Karen E. Sohn

Amphiphilic Surface Active Triblock Copolymers with Mixed Hydrophobic and Hydrophilic Side Chains for Tuned Marine Fouling-Release Properties

Two series of amphiphilic triblock surface active block copolymers (SABCs) were prepared through chemical modification of two polystyrene-block-poly(ethylene-ran-butylene)-block-polyisoprene ABC triblock copolymer precursors. The methyl ether of poly(ethylene glycol) [M(n) approximately 550 g/mol (PEG550)] and a semifluorinated alcohol (CF(3)(CF(2))(9)(CH(2))(10)OH) [F10H10] were attached at different molar ratios to impart both hydrophobic and hydrophilic groups to the isoprene segment. Coatings on glass slides consisting of a thin layer of the amphiphilic SABC deposited on a thicker layer of an ABA polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene thermoplastic elastomer were prepared for biofouling assays with algae. Dynamic water contact angle analysis, X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) measurements were utilized to characterize the surfaces. Clear differences in surface structure were realized as the composition of attached side chains was varied. In biofouling assays, the settlement (attachment) of zoospores of the green alga Ulva was higher for surfaces incorporating a large proportion of the hydrophobic F10H10 side chains, while surfaces with a large proportion of the PEG550 side chains inhibited settlement. The trend in attachment strength of sporelings (young plants) of Ulva did not show such an obvious pattern. However, amphiphilic SABCs incorporating a mixture of PEG550 and F10H10 side chains performed the best. The number of cells of the diatom Navicula attached after exposure to flow decreased as the content of PEG550 to F10H10 side chains increased.

Protein adsorption resistance of anti-biofouling block copolymers containing amphiphilic side chains

Surface active block copolymers (SABCs) with amphiphilic side chains containing ethoxylated fluoroalkyl groups have previously demonstrated advantageous properties with regard to marine fouling resistance and release. While it was previously postulated that the ability of the block copolymer surface to undergo an environment-dependent transformation in surface structure aided this behaviour, protein adsorption characteristics of the surface were never explored. This study aims to expand our knowledge of protein interaction with the amphiphilic surface active block copolymer in an aqueous environment through experiments with bovine serum albumin (BSA), a widely utilized test protein. Fluorescence microscopy analysis using BSA labelled with fluorescein isothiocyanate (BSA–FITC) was performed on a SABC test surface to establish the polymers protein adsorption resistance. Additionally, atomic force microscopy (AFM) based chemical force microscopy (CFM) was utilized to examine the force of adhesion of an AFM tip functionalized with strands of BSA protein with the SABC. No measurable force of adhesion was detected for 58% of the measurements of adhesion force taken for a BSA coated AFM tip interacting with the surface of the amphiphilic SABC in a PBS buffer. Furthermore, no measurements of force of adhesion were made in excess of 0.15 nN. This was in contrast to the non-zero mean adhesion force seen for several control surfaces in PBS buffer.

Determination of the Electron Escape Depth for NEXAFS Spectroscopy

A novel method was developed to determine carbon atom density as a function of depth by analyzing the postedge signal in near-edge X-ray absorption fine structure (NEXAFS) spectra. We show that the common assumption in the analysis of NEXAFS data from polymer films, namely, that the carbon atom density is constant as a function of depth, is not valid. This analysis method is then used to calculate the electron escape depth (EED) for NEXAFS in a model bilayer system that contains a perfluorinated polyether (PFPE) on top of a highly oriented pyrolitic graphite (HOPG) sample. Because the carbon atom densitites of both layers are known, in addition to the PFPE surface layer thickness, the EED is determined to be 1.95 nm. This EED is then used to measure the thickness of the perfluorinated surface layer of poly(4-(1H,1H,2H,2H-perfluorodecyl)oxymethylstyrene) (PFPS).

Antimicrobial Behavior of Semifluorinated-Quaternized Triblock Copolymers against Airborne and Marine Microorganisms

Semifluorinated-quaternized triblock copolymers (SQTCs) were synthesized by chemical modification of polystyrene-block-poly(ethylene-ran-butylene)-block-polyisoprene ABC triblock copolymers. Surface characterization of the polymers was performed by X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) analysis. The surface of the SQTC showed very high antibacterial activity against the airborne bacterium Staphylococcus aureus with >99 % inhibition of growth. In contrast in marine fouling assays, zoospores of the green alga Ulva settled on the SQTC, which can be attributed to the positively charged surface. The adhesion strength of sporelings (young plants) of Ulva and Navicula diatoms (a unicellular alga) was high. The SQTC did not show marked algicidal activity.

Interfaces in organic devices studied with resonant soft x-ray reflectivity

Interfaces between donor and acceptor semiconducting polymers are critical to the performance of polymer light-emitting diodes and organic solar cells. Similarly, interfaces between a conjugated polymer and a dielectric play a critical role in organic thin-film transistors. Often, these interfaces are difficult to characterize with conventional methods. Resonant soft x-ray reflectivity (R-SoXR) is a unique and relatively simple method to investigate such interfaces. R-SoXR capabilities are exemplified by presenting or discussing results from systems spanning all three device categories. We also demonstrate that the interfacial widths between active layers can be controlled by annealing at elevated temperature, pre-annealing of the bottom layer, or casting from different solvent mixtures. The extension of R-SoXR to the fluorine K absorption edge near 698 eV is also demonstrated.

Structural characterization of an elevated lipid bilayer obtained by stepwise functionalization of a self-assembled alkenyl silane film

This work reports a novel tethered lipid membrane supported on silicon oxide providing an improved model cell membrane. There is an increasing need for robust solid supported fluid model membranes that can be easily deposited on soft cushions. In such architecture the space between the membrane and the substrate should be tunable in the nanometer range. For this purpose a SiO2 surface was functionalized with poly(ethylene glycol) (PEG)-lipid tethers and further modified with poly(ethylene glycol) making a biologically passivated substrate available for lipid bilayer deposition. First, a short chain self-assembled alkenyl silane film was oxidized to yield terminal COOH groups and then functionalized with amino-terminated PEG-lipids via N-hydroxysuccinimide chemistry. The functionalized silane film was then additionally passivated by functionalization of unreacted COOH groups with amino-terminated PEG of variable chain length. X-ray photoelectron spectroscopy (XPS) analysis of dry films, carried out near the C 1s ionization edge to characterize chemical groups formed in the near-surface region, confirmed binding of PEG-lipid tethers to the silane film. XPS further indicated that backfilling with PEG caused the lipid tails to stick up above the PEG layer which was confirmed by the x-ray reflectivity measurements. Lipid vesicle fusion on these surfaces in the presence of excess water resulted in the formation of supported membranes characterized by very high homogeneity and long range mobility, as confirmed by fluorescence bleaching experiments. Even after repeated drying-hydrating cycles, these robust surfaces provided good templates for high fluidity elevated membranes. X-ray reflectivity measurements of the tethered membranes, with a resolution of 0.6 nm in water, showed that these fluid membranes are elevated up to 8 nm above the silicon oxide surface.