Direct Measurement of Molecular Weight and Grafting Density by Controlled and Quantitative Degrafting of Surface-Anchored Poly(methyl methacrylate)

Abstract

We report on quantitative determination of the molecular weight distribution (MWD) and grafting density (σ_P) of polymer assemblies grown by controlled radical polymerization from flat substrates as a function of polymerization time and the ratio between the inhibitor and catalyst species. Specifically, we grow poly(methyl methacrylate) (PMMA) brushes on flat silica-based surfaces by surface-initiated atom transfer radical polymerization (SI-ATRP), cleave the PMMA grafts quantitatively using tetrabutyl ammonium fluoride (TBAF), and analyze their MWD by size exclusion chromatography equipped with a high-sensitivity differential refractive index detector. The polymer growth and degrafting processes are followed by ellipsometry, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry. The σ_P is independent of polymerization time and increases with increasing SI-ATRP inhibitor/catalyst ratio. Specifically, σ_P increases from 0.48 ± 0.06 to 0.58 ± 0.06 chains/nm² as the inhibitor/catalyst molar ratio increases from 0 to 0.015, respectively, providing evidence that high inhibitor/catalyst ratio offers better control of the SI-ATRP reaction, by lowering number of terminations, and leading to denser PMMA brush assemblies.