Chain Confinement Entropy and Nanoconfined Polymerization Thermodynamics

The equilibrium between monomer and polymer in free radical polymerization is shifted towards monomer under nanoconfinement, resulting in a decrease in the ceiling temperature. The magnitude of the shift is related to the confinement entropy, i.e., the entropy loss on confining a chain (ΔS_chain). The confinement entropy scales with both the polymer chain length and the length scale of the nanoconfinent. Here, we report the results of an investigation of n-alkyl methacrylates confined to the pores of controlled pore glass using differential scanning calorimetry, with n ranging from methyl to ethyl to butyl to dodecyl. The confinement entropy is experimentally determined from the difference in the equilibrium conversion at a given reaction temperature between the bulk and nanoconfined cases. Confinement entropy decreases as the n-alkyl group increases from methyl to butyl, but there is an unexpected increase for dodecyl methacrylate. The scaling of confinement entropy with molecular weight and nanopore size will be discussed and compared to the literature.