Studies On The Reaction-Induced Phase Separation In Poly (Ether Imide) Modified Cyanate Ester Resin

The phase separation of cyanate ester resin modified with polyetherimide (PEI) was investigated by differential scanning calorimeter (DSC), scanning electron microscopy (SEM), time resolved light scattering (TRLS) and so on. The viscoelastic effects on the phase separation of the blend were studied.A novel PEI(PIP) was used to blend with cyanate ester resin. The curing conversion of cyanate ester monomer was slightly affected by the composition in the blend. The phase structure of the blend changed from separated phase, via co-continuous phase, to phase inversion with the increase of PIP content. DSC was also employed to measure the phase diagram of the blend. The results indicated that the PIP content of critical point was around 15wt%. TRLS was used to monitor the phase separation process of the PIP/cyanate(15wt/85wt) blend and the evolution of qm and Im was explained. The Theological behavior of the PIP/cyanate(10wt/90wt) blend could be related with its morphological evolution. It was found by tensile test that the blend with 15wt% PIP had higher tensile strength and elongation at break than that without PIP.The effect of molecular weight of PIP on phase separation was studied. It was shown that the curing conversion and reaction activation energy of the blend withhigh molecular weight PIP were higher than that with low molecular weight PIP. Lower content PIP was needed to obtain the phase inversion in the blend with high molecular weight PIP compared with that with low molecular weight PIP. The tensile mechanical properties of the blend with low molecular weight PIP was better than that with high molecular weight PIP due to the variation of the phase structure. After the cyanate resin was precured at 120C for various time, it was mixed with PIP. It was found that after the cyanate resin was precured at 120C for one hour, the phase structure of the PIP/cyanate(15wt/85wt) blend changed from phase inversion to double phase structure.The curing conversion of the blend cured at 120C for 6 hours only ranged from 10% to 20% while the conversion at gelation ranged from 40%~50%. Therefore, the phase structure of the blend cured at 120C for 6 hours would develop further after postcuring at 210C for 2 hours. The effect of catalyst on the phase structure was similar to the increase of curing temperature.TRLS was used to monitor the phase separation process of the blend with various PIP content. It displayed clearly that the phase separation took place according to spinodal decomposition mechanism and the evolution of scattering vector qm followed Maxwell-type relaxation equation qm(t)=qo+A0exp(-t/ ). The values of relaxation time r at various temperatures were fitted with Williams-Landel-Ferry equation. It was found that the experiment values fitted the equation very well and the reference temperature Ts obtained from the fitting was 34K-62K higher above Tg of the blend. It demonstrated experimentally that viscoelastic effect existed in the phase separation processes.The TRLS results of the Ultem/cyanate blend demonstrated that both from qm and from Im at various temperatures could be fitted with WLF equation, which suggested that viscoelastic phase separation was a universal phenomenon in PEI/cyanate blends.