(488c) Covalently-Immobilized Cleavable Triblock Surfactants for Determination of Triblock Distribution

Poly(ethylene oxide) (PEO) brush layers exhibit desirable cell- and protein-repellant characteristics, and can be easily produced by adsorption of the poly(propylene oxide) backbone of PEO-PPO-PEO triblock copolymers (Pluronic® surfactants) onto hydrophobic surfaces. This technique can be used to increase the biocompatibility of medical devices such as central venous catheters. A key advantage of this method is easy application by simple adsorption of surfactants from aqueous solution to the device surface. However, the adsorbed molecules are subject to desorption and displacement by plasma proteins, leaving coated surfaces devoid of the protective PEO layer. In addition, small peptides have been shown to integrate into the PEO brush, and defects in the brush can allow adsorption of large proteins such as fibrinogen onto "bare spots".

Little is currently known about the morphology of PEO-PPO-PEO triblock copolymer brushes at a hydrophobic surface. The brush layer itself is a two-dimensional fluid and is thus difficult to image using AFM and other traditional surface analysis methods. The non-covalent association of the PPO center-block with the hydrophobic surface also allows rearrangement and relaxation of the adsorbed surfactant with time and changing conditions.

In order to address these analytical issues, we have synthesized Pluronic®-analogues that contain cleavable linkages between the hydrophobic PPO center-block and the two hydrophilic PEO side arms. These triblocks are adsorbed to model surfaces consisting of polished silicon wafers functionalized with vinyl groups. Exposure of the double bonds to gamma radiation produces radicals that covalently immobilize the PPO center-blocks to the wafer surface. After cleaving the PEO side-chains, the resulting PPO-decorated surface can be imaged using AFM and other surface-analytical techniques to determine the packing behavior and surface density.

Incorporation of the radiation-activated double bonds into the backbone of a triblock polymer offers the possibility of using the same immobilization chemistry to produce permanent dense PEO brushes on a variety of "unreactive" surfaces. To demonstrate this method, we synthesized triblock copolymers containing a double bond-rich polybutadiene (PBD) backbone and pendant PEO chains. These PEO-PBD-PEO surfactants were adsorbed onto polyurethane catheter sections from aqueous solution, and then irradiated to covalently immobilize the PBD centerblocks. The morphology and density of the resulting PBD backbones of the surfactant layer was determined by AFM after hydrolysis of the PEO arms. We also used XPS to determine the surface density of the immobilized chains.

Subduction of adsorbed groups under the polymer surface by molecular rearrangements can be observed in polyurethane. This was investigated by including a fluorine tag between the PBD and PEO chains, and using XPS depth profiling to locate the tagged polymers within the bulk polyurethane. These methods provide a valuable tool for investigating the conformation and uniformity of adsorbed surfactants used in biomedical applications.