The effects of nanoconfinement on the kinetics and thermodynamics of free radical polymerization reactions
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The influence of nanoconfinement on the kinetics and thermodynamics of free radical polymerization reactions is investigated considering the polymerization of methyl methacrylate (MMA) and sulfur. The mathematical model described by Verros et al. for diffusion controlled bulk polymerization of methyl methacrylate is extended to account for polymerization in nanopores by incorporating the effect of nanoconfinement on diffusivity using the scaling reported in the literature. The calculations indicate that nanoconfinement will lead to higher molecular weights and lower polydispersity, and that the gel effect will occur at earlier times. The results qualitatively describe the literature experimental observations for molecular weights and polydispersity. The decrease in the time for the onset of the gel effect was verified in subsequent work in our laboratory on methyl methacrylate polymerization in nanoporous confinement. Experimental calorimetric conversion versus time data in hydrophobic nanopores was described using a simplified kinetic model in which different effects are incorporated using the Doolittle free volume theory. In the case of hydrophobic nanopores, the model uses the same parameters as used for the bulk reaction, plus two additional parameters, one accounting for the change in chain diffusivity on confinement and the another accounting for the modest increase in Tg under nanoconfinement. One additional consideration, catalysis by surface silanol groups, is accounted for in the polymerization in native hydrophilic nanopores. The diffusivity of chains in good solvent scales with molecular size to the –3 power and with nanopore diameter to the 1.3 power. The effect of nanoconfinement on the polymerization of sulfur and its ring/chain equilibrium is examined. We extend Tobolsky and Eisenberg's model of sulfur polymerization to nanopores accounting for the entropy change of chain and ring on nanoconfinement using scaling in literature. The data in the literature, which shows the transition temperature is shifted to higher temperatures with decreasing pore size and the limiting conversion at high temperatures decreases, are quantitatively captured assuming the change of entropy is proportional with chain length to the 2 power and with nanopore diameter to the –3 power. Another prediction of the model is that the maximum average chain length decreases with decreasing pore size.