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Cyanate ester (CE) foams with different chemical structures were prepared using bisphenol A dicyanate ester (BADCy), bisphenol E dicyanate ester (BECy), and tetramethyl bisphenol F dicyanate ester (TBFDCy) as monomers, through a two-step process. Rheological tests were performed to investigate the optimal conditions for the preparation of these foams. The results of morphology by scanning electron microscopy showed that cells are in the form of nearly spherical shape in foams from TBFDCy and BADCy and oval in foam from BECy. The thermal properties of the three CE foams were studied by methods of dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and thermogravimetry/differential thermogravimetry analysis. The glass transition temperature (Tg) obtained from DMA tests are 274, 264, and 241°C for the foams from TBFDCy, BADCy, and BECy, respectively, which are apparently higher than that tested by DSC method. The Tg, compressive properties, and thermal stabilities of the foams are improved after the introduction of the alkyl-substituent groups to the same aromatic ring of –OCN functionality, and the chemical structure–properties relationships are explained according to the monomer chemical structures. [ABSTRACT FROM AUTHOR]

A new type of cyanate (CE)/epoxy (EP) foam with bisphenol-A dicyanate ester prepolymer and diglycidyl ether of bisphenol-A (BADCy/DGEBA) has been successfully prepared through a two-step process. The structure and properties of CE/EP foam were studied. The results reveal that the CE/EP foams, with relatively uniform cell structure, were composed of closed cells as confirmed by scanning electron microscopy. The compressive strength increased from 0.507 MPa to 3.021 MPa, and the compressive modulus (E) increased from 15 MPa to 123 MPa as the density increased from 0.103 g cm−3 to 0.305 g cm−3. Dynamic mechanical analysis revealed that the CE/EP foams possessed a high glass transition temperature (Tg) (203°C) and that density had only a little impact on Tg. Moreover, the excellent thermal stability presented with the onset of weight loss taken at 5% value was above 320°C, and the residual weight of the foam was more than 21.6% at 800°C. With increase in the density of CE/EP foams, the dielectric constants (ε) gradually decreased. For the foam with density of ρ = 0.162 g cm−3, the value of ε was as low as 2.28 at the frequency of 10 kHz. [ABSTRACT FROM AUTHOR]