Trimerization Reaction Kinetics and Tg 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 (Tg) 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 Tg is 5–9 K higher than in the bulk. However, in the 8 nm diameter pores, a Tg 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 Tg, 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 Tg differ, with changes in reactivity in this system arising from surface layering or ordering and Tg depressions arising from intrinsic size effects.