Tunable properties and morphologies of biodegradable polymer thin films
Grubbs, Joe Brown
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In this dissertation, homopolymer and block copolymer (BCP) brushes were fabricated using surface-initiated ring-opening polymerization (SI-ROP) of polycaprolactone (PCL) and polylactide (PLA). These two polymer brush coatings exhibited controlled degradation rates. PCL brushes were polymerized from hydroxyl-terminated monolayers utilizing the bifunctional catalyst triazabicyclodecene (TBD). A brush thickness of 40 nm was achieved with a reproducible and unique crystalline morphology. The organocatalyzed PCL brushes were chain extended using lactide in the presence of zirconium n-butoxide to successfully grow PCL/PLA block copolymer (PCL-b-PLA) brushes with a final thickness of 55 nm. The degradation properties of the “grafted to” and “grafted from” brushes were probed using aqueous methanol solutions with elevated pH. Degradation kinetic studies elucidated that the brush density plays a major role in the rate of hydrolysis for the different PCL-based systems. In addition, BCPs using polylactide as a biodegradable sacrificial component and polystyrene (PS) (PS-b-PLA) were studied in order to produce patterned thin films. The PS-b-PLA polymers were prepared using the bifunctional initiator 2-hydroxyethyl 2-bromoisobutyrate (HEBIB). Atom transfer radical polymerization (ATRP) was utilized to synthesize the PS macroinitiator with low dispersity from the bromine end of HEBIB. PLA was then synthesized using the hydroxyl end of HEBIB to fabricate a diblock copolymer that consisted of immiscible covalently attached homopolymers. The covalently attached molecules enabled the formation of phase separated morphological cylinders to create a framework for nanoporous membranes. Solvent annealing using THF was employed to create the highly ordered perpendicular cylinder domains needed to develop nanopores that are desirable in applications such as drug delivery and water filtration. Film thicknesses ranging from 20 to 130 nm were investigated to elucidate the role of thickness in the long-range ordering of cylinders during solvent annealing. In addition, two stereoisomers of lactide were studied as sacrificial components to probe morphological changes in the thin films due to the addition of amorphous and crystalline lactide isomers. Degradation studies of the BCP brushes and phase separated films were carried out using a basic solution (pH 14) in order to promote degradation of the polyester backbone by hydrolysis that yields selective etching of only the biodegradable component. This framework also enables tunability over the degradation kinetics.