Poly(styrene sulfonate) layered double hydroxide nanocomposites. Stability and subsequent structural transformation with changes in temperature

Abstract

Poly(styrene sulfonate)–Zn₂Al(OH)_6·nH₂O (PSS–LDH) layered double hydroxide nanocomposites have been prepared by the polymer templating method or by an exchange reaction followed by in situ polymerization of the monomers. The two well-defined samples were characterized by a combination of techniques and the polymerization of styrene sulfonate evidenced by ¹³C CP-MAS experiments. The nanocomposites were further characterized by in situ XRD measurements at different temperatures. The basal spacing differs in the two samples; it is stable up to 450 °C for the material synthesized by the polymer templating method, whereas a large contraction is observed for the other sample. Grafting reactions were not observed during thermal treatment. The pristine chlorine LDH material first underwent a contraction attributed to the loss of intracrystalline water molecules followed by an expansion before the breakdown of the lamellar framework at 200 °C. The microstructural transformations and the nature of the by-products are independent of the atmosphere. In contrast, drastic changes occur in the nanocomposites. A ZnS-like phase is evidenced by EDX analysis and an EXAFS study at the sulfur K-edge after treatment at 600 °C under a nitrogen atmosphere; ZnS was observed to crystallize at higher temperatures. For the templating assisted nanocomposite, an intermediate phase Zn₃O(SO₄)₂ was observed in air. The organic moiety is found to delay the crystallization of the metal oxides, ZnO (zincite) and ZnAl₂O₄ (gahnite); treatment under N₂ atmosphere also shows this trend.