Improvement of thermal stability of poly(methyl methacrylate) by incorporation of colloidal TiO2 nanorods

Polymer Degradation and Stability 96 (2011) 1377e1381

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Improvement of thermal stability of poly(methyl methacrylate) by incorporation of colloidal TiO2 nanorods Niranjan Patra a, b, *, Marco Salerno a, M. Malerba a, P. Davide Cozzoli c, Athanassia Athanassiou c, d a

Italian Institute of Technology, via Morego 30, 16163 Genova, Italy University of Genova, Viale Causa 13, 16145 Genova, Italy c National Nanotechnology Laboratory (NNL), CNR e Istituto di Nanoscienze, via per Arnesano, 73100 Lecce, Italy d Center for Biomolecular Nanotechnologies (CBN) of IIT@UniLe, via Barsanti 1, 73010 Arnesano, Lecce, Italy b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 December 2010 Received in revised form 23 February 2011 Accepted 27 March 2011 Available online 5 April 2011

The thermal degradation behaviour of oleic acid-capped colloidal anatase TiO2 nanorods, poly(methyl methacrylate), and their nanocomposites has been studied. Thermogravimetric and differential thermal analysis have been carried out in nitrogen atmosphere for both nanorods, and nanocomposites with nanorod loading from 5 to 30 wt% relative to the polymer. Our study shows that the degradation of the oleic acid-capped nanorods in nitrogen is mainly endothermic and occurs in two steps. The thermal stability of the nanocomposites is improved on increasing the filler loading in the considered range, as the nanorods prevent rapid heat diffusion and limit further degradation. This effect seems to be favoured by the nanorods increased mobility, leading to enhanced dispersion in the matrix upon heating the samples during the thermal analysis. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Poly(methyl methacrylate) Colloidal TiO2 nanorods Oleic acid Nanocomposites TGA Degradation

1. Introduction Polymeric nanocomposite materials have been intensively studied in recent years, with the expectation that they will serve as a means of achieving properties that cannot be realized with single materials [1e3]. Nanocomposites of glassy polymers and functional nanoparticles of different oxides are important materials for nanoelectronics and miniaturized electromechanical devices, which are also increasingly being used in various biotechnology applications such as lab-on-chip. Among the unique properties arising in these materials as a consequence of their nanometre scale fillers are the high gas barrier performance, excellent solvent resistance, high flame resistance and high optical transparency. Poly(methyl methacrylate) (PMMA) is a thermoplastic polymer commonly employed as the main component of positive resists for both electron and UV photo-lithography [4], as well as an imprintable material for hot embossing soft lithography, optical fibres, disks and lenses [5]. Due to its excellent transparency in the visible region

* Corresponding author. Italian Institute of Technology, via Morego 30, 16163 Genova, Italy. Tel.: þ39 (0) 10 71 781 756; fax: þ39 (0) 10 72 03 21. E-mail address: [email protected] (N. Patra). 0141-3910/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2011.03.020

of the electromagnetic spectrum, PMMA is widely used in optical applications, especially as a matrix for non-linear optical composite materials. Other important applications are in the field of microelectronics, food packaging, medicine, dentistry and cosmetics [6]. On the other hand, nanostructured TiO2 stands out among transition metal oxides since it offers a variety of appealing physicalechemical properties that coexist with the potential for lowcost and environmentally benign processes. Apart from its traditional use in cosmetics and ceramics, nanoscale TiO2 has stimulated both fundamental investigations and practical applications in fields as diverse as optoelectronics, photovoltaics, catalysis, ambient detoxification, sensing, fuel cells, batteries, and hydrogen storage/production [7]. Presently, advanced fabrication strategies aim to extend the potential of TiO2-based materials to realize photocatalytic systems with controlled spatial organization of selected titania polymorphs [8], and light responsive coatings with antireflective, antibacterial, self-cleaning, and antifogging properties, or a combination thereof [9,10]. In this work, hybrid organiceinorganic nanocomposites made of PMMA and oleic acid (OLAC) capped colloidal TiO2 nanorods (NRs) have been prepared by blending in toluene. The thermal degradation in nitrogen atmosphere of both bare NRs and their nanocomposites with PMMA have been investigated by thermogravimetric analysis

