New Insights into Structure–Property Relationships in Thermosetting Polymers from Studies of Cocured Polycyanurate Networks

Studies of the physical properties of the cocured networks formed from three similar dicyanate ester monomers revealed a number of unexpected variations from simple linear mixing rules. These variations shed light on important synergistic effects in cocured thermosetting networks and their possible causes. The monomers utilized were the dicyanate esters of Bisphenol A (BADCy) and Bisphenol E (LECy) and the silicon-containing analogue of Bisphenol A (SiMCy). The most important of the synergistic effects was a decrease of ∼25% in moisture uptake seen only in conetworks of LECy and SiMCy. For all other systems, a clear relationship between moisture uptake and the number density of cyanurate rings was observed. This relationship generally applies to many types of cyanate esters and gives an indication of the importance of specific sites (as opposed to free volume alone) in moisture uptake. Numerous additional examples of nonlinear mixing relations were observed in the glass transition temperature, density, and thermochemical stability of fully cured networks. Interestingly, the most widespread deviations from linear behavior were observed for conetworks of SiMCy and LECy, suggesting that factors such as the mismatch in network segment size may be more important than differences in flexibility or symmetry in driving significant physical interactions among conetwork components

Hansen Solubility Parameters for Octahedral Oligomeric Silsesquioxanes

The Hansen Solubility Parameters (HSP) for several polyhedral oligomeric silsesquioxane (POSS) compounds were successfully determined, demonstrating the applicability of the HSP approach for selected types of organic–inorganic compounds. As commonly practiced with organic polymers, a set of simple “pass/fail” tests for complete solubility at a fixed concentration (100 mg/mL) was conducted for an array of five octameric POSS compounds, octa­(phenethyl), octa­(styrenyl), octa­(<i>iso</i>butyl), octakis­(hexafluoro<i>iso</i>butyl), and (1-naphthyl)­heptaphenyl), and 45 test solvents. Group contributions for the octameric POSS cage were determined using three different approaches, which produced similar results. The best cage contribution estimate for the dispersive, polar, and hydrogen-bonding components δ_D, δ_P,, and δ_H of the total solubility parameter was determined to be δ_D = 22 (J/cc)^1/2, δ_P = 19 (J/cc)^1/2, and δ_H = 15 (J/cc)^1/2, with an estimated uncertainty of approximately 5 (J/cc)^1/2. The utility of the HSP approach was demonstrated by successfully identifying mixtures of poor solvents that provided significantly enhanced solubility for octa­(<i>iso</i>butyl) POSS, and by successfully estimating the HSP of octakis­(trifluoropropyl) POSS from group contributions derived solely from aromatic POSS compounds

Effect of Nanoparticle Functionalization on the Performance of Polycyanurate/Silica Nanocomposites

The impact of silica functionalization in determining the performance of polycyanurate networks polymerized from 1,1-bis­(4-cyanatophenyl)­ethane, known commercially as Primaset LECy, reinforced with modified fumed silica, was elucidated through systematic comparison of the properties of nanocomposite networks in which the silica surface treatment was altered. Three types of surfaces were investigated: moderately acidic (unmodified silanol), neutral (alkylsilane modified), and slightly basic (3-aminopropylsilane modified). In terms of cyanate ester cure, the acidic surface proved to be moderately catalytic, the neutral surface mildly catalytic due to slight residual silanol content, and the basic amino-functional surface mildly inhibitory. In terms of network performance, the amino-functional surface led to significant degradation of the network at elevated temperatures, while the silanol-functional surface outperformed the alkyl-functional surface in terms of protection against hydrolytic degradation. In agreement with expectations, the addition of 2–5 wt% of relatively well-dispersed silica nanoparticles had negligible impact on the fracture toughness of the cyanurate networks. Overall, these results demonstrate that the functionalization of nanoparticle additives for polycyanurate networks is an important determinant of performance and must be taken into consideration in the development of polycyanurate nanocomposites, even at levels that are too low to strongly affect mechanical properties

Di(cyanate Ester) Networks Based on Alternative Fluorinated Bisphenols with Extremely Low Water Uptake

A new polycyanurate network exhibiting extremely low moisture uptake has been produced via the treatment of perfluorocyclobutane-containing Bisphenol T with cyanogen bromide and subsequent thermal cyclotrimerization. The water uptake, at 0.56 ± 0.10% after immersion in water at 85 °C for 96 h, represents some of the most promising moisture resistance observed to date in polycyanurate networks. This excellent performance derives from a near optimal value of the glass transition at 190 °C at full cure. Superior dielectric loss characteristics compared to commercial polycyanurate networks based on Bisphenol E were also observed. Polycyanurate networks derived from this new monomer appear particularly well-suited for applications such as radomes and spacecrafts where polycyanurates are already widely recognized as providing outstanding properties

Effects of <i>o</i>‑Methoxy Groups on the Properties and Thermal Stability of Renewable High-Temperature Cyanate Ester Resins

