Development of Polymer Nanocomposites Based on Layered Double Hydroxides

Polymeric nanocomposites are commonly considered as systems composed of a polymeric matrix and - usually inorganic - filler. The types of nanofillers are indicated in Fig. 1. Beside well-known layered silicate fillers, recent attention is attracted to layered double hydroxide fillers (LDH), mainly of synthetic origin. The structure of LDH is based on brucite, or magnesium hydroxide, Mg(OH)(2), and is illustrated in Fig. 2. The modification of LDHs is commonly done by organic anions, to increase the original interlayer distance and to improve the organophilicity of the filler, keeping in mind their final application as fillers for, usually hydrophobic, polymer matrices. We have used the modified rehydration procedure for preparing organically modified LDH. The stoichiometric quantities of Ca3Al2O6, CaO and benzoic (B) (or undecenoic (U)) acid were mixed with water and some acetone. After long and vigorous shaking, the precipitated fillers were washed, dried and characterized. X- ray diffraction method (XRD) has shown the increase of the original interlayer distance for unmodified LDH (OH-- saturated) of 0.76 nm to the 1.6 nm in LDH- B or LDH- U fillers (Fig. 3). Infrared spectroscopy method (FTIR) has confirmed the incorporation of benzoic anion within the filler layers (Fig. 4). For the preparation of LDH- B and LDH- U composites with polystyrene (PS), poly(methyl methacrylate) (PMMA) and copolymer (SMMA) matrices, a two- step in situ bulk radical polymerization was selected (Table 1 for recipes, azobisisobutyronitrile as initiator), using conventional stirred tank reactor in the first step, and heated mold with the movable wall (Fig. 6) in the second step of polymerization. All the prepared composites with LDH- U fillers were macroscopically phase- separated, as was the PMMA/ LDH- B composite. PS/ LDH- B and SMMA/ LDH- B samples were found to be transparent and were further examined for deduction of their structure (Fig. 5) and thermal properties. FTIR measurements showed that there is some filler present in the nanocomposites (Fig. 7). XRD measurements pointed to the disturbance of the characteristic layered structure of the filler in the obtained composites (Fig. 8). Transmission electron microscopy (TEM) images showed that the filler was not homogeneously dispersed within the matrix (Fig. 9). However, the dispersion was quite good, and a high degree of exfoliation was obtained for PS/ LDH- B composites (Fig. 10); the predominantly intercalated structure was found for SMMA/ LDH- B composites (Fig. 11). Thus, in both cases nanocomposites were prepared. The thermal characterization by the differential scanning calorimetry (DSC) showed the increase of glass transition point of 10 degrees C for PS/ LDH- B nanocomposite with intermediate (w = 2.5 or 5.0 %) filler content (in comparison with neat PS), a feature that is characteristic for exfoliated nanocomposites. No such increase was obtained for SMMA/ LDH- B nanocomposites. The thermal degradation in the inert nitrogen atmosphere was studied by thermal gravimetric analysis (TGA) method. The improvement of thermal stability of PS/ LDH- B in comparison with neat PS was found only for the nanocomposites with intermediate (w = 2.5 or 5.0 %) filler content (Fig. 12), again proving the exfoliated structure. The half- weight loss temperature of SMMA/ LDH- B nanocomposites continuously increases with the increase of filler content (Fig. 13), a feature that is characteristic for intercalated nanocomposites. In conclusion, the described methods were found satisfactory for preparing the exfoliated nanocomposites of LDH- B and PS. New organic modifiers are to be sought, if exfoliated nanocomposites of SMMA and PMMA matrices are to be prepared. Further investigation will include the deduction of mechanical properties of prepared materials.