Structural, thermal, and rheological characterization of bisphenol A-based cyanate ester resin systems

The relations between the cure chemistry and the physical properties of bisphenol A-based cyanate ester resin systems were examined using a variety of analytical techniques. The bisphenol A dicyanate was cured with the octoates and naphthenates of zinc and manganese and with cobalt acetylacetonate at concentrations ranging from 0 to 750 ppm metal. Nonylphenol was used as a cocatalyst. The mechanism and kinetics of cure were studied using Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), $\sp{13}$C nuclear magnetic resonance (NMR), and size exclusion chromatography (SEC). Cyclotrimerization is the dominant reaction during the cure of cyanate esters. Empirical rate laws were used to predict the cyanate concentration profiles for catalyzed and uncatalyzed systems. A mechanism, based on cyanate-metal complexes and consistent with the kinetics of the reaction, was proposed to describe the pathway to cyclotrimerization. The behavior of catalyzed and uncatalyzed cyanate resins in solution at room temperature conditions was investigated in three different solvents: tetrahydrofuran, methyl ethyl ketone, and chlorobenzene. Both trimerization reactions and the reaction of cyanate groups with water forming carbamate occur under these conditions. The extent of the reaction was dependent on the solvent, the type of catalyst, the concentration of catalyst, and the time the sample remained in solution. The distribution of species in both the pregel and postgel regions during cure was examined using SEC. Apparent molecular weights of species exceeding 40,000 are attained prior to gelation. A kinetic scheme based on the SEC data was used to predict the species distribution in the reaction medium in the pregel region. After gelation, the molecular species undergo retroversion as the high molecular weight species, with higher levels of reactive groups, are predominantly attached to the growing network structure. Gelation in the cyanate systems was examined using extraction studies and rheological measurements. The gel point was characterized by the crossover of the loss and storage moduli. Branching theory based on the recursive nature of the branching process and the laws of conditional probability was used to predict the network parameters. The calculated gel conversion was lower than the value obtained from the experimental data. This may be due to intrachain polymerization that occurs in the system or diffusional limitations which cause the reaction to occur in localized regions. The effects of catalyst type and concentration on the glass transition temperature, the thermal stability, and the rheological properties of the resin were determined using thermogravimetry, dynamic mechanical analysis, and the DSC. A concentration window to achieve the optimum properties was determined by the amount of zinc used in the curing process. No concentration effect was observed for the manganese and cobalt catalysts in the concentration range that was studied