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(TGA) combined with differential thermal analysis (DTA). These experiments complement our previous study of similar samples carried out in air [11]. 2. Experimental 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 SigmaeAldrich (Milan, Italy). All solvents used were of analytical grade and were also purchased from SigmaeAldrich. 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 [12,13]. Prolate NRs with average equatorial diameter of 3e4 nm and average length of 25e30 nm were obtained by low temperature TMAO-catalyzed hydrolysis of TTIP [12]. 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 toluene and precipitation with acetone to wash out surfactant residuals. Finally, optically clear NR stock solutions in toluene were prepared to be used as the filler precursors 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 (ReCOOe) in a bidentate configuration. NR samples to be investigated by TGA were prepared by precipitating the NRs from their stock solutions upon methanol addition and centrifuging the resulting flocculated suspension for 10 min at 3000 rpm. The samples were then dried under vacuum for 2 h to remove any remaining solvent. Finally a brownish precipitate was obtained. 2.3. Preparation of the nanocomposites PMMA nanocomposites based on anatase TiO2 colloidal NRs were prepared by blending the polymer and the NRs in toluene as the solvent. Varying volume amounts of TiO2 NR stock solutions were added to a fixed volume of a PMMA solution. The bare PMMA sample is coded as PMMA, whereas the PMMA/NR nanocomposites samples are coded as PMMAXNRs, with ‘X’ values of 5, 10, 20 and 30, respectively, representing the weight percent concentration relative to the fixed amount of PMMA in the starting solution (100% initially). Therefore, the above numbers report the relative wt% concentration of NRs/PMMA, defined as F ¼ mNR/mPMMA. This is different from the concentration relative to the total mass in the solid samples, defined as C ¼ mNR/(mPMMA þ mNR), which turns out to be C ¼ 4.8, 9.1, 16.7, and 23.1%, for the above F values, respectively. For each sample approximately 20 mL of nanocomposite solution was dispensed by micropipette into a small platinum pan and left to dry in ambient air. After 3 days the filled pan was used for the TGA experiment. Films were also spin coated from the same solutions onto glass substrates for morphological analysis of similar nanocomposites surface by means of atomic force microscopy. The respective results are available as supporting information. For the Transmission Electron Microscopy (TEM) investigation the samples were prepared starting from the nanocomposite solutions of

10 wt% TiO2 in PMMA. The nanocomposite solutions were drop casted onto clean Teflon sheets to prevent strong adhesion of the nanocomposite film to the substrate. The films were treated in a vacuum oven at 100  C for 15 h. The obtained films had a thickness of w0.10 mm, measured by a digital micrometre (Mitutoyo USA). Then, the nanocomposite samples were prepared for TEM analysis using an ultramicrotome with a Leica Ultracut UC6 (Leica Mikrosysteme GmbH, Austria). A clean diamond knife with cutting edge of 45 was used to obtain cross-sections of w70 nm thickness-specimens, at ambient temperature 25  C placed on a 150-mesh carbon coated copper grid. 2.4. Thermal analysis TGA measurements were carried out on a Mettler-Toledo TGA/ DSC instrument, working in N2 atmosphere between 30 and 500  C, with a heating rate of 10  C/min and a flow rate of 20 mL/min. From the TGA traces, differential thermogravimetric (DTG) plots were calculated as the first derivative of the TGA curve. The TGA instrument allowed also for simultaneous DTA, obtained by recording the temperature difference between the sample and an inert standard reference material (a-alumina) operated under identical conditions. 2.5. TEM analysis The nanocomposites samples for TEM analysis were prepared using an ultramicrotome with a Leica Ultracut UC6 (Leica Mikrosysteme GmbH, Austria). A clean diamond knife with cutting edge of 45 was used to obtain cross-sections of w70 nm thicknessspecimens at ambient temperature 25  C placed on a 150-mesh carbon coated copper grid. The distribution of TiO2 NRs particles into the polymer matrix was studied using a transmission electron microscope (model JEM 1011, JEOL, Japan) operated at an accelerating voltage of 100 kV. 3. Results and discussion 3.1. Thermal analysis of pure TiO2 nanorods Fig. 1(a) shows the TGA/DTG analysis of the precipitated solid TiO2 NRs, which allows to evaluate their thermal stability and composition (NRs surfactant, impurities etc.). The TGA measures the weight changes (weight loss) in the material while the material is heated at a constant rate, and the quantity represented in the DTG thermograms is a measure of the mass loss rate. Therefore, the DTG trace highlights the temperatures at which the weight loss rate was maximum (DTGmax is the temperature at the minima of the curve, as the mass change is negative). The horizontal tangent points in the DTG trace are instead the inflexion points (first derivative zero) of the TGA curve. At these points the loss rate is steady at a given value (zero initially). Therefore, the corresponding temperatures shown in the TGA curve of Fig. 1(a) can be identified as the limit points of subsequent degradation steps, namely Tonset firstly, when the first mass loss starts, and Tend at later steps, where the beginning of each new step is also assumed as the end point of the previous step. One can see that after evaporation of the physically adsorbed water and residual solvent content (from Tonset w 54  C up to Tend1 w 175  C, corresponding to a 2.2% final mass loss), only one major degradation step occurs (from Tend1 up to Tend2w483  C), which can be assigned to degradation of the OLAC surfactant [14]. No other degradation steps associated with removal of contaminants are observed, indicating a high purity of the filler material. In particular, combustion of the amorphous carbon between 472  C and 525  C observed in previous measurements carried out in air

