Polyelectrolyte multilayers incorporating photocrosslinking polymers for controlling 2- and 3-dimensional structure

Abstract

Polyelectrolyte multilayer (PEM) assembly is a technology for building thin films with a number of useful and novel functionalities. PEMs interact favorably with proteins and other biomolecules making them potentially valuable as biomaterials. Many systems of polymers have been studied for use in PEMs, including weak polyelectrolytes such as those containing poly(acrylic acid) (PAA) and poly(allyamine hydrochloride) (PAH), which exhibit pH-dependent behavior. Properties such as thickness, swellability, mechanical stiffness, porosity (and by extention optical properties), and ability to adsorb small molecules are all controlled by the charge density of these polymers in the film and their resultant binding state. Patterning covalent binding through photo-crosslinking in addition to electrostatic binding, can locally override the native tendency to change structure with changes in pH. To achieve this, poly(acrylic acid) was chemically modified through a halide esterification reaction to incorporate photo-crosslinkable vinylbenzyl side groups, making poly(acrylic acid-ran-vinylbenzyl acrylate) (PAArVBA). The chemical modification was characterized by nuclear magnetic resonance spectroscopy (NMR) and light spectroscopy.

(cont.) NMR revealed that up to 6% of vinylbenzyl groups could be appended to PAA while maintaining the viability of the crosslinking group and the solubility of PAA in aqueous solution. Light spectroscopy indicated the location of the absorbance peak of the vinylbenzyl groups at 254 nm and the generation of crosslinking radicals was achieved with a quantum yield of 0.013. These parameters allow the polymer to be used in PEM films and to be crosslinked with a standard UV lamp, useful for practical applications. The effects of crosslinking on film thickness, swelling in aqueous solution, and mechanical stiffness were measured with ellipsometry, atomic force microscopy (AFM) and nanoindentation. Crosslinking was observed to limit the degree of film swelling in neutral and basic solutions. The patterning of swelling through UV exposure with masking was achieved with a resolution demonstrated down to 3 /m feature sizes. The mechanical stiffness of both crosslinked and as-built films was found to be less than that for equivalent films built with PAA, attributed to charge effects from the quarternary ammonium residue generated during the esterification reaction. The ability of crosslinked PAH/PAArVBA films to locally suppress induced film porosity was confirmed with ellipsometry, AFM and optical microscopy.

(cont.) The suppression effect was used to build two types of films with patterned optical Bragg reflection through alternation of porous and nonporous stacks in the direction normal to the film plane. Maximum reflectivities higher than 70% were generated from 3.5 stack films due to the unusually low refractive index in the porous regions, modeled as 1.09. Patterned structures, both porous and nonporous, were used to control the location of uptake of fluorescent dyes through increased affinity and surface area, as observed by fluorescence microscopy. Porous channel structures were used to direct dye uptake by controlling solvent transport through capillarity, enabling placement of two separate dyes in close proximity on a film surface without mixing. Finally, bovine serum albumin was used as a model protein to examine the effects of photocrosslinking and porosity on uptake and immobilization of proteins within PEM structures via fluorescence microscopy and radiolabeling. Porous thermally crosslinked films were found to adsorb significantly more protein than most other processing conditions and all films demonstrated retention of 80-95% of the adsorbed protein. Patterning of porosity gave selective control of the location of protein uptake.