PhD thesis defense of Joachim Frieder Laun
Dec 08, 2017 - 15.00 uur
Agoralaan Gebouw D
Lokaal auditorium H5
Joachim Frieder Laun invites you to the public defense of his doctoral thesis entitled: "Development of polymer grafting methodologies for advanced surface engineering".
Promoter is Prof. Dr. Tanja Junkers.
Co-promoter is Prof. Dr. Christopher Barner-Kowollik - Queensland University of technology.
Today’s needs for complex functions and steady miniaturization of devices challenge materials scientists towards constant improvement. Surface properties determine the interaction of two materials. Methodologies that allow for tunable surface properties consequently possess a high importance in materials science. With the rise of thermal reversible deactivation radical polymerizations such as ATRP, RAFT, or NMP, polymers with almost any desired architecture and composition have become accessible. However, these thermal techniques either lack spatial control on surface in ‘grafting-from’ approaches or require post-polymerization modification when employed in ‘grafting-to’ approaches. To overcome these two constraints, three polymer grafting protocols have been developed.
Surface-initiated photoinduced copper-mediated radical polymerization was employed to graft a wide range of polyacrylate brushes from silicon substrates at extremely low catalyst concentrations. This is the first time the controlled nature of the reported process has been demonstrated via block copolymer formation and reinitiation experiments. In addition to unmatched copper catalyst concentrations in the range of few ppb, film thicknesses up to almost 1 µm were achieved within only one-hour illumination time.
The above described protocol was subsequently further improved to the first 2D laser lithography protocol for controlled grafting of polymer brushes. Various polyacrylates were grafted from silicon substrates via laser-induced copper-mediated radical polymerization. Film thicknesses up to 39 nm were reached within 125 µs of exposure to UV laser light (351 nm). Successful block copolymerization underpinned the controlled nature of the grafting methodology. The resolution of a small structure of grafted poly(2 hydroxyethyl acrylate) was assessed to be approximately 270 µm and was limited by the type of laser used in the study. Further, a checkerboard pattern of poly(t butyl acrylate) and poly(oligo(ethylene glycol) methyl ether acrylate) was produced and imaged via time of flight secondary ion mass spectrometry (ToF-SIMS), and X-ray photoelectron spectroscopy (XPS).
For polymer surface grafting, post polymerization modifications are often required, which can impose a significant synthetic hurdle. Two strategies that allow for reversible surface engineering via nitrone-mediated radical coupling (NMRC) are reported. Macroradicals stemming from activation of polymers generated by copper-mediated radical polymerization are grafted via radical trapping with a surface-immobilized nitrone or a solution-borne nitrone. Since the product of an NMRC coupling features an alkoxyamine linker, the grafting reactions can be reversed or chain insertions be performed via nitroxide mediated polymerization (NMP). Poly(n-butyl acrylate) (Mn = 1570 g·mol -1, Ɖ = 1.12) with bromine terminus was reversibly grafted to planar silicon substrates or silica nanoparticles as successfully evidenced via X ray photoelectron spectroscopy (XPS), time of flight secondary ion mass spectrometry, and grazing angle attenuated total reflection Fourier-transform infrared spectroscopy (GAATR-FTIR). NMP chain insertions of styrene is evidenced via GAATR-FTIR. On silica nanoparticles, an NMRC grafting density of close to 0.21 chains per nm² was determined by dynamic light scattering and thermogravimetric analysis. Concomitantly, a simple way to decorate particles with nitroxide radicals with precise control over the radical concentration is introduced. Silica microparticles and zinc oxide, barium titanate, and silicon nanoparticles were successfully functionalized.