The effect of nanoconfinement on free radical polymerization

Date

2021-05

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Abstract

The effect of nanoconfinement on the free radical polymerization of ethyl methacrylate (EMA), butyl mehtharcylate (BMA), benzyl methacrylate (BzMA) and dodecyl methacrylate (DMA) is investigated using differential scanning calorimetry (DSC) and gel permeation chromatography (GPC). For nanoconfined cases, the initial reaction rate for monomer confined in the pores of controlled pore glass is enhanced, with larger effects observed in native pores compared to pores in which the native silanol was converted to trimethyl silyl. The onset of autoacceleration also occurs earlier under nanoconfinement, with decreases in both the conversion and the time required to reach autoacceleration and , respectively, and larger changes for the native pores. At polymerization temperatures above 160 °C, depropogation becomes important as the ceiling temperature is approached and seems to be more pronounced under nanoconfinement than in the bulk.

As the alkyl group gets longer for the DMA monomer, autoacceleration is strongly suppressed during polymerization compared to the methyl methacrylate polymerization. For bulk polymerization, the molecular weight increases as temperature decreases with an infinite molecular weight, crosslinked product obtained at temperatures of 140 °C and below due to chain transfer to polymer. Under nanoconfinement, molecular weight decreases with decreasing pore size, and at all polymerization temperatures, the molecular weight of polymer synthesized in nanopores is smaller compared to that synthesized in bulk conditions, which is contrast to the MMA monomer with short alkyl segments. The fact that crosslinked sample is not obtained in the nanopores for DMA indicates that nanoconfinement suppresses branching caused by chain transfer to polymer.

From the perspective of thermodynamics, the changes in the limiting conversion for nanoconfined BMA and BzMA free radical polymerization are compared to the bulk case in order to determine the chain confinement entropy, which is the entropy lost on confining a chain to a pore. This entropy loss can be as much as forty percent of the entropy change on bulk polymerization (50 J/mol/K compared to 130 J/mol/K). The chain confinement entropy is found to scale with chain length N to the 0.7 ± 0.3 power and to scale with pore size D to -0.9 ± 0.3 for BMA and -1.1 ± 0.1 for BzMA. Implications and comparison to theoretical predictions are discussed.


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Keywords

Nanoconfinement, Free Radical Polymerization, Reaction Kinetics, Thermodynamics

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