CHARACTERIZATION OF HIGH DENSITY POLYETHYLENE/ORGANOCLAY NANOCOMPOSITE BY X-RAY DIFFRACTION AND LOW FIELD NMR Tathiane C. Rodrigues 1 *, Maria I. B. Tavares 1, Victor J. R. R. Pita 1, Igor L. Soares 1 and Ana Moreira 2 1 Instituto de Macromoléculas Professora Eloisa Mano Universidade Federal do Rio de Janeiro (IMA/UFRJ). Centro de Tecnologia, Bloco J, Ilha do Fundão. PO Box: 68525, 21945-970, Rio de Janeiro, RJ, Brazil 2 Rio Polímeros S/A Rua Marumbi 1001 Duque de Caxias, RJ, Brazil tathiquimcr@uol.com.br The purpose of this study was preparing polymer/organoclay nanocomposite based on high-density polyethylene and organically modified montmorillonite by melt processing using twin-screw extruder at different processing parameters (60 and 90 rpm). The x-ray diffraction (XRD) was employed to characterize the formation of the nanocomposite and organoclay dispersions. A new technique was applied by low-field nuclear relaxation study using the proton spin-lattice relaxation time (T 1 H) to understand changes in the molecular mobility after processing, when organoclay was incorporated on polymer matrix. From the T 1 H results, it was observed that samples presented different molecular domains after processing and after clay dispersion on polymer matrix. The XRD characterization of organoclay dispersion on polyethylene matrix indicated that an effective surface layer and delamination at nanoscale level could be obtained from different processing parameters applied. Introduction The technology of polymer nanocomposites is described as one of the most important frontier of materials science since sensible improvements in mechanical, thermal, dimensional, and barrier properties can be obtained with minimum amounts of filler [1-3]. The properties of a nanocomposite are strongly dependent on the final morphology of the material, which depends on the dispersion of the nano particles in the polymer matrix. Polymer/organoclay composites can be intercalated, but the best nanocomposite properties only would be obtained from completely exfoliated systems [4-6], because the increase in the interfacial contact results in a large improvement of the properties. Melt processing is the most appropriate technique for the industrial preparation of polymer-layered silicate nanocomposites of thermoplastic polymers [7]. The success of this way is related with the identification of optimal melt processing conditions; which should be able to promote the intercalation and/or exfoliation of organic layered silicate. In this case, the final level of filler dispersion could characterize the effect of processing condition on the material produced. Interest in polyolefin nanocomposites has emerged due to their promise of improved performance in packaging and engineering applications [8]. Consequently, polyethylene (PE) is one
of the most widely employed polyolefin and high-density polyethylene (HDPE) represents an important class of polymer commodities investigated [9]. The main techniques used for the characterization of these materials are x-ray diffraction (XRD) and transmission electron microscopy (TEM). So, the knowledge of the molecular structure and dynamic of these compounds become very important in order to obtain the response of the materials behavior. In this context, the Nuclear Magnetic Resonance (NMR) spectroscopy is one of the most important tools employed to study changes in the structural and dynamical behavior in the polymer chain and nanocomposite material [10-14]. The purpose of this work was to prepare high density polyethylene/organoclay nanocomposite obtained by polymer melt compounding from twin screw extruder (TS), at different processing parameters. The XRD was employed to characterize the formation of the nanocomposite and organoclay dispersions. A new technique using low field NMR was applied to understand changes in the molecular mobility of polyethylene (polymer matrix) when organoclay was incorporated. Experimental Samples One commercial grade of high-density polyethylene sample was used: EI-60070 (MFI=7.0g/min,190 /2.16kg), provided by RioPol S/A. The commercial montmorillonite organoclay was purchased by Bentec S/A. Both polymer and organoclay were used as received. Nanocomposite preparation After a premix, melt compounding was performed using a torque