Effect of ionic liquids on kinetic parameters of dicyanate ester polycyclotrimerization and on thermal and viscoelastic properties of resulting cyanate ester resins

Abstract

Cyanate ester resins (CERs) represent a family of thermosetting polymers possessing attractive intrinsic features, such as excellent dimensional stability, high glass transition temperature (T g > 250°С), low dielectric constants (2.5-3.2), flame-retardancy, and high adhesion to conductor metals and composites. Therefore, they are promising materials for aerospace and microelectronic applications, especially as polymer matrices for structural composites, adhesives, potting resins, and coatings that work under severe conditions (high temperature, humidity, corrosive media, etc) [1][2][3][4][5][6]. Dicyanate ester monomers undergo thermal polycyclotrimerization to generate high T g polycyanurate networks (PCNs), i.e. cyanate ester resins (CERs), without releasing volatile products. Figure 1 describes the reaction scheme of polycyclotrimerization of one of the widely used monomers, i.e. dicyanate ester of bisphenol E (DCBE). Dicyanate ester homopolymerization occurs at high temperature in the presence or the absence of a specific catalyst. The rate of non-catalyzed polycyclo -Abstract. A strong catalytic effect of 1.0 wt% ionic liquids (ILs) on kinetic parameters of dicyanate ester of bisphenol E (DCBE) polycyclotrimerization was evidenced, and structure-property relationships of resulting densely cross-linked cyanate ester resins (CERs) were investigated. Three different ILs with contrasted reactivity were employed as a catalysts: an aprotic IL, i.e. 1-octyl-3-methyl imidazolium tetrafluoroborate ([OMIm][BF 4 ]), a protic IL, i.e. 2-(hydroxyethylamino) imidazolinium chloride ([HEAIm][Cl]), and a protic polymeric IL, i.e. poly(hexamethylene guanidine) toluene sulfonate ([PHMG][TS]). Both [HEAIm][Cl] and [PHMG][TS] were reactive towards DCBE monomer, whereas [OMIm] [BF 4 ] was chemically inert, as confirmed by Fourier Transform Infrared (FTIR) spectroscopy. Noticeably, the conversion (α c ) of cyanate groups in the presence of ILs dramatically increased, and a significant dependence of α c values on IL chemical structure was found. The corresponding mechanisms of DCBE polycyclotrimerization in the presence of different ILs were proposed. All the CER/IL networks exhibited a high thermal stability inherent to neat CER, as shown by TGA, whereas unexpected significant changes of the viscoelastic characteristics for CER/IL networks compared to pure CER analogue were observed using DMTA.

Kinetic Study of Curing Bisphenol A Dicyanate Ester with Ionic Liquid Additive

“…It is also known that water can accelerate the cure of cyanate ester . Indeed, as shown in the results in Figure , we find that addition of 1% water along with 8% aliphatic IL resulted in an acceleration by a factor of 2.5 over the neat resin, which is very similar to Fainleib's more recent work . Although considerable care was taken in this work to ensure that the ILs did not absorb adventitious moisture, the water content of our ILs after storage for several months over dessicant is 0.2–0.3% based on Karl‐Fisher titration.…”

“…As mentioned, the acceleration of the cyanate ester reaction on addition of the aromatic [BnzC 1 im][NTf 2 ] IL that is observed in the present work depends on temperature and is smaller than the acceleration reported by Fainleib and co‐workers for an aliphatic imidazolium IL, 1‐octyl‐3‐methylimidazolium tetrafluoroborate [C 8 C 1 im][BF 4 ] that increased the reaction rate at 150 °C of BEDCy by a factor of three or up to several orders of magnitude . We use a similarly structured cation aliphatic‐based IL but with a different anion, [C 9 C 1 im][NTf 2 ] to investigate the aromatic versus aliphatic nature of IL, and we found that the reaction rate was accelerated by a factor of approximately two, consistent with but slightly less than Fainleib's more recent work.…”

“…In contrast, Fainleib and coworkers found a greater acceleration for an aliphatic IL in BEDCy and at lower temperatures, with no perceptible reaction for the neat resin in 6 h at 150 °C compared to over 80% conversion when 5% aliphatic IL was added . In their recent work, the cure of BEDCy was three times faster when 1% aliphatic IL was added. Thus, we examine the influence of the aromatic versus aliphatic nature of the IL additive on the reaction kinetics of our BADCy by comparing dynamic curing scans at 10 K/min for the two systems.…”

“…However, the acidity of [C 9 C 1 im][NTf 2 ] used in present work is expected to be the same as the [C 8 C 1 im][BF 4 ] used in ref. 31 and 32 according to the Kamlet‐Taft hydrogen‐bond donor (HBD) α parameters of [C 8 C 1 ][NTf 2 ] and [C 8 C 1 im][BF 4 ], which are 0.60 and 0.62, respectively as reported by Welton et al . Interestingly, quantum chemistry calculations indicate that a water molecule can affect the Lewis acidity/basicity pattern of an IL depending upon its location with respect to the ion‐pair .…”

“…Similarly, Throckmorton and Palmese found that the cure of bisphenol E dicyanate ester (BEDCy) was accelerated with a reactive dicyanamide‐containing IL. In the case of non‐reactive IL additives, Fainleib and coworkers reported acceleration of BEDCy trimerization with the incorporation of an aliphatic imidazolium IL and an aliphatic pyridinium IL, as well as with hydroxy‐terminated and polymeric ILs, and they proposed a mechanism to explain the acceleration which involved formation of a cyanate ester/IL complex.…”