Complex Phase Separation In Thermoplastics Modified Epoxy Systems

Viscoelastic phase separation and double phase separation process were choosed to study the complex phase separation processes in thermoplastics modified epoxy systems. With different hardeners, polyesterimide, polyetherimide, polyethersulfone and their blend modified epoxy systems were used to study the relationship between thermoplastics and their complex phase separation behaviors. The development of morphologies, activate energy of curing reaction and the relationship between rheological behavior and phase separation were followed by Optical Microscopy (OM), Scanning Electron Microscopy (SEM), Time-Resolved Light Scattering (TRLS) and rheolometry.The differences in dynamic asymmetry, or compactions between the characteristic rheological time and characteristic deformation time in the phase separation process of polyethersulfone modified systems, were caused by the diffusion/flow ability of epoxy monomer and growing-epoxy due to the cure mechanism: chainwise and step wise polymerization. Dynamic asymmetry in chainwise system caused viscoelastic phase separation while dynamic equiluim in stepwise system with low thermoplastic concentration caused double phase separation at the early stage of phase separation.A structure evolution process from phase inversion to bicontinuous structure was observed in the viscoelastic phase separation by opitical microscopy. While in double phase separation, the high hydrodynamic flow due to interface motion causes the geometrical coarsening too fast for diffusion to follow, thus it will bring the system out of equilibrium and leads to secondary phase separation.Light scattering results with final phase inversion morphology show a typical exponential decay procedure of scattering vector qm and light intensity Im. The temperature-dependent relaxation time r was thus obtained.The values of relaxation time r had been fitted with Williams-Landel-Ferry equation separately. It was found that the reference temperature Ts obtained from the fitting, about 50K higher above Tg of epoxy-anhydride blend, consisted with each other. It demonstrates experimentally that the coarsening processes of epoxy droplets and the final morphologies obtained in these thermoplastic-epoxy systems are affected by viscoelastic behavior and the viscoelastic behavior could be attributed to the escape movement of epoxy monomer or growing epoxy chain from the PEI-rich phase.The rheological behavior during phase separation corresponded well with the morphology evolution. The viscosity increase at the beginning of phase separation was caused by the formation of thermoplastic-rich continuous phase structure. And the physical gelation, which corresponds to the crosspoint of modulus G' and G", was resulted from the vitrification of thermoplastic-rich phase.Viscoelastic phase separation process was demonstrated by the transformation of phase inversion to bicontinuous structure due to the earlier chemical gelation of epoxy resin cured with imidazole. And systems cured with imidazole could obtain homogenous structure due to the fast gelation before phase separation.