MOLECULAR MODELING OF VERMICULITE/POLYETHYLENE NANOCOMPOSITE

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1 MOLECULAR MODELING OF VERMICULITE/POLYETHYLENE NANOCOMPOSITE Jonáš TOKARSKÝ a,b, Marta VALÁŠKOVÁ a,b a NANOTECHNOLOGY CENTRE, VŠB - TECHNICAL UNIVERSITY OF OSTRAVA, 17. listopadu 15, Ostrava - Poruba, Czech Republic, jonas.tokarsky@vsb.cz b IT4INNOVATIONS CENTRE OF EXCELLENCE, VŠB - TECHNICAL UNIVERSITY OF OSTRAVA, 17. listopadu 15, Ostrava - Poruba, Czech Republic. Abstract The finding that an addition of small amount of vermiculite (VMT) into polyethylene (PE) matrix (without any additives) leads to the material exhibiting antibacterial poperties was the motivation for the preparation of VMT/PE nanocomposite. X-ray diffraction analysis together with molecular modeling using Universal force field as implemented in Materials Studio modeling environment allows us to study the nanocomposite structure, even in the interlayer space of VMT. Three sets of models containing PE chains of various lengths on VMT(001) and VMT(100) surfaces and in the VMT interlayer space have been prepared and optimized. For the characterization of optimized models, an attention has been paid to the three following structural and energy parameters: (1) position of PE chains against the VMT layers, (2) change of VMT basal spacing, and (3) total potential energy of the VMT/PE model. Keywords: Polyethylene, vermiculite, molecular modeling, structural parameters 1. INTRODUCTION Clay/polymer nanocomposites, prepared by an addition of a clay mineral filler (having at least one nanodimension, i.e. the thickness of clay layers) to a polymer matrix, belong to the large group termed hybrid organic inorganic composite nanomaterials [1,2]. These nanocomposites exhibit new and/or improved mechanical properties, thermal stability and suppression of flammability, that are not observed in pure compounds [3, 4, 5]. The high aspect ratio of clay nanoparticles is very suitable for the modification of polymer properties but the hydrophilicity of clays does not allow the homogeneous dispersion in the polymer matrix. Therefore, many authors described the usage of compatibilizer ensuring good incorporation of polymeric chains between the clay particles. For the clay mineral vermiculite (VMT) the maleic anhydride is often used in order to incorporate the VMT into polypropylene [6], polyamide [7], or polyethylene [8]. It was also found that in case of polyethylene the polar additives may induce the dipole moments in the polymeric chains [9, 10, 11]. In 2006 Shao et al. [12] showed how to add the VMT into polypropylene matrix without any compatibilizer using solid-state shear compounding. Present work is a part of large study focused on VMT/polyethylene (VMT/PE) nanocomposite, its preparation without compatibilizer, characterization and utilization as an antibacterial nanomaterial. Here we present the results of characterization using the computer molecular modeling, energy dispersive X-ray fluorescence spectroscopy, and X-ray diffraction analysis. 2. EXPERIMENT AND STRATEGY OF MOLECULAR MODELING 2.1 Preparation and characterization of real samples The additive free industrial Low-Density PE Bralen VA was obtained from Slovnaft (Slovak Republic). Brazilian VMT was obtained from Grena a.s., Czech Republic, and the ball milled fraction was used as clay

