Polycyanurate Networks with Enhanced Segmental Flexibility and Outstanding Thermochemical Stability

Abstract

The synthesis and physical properties of cyanurate networks formed from two new tricyanate monomers, 1,3,5tris[(4-cyanatophenylmethyl]benzene and 3,5-bis [(4-cyanatophenylmethyl)]phenylcyanate, are reported and compared to those of 1,1,1-tris [(4-cyanatophenyl)]ethane (also known as ESR-255). All three networks possessed somewhat different aromatic contents and cross-link densities; however, the thermochemical stability of these networks, as determined by TGA, was outstanding, with that of 1,3,5-tris[(4-cyanatophenylmethyl)]benzene being among the best known for organic cyanate esters despite its comparatively high segmental flexibility. Moreover, the moisture uptake of cured 1,3,5-tris[(4-cyanatophenylmethyl)]benzene, at 2.2% after 96 h immersed in 85 °C water, was comparatively low for a cyanate ester network with a glass transition temperature of 320 °C at full cure. When cured for 24 h at 210 °C, the dry glass transition temperatures of the networks ranged from 245 to 285 °C, while the wet glass transition temperatures ranged from 225 to 240 °C. The similarity in glass transition temperatures resulted from a lower extent of cure in the networks with more rigid segments. In essence, for networks with very high glass transition temperatures at full cure, the process conditions, rather than the rigidity of the network, determined the attainable glass transition temperature. Because networks with a higher extent of cure tend to exhibit slower long-term degradation, in this case, the networks with greater segment flexibility enabled superior performance despite exhibiting a lower glass transition temperature at full cure. These results illustrate that, in contrast to the prevailing heuristics for improving the performance of high-temperature thermosetting polymer networks, a more flexible network with a lower glass transition temperature at full cure can offer an optimal combination of thermomechanical and thermochemical performance.

Organic Crystal Engineering of Thermosetting Cyanate Ester Monomers: Influence of Structure on Melting Point

“…36 The material used for analysis was synthesized and kindly provided by Dr. Matthew Davis at the Naval Air Warfare Center. 37 The ESR-255 sample was consistent with the reported melting point of 388.5 ± 0.3K, heat of melting of 72 ± 4 J/g, and purity of 99.4 ± 0.2%. 27 The silicon-containing analog of ESR-255, tris(4-cyanatophenyl)methylsilane, also known as SiCy-3, was synthesized at AFRL with a reported melting point of 391.3 ± 0.1 K, heat of melting of 74 ± 2 J/g, and purity of 99.21 ± 0.04%.…”

Silicon-containing trifunctional and tetrafunctional cyanate esters: Synthesis, cure kinetics, and network properties

“…Cured networks were prepared from the newly synthesized cyanate esters ( 3 ) and ( 6 ), blends of ( 3 ) and ( 6 ) with the commercial dicyanate ester Primaset ® LECy (1,1‐bis(4‐cyanatophenol)ethane), and three different “control” materials: (1) Primaset ® BADCy (2,2‐bis(4‐cyanatophenyl)propane), (2) SiMCy (bis(4‐cyanatophenyl)dimethylsilane), and (3) ESR‐255 89 (1,1,1‐tris(4‐cyanatophenyl)ethane). The properties of SiMCy and ESR‐255 have been described in numerous previous publications. To assess the effects of purity, some batches of ( 3 ) and SiMCy were purified by passing solutions in dichloromethane through a W‐Prep2XY Yamazen flash chromatograph, followed by rotary evaporation.…”

Synthesis and characterization of siloxane‐based cyanate ester elastomers from readily available materials: a top‐down approach

“…27,28 Additionally, it has been shown that high conversions of the cyanate groups yields networks with more desirable physical properties, including reduced water uptake. 29,30 When unreacted cyanate groups are present, they can decompose or react with water leading to further degradation and blistering within the network. 7,31 One avenue for circumventing the issue of incomplete curing of the CE groups would be to utilize a top-down approach and prepare the CEs from monomers where the triazine cores are pre-formed prior to obtaining the network.…”