Thermal and Mechanical Characterization of PMMA ...

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increase in surface roughness, and no major effects on the top film features. Introduction ... publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net.
Advanced Materials Research Vol. 67 (2009) pp 209-214 online at http://www.scientific.net © (2009) Trans Tech Publications, Switzerland Online available since 2009/Apr/01

Thermal and Mechanical Characterization of PMMA-TiO2 Nanocomposites Niranjan Patra1,2,a, Alberto C. Barone2,b, Marco Salerno2,c, Gianvito Caputo3,d, Davide Cozzoli3,e, Athanassia Athanassiou2,3,f 1

University of Genova, viale Causa 13, I-16145 Genova, Italy.

2

Italian Institute of Technology, via Morego 30, I-16163 Genova, Italy.

3

National Nanotechnology Laboratory of CNR-INFM, via per Arnesano 16, I-73100 Lecce, Italy. a

[email protected], [email protected], [email protected],

d

[email protected], [email protected], [email protected]

Keywords: TiO2, PMMA, Nanocomposites, AFM, Nanoindentation

Abstract. Hybrid composite films made of an organic polymeric matrix and an inorganic nanosized filler, namely titania prolate nanoparticles, have been prepared and characterized both thermally and mechanically. The filler content has been varied, while still being kept in the regime of unsaturated, homogeneous nanocomposites. On increasing, the filler content there is an abrupt increase in hardness at an intermediate load, while the elastic modulus increases almost linearly. The glass transition temperature is also increased, with a tendency to saturation. The morphological characterization of the films confirms a lack of phase separation, with only a continuous, slight increase in surface roughness, and no major effects on the top film features. Introduction Composites of glassy polymers and functional nanoparticles of different oxides - i.e. nanocomposites - are important materials for nanoelectronics and miniaturized electromechanical devices, which are also increasingly used in various bio-technology applications such as lab-on-chip. The efficacy of nanoimprint lithography techniques applied on thin films of these nanocomposites greatly depends on process steps that involve compression, elongation and viscoelastic properties of the materials that are used. Therefore, both the nanomechanical and thermal behaviours of these nanocomposites materials are of critical importance. In this work, hybrid organic–inorganic nanocomposites made of polymethyl methacrylate (PMMA) and TiO2 nanorods (NRs) have prepared by solvent spin coating. The top surface morphology of the nanocomposites has been characterized by atomic force microscopy (AFM), whereas the thermal and mechanical effects of the nanocomposites have been investigated by thermogravimetric/differential thermal analysis (TGA/DTA), differential scanning calorimetry (DSC) and nanoindentation respectively. Colloidal Synthesis of TiO2 Rs Materials. All chemicals were of the highest purity available and were used as received. Titanium tetraisopropoxide (TTIP, 97%), titanium tetrachloride (TiCl4, 99.999%), trimethylamine -oxide dihydrate (TMAO, 98%), oleic acid (OLAC, 90%), 1-octadecene (ODE, 90%), oleyl amine (OLAM, 70%), polymethylmethacrylate (PMMA, =120.000 D) were purchased from Sigma-Aldrich (Milan, Italy). All solvents used were of analytical grade and were also purchased from Sigma-Aldrich. Water was bi-distilled (Millipore Q). Preparation of the Rs. All synthesis were carried out under air-free conditions using a standard Schlenk line setup. Organic-capped anatase TiO2 NRs were synthesized by modified literature protocols [1, 2]. NRs with an average diameter of 3-4 nm and a mean length of 25-30 nm were obtained by low-temperature TMAO-catalyzed hydrolysis of TTIP [1]. In a typical synthesis, 15 mmols of TTIP was dissolved in 70 g of degassed OLAC, and the resulting solution was then reacted with 5 mL of an aqueous 2M TMAO solution at 100°C for 72 h. The TiO2 NRs were All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 194.244.27.162-01/04/09,12:11:02)

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Nanomaterials and Devices: Processing and Applications

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 NRs stock solutions in toluene were prepared as the filler for the nanocomposites. The effective resulting content of TiO2 NRs core as well as of NRs capping layer shell was determined by TGA, for which the sample is prepared by precipitating the TiO2 stock solutions in methanol and centrifuging the solution for 10 mins. The samples were then dried under vacuum for 2 h to remove any remaining solvent. Finally a brownish color residual precipitate was obtained. Preparation of the TiO2-PMMA nanocomposites. Nanocomposite films with TiO2 NRs dispersed in the polymer were fabricated by adding powdered PMMA to toluene solutions of NRs with different NRs content. The mixture was stirred until complete dissolution of the polymer in the solvent. The nanocomposite thin films for AFM and nanoindentation measurements were prepared by spin coating the NRs–PMMA mixture onto cleaned glass substrates at 1000 rpm for 60 s, (using a Sawatec SM-180-BT spinner, Germany). The relative mass concentration Φ of TiO2 NRs to PMMA in the spin coating solution was varied from 5 to 30%. However, the total mass concentration of the solutions with different Φ was always maintained constant, in order to minimize the change in viscosity and thus in thickness of the film. In fact, all the films had thickness ~0.5 µm, as measured by a profilometer (XP-2, AMBIOS Technology, USA), which is sufficiently high to provide reliable nanoindentation measurements. Characterization techniques Thermal analysis. TGA was carried out on a Mettler-Toledo TGA/DSC 1 instrument, working between 30 to 600oC with a heating rate of 10oC/min in air atmosphere (flow rate 50 mL/min). This instrument allows simultaneous TGA and DTA (differential thermal analysis) measurements. DSC was carried out on a Pyris Diamond DSC (Perkin-Elmer make, Model SII) from 25 to 200oC with a heating rate of 10oC/min in a nitrogen atmosphere (flow rate 10 mL/min). The sample weights were 7-8 mg in all the DSC experiments. The DSC instrument was calibrated using indium and zinc as the standard materials. Morphological characterization. The top surface morphology of the nanocomposites films was investigated with a MFP-3D AFM (Asylum Research, USA). The instrument was operated in ambient air in Tapping (i.e. intermittent contact) mode, using standard gold coated silicon probes (NSG10, NT-MDT, Russia) with a resonant frequency of ~250 kHz. Tapping mode was chosen instead of e.g. contact mode as the former is known to allow for the lowest degree of surface modification during interaction due to imaging, mainly thanks to the suppression of the lateral shear forces between probe tip and sample occurring on constant contact [3]. On the other hand contact mode operation was not required as all the mechanical properties of interest were measured by the complementary nanoindentation setup. anoindentation. Experiments were performed by using a NanoTest™ equipment (Micro Materials Ltd., UK). The instrument is equipped with a Berkovich pyramidal indenter, with a tip curvature radius ~50 nm. The equipment was calibrated by iterative cycles of indentations into synthetic fused silica (with loads ranging from 0.5 to 200 mN) in order to obtain an accurate indenter area function and to correct for the instrument compliance [4,5]. The experiments were performed in a cabinet controlled temperature of 25°C, in mechanical and electrical low-noise conditions, (signal vibration level RMS