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Fig. 1. Thermal analysis of precipitated solid TiO2 NRs in N2 atmosphere: (a) TGA (black line) and its derivative trace DTG (red line); (b) DTA. Details see text. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

atmosphere [11] does not appear in Fig. 1(a). Therefore, the degradation of the TiO2 NRs-capping molecules is dominant in N2 atmosphere, and the total relative amount of NRs oxide left at the end of the highest temperature degradation step (Tend2) is higher than in air, resulting in a residual mass of w77% instead of the previously observed w70% [11]. In Fig. 1(b) the DTA curve of solid TiO2 NRs in N2 atmosphere is presented. Three maxima are observed, all corresponding to endothermic reactions, at T1 ¼ 105  C, T2 w 265  C and T3 ¼ 436  C, respectively. The first reaction corresponds to moisture and solvent removal, the second reaction corresponds to OLAC chain fragmentation, and the last and dominant reaction corresponds to complete OLAC removal. The intermediate reaction around T2 is actually a convolution of different peaks, as shown in the inset, which could be due to the detachment of OLAC surfactant from the TiO2 backbone and to the fragmentation of its chains. Our previous measurements carried out in air atmosphere had pointed out reactions occurring above 340  C assigned to decomposition of OLAC and of impurities, whereas a peak at w485  C was assigned to combustion of the resulting amorphous carbon [11]. In the present case of inert N2 atmosphere, the combustion peak of Ref. [11] is obviously missing, and in the absence of major contamination both the multiple peaks around T2 and the sharp dominant peak at T3 in Fig. 1(b) should be assigned to successive steps of OLAC decomposition. Probably in contaminant free N2 atmosphere the OLAC chain is first decomposed into smaller subunits, without an actual loss of mass appearing around T2 in Fig. 1(a). This reaction occurs more easily than the reactions observed in air, since it is characterized by significantly lower temperature and peak intensity. On the contrary, most of the adsorbed heat is spent in decomposing these subunits at a later stage (T3 peak).

from Tend2 up to Tend3 w 455  C, and is due to random scissions within the polymer chain [15,16]. A similar three-steps process is also observed in Fig. 2(a) for the nanocomposite samples (colour curves in the web version). However, the respective peculiar temperatures (Tonset and Tend1,2,3, not pointed out on the colour curves of Fig. 2(a) for the sake of clarity) are all shifted to higher values with respect to PMMA (black curve), showing higher thermal stability compared to the bare polymer. From Fig. 2(a) it is also clear that the residual material obtained at 500  C increases with increasing TiO2 NRs loading F, as expected. Fig. 2(b) shows the DTG thermograms of both PMMA and PMMA nanocomposites. Similarly to Fig. 2(a), with respect to bare PMMA the nanocomposites exhibit much lower mass loss rate occurring at the second peak (DTGmax2), whereas it is finally higher at the third peak (DTGmax3). The mass loss rate close to 400  C is higher for the nanocomposite samples compared to the pure polymer possibly because around this temperature complete OLAC removal also occurs, as previously shown in Fig. 1. On the other hand, this could

3.2. Thermal analysis of PMMA/TiO2 nanocomposites In Fig. 2 the results of the thermal analysis under N2 atmosphere for both bare PMMA and PMMA nanocomposites are reported. Fig. 2(a) shows the TGA thermograms. For pure PMMA (black curve) an initial water content and residual solvent removal first occurs below w100  C, which is hardly visible in the figure due to the compressed vertical scale. At higher temperatures the material degradation occurs, which can be separated in three major steps, in agreement with the literature. The first step proceeds from Tonset w 110  C up to Tend1 w 213  C, and is due to the scissions of head-to-head linkages (HeH); the second step proceeds from Tend1 up to Tend2 w 323  C, and is due to scissions at unsaturated chain ends (resulting from termination by disproportionation) involving a homolytic scission b to the vinyl group; the third step proceeds

Fig. 2. Thermal analysis of PMMA/TiO2 nanocomposites in N2 atmosphere: (a) TGA and (b) corresponding DTG.

