Polymerization: Effect of nanoconfinement and ionic liquid additive
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The kinetics, thermodynamics, and product properties of polymerization are known to be influenced by properties including the reaction temperature, monomer concentration, initiator, and solvent. In addition, nanoreactors and specific additives may be used as synthetic tools to optimize the polymerization. This work aims to investigate the effect of nanoconfinement on the free radical polymerization of benzyl methacrylate and to study the effect of ionic liquid additive on the step-growth polymerization of bisphenol A dicyanate ester. The effect of nanoconfinement on the kinetics and thermodynamics of benzyl methacrylate free radical polymerization is investigated using differential scanning calorimetry (DSC). Controlled pore glass (CPG) and ordered mesoporous carbon with pore size ranging from 3 to 8 nm are used as confinement media. The propagation rate in CPG increases as pore size decreases, whereas in mesoporous carbon, the rate decreases. For nanoconfined polymerizations, the rate is proportional to initiator concentration to the one-half power, indicating reaction kinetics similar to the bulk, but longer induction time are observed. Lower conversion is required to reach autoacceleration under nanoconfinement, presumably due to the limited diffusivity and lower termination rate for the confined polymer chains. The molecular weight of the polymer synthesized in the nanopores is generally higher than that obtained in the bulk except at the lowest temperatures investigated. In addition, the apparent activation energy of nanoconfined polymerization is lower than bulk. From the perspective of thermodynamics, the dependence of equilibrium conversion on the temperature is investigated. The entropy loss of the polymer chains due to nanoconfinement is determined using an activity model that relates the entropy loss to the chain length and the size of nanopores. The results are compared with the polymerization of methyl methacrylate. The kinetics of the trimerization reaction of bisphenol A dicyanate ester with an aromatic imidazolium-based ionic liquid as additive is also studied using DSC. The reaction follows second-order autocatalytic kinetics, and a slight acceleration effect is observed in the presence of the aromatic ionic liquid relative to the neat resin. The activation energy also increases with the ionic liquid additive, whereas the glass transition temperature (Tg) is depressed. A model incorporating diffusion effects is able to describe the dynamic and isothermal curing data for both the neat resin system and that containing aromatic ionic liquid. A comparison with aliphatic-based IL additive indicates that the reaction is more accelerated with aliphatic IL than with the aromatic IL in spite of the fact that the aliphatic additive phase separates during cure.Embargo status: Restricted until 06/2022. To request the author grant access, click on the PDF link to the left.