Mechanisms of Decreased Moisture Uptake in Ortho-Methylated Di(cyanate ester) Networks

Abstract

Decreases of up to 50% in the moisture uptake of polycyanurate networks based on 2,2-bis(4-cyanatophenyl)propane (BADCy) and 1,1-bis(4-cyanatophenyl)ethane (LECy) were observed when analogous networks containing a single methyl group ortho-to each aryl−cyanurate linkage were prepared by reduction and acid-catalyzed coupling of salicylic acid followed by treatment with cyanogen bromide and subsequent cyclotrimerization. The differences in water uptake were observed despite similar decreases in packing fraction as conversion proceeded in all networks studied. Conversely, the presence or absence of methyl groups at arylene bridges, remote from the cyanurate oxygen, had no influence on water uptake. Vitrification during cure had little effect on either free volume development or moisture uptake. These results confirm that steric hindrance from ortho-methyl groups inhibits absorption of water presumably by decreasing the thermodynamic favorability of sterically permitted interaction with the cyanurate oxygen. A further examination of the effect of two different catalysts, 2 parts per hundred of a 30:1 by weight mixture of nonylphenol and copper(II) acetylacetonate and 500 ppm of dibutyltin dilaurate (DBTDL), compared to analogous uncatalyzed networks, showed that hydrolytic stability was dramatically affected by catalyst choice, while thermochemical stability was also impacted. These results provide important insights into the mechanisms that determine structure−property relationships in polycyanurate networks.

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

“…23 -25 Water uptake can be alleviated in part through CE monomer development and the incorporation of functional groups that inhibit water absorption through steric or hydrophobic interactions. 26 For example, our research group and others have shown that incorporating some degree of fluorination within CE monomers results in CEs that display some of the lowest water uptake reported to date. 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.…”

“…A brief review of the current literature demonstrates that the mechanism of network hydrolysis appears to be multifaceted and increased water uptake has been assigned to detrimental effects of catalysts, pore sizes within the network that can accommodate water molecules, incomplete curing of the cyanate groups and others . Water uptake can be alleviated in part through CE monomer development and the incorporation of functional groups that inhibit water absorption through steric or hydrophobic interactions . For example, our research group and others have shown that incorporating some degree of fluorination within CE monomers results in CEs that display some of the lowest water uptake reported to date .…”

Structure-Property Relationships for Polycyanurate Networks Derived from Renewable Resources

“…The networks chosen for comparison have all been analyzed using either identical or nearly identical protocols in our laboratories, thereby helping to ensure that comparisons reflect meaningful differences. Table 1 lists the properties of interest for all of these materials, along with the original source publications (9)(10)(11)(12)(13)(14)(15), where available. In some cases, as noted in the table, publication is in progress.…”

Thermal and water absorption properties of cyanate ester resins modified by fluoride‐containing and silicone‐containing components

“…Dai et al reported an epoxidized polysiloxane with amino silicone and used it as a modifier to effectively reduce the dielectric constant of CE resin . Guenthner et al reported the preparation of a novel silyl cyanate monomer from bis(4‐cyanophenyl) dimethylsilane (Simcy), and moisture absorption of the as‐prepared CE resin was reduced by 50%, when compared with pure CE resin . Gu's group synthesized high‐quality and stable fluoride‐containing compound 2‐((3‐(trifluoromethyl)phenoxy)methyl) oxirane (TFMPMO), and the results showed that TFMPMO was gifted with lower moisture absorption than cyanate ester resins .…”