Viscoelastic effects in thermoplastic poly(styrene-acrylonitrile)-modified epoxy-DDM system during reaction induced phase separation

The viscoelastic phase separation of a poly(styrene-co-acrylonitrile) (SAN) modified epoxy system based on the diglycidyl ether of bisphenol A (DGEBA) cured with 4,4'-diaminodiphenylmethane (DDM) has been monitored in situ using rheometry, optical microscopy (OM) and small angle laser light scattering (SALLS). The amount of SAN in the epoxy blends were 3.6, 6.9, 10, and 12.9 wt%. The relationship between rheological properties and phase separation was carefully explored. The evolution of storage modulus, loss modulus, and tan delta were found to be closely related to the evolution of complex phase separation. From the rheological profile, two gel points are identified, corresponding to physical gelation and chemical gelation, the first one because of viscoelastic phase separation and the second one related to crosslinking of the epoxy resin, these depend on the cure temperature and amount of thermoplastic. Further SALLS investigations investigated the mechanism of phase separation. The time-dependent peak scattering vector was simulated with a Maxwell-type viscoelastic relaxation equation. Relaxation times obtained at different temperatures for the blends could be described by the Williams-Landel-Ferry equation. Moreover, the development of light scattering profile follows the Tanaka model of viscoelastic phase separation.