Polypropylene nanocomposites containing silica nanospheres based on the sol-gel methods were produced via
Metallocene-based catalysts offer versatility and flexibility for the synthesis of polyolefins with control of their structure [
The nature of the support and the method used for supporting the metallocene influence the catalytic activity, the final properties of the polymer, and especially the polymer particle morphology and molecular weight distribution [
The incorporation on nanoparticles in the polymerization reactions has the advantage that the nanoparticles can be distributed in the polymer generating a polymer nanocomposite by
It is well known that nanotechnology is widely considered as a major area that will undergo great technological progress in the future. In materials category as polymer nanocomposites, it is regarded as a radical alternative to conventional filled polymers or polymer blends, and new classes of materials will be available with improved properties.
Our group have prepared PE/silica nanocomposites by
Although different aspects of the preparation of PP nanocomposites by
The metallocene catalyst, rac-ethylenebis(indenyl) zirconium (rac-Et(Ind)2ZrCl2) (Aldrich) was used as the propene polymerization catalyst, and MAO (Witco) as cocatalyst. Propylene was deoxygenated and dried by passage through columns of Cu catalyst (BASF) and activated molecular sieves (13X), respectively. All manipulations were carried out in an inert nitrogen atmosphere using standard Schlenk techniques.
Silica nanospheres (SN) were synthesized using the sol-gel method as reported previously [
The nanoparticles were calcined for 4 h at 450°C in order to control the OH groups on the silica surface. One gram of nanospheres was placed in contact with 4 wt% MAO (0.88 mL) and then contacted with metallocene (rac-Et(Ind)2ZrCl2) in toluene for 3 h at 60°C. This method has been reported previously [
Polymerization reactions were performed in a slurry system using a 1 L Büchi glass reactor. Toluene, MAO, and propene were added to the reactor, followed by the addition of metallocene catalysts. For each experiment 1.2 × 10−5 mol of metallocene catalyst and 10.4 mL of MAO ([Al]/[Zr] = 1000) were used. The final volume of the solution in the reactor was 500 mL. The polymerization reaction was carried out at 60°C and 2 bar propene for 30 min, while stirring at 1000 rpm.
The polymerization was terminated by the addition of acidified methanol (10% HCl, 20 mL). The polypropylene product was recovered by filtration, washed, and then dried at room temperature. Catalytic activity was expressed as the mass of polypropylene (PP) produced per unit time per mol of Zr and per-unit pressure (kg·mol−1·bar−1·h−1).
Polymerization in the presence of SN was carried out using the same reaction conditions as those used in the homogeneous polymerization (neat). The polymerization was performed by first adding toluene, then MAO solution, and the SN (dispersed in toluene) into the reactor and mixing for 2 min. The SN were precontacted with MAO in the reactor to decrease the population of catalyst-deactivating OH groups on the silica surface. The catalyst solution was then added and mixed for 2 min and finally saturated with propene. 2 wt% loads of nanospheres were used.
Metallocene catalyst supported on MAO-treated silica nanospheres (Cat/MAO/SN) was used for heterogeneous polymerizations. It was used under the same conditions and according to the procedure described above. Heterogeneous polymerizations were carried out by first adding toluene and 10.4 mL MAO to the reactor, followed by the silica nanosphere-supported catalyst (Cat/MAO/SN), mixing for 5 min, and finally saturating with propene for 30 minutes.
Al and Zr content in the supported catalyst was measured by Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) on a Perkin Elmer P-400 instrument.
The viscosimeter molecular weight (Mv) was calculated from intrinsic viscosities determined in decahydronaphthalene (decaline) at 135°C using a Viscosimatic-Sofica viscometer. The Mv values were obtained from the Mark-Kuhn-Houwink equations.
The melting temperature and enthalpy of fusion of the neat and nanocomposite PP samples were measured by differential scanning calorimetry (DSC) on a TA Instruments DSC 2920. Percent crystallinity of the polymer was calculated using the enthalpy of fusion of an ideal polypropylene having 100% crystallinity (207 J/g) as reference [
Tacticity was determined by 13C nuclear magnetic resonance spectra (13CNMR) recorded on a Varian Inova 300 instrument operating at 75 MHz.
Polymer apparent density was measurement in a test tube by measuring a given volume of the polymer and then weighing it.
Polymer particle morphology was examined by scanning electron microscopy (SEM) on a Tesla BS 343A SEM instrument. The dispersion of the silica nanospheres in the composites was analyzed by TEM on a JEOL 1011 microscope operated at 100 Kv.
The SiO2 nanoparticles synthesized by the sol-gel method presented spherical morphology with diameters of ca. 80–100 nm observed by TEM, as reported in previous work [
The PP properties obtained by the homogeneous system (pure PP), routes 1 and 2, are shown in Table
Results of propene polymerization in the presence of SN used as support of the metallocene catalyst.
Process | Zr (%) (fixed) | Mv (Kg·Mol −1) | Tm (°C) | Tc (°C) | SN content (%) | |||
---|---|---|---|---|---|---|---|---|
Pure PP | — | 16 | 125 | 92 | 31 | 0.25 | 92 | — |
Route 1 | — | 16 | 126 | 95 | 33 | 0.28 | 91 | 2.0 |
Route 2 Cat/MAO/SN | 0.45 | 12 | 126 | 97 | 34 | 0.30 | 91 | 2.0 |
Mv: Molecular Weight; Tm: Melting temperature,
The metallocene (rac-Et(Ind)2ZrCl2) has C2 symmetry with homotopic sites; isotactic polypropylene can be obtained. The tacticity of polypropylene was calculated by 13C NMR and the percentage of dyads (
The polymers obtained by routes 1 and 2 presented a slight increase in crystallinity and crystallization temperature compared with pure PP (Table
The melting temperature of the PP obtained by both routes is comparable with that of pure PP. The apparent density of the heterogeneous system (route 2) is slightly higher than pure PP, showing a positive effect for using nanospheres as support (route 2); it can be related with the increase of the percent crystallinity.
Figure
XRD patterns of (a) Pure PP and (b) PP obtained by route 2 (PP/Cat/MAO/SN).
Figure
SEM image of the (a) Pure PP, (b) PP obtained by route 1 and (c) PP obtained by route 2 (PP/Cat/MAO/SN).
TEM images of PE/SN nanocomposites prepared by
TEM images of (a) PP/SN nanocomposites obtained by route 1 at 1
The mechanical properties of the polymers could not be measured because the PP obtained with this metallocene catalyst has a low molecular weight and the polymer is too fragile.
PP nanocomposites were prepared by
The polypropylene obtained in the presence of nanospheres had slightly higher percent crystallinity and crystallinity temperature compared with pure PP. In general, the dispersion of the nanoparticles improved for PP obtained when the support system was used (route 2).
The authors acknowledge the financial support of CONICYT under FONDECYT Project 1100058 “Polymer nanocomposites based on sol-gel synthesized nanoparticles for enhanced barrier properties”. P. Zapata acknowledges the partial financial support under CONICYT insertion Project 79100010. Special thanks are due to Professor José Luis Arias, of the Facultad de Medicina Veterinaria of the Universidad de Chile, for assistance with the SEM observations; to Professor Griselda B. Galland for the 13C-RMN determinations; to Professor René Rojas and to Juan Benavides for assistance in the polymer synthesis.