Renewable phenols derived from biomass sources often contain methoxy groups that alter the properties of derivative polymers. To evaluate the impact of <i>o</i>-methoxy groups on the performance characteristics of cyanate ester resins, three bisphenols derived from the renewable phenol creosol were deoxygenated by conversion to ditriflates followed by palladium-catalyzed elimination and hydrolysis of the methoxy groups. The deoxygenated bisphenols were then converted to the following cyanate ester resins: bis­(4-cyanato-2-methylphenyl)­methane (<b>16</b>), 4,4′-(ethane-1,1′-diyl)­bis­(1-cyanato-3-methylbenzene) (<b>17</b>), and 4,4′-(propane-1,1′-diyl)­bis­(1-cyanato-3-methylbenzene) (<b>18</b>). The physical properties, cure chemistry, and thermal stability of these resins were evaluated and compared to those of cyanate esters derived from the oxygenated bisphenols. <b>16</b> and <b>18</b> had melting points 37 and >95 °C lower, respectively, than the oxygenated versions, while <b>17</b> had a melting point 14 °C higher. The <i>T</i>_g’s of thermosets generated from the deoxygenated resins ranged from 267 to 283 °C, up to 30 °C higher than the oxygenated resins, while the onset of thermal degradation was 50–80 °C higher. The deoxygenated resins also exhibited water uptakes up to 43% lower and wet <i>T</i>_gs up to 37 °C higher than the oxygenated resins. TGA-FTIR of thermoset networks derived from <b>16</b>–<b>18</b> revealed a different decomposition mechanism compared to the oxygenated resins. Instead of a low-temperature pathway that resulted in the evolution of phenolic compounds, <b>16</b>–<b>18</b> had significantly higher char yields and decomposed via evolution of small molecules including isocyanic acid, CH₄, CO₂, and NH₃

Di(cyanate Ester) Networks Based on Alternative Fluorinated Bisphenols with Extremely Low Water Uptake

A new polycyanurate network exhibiting extremely low moisture uptake has been produced via the treatment of perfluorocyclobutane-containing Bisphenol T with cyanogen bromide and subsequent thermal cyclotrimerization. The water uptake, at 0.56 ± 0.10% after immersion in water at 85 °C for 96 h, represents some of the most promising moisture resistance observed to date in polycyanurate networks. This excellent performance derives from a near optimal value of the glass transition at 190 °C at full cure. Superior dielectric loss characteristics compared to commercial polycyanurate networks based on Bisphenol E were also observed. Polycyanurate networks derived from this new monomer appear particularly well-suited for applications such as radomes and spacecrafts where polycyanurates are already widely recognized as providing outstanding properties

Synergistic Physical Properties of Cocured Networks Formed from Di- and Tricyanate Esters

The co-cyclotrimerization of two tricyanate ester monomers, Primaset PT-30 and 1,2,3-tris­(4-cyanato)­propane (FlexCy) in equal parts by weight with Primaset LECy, a liquid dicyanate ester, was investigated for the purpose of exploring synergistic performance benefits. The monomer mixtures formed stable, homogeneous blends that remained in the supercooled liquid state for long periods at room temperature, thereby providing many of the processing advantages of LECy in combination with significantly higher glass transition temperatures (315–360 °C at full cure) due to the presence of the tricyanate-derived segments in the conetwork. Interestingly, the glass transition temperatures of the conetworks after cure at 210 °C, at full cure, and after immersion in 85 °C water for 96 h were all higher than predicted by the Flory–Fox equation, most significantly for the samples immersed in hot water. Conetworks comprising equal parts by weight of PT-30 and LECy retained a “wet” glass transition temperature near 270 °C. The onset of thermochemical degradation for conetworks was dominated by that of the thermally less stable component, while char yields after the initial degradation step were close to values predicted by a linear rule of mixtures. Values for moisture uptake and density in the conetworks also showed behavior that was not clearly different from a linear rule of mixtures. An analysis of the flexural properties of catalyzed versions of these conetworks revealed that, when cured under the same conditions, conetworks containing 50 wt % PT-30 and 50 wt % LECy exhibited higher modulus than networks containing only LECy while conetworks containing 50 wt % FlexCy and 50 wt % LECy exhibited a lower modulus but significantly higher flexural strength and strain to failure. Thus, in the case of “FlexCy”, LECy was copolymerized with a tricyanate that provided both improved toughness and a higher glass transition temperature

Effect of Segmental Configuration on Properties of <i>n</i>‑Propyl-Bridged Polycyanurate Networks

The effect of the chemical configuration of network segments on the physical properties, cure properties, mechanical performance, and chemical stability of polycyanurate networks was investigated via synthesis, network formation, and characterization of an isomeric series of <i>n</i>-propyl-bridged cyanate ester monomers. Configurations that provide cyanurate oxygen atoms with either nearby methyl groups or nearby bridge groups exhibited decreased moisture uptake by up to 50%, along with a roughly 20–40 °C reduction in the loss in glass transition temperatures due to hydrolysis, for networks immersed in 85 °C water for 96 h. In <i>ortho</i>,<i>para</i>-linked aryl cyanates, dry glass transition temperatures of cured networks were reduced compared to analogous <i>para</i>,<i>para</i>-linked networks by only about 10 °C, compared to a reduction of 30 °C in <i>ortho</i>-methylated cyanate ester networks, leading to higher “wet” glass transition temperatures in the <i>ortho</i>,<i>para</i>-linked networks. Neither methyl groups nor bridge groups in a position <i>ortho</i> to the reactive cyanate ester groups prevented the creation of networks with >99% conversion at cure temperatures of 230 °C. Networks with placement of methyl groups in a position <i>ortho</i> to the cyanate ester exhibited char yields in nitrogen at 600 °C of 46–47% compared to 43% for networks with methyl groups in the corresponding <i>meta</i> position, regardless of whether a sterically hindered environment was present around the cyanurate oxygen. These results illustrate the manner in which the chemical configuration around reactive groups can substantially modify the properties of networks even when the number density and type of reactive group present do not change

Mechanisms of Decreased Moisture Uptake in <i>Ortho</i>-Methylated Di(cyanate ester) Networks

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 <i>ortho</i>- 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 <i>ortho</i>-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