Trimerization Reaction Kinetics and T_g Depression of Polycyanurate under Nanoconfinement

Abstract

Trimerization of a mixture containing a mono-and difunctional cyanate ester is investigated under the nanoporous confinement of silanized hydrophobic controlled pore glass using differential scanning calorimetry. The trimerization reaction of the nanoconfined monomer mixture is accelerated relative to the bulk by as much as 12 times in 8 nm pores, but this acceleration is less than half that observed for nanoconfinement of the individual monomers. The absolute reaction rate of the monomer mixture lies between those of the individual species, being slower than the monocyanate ester and faster than the dicyanate ester. The results are consistent with the hypothesis that the reaction acceleration is due to monomer ordering or layering at the pore surface, leading to a local concentration of reactive groups higher than in the bulk. In addition to the influence of nanoconfinement on trimerization kinetics, the molecular weight and glass transition temperature (T g ) of the polycyanurate formed in the nanopores are investigated. The molecular weight decreases approximately 20% for synthesis in the smallest 8 nm pores relative to the bulk value of 5200 g/mol. Upon extraction from the pores, the polymer T g is 5−9 K higher than in the bulk. However, in the 8 nm diameter pores, a T g depression of 44 K is observed relative to the value of the material after extraction from the pores. This depression lies between the values previously observed for the products of the individual cyanate esters which formed a low molecular weight trimer and a cross-linked polymer network. A secondary T g , associated with a less mobile layer at the pore wall, is 26−40 K above the primary value. The implication is that the origin of confinement effects on reactivity and T g differ, with changes in reactivity in this system arising from surface layering or ordering and T g depressions arising from intrinsic size effects. ■ INTRODUCTIONNanoconfinement is known to influence material properties, including changes in reactivity. Previous work in our laboratory has focused on changes in reactivity in nanopores for the free radical polymerization of methyl methacrylate and the trimerization reaction of cyanate esters. In the case of methyl methacrylate, the higher reactivity under nanoconfinement was explained primarily by a decrease in diffusivity of nanoconfined chains which results in a decrease in the rate of termination and earlier autoacceleration. 1,2 On the other hand, the origin of the acceleration in the trimerization reaction is not as clear. Acceleration of the reaction rate is observed for both a monofunctional monomer that forms a low molecular weight trimer and for a difunctional monomer that forms a crosslinked polycyanurate network; the reaction acceleration is similar for the two monomers although it does depend slightly on reaction conditions. 3−7 The acceleration may be attributable to layering of functional groups near the pore wall, as suggested by simulations of Malvaldi et al. 8 where such ordering led to an increase in the ...

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

“…The higher activation energies E 1 and E 2 also result in acceleration of the 8 wt % BADCy/aromatic IL reaction and higher d x /d T in dynamic runs, such that the BADCy reaction is completed within a shorter time. As a caveat, the authors acknowledge that although the reaction model well describes the cure of dicyanate ester in this work and in the literature, validation of the physical significance of model parameters would require further work.…”

“…The dicyanate ester trimerization reaction is described by a second‐order autocatalytic model: where k 1 and k 2 are rate constants of second‐order and second‐order autocatalytic reactions, respectively. As the reaction progresses, the glass transition temperature T g of the polycyanurate network increases and vitrification occurs when T g reaches the cure temperature.…”

Efficient metal-free strategies for polymerization of a sterically hindered ionic monomer through the application of hard confinement and high pressure

“…It should be highlighted that the presence of two glass transition temperature within polymerized systems is widely reported in the literature and has been previously reported, i.e., for curing of bisphenol M dicyanate ester 40,43 and trimerization of a mixture containing mono-and difunctional cyanate ester. 44 Values of all recorded T g s plotted as a function of pore diameter are presented in Fig. 5c and d.…”

Polymerization Kinetics of Cyanate Ester Confined to Hydrophilic Nanopores of Silica Colloidal Crystals with Different Surface-Grafted Groups

“…For this type of polymerization the changes in the reactivity of monomers have been explained as the effects of confinement on the rates of the initiation [ 11 ] and termination [ 17 , 22 ]. Polycyclotrimerization of cyanate esters is another type of reaction for which the effect of confinement on both reactivity of the reactant monomer and product polymer has been well documented [ 23 , 24 , 25 , 26 , 27 , 28 ]. For these reactions, there have been no specific studies of the significant acceleration in terms of the reaction mechanisms.…”

“…For these reactions, there have been no specific studies of the significant acceleration in terms of the reaction mechanisms. The increased reactivity of monomers has been attributed mostly to increased collision efficiency induced by the nanopore surface in the layer of the adsorbed monomer [ 25 , 26 , 28 ]. Although the effect of the surface silanols groups on the polymerization of cyanate esters has been invoked, it has never been studied in detail [ 29 , 30 ].…”