canale C, Caputo G, Cozzoli PD, Cingolani R, Fragouli D, et al. Controlled swapping of nanocomposite surface wettability by multilayer photopolymerization. Langmuir : ACS J Surface Colloid 2011;27(13):8522–9.

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