2 filler to the polymer matrix. VMT/PE nanocomposite was prepared from the mixture (93.0 wt.% of PE and 7.0 wt.% of VMT filler) blended in the Brabender kneading chamber (Brabender GmbH & Co. KG, ) at 160 C for 10 min in two velocity intervals (10 rpm for 2 min and subsequently 50 rpm for 8 min). The energy dispersive X-ray fluorescence spectroscopy (XRF) (SPECTRO XEPOS) was used in order to determine the elemental composition of VMT. The X-ray diffraction (XRD) patterns were measured using the powder X-ray diffractometer INEL equipped with a curved position-sensitive detector CPSD 120 (Gemonochromator, CuKα1 radiation). Microstructure of the samples was observed using atomic force microscope SOLVER NEXT (NT-MDT Co., Russia) equipped with contact probes CSG10 (NT-MDT Co.). Nanostructural arrangement of the PE chains near the VER was investigated by molecular modeling using an empirical force field in Accelrys Materials Studio modeling environment (Accelrys, Inc., USA). 2.2 Preparation of initial models Model of VMT unit cell was prepared using the data published by Shirozu and Bailey [13], i.e. a= nm, b= nm, and c= nm, and the structural formula calculated from the elemental analysis: (Na0.16K0.36Ca0.13) (Si6.16Al1.84) (Al0.08Fe Fe Mg5.02Ti0.12) O20 (OH)4. This allowed us to build the crystal supercell 10a 3b 1c with the crystallochemical formula (Si185Al55) (Al2Fe 3+ 22Fe 2+ 1Mg151Ti4) O600 (OH)120. The corresponding layer charge 23 el. was compensated by 5Na +, 10K + and 4Ca 2+ interlayer cations. Three following VMT initial models were built: (1) VMT interlayer (denoted as VMT-i), (2) VMT surface (denoted as VMT-s) and (3) VMT edge (denoted as VMT-e). Models of PE chains were prepared according to the data published by Keller [14]. The models of VMT/PE were detoned as VMT-i/XPEY, VMT-s/ XPEY and VMT-e/XPEY, where X and Y represent the number of PE chains and the number of carbon atoms in the chain, respectively. In addition, these models contained water molecules (see Table 1). The VMT and PE charges were calculated using the QEq (charge equilibration) [15] and Gasteiger [16] methods, respectively. Both the geometry optimization and the energy calculation were carried out in Accelrys Materials Studio modeling environment (Forcite module). The Smart algorithm was used for the geometry optimization with iterations. Values of convergence criteria were: kcal for the energy and nm for the displacement. All atoms were parameterized using Universal force field (UFF) [17]. Table 1. Interlayer distances and total potential energies (Etot) per one atom for VMT-i models. models of VMT-i VMT-i / 1PE60 VMT-i / 2PE30 number of H 2O molecules interlayer distance (nm) (E tot per one atom (kj)) (-28.66) (-28.66) (-27.47) (-27.47) (-26.51) (-26.51) (-24.84) (-24.60) (-24.12) (-24.12) (-20.54) (-20.54) 3. RESULTS AND DISCUSSION Fig. 1a showing XRD patterns of VMT and VMT/PE revealed the interstratified layered structure with various content of water in the interlayer space. Three main interlayer distances can be easily distinguished by the basal reflections with the values d002 = 1.24 nm, d002 = 1.43 nm, and d001 = 2.30 nm. In case of VMT/PE the formation of one layer of water molecules in the interlayer space of VMT can be seen (new reflection corresponding to interlayer distance 1.04 nm) [18]. In the small angles area of VMT/PE XRD pattern one can find several new reflections (corresponding to interlayer distances ~ 5.1 nm) demonstrating formation of mixed-layered structures in which the PE chains can be present in the VMT interlayer space. The area of PE containing both crystalline and amorphous regions can be found above angle 2θ = 16 and two significant reflections (d110 = 0.42 nm and d200 = 0.38 nm) correspond to PE orthorhombic crystal structure [19].

