Synthesis and characterisation of thermoset-layered silicate nanocomposites

Abstract: Thermoset-layered silicate nanocomposites are materials reinforced on a molecular scale by 1 nm-thick silicate layers of high aspect ratio dispersed in a thermoset matrix. These new materials were synthesised from unsaturated polyester and several epoxy resins (DGEBA/diamines and TGDDM/DDS) using two different approaches. Unsaturated polyester-clay nanocomposites were synthesised by treating clay with silanes and mixing it with the resin prior to curing. In this case, the clay was partially delaminated. Epoxy-layered silicate nanocomposites were synthesised by swelling organophilic layered silicates in epoxy with subsequent polymerisation. Layered silicates were rendered organophilic by means of interlayer cation exchange of sodium cations for surface modifiers such as protonated amines and dihydroimidazolines. The nature of the layered silicate, the choice of the curing agent and the nature of the surface modifier showed significant effect on the resulting morphology of nanocomposites, which is mainly governed by cure kinetics and diffusion mechanisms. The cation-exchange capacity of the layered silicate possibly influenced a self-polymerisation of the epoxy resin between the silicate layers induced by alkylammonium ions. The reactivity and the diffusion rate of the curing agent had a direct influence on the balance between the polymerisation occurring between the layers (intragallery polymerisation) and the polymerisation occurring outside (extragallery polymerisation). Curing temperature permitted to adjust this balance in order to separate silicate layers in the matrix. The nature of the surface modifier had also an important influence on the morphology of epoxy-layered silicate nanocomposites. Polar surface modifiers such as protonated polyetheramines prevented the corresponding organosilicates from swelling in the epoxy. In contrast, the presence of OH groups in the molecular structure of surface modifiers favoured the separation of the silicate layers in epoxy matrix. OH groups apparently catalyse intragallery polymerisation and promote diffusion of epoxy and curing agent molecules between the silicate layers. Microscopy observations revealed that epoxy-layered silicate nanocomposites are constituted of microscale domains of parallel silicate layers separated in most cases by several nanometers. Nanocomposites synthesised with high Tg epoxies showed a systematic decrease of glass transition temperature as the organosilicate content increased. The molecular mobility of the polymer was also reduced by the presence of the silicate layers. Large stiffness improvements caused apparently by an increase of effective particle volume fraction were observed in all the nanocomposites as compared with conventionally filled composites. Fracture toughness measurements suggested that mechanisms governing fracture properties of thermoset-layered silicate nanocomposites are mainly occurring on a microscale.