EVALUATION OF CLAY DISPERSION IN NANOCOMPOSITES OF STYRENIC POLYMERS

Transcription

1 EVALUATION OF CLAY DISPERSION IN NANOCOMPOSITES OF STYRENIC POLYMERS D. J. Carastan 1, A. Vermogen 2, K. Masenelli-Varlot 3, N. R. Demarquette 4 * 1 Metallurgical and Materials Engineering Department Polytechnic School University of São Paulo - danilo.carastan@poli.usp.br; 2 MSE Laboratory - Cornell University, Ithaca, NY - US; MATEIS INSA-Lyon France - karine.masenelli-varlot@insa-lyon.fr; 4* Metallurgical and Materials Engineering Department Polytechnic School University of São Paulo, Av. Prof. Mello Moraes, 2463, CEP: , Cidade Universitária, São Paulo SP Brazil - nick@usp.br Nanocomposites of polystyrene and styrenic block copolymers were prepared using three different methods and their clay dispersion was evaluated using a TEM-based image analysis. The materials showed different degrees of clay dispersion, and the best results were obtained by the solution casting technique. The highest clay dispersion was, in general, obtained by the SEBS nanocomposites, followed by the SBS and PS samples. Introduction Polymer nanocomposites are one of the main subjects in study in polymer science nowadays. The objective of most studies is to obtain a good dispersion of nanoparticles within a polymeric matrix, in order to improve its properties. The most popular nanoparticles are smectitic clays, because of their natural availability and the possibility of being compatibilized with polymers through organic modification [1]. It is common to classify the morphology of polymerclay nanocomposites as either intercalated or exfoliated. In practice there is usually the coexistence of particles with different sizes and degrees of dispersion, from individually exfoliated platelets to large micron-sized aggregates. In most studies the microstructure of nanocomposites is only qualitatively described, usually based on X-Ray Diffraction (XRD) analyses and Transmission Electron Microscopy (TEM) observations. Styrenic compounds such as polystyrene (PS) and styrene-based copolymers are known to intercalate organoclays [2,3,4] and, depending on the method of preparation, exfoliated nanocomposites can be obtained [5]. In this work nanocomposites of PS and two styrenic block copolymers were prepared by three different methods (melt mixing, solution casting and a hybrid masterbatch technique)[6]. Their morphologies were analyzed qualitatively using diverse techniques (XRD, TEM) and also quantitatively by the mean of a TEM image analysis [7]. Experimental The polymers used in this study were polystyrene (N1841 from Innova), a styrene-butadiene-styrene triblock copolymer with 80 wt% of PS blocks (SBS Styrolux 648D from BASF) and a styrene-butylene/ethylene-styrene triblock copolymer with 30 wt% of PS blocks (SEBS Kraton G1652 from Kraton Polymers). The clay used, Cloisite 15A provided by Southern Clay, is a montmorillonite modified with dimethyl dihydrogenated tallow ammonium. The composites of each polymer were prepared using three different techniques: melt mixing, solution casting and a hybrid masterbatch technique [6]. The clay content in each sample was set to 5wt%. The melt mixing samples were prepared in an internal mixer (Thermo Haake s PolyLab 900 / Rheomix 600p) at 200 C and 50 rpm for 5 min. In the case of solution casting, the polymer and the clay were dispersed in toluene and cast in a bowl to evaporate the solvent. The hybrid technique consisted in preparing a 25 wt% clay masterbatch by toluene solution and mixing it with pure polymer in the internal mixer to form a composite with final clay content of 5wt%. A preliminary microstructural analysis was done by Optical Microscopy (OM), which was useful to observe large clay aggregates. Each nanocomposite was then analyzed by XRD and TEM. Following the procedure developed by Vermogen et al. [7], 15 TEM micrographs were taken for each sample at different magnifications (one at , four at and ten at ). An image analysis was performed on these micrographs, in order to evaluate the degree of dispersion of the clay particles.

