Influence of physical state of intercalating agents on intercalation process of high speed airflow pretreated montmorillonite in supercritical carbon dioxide

• The pretreated MMT with high speed airflow was modified with surfactants in scCO2.
• Whether the intercalation can be realized depends on the surfactant type.
• The scCO2 processing benefits the formation of more dispersed structures.
• Phosphorus salt-modified MMT presents more dispersed structure than ammonium salt.

The work aimed at studying the effect of scCO2 processing technology on basal spacing, surface morphology and thermal stability of sodium montmorillonite (MMT) that were pretreated with high speed airflow pulverization method and then modified using myristyltrimethylammonium bromide (MTAB) and tetradecyltrihexylphosphonium chloride (TDTHP) with scCO2 as the medium. X-ray diffraction (XRD) showed that physical state of intercalating agents played an important role on the intercalation process in scCO2. Solid-state MTAB could hardly intercalate into the interlayer of pretreated MMT (PMMT), though addition of co-solvent benefited the intercalation to some extent. However, liquid TDTHP could intercalate into the interlayer of PMMT easily even without co-solvent and the basal spacing of TDTHP-modified PMMT was larger than that of MTAB-modified PMMT. Scanning electron micrographic (SEM) showed the large compact structure for MMT broke into small random structures after airflow processing and some smaller tactoids and more dispersed structures can be observed for both MTAB-modified PMMT and TDTHP-modified PMMT compared to unprocessed MMT. For TDTHP–PMMT, many clay platelets that were separated from the tactoidal structure and more dispersed structure were observed, which may be helpful for MMT exfoliation and dispersion in polymers. Thermogravimetric analysis (TG) demonstrated that TDTHP-modified PMMT was up to 100 °C more stable than MTAB-modified PMMT. These results are very important and relevant to the preparation and application of MMT/polymer nanocomposites.

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