Mohammad Divandari

ATR-IR Investigation of Solvent Interactions with Surface-Bound Polymers

Solvent interactions with bulk and surface-bound polymer brushes are crucial for functionalities such as controlled friction and thermoresponsive adhesion. To study such interactions, the temperature-induced solvent-quality changes and the effect of surface tethering on the mechanical and tribological properties of poly(dodecyl methacrylate) (P12MA) brushes have been investigated by means of attenuated total reflection infrared spectroscopy (ATR-IR), as well as atomic force microscopy (AFM) and lateral force microscopy (LFM). These results have been compared with temperature-dependent UV–visible spectrophotometry (UV–vis) data for the corresponding bulk polymer solutions. The ATR-IR results clearly show that increasing temperature enhances ethanol uptake in P12MA, which results in film swelling. This is accompanied by a marked increase in both adhesion and friction. We have also shown that a combination of solvents, such as toluene and ethanol, can lead to a temperature-dependent solvent partitioning within the polymer brush. To our knowledge this is the first time preferential solvent uptake in a grafted-from brush has been monitored via in situ ATR-IR. Moreover, we have observed remarkably different behavior for polymer chains in solution compared to the behavior of similar chains bound to a surface. The presented findings on the temperature-dependent solvent interactions of surface-grafted P12MA reveal previously unknown solvation phenomena and open up a range of possible applications in the area of stimuli-responsive materials.

Modulation of Surface-Initiated ATRP by Confinement: Mechanism and Applications

The mechanism of surface-initiated atom transfer polymerization (SI-ATRP) of methacrylates in confined volumes is systematically investigated by finely tuning the distance between a grafting surface and an inert plane by means of nanosized patterns and micrometer thick foils. The polymers were synthesized from monolayers of photocleavable initiators, which allow the analysis of detached brushes by size-exclusion chromatography (SEC). Compared to brushes synthesized under “open” polymerization mixtures, nearly a 4-fold increase in brush molar mass was recorded when SI-ATRP was performed within highly confined reaction volumes. Correlating the SI-ATRP of methyl methacrylate (MMA), with and without “sacrificial” initiator, to that of lauryl methacrylate (LMA) and analyzing the brush growth rates within differently confined volumes, we demonstrate faster grafting kinetics with increasing confinement due to the progressive hindering of CuII-based deactivators from the brush propagating front. This effect is especially noticeable when viscous polymerization mixtures are generated and enables the synthesis of several hundred nanometer thick brushes within relatively short polymerization times. The faster rates of confined SI-ATRP can be additionally used to fabricate, in one pot, precisely structured brush gradients, when volume confinement is continuously varied across a single substrate by spatially tuning the vertical distance between the grafting and the confining surfaces.

Design and characterization of ultrastable, biopassive and lubricious cyclic poly(2-alkyl-2-oxazoline) brushes

Bilayer polymer brushes presenting surface-bound poly(glycidyl methacrylate) (PGMA) films and interfacial cyclic poly(2-alkyl-2-oxazoline) (PAOXA) brushes show excellent biopassivity and lubrication, while displaying long-term stability in chemically harsh aqueous environments. Due to their lower radii of gyration (Rg), cyclic poly(2-methyl-2-oxazoline)s (PMOXAs) and poly(2-ethyl-2-oxazoline)s (PEOXAs) react at high temperatures with PGMA grafts producing ∼50% denser brushes compared to linear analogues featuring comparable molar masses. This generates significantly more hydrated brush interfaces, which quantitatively prevent unspecific surface contamination by biomolecules after several hours of exposure. In addition, the more compact and denser character of cyclic brushes imparts excellent lubricating properties to the bilayered coatings, with the more hydrophilic cyclic PMOXA interfaces reaching a coefficient of friction (μ) of 0.05 against a silica AFM probe in aqueous medium. In addition to their unique physicochemical properties, cyclic PMOXA and PEOXA brushes grafted on PGMA layers demonstrate extremely robust films, which could withstand one month incubation in phosphate buffered saline (PBS) solution, tap water or water from Lake Zurich.