Double phase separation induced by polymerization in ternary blends of epoxies with polystyrene and poly(methyl methacrylate)

Ternary blends based on a stoichiometric mixture of diglycidyl ether of bis phenol A (DGEBA) and 4,4'-diaminodiphenyl sulfone (DDS), monodisperse polys tyrene (PS, 8.3 x 10(4) g/mol), and monodisperse poly(methyl methacrylate) (PMMA, 8.86 x 10(4) g/mol) were polymerized at constant temperature. Blends containing a total amount of 8 wt % of both thermoplastics (with different PS/PMMA ratios), were initially homogeneous at 135 degreesC. When they wer e polymerized at this temperature, a double phase separation was clearly ob served by light transmission. At conversions close to 0.08, the system beca me opalescent due to the generation of a PS-rich phase; at conversions clos e to 0.27, a new phase separation process took place, generating a PMMA-ric h phase. TEM (transmission electron microscopy) observations confirmed the existence of three different phases: the epoxy matrix, PS-rich particles wi th a broad size distribution, and PMMA-rich particles. Some of the PS parti cles appeared encapsulated by the PMMA-rich phase (core-shell particles). T he double phase separation process was analyzed using a Flory-Huggins model . Experimental results could be explained by following the evolution of pha se diagrams with conversion. The initial composition was located in the reg ion of homogeneous solutions. As conversion increased, the system entered t he equilibrium region between two phases (first phase separation process), and at higher conversions it attained the region where three phases coexist at equilibrium (second phase separation process). Depending on the PS/PMMA ratio, the model predicted the possibility of a phase inversion following the first phase separation process. This was also inferred from measurement s of the evolution of the complex viscosity during polymerization.