3 Simulated XRD pattern of the PE orthorhombic crystal structure (a= nm, b = nm, c = nm, Schoenflies space group D2H-16) also proved this fact (Fig. 1b). Fig. 1. (a) XRD patterns of VMT and VMT/PE real samples; (b) simulated XRD pattern of pure PE. Interlayer distances and the values of total potential energy (Etot) per one atom (because each model contains different number of atoms) in dependence on various numbers of water molecules and PE chains are listed in Table 1. It can be seen that the Etot of models increases with increasing number of water molecules. Therefore, the joint presence if PE and water is not energetically favorable and not probable in the real samples. Nevertheless, VMT-e/PE models revealed that PE chains have a tendency to partially enter the interlayer space as suggested by a significant decrease in Etot (Table 2). It can be also seen that PE chains partially entering the interlayer space produced the disorder of VMT layers (see interlayer distances in Table 2 and Figs. 2a,d) while free PE chains outside the VMT structure are close together. PE chains on VMT surface (Fig. 2c) exhibit similar behavior, i.e. free ends of the long PE chains vertically oriented to the VMT surface lie parallel next to each other and form thicker bundles. This arrangement is a contribution to the formation of crystalline domains of PE as observed from XRD pattern (Fig. 1a). Table 2. Interlayer distances and total potential energies (Etot) per one atom for VMT-i models. composition of models E tot per 1 atom (kcal) interlayer distance (nm) orientation of PE against VMT-e VMT-e/6PE partially entered VMT-e/6PE partially entered VMT-e/10PE horizontally VMT-e/10PE vertically VMT-e/6PE horizontally VMT-e/6PE vertically composition of models E tot per 1 atom (kcal) interlayer distance (nm) orientation of PE against VMT-s VMT-s/10PE horizontally VMT-s/10PE vertically VMT-s/10PE randomly Visual observation of the optimized models revealed that hydrogen atoms in the last sub-unit of PE chain (i.e. C=CH2, see Fig. 2c) tend to get close to OH groups in the VMT octahedral sheet. The average distance d(c=ch2 HO) was found to be about nm, the shortest observed distance (model VMTe/6PE60) was nm (Fig. 2c). These mutual PE-VMT interactions, similar to a weak carbon-donor hydrogen bond [20]. can influence the properties of composite.

4 Fig. 2. Optimized VMT/PE models. (a) VMT-e/6PE60 (partially entered); (b) VMT-s/10PE60 (vertically); (c) end of PE chain close to OH group of VMT layer; (d) VMT-e/6PE30 (partially entered). 4. CONCLUSIONS Real sample of VMT/PE nanocomposite was characterized by a combination of computer molecular modeling with XRF and XRD analyses. XRD analysis revealed increased crystallinity of PE in the VMT/PE nanocomposite and changes in the stratification of VMT layers. Molecular modeling showed that PE chains at the VMT edges enter partially into the interlayer space due to the attractive interaction between the chain ends (C=CH2) and OH groups of VMT octahedra. Entering of PE chains into the interlayer space of VMT showed by molecular modeling results and increased crystallinity of PE after melt mixing with VMT proved by XRD analysis may be a confirmation of the previously published hypothesis that the charge carrier VMT introduces dipole moments in the PE chains [10,11]. The combination of molecular modeling with XRD analysis is not used very often but present results show that this approach can be an efficient tool for the characterization of the clay/polymer nanocomposites. ACKNOWLEDGEMENT This work was supported by the Grant Agency of Czech Republic (GA P210/11/2215) and the European Regional Development Fund in the IT4Innovations Centre of Excellence project (CZ.1.05/1.1.00/ ). REFERENCES [1] RUITZ-HITZKY, E., van MEERBEEK, A. Clay mineral- and organoclay-polymer nanocomposites. In: BERGAYA, F., THENG, B.K.G., LAGALY, G. (Eds.), Handbook of Clay Science, Developments in Clay Science, vol. 1. Elsevier Ltd., Amsterdam, 2006, 1224 pages. [2] GIANNELIS, E.P. Polymer layered silicate nanocomposites. Advanced Materials, 1996, vol. 8, iss. 1, p [3] XIDAS, P.I., TRIANTAFYLLIDIS, K.S. Effect of the type of alkylammonium ion clay modifier on the structure and thermal/mechanical properties of glassy and rubbery epoxy clay nanocomposites. European Polymer Journal, 2010, vol. 46, iss. 3, p

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