Cyanate resins have recently received attention as desired materials for many applications due to their excellent ultimate properties such as high $T_g$, low dielectric constant, and low water absorption as well as no volatile evolution during cure via cyclotrimerization.
Though cyanate resins are known to be relatively tough compared to other thermosets, some applications require improved fracture resistance. Incorporation of thermoplastics with high $T_g$ and high toughness has been recently highlighted as a new approach to enhance the toughness of thermosets without significantly lowering the desirable properties. Formation of semi-IPN, an expanded concept of incorporation of thermoplastics into a thermoset matrix, can be a useful way to combine the toughness of themoplastics and the thermal and chemical resistance of thermosets.
Properties of polymer blends including semi-IPNs are so closely related to the morphology that the analysis on the phase separation behavior during cure is essentially needed.
In this work, semi-IPNs were synthesized from the mixture of Bisphenol-A dicyanate(BPACY) and polyetherimide(PEI) and the effect of the curing conditions(composition, cure temperature, catalyst) on the phase separation behavior as well as on the final morphology and properties was analyzed.
1. Cure Kinetics
Cure kinetics for the cyclotrimerization was studied by DSC. Kinetic equation was obtained through the analysis of the reaction mechanism and the parameters were determined by Marquart non-linear regression. Retardation in reaction rate due to gelation and/or vitrification was described by introducing the WLF-type rate constant. Kinetic equation for catalyzed reaction was also obtained by the sum of the contribution of uncatalyzed reaction and that of the catalyzed reaction. The calculated data from these equations showed good agreement with the experimental data.
2. Semi-IPNs synthesized by uncatalyzed reaction
The mixtures of PEI and BPACY monomer showed UCST behavior. The semi-IPNs formed had sea-island morphology in 1-14wt% PEI composition, dual-phase morphology in 15-19wt% PEI composition and nodular morphology in 20-60wt% PEI composition, respectively.
Cure temperature did not influence the macroscopic morphology, but the domain size changed with temperature. As cure temperature was increased, the PEI domain size in the sea-island morphology decreased, while the BPACY nodule size increased in the nodular morphology.
The loss modulus behavior from the dynamic mechanical analysis reflected the morphology and the relative area of the loss peaks corresponding to each separated phase provided the information about the phase volume ratio.
$T_g$ behavior of the semi-IPN as a function of composition showed different trend reflecting different phase separation mechanisms below and above 15wt% PEI composition.
This feature could be analyzed with the free energy diagram. Analysis on $T_g$ behavior indicated that sea-island morphology in 1-14wt% PEI was formed via nucleation and groqth machanism, while dual-phase morphology and nodular morphology were formed dominantly via spinodal decomposition.
The different phase separation mechanism also confirmed by the light scattering studies. In addition, light scattering profile of the mixture with 15wt% PEI during isothermal cure indicated the existence of macroscale domain in dual-phase morphology. The macroscale domain was formed by the rapid growth from the initial small spinodal structure(5.4$μm) and the dual-phase morphology was formed by the secondary phase separation in the already formed large scale domains.
Mechanical and thermal properties were so strongly dependent upon the morphology that they changed dramatically near the phase inversion point.
3. Semi-IPNs synthesized by catalyzed reaction. Semi-IPNs cured with zinc stearate showed different morphology from that of the uncatalyzed system. That is, sea-island morphology in 10wt% PEI, dual-phase morphology in 15wt% PEI in uncatalyzed reaction changed to dual-phase morphology and nodular morphology in catalyzed system, respectively. This change was attributed to the change of the phase separation mechanism by the rapid movement of the binodal line in the phase diagram due to the increase in the reaction rate by the incorporation of catalyst.
Dynamic mechanical analysis on the catalyzed system also reflected the morphology change.
$T_g$ behavior showed a similar trend to that of the uncatalyzed system although the boundary composition separating the nucleation and growth and spinodal decomposition moved to lower PEI composition. It was also found through the comparison of $T_g$ behavior of catalyzed system with that of uncatalyzed system that the dual-phase morphology was formed by 3-stage phase separation ; the nucleation and growth/first, spinodal decomposition/second, and each mechanism within the domains formed by spinodal decomposition at second stage.
Mechanical and thermal properties were also strongly dependent upon the morphology similar to the uncatalyzed system.