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be a hint that the degradation of the PMMA molecular backbone is dominant in the nanocomposites compared to the bare PMMA. The improvement in thermal stability of PMMA on introduction of the NRs is partly due to the inorganic material preventing quick and full diffusion of the heat into the polymer matrix alone, which limits material degradation [17] providing a so-called barrier effect. The efficiency of this effect of the NRs may be associated with the lack of major NRs aggregates and to a rather homogeneous NRs distribution in the matrix, as also observed on the surface of spincoated films by atomic force microscopy imaging (see additional information). In fact, it is known that the in-depth NRs distribution is not uniform and the NRs tend to accumulate close to the film surface [10]. However, heating the samples above the glass transition temperature during the thermal analysis increases the NRs mobility and leads to more homogeneous distribution of the NRs in the matrix with respect to the distribution originally occurring in the solid sample upon drying. Indeed, cross-section TEM measurements of TiO2ePMMA nanocomposites after treatment above glass transition temperature (around 100  C) for 15 h show a distribution of the NRs throughout the bulk of the sample (Fig. 3). For these reasons the presented results clearly demonstrate an apparently improved thermal stability of the nanocomposite samples. Additionally, part of the improvement in the thermal stability of the nanocomposite with respect to the bare polymer may arise from the increased escape, and the associated loss of efficiency, of the thermally generated radicals. In fact, the radicals meeting the inert nanorods are ineffective in promoting matrix depolymerization along the part of their path taken by the nanorods volume [Pielichowski]. Fig. 4 shows the DTA curve of pure PMMA and PMMA/TiO2 nanocomposites in N2 atmosphere. By inspecting this curve we can

Fig. 4. DTA analysis of pure PMMA and PMMA/TiO2 nanocomposites in N2 atmosphere.

see that three reactions occur on increasing temperature at T1, T2 and T3. The first two reactions are endothermic, whereas the last one is exothermic. The first reaction is observed for PMMA at T1 w 280  C, whereas on increasing the filler loading F in the nanocomposites T1 it is progressively shifted to higher temperatures, from 289 up to 304  C. Concurrently, the reaction peak is also decreased in intensity. The second reaction is at T2 ¼ 383  C for PMMA and is approximately constant both in position (T2 ¼ 383e388  C)and in intensity for all the nanocomposites. Finally, a third reaction clearly occurs at T3 w 437  C for PMMA. For the nanocomposites this reaction occurs around the same temperature as for PMMA but fluctuates in intensity between a sharp peak (for F ¼ 20) and wide bands (for F ¼ 10 and 30). This reaction is probably due to degradation of the residuals organic moieties. The above findings show that the thermal stability of the nanocomposites is increased on increasing the NRs loading F for the presented NRs concentrations and when the NRs are well dispersed in the matrix. 4. Conclusions In this work the thermal degradation of PMMA nanocomposites filled with OLAC capped TiO2 nanorods up to w30% wt relative filler to matrix loading is presented and discussed. Our study points out that the mass loss rate in pure nitrogen can be modelled as threestep degradation, with a dominant effect of the highest temperature step for the nanocomposites. When the nanorods loading is increased from 5 to 30%, the first degradation step is progressively shifted to higher temperature and decreased in intensity. Therefore, an increasing thermal stability appears in the nanocomposites as compared to the bare polymer. This improvement is probably enhanced by the homogeneous redistribution of the nanorods in the matrix on heating the samples during the thermal analysis with respect to their original solid form. Consequently, to obtain a similar improved thermal stability in solid nanocomposites at room temperature, a homogeneous nanorods distribution should be achieved. This could make the investigated nanocomposites suitable materials for thermally resistive coatings or other applications of PMMA that require increased thermal stability of the polymer without causing major material degradation. Acknowledgements

Fig. 3. TEM image of (a) pure TiO2 NRs (b) Cross-section TEM of 10% wt. TiO2ePMMA nanocomposite after treatment above glass transition temperature for 15 h.

Dr. Pierpaolo Pustianaz (Mettler-Toledo, Milan) is gratefully acknowledged for his kind support in doing the TGA measurements.

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Appendix. Supplementary information [9]

Supplementary information associated with this article can be found in the online version, at doi:10.1016/j.polymdegradstab.2011. 03.020.

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