Organoclays with hexagonal rotator order for the paraffinic chains of the compensating cation. Implications on the structure of clay polymer nanocomposites

Abstract Organically modified clays (OC) were prepared by reacting montmorillonite (Mt) and dimethyl ditallow ammonium chloride (2HTCl), optionally adding either stearic acid (SA) or 2-stearamidoethyl stearate (SAES). Reactions were performed either at the solid state, in the absence of any other chemical substance, or with the help of solvents such as water and alcohol, or in poly(1,4-cis-isoprene) as the reaction medium. Characterization was performed by means of TGA, XRD, DSC, IR analyses. Two types of OC were identified, with 3.6–4.0 nm and about 6 nm as d 001 basal spacing, independently of the synthetic route. The 3.6–4 nm value could be explained with the bilayer placement of the tilted paraffinic chains of the ammonium cation. The 6 nm value could arise from the perpendicular placement of the paraffinic chains, associated with the presence in the interlayer space of either the acid or the amide. For the first time, it is documented the occurrence of hexagonal rotator order in the packing of the long hydrocarbon tails of the 2HT compensating cation. SA and, in particular, SAES were found to enhance this type of order. A third type of OC, with 2.5 nm as d 001 basal spacing was obtained through solvent extraction of the above mentioned OC. Results reported in this work show that the crystalline structure of an OC depends on type and amount of low molecular mass substances present in the interlayer space: compensating cations and guests such as an acid and/or an amide. For a given compensating cation, the interlayer distance can be varied by changing the amount of the cation and by adding a guest. The Mt/2HT organoclay studied in this work reveals 2.5, 3.6–4.0 and 6.0 nm as the interlayer distances of the minima of energy. Polymer chains are not involved in the OC crystalline structure, even if the OC synthesis is performed in the polymer matrix. This work strongly suggests that the best explanation for most interlayer spacing variation observed for clays in polymer nanocomposites is the different placement of intercalated low molecular mass substances, rather than the generally assumed polymer chain intercalation. Moreover, it presents the synthesis of OC at the solid state as a versatile method for preparing a large variety of clay polymer nanocomposites.