Sol–gel processing at increased temperatures: the formation of polyester nanocomposites

Abstract

Polyester nanocomposites were prepared using sol–gel precursors, prehydrolyzed sols, or nanoparticles in polyester formulations. The different inorganic components were introduced in the early stages of the esterification reaction and a typical polymerization temperature program was applied leading to temperatures up to 240 °C at low pressures. The structural and physical properties of the final materials depend on the applied method for the introduction of the sol–gel materials. Silicon atoms were incorporated into the polyester chain if silicon tetraalkoxide was used as precursor. The silicon atoms represent branching points in the polymer structure. Prehydrolyzed sols that were prepared under acidic conditions were another source of silicon and formed larger inorganic aggregates in the polymer matrix. Nanoparticles prepared via the Stober process were the third inorganic species in polyester formation. All three processing pathways produced different kinds of materials depending on the type of silica incorporated in the polyester networks but also with regard to the nanoscale structure of the materials. Both, composition and structure have a major influence on the final polyester nanocomposite properties. Model reactions between silicon tetraalkoxides and diols or diacids using the temperature program for the polyester formation showed that exchange reactions of the alkoxides and the alcohols or acids can occur and the obtained products can carry out side reactions in the polyester formation. The final materials show a homogeneous distribution of the silicon containing moieties in the polyester matrix. The viscosities and the branching degrees of the polymers changed dramatically compared to the pristine polymers by incorporation of the sol–gel precursors.