Abstract
Supercritical carbon dioxide (scCO2) treatments of a montmorillonite (MMT) intercalated with ammonium cations bearing two long hydrocarbon tails (organo-modified MMT, OMMT) led to OMMT exfoliation, with loss of the long-range order in the packing of the hydrocarbon tails and maintenance of the long-range order in the clay layers. The intercalated and the derived exfoliated OMMT have been deeply characterized, mainly by X-ray diffraction analyses. Monolithic composite aerogels, with large amounts of both intercalated and exfoliated OMMT and including the nanoporous-crystalline δ form of syndiotactic polystyrene (s-PS), have been prepared, by scCO2 extractions of s-PS-based gels. Also for high OMMT content, the gel and aerogel preparation procedures occur without re-aggregation of the exfoliated clay, which is instead observed for other kinds of polymer processing. Aerogels with the exfoliated OMMT have more even dispersion of the clay layers, higher elastic modulus and larger surface area than aerogels with the intercalated OMMT. Extremely light materials with relevant transport properties could be prepared. Moreover, s-PS-based aerogels with exfoliated OMMT could be helpful for the handling of exfoliated clay minerals.
Highlights
Over the last decades, clay polymer nanocomposites (CPN) (LeBaron et al, 1999; Alexandre and Dubois, 2000; Ray and Okamoto, 2003; Bergaya, 2008; Chen et al, 2008; Galimberti, 2011) have been steadily increasing their importance in the field of material science, as they substantially improve polymer properties such as mechanical reinforcement, impermeability, thermal stability (Paul and Robeson, 2008; Bergaya and Lagaly, 2013)
Besides many 00l reflections that indicate a high degree of order perpendicular to the clay layers and an interlayer spacing of 3.5 nm, the pattern shows well defined weak peaks, corresponding to the typical 020, 210, and 060 in-plane MMT periodicities (Galimberti et al, 2007; Powder diffraction database 70-2151 International Centre for Diffraction Data, 1999)
It is worth adding that a well defined narrow peak is present at 2θ = 21.7◦, corresponding to d = 0.41 nm, i.e., the distance between long hydrocarbon chains in their rotator order, analogous to those observed for longchain alkylammoniums intercalated in anionic clays (Itoh et al, 2003) as well as in graphite oxide (Mauro et al, 2013) The thickness of the clay layer (≈1 nm) and the length of the alkylammoniums (≈5 nm) (Osman et al, 2002) indicate that the tilting angle of the hydrocarbon chains is not far from α = 60◦
Summary
Clay polymer nanocomposites (CPN) (LeBaron et al, 1999; Alexandre and Dubois, 2000; Ray and Okamoto, 2003; Bergaya, 2008; Chen et al, 2008; Galimberti, 2011) have been steadily increasing their importance in the field of material science, as they substantially improve polymer properties such as mechanical reinforcement, impermeability, thermal stability (Paul and Robeson, 2008; Bergaya and Lagaly, 2013). Many reports show that different processing techniques based on supercritical carbon dioxide (scCO2) constitute effective ways to increase dispersion and delamination in polymer/clay nanocomposites (Ma et al, 2007; Nguyen and Baird, 2007; Treece and Oberhauser, 2007; Samaniuk et al, 2009; Baker et al, 2011; Feng-hua et al, 2011; Chen et al, 2012). As a consequence of preparation of polymer nanocomposites, the 00l reflections reappear with peak height and location essentially independent of the processing conditions (Manitiu et al, 2008)
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