2 Results and Discussion The XRD results indicated that all samples formed intercalated structures, with values of clay interlayer spacing ranging from 3.1 to 3.8 nm. TEM observations confirmed the presence of intercalated clay tactoids (Figure 1a), although in some samples exfoliated platelets could also be observed (Figure 1b). Table 1 presents parameters obtained by the image analysis. The clay particles can be divided in different size classes, according to the measured thickness from the image analysis [7]. Figure 2 shows the frequency of six different particle classes for each samples studied. a) b) Figure 1 Examples of microstructures observed by TEM in the nanocomposites of this study: a) intercalated tactoids (PS+5%clay prepared by the masterbatch technique), b) exfoliated platelets (SBS+5%clay prepared by the masterbatch technique). Table 1 Parameters obtained from the image analysis of the materials studied: µ agglo estimated fraction of micrometric agglomerates, <t> weight average nanotactoid thickness, <ε // > average distance between tactoids in the direction parallel to the tactoids plane, <ε > average distance between tactoids in the direction perpendicular to the tactoids plane. Nanocomposite µ agglo (wt%) <t> (nm) <ε // > (nm) <ε > (nm) PS melt mixing PS solution n.m.* PS masterbatch SBS melt mixing SBS solution n.m. n.m. SBS masterbatch SEBS melt mixing SEBS solution SEBS masterbatch * not measured

3

4 Figure 2 Frequency of each class of tactoids in: a) PS nanocomposites, b)sbs nanocomposites, c) SEBS nanocomposites. The results obtained from image analysis revealed some interesting features of the preparation techniques. The estimated values of the amount of micrometric agglomerates presented in Table 1 suggest that in most cases the solution technique is efficient in breaking down these agglomerates. These results corroborate OM observation, in which almost no microscopic agglomerate can be seen in optical micrographs, whereas the samples prepared by melt mixing show a considerable amount of large particles. The microstructure seen through OM of the samples prepared by the masterbatch technique was similar to the ones prepared by solution. However, the image analysis of these samples reveals a higher content of agglomerates, with exception of the SBS sample. This suggests that the masterbatch technique might be an intermediate technique between melt mixing and solution casting. From Figure 2, it can be seen that there is a higher frequency of small tactoids and exfoliated platelets in SBS and SEBS nanocomposites, showing that these copolymers tend to form exfoliated nanocomposites more easily than PS. The interparticular distances shown in Table 1 are larger for the PS nanocomposites, which is also an indication of poorer clay dispersion and more heterogeneous microstructure in these materials. The distances could not be easily measured for the PS and SBS nanocomposites prepared by solution, because it was more difficult to define the particle boundaries and the main clay orientation in these samples. The TEM micrographs show that the materials prepared by solution have a more random distribution of clay particles. This is expected, as these samples were formed statically during the solvent evaporation, whereas the samples prepared by melt mixing and the masterbatch technique were aligned by shear in the mixing chamber. Thus, the samples prepared by solution have a house of cards structure, with clay platelets forming a three-dimensional network [8]. Conclusion A TEM-based image analysis was performed on nine different nanocomposites in order to evaluate their clay dispersion. Through this technique the clay tactoids were classified in six size classes, to map the overall microstructure of the samples. In combination to other techniques, it was possible to verify that solution casting was the most efficient technique to disperse clay, followed by the masterbatch technique and melt mixing. The block copolymers, especially SEBS presented better clay dispersion than PS, indicating that the middle block would help exfoliation. Acknowledgements To Fapesp for financial support, to Kraton, BASF and Innova for materials supply. The Centre Technologique des Microstructures of the Université Lyon I (Lyon, France) is also thanked for the access to the cryoultramicrotomes.

5 References 1. M. Alexandre, P. Dubois Mat. Sci. Eng. 2000, 28, R. A. Vaia, K. D. Jandt, E. J. Kramer, E. P. Giannelis Macromolecules 1995, 28, Y.-H. Ha, E. L. Thomas Macromolecules 2002, 35, D. J. Carastan, N. R. Demarquette Int. Mater. Rev. Accepted. 5. B. Hoffmann, C. Dietrich, R. Thomann, C. Friedrich, R. Mülhaupt Macomol. Rapid Commun. 2000, 21, D. J. Carastan, N. R. Demarquette Macromol. Symp. 2006, 233, A. Vermogen, K. Masenelli-Varlot, R. Séguéla, J. Duchet-Rumeau, S. Boucard, P. Prele Macromolecules 2005, 38, J. Ren, A. S. Silva, R. Krishnamoorti Macromolecules 2000, 33, 3739.