rheometer, Rheocord 9000, Haake, equipped with twin screw extruder; conical intermeshing counter rotate twin screw, at different processing parameters (60 and 90 rpm) and characteristic temperature profile. The dry blends were prepared with a content of 5wt % organoclay. After processing, the composites were manufactured with a hot and cold press to obtain films for characterization. Characterization of the materials The extent of organoclay intercalation and/or exfoliation was determinate by XRD analysis. The films were characterized using a Shimadzu XRD 6000 diffractometer with nickel filtered CuKα (λ=1,54 Å) radiation operated at 40 KV and 30 ma. The data were recorded at 2θ rates of 2 per
minute. The Bragg equation [15], λ = 2 d sinθ, was applied to obtain the organoclay basal spacing, d, from pure organoclay (OMMT) and into the composites. The analyses of spin-lattice relaxation time, T 1, using a Low field NMR were made in a Resonance Maran Ultra at 23 MHz for the hydrogen nucleus, using the inversion recovery pulse sequence (180 - τ - 90 ). The conditions used to measure this parameter were: temperature 27ºC, with a range of τ varying from 0.1 to 5.000 ms, with a recycle delay 5s, using 20 data point and 4 scans. Results and Discussions Figures 1 show the XRD pattern for the OMMT and the EI-60070 composites made from it at different processing parameters. From the XRD results it was observed that the organoclay was incorporated into the polyethylene matrices, at distinct levels of OMMT delamination. The results indicated that intercalated and exfoliated nanocomposite system could be obtained from only one processing parameter. The OMMT pattern shows an intense (001) peak at around 2θ-3.2º, corresponding to a basal spacing of 27.6 Å. As shown in the Table 1, the specific shifts of the (001) peaks position, to lower angles, was observed for the EI-60070/OMMT compounding, at 60 rpm, indicating nanocomposite formation. When 90 rpm was applied, the basal spacing of organoclay was very similar to the pure organoclay. But, the efficiency of organoclay delamination process not should be attributed only to the processing parameters effects. The increase of the montmorillonite basal spacing could be affected by the structural and mobility characteristics of polyethylene resin. The hydrogen relaxation data (T 1 ) for the samples, measured by Low field NMR is shown in Table 2. The T 1 results showed that the samples presented different molecular domains. Analyzing the polyethylene (resin and after processing), two values of relaxation parameter were found and they were attributed to mobile region (low value) and rigid region (high value) that seems be responsible for the controlling of relaxation process. For the polyethylene/organoclay nanocomposite four values were detected. These values were attributed to the mobile region, interface region (amorphous region) and two crystalline regions, because of the clay dispersion in the polymer matrix. One crystalline region is from the polymer far from the clay lamellae and the other one is for the crystalline region of the polymer matrix.
Table 1: The basal spacing of organoclay and polyethylene/organoclay nanocomposites Sample Rotations (rpm) Extruder Type XRD 2θ d-spacing (Å) OMMT - - 3.2 27.6 EI-60070/OMMT 60 TS 2.80 31.5 EI-60070/OMMT 90 TS 3.1 28.5 Table 2. The low field NMR relaxation data of polyethylenes (resin and after processing) and polyethylene/organoclay Sample T 1 H (ms) Assignments Domains EI-60070 (Polyethylene resin) EI-60070 (Polyethylene resin after processing) EI-60070 (Polyethylene resin after processing with organoclay) 26 Mobile Region 334 Rigid Region 18 Mobile Region 331 Rigid Region 12 Mobile Region 174 Interface Region (amorphous region) 337 Cristalline region (near to clay lamellae) 367 Cristalline region (polymer matrix) 2 2 4 25000 20000 OMMT EI-60070/OMMT 60 rpm EI-60070/OMMT 90 rpm Intensity (cps) 15000 10000 5000 0 0 5 10 15 20 25 30 35 2θ Figure 1: XRD patterns of OMMT and EI-60070/OMMT composites after processing with SS and TS extruder, at 60 rpm. Conclusions The XRD characterization suggested that intercalation and/or exfoliation of the silicate layers could be obtained for twin-screw extruder processing condition in the presence of
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