Rheological Behaviors During Phase Separation In Thermoplastic Modified Epoxy Systems

In this work the rheological behaviors during phase separation in thermoplastic modified epoxy resin were systemically investigated. The development of phase separation were followed by optical microscopy (OM), scanning electron microscopy (SEM), transmission electronic microscopy (TEM), Synchrotron Radiation soft X-ray contact microscopy, time resolved light scattering (TRLS), differential scanning calorimeters (DSC) and rheometer. The relationship between rheological behavior and phase separation was discussed.Based on the relationship between the gel time and the onset of phase separation , there are three typical modified systems. The PES modified epoxy/anhydride system was selected to go ahead because that the onset of phase separation in this system was much earlier than gel time so that we have enough time to collect data through various kinds of test equipments.The three typical morphologies (dispersed phase, co-continuous phase, phase inversion) were obtained through changing the epoxy cure mechanism or PES content. And Optical microscopy was employed to monitor the phase separation evolution compared with the rheological behaviors. The results indicated that at the beginning of phase separation, the complex viscosity abruptly dropped if PES dispersed phase structure formed, while the complex viscosity steadily increased if PES inversion structure exhibited and the complex viscosity firstly increased then dropped if co-continuous phase structure formed. The phenomenon can be explained by Einstein equation.The viscoelatic behaviors play an important role in the viscosity evolution during phase separation with final phase inversion morphology. The viscosity show atypical exponential increase procedure η~*(t) =η~* 0 + A_η exp(t/τ_η). And thetemperature-dependent relaxation time τ was thus obtained. The values of relaxation time τ had been fitted with Williams-Landel-Ferry equation separately. It was found that the reference temperature T_s obtained from the fitting was related to T_g of epoxy-rich phase. 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 PES-rich phase.Particularly, two critical gel points were observed in the isothermal curing of the PES modified epoxy systems and were identified as the fixing of the phase structure and the chemical gelation of the cross-link reaction of epoxy, respectively. The two gel times depend on temperature can be fitted with Arrhenius equation separately and then the two kinds of active energy were obtained.We also studied the effect of PES molecular weight, epoxy molecular weight and cure mechanism and the results confirmed further how the rheological behaviors were influenced significantly by viscoelastic phase separation.