In-Situ Generation of a Dispersion of POSS Crystalline Platelets in an Epoxy Matrix Induced by Polymerization

The dispersion of intercalated/exfoliated clays in polymers imparts some desired properties to the neat matrix, such as a decrease in permeability due to geometrical effects and an increase in the fire resistance due to the inorganic character of the clay. However, processing is difficult mainly due to the high viscosities of the starting dispersions. In this article we explored the possibility of producing a dispersion of crystalline platelets in situ during polymerization, starting from homogeneous solutions. For this purpose, we replaced the clays with polyhedral oligomeric silsesquioxanes (POSS) because they can be dissolved in adequate polymer precursors and can be phase-separated in the course of polymerization. The aim was to find conditions where a crystal liquid (C-L) phase separation could take place instead of a conventional L-L phase separation. The in-situ generation of POSS crystalline platelets can impart similar characteristics to those observed in clay-modified polymers (except for the nanoscopic size of thickness), with the advantage of a much easier processing. The selected formulation was based on glycidyloxypropyl-heptaisobutyl POSS dissolved in a stoichiometric mixture of diglycidyl ether of bisphenol A (DGEBA) and 4,4'-methylenebis(2,6-diethylaniline) (MDEA). In a specific range of POSS concentrations and polymerization temperatures, a C-L phase separation was observed generating POSS crystalline platelets with sizes in the range of the micrometers. Following this primary phase separation, a dispersion of POSS-rich droplets was produced when the residual liquid phase entered the L-L immiscibility region. The final material exhibited a dual dispersion of POSS platelets and spherical POSS-rich domains uniformly dispersed in the matrix. A thermodynamic model enabled to provide an explanation of